Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry

Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry Zeldah Ngwanakgagane Sentsho, Pieter Pistorius, Andrew Venter, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8317506/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract This study explored phase transformations and thermal expansion coefficients in 9%Cr steels; P91 and P92 base metals, and P91 welds, on heating to 1000°C. Neutron powder diffraction was employed to determine transformation temperatures, lattice parameters, and phase fractions. Austenite onset (A c1 ) and completion (A c3 ) temperatures were 800–805°C and 885–890°C for P91 base metal, and 814–816°C and 928–930°C for P92. The weld metals were evaluated in the as-welded condition. At room temperature, weld metals contained ferrite, martensite, and residual austenite. Residual austenite in weld 1 was retained on heating until fresh austenite formed at 765–770°C, whereas in weld 2 it disappeared at 655–660°C. The A c1 and A c3 temperatures of weld 2 were 815–820°C and 920–922°C. P92 transformed at lower temperatures and thermal expansion during the α-BCC-to-austenite transformation than P91. Weld 1 showed higher transformation temperatures and expansion than the base metal. Dilatometry measured transition temperatures of 817°C and 880°C for P91 base metal, and 836°C and 923°C for P92. As-welded curve exhibited multiple transitions between 697–904°C. Calorimetry showed endothermic α-ferrite-to-austenite transitions with A c1 and A c3 at 824°C and 874°C for P91 base metal, 839°C and 893°C for P92, and 831°C and 876°C for weld 1. This study demonstrates the advantages of integrating neutron diffraction, dilatometry, and calorimetry to improve transformation analysis in 9%Cr steels; neutron diffraction measured A c1 up to 20°C lower than dilatometry, indicating current post-weld heat treatment temperature ranges may be insufficiently conservative. Residual austenite in weld metal and its behaviour on heating may also influence weld heat-treatment response. Creep-strength-enhanced ferritic steel Phase transition Thermal expansion Endothermicity Exothermicity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Creep-strength-enhanced ferritic (CSEF) steels with compositions of 9%–12%Cr are widely used in power plants, mainly owing to their creep resistance and stability to oxidation at temperatures up to 600°C [ 1 – 3 ]. Understanding the phase transitions from martensite to austenite during heating and back to martensite during cooling are critical for optimising their performance [ 4 – 6 ]. This work focused on ASTM A335 Grade P91 and 92 base metals, and P91 weld metals. The metals were heated to 1000°C to characterise phase transitions between α-ferrite (body-centred cubic; bcc) and ɣ-austenite (face-centred cubic; fcc). α‑Ferrite is ferromagnetic at room temperature, but changes to paramagnetic at the Curie point (Tc) [ 7 – 11 ]. On heating through the A c1 temperature, α-ferrite begins to transform to austenite. The A c3 temperature represents the upper critical temperature at which α-ferrite has completely transformed to austenite on heating. Post-weld heat treatment (PWHT) is usually carried out just below the A c1 temperature, at 740–770°C [ 12 ]. Dilatometry is commonly used to measure A c1 and A c3 transformation temperatures in steels, but its accuracy is compromised by the assumption of constant thermal expansion coefficients, which does not account for the temperature-dependent and phase-specific nature of thermal expansion, particularly during the coexistence of ferrite and austenite. As a result, the onset of austenite formation during heating may be inaccurately identified, leading to a shift in the transformation temperature. This limitation has been highlighted in both experimental studies and round-robin tests, often leading to underestimation of the true onset of austenite phase formation on heating [ 13 – 15 ]. Nitsche et al. [ 13 ] reported the results of a round-robin test measured on 9%Cr base metal at twelve laboratories that involved slow heating (at 28°C/h) from 700–1000°C to measure the A c1 and A c3 temperatures under conditions approaching equilibrium, using either dilatometry or differential thermal analysis. Ten laboratories reported an A c1 temperature of 820°C ± 10°C. Two laboratories reported a significantly lower A c1 temperature of approximately 770°C. No single explanation for this difference was presented. There is therefore a need for complementary techniques to improve the characterisation of phase transformations and thermal expansion of materials in high-temperature environments. This study aimed to compare neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC) techniques for measuring phase transformation temperatures during heating. While dilatometry provides macroscopic data on thermal expansion, it may fail to account for non-linear lattice changes [ 16 ]. In CSEF steels, slope changes are associated with the precipitation of large quantities of carbonitrides in the austenite nucleation range [ 17 ]. The threshold quantity of these carbonitrides required for a slope change is not known, therefore phase transitions may occur prior to detection of the slope change. When phase transitions occur from a multiphase material during a heat treatment cycle, interpretation of the dilatometry curve becomes complex. Additionally, length changes due to contamination or oxidation of the material may occur [ 18 ]. At the Curie temperature, ferromagnetic steels undergo a transition to the paramagnetic state, resulting in slight lattice expansion and an increase in the thermal expansion coefficient without any change in crystal structure. For pure iron, this transition occurs at approximately 770°C [ 9 , 10 , 19 ]. In 9%Cr ferritic steels, alloying reduces the magnetic moment, lowering the Curie temperature to 740–750°C [ 18 , 20 ]. NPD directly measures the atomic structure that enables determination of the phase fractions and lattice parameters, offering detailed insight into phase transformations [ 21 – 23 ]. Hosemann et al. [ 24 ] investigated T91 and HT-9 steels during normalisation and tempering cycles, heating samples to 1050°C and 760°C, respectively, at 200°C/h, with diffraction measurements taken at 5 min intervals with constant temperature holds, and at 2 min intervals during continuous cooling, resulting in a temperature resolution of approximately 30°C. Tomota et al. [ 25 ] investigated ferrite and pearlite transformations in a 1.5Mn–1.5Si–0.2C steel using in-situ neutron diffraction during continuous cooling from 900°C. The study enabled real-time tracking of transformation kinetics without temperature holds. Gong et al. [ 26 ] studied Fe–Ni–C alloys using time-resolved neutron diffraction during cryogenic cooling from room temperature to − 269°C, focusing on martensitic transformation and lattice evolution. Their findings revealed deviations in austenite lattice constants and highlighted the role of carbon in maintaining martensite tetragonality. Tomota et al. [ 27 ] reviewed neutron diffraction applications in steels, covering transformation behaviour from ambient temperature to 1000°C under both continuous and isothermal conditions. DSC provides an understanding of thermodynamic properties and transformations by examining heat flow as a function of temperature and time [ 28 , 29 ]. The heat flow measurements detect energy absorption, which characterises an endothermic phase transition, or the release of heat, an exothermic phase transition, by changes in sample temperature. Endothermic transitions are associated with processes like decomposition, melting, and vaporization; exothermic transitions are indicative of crystallisation, oxidation, and combustion. In addition to determining the endothermic and exothermic phase transitions corresponding to heat absorption or release phenomena [ 29 ], DSC can detect the Curie temperature of a transition from ferromagnetic to paramagnetic behaviour in 9%Cr steels [ 9 , 11 ]. Established alternative methods for determining transition temperatures in ferritic steels involve deriving predictive formulas based on thermodynamic simulations using Thermo-Calc or JMat-Pro and alloy composition [ 2 , 4 , 20 , 30 ]. Andrew [ 30 ] developed empirical formulas based on alloy composition to determine A c1 and A c3 temperatures [ 30 ]. Santella proposed equations for predicting A 1 temperatures for P91 and P92 based on thermodynamic simulations [ 20 ]. Wang et al. [ 2 ] developed a design-of-experiment approach using single-sensor differential thermal analysis measurements to determine the A 1 temperature in P91 and P92 steels, resulting in a least-squares fitting model that correlates A 1 with alloy composition. These formulas are used in this study as first-order approximations of the A c1 and A c3 temperatures. This study compares information obtained from NPD, dilatometry, and DSC techniques in determining the characteristics of phase transitions in ferritic steels over the temperature range from ambient to 1000°C. 2. Materials and methods 2.1 Sample geometry Table 1 summarises the sample geometries employed using the various techniques. Table 1 Sample dimensions (diameter and length in mm) for the three characterisation techniques Sample NPD Dilatometry DSC Pure Fe Ø5 × 40 Ø5 × 10 — P91 base metal Ø5 × 40 Ø5 × 10 Shavings P92 base metal Ø5 × 40 Ø5 × 10 Ø5 × 2 P91 weld 1 Ø5 × 40 Ø5 × 10 Ø5 × 2 P91 weld 2 Ø5 × 40 — — 2.2 Pure iron rod A 99.99% pure iron rod (Goodfellow, England) was utilised as a standard to calibrate sample temperature measurements in both the neutron diffraction vacuum furnace and the dilatometer. The sum of all reported alloying elements in the rod was less than 50 ppm. 2.3 9%Cr base and weld metals The primary samples of this study were P91 and P92 base metal tubes with wall thickness of 35 mm and P91 weld metals. The elemental compositions, determined by optical emission spectroscopy, are listed in Table 2 . The base metals were received in the normalised and tempered condition. Table 2 Elemental compositions of ASTM A 335 Grade 91 and 92 base metals Element ASTM A 335 Grade P91 range [mass%] Actual composition Grade P91 [mass%] ASTM A 335 Grade P92 range [mass%] Actual composition Grade P92 [mass%] Al < 0.04 0.02 < 0.04 ≤ 0.01 B — 0 0.00–0.01 0 C 0.08–0.12 0.11 0.07–0.13 0.09 Cr 8.00–9.50 8.17 8.50–9.50 8.6 Cu 0.3 0.12 — 0.1 Mn 0.30–0.60 0.51 0.30–0.60 0.43 Mo 0.85–1.05 0.89 0.30–0.60 0.54 N 0.03–0.07 0.05 0.03–0.07 0.05 Nb 0.06–0.10 0.08 0.04–0.09 0.09 Ni < 0.40 0.2 < 0.40 0.19 P < 0.02 0.01 — 0.01 Si 0.20–0.50 0.24 < 0.50 0.14 V 0.18–0.25 0.19 0.30–0.60 0.22 W — — 1.50–20 1.87 Table 3 Elemental compositions of Grade 91 filler metal and weld metal Element Manufacturer composition ECrMo 91 B 42 H5 [mass%] Measured composition P91 weld 1 [mass%] Manufacturer composition CrMo91 EB91 [mass%] Measured composition P91 weld 2 [mass%] Al — ≤ 0.005 — ≤ 0.005 C 0.1 0.09 0.1 0.08 Co — 0.012 — 0.007 Cr 9 9.85 8.8 8.42 Cu — 0.03 0.03 0.07 Fe Balance Balance Balance Balance Mn 0.7 0.68 0.6 0.44 Mo 1 0.96 0.94 1.06 N 0.05 — 0.04 — Nb 0.06 0.05 0.06 0.07 Ni 0.7 0.66 0.5 0.34 P — 0.013 0.007 ≤ 0.005 S — ≤ 0.005 0.003 0.008 Si 0.4 0.29 0.24 0.25 Ti — ≤ 0.005 — ≤ 0.005 V 0.2 0.28 0.2 1 Single-pass bead-on-plate welds 1 and 2 (designated as P91 weld metal 1 and P91 weld metal 2) were produced by shielded metal arc welding using electrodes with a diameter of 3.2 mm that met the requirements of EN ISO 1599 ECrMo 91 B 42 H5 and EN ISO 24598-A-S CrMo91 on a substrate of Grade P91 base metal. The heat input varied between 0.8 and 1.5 kJ/mm. The preheating temperature was maintained above 250°C. The interpass temperature was kept below 400°C. Samples were sectioned parallel to the welding direction through the centre of the overlaid weld beads. Although PWHT is a standard practice for P91 welded joints, the weld samples were examined in the as-welded condition in this study to enable assessment of phase transformation behaviour during heating up to the onset of austenite. Table 3 provides the elemental compositions of the welds determined by optical emission spectroscopy. 2.4 Experimental techniques 2.4.1 Neutron powder diffraction The NPD experiments were performed using the Wombat instrument at ANSTO, Australia, which uses the (115) reflection of a germanium crystal monochromator to produce a 0.1170 nm neutron beam [ 31 , 32 ]. Diffraction data were collected over the 2θ range of 28° to 148°. A certified NIST Al 2 O 3 powder sample was used as reference standard to calibrate instrument parameters, such as wavelength and 2θ offset. Data analysis was conducted using Rietveld refinement in GSAS II, leveraging least-squares fitting algorithms to model crystal structures [ 33 , 34 ]. Samples were stationary during measurements conducted in a niobium element furnace optimised for neutron scattering measurements. The temperature ranged from room temperature to 1000°C, with heating rates of 600°C/h up to 700°C and 60°C/h from 700–1000°C. At each discrete target temperature, samples were temperature-stabilised for 5 min before diffraction patterns were measured under isothermal control for 5 min. In-situ NPD experiments with pure iron rods were conducted to calibrate the furnace from room temperature to 1000°C. Pure iron has well-defined and extensively characterised phase transformation temperatures under equilibrium conditions. In this study, all diffraction peaks were measured at the selected neutron beam wavelength, with no peak overlap. The sensitivity of phase quantification using Rietveld refinement methods is dependent on the fraction present for the specific phase. Leon-Reina et al. [ 35 ] reported a quantification limit near 0.10 mass% for well-crystallised inorganic phases in laboratory powder diffraction, although accuracy was poor, with relative errors of approximately 100%. Reliable analyses, with errors below 20%, were only achieved for phase fractions exceeding 1.0 mass%, consistent with reported detection limits of approximately 1–2 mass% for well-resolved crystalline phases [ 34 , 36 – 38 ]. Below this threshold, quantification becomes increasingly uncertain due to limitations in signal-to-noise ratio and peak resolution. 2.4.2 Dilatometry A DIL805 quenching dilatometer with a length resolution of 50 nm and temperature resolution of 0.05°C was used, with measurements for length changes accurate to 0.01 µm. Temperature was monitored by two type K (Ni–Cr/ Ni–Al) thermocouples spot-welded to the middle of each sample [ 2 ]. Continuous length-change measurements were conducted in a helium gas environment over the temperature range of 25–1000°C. Heating rates were set at 600°C/h from ambient to 700°C, followed by a slower rate of 28°C/h from 700–1000°C. Data acquisition adhered to ASTM A1033-18 [ 16 ], with constructed tangents applied at phase transformation points to determine A c1 and A c3 temperatures. Phase fractions within the transformation zone were calculated by fitting additional tangents, firstly to establish the temperature at which the ɣ-austenite phase reached 50%, and then subsequent phase fractions determined using the same methodology at temperatures between A c1 and A c3 . 2.4.3 Differential scanning calorimetry DSC experiments were conducted in a TG/DSC TA instrument to record the heat flow of the samples as a function of temperature and time. At ambient temperature, recorded sample masses were 275.99 mg, 23.33 mg, and 19.46 mg for P91 base metal, P92 base metal, and P91 weld metal, respectively. Samples were heated from ambient temperature to 1000°C at 20°C/min in a controlled nitrogen atmosphere, with measurements continuously recorded throughout the heating process. 3. Results and discussion 3.1 Phase transformation temperatures Comparison of the phase fractions determined using NPD and dilatometer is shown in Fig. 1 . Table 4 summarises the A c1 and A c3 temperatures determined from the three experimental techniques and from the prediction models [ 2 , 31 , 32 ]. In pure Fe, the fraction of austenite changed rapidly with temperature, as observed from the NPD data at 900–915°C, with start and end transformation temperatures of 900–905°C and 910–915°C, respectively. The onset temperatures determined by NPD and dilatometry align closely with the reported α-BCC → γ-FCC phase transformation temperatures for pure iron of 910°C and 912°C [ 10 , 19 , 39 ]. The deviation observed in this study was less than 5°C, confirming the reliability of the measurements and procedures employed during the NPD experiments. Additionally, the Curie temperature of 776°C obtained from dilatometry corresponds well with the value of 770°C reported by Touloukian et al. [ 10 ], indicating a variation of approximately ± 6°C. In NPD, the nuclear scattering contribution remains visible in diffraction patterns above Tc; however, because magnetic effects were not the primary focus of this work, data in this temperature region were collected at 50°C intervals. In contrast, the continuous heating modes used in dilatometry and DSC captured the effect of this magnetic transition more clearly within their thermal response curves. Table 4 Comparison of experimental transformation temperatures obtained from neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC) with values calculated using model formulae reported in literature for pure Fe, P91 base metal, P92 base metal, P91 weld metal 1, and P91 weld metal 2 Source A c1 [°C] A c3 [°C] Tc [°C] Pure Fe NPD 905–910 910–915 — Dilatometry 912 942 776 Basinki et al. [ 19 ] & ASM International [ 39 ] 910 910 — Touloukian et al. [ 10 ] 912 912 770 9%Cr steels Phase diagram Bungardt et al. [ 40 ] 800 870 — P91 base metal NPD 805 890 — Dilatometry 817 880 750 DSC 824 874 749 Andrew [ 30 ] 859 889 — Santella [ 20 ] 795 — — Wang et al. [ 2 ] 813 — — P92 base metal NPD 816 930 — Dilatometry 836 923 750 DSC 839 893 741 Andrew [ 30 ] 877 908 — Santella [ 20 ] 816 — — Wang et al. [ 2 ] 797 — — P91 weld metal 1 NPD 770 915 — Dilatometry 797 904 747 DSC 831 876 737 Andrew [ 30 ] 879 903 — Santella [ 20 ] 762 — — Wang et al. [ 2 ] 783 — — P91 weld metal 2 NPD 820 922 — Andrew [ 30 ] 862 905 — Santella [ 20 ] 796 — — Wang et al. [ 2 ] 811 — — 3.1.1 Base metal Comparisons of the A c1 and A c3 transformation temperatures obtained from NPD, dilatometry, and DSC for the P91 and P92 base metals are shown in Fig. 2 and Fig. 3 , respectively. For the P91 base metal, NPD revealed austenite formation over a broad temperature range of approximately 800–890°C, with the A c1 and A c3 of transformation occurring at 800–805°C and 885–890°C, respectively. Dilatometry and DSC measurements showed similar transformation behaviour, albeit with slightly shifted temperature intervals: A c1 and A c3 were recorded at 817°C and 880°C with dilatometry, and at 824°C and 874°C by DSC. The A c1 temperature from dilatometry was approximately 17°C higher than that from NPD, while A c3 was about 10°C lower, reflecting the greater sensitivity of NPD to the initial appearance of austenite and the bulk-averaged nature of dilatometric detection. DSC, which detects thermal events rather than structural changes, consistently recorded the highest A c1 and lowest A c3 values, indicating delayed thermal response relative to phase transformation onset. The A c1 temperatures determined in this study are in good agreement with the predictive models developed by Wang et al. [ 2 ] and Santella [ 20 ], which consider the effects of alloying elements such as Cr, Mo, and Ni on the ferrite-to-austenite transformation. These models, tailored for CSEF steels, predict A c1 temperatures in the range of 790–820°C for compositions similar to P91. Santella [ 20 ] emphasised that increased Cr and Mo stabilise the ferritic matrix, delaying austenite formation, while low Ni and C levels further depress A c1 . The model of Wang et al. [ 2 ] similarly attributes the broad α + γ coexistence region to strong ferrite stabilisers, explaining the gradual transformation observed in this study. The measured A c1 values also align with the Fe–Cr–C phase diagram presented by Bungardt et al. [ 40 ], which shows that Cr additions up to 9% significantly expand the dual-phase region and enhance ferrite stability. The experimentally measured A c1 values (800–805°C) fall within the A 1 limits (788–813°C) reported by Alexandrov et al. [ 4 ] for normalised and tempered P91 steels, confirming that the observed transformation behaviour is typical for this alloy. The A c3 temperature determined by NPD (885–890°C) closely matches the value predicted by Andrew’s empirical model [ 30 ] (889°C), which incorporates the effects of C and ferrite stabilisers such as Cr, Mo, and W. This agreement reinforces the accuracy of the experimental temperature calibration and the applicability of Andrew’s formulation to P91 compositions. Table 4 provides a comprehensive comparison of experimentally measured A c1 , A c3 , and Curie temperatures (Tc) obtained from NPD, dilatometry, and DSC, with values calculated using the models of Andrew [ 31 ], Santella [ 32 ], and Wang et al. [ 2 ]. Across all datasets, deviations are generally within ± 20°C, demonstrating the robustness of these predictive approaches for modern P91 steels. The close match between NPD and model-predicted values confirms that the alloy chemistry and transformation responses of these samples are consistent with the assumptions underlying these empirical correlations. In contrast, the P92 base metal exhibited slightly higher A c1 and A c3 temperatures, ranging from 816–839°C and 893–930°C, respectively, indicating enhanced ferrite stability. This shift is attributed to the substitution of W for Mo, as discussed by Santella [ 20 ] and Wang et al. [ 2 ], which increases the α→γ transformation temperatures. The experimental A c1 values (816°C from NPD and 836°C from dilatometry) correspond well with the prediction of Santella [ 32 ] (816°C), while the model of Wang et al. [ 2 ] (797°C) slightly underestimates the onset temperature, likely due to compositional nuances. The A c3 temperature predicted by Andrew [ 31 ] (908°C) aligns with the NPD-measured value (930°C), confirming the model’s applicability to high-Cr W-containing steels. For P91 base metal, Tc was measured as 750°C by dilatometry and 749°C with DSC; for P92 base metal, Tc was recorded at 750°C and 741°C, respectively. These values fall within the expected range for 9%Cr ferritic steels and are consistent with the Tc values reported by Nitsche and Mayr [ 13 ] of approximately 770°C for 9Cr1Mo steels. Alexandrov et al. [ 4 ] reported Tc values between 737–750°C for P91 and P92 steels. The small deviations observed between results obtained from DSC and dilatometry, typically within ± 10°C, are attributed to the differing detection mechanisms of these techniques. The differences in detection thresholds for NPD, dilatometry, and DSC reflect their underlying principles. NPD, being a crystallographic technique, is highly sensitive to the initial formation of austenite, detecting subtle changes in lattice structure. Dilatometry measures bulk dimensional changes, which occur once a sufficient volume fraction of austenite has formed. DSC detects endothermic events associated with phase transformations, such as the α-BCC to γ-FCC transition but may miss early structural changes due to its reliance on heat flow rather than phase fraction. This explains why DSC consistently reports higher A c1 and lower A c3 values, because it captures the thermal signature of transformation rather than its onset or completion in the microstructure. Overall, the transformation temperatures observed for P91 and P92 base metals are consistent with thermodynamic expectations and literature for 9%–12%Cr steels. The agreement between experimental results and compositional models validates the reliability of the temperature calibration, the sensitivity of NPD for detecting phase transitions, and the predictive accuracy of established empirical formulae. These findings are critical for optimising heat treatment protocols and ensuring the desired microstructural evolution in CSEF steels. 3.1.2 Weld metal The transformation behaviours of P91 weld metals investigated using NPD, dilatometry, and DSC are presented in Fig. 4 to Fig. 6 , respectively. Compared with the base metal, the weld metal exhibited more complex transformation characteristics, influenced by residual austenite, compositional variations, and microstructural heterogeneity. At room temperature, P91 weld metal 1 contained less than 1% residual austenite, while P91 weld metal 2 retained approximately 8% austenite. During heating, P91 weld metal 1 showed early transformation behaviour, with austenite nucleating directly on the limited retained austenite phase, contributing to a gradual and complex transformation path. In contrast, weld metal 2 underwent complete transformation of residual austenite to martensite between 655–660°C, prior to reaching A c1 . This resulted in a fully martensitic structure at the onset of austenite formation and a higher A c1 temperature. For P91 weld metal 1, NPD detected A c1 and A c3 temperatures at 770 and 915°C, respectively. Dilatometry recorded A c1 at 797°C and A c3 at 904°C; DSC measured A c1 at 831°C and A c3 at 876°C. Dilatometry measured A c1 at 27°C higher and A c3 at 11°C lower than NPD. This reflects the delayed response of bulk dimensional change relative to the microscopic onset of austenite formation and its apparent completion once expansion stabilises, even if some ferrite remains. DSC consistently reported the highest A c1 and lowest A c3 values, confirming its sensitivity to heat flow rather than phase fraction. The measured A c1 of P91 weld metal 1 aligns closely with the value predicted by Santella’s model (762°C) [ 20 ]. The proximity of A c1 to common PWHT ranges (730–780°C) raises concerns about partial re-austenitisation during PWHT, potentially leading to the formation of untempered martensite, which reduces creep strength and toughness [ 5 , 41 – 43 ]. In contrast, P91 weld metal 2 exhibited A c1 and A c3 temperatures at 815–820°C and 920–922°C, respectively, as measured by NPD. These values are comparable with those of the P92 base metal, suggesting improved chemical homogeneity and reduced Cr segregation. According to a study by Vimalan et al. [ 44 ], P91 weld metal 2, with a combined Ni + Mn content of 0.78 mass%, was expected to exhibit a higher A c1 temperature than P91 weld metal 1. Complete transformation of residual austenite to martensite was observed at 655–660°C, indicating that PWHT temperatures should exceed this threshold to ensure full martensitic transformation of retained austenite. This is particularly relevant given that the common PWHT range falls between 730–780°C [ 41 , 43 ], which is sufficiently above the transformation threshold. The transformation path of P91 weld metal 2, with all residual austenite transformed to martensite prior to A c1 , differs from P91 weld metal 1 where A c1 started in the presence of retained austenite. This distinction highlights the role of residual phase content in shaping transformation behaviour and reinforces the importance of controlling austenite retention during welding and PWHT. The observations corroborate literature findings that weld metals may display distinct transformation behaviour compared with their corresponding base metals [ 45 , 46 ]. Overall, the transformation behaviour of P91 weld metals seems to be strongly influenced by residual phase content, alloying element distribution, and thermal history. The consistency between experimental results and predictive models (Andrew [ 30 ], Santella [ 20 ], Wang et al. [ 2 ]) validates the applicability of these formulations to weld metal compositions. The use of NPD, dilatometry, and DSC provides complementary insights into both structural and thermal aspects of phase transformations, reinforcing the robustness of the experimental approach. The Curie temperatures for P91 weld metal 1 were determined to be 747°C and 737°C with dilatometry and DSC, respectively. These values are consistent with those reported for 9%Cr steels by Alexandrov et al. [ 4 ] at 737–750°C and Nitsche & Mayr [ 13 ] at 735–755°C. The 10°C deviation between the DSC and dilatometry values reflects their respective sensitivities: DSC detects the endothermic magnetic transition, while dilatometry captures the associated dimensional change. The general agreement across methods confirms their reliability in identifying magnetic transformation thresholds in weld metals. 3.2 Thermal strain and expansion Thermal strain behaviour was evaluated by comparing the lattice parameter evolution of the BCC phase with dilatometry measurements for pure Fe, P91 base metal, P92 base metal ,and P91 weld metal 1, as illustrated in Fig. 7 . Below A c1 , the thermal strain of pure Fe showed good agreement with dilatometry, while the base metals exhibited slight underestimation and P91 weld metal 1 showed overestimation of dilatometry curve. These discrepancies are attributed to microstructural complexity and residual phase content. In P91 weld metal 1, multiple transition zones are observed, complicating the strain response. Above A c1 , the lattice parameter evolution became inconsistent due to dual-phase interactions, whereas dilatometry continued to show a steady decline in sample length. Thermal expansion behaviour was assessed by comparing the coefficients of thermal expansion (CTE) derived from α-BCC lattice strain and dilatometry for pure Fe, P91 and P92 base metals and P91 weld metals, as given in Table 5 . Pure Fe exhibited the lowest CTE at 12.8 × 10 − 6 /°C, consistent with literature [ 10 ]. Table 5 Thermal expansion coefficients of ferritic steels determined by linear fitting of α-BCC thermal strain and dilatometry curves Material α-BCC + carbides + / residual FCC [/°C] Temperature range α-BCC [°C] Pure Fe rod 12.8 × 10 − 6 25–910 P91 base metal 14.1 × 10 − 6 25–985 P92 base metal 16.4 × 10 − 6 25–928 P91 weld 1 15.5 × 10 − 6 44–910 P91 weld 2 12.4 × 10 − 6 60–900 The ferritic steels showed higher CTE than pure Fe due to solid-solution alloying with elements like Cr, Mo, and W, which increase lattice sensitivity to temperature [ 47 ]. P92 base metal showed a high CTE of 16.4 × 10 − 6 \(\:\:\) /°C − 6 compared with the value for P91 of 14.1 × 10 − 6 \(\:\:\) /°C. P91 weld metal 1 displayed a CTE of 15.5 × 10 − 6 \(\:\:\) /°C, which shows larger expansion than the P91 base metal, attributed to its 8% residual FCC phase [ 48 , 49 ]. In contrast, P91 weld metal 2 showed a lower CTE of 12.4 × 10 − 6 /°C, consistent with its fully martensitic structure prior to the α-BCC → FCC transformation. These results confirm that residual phase content and alloying significantly influence thermal expansion behaviour in 9%–12%Cr steels [ 49 ]. Given the low austenite phase fraction at low temperatures (zero in base metal and up to 8% in weld metal), which made lattice parameter measurements unreliable, its coefficient of thermal expansion, although similar to ferrite (presented in Fig. 7 ), is omitted here due to limited practical relevance and because these steels are never used when austenite is present. 4. Conclusions Accurate determination of A c1 and A c3 temperatures is crucial for optimising heat treatment processes in ferritic steels. These temperatures define the austenitisation phase transformation, which influences microstructure and subsequent mechanical properties. This analysis of A c1 and A c3 temperatures using NPD, dilatometry, and DSC techniques highlighted significant differences in detection thresholds for various samples. NPD mostly showed lower A c1 and higher A c3 temperatures, indicating its sensitivity to initial phase changes until completion of the α-BCC to ɣ-FCC transformation. This sensitivity is evident in the analysis of the austenite onset across different materials, as seen in the transformation ranges for pure Fe, P91 base metal, and P92 base metal. Dilatometry, while indicating high A c3 temperatures, demonstrates a slow transformation process and a consistent decline in sample dilation at the transition zone. DSC provides additional insights into thermal events associated with phase formation, highlighting endothermic transitions for the α-BCC to FCC transformation. Overall, the detailed structural sensitivity of NPD makes it a more reliable technique for detecting phase transformation temperatures across diverse samples. For the two base metal samples, the A c1 temperature as measured using NPD was consistently significantly lower than the corresponding dilatometry measurement. For the P91 base metal, the NPD measurement (805°C) was 12°C lower than the dilatometry measurement (817°C). For P92 base metal, the difference was 20°C (NPD: 816°C; dilatometry: 836°C). It is reasonable to assume that most empirical relationships between composition, A c1 and A c3 temperatures are based on dilatometry measurements, because this technique is more accessible than NPD. The specified PWHT temperature (usually given as 740–770°C) may therefore not be adequately conservative, particularly if the low A c1 temperatures measured for the weld metal are considered. Weld metals in 9% Cr steels exhibit significant deviations in thermal expansion compared with their base metals, probably due to the presence of residual FCC austenite, which has a higher thermal expansion than the martensitic or ferritic matrix. In contrast, the elevated CTE values observed in P91 and P92 base metals arise mainly from solid-solution alloying of elements such as Cr, Mo, and W within the ferritic matrix. This distinction is highlighted by the difference between P91 weld metal 1, which contains about 8% residual FCC phase and exhibits a larger CTE of 15.5 × 10 − 6 /°C, and P91 weld 2, which undergoes a complete martensitic transformation and shows a lower CTE of 12.4 × 10 − 6 /°C. These findings confirm that residual FCC phases significantly influence thermal expansion in weld metals, causing deviations from the base metal behaviour in 9% Cr steels. Declarations Competing interest The authors declare no competing interest. Funding The research leading to these results received funding from the South African National Research Foundation (Grant number FUNNRF118109), the South African Nuclear Energy Corporation, and the Southern African Institute of Welding. Authors’ contribution statements ZNS carried out the experimental work, performed data analysis, interpreted the results, and prepared initial manuscript draft. PGHP supervised the research, and contributed to the theoretical framework, data interpretation, and visualisation. AMV, JRH, and HEM-C assisted in conducting the experimental work. All authors reviewed, edited and approved the final manuscript. Acknowledgements This project was funded by the South African National Research Foundation (Grant number FUNNRF118109), the South African Nuclear Energy Corporation, and the Southern African Institute of Welding. The Australian Nuclear Science and Technology Organisation is acknowledged for access to the Wombat instrument under proposal number P13600. Prof. J.P.R. de Villiers is acknowledged for his advice in crystallography. The Industrial Metals and Minerals Research Institute at the University of Pretoria is acknowledged for use of the dilatometer. Delta F at Necsa is acknowledged for differential scanning calorimetry measurements. Prof. Kathryn Sole is thanked for English editing of this manuscript. Data availability statement The data supporting the findings of this study are included within the manuscript. Any other data can be made available by the corresponding author upon request. 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Int J Press Vessel Pip 85:30–37 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 13 Feb, 2026 Reviewers invited by journal 13 Feb, 2026 Editor invited by journal 15 Dec, 2025 Editor assigned by journal 11 Dec, 2025 First submitted to journal 10 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8317506","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":590695996,"identity":"5f57f0f7-1796-4924-bbbf-3690b3ed683a","order_by":0,"name":"Zeldah Ngwanakgagane 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Organisation","correspondingAuthor":false,"prefix":"","firstName":"Helen","middleName":"","lastName":"Maynard-Casely","suffix":""}],"badges":[],"createdAt":"2025-12-09 12:16:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8317506/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8317506/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102964017,"identity":"de8649b1-791a-4822-af8c-585c87d945e1","added_by":"auto","created_at":"2026-02-19 04:21:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":67606,"visible":true,"origin":"","legend":"\u003cp\u003ePhase fractions of pure Fe, P91 base metal, P92 base metal, and P91 weld metal as a function of sample temperature, determined using neutron powder diffraction (NPD) and dilatometry\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/a5f683c0703f1f9bea83f4b8.png"},{"id":102923387,"identity":"1f36792f-2f1b-4c9a-ab22-e88832aef3a4","added_by":"auto","created_at":"2026-02-18 13:12:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":26883,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between phase fractions and heat flow in P91 base metal, showing the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures measured by neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC)\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/83ec0b54a47239aeb0a2ecc0.png"},{"id":102964177,"identity":"cfdfa980-53e0-45cc-a06c-f30c8cfa9e2b","added_by":"auto","created_at":"2026-02-19 04:21:39","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":27318,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between phase fractions and heat flow in P92 base metal, showing the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures measured by neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/b6e5869272e7a45fc308c4bd.png"},{"id":102964640,"identity":"8cd8db85-5b14-42d8-b4d7-cb1d7d605ed8","added_by":"auto","created_at":"2026-02-19 04:23:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":24317,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eC\u003c/strong\u003eomparison between phase fractions and heat flow in P91 weld 1, showing the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures measured with neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC)\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/7af7aec99128d42fc542f953.png"},{"id":102923393,"identity":"24b6ab0e-97e6-4137-b74d-9093d104788e","added_by":"auto","created_at":"2026-02-18 13:12:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":21723,"visible":true,"origin":"","legend":"\u003cp\u003eFraction of body-centred cubic (BCC) phase in P91 base metal, weld metal 1, and weld metal 2 as a function of temperature (600–1000°C) determined from neutron powder diffraction (NPD) data. The onset of BCC transformation in P91 weld metal 2 is observed between 622–632°C and progresses up to 660–665°C. In P91 weld metal 1, a change in BCC content is also evident between 637–665°C; however, complete transformation did not occur. Fresh austenite formed at the A\u003csub\u003ec1\u003c/sub\u003e temperature, nucleating directly on the reduced retained austenite\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/2b7684a908fccdfcef189a4b.png"},{"id":102923392,"identity":"3bbf9a10-05a0-44d9-bc4e-28268e0b7076","added_by":"auto","created_at":"2026-02-18 13:12:49","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":12230,"visible":true,"origin":"","legend":"\u003cp\u003eFraction of P91 weld 1 as a function of temperature, illustrating the body-centred cubic (BCC) transformation in the presence of residual face-centred cubic (FCC), compared with P91 base metal and P91 weld 2. Between 750 and 755°C, the BCC fraction initially increased as retained austenite (FCC) transformed to BCC, preceding the onset of the A\u003csub\u003ec1\u003c/sub\u003e transformation observed at 755–760°C\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/9039ad48fbc7c246c7791694.png"},{"id":102923391,"identity":"cc2adc4b-6376-4ad2-b61b-6bbc9b04bda6","added_by":"auto","created_at":"2026-02-18 13:12:49","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":81860,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of thermal strain calculated from lattice parameters of body-centred cubic (BCC) phase and change in length of the samples, as measured with NPD and dilatometry\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/25c7a1f381cc6a3f01123d17.png"},{"id":102965692,"identity":"b92cca37-0510-44b5-98a8-80f10c98cbf6","added_by":"auto","created_at":"2026-02-19 04:32:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1410184,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8317506/v1/95dc6305-1fb9-4c02-a0bc-2438ac38ffa6.pdf"}],"financialInterests":"","formattedTitle":"Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eCreep-strength-enhanced ferritic (CSEF) steels with compositions of 9%\u0026ndash;12%Cr are widely used in power plants, mainly owing to their creep resistance and stability to oxidation at temperatures up to 600\u0026deg;C [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Understanding the phase transitions from martensite to austenite during heating and back to martensite during cooling are critical for optimising their performance [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. This work focused on ASTM A335 Grade P91 and 92 base metals, and P91 weld metals. The metals were heated to 1000\u0026deg;C to characterise phase transitions between α-ferrite (body-centred cubic; bcc) and ɣ-austenite (face-centred cubic; fcc). α‑Ferrite is ferromagnetic at room temperature, but changes to paramagnetic at the Curie point (Tc) [\u003cspan additionalcitationids=\"CR8 CR9 CR10\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. On heating through the A\u003csub\u003ec1\u003c/sub\u003e temperature, α-ferrite begins to transform to austenite. The A\u003csub\u003ec3\u003c/sub\u003e temperature represents the upper critical temperature at which α-ferrite has completely transformed to austenite on heating. Post-weld heat treatment (PWHT) is usually carried out just below the A\u003csub\u003ec1\u003c/sub\u003e temperature, at 740\u0026ndash;770\u0026deg;C [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDilatometry is commonly used to measure A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e transformation temperatures in steels, but its accuracy is compromised by the assumption of constant thermal expansion coefficients, which does not account for the temperature-dependent and phase-specific nature of thermal expansion, particularly during the coexistence of ferrite and austenite. As a result, the onset of austenite formation during heating may be inaccurately identified, leading to a shift in the transformation temperature. This limitation has been highlighted in both experimental studies and round-robin tests, often leading to underestimation of the true onset of austenite phase formation on heating [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Nitsche et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] reported the results of a round-robin test measured on 9%Cr base metal at twelve laboratories that involved slow heating (at 28\u0026deg;C/h) from 700\u0026ndash;1000\u0026deg;C to measure the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures under conditions approaching equilibrium, using either dilatometry or differential thermal analysis. Ten laboratories reported an A\u003csub\u003ec1\u003c/sub\u003e temperature of 820\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u0026deg;C. Two laboratories reported a significantly lower A\u003csub\u003ec1\u003c/sub\u003e temperature of approximately 770\u0026deg;C. No single explanation for this difference was presented. There is therefore a need for complementary techniques to improve the characterisation of phase transformations and thermal expansion of materials in high-temperature environments. This study aimed to compare neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC) techniques for measuring phase transformation temperatures during heating.\u003c/p\u003e \u003cp\u003eWhile dilatometry provides macroscopic data on thermal expansion, it may fail to account for non-linear lattice changes [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. In CSEF steels, slope changes are associated with the precipitation of large quantities of carbonitrides in the austenite nucleation range [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The threshold quantity of these carbonitrides required for a slope change is not known, therefore phase transitions may occur prior to detection of the slope change. When phase transitions occur from a multiphase material during a heat treatment cycle, interpretation of the dilatometry curve becomes complex. Additionally, length changes due to contamination or oxidation of the material may occur [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. At the Curie temperature, ferromagnetic steels undergo a transition to the paramagnetic state, resulting in slight lattice expansion and an increase in the thermal expansion coefficient without any change in crystal structure. For pure iron, this transition occurs at approximately 770\u0026deg;C [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. In 9%Cr ferritic steels, alloying reduces the magnetic moment, lowering the Curie temperature to 740\u0026ndash;750\u0026deg;C [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNPD directly measures the atomic structure that enables determination of the phase fractions and lattice parameters, offering detailed insight into phase transformations [\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Hosemann et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] investigated T91 and HT-9 steels during normalisation and tempering cycles, heating samples to 1050\u0026deg;C and 760\u0026deg;C, respectively, at 200\u0026deg;C/h, with diffraction measurements taken at 5 min intervals with constant temperature holds, and at 2 min intervals during continuous cooling, resulting in a temperature resolution of approximately 30\u0026deg;C. Tomota et al. [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] investigated ferrite and pearlite transformations in a 1.5Mn\u0026ndash;1.5Si\u0026ndash;0.2C steel using in-situ neutron diffraction during continuous cooling from 900\u0026deg;C. The study enabled real-time tracking of transformation kinetics without temperature holds. Gong et al. [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] studied Fe\u0026ndash;Ni\u0026ndash;C alloys using time-resolved neutron diffraction during cryogenic cooling from room temperature to \u0026minus;\u0026thinsp;269\u0026deg;C, focusing on martensitic transformation and lattice evolution. Their findings revealed deviations in austenite lattice constants and highlighted the role of carbon in maintaining martensite tetragonality. Tomota et al. [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] reviewed neutron diffraction applications in steels, covering transformation behaviour from ambient temperature to 1000\u0026deg;C under both continuous and isothermal conditions.\u003c/p\u003e \u003cp\u003eDSC provides an understanding of thermodynamic properties and transformations by examining heat flow as a function of temperature and time [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The heat flow measurements detect energy absorption, which characterises an endothermic phase transition, or the release of heat, an exothermic phase transition, by changes in sample temperature. Endothermic transitions are associated with processes like decomposition, melting, and vaporization; exothermic transitions are indicative of crystallisation, oxidation, and combustion. In addition to determining the endothermic and exothermic phase transitions corresponding to heat absorption or release phenomena [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], DSC can detect the Curie temperature of a transition from ferromagnetic to paramagnetic behaviour in 9%Cr steels [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eEstablished alternative methods for determining transition temperatures in ferritic steels involve deriving predictive formulas based on thermodynamic simulations using Thermo-Calc or JMat-Pro and alloy composition [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Andrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] developed empirical formulas based on alloy composition to determine A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Santella proposed equations for predicting A\u003csub\u003e1\u003c/sub\u003e temperatures for P91 and P92 based on thermodynamic simulations [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] developed a design-of-experiment approach using single-sensor differential thermal analysis measurements to determine the A\u003csub\u003e1\u003c/sub\u003e temperature in P91 and P92 steels, resulting in a least-squares fitting model that correlates A\u003csub\u003e1\u003c/sub\u003e with alloy composition. These formulas are used in this study as first-order approximations of the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures.\u003c/p\u003e \u003cp\u003eThis study compares information obtained from NPD, dilatometry, and DSC techniques in determining the characteristics of phase transitions in ferritic steels over the temperature range from ambient to 1000\u0026deg;C.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Sample geometry\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarises the sample geometries employed using the various techniques.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSample dimensions (diameter and length in mm) for the three characterisation techniques\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDilatometry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePure Fe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 base metal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eShavings\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP92 base metal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 weld 1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 weld 2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e\u0026Oslash;5 \u0026times; 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Pure iron rod\u003c/h2\u003e \u003cp\u003eA 99.99% pure iron rod (Goodfellow, England) was utilised as a standard to calibrate sample temperature measurements in both the neutron diffraction vacuum furnace and the dilatometer. The sum of all reported alloying elements in the rod was less than 50 ppm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 9%Cr base and weld metals\u003c/h2\u003e \u003cp\u003eThe primary samples of this study were P91 and P92 base metal tubes with wall thickness of 35 mm and P91 weld metals. The elemental compositions, determined by optical emission spectroscopy, are listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The base metals were received in the normalised and tempered condition.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElemental compositions of ASTM A 335 Grade 91 and 92 base metals\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eASTM A 335 Grade P91 range [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eActual composition Grade P91 [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eASTM A 335 Grade P92 range [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eActual composition Grade P92 [mass%]\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.00\u0026ndash;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.08\u0026ndash;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.07\u0026ndash;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.00\u0026ndash;9.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.50\u0026ndash;9.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.30\u0026ndash;0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.30\u0026ndash;0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.85\u0026ndash;1.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.30\u0026ndash;0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.03\u0026ndash;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.03\u0026ndash;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.06\u0026ndash;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.04\u0026ndash;0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.20\u0026ndash;0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.18\u0026ndash;0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.30\u0026ndash;0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.50\u0026ndash;20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.87\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElemental compositions of Grade 91 filler metal and weld metal\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eManufacturer composition ECrMo 91 B 42 H5 [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMeasured composition P91 weld 1 [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eManufacturer composition CrMo91 EB91 [mass%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMeasured composition P91 weld 2 [mass%]\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.08\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.012\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.007\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.42\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.013\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.003\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eV\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSingle-pass bead-on-plate welds 1 and 2 (designated as P91 weld metal 1 and P91 weld metal 2) were produced by shielded metal arc welding using electrodes with a diameter of 3.2 mm that met the requirements of EN ISO 1599 ECrMo 91 B 42 H5 and EN ISO 24598-A-S CrMo91 on a substrate of Grade P91 base metal. The heat input varied between 0.8 and 1.5 kJ/mm. The preheating temperature was maintained above 250\u0026deg;C. The interpass temperature was kept below 400\u0026deg;C. Samples were sectioned parallel to the welding direction through the centre of the overlaid weld beads. Although PWHT is a standard practice for P91 welded joints, the weld samples were examined in the as-welded condition in this study to enable assessment of phase transformation behaviour during heating up to the onset of austenite. Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e provides the elemental compositions of the welds determined by optical emission spectroscopy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Experimental techniques\u003c/h2\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.4.1 Neutron powder diffraction\u003c/h2\u003e \u003cp\u003eThe NPD experiments were performed using the Wombat instrument at ANSTO, Australia, which uses the (115) reflection of a germanium crystal monochromator to produce a 0.1170 nm neutron beam [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Diffraction data were collected over the 2θ range of 28\u0026deg; to 148\u0026deg;. A certified NIST Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder sample was used as reference standard to calibrate instrument parameters, such as wavelength and 2θ offset. Data analysis was conducted using Rietveld refinement in GSAS II, leveraging least-squares fitting algorithms to model crystal structures [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Samples were stationary during measurements conducted in a niobium element furnace optimised for neutron scattering measurements. The temperature ranged from room temperature to 1000\u0026deg;C, with heating rates of 600\u0026deg;C/h up to 700\u0026deg;C and 60\u0026deg;C/h from 700\u0026ndash;1000\u0026deg;C. At each discrete target temperature, samples were temperature-stabilised for 5 min before diffraction patterns were measured under isothermal control for 5 min. In-situ NPD experiments with pure iron rods were conducted to calibrate the furnace from room temperature to 1000\u0026deg;C. Pure iron has well-defined and extensively characterised phase transformation temperatures under equilibrium conditions. In this study, all diffraction peaks were measured at the selected neutron beam wavelength, with no peak overlap. The sensitivity of phase quantification using Rietveld refinement methods is dependent on the fraction present for the specific phase. Leon-Reina et al. [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] reported a quantification limit near 0.10 mass% for well-crystallised inorganic phases in laboratory powder diffraction, although accuracy was poor, with relative errors of approximately 100%. Reliable analyses, with errors below 20%, were only achieved for phase fractions exceeding 1.0 mass%, consistent with reported detection limits of approximately 1\u0026ndash;2 mass% for well-resolved crystalline phases [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan additionalcitationids=\"CR37\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Below this threshold, quantification becomes increasingly uncertain due to limitations in signal-to-noise ratio and peak resolution.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.4.2 Dilatometry\u003c/h2\u003e \u003cp\u003eA DIL805 quenching dilatometer with a length resolution of 50 nm and temperature resolution of 0.05\u0026deg;C was used, with measurements for length changes accurate to 0.01 \u0026micro;m. Temperature was monitored by two type K (Ni\u0026ndash;Cr/ Ni\u0026ndash;Al) thermocouples spot-welded to the middle of each sample [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Continuous length-change measurements were conducted in a helium gas environment over the temperature range of 25\u0026ndash;1000\u0026deg;C. Heating rates were set at 600\u0026deg;C/h from ambient to 700\u0026deg;C, followed by a slower rate of 28\u0026deg;C/h from 700\u0026ndash;1000\u0026deg;C. Data acquisition adhered to ASTM A1033-18 [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], with constructed tangents applied at phase transformation points to determine A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures.\u003c/p\u003e \u003cp\u003ePhase fractions within the transformation zone were calculated by fitting additional tangents, firstly to establish the temperature at which the ɣ-austenite phase reached 50%, and then subsequent phase fractions determined using the same methodology at temperatures between A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.4.3 Differential scanning calorimetry\u003c/h2\u003e \u003cp\u003eDSC experiments were conducted in a TG/DSC TA instrument to record the heat flow of the samples as a function of temperature and time. At ambient temperature, recorded sample masses were 275.99 mg, 23.33 mg, and 19.46 mg for P91 base metal, P92 base metal, and P91 weld metal, respectively. Samples were heated from ambient temperature to 1000\u0026deg;C at 20\u0026deg;C/min in a controlled nitrogen atmosphere, with measurements continuously recorded throughout the heating process.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Phase transformation temperatures\u003c/h2\u003e \u003cp\u003eComparison of the phase fractions determined using NPD and dilatometer is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e summarises the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures determined from the three experimental techniques and from the prediction models [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. In pure Fe, the fraction of austenite changed rapidly with temperature, as observed from the NPD data at 900\u0026ndash;915\u0026deg;C, with start and end transformation temperatures of 900\u0026ndash;905\u0026deg;C and 910\u0026ndash;915\u0026deg;C, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe onset temperatures determined by NPD and dilatometry align closely with the reported α-BCC \u0026rarr; γ-FCC phase transformation temperatures for pure iron of 910\u0026deg;C and 912\u0026deg;C [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. The deviation observed in this study was less than 5\u0026deg;C, confirming the reliability of the measurements and procedures employed during the NPD experiments. Additionally, the Curie temperature of 776\u0026deg;C obtained from dilatometry corresponds well with the value of 770\u0026deg;C reported by Touloukian et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], indicating a variation of approximately\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u0026deg;C. In NPD, the nuclear scattering contribution remains visible in diffraction patterns above Tc; however, because magnetic effects were not the primary focus of this work, data in this temperature region were collected at 50\u0026deg;C intervals. In contrast, the continuous heating modes used in dilatometry and DSC captured the effect of this magnetic transition more clearly within their thermal response curves.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of experimental transformation temperatures obtained from neutron powder diffraction (NPD), dilatometry, and differential scanning calorimetry (DSC) with values calculated using model formulae reported in literature for pure Fe, P91 base metal, P92 base metal, P91 weld metal 1, and P91 weld metal 2\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSource\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eA\u003csub\u003ec1\u003c/sub\u003e [\u0026deg;C]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eA\u003csub\u003ec3\u003c/sub\u003e [\u0026deg;C]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTc [\u0026deg;C]\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003ePure Fe\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e905\u0026ndash;910\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e910\u0026ndash;915\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDilatometry\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e912\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e942\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e776\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBasinki et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] \u0026amp; ASM International [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e910\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e910\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTouloukian et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e912\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e912\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e770\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003e9%Cr steels\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhase diagram Bungardt et al. [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e870\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eP91 base metal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e805\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e890\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDilatometry\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e817\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e880\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e750\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e824\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e874\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e749\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAndrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e859\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e889\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSantella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e795\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e813\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eP92 base metal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e816\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e930\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDilatometry\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e836\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e923\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e750\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e839\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e893\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e741\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAndrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e877\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e908\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSantella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e816\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e797\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eP91 weld metal 1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e770\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e915\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDilatometry\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e797\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e904\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e747\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e831\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e876\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e737\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAndrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e879\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e903\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSantella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e762\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e783\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eP91 weld metal 2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNPD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e820\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e922\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAndrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e862\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e905\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSantella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e796\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e811\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Base metal\u003c/h2\u003e \u003cp\u003eComparisons of the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e transformation temperatures obtained from NPD, dilatometry, and DSC for the P91 and P92 base metals are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, respectively. For the P91 base metal, NPD revealed austenite formation over a broad temperature range of approximately 800\u0026ndash;890\u0026deg;C, with the A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e of transformation occurring at 800\u0026ndash;805\u0026deg;C and 885\u0026ndash;890\u0026deg;C, respectively. Dilatometry and DSC measurements showed similar transformation behaviour, albeit with slightly shifted temperature intervals: A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e were recorded at 817\u0026deg;C and 880\u0026deg;C with dilatometry, and at 824\u0026deg;C and 874\u0026deg;C by DSC. The A\u003csub\u003ec1\u003c/sub\u003e temperature from dilatometry was approximately 17\u0026deg;C higher than that from NPD, while A\u003csub\u003ec3\u003c/sub\u003e was about 10\u0026deg;C lower, reflecting the greater sensitivity of NPD to the initial appearance of austenite and the bulk-averaged nature of dilatometric detection. DSC, which detects thermal events rather than structural changes, consistently recorded the highest A\u003csub\u003ec1\u003c/sub\u003e and lowest A\u003csub\u003ec3\u003c/sub\u003e values, indicating delayed thermal response relative to phase transformation onset.\u003c/p\u003e \u003cp\u003eThe A\u003csub\u003ec1\u003c/sub\u003e temperatures determined in this study are in good agreement with the predictive models developed by Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] and Santella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], which consider the effects of alloying elements such as Cr, Mo, and Ni on the ferrite-to-austenite transformation. These models, tailored for CSEF steels, predict A\u003csub\u003ec1\u003c/sub\u003e temperatures in the range of 790\u0026ndash;820\u0026deg;C for compositions similar to P91. Santella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] emphasised that increased Cr and Mo stabilise the ferritic matrix, delaying austenite formation, while low Ni and C levels further depress A\u003csub\u003ec1\u003c/sub\u003e. The model of Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] similarly attributes the broad α\u0026thinsp;+\u0026thinsp;γ coexistence region to strong ferrite stabilisers, explaining the gradual transformation observed in this study. The measured A\u003csub\u003ec1\u003c/sub\u003e values also align with the Fe\u0026ndash;Cr\u0026ndash;C phase diagram presented by Bungardt et al. [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], which shows that Cr additions up to 9% significantly expand the dual-phase region and enhance ferrite stability.\u003c/p\u003e \u003cp\u003eThe experimentally measured A\u003csub\u003ec1\u003c/sub\u003e values (800\u0026ndash;805\u0026deg;C) fall within the A\u003csub\u003e1\u003c/sub\u003e limits (788\u0026ndash;813\u0026deg;C) reported by Alexandrov et al. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] for normalised and tempered P91 steels, confirming that the observed transformation behaviour is typical for this alloy. The A\u003csub\u003ec3\u003c/sub\u003e temperature determined by NPD (885\u0026ndash;890\u0026deg;C) closely matches the value predicted by Andrew\u0026rsquo;s empirical model [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] (889\u0026deg;C), which incorporates the effects of C and ferrite stabilisers such as Cr, Mo, and W. This agreement reinforces the accuracy of the experimental temperature calibration and the applicability of Andrew\u0026rsquo;s formulation to P91 compositions.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e provides a comprehensive comparison of experimentally measured A\u003csub\u003ec1\u003c/sub\u003e, A\u003csub\u003ec3\u003c/sub\u003e, and Curie temperatures (Tc) obtained from NPD, dilatometry, and DSC, with values calculated using the models of Andrew [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], Santella [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], and Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Across all datasets, deviations are generally within \u0026plusmn;\u0026thinsp;20\u0026deg;C, demonstrating the robustness of these predictive approaches for modern P91 steels. The close match between NPD and model-predicted values confirms that the alloy chemistry and transformation responses of these samples are consistent with the assumptions underlying these empirical correlations.\u003c/p\u003e \u003cp\u003eIn contrast, the P92 base metal exhibited slightly higher A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures, ranging from 816\u0026ndash;839\u0026deg;C and 893\u0026ndash;930\u0026deg;C, respectively, indicating enhanced ferrite stability. This shift is attributed to the substitution of W for Mo, as discussed by Santella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] and Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], which increases the α\u0026rarr;γ transformation temperatures. The experimental A\u003csub\u003ec1\u003c/sub\u003e values (816\u0026deg;C from NPD and 836\u0026deg;C from dilatometry) correspond well with the prediction of Santella [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] (816\u0026deg;C), while the model of Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] (797\u0026deg;C) slightly underestimates the onset temperature, likely due to compositional nuances. The A\u003csub\u003ec3\u003c/sub\u003e temperature predicted by Andrew [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] (908\u0026deg;C) aligns with the NPD-measured value (930\u0026deg;C), confirming the model\u0026rsquo;s applicability to high-Cr W-containing steels.\u003c/p\u003e \u003cp\u003eFor P91 base metal, Tc was measured as 750\u0026deg;C by dilatometry and 749\u0026deg;C with DSC; for P92 base metal, Tc was recorded at 750\u0026deg;C and 741\u0026deg;C, respectively. These values fall within the expected range for 9%Cr ferritic steels and are consistent with the Tc values reported by Nitsche and Mayr [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] of approximately 770\u0026deg;C for 9Cr1Mo steels. Alexandrov et al. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] reported Tc values between 737\u0026ndash;750\u0026deg;C for P91 and P92 steels. The small deviations observed between results obtained from DSC and dilatometry, typically within \u0026plusmn;\u0026thinsp;10\u0026deg;C, are attributed to the differing detection mechanisms of these techniques.\u003c/p\u003e \u003cp\u003eThe differences in detection thresholds for NPD, dilatometry, and DSC reflect their underlying principles. NPD, being a crystallographic technique, is highly sensitive to the initial formation of austenite, detecting subtle changes in lattice structure. Dilatometry measures bulk dimensional changes, which occur once a sufficient volume fraction of austenite has formed. DSC detects endothermic events associated with phase transformations, such as the α-BCC to γ-FCC transition but may miss early structural changes due to its reliance on heat flow rather than phase fraction. This explains why DSC consistently reports higher A\u003csub\u003ec1\u003c/sub\u003e and lower A\u003csub\u003ec3\u003c/sub\u003e values, because it captures the thermal signature of transformation rather than its onset or completion in the microstructure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, the transformation temperatures observed for P91 and P92 base metals are consistent with thermodynamic expectations and literature for 9%\u0026ndash;12%Cr steels. The agreement between experimental results and compositional models validates the reliability of the temperature calibration, the sensitivity of NPD for detecting phase transitions, and the predictive accuracy of established empirical formulae. These findings are critical for optimising heat treatment protocols and ensuring the desired microstructural evolution in CSEF steels.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Weld metal\u003c/h2\u003e \u003cp\u003eThe transformation behaviours of P91 weld metals investigated using NPD, dilatometry, and DSC are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e to Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, respectively. Compared with the base metal, the weld metal exhibited more complex transformation characteristics, influenced by residual austenite, compositional variations, and microstructural heterogeneity.\u003c/p\u003e \u003cp\u003eAt room temperature, P91 weld metal 1 contained less than 1% residual austenite, while P91 weld metal 2 retained approximately 8% austenite. During heating, P91 weld metal 1 showed early transformation behaviour, with austenite nucleating directly on the limited retained austenite phase, contributing to a gradual and complex transformation path. In contrast, weld metal 2 underwent complete transformation of residual austenite to martensite between 655\u0026ndash;660\u0026deg;C, prior to reaching A\u003csub\u003ec1\u003c/sub\u003e. This resulted in a fully martensitic structure at the onset of austenite formation and a higher A\u003csub\u003ec1\u003c/sub\u003e temperature.\u003c/p\u003e \u003cp\u003eFor P91 weld metal 1, NPD detected A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures at 770 and 915\u0026deg;C, respectively. Dilatometry recorded A\u003csub\u003ec1\u003c/sub\u003e at 797\u0026deg;C and A\u003csub\u003ec3\u003c/sub\u003e at 904\u0026deg;C; DSC measured A\u003csub\u003ec1\u003c/sub\u003e at 831\u0026deg;C and A\u003csub\u003ec3\u003c/sub\u003e at 876\u0026deg;C. Dilatometry measured A\u003csub\u003ec1\u003c/sub\u003e at 27\u0026deg;C higher and A\u003csub\u003ec3\u003c/sub\u003e at 11\u0026deg;C lower than NPD. This reflects the delayed response of bulk dimensional change relative to the microscopic onset of austenite formation and its apparent completion once expansion stabilises, even if some ferrite remains. DSC consistently reported the highest A\u003csub\u003ec1\u003c/sub\u003e and lowest A\u003csub\u003ec3\u003c/sub\u003e values, confirming its sensitivity to heat flow rather than phase fraction.\u003c/p\u003e \u003cp\u003eThe measured A\u003csub\u003ec1\u003c/sub\u003e of P91 weld metal 1 aligns closely with the value predicted by Santella\u0026rsquo;s model (762\u0026deg;C) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The proximity of A\u003csub\u003ec1\u003c/sub\u003e to common PWHT ranges (730\u0026ndash;780\u0026deg;C) raises concerns about partial re-austenitisation during PWHT, potentially leading to the formation of untempered martensite, which reduces creep strength and toughness [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn contrast, P91 weld metal 2 exhibited A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures at 815\u0026ndash;820\u0026deg;C and 920\u0026ndash;922\u0026deg;C, respectively, as measured by NPD. These values are comparable with those of the P92 base metal, suggesting improved chemical homogeneity and reduced Cr segregation. According to a study by Vimalan et al. [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e], P91 weld metal 2, with a combined Ni\u0026thinsp;+\u0026thinsp;Mn content of 0.78 mass%, was expected to exhibit a higher A\u003csub\u003ec1\u003c/sub\u003e temperature than P91 weld metal 1. Complete transformation of residual austenite to martensite was observed at 655\u0026ndash;660\u0026deg;C, indicating that PWHT temperatures should exceed this threshold to ensure full martensitic transformation of retained austenite. This is particularly relevant given that the common PWHT range falls between 730\u0026ndash;780\u0026deg;C [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e], which is sufficiently above the transformation threshold.\u003c/p\u003e \u003cp\u003eThe transformation path of P91 weld metal 2, with all residual austenite transformed to martensite prior to A\u003csub\u003ec1\u003c/sub\u003e, differs from P91 weld metal 1 where A\u003csub\u003ec1\u003c/sub\u003e started in the presence of retained austenite. This distinction highlights the role of residual phase content in shaping transformation behaviour and reinforces the importance of controlling austenite retention during welding and PWHT.\u003c/p\u003e \u003cp\u003eThe observations corroborate literature findings that weld metals may display distinct transformation behaviour compared with their corresponding base metals [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, the transformation behaviour of P91 weld metals seems to be strongly influenced by residual phase content, alloying element distribution, and thermal history. The consistency between experimental results and predictive models (Andrew [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], Santella [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], Wang et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]) validates the applicability of these formulations to weld metal compositions. The use of NPD, dilatometry, and DSC provides complementary insights into both structural and thermal aspects of phase transformations, reinforcing the robustness of the experimental approach.\u003c/p\u003e \u003cp\u003eThe Curie temperatures for P91 weld metal 1 were determined to be 747\u0026deg;C and 737\u0026deg;C with dilatometry and DSC, respectively. These values are consistent with those reported for 9%Cr steels by Alexandrov et al. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] at 737\u0026ndash;750\u0026deg;C and Nitsche \u0026amp; Mayr [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] at 735\u0026ndash;755\u0026deg;C. The 10\u0026deg;C deviation between the DSC and dilatometry values reflects their respective sensitivities: DSC detects the endothermic magnetic transition, while dilatometry captures the associated dimensional change. The general agreement across methods confirms their reliability in identifying magnetic transformation thresholds in weld metals.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Thermal strain and expansion\u003c/h2\u003e \u003cp\u003eThermal strain behaviour was evaluated by comparing the lattice parameter evolution of the BCC phase with dilatometry measurements for pure Fe, P91 base metal, P92 base metal ,and P91 weld metal 1, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Below A\u003csub\u003ec1\u003c/sub\u003e, the thermal strain of pure Fe showed good agreement with dilatometry, while the base metals exhibited slight underestimation and P91 weld metal 1 showed overestimation of dilatometry curve. These discrepancies are attributed to microstructural complexity and residual phase content.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn P91 weld metal 1, multiple transition zones are observed, complicating the strain response. Above A\u003csub\u003ec1\u003c/sub\u003e, the lattice parameter evolution became inconsistent due to dual-phase interactions, whereas dilatometry continued to show a steady decline in sample length.\u003c/p\u003e \u003cp\u003eThermal expansion behaviour was assessed by comparing the coefficients of thermal expansion (CTE) derived from α-BCC lattice strain and dilatometry for pure Fe, P91 and P92 base metals and P91 weld metals, as given in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Pure Fe exhibited the lowest CTE at 12.8 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e /\u0026deg;C, consistent with literature [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThermal expansion coefficients of ferritic steels determined by linear fitting of α-BCC thermal strain and dilatometry curves\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eα-BCC\u0026thinsp;+\u0026thinsp;carbides + /\u003c/p\u003e \u003cp\u003eresidual FCC\u003c/p\u003e\u003cp\u003e[/\u0026deg;C]\u003c/p\u003e\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTemperature range\u003c/p\u003e \u003cp\u003eα-BCC\u003c/p\u003e \u003cp\u003e[\u0026deg;C]\u003c/p\u003e\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePure Fe rod\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e12.8 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u0026ndash;910\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 base metal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e14.1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u0026ndash;985\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP92 base metal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e16.4 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u0026ndash;928\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 weld 1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e15.5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e44\u0026ndash;910\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP91 weld 2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e12.4 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e60\u0026ndash;900\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe ferritic steels showed higher CTE than pure Fe due to solid-solution alloying with elements like Cr, Mo, and W, which increase lattice sensitivity to temperature [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. P92 base metal showed a high CTE of 16.4 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\)\u003c/span\u003e\u003c/span\u003e/\u0026deg;C\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e compared with the value for P91 of 14.1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\)\u003c/span\u003e\u003c/span\u003e/\u0026deg;C. P91 weld metal 1 displayed a CTE of 15.5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\)\u003c/span\u003e\u003c/span\u003e/\u0026deg;C, which shows larger expansion than the P91 base metal, attributed to its 8% residual FCC phase [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. In contrast, P91 weld metal 2 showed a lower CTE of 12.4 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e /\u0026deg;C, consistent with its fully martensitic structure prior to the α-BCC \u0026rarr; FCC transformation. These results confirm that residual phase content and alloying significantly influence thermal expansion behaviour in 9%\u0026ndash;12%Cr steels [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGiven the low austenite phase fraction at low temperatures (zero in base metal and up to 8% in weld metal), which made lattice parameter measurements unreliable, its coefficient of thermal expansion, although similar to ferrite (presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), is omitted here due to limited practical relevance and because these steels are never used when austenite is present.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eAccurate determination of A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures is crucial for optimising heat treatment processes in ferritic steels. These temperatures define the austenitisation phase transformation, which influences microstructure and subsequent mechanical properties. This analysis of A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures using NPD, dilatometry, and DSC techniques highlighted significant differences in detection thresholds for various samples. NPD mostly showed lower A\u003csub\u003ec1\u003c/sub\u003e and higher A\u003csub\u003ec3\u003c/sub\u003e temperatures, indicating its sensitivity to initial phase changes until completion of the α-BCC to ɣ-FCC transformation. This sensitivity is evident in the analysis of the austenite onset across different materials, as seen in the transformation ranges for pure Fe, P91 base metal, and P92 base metal. Dilatometry, while indicating high A\u003csub\u003ec3\u003c/sub\u003e temperatures, demonstrates a slow transformation process and a consistent decline in sample dilation at the transition zone. DSC provides additional insights into thermal events associated with phase formation, highlighting endothermic transitions for the α-BCC to FCC transformation. Overall, the detailed structural sensitivity of NPD makes it a more reliable technique for detecting phase transformation temperatures across diverse samples.\u003c/p\u003e \u003cp\u003eFor the two base metal samples, the A\u003csub\u003ec1\u003c/sub\u003e temperature as measured using NPD was consistently significantly lower than the corresponding dilatometry measurement. For the P91 base metal, the NPD measurement (805\u0026deg;C) was 12\u0026deg;C lower than the dilatometry measurement (817\u0026deg;C). For P92 base metal, the difference was 20\u0026deg;C (NPD: 816\u0026deg;C; dilatometry: 836\u0026deg;C). It is reasonable to assume that most empirical relationships between composition, A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures are based on dilatometry measurements, because this technique is more accessible than NPD. The specified PWHT temperature (usually given as 740\u0026ndash;770\u0026deg;C) may therefore not be adequately conservative, particularly if the low A\u003csub\u003ec1\u003c/sub\u003e temperatures measured for the weld metal are considered.\u003c/p\u003e \u003cp\u003eWeld metals in 9% Cr steels exhibit significant deviations in thermal expansion compared with their base metals, probably due to the presence of residual FCC austenite, which has a higher thermal expansion than the martensitic or ferritic matrix. In contrast, the elevated CTE values observed in P91 and P92 base metals arise mainly from solid-solution alloying of elements such as Cr, Mo, and W within the ferritic matrix. This distinction is highlighted by the difference between P91 weld metal 1, which contains about 8% residual FCC phase and exhibits a larger CTE of 15.5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e /\u0026deg;C, and P91 weld 2, which undergoes a complete martensitic transformation and shows a lower CTE of 12.4 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e /\u0026deg;C. These findings confirm that residual FCC phases significantly influence thermal expansion in weld metals, causing deviations from the base metal behaviour in 9% Cr steels.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interest\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interest.\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThe research leading to these results received funding from the South African National Research Foundation (Grant number FUNNRF118109), the South African Nuclear Energy Corporation, and the Southern African Institute of Welding.\u003c/p\u003e\u003ch2\u003eAuthors\u0026rsquo; contribution statements\u003c/h2\u003e \u003cp\u003eZNS carried out the experimental work, performed data analysis, interpreted the results, and prepared initial manuscript draft. PGHP supervised the research, and contributed to the theoretical framework, data interpretation, and visualisation. AMV, JRH, and HEM-C assisted in conducting the experimental work. All authors reviewed, edited and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis project was funded by the South African National Research Foundation (Grant number FUNNRF118109), the South African Nuclear Energy Corporation, and the Southern African Institute of Welding. The Australian Nuclear Science and Technology Organisation is acknowledged for access to the Wombat instrument under proposal number P13600. Prof. J.P.R. de Villiers is acknowledged for his advice in crystallography. The Industrial Metals and Minerals Research Institute at the University of Pretoria is acknowledged for use of the dilatometer. Delta F at Necsa is acknowledged for differential scanning calorimetry measurements. Prof. Kathryn Sole is thanked for English editing of this manuscript.\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eThe data supporting the findings of this study are included within the manuscript. Any other data can be made available by the corresponding author upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLundin CD, Liu P, Cui Y (2000) A literature review on characteristics of high temperature ferritic Cr-Mo steels and weldments. Welding Research Council, New York, pp 1\u0026ndash;36\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang L (2010) Development of predictive formulae for the A1 temperature in creep strength enhanced ferritic steels. PhD dissertation. The Ohio State University\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCerjak H, Letofsky E (1996) The effect of welding on the properties of advanced 9\u0026ndash;12%Cr steels. 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Int J Press Vessel Pip 85:30\u0026ndash;37\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Creep-strength-enhanced ferritic steel, Phase transition, Thermal expansion, Endothermicity, Exothermicity","lastPublishedDoi":"10.21203/rs.3.rs-8317506/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8317506/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study explored phase transformations and thermal expansion coefficients in 9%Cr steels; P91 and P92 base metals, and P91 welds, on heating to 1000\u0026deg;C. Neutron powder diffraction was employed to determine transformation temperatures, lattice parameters, and phase fractions. Austenite onset (A\u003csub\u003ec1\u003c/sub\u003e) and completion (A\u003csub\u003ec3\u003c/sub\u003e) temperatures were 800\u0026ndash;805\u0026deg;C and 885\u0026ndash;890\u0026deg;C for P91 base metal, and 814\u0026ndash;816\u0026deg;C and 928\u0026ndash;930\u0026deg;C for P92. The weld metals were evaluated in the as-welded condition. At room temperature, weld metals contained ferrite, martensite, and residual austenite. Residual austenite in weld 1 was retained on heating until fresh austenite formed at 765\u0026ndash;770\u0026deg;C, whereas in weld 2 it disappeared at 655\u0026ndash;660\u0026deg;C. The A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e temperatures of weld 2 were 815\u0026ndash;820\u0026deg;C and 920\u0026ndash;922\u0026deg;C. P92 transformed at lower temperatures and thermal expansion during the α-BCC-to-austenite transformation than P91. Weld 1 showed higher transformation temperatures and expansion than the base metal. Dilatometry measured transition temperatures of 817\u0026deg;C and 880\u0026deg;C for P91 base metal, and 836\u0026deg;C and 923\u0026deg;C for P92. As-welded curve exhibited multiple transitions between 697\u0026ndash;904\u0026deg;C. Calorimetry showed endothermic α-ferrite-to-austenite transitions with A\u003csub\u003ec1\u003c/sub\u003e and A\u003csub\u003ec3\u003c/sub\u003e at 824\u0026deg;C and 874\u0026deg;C for P91 base metal, 839\u0026deg;C and 893\u0026deg;C for P92, and 831\u0026deg;C and 876\u0026deg;C for weld 1. This study demonstrates the advantages of integrating neutron diffraction, dilatometry, and calorimetry to improve transformation analysis in 9%Cr steels; neutron diffraction measured A\u003csub\u003ec1\u003c/sub\u003e up to 20\u0026deg;C lower than dilatometry, indicating current post-weld heat treatment temperature ranges may be insufficiently conservative. Residual austenite in weld metal and its behaviour on heating may also influence weld heat-treatment response.\u003c/p\u003e","manuscriptTitle":"Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-18 13:12:44","doi":"10.21203/rs.3.rs-8317506/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-02-13T14:47:50+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-13T07:17:34+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-12-15T09:50:31+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-12T02:12:57+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-12-11T02:18:12+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d6cca10d-7995-409f-9ce7-abf7d4ca3120","owner":[],"postedDate":"February 18th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-04T09:31:06+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-18 13:12:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8317506","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8317506","identity":"rs-8317506","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}