Effect of welding heat input on microstructure evolution and mechanical properties of welded joints of 1000MPa steel for hydropower engineering

Effect of welding heat input on microstructure evolution and mechanical properties of welded joints of 1000MPa steel for hydropower engineering | 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 Effect of welding heat input on microstructure evolution and mechanical properties of welded joints of 1000MPa steel for hydropower engineering Qiuju Bu, Fan Wang, Xiuhua Gao, Guoqing Feng, Zhiyong Chang, Xincheng Chen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5222267/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 Gas metal arc welding experiments were carried out on 1000MPa hydropower steel with different welding heat inputs. The changing laws of microstructure, hardness, strength and toughness of welded joints of 1000MPa hydropower steel under different heat inputs were studied. The highest hardness of the experimental steel under different conditions was measured through the highest hardness experiment. The results show that with the increase of heat input and the preheating of the specimen at 100℃, the hardness of the heat affected zone decreases to some extent, and the width of the heat affected zone increases, which reduces the crack sensitivity. There are a large number of inclusions in the weld metal (WM), some of which are particles for heterogeneous nucleation of acicular ferrite (AF). With the increase of heat input, the AF of WM structure is coarse, the content of granular bainite (GB) increases, the proportion of high angle grain boundaries (HAGBs) in WM structure decreases gradually, and the kernel average misorientation (KAM) value decreases. Because of the high temperature, the microstructure in the coarse grain heat-affected zone (CGHAZ) is lath bainite (LB) and GB, and the recrystallization of GB in the fine grain heat-affected zone (FGHAZ) is beneficial to the refinement of the original austenite grains. The diffusion of alloying elements near the fusion zone (FZ) and the GB phase transformation of FGHAZ lead to the decrease of microhardness. The mechanical properties of welded joints all meet the requirements of 1000MPa steel for hydropower station. 1000MPa hydropower steel Acicular ferrite Inclusion Gas metal arc welding Heat input Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction As an important part of clean energy, hydropower is of great significance for building a modern energy system, boosting the transformation of energy in low-carbon life and achieving the strategic goal of "double carbon". With the increasing installed capacity of hydropower station, the maximum wall thickness of pressure pipe has reached 90 mm, and the thickness of steel plate on crescent rib plate has reached 150 mm, which has caused great problems for subsequent welding, transportation and installation, which requires hydropower steel to develop in the direction of high strength, energy saving, green and lightweight [1,2] . Welding is mainly applied to important parts of large hydropower units in hydropower industry, such as hydraulic turbines, spiral cases and steel bifurcated pipes [3] . The tensile strength of deposited metal is required to reach 940～1130MPa, and the yield strength is greater than 885MPa. In order to meet the requirements of high head, high speed, high efficiency and large capacity of hydropower units, the steel grade used in hydropower projects has been raised to 1000MPa, especially in special environments such as high-altitude hydropower stations, and the construction of large-capacity pumped storage power stations has further increased the demand for 1000MPa hydropower steel. Therefore, more and more research is being carried out on the manufacture of ultra-high strength hydropower steel [4,5,6] . WM is considered as the most critical area in the whole welded joint, because failure usually begins in it. Compared with conventional low-strength carbon steel, the strength of hydropower steel is improved, so it is necessary to ensure its low-temperature toughness, which will inevitably lead to the improvement of welding crack sensitivity and the difficulty of welding technology. During the welding process, the welding thermal cycles with different peak temperatures have led to drastic changes in the microstructure of the welded joint. Because of the high strength of the parent material (BM), it is difficult to ensure that the welded joint has the same strength and toughness as BM. Therefore, in order to improve the toughness of WM, inducing the formation of acicular ferrite (AF) is the key, because AF has a good combination of strength and toughness, and its characteristics are intertwined, which can hinder the crack propagation and significantly improve the toughness [7,8] . Yang [9] studied the transformation from parent austenite to ferrite in accordance with the K-S relationship, that is, ferrite transformed according to the orientation relationship of [011]γ parallel [111]α and (111)γ parallel (011)α. Wang [10] studied the nucleation and growth of AF in low carbon steel. The formation of AF was promoted by oxide inclusions, and the evolution of the whole weld structure could be controlled by changing the cooling rate. Chen [11] added nano-TiO2 particles to WM during the welding process, which resulted in the formation of 0.67 μm Mn-Ti-O composite inclusions in WM, resulting in AF and high and low temperature toughness. A large number of studies have shown that inclusions can promote the formation of AF when the chemical composition, austenite grain size and undercooling degree are determined. Santos [12] found that too high heat input will lead to the degradation of upper bainite, increase the carbide content, and appear a large number of coarse M-A islands, which is not conducive to impact toughness. On the contrary, the welded joints with low heat input show a lot of AF and GB structures, which is beneficial to impact toughness. The research of Kumar [13] shows that the impact toughness and hardness decrease with the increase of heat input, and the higher volume fraction of AF improves the impact toughness of the weld, but the coarser microcrystalline ferrite AF and M-A chain components reduce the toughness of the weld. The change of heat input during welding has an influence on the formation, development and final properties of welded joints [14,15] . Therefore, it is of great practical significance to study the microstructure and mechanical properties of welded joints under different heat inputs. In this paper, the welding experimental of 1000MPa grade hydropower engineering steel is carried out, and the formation mechanism of AF, microstructure evolution of welded joints and mechanical properties under different heat inputs are studied. Experimental materials and methods 1.1 experimental materials The parent material used in the welding experiment is 1000MPa hydro-electric steel, and the compositions of the experimental steel and deposited metal are shown in Table 1 . According to the calculation formula, the welding crack sensitivity index Pcm and carbon equivalent Ceq [ 16 ] of the experimental steel are calculated to be 0.24% and 0.59% respectively. $$\:{\text{C}}_{\text{eq}}\text{=C+}\frac{\text{Mn}}{\text{6}}\text{+}\frac{\text{Cu+Ni}}{\text{15}}\text{+}\frac{\text{Cr+Mo+V}}{\text{5}}$$ 1 $$\:{\text{P}}_{\text{cm}}\text{=C+}\frac{\text{Si}}{\text{30}}\text{+}\frac{\text{Mn+Cu+Cr}}{\text{20}}\text{+}\frac{\text{Ni}}{\text{60}}\text{+}\frac{\text{Mo}}{\text{15}}\text{+}\frac{\text{V}}{\text{10}}\text{+5B}$$ 2 Table 1 Chemical composition of experimental steel (mass fraction,%) C Si Mn P S Alt Nb V Ti Cr Ni Cu Mo B 0.068 0.21 1.19 0.004 0.0028 0.021 0.059 0.06 0.013 0.51 1.699 0.19 0.50 0.001 1.2 experimental methods After controlled rolling, controlled cooling and modulation, the yield strength, tensile strength, elongation and impact energy of BM are 997 MPa, 1019 MPa, 16.0% and 181 J respectively. Cut a welding sample with the size of 250mm (length) × 120mm (width) × 14mm along the longitudinal direction of the steel plate. Firstly, the highest hardness experiment of heat affected zone is carried out to study the weldability. On the basis of equal matching welding joints, YM-100A welding wire with diameter of 1.2mm was selected for butt welding. See Table 2 for the chemical composition and mechanical properties of YM-100A welding wire. Table 3 shows the experimental parameters of the highest hardness. After preheating at 120℃, multi-layer multi-pass welding was carried out with shielding gas of 80% Ar and 20% CO 2 and gas flux of 20L/min, followed by heat preservation at 250℃ for 2h. The main parameters of welding process are shown in Table 4 , and the groove size is shown in Fig. 1 . The microstructures of solder, HAZ and WM were studied by using Leica DMIRM optical microscope (OM) and scanning electron microscope (SEM) equipped with EBSD system. Cut the metallographic specimen from the welded steel plate along the vertical welding direction. The samples used for OM observation were mechanically polished and then corroded in 4% nitric alcohol solution. Electrolytic polishing was performed in an electrolyte consisting of 12.5% perchloric acid and 87.5% ethanol for EBSD analysis, and electropolishing was performed at 25 V for 20 s with a scanning step of 0.15m m. The collected EBSD data were post-processed by AZtecCrystal software. Tensile and impact tests refer to GB2651-2008 Tensile Test Method for Welded Joints and GB/T 2650 − 2008 Impact Test Method for Welded Joints respectively. Using FM700 hardness tester, the microhardness distribution of welded joints was measured under the conditions of 500 g load and 15s residence time, and the distance between adjacent detection points of microhardness was 0.5 mm. The location of microhardness test is about 2mm away from the upper surface of welded joint. Table 2 Chemical composition of YM-100a welding wire (mass fraction,%) C Si Mn P S Cr Ni Mo 0.09 0.36 1.43 < 0.004 < 0.004 0.99 2.58 0.59 Table 3 Maximum hardness test Welding process parameters number Current/A Voltage/V speed/cm·min − 1 preheat temperature/℃ Heat input/kJ·cm 1 250 27.5 40 20 10 2 260 29 30 20 15 3 260 27.9 40 100 10 Table 4 Welding process parameters number Current/A Voltage/V speed/cm·min − 1 Preheat temperature/℃ Inter-channel temperature/℃ Heat input/kJ·cm 1 230 26.5 37 100 100 10 2 260 29 35 100 150 13 3 285 30.9 33 100 150 16 4 300 31.8 30 100 150 19 Results and discussion 2.1 Highest hardness experimental of heat affect zone HAZ maximum hardness test is used to indirectly judge the hardening tendency and cold cracking sensitivity of welded steel, and it is an evaluation index of weldability of metal materials. The test results shown in Fig. 2 show that after preheating at 100℃, the hardness of the heat affected zone decreases from 369HV to 359HV, and the width of the heat affected zone slightly increases. Therefore, the sensitivity of welding cold cracks can be further reduced at a proper preheating temperature. When the experimental steel is welded at room temperature with the welding line energy of 10kJ/cm and 15kJ/cm, respectively, the highest hardness of the welding heat affected zone of the experimental steel plate is 2.5 ~ 4mm from both sides of the bottom tangent point (O point) of the fusion line, which is 369HV and 361HV respectively, and the cold cracking tendency of the sample heat affected zone is small. At the same time, the HAZ of welded joint is softened to some extent. With the increase of heat input, the cooling rate of the specimen decreases, the softening zone is far away from the fusion line, the width of the heat affected zone increases, the hardness decreases, and the softening trend is more obvious. However, when the heat input is too large, it is easy to cause HAZ softening and lead to the decrease of joint strength.Therefore, the welding heat input should be strictly controlled in actual production. Because the highest hardness of welding HAZ mainly depends on the chemical composition of base metal and welding cooling conditions, its essence is to reflect the microstructure and properties of different metallographic phases. When welding, the cooling time t 8/5 can be increased to reduce the maximum hardness of welding HAZ, but excessively extending t 8/5 will make the high temperature stay longer, which will lead to grain coarsening and precipitation of the second phase. Therefore, the heat input is mainly controlled, and the maximum hardness of welding HAZ can be controlled with the technological measures such as preheating before welding and slow cooling after welding, without coarsening the grains of welding HAZ, which will lead to the decrease of mechanical properties. 2.2 Influence of welding heat input on weld microstructure In the welding process, the center temperature of the weld is the highest, and the peak temperature decreases with the distance from the center line of the weld. Welding thermal cycle is an important parameter affecting the microstructure and mechanical properties of welded joints. HAZ can be divided into fusion zone (FZ), coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ) and intercritical HAZ (ICHAZ). Figure 3 shows the macroscopic and metallographic morphology of the welded joint, and there is no defect on the weld surface. FZ is relatively irregular because of the irregular combination of WM and BM. In addition, the width of FZ is only about 80µm. The optical micrograph of weld microstructure after multi-pass welding under different heat input is shown in Fig. 4 . The microstructure of WM is mainly composed of AF and granular bainite (GB) and a small amount of proeutectoid ferrite. The transformation of AF generally has no long-range diffusion of replacement atoms, and it is generally believed that the growth mechanism of acicular ferrite is displacement mechanism. The orientation difference between acicular ferrite sheets is great, and the crack propagation will be deflected when encountering acicular ferrite with different orientations, which is beneficial to toughness [ 17 , 18 ] . As can be seen from Fig. 4 , there are some fine micron-sized nonmetallic inclusions in the weld metal, the size of which is mostly about 0.5 ~ 2 µm. Figure 5 shows some inclusions in welded joints. According to EDS analysis, the main elements of WM inclusions in experimental steel are Ti, Mn, O, Si and S, etc. Many studies show that high-density inclusions such as Al2O3, Ti2O3, MnO, SiO2, MnS, ZrO2, TiN and VN in WM can provide high-density nucleation sites for AF in weld metal [ 19 , 20 ] . Inclusions in Fig. 4 are the origin of partial AF nucleation. AF nucleates unevenly at smaller inclusions, and grows radially from the nucleation point in many directions. There are still differences in the morphology and quantity of WM structures with different heat inputs. With the increase of heat input, the cooling rate increases appropriately. When the linear energy is 10kJ/cm, the WM structure is obviously dominated by AF, and GB also occupies a certain proportion, as shown in Fig. 4 (a). When the linear energy is 13kJ/cm, the structure of WM is still dominated by fine acicular ferrite, and the amount of GB also increases, as shown in Fig. 4 (b). When the linear energy is greater than 16kJ/cm, the cooling rate of the joint is further slowed down, and there are still a lot of AF in the WM structure, but the AF has been slightly coarsened, and the austenite decomposition reaction moves to a higher temperature, as shown in Fig. 4 (c). When the heat input reaches 19kJ/cm, the AF has been seriously coarsened, and due to the high temperature, austenite tends to be transformed into GB as shown in Fig. 4 (d). Figure 6 shows the grain orientation distribution, high and small low grain boundary distribution and the kernel average misorientation(KAM) diagram of WM structure. Figure 7 shows the angular grain boundary distribution of CGHAZ structure. Among them, the orientation boundary of black marks is greater than the high angle grain boundary (HAGBs) of 15, and the orientation boundary of red marks is the low angle grain boundary (LAGBs) of 2 ~ 15. When the heat input is 10kJ/cm, the proportion of HAGBs in WM tissue is the highest, which is 73.7%. With the increase of heat input, the proportion of HAGBs decreases gradually. Generally, a higher proportion of HAGBs can hinder the crack propagation or change the crack propagation direction, which is beneficial to impact toughness [ 21 , 22 ] . Figure 8 is a statistical graph of large-angle grain boundary ratio and KAM statistical data of WM and CGHAZ under different heat inputs.It can be seen that the proportion of HAGBs in CGHAZ is above 75%, bainite belongs to large-angle grain boundary structure, and the proportion of HAGBs does not change obviously with the increase of heat input, and it increases with the increase of heat input as a whole, reaching 79.6% at 19kJ/cm, which is obviously higher than WM.By comparing the KAM diagram and KAM statistical data diagram with different heat inputs, we can see the dislocation density and strain distribution. It is obvious from the KAM diagram that most KAM values are in the range of 0 to 2, and higher values are observed between slats. When the heat input increases, the KAM value increases significantly in the range of 0 ~ 1. Higher geometric dislocation density is related to higher strain level, which leads to grain fracture and deformation, and the increase of residual stress in metal. Crack initiation is more likely to occur at a higher KAM value. Therefore, this impairs the plastic deformation ability in the impact test and reduces the impact toughness [ 23 ] . Figure 9 shows the HAZ structure of welded joint, including CGHAZ, FGHAZ and ICHAZ. The structure of CGHAZ is composed of lath bainite (LB) and GB. Because the solid-state heating temperature is above 1100℃, there are large original austenite grains in CGHAZ. The heating temperature of FGHAZ is above A3, but it is lower than that of CGHAZ. Austenite transformation occurs during welding, which promotes the refinement of original austenite grains through recrystallization. At the same time, in the subsequent air cooling process, austenite is transformed into GB. When the heat input is 10kJ/cm, the cooling rate of CGHAZ is faster, and the coarse outline of the original austenite grain boundary can be seen at the grain boundary, and almost all the grains are fine LB with a small amount of martensite structure. Bainite ferrite laths grow from the original austenite grain boundary to the grain, and the boundaries are straight, parallel and continuous, forming lath bundles. The spatial arrangement between the lath bundles is uncertain and irregular, and one lath bundle is cut off by other lath bundles, so that it cannot pass through the whole austenite grain. With the increase of heat input, GB gradually increased. When the heat input reached 19kJ/cm, some grains grew abnormally, and the structure was mainly a mixture of LB and GB, and the effective grain size increased. Therefore, with the increase of linear energy and the extension of high temperature residence time, t8/5 increases correspondingly, the cooling rate decreases, and the original austenite grain size increases. At the same time, the number of plates LB decreases gradually and the number of GB increases. 2.3 Influence of welding heat input on hardness of welded joints Figure 10 shows the micro Vickers hardness (the distance between each test point is 0.5mm) measured from the center of the weld to the base metal. It can be seen that in the distance from the measured weld to the base metal, the Vickers hardness values at different positions are high and low, and change in waves. During the distance from the weld center to the base metal, because the welding pool is in the melting state for a very short time and the cooling rate is very fast, the welding metallurgical reaction and the crystallization of the weld metal are carried out in a serious unbalanced state, which leads to the obvious uneven distribution of chemical composition, microstructure and morphology in the whole weld metal, and also leads to the uneven distribution of hardness on the weld section. Obviously, the hardness of FZ and FGHAZ is obviously decreased. Due to the diffusion of alloying elements, the content of alloying elements in WM near FZ is lower than that in other parts of WM. High temperature tempering mainly occurred in FGHAZ region, which made the microstructure evolve towards tempered sorbite. CGHAZ has the highest overall hardness, which is due to its high dislocation density and a large number of bainite structures. Due to different heat input, the hardness of welded joints generally tends to decrease with the increase of welding heat input. The reason is that with high welding heat input, the cooling rate of weld is slow, and it is not easy to generate higher hardness structure. The weld structure coarsens with the increase of welding line energy, and the fine grain strengthening effect is weakened, which makes the Vickers hardness value decrease. 2.4 Influence of welding heat input on tensile properties of welded joints Figure 11 show the tensile properties of experimental steel welded joints. The tensile strength of welded joints with heat input of 10 ~ 16kJ/cm is within the requirements of 940 ~ 1130MPa and the tensile strength of 1000MPa hydropower steel plates. Secondly, the fracture positions of tensile samples are all in the weld, and the strength of the base metal is high, but the strength of the welding material does not match. The reason may be that there is a certain amount of proeutectoid ferrite in the weld, and cracks tend to spread and spread at the proeutectoid ferrite during tensile deformation [ 24 ] . With the increase of welding heat input, the welded joint shows a downward trend. Increasing the welding heat input will inevitably lead to the burning loss and element transition of alloy elements, resulting in the decrease of the concentration of alloy elements in the weld, leading to the decline of other strengthening effects such as solid solution strengthening. On the other hand, increasing the welding heat input will also reduce the cooling rate of the welded joint, which will prolong the welding thermal cycle above the phase transformation, help the grain growth, change the metallographic structure, reduce the weld performance, enlarge the heat affected zone and widen the softening zone accordingly. 2.5 Influence of welding heat input on impact properties of welded joints Charpy impact energy of different welding joints under different welding heat input is shown in Fig. 12 . With the increase of heat input, the impact absorption energy generally shows a downward trend. For different welding areas, the impact absorption energy of weld is the lowest, followed by fusion line, and the impact absorption energy of heat affected zone is the best. The difference of impact absorption work at different welding joint positions is attributed to its microstructure. With the change of sampling position, BM ratio gradually increases, corresponding to the increase of impact absorption work. Therefore, the difference of impact toughness at different sampling positions is mainly caused by the ratio of different regions on the fracture surface. Secondly, the proportion of HAGBs in CGHAZ is above 75%, and the proportion of HAGBs increases with the increase of heat input, reaching 79.6% at 19kJ/cm, which is obviously higher than WM, so it is beneficial to toughness. With the increase of heat input, alloying elements burn and diffuse into BM, which leads to the decrease of impact absorption work of weld. The thermal influence is little influenced by welding materials, mainly due to the change of original structure. BM itself has fine grains and high impact absorption work, so the impact absorption work in the heat affected zone is higher than that in WM. With the increase of heat input, t8/5 increases correspondingly, and the original austenite grain size in the coarse-grained heat affected zone increases, and the effective grain size coarsens and brittleness increases. Conclusion With the increase of heat input and the preheating of the specimen at 100℃, the hardness of the heat affected zone decreases to some extent, and the width of the heat affected zone increases, which reduces the crack sensitivity. There are a large number of inclusions in WM, some of which are particles for heterogeneous nucleation of AF. The structure of WM consists of AF, GB and a small amount of proeutectoid ferrite. With the increase of heat input, the AF in WM structure is coarse, the content of GB increases, and the proportion of HAGBs in WM structure decreases gradually. The microstructure of CG is LB and GB at higher temperature, and the recrystallization of GB in FGHAZ is beneficial to the refinement of original austenite grains. The strength, hardness and impact energy of welded joints generally decrease with the increase of welding heat input, and the diffusion of alloying elements near FZ and granular bainite transformation of FGHAZ lead to the decrease of microhardness. Due to the burning loss and diffusion of alloying elements, the tensile samples eventually fracture at WM, and the cracks tend to nucleate and propagate in the eutectoid ferrite. In WM, the element spread, burning loss and non-uniform composition or structure near FZ reduce the impact toughness, BM itself has higher impact energy, and HAZ impact absorption energy is higher than WM. When the heat input is 10~16kJ/cm, the welded joints can meet the mechanical properties requirements of 1000MPa grade steel plate in hydropower station. Declarations Conflicts of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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Journal Material Research and Technology, 2020;9(6):13793–13800 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 06 Nov, 2024 Reviewers invited by journal 01 Nov, 2024 Editor invited by journal 28 Oct, 2024 Editor assigned by journal 17 Oct, 2024 First submitted to journal 17 Oct, 2024 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-5222267","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":373016449,"identity":"d3d8c548-856b-4ca8-8c3b-ae547c453e15","order_by":0,"name":"Qiuju Bu","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Qiuju","middleName":"","lastName":"Bu","suffix":""},{"id":373016450,"identity":"c0d09b12-dc07-4bf1-ba6e-1d44a185debd","order_by":1,"name":"Fan Wang","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Fan","middleName":"","lastName":"Wang","suffix":""},{"id":373016451,"identity":"ddb82f88-e3dc-4312-858f-02b4e2ec83ba","order_by":2,"name":"Xiuhua Gao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3klEQVRIiWNgGAWjYDACZgY2BOeDgY0caVoYZxSkGRNjD0ILM8+Hw4kE1Rsc5zF7zFNTK88v3XxM2saAOYGB/fDRDfi0SDbzmBvzHDtuOHPOsTTpHAO2PAaetLQb+LTwM/OYSfOwHWPccCPHDKiFp5hBgscMrxY2sJZ/x+w33Mj/Jm1hIJHYQEgL2BbetppEoC1s0gwGBoS1SDazlUnO7TuQPHNGmrFlj0GCMRshvxicP7xN4s23Ott+ieSHN378+S/Hz374GF4tUHAYyXdEKAeBOiLVjYJRMApGwYgEAMg4QOMMSJLJAAAAAElFTkSuQmCC","orcid":"","institution":"Northeastern University","correspondingAuthor":true,"prefix":"","firstName":"Xiuhua","middleName":"","lastName":"Gao","suffix":""},{"id":373016452,"identity":"33c2297c-e253-4393-a1ef-18e0c6f8b127","order_by":3,"name":"Guoqing Feng","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Guoqing","middleName":"","lastName":"Feng","suffix":""},{"id":373016453,"identity":"d497aac6-af48-47f1-af19-611da8fb1b26","order_by":4,"name":"Zhiyong Chang","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Zhiyong","middleName":"","lastName":"Chang","suffix":""},{"id":373016454,"identity":"3e0af25c-5765-433f-a428-0e872dfe5e5e","order_by":5,"name":"Xincheng Chen","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Xincheng","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2024-10-08 05:55:58","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5222267/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5222267/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":68915578,"identity":"9caa1028-c2b9-4597-b438-7eec699c2108","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":50161,"visible":true,"origin":"","legend":"\u003cp\u003eGroove Form and Size\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/5064a5cb2ead4857c64924e5.png"},{"id":68915983,"identity":"eb0458cf-a1c8-47a0-9dea-6169d7148c37","added_by":"auto","created_at":"2024-11-13 12:53:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":33334,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental results of maximum hardness in heat affected zone\u003c/p\u003e\n\u003cp\u003e(a) Hardness distribution at different preheating temperatures; (b) Hardness distribution under different linear energies\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/f61caed0cfa19c6ca50e4d62.png"},{"id":68915979,"identity":"354a2820-5ee7-4a8f-9785-68a9d1710819","added_by":"auto","created_at":"2024-11-13 12:53:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":125906,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of Welded Joints\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/93e7fb954a3c7b6f60dfa339.png"},{"id":68915978,"identity":"a1021dbe-85e0-4b6a-b6eb-95cffbb30fb1","added_by":"auto","created_at":"2024-11-13 12:53:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":434472,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of weld under different heat input\u003c/p\u003e\n\u003cp\u003e(a)10kJ/cm; (b)13kJ/cm; (c)16kJ/cm; (d)19kJ/cm\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/0827891e7871db34535dc1ad.png"},{"id":68915580,"identity":"7c46ae03-73aa-47d9-80ad-3f9a284a36fa","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":77916,"visible":true,"origin":"","legend":"\u003cp\u003eInclusions and their compositions in welded joints\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/35bdb4d7908c1d696e416b58.png"},{"id":68915976,"identity":"0249faad-53df-4d80-aded-eb00dddaa7a9","added_by":"auto","created_at":"2024-11-13 12:53:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":984870,"visible":true,"origin":"","legend":"\u003cp\u003eGrain orientation distribution, large and small angle grain boundary distribution and KAM diagram of WM structure\u003c/p\u003e\n\u003cp\u003e（a）：E=10kJ/cm；（b）：E=13kJ/cm；（c）：E=16kJ/cm；（d）：E=19kJ/cm\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/86306913b421f330b039ac00.png"},{"id":68915589,"identity":"c50360bb-63f9-4213-8799-4b6c7bdfee7f","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":427258,"visible":true,"origin":"","legend":"\u003cp\u003eCGHAZ structure size angle grain boundary distribution map\u003c/p\u003e\n\u003cp\u003e（a）：E=10kJ/cm；（b）：E=13kJ/cm；（c）：E=16kJ/cm；（d）：E=19kJ/cm\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/0baf524aebb463e45f2bd425.png"},{"id":68915582,"identity":"1bc8a24c-b693-41d4-bafd-eda9424c4284","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":29059,"visible":true,"origin":"","legend":"\u003cp\u003eLarge-angle grain boundary ratio and KAM distribution of WM and CG under different heat inputs\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/e56e36620f9c50c15f730b4c.png"},{"id":68915583,"identity":"956bb4f1-ff13-445b-978c-97fb7fa33b74","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":496809,"visible":true,"origin":"","legend":"\u003cp\u003emicrostructure and morphology of HAZ of welded joint (CGHAZ, FGHAZ, ICHAZ)\u003c/p\u003e\n\u003cp\u003e（a1）、（a2）、（a3）：E=10kJ/cm；（b1）、（b2）、（b3）：E=13kJ/cm；\u003c/p\u003e\n\u003cp\u003e（c1）、（c2）、（c3）：E=16kJ/cm；（d1）、（d2）、（d3）：E=19kJ/cm\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/6f58ffc1e25b86ecc19618b0.png"},{"id":68915586,"identity":"7299aa45-8f69-4c5a-9627-5f994a9c3cb7","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":98122,"visible":true,"origin":"","legend":"\u003cp\u003eHardness distribution of welded joints under different heat inputs.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/4be90576f84dd82a73acb398.png"},{"id":68915584,"identity":"9f6f54ef-109a-4162-b4f0-ffdabed037df","added_by":"auto","created_at":"2024-11-13 12:45:28","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":39601,"visible":true,"origin":"","legend":"\u003cp\u003eTensile properties of welded joints under different heat inputs\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/18884b4ebecced859f3f289a.png"},{"id":68916825,"identity":"47e61ad0-5e8b-4e4d-b654-36975d9b8952","added_by":"auto","created_at":"2024-11-13 13:01:28","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":26321,"visible":true,"origin":"","legend":"\u003cp\u003eImpact toughness change under different heat input\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/5c7f39cce1b6d321884e0188.png"},{"id":68917624,"identity":"1d16aad9-f36e-4438-8c56-f46a1b61c794","added_by":"auto","created_at":"2024-11-13 13:09:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3531266,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5222267/v1/03221d48-711f-4df6-a97a-9cdc3b5f5185.pdf"}],"financialInterests":"","formattedTitle":"Effect of welding heat input on microstructure evolution and mechanical properties of welded joints of 1000MPa steel for hydropower engineering","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAs an important part of clean energy, hydropower is of great significance for building a modern energy system, boosting the transformation of energy in low-carbon life and achieving the strategic goal of \"double carbon\". With the increasing installed capacity of hydropower station, the maximum wall thickness of pressure pipe has reached 90 mm, and the thickness of steel plate on crescent rib plate has reached 150 mm, which has caused great problems for subsequent welding, transportation and installation, which requires hydropower steel to develop in the direction of high strength, energy saving, green and lightweight\u003csup\u003e\u0026nbsp;[1,2]\u003c/sup\u003e. Welding is mainly applied to important parts of large hydropower units in hydropower industry, such as hydraulic turbines, spiral cases and steel bifurcated pipes\u003csup\u003e\u0026nbsp;[3]\u003c/sup\u003e. The tensile strength of deposited metal is required to reach 940～1130MPa, and the yield strength is greater than 885MPa. In order to meet the requirements of high head, high speed, high efficiency and large capacity of hydropower units, the steel grade used in hydropower projects has been raised to 1000MPa, especially in special environments such as high-altitude hydropower stations, and the construction of large-capacity pumped storage power stations has further increased the demand for 1000MPa hydropower steel. Therefore, more and more research is being carried out on the manufacture of ultra-high strength hydropower steel \u003csup\u003e[4,5,6]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eWM is considered as the most critical area in the whole welded joint, because failure usually begins in it. Compared with conventional low-strength carbon steel, the strength of hydropower steel is improved, so it is necessary to ensure its low-temperature toughness, which will inevitably lead to the improvement of welding crack sensitivity and the difficulty of welding technology. During the welding process, the welding thermal cycles with different peak temperatures have led to drastic changes in the microstructure of the welded joint. Because of the high strength of the parent material (BM), it is difficult to ensure that the welded joint has the same strength and toughness as BM. Therefore, in order to improve the toughness of WM, inducing the formation of acicular ferrite (AF) is the key, because AF has a good combination of strength and toughness, and its characteristics are intertwined, which can hinder the crack propagation and significantly improve the toughness\u003csup\u003e\u0026nbsp;[7,8]\u003c/sup\u003e. Yang \u003csup\u003e[9]\u003c/sup\u003e studied the transformation from parent austenite to ferrite in accordance with the K-S relationship, that is, ferrite transformed according to the orientation relationship of [011]γ parallel [111]α and (111)γ parallel (011)α. Wang \u003csup\u003e[10]\u003c/sup\u003e studied the nucleation and growth of AF in low carbon steel. The formation of AF was promoted by oxide inclusions, and the evolution of the whole weld structure could be controlled by changing the cooling rate. Chen \u003csup\u003e[11]\u0026nbsp;\u003c/sup\u003eadded nano-TiO2 particles to WM during the welding process, which resulted in the formation of 0.67 μm Mn-Ti-O composite inclusions in WM, resulting in AF and high and low temperature toughness. A large number of studies have shown that inclusions can promote the formation of AF when the chemical composition, austenite grain size and undercooling degree are determined. Santos \u003csup\u003e[12]\u0026nbsp;\u003c/sup\u003efound that too high heat input will lead to the degradation of upper bainite, increase the carbide content, and appear a large number of coarse M-A islands, which is not conducive to impact toughness. On the contrary, the welded joints with low heat input show a lot of AF and GB structures, which is beneficial to impact toughness. The research of Kumar \u003csup\u003e[13]\u0026nbsp;\u003c/sup\u003eshows that the impact toughness and hardness decrease with the increase of heat input, and the higher volume fraction of AF improves the impact toughness of the weld, but the coarser microcrystalline ferrite AF and M-A chain components reduce the toughness of the weld. The change of heat input during welding has an influence on the formation, development and final properties of welded joints \u003csup\u003e[14,15]\u003c/sup\u003e. Therefore, it is of great practical significance to study the microstructure and mechanical properties of welded joints under different heat inputs.\u003c/p\u003e\n\u003cp\u003eIn this paper, the welding experimental of 1000MPa grade hydropower engineering steel is carried out, and the formation mechanism of AF, microstructure evolution of welded joints and mechanical properties under different heat inputs are studied.\u003c/p\u003e"},{"header":"Experimental materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e1.1 experimental materials\u003c/h2\u003e\n \u003cp\u003eThe parent material used in the welding experiment is 1000MPa hydro-electric steel, and the compositions of the experimental steel and deposited metal are shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. According to the calculation formula, the welding crack sensitivity index Pcm and carbon equivalent Ceq\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e of the experimental steel are calculated to be 0.24% and 0.59% respectively.\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:{\\text{C}}_{\\text{eq}}\\text{=C+}\\frac{\\text{Mn}}{\\text{6}}\\text{+}\\frac{\\text{Cu+Ni}}{\\text{15}}\\text{+}\\frac{\\text{Cr+Mo+V}}{\\text{5}}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e$$\\:{\\text{P}}_{\\text{cm}}\\text{=C+}\\frac{\\text{Si}}{\\text{30}}\\text{+}\\frac{\\text{Mn+Cu+Cr}}{\\text{20}}\\text{+}\\frac{\\text{Ni}}{\\text{60}}\\text{+}\\frac{\\text{Mo}}{\\text{15}}\\text{+}\\frac{\\text{V}}{\\text{10}}\\text{+5B}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\n \u003c/div\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eChemical composition of experimental steel (mass fraction,%)\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"14\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSi\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMn\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAlt\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNb\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eV\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCr\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNi\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCu\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMo\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eB\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.068\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.0028\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.021\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.059\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.013\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.51\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.699\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003ch3\u003e1.2 experimental methods\u003c/h3\u003e\n\u003cp\u003eAfter controlled rolling, controlled cooling and modulation, the yield strength, tensile strength, elongation and impact energy of BM are 997 MPa, 1019 MPa, 16.0% and 181 J respectively. Cut a welding sample with the size of 250mm (length) \u0026times; 120mm (width) \u0026times; 14mm along the longitudinal direction of the steel plate. Firstly, the highest hardness experiment of heat affected zone is carried out to study the weldability. On the basis of equal matching welding joints, YM-100A welding wire with diameter of 1.2mm was selected for butt welding. See Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e for the chemical composition and mechanical properties of YM-100A welding wire. Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the experimental parameters of the highest hardness. After preheating at 120℃, multi-layer multi-pass welding was carried out with shielding gas of 80% Ar and 20% CO\u003csub\u003e2\u003c/sub\u003e and gas flux of 20L/min, followed by heat preservation at 250℃ for 2h. The main parameters of welding process are shown in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, and the groove size is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eThe microstructures of solder, HAZ and WM were studied by using Leica DMIRM optical microscope (OM) and scanning electron microscope (SEM) equipped with EBSD system. Cut the metallographic specimen from the welded steel plate along the vertical welding direction. The samples used for OM observation were mechanically polished and then corroded in 4% nitric alcohol solution. Electrolytic polishing was performed in an electrolyte consisting of 12.5% perchloric acid and 87.5% ethanol for EBSD analysis, and electropolishing was performed at 25 V for 20 s with a scanning step of 0.15m m. The collected EBSD data were post-processed by AZtecCrystal software. Tensile and impact tests refer to GB2651-2008 Tensile Test Method for Welded Joints and GB/T 2650\u0026thinsp;\u0026minus;\u0026thinsp;2008 Impact Test Method for Welded Joints respectively. Using FM700 hardness tester, the microhardness distribution of welded joints was measured under the conditions of 500 g load and 15s residence time, and the distance between adjacent detection points of microhardness was 0.5 mm. The location of microhardness test is about 2mm away from the upper surface of welded joint.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eChemical composition of YM-100a welding wire (mass fraction,%)\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"8\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSi\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMn\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCr\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNi\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMo\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.59\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"char\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eMaximum hardness test Welding process parameters\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003enumber\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCurrent/A\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVoltage/V\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003espeed/cm\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003epreheat temperature/℃\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHeat input/kJ\u0026middot;cm\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e27.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e260\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e260\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e27.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"char\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eWelding process parameters\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"7\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003enumber\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCurrent/A\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVoltage/V\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003espeed/cm\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePreheat temperature/℃\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eInter-channel temperature/℃\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHeat input/kJ\u0026middot;cm\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e230\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e26.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e37\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e260\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e150\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e285\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e150\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e16\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e150\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e19\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Highest hardness experimental of heat affect zone\u003c/h2\u003e\n \u003cp\u003eHAZ maximum hardness test is used to indirectly judge the hardening tendency and cold cracking sensitivity of welded steel, and it is an evaluation index of weldability of metal materials. The test results shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e show that after preheating at 100℃, the hardness of the heat affected zone decreases from 369HV to 359HV, and the width of the heat affected zone slightly increases. Therefore, the sensitivity of welding cold cracks can be further reduced at a proper preheating temperature. When the experimental steel is welded at room temperature with the welding line energy of 10kJ/cm and 15kJ/cm, respectively, the highest hardness of the welding heat affected zone of the experimental steel plate is 2.5\u0026thinsp;~\u0026thinsp;4mm from both sides of the bottom tangent point (O point) of the fusion line, which is 369HV and 361HV respectively, and the cold cracking tendency of the sample heat affected zone is small. At the same time, the HAZ of welded joint is softened to some extent. With the increase of heat input, the cooling rate of the specimen decreases, the softening zone is far away from the fusion line, the width of the heat affected zone increases, the hardness decreases, and the softening trend is more obvious. However, when the heat input is too large, it is easy to cause HAZ softening and lead to the decrease of joint strength.Therefore, the welding heat input should be strictly controlled in actual production.\u003c/p\u003e\n \u003cp\u003eBecause the highest hardness of welding HAZ mainly depends on the chemical composition of base metal and welding cooling conditions, its essence is to reflect the microstructure and properties of different metallographic phases. When welding, the cooling time t\u003csub\u003e8/5\u003c/sub\u003e can be increased to reduce the maximum hardness of welding HAZ, but excessively extending t\u003csub\u003e8/5\u003c/sub\u003e will make the high temperature stay longer, which will lead to grain coarsening and precipitation of the second phase. Therefore, the heat input is mainly controlled, and the maximum hardness of welding HAZ can be controlled with the technological measures such as preheating before welding and slow cooling after welding, without coarsening the grains of welding HAZ, which will lead to the decrease of mechanical properties.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003e2.2 Influence of welding heat input on weld microstructure\u003c/h3\u003e\n\u003cp\u003eIn the welding process, the center temperature of the weld is the highest, and the peak temperature decreases with the distance from the center line of the weld. Welding thermal cycle is an important parameter affecting the microstructure and mechanical properties of welded joints. HAZ can be divided into fusion zone (FZ), coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ) and intercritical HAZ (ICHAZ). Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the macroscopic and metallographic morphology of the welded joint, and there is no defect on the weld surface. FZ is relatively irregular because of the irregular combination of WM and BM. In addition, the width of FZ is only about 80\u0026micro;m.\u003c/p\u003e\n\u003cp\u003eThe optical micrograph of weld microstructure after multi-pass welding under different heat input is shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. The microstructure of WM is mainly composed of AF and granular bainite (GB) and a small amount of proeutectoid ferrite. The transformation of AF generally has no long-range diffusion of replacement atoms, and it is generally believed that the growth mechanism of acicular ferrite is displacement mechanism. The orientation difference between acicular ferrite sheets is great, and the crack propagation will be deflected when encountering acicular ferrite with different orientations, which is beneficial to toughness \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAs can be seen from Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, there are some fine micron-sized nonmetallic inclusions in the weld metal, the size of which is mostly about 0.5\u0026thinsp;~\u0026thinsp;2 \u0026micro;m. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e shows some inclusions in welded joints. According to EDS analysis, the main elements of WM inclusions in experimental steel are Ti, Mn, O, Si and S, etc. Many studies show that high-density inclusions such as Al2O3, Ti2O3, MnO, SiO2, MnS, ZrO2, TiN and VN in WM can provide high-density nucleation sites for AF in weld metal \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Inclusions in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e are the origin of partial AF nucleation. AF nucleates unevenly at smaller inclusions, and grows radially from the nucleation point in many directions. There are still differences in the morphology and quantity of WM structures with different heat inputs. With the increase of heat input, the cooling rate increases appropriately. When the linear energy is 10kJ/cm, the WM structure is obviously dominated by AF, and GB also occupies a certain proportion, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(a). When the linear energy is 13kJ/cm, the structure of WM is still dominated by fine acicular ferrite, and the amount of GB also increases, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(b). When the linear energy is greater than 16kJ/cm, the cooling rate of the joint is further slowed down, and there are still a lot of AF in the WM structure, but the AF has been slightly coarsened, and the austenite decomposition reaction moves to a higher temperature, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(c). When the heat input reaches 19kJ/cm, the AF has been seriously coarsened, and due to the high temperature, austenite tends to be transformed into GB as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(d).\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e shows the grain orientation distribution, high and small low grain boundary distribution and the kernel average misorientation(KAM) diagram of WM structure. Figure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e shows the angular grain boundary distribution of CGHAZ structure. Among them, the orientation boundary of black marks is greater than the high angle grain boundary (HAGBs) of 15, and the orientation boundary of red marks is the low angle grain boundary (LAGBs) of 2\u0026thinsp;~\u0026thinsp;15. When the heat input is 10kJ/cm, the proportion of HAGBs in WM tissue is the highest, which is 73.7%. With the increase of heat input, the proportion of HAGBs decreases gradually. Generally, a higher proportion of HAGBs can hinder the crack propagation or change the crack propagation direction, which is beneficial to impact toughness \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Figure \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e is a statistical graph of large-angle grain boundary ratio and KAM statistical data of WM and CGHAZ under different heat inputs.It can be seen that the proportion of HAGBs in CGHAZ is above 75%, bainite belongs to large-angle grain boundary structure, and the proportion of HAGBs does not change obviously with the increase of heat input, and it increases with the increase of heat input as a whole, reaching 79.6% at 19kJ/cm, which is obviously higher than WM.By comparing the KAM diagram and KAM statistical data diagram with different heat inputs, we can see the dislocation density and strain distribution. It is obvious from the KAM diagram that most KAM values are in the range of 0 to 2, and higher values are observed between slats. When the heat input increases, the KAM value increases significantly in the range of 0\u0026thinsp;~\u0026thinsp;1. Higher geometric dislocation density is related to higher strain level, which leads to grain fracture and deformation, and the increase of residual stress in metal. Crack initiation is more likely to occur at a higher KAM value. Therefore, this impairs the plastic deformation ability in the impact test and reduces the impact toughness \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e shows the HAZ structure of welded joint, including CGHAZ, FGHAZ and ICHAZ. The structure of CGHAZ is composed of lath bainite (LB) and GB. Because the solid-state heating temperature is above 1100℃, there are large original austenite grains in CGHAZ. The heating temperature of FGHAZ is above A3, but it is lower than that of CGHAZ. Austenite transformation occurs during welding, which promotes the refinement of original austenite grains through recrystallization. At the same time, in the subsequent air cooling process, austenite is transformed into GB.\u003c/p\u003e\n\u003cp\u003eWhen the heat input is 10kJ/cm, the cooling rate of CGHAZ is faster, and the coarse outline of the original austenite grain boundary can be seen at the grain boundary, and almost all the grains are fine LB with a small amount of martensite structure. Bainite ferrite laths grow from the original austenite grain boundary to the grain, and the boundaries are straight, parallel and continuous, forming lath bundles. The spatial arrangement between the lath bundles is uncertain and irregular, and one lath bundle is cut off by other lath bundles, so that it cannot pass through the whole austenite grain. With the increase of heat input, GB gradually increased. When the heat input reached 19kJ/cm, some grains grew abnormally, and the structure was mainly a mixture of LB and GB, and the effective grain size increased. Therefore, with the increase of linear energy and the extension of high temperature residence time, t8/5 increases correspondingly, the cooling rate decreases, and the original austenite grain size increases. At the same time, the number of plates LB decreases gradually and the number of GB increases.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 Influence of welding heat input on hardness of welded joints\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e shows the micro Vickers hardness (the distance between each test point is 0.5mm) measured from the center of the weld to the base metal. It can be seen that in the distance from the measured weld to the base metal, the Vickers hardness values at different positions are high and low, and change in waves.\u003c/p\u003e\n \u003cp\u003eDuring the distance from the weld center to the base metal, because the welding pool is in the melting state for a very short time and the cooling rate is very fast, the welding metallurgical reaction and the crystallization of the weld metal are carried out in a serious unbalanced state, which leads to the obvious uneven distribution of chemical composition, microstructure and morphology in the whole weld metal, and also leads to the uneven distribution of hardness on the weld section. Obviously, the hardness of FZ and FGHAZ is obviously decreased. Due to the diffusion of alloying elements, the content of alloying elements in WM near FZ is lower than that in other parts of WM. High temperature tempering mainly occurred in FGHAZ region, which made the microstructure evolve towards tempered sorbite. CGHAZ has the highest overall hardness, which is due to its high dislocation density and a large number of bainite structures. Due to different heat input, the hardness of welded joints generally tends to decrease with the increase of welding heat input. The reason is that with high welding heat input, the cooling rate of weld is slow, and it is not easy to generate higher hardness structure. The weld structure coarsens with the increase of welding line energy, and the fine grain strengthening effect is weakened, which makes the Vickers hardness value decrease.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003e2.4 Influence of welding heat input on tensile properties of welded joints\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e show the tensile properties of experimental steel welded joints. The tensile strength of welded joints with heat input of 10\u0026thinsp;~\u0026thinsp;16kJ/cm is within the requirements of 940\u0026thinsp;~\u0026thinsp;1130MPa and the tensile strength of 1000MPa hydropower steel plates. Secondly, the fracture positions of tensile samples are all in the weld, and the strength of the base metal is high, but the strength of the welding material does not match. The reason may be that there is a certain amount of proeutectoid ferrite in the weld, and cracks tend to spread and spread at the proeutectoid ferrite during tensile deformation \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eWith the increase of welding heat input, the welded joint shows a downward trend. Increasing the welding heat input will inevitably lead to the burning loss and element transition of alloy elements, resulting in the decrease of the concentration of alloy elements in the weld, leading to the decline of other strengthening effects such as solid solution strengthening. On the other hand, increasing the welding heat input will also reduce the cooling rate of the welded joint, which will prolong the welding thermal cycle above the phase transformation, help the grain growth, change the metallographic structure, reduce the weld performance, enlarge the heat affected zone and widen the softening zone accordingly.\u003c/p\u003e\n\u003ch3\u003e2.5 Influence of welding heat input on impact properties of welded joints\u003c/h3\u003e\n\u003cp\u003eCharpy impact energy of different welding joints under different welding heat input is shown in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e. With the increase of heat input, the impact absorption energy generally shows a downward trend. For different welding areas, the impact absorption energy of weld is the lowest, followed by fusion line, and the impact absorption energy of heat affected zone is the best.\u003c/p\u003e\n\u003cp\u003eThe difference of impact absorption work at different welding joint positions is attributed to its microstructure. With the change of sampling position, BM ratio gradually increases, corresponding to the increase of impact absorption work. Therefore, the difference of impact toughness at different sampling positions is mainly caused by the ratio of different regions on the fracture surface. Secondly, the proportion of HAGBs in CGHAZ is above 75%, and the proportion of HAGBs increases with the increase of heat input, reaching 79.6% at 19kJ/cm, which is obviously higher than WM, so it is beneficial to toughness.\u003c/p\u003e\n\u003cp\u003eWith the increase of heat input, alloying elements burn and diffuse into BM, which leads to the decrease of impact absorption work of weld. The thermal influence is little influenced by welding materials, mainly due to the change of original structure. BM itself has fine grains and high impact absorption work, so the impact absorption work in the heat affected zone is higher than that in WM. With the increase of heat input, t8/5 increases correspondingly, and the original austenite grain size in the coarse-grained heat affected zone increases, and the effective grain size coarsens and brittleness increases.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003col\u003e\n \u003cli\u003eWith the increase of heat input and the preheating of the specimen at 100℃, the hardness of the heat affected zone decreases to some extent, and the width of the heat affected zone increases, which reduces the crack sensitivity.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eThere are a large number of inclusions in WM, some of which are particles for heterogeneous nucleation of AF. The structure of WM consists of AF, GB and a small amount of proeutectoid ferrite. With the increase of heat input, the AF in WM structure is coarse, the content of GB increases, and the proportion of HAGBs in WM structure decreases gradually. The microstructure of CG is LB and GB at higher temperature, and the recrystallization of GB in FGHAZ is beneficial to the refinement of original austenite grains.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eThe strength, hardness and impact energy of welded joints generally decrease with the increase of welding heat input, and the diffusion of alloying elements near FZ and granular bainite transformation of FGHAZ lead to the decrease of microhardness. Due to the burning loss and diffusion of alloying elements, the tensile samples eventually fracture at WM, and the cracks tend to nucleate and propagate in the eutectoid ferrite. In WM, the element spread, burning loss and non-uniform composition or structure near FZ reduce the impact toughness, BM itself has higher impact energy, and HAZ impact absorption energy is higher than WM. When the heat input is 10~16kJ/cm, the welded joints can meet the mechanical properties requirements of 1000MPa grade steel plate in hydropower station.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Key Technology Research and Development Program of China (Grant No. 2022YFB3706400).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eBoting Zhang. The development of hydropower and the power transformation of Energy Revolution in China [J]. 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Wang, An in-situ microscopy study on nucleation and growth of acicular ferrite in Ti\u0026ndash;Ca\u0026ndash;Zr deoxidized low-carbon steel. Mater. Charact. 165, 110381 (2020)\u003c/li\u003e\n \u003cli\u003eC. X. Chen, H. T. Xue, H. F. Peng, L. Yan, L. Zhi, S. X. Wang, J. Nanomater. 2014, 2014, 1.\u003c/li\u003e\n \u003cli\u003eT. F. A. Santos, T. F. C. Hermenegildo, C. R. M. Afonso, R. R. Marinho, M. T. P. Paes, A. J. Ramirez, Eng. Fract. Mech. 2010, 77, 2937.\u003c/li\u003e\n \u003cli\u003eS. Kumar, S. K. Nath, J. Mater. Process. Technol. 2016, 236, 216.\u003c/li\u003e\n \u003cli\u003eAmer AE, Koo MY, Lee KH, Kim SH, Hong SH. Effect of welding heat input on microstructure and mechanical properties of simulated HAZ in Cu containing microalloyed steel. J Mater Sci 2010; 45:1248\u0026ndash;54.\u003c/li\u003e\n \u003cli\u003eJiang Q, Zhang X, Chen L. Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel[J]. 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Vol. 59(2009), p. 134.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eT. Koseki, G. Thewlis: Mater. Sci. Tech. Vol. 21 (2005), p.867.\u003c/li\u003e\n \u003cli\u003eY.M. Kim, S.Y. Shin, H. Lee, B. Hwang, S. Lee, N.J. Kim: Metall. Mater. Trans. A, Vol. 38(2007), p.1731.\u003c/li\u003e\n \u003cli\u003eM. Diaz-fuentes, A. Iza-mendia, I. Gutierrez: Metall. Mater. Trans. A, Vol. 34(2003), p.2505.\u003c/li\u003e\n \u003cli\u003eSun Q, Li W, Li T., Gao Q, Wang, C, Zhang X. Significant influence of heat input on microstructure evolution and mechanical properties of the simulated CGHAZ in a 1000 MPa grade ultra-high strength steel. Materials Technology, (2024)39(1).\u003c/li\u003e\n \u003cli\u003eJie Chen, Changsheng Li, Jinyi Ren. Evaluation of microstructure and mechanical properties of Fe-1.2Mn-0.3Cr-1.4Ni-0.4Mo-C steel welded joints. Journal Material Research and Technology, 2020;9(6):13793\u0026ndash;13800\u003c/li\u003e\n\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":"1000MPa hydropower steel, Acicular ferrite, Inclusion, Gas metal arc welding, Heat input","lastPublishedDoi":"10.21203/rs.3.rs-5222267/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5222267/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGas metal arc welding experiments were carried out on 1000MPa hydropower steel with different welding heat inputs. The changing laws of microstructure, hardness, strength and toughness of welded joints of 1000MPa hydropower steel under different heat inputs were studied. The highest hardness of the experimental steel under different conditions was measured through the highest hardness experiment. The results show that with the increase of heat input and the preheating of the specimen at 100℃, the hardness of the heat affected zone decreases to some extent, and the width of the heat affected zone increases, which reduces the crack sensitivity. There are a large number of inclusions in the weld metal (WM), some of which are particles for heterogeneous nucleation of acicular ferrite (AF). With the increase of heat input, the AF of WM structure is coarse, the content of granular bainite (GB) increases, the proportion of high angle grain boundaries (HAGBs) in WM structure decreases gradually, and the kernel average misorientation (KAM) value decreases. Because of the high temperature, the microstructure in the coarse grain heat-affected zone (CGHAZ) is lath bainite (LB) and GB, and the recrystallization of GB in the fine grain heat-affected zone (FGHAZ) is beneficial to the refinement of the original austenite grains. The diffusion of alloying elements near the fusion zone (FZ) and the GB phase transformation of FGHAZ lead to the decrease of microhardness. The mechanical properties of welded joints all meet the requirements of 1000MPa steel for hydropower station.\u003c/p\u003e","manuscriptTitle":"Effect of welding heat input on microstructure evolution and mechanical properties of welded joints of 1000MPa steel for hydropower engineering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-13 12:45:23","doi":"10.21203/rs.3.rs-5222267/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-11-06T08:41:35+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-01T13:45:17+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2024-10-28T07:30:20+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-17T22:36:59+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2024-10-17T08:54:58+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":"7fae8db4-80b1-460d-9c17-97b1ec9ca367","owner":[],"postedDate":"November 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-02-03T08:09:17+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-13 12:45:23","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5222267","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5222267","identity":"rs-5222267","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}