Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels

Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels | 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 Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels Kemal AYDIN, Mehtap HIDIROĞLU, Nizamettin KAHRAMAN This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2952877/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract STRENX and DP steels are advanced high-strength dual-phase steel types used in the automotive industry for producing lightweight vehicles that have low cost, high fuel efficiency, and can carry more load volume. In this study, STRENX 700 CR and DP 800 steels were joined by resistance spot welding (RSW) using medium frequency direct current (MFDC). Some parts were joined under atmospheric conditions, while others were joined by applying regional pre-heating (RPH) to their heat-affected zones (HAZ) before welding. The design, manufacturing, and adaptation of the RPH system to the RSW machine were carried out as part of this study, considering it as a new system. After the joining process, non-destructive (visual inspection, magnetic particle) and destructive (microhardness, tensile-shear, cross-tension, fatigue) tests were performed on RSW connections between STRENX 700 CR and DP 800 steels. Hardness tests revealed that the RPH applied to the HAZ before welding reduced the hardness of the weld metal by approximately 8%, while the microstructure analysis showed that the RPH increased both the weld metal and the HAZ width. The tensile-shear strength increased by about 7%, and the cross-tension test strength increased by about 5% with the RPH applied to the HAZ before welding. In both the tensile-shear and cross-tension tests, all failures occurred in the form of button shearing from the HAZ. The RPH treated specimens showed the highest fatigue life, with an average of 947,632 cycles. Resistance Spot Welding MFDC AHSS STRENX 700 CR DP 800 Dual-Phase Steel 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 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 1. Introduction Third-generation advanced high-strength steels (3N-AHSS), developed for vehicle safety, fuel efficiency, and lightweight design, are becoming increasingly prevalent in automotive bodies [ 1 , 2 ]. The need for these types of steel is growing due to the increased crash resistance provided for passenger safety and the desire to reduce carbon emissions to address environmental concerns [ 3 , 4 ]. DP steels, which belong to the high-strength steel group, consist of ferrite and martensite structures [ 5 – 7 ]. Dual-phase steels contain 20% martensite within the ferrite phase [ 8 , 9 ]. The martensite phase in the structure increases the necessary strength, while the ferrite phase ensures the desired ductility and improves the formability of the structure. Additionally, the strength-to-weight ratio is also important [ 10 – 12 ]. With the introduction of advanced high-strength steels (AHSS), a weight reduction of approximately 25% in vehicles has been observed [ 13 ]. Advanced high-strength steels (AHSS) also encompass high-strength (HSS) and ultra-high-strength (UHSS) steel groups [ 14 , 15 ]. AHSS steels have higher yield, tensile, and elastic modulus compared to other steel groups. While the tensile strength of high-strength (HSS) steels ranges from 270–700 MPa, the tensile strength of the newly developed high-strength AHSS steel groups exceeds 1000 MPa [ 16 – 18 ]. In the automotive sector, DP 800 is used for A-B-C pillars, roof rails, reinforcements for bumpers and sills, while STRENX 700 CR is used for seat frames, seat rails, heavy-duty machinery, load-bearing structures, and is also considered for use in protecting electric vehicle batteries. When it comes to joining STRENX and DP steels, it can be seen that methods such as MIG-MAG, TIG, and to a lesser extent, RSW are used [ 19 , 20 ]. Numerous studies have been conducted on the weldability and mechanical properties of various types of AHSS steel using the RSW method. Rajarajan et al. [ 4 ], joined 1.6 mm DP 800 steel using the RSW method and examined the micro/macro structures, hardness, cross-tension, and tensile-shear properties at different electrode pressures (3.75–4.75 MPa) using a medium frequency current welding machine. It was found that the fusion zone widened as the electrode force increased, and the changes in mechanical properties were due to the martensitic structure observed in SEM examinations. Rajarajan et al. [ 21 ], also joined DP 800 dual-phase steel using the RSW method and examined the mechanical and microstructure properties of the welds. It was determined that the tensile-shear strength increased up to a certain point with increasing welding current and then decreased. Moreover, an increase in welding current led to an increase in the fusion zone and the highest hardness value was observed in the fusion zone according to microhardness measurements. Sun et al. [ 22 ], examined the failure modes of welded joints when joining DP 800 and TRIP 800 high-strength steels using the RSW method and observed partial interface, interface, and nugget pullout damage modes. Yaghoobi et al. [ 23 ], joined high-strength DP dual-phase steel with fine-grain interstitial-free (IF) steel using the RSW method and found that the fusion zone diameter increased with increasing welding current. Aydın et al. [ 24 ], joined DP600 and DP 800 high-strength dual-phase steels using RSW method and reported that the strength increased with increasing welding current based on the tensile-shear and cross-tension test results of the welded joints. The increasing cost, the effect of energy losses, and, most importantly, the controllability of the welding current have led to the widespread use of MFDC welding machines [ 25 ]. As a result, MFDC technology has started to be used in the automotive sector compared to AA technology, and significant improvements in welding quality have been observed [ 26 ]. In this study, 1.2 mm thickness STRENX 700 CR and DP 800 steel sheet materials belonging to the group of advanced high strength steels (AHSS) were joined by the RSW method in accordance with the needs and demands of the developing automotive sector. Medium frequency direct current (MFDC) was used in the joining process and RPH process with a specific design and manufacturing was applied to the area under the heat influence (HAZ) of the parts to be joined prior to welding. Subsequently, the microstructure, mechanical properties, and fatigue behavior of the resistance spot welded joints between STRENX 700 CR and DP 800 steels were investigated and characterized. 2. Materials and methods In the experimental study, advanced high-strength steel types, which are preferred in the automotive sector due to factors such as strength, cost, weight, and CO 2 emissions, and produced by the SSAB company, namely DP 800 - STRENX 700 CR sheet materials, were used. The chemical properties of the high-strength steels used for welding are given in Table 1 , and the mechanical properties are given in Table 2 [ 27 – 29 ]. Table 1 Chemical content of STRENX 700 CR and DP800 steels (%) Material C Si Mn P S Al Nb + Ti Cr Ni + Cu Fe STRENX 700 CR 0,16 0,40 1,80 0,02 0,01 0,015 0,10 - - Balance DP 800 0,15 0,20 1,72 0,012 0,003 0,040 0,20 0,42 0,050 Balance Table 2 Mechanical properties of STRENX 700 CR and DP800 steels Materials Yield strength (MPa) Tensile strength (MPa) Elongation (min %) STRENX 700 CR 700 1000–1200 7 DP 800 620–770 800–950 10 STRENX and DP steel sheets with a thickness of 1.2 mm were prepared for welded joint with laser cutting method in the dimensions of 100x30x1.2 mm. The welding samples were prepared in accordance with EN ISO 14273 standards for tensile-shear and cross-tension testing. The dimensions of the tensile-shear and cross-tension specimens are shown in Fig. 1 . In the conducted study, STRENX 700 CR and DP 800 steel sheet from the high-strength steel group were joined to each other using the RSW method. The fixture design was made to overcome the regional hardness after the welding of STRENX 700 CR and DP 800 dual-phase steels and to provide ductility for shaping, and it was adapted to the welding machine. In contrast to previous studies, no operation was performed on the welding core in this study, and only the preheat treatment was applied to the region under the heat effect. The most unique aspect of the study is the design and manufacturing of this system. The schematic and post-manufacturing images of the designed electrode (dimensions in mm) are given in Fig. 2 . The 3D models and post-manufacturing images of the fixture designs are shown in Fig. 3 . A second system was mounted on the fixture designed and manufactured to perform special tensile-shearing and cross-tensile tests. A second head has been added to the RSW machine, which has a copper electrode fixed for a heating system. During the welding process, after the heating stage (0.2 seconds) is completed, the lower table moves suddenly within about 0.7 seconds, sending the part to the other welding head to complete the welding process. Various fixtures were designed and assembled for the welding machine, and the images of the test samples obtained after the welding process are given in Fig. 4 . Optimal welding parameters for joining STRENX and DP steel sheets have been determined through preliminary experiments (Table 3 ). These parameters include welding current, electrode force, and welding time. The determined parameters have been integrated into the RSW machine using software. This integration has enabled precise and consistent results to be obtained by avoiding application errors with different parameter values. This minimizes the margin of error and ensures more accurate results. Table 3 Welding parameters used in the experimental study Atmosphere Pre-Heating Pulse Welding Pulse Current (kA) Electrode force(bar) Welding time(ms) Welding current (kA) Electrode force (bar) Pulse Welding time (ms) Normal 3 3,6 100 7 3,6 1 500 RPH 3 3,6 100 7 3,6 1 500 Tensile-shear, cross-tension, and fatigue tests were performed to determine the mechanical properties of the welded samples, while microstructure analysis and hardness testing were carried out for the characterization of the weld zone. For microstructure analysis and hardness measurement, the samples were first subjected to grinding, polishing, and etching processes. The etching process was carried out in a 2% nitric acid solution for 5 seconds. After the samples were etched, the microstructure images of the weld zone were examined using a Nikon digital camera-connected optical microscope at different magnification ratios. Hardness measurements were performed in HV units using a Leco 247 AT device. Tensile-shear and cross-tension tests were conducted in accordance with DIN EN ISO 14272 standard using a SHIMADZU tensile testing machine with a capacity of 50 kN and a testing speed of 2 mm/min. The fatigue tests of the welded connections were performed by applying different load values with the help of an MTS modular hydraulic power and control unit. The morphological structure of the welded connections after rupture was examined using a Zeiss scanning electron microscope (SEM). 3. Results and Discussion 3.1. Visual Inspection and Magnetic Particle Testing Visual inspection tests were performed on the joined samples of RSW to investigate whether there were any surface defects. Possible surface deformations, cracks, etc. around the welding area were examined, and the welding core diameter, height, electrode penetration depth, and spot diameter were measured in mm using calipers and micrometers. The images of the measurements are presented in Fig. 5 , and the data in tabular form are given in Table 4 . Figure 5 represents a-normal and b-RPH processes. Table 4 Dimensions of the welding nugget and spot. Nugget diameter (mm) Nugget height (mm) Electrode immersion depth (mm) Spot diameter (mm) Normal 5,04 1,18 0,52 6,13 RPH 5,32 1,27 0,58 6,18 There are different standards for determining the minimum size of a weld in the automotive sector [ 30 , 31 ]. In order for the welded joint to be able to withstand the desired load, it is required that the size of the weld nugget be as large as possible. When examining Table 4 , it can be seen that while the electrode penetration depth is 0.52 mm, the nugget diameter is 5.04 mm, and similarly, when the electrode penetration depth is 0.58 mm, the nugget diameter is 5.32 mm. It can be observed that as the electrode penetration depth increases, the weld nugget diameters also increase. When the welded joints were visually inspected, it was determined that there were no defects such as cracks, superficial deformations, or interface protrusions in the welding zone. In this context, it can be inferred that the parameter selections were appropriate in the DNA method that was performed. Of course, the fact that no welding errors were encountered is due to the adaptation of the welding machine to today's technology, allowing for software applications to minimize errors that could occur due to human factors. After visual inspection, magnetic particle (MP) testing was performed to detect discontinuities that cannot be detected by the naked eye on the surface or near-surface internal structures of the test pieces joined by RSW. MP testing was performed on both sides of the STRENX and DP welded connections, and the obtained images are given in Fig. 6 . Figures 6 -a and 6 -b represent the normal welded connections, while Figs. 6 c and 6 d represent the RPH-treated ones. Upon examining Fig. 6 , it was determined that liquid metal brittleness, which can occur due to the formation of intermetallics between the zinc coating and the steel, did not occur due to the residual austenite. As a result, it is believed that the absence of the austenite phase in the joined steels or the inability of residual austenite, which may exist in very small proportions, to diffuse with the melted liquid zinc, and the suHAZility of the selected parameters for the chemical and mechanical properties of the steels and welding processes support a flawless weld. Başer [ 32 ], joined galvanized beynitic ferrite-supported TRIP (TBF) steel using direct current medium frequency technology (MFDC) with the RSW method and applied magnetic particle (MP) testing to detect liquid metal embrittlement in the joints. As a result of the magnetic particle testing, it was reported that no liquid metal embrittlement was detected in the weld core and HAZ. 3.2. Microstructure Investigations In all welding methods, microstructure studies performed in the welding zone are crucial in order to understand the changes that occur in the microstructure or the joining effects after the welding process. Therefore, macro and microstructure studies were performed on the welded specimens to determine the effects of the RPH process on the welding core and especially on the surrounding HAZ. Macro-microstructure images for normal welded connections are shown in Fig. 7 , while welded connections obtained using the RPH process are shown in Fig. 8 . In the microstructure analyses of the welded samples, which were obtained using normal welding and RPH, in the cross-sections of the samples, as shown in Fig. 7 , 1 represents the weld metal, 2 and 3 represent the transition zone between the weld core and the HAZ, and finally, 4 and 5 represent the transition zone from the HAZ to the base material. When the macro and microstructures of the welded samples subjected to RPH, as shown in Fig. 8 , were examined, it was observed that similar transition zones existed as in Fig. 7 . In addition, as shown in Fig. 8 (visual 3), the weld core-HAZ was divided into 4 different regions. On Fig. 8 , a represents fine-grained HAZ, b represents coarse-grained HAZ, c represents the fusion boundary zone, and d represents the fusion zone. When examining the microstructure images of the welded joint with RPH applied in Fig. 8 , which belongs to the transition area from HAZ to the base metal, relatively lighter-colored perlitic-ferritic structures are observed in these areas, while the formation of tempered martensitic structures around this region is noteworthy. Nikosohbat et al. [ 33 ], welded DP980 steel using the RSW method and examined the macro and microstructures of the welded specimens by taking sections. As a result of the examinations, they stated that the HAZ was composed of ferrite and martensite phases, there was softening in the HAZ as it went from the base metal to the welding zone, and this situation was tempered with the martensite phase due to the heat effect. Although the chemical compositions of STRENX 700 CR and DP 800 high-strength steels are different, the cross-section images show similarities due to their similar resistances. It has been determined that the effect of STRENX 700 CR material on HAZ and the welding core is greater than that of DP800. When the microstructure images of the welds produced in Fig. 8 are examined, it is thought that the coarse and fine-grained structures observed in HAZ were transformed into austenite phase due to the peak temperature reached during the DNA process being above the Ac 3 temperature, and then coarse martensitic phases were formed due to cooling. It is also believed that this contributed to the formation of coarse grains in the welded joint due to the delayed cooling of the joint caused by the RPH process. The region where the highest temperature is reached in coarse-grained HAZ is above the Ac 3 value and is the area where there is enough time for the growth of austenite grains [ 34 , 35 ]. 3.3. Hardness Test Hardness measurements were taken along a line on the test pieces joined by the DNA method to investigate the effect of hardness changes in the welding area on mechanical properties. In this context, measurements were made from DP 800 and STRENX 700 CR materials and welding core regions to determine the hardness profiles of welded joints, and graphs were created. Graphs showing the changes in hardness strengths of RPH and normally joined DNA welds are shown in Figs. 9 and 10 , respectively, to examine the changes in hardness strengths of welded joints. These graphs, with and without RPH, were obtained at 120 µm intervals along the cross-sectional directions of the welded joints and from an average of 145 hardness measurement points. The two hardness graphs given below are very similar to each other regardless of the values. When the graphs are evaluated in general, it is seen that the highest hardness is in the normal welded joints of DP800 HAZ, followed by the welding metal. The lowest hardness values are found in the welded joints subjected to the RPH process, and it is determined that the lowest hardness is observed in STRENX 700 CR HAZ, and then hardness values are formed in the welding metal. Sanchez et al. [ 36 ], stated in their study that the hardness values of high-strength steels after DNA are caused by changes in ferrite and martensite phases in the structure, especially in HAZ. They also stated that the decrease in hardness in HAZ occurred as a result of the tempering of the martensite phase. When Figs. 9 and 10 were examined, it was found that the highest hardness value in the weld metal in normally welded joints was 489 HV, while the highest hardness value in the weld metal after RPH application was found to be 465 HV. This result showed that RPH caused approximately a 5% reduction in hardness in the weld metal. Thus, it was seen that the reduction in hardness values in this region with the RPH process allowed for a more homogeneous structure by bringing it closer to the hardness of the base metal. It was observed that the lowest hardness of RSW-joined normal sources in STRENX 700 CR HAZ was 291 HV, while the highest hardness was 487 HV. Welded joints made with RPH showed that the lowest hardness in STRENX 700 CR HAZ was 287 HV, while the highest hardness was 454 HV, as seen from the hardness measurements taken and the graphs created. According to these results, there was almost no change in hardness values measured in the martensitic softening zone of HAZ in STRENX 700 CR (Figs. 9 and 10 ), while there was a 33 HV decrease in intercritical HAZ. As a result, a hardness decrease of approximately 6.8% occurred in STRENX 700 CR HAZ due to RPH prior to welding. Thus, it can be seen that the RPH process is effective in MFDC technology and reduces the upper critical hardness of HAZ, enabling more ductile ruptures to occur. At the same time, the changes in hardness values in test samples subjected to both normal and RPH treatments in critical sub-HAZ were negligible, and it is thought that the RPH treatment causes the forces applied to welded joints to be distributed over a wider area with the expansion of HAZ. It is believed that this prevents the formation of a sharper zone for the starting point of deformation when a mechanical force is applied to RSW joint, and thus makes welded joints more resistant under mechanical stress. When the DP 800 HAZ was examined through the graphs, it was seen that the martensite softening observed in the STRENX 700 CR HAZ did not occur here. The highest hardness values observed in the DP 800 HAZ were 493 HV in normal welded joints and 458 HV in welded joints with RPH. The measurement results showed that a 7.8% decrease in hardness occurred in the DP HAZ with the application of the RPH process. When the lowest hardness values observed in the welding metal were compared with each other through the graphs (Figs. 9 and 10 ), a 35 HV decrease in hardness values was observed. Similarly, in the measurements, it was observed that the difference between the highest hardness values observed in the welding metal was 24 HV. As a result, a decrease of approximately 8% in the lowest hardness and 5% in the highest hardness values of the welding metal was achieved as a result of the heat treatment applied in the joints. Chabok vd. [ 37 ], reported in their study that intercritical HAZ was present in the Ac 1 and Ac 3 range, which they considered as peak temperatures, and that the volume fraction of martensite in the weld zone was higher than in the base metal. In a similar study, Jonardhan et al. [ 38 ], stated that hardness values increased as they moved from the base metal towards the fusion zone, and that this was due to the presence of martensite and different zones formed in HAZ. Khan et al.[ 39 ] also found that hardness increased from the base metal towards the fusion zone, and attributed this to the chemical composition of the steels and the rapid cooling process. 3.4. Tensile-Shear Test Three welded samples were made for each parameter by joining the welded samples, and the graphs of all samples were created separately for each. As an example, the pulling-shearing graph obtained from the samples processed with PWHT is given in Fig. 11 -a, and the pulling-shearing graph created to better understand the effects of welding parameters used during welding is given in Fig. 11 -b. It can be seen that the pulling-shearing measurements obtained from all welded connections with both normal and PWHT treatments gave similar results in both force (kN) and extension (mm). When Fig. 11 -a is examined, the highest pulling-shearing strength in the welded joints performed with PWHT was measured as 15.58 kN. It is thought that the increase in strength was due to the expansion of HAZ as a result of the slow cooling of the welded connections after the PWHT process, and as a result of the wider area of reaction of the applied force against the pulling-shearing strength. As a result, it was understood that the RPH process contributed significantly to the pulling-shearing strength. Wang et al. [ 40 ], stated in their study that the pre-heating process increased the pulling-shearing strength, while Manladan et al. [ 41 ] stated that the pre-heating process helped to clean the oxide layer and improved the contact resistance and welding quality. Lia et al. [ 42 ] reported that pre-heating in spot welding promoted the formation of a fusion region and also increased the welding strength. When Fig. 11 -b is examined, it is clearly seen that the application of RPH to the joined welded specimens led to an increase in their strengths compared to the welded specimens without RPH. Looking at the tensile-shear strengths in the graph, it can be observed that the strength increase in the RPH-applied welded specimens (15.45 kN) compared to the non-RPH applied joined specimen (14.51 kN) was approximately 6.7%. When evaluated as a whole, the tensile-shear results showed parallelism with hardness studies and it was seen that the changes in hardness strengths had an effect on the tensile-shear strength capacities [ 43 ]. At the same time, in order to determine the tensile-shear strengths, it is necessary to have a good knowledge of the morphology of the welded connections. High-strength dual-phase steels generally experience tensile-shear damage not from the base metal but from the HAZ, unlike other steels (low-carbon steels, high-strength low-alloy steels, etc.), and it is believed that this situation occurs due to HAZ softening, which is different from other steels [ 44 ]. In conclusion, when looking at the tensile-shear strength results, it is observed that RPH, by reducing the hardness in the upper critical HAZ and allowing it to expand, causes the mechanically applied tensile-shear force to spread over a wider area, thereby increasing the tensile-shear strength, in other words, the load-carrying capacity, with the application of RPH compared to normal welded connections. 3.4.1. Tensile-Shear Failure Modes The rupture images, or in other words, the failure modes obtained after the tensile-shear tests on welded connections are shown in Fig. 12 . In Fig. 12 , a and b represent the RPH process, while c represents the normal welded connections. As seen in Fig. 12 , the post-rupture failure mode is buttoning, and the ruptures occur between the HAZ and the main material. When the morphological structure of the damage modes was examined, it was concluded that the martensite phase, especially those present within the structure, was significant in thermal cycling, and that the softening of the tempered martensite that occured below the critical HAZ in microhardness measurements played a role in the buttoning-type ruptures. Additionally, the weakest area, the HAZ, was where the buttoning-type damage modes seen in Figs. 12 -a, b, and c occured, and it was understood that the broader HAZ resulting from the RPH process not only led to a more homogeneous structure due to the reduction in upper critical HAZ hardness between Ac3 and Ac1 seen in hardness test results but also enabled the load stresses applied during the tensile-shear strength testing to be borne in a broader area. Hernandez [ 45 ] and Shojaee [ 46 ] reported that softening zones occured due to martensite tempering in the non-critical heat-affected zone (HAZ) close to the base metal and that the presence of these softening zones could affect the type of ruptures that occur during tensile-shear strength testing. SEM images of the rupture damages obtained from tensile-shear tests conducted on STRENX 700 CR and DP 800 steels after different process treatments are given in Figs. 13 and 14 , respectively. Figure 13 includes SEM images of the rupture damages of normal specimens, while Fig. 14 shows the SEM images of the rupture damages of specimens treated with RPH. Upon examination of all the SEM images provided in Figs. 13 and 14 , it was seen that the ductile-brittle and brittle rupture modes occured due to the formation of shear rupture. When the structure morphology was examined, it was evaluated that the results obtained were one of the consequences of martensitic softening, which could vary depending on the martensite volume fraction, tempering degree, and severity in critical sub-HAZ in dual-phase steels. The region where martensitic softening occured was surrounded by the fusion boundary and the martensite phase that occured due to rapid cooling, which led to shear rupture in the form of failure after the tensile-shear strength test. The SEM images of resistance spot welded joints were further examined, and it was observed that cracks started from the lower critical HAZ of STRENX 700 CR, continued to the upper critical HAZ where high microhardness values were obtained, and then led to rupture by surrounding the fusion zone. It was also considered that the presence of heterogeneous structures in HAZ triggered internal stresses and led to crack formation. Similar studies support this finding [ 47 ]. 3.5. Cross-Tension Test Tensile-shear testing is one of the commonly used methods for determining the mechanical behavior of resistance spot welded joints [ 48 , 49 ]. Therefore, tensile-shear tests were conducted on all normal and PEO-treated welded joints, and the results were used to create a force (kN)- displacement (mm) graph (Fig. 15 ). For the tensile-shear testing, three welded joints were made for each parameter, and the normal graph was created as an example, which is presented in Fig. 15 -a. When the averages of the cross-tensile strengths were examined, the lowest strength value was measured as 4.73 kN in the normal samples, while the highest strength value was obtained as 4.97 kN in the samples where the PWHT process was applied. When a general comparison was made between the welding operations, it was determined that an increase of approximately 5.3% was achieved in the cross-tensile strengths of the samples where the PWHT process was applied. A graph, composed of the triple averages of the welded samples (a total of 6) in both different parameters, is given in Fig. 15 -b to be able to see the cross-tensile results together and for easy understanding. When the results are examined, it is clearly seen that the PWHT process contributes significantly to the increase in cross-tensile strength. The graph obtained in this way was actually an expected outcome. It was believed that the increase in cross-tensile strength was due to the changes that occured as a result of the effect of martensitic softening that occured after welding, critical sub-HAZ formation, upper critical HAZ, and also due to the lower heat input to the welding zone with the use of medium-frequency direct current technology. At the same time, the high cross-tensile strength values of the welded connections where the PWHT process was applied, as seen in Fig. 15 -b, were attributed to the fact that the force applied to the test samples was distributed over a wider area due to the expansion of the critical sub-HAZ zone (soft zone) observed in HAZ with the PWHT process. As a result, it was predicted that the tears spread over a wider area compared to normal welded connections, and thus, the cross-tensile load-bearing capacity increased. In addition, it was found that the cross-tensile strength values obtained from the welded samples were approximately three times lower than the tensile-shear strength values. It is known that the way the force was applied during testing was the reason for this situation in the joints made using the RSW method. Hernandez [ 50 ], joined DP steels using the RSW method and stated that the cross-tensile values were lower than the tensile-shear test values. Chao [ 51 ] applied tensile-shear and cross-tensile tests to welded connections by joining high-strength steels with the RSW method in his study. According to the results obtained, he stated that the cross-tensile strength was lower than the tensile-shear strength. 3.5.1. Cross-Tension Failure Modes In the automotive sector, the maximum load carrying capacity during collisions is determined using the cross-tension test, and the reliability of vehicles produced in this way is determined, and this method is frequently used. Therefore, the rupture surfaces of DNA samples obtained after the cross-tension test were examined macroscopically in detail, and images taken from the rupture regions of the samples after the test are given in Fig. 16, where a represents the RPH and b, c represent normal operations. Cross-tension tests were applied to all welded samples combined in two different parameters, and the rupture images of all cross-tension test samples (normal) made in a series are given in Fig. 16-a for an example. The rupture images obtained from the samples subjected to cross-tension tests under normal conditions are shown in Fig. 16-b, and the rupture images obtained from the samples subjected to RPH are shown in Fig. 16-c. Rupture damage modes are crucial in determining the toughness and load-carrying capacities of DNA connections. Looking at Fig. 16, it can be seen that all damage modes occured in the form of buttoning. The buttoning damage mode seen as a result of the cross-tension test indicates that it occurred around the weld or HAZ. It is believed that the formation of buttoning damage mode begins with cracking around the weld core due to the applied force, and it progresses as the force continues to load. Damage modes that occur in this way indicate that the welded joint will show the necessary strength when the structure is forced to separate under the static force application. The ruptures that occured as a result of the cross-tension test are seen to occur in a brittle and semi-brittle manner on the STRENX 700 CR side. When the structure morphology was examined, it was thought that factors such as martensite softening and upper critical HAZ triggered these types of ruptures. In this context, it was confirmed that the cross-tension results obtained were parallel to the tensile-shear results, and therefore, the optimum welding parameters determined by the tensile-shear test were also confirmed by the cross-tension test results. Tamizi et al. [ 52 ], stated that during the formation of cross-tension damage modes, the crack starts at the upper critical HAZ and then continues towards the lower critical HAZ. 3.6. Fatigue Test Results The effects of welding parameters on the fatigue life of DP and STRENX steels joined by RSW were investigated by subjecting them to fatigue tests. Fatigue tests were conducted at different load levels (0.2, 0.3, 0.5, 0.75 kN) and a constant frequency of 5 Hz to predict possible discontinuities (such as crack formation) that may occur as a result of continuous cyclic loading under future usage conditions. Force-life curves were created to understand the fatigue behavior of the materials using the data obtained from the fatigue tests. The result graph produced based on the numerical data obtained from the fatigue tests of welded connections (normal and RPH) is shown in Fig. 17 . When the force-cycle relationship of the RPH process was examined, it was determined that there were significant increases in fatigue cycle numbers due to load reductions. When the welded joints were examined among themselves, it was observed that the average cycle numbers of the normal processed samples under 0.75 kN load were 4,850, while this rate increased by 109% to 10,179 cycle numbers in the fatigue strength of the welded joints with HSLA steel and DP steel joined by the RPH process. Similarly, while the average fatigue cycle numbers of normal processed joints under 0.2 kN load were 327,116, this rate increased by 189% to 947,632 cycle numbers in the welded joints joined by the RPH process. As a result, it was determined that there was a significant increase in the number of cycles due to the decrease in the applied force during the fatigue strength testing. The RPH process before welding was clearly effective in the formation of increased cycle numbers. Considering all these results, it is thought that the high fatigue strengths of welded connections with the RPH treatment compared to normal welded connections, which was previously observed during hardness measurements and explained in detail as the widening of the soft zone in HAZ, can be attributed to this soft zone. Ordonez et al. [ 53 ], joined DP980 dual-phase steel using RSW and applied fatigue testing to the resulting welded joints. They stated that a soft zone was formed due to the tempering of martensite as it could not reach the over-tempering temperature within the structure after welding, and that the soft zone and the decrease in hardness values due to softening could improve fatigue damage that could occur on HAZ. Banarjee et al. [ 54 ], examined the effects of weld size and geometry on the fatigue performance of DP590 steel welded joints using RSW. According to the results obtained from the fatigue tests, it was observed that the applied force and weld size had an effect on the fatigue strength, and insufficient fatigue strengths were obtained in cases of small weld sizes and high stress loads. 3.6.1. Examination of Ruptured Surfaces after Fatigue Testing The examination of rupture surfaces obtained after the fatigue strength tests revealed that ruptures occurred at the weld core and HAZ interface in all welded joints. It is known that in RSW studies in the automotive industry, fatigue strengths are important along with material performance, microstructure, and geometric properties [ 54 , 55 ]. To analyze the detected ruptures after fatigue, one sample from each series was examined. It can be said that the crack initiation occurred around the weld core during fatigue testing, and due to the increase in dislocation density with the heat effect, the crack continued from the HAZ and then progressed along the material. Xu et al. [ 56 ], investigated the fatigue performance of dual-phase steels using RSW and found that the fatigue crack initially started around the core. The rupture patterns observed in welded joints after fatigue testing appeared to be very similar and consistent. It is believed that all ruptures occured due to the softening of the weld nugget and the HAZ interface that occured after welding, followed by crack propagation. The softening caused by thermal transformations in the STRENX 700 CR region could be considered as a significant factor in crack initiation and rupture formation, as well as HAZ expansion and martensite softening. Figures 18 and 19 show SEM images of ruptured surfaces of samples subjected to normal and RPH treatment, respectively. Upon examination of the SEM images (Figs. 18 – 19 ), it is believed that cracks formed during the fatigue test started from the tempered zone, i.e., STRENX 700 CR HAZ, and then continued along the cross-section, reaching the coarse-grained HAZ and surface coating, causing brittle ruptures. It is also highly likely that there are areas of ductile-brittle rupture formation. When a literature search was conducted, it was suggested that the decrease in fatigue strength after the RSW process could be due to the notch effect that occurred in the RSW and the microstructure that was created in the base metal, HAZ, and welding metal [ 55 , 57 , 58 ]. Therefore, due to the heterogeneous microstructure present in the structure, the rupture images of the areas taken for SEM analysis may vary. Holovenko et al. [ 59 ], investigated the fatigue life of welded samples in their study and found that welds made by the RSW method showed a noticeable decrease in fatigue life compared to laser welding. The researchers explained that the cause of this decrease was the notch effect caused by the welding geometry, the different mechanical properties exhibited by the microstructures, and the presence of HAZ. According to the SEM images, fatigue cracks are thought to occur as a result of the breaking of interatomic bonds in certain planes, and these crack progressions are called cleavage mechanisms. These progressions can be seen within grains or at grain boundaries. It is known that the yield strength of the ferrite phase in the structure is lower than that of the martensite phase. Therefore, while the martensite phase remains in the elastic state, the ferrite phase is subjected to plastic deformation. With this effect, the stress in the ferrite increases, and thus, regional deformations can be observed. As a result, ductile or cleavage-type brittle ruptures can be seen depending on different morphologies. In Fig. 18 , the width of one measured crack was 380 µm, while the length of another crack was measured as 4,435 µm. In this context, it can be seen from SEM images that cleavage cracks occur in related fatigue ruptures, and it is also possible to observe cracks occurring at grain boundaries, as seen in Fig. 19 . 4. Conclusions In this study, commonly used high-strength steels in the automotive sector, STRENX 700 CR and DP800, were joined by the RSW method. Only HAZ was subjected to a pre-heat treatment, and the results obtained are summarized below. According to macro and microstructure examinations of all resistance spot welded samples, it was determined that the microstructures of the weld metal consisted of martensitic phase, but a small amount of regional bainitic structures also formed as a result of the applied pre-welding heat treatment process. The chemical element contents and the inherent resistance differences of STRENX 700 CR steel and DP 800 steel was shown to have an effect on the weld nugget during the fusion-solidification process during welding. When the welded joints were examined, it was determined that the changes in HAZ were more pronounced on the STRENX 700 CR side compared to DP800, and HAZ expanded with the application of RPH. After examining the hardness values of welded connections made on STRENX 700 CR by HAZ, it was observed that there was a reduction in hardness of approximately 7% in connections treated with RPH compared to normal welded connections. These results indicate that HAZ can transition to a more ductile structure, thereby reducing its brittleness. According to the results of the hardness analysis, it was determined that the soft region, called critical zone 6 HAZ, did not undergo significant changes in hardness with the RPH treatment. However, the hardness of the upper critical HAZ in the range of Ac 1 and Ac 3 was found to decrease, contributing to the formation of a more homogeneous structure. When the tensile-shear strength results of the joined samples were analyzed, it was observed that a strength increase of approximately 6.7% was achieved when the RPH treatment was applied compared to normal welded connections. These results demonstrate that the RPH treatment is an effective method for increasing the strength of the connection. Upon examination of the cross-tensile strength of the welded connections, it was determined that the welded samples treated with RPH showed an increase of approximately 5.3% in cross-tensile strength compared to normal welded samples. Looking at the results of the tensile-shear and cross-tensile strength tests, it was determined that the application of the RPH treatment caused the HAZ to expand, resulting in a wider area of deformation after the application of force on the welded connection. As a result, an increase in both the tensile-shear and cross-tensile strength was observed. When the failure modes after the tensile-shear and cross-tensile tests were examined, it was determined that all welded connections failed in a button-pull mode, and the failures occurred between the weld nugget and STRENX 700 CR HAZ. After the fatigue test, it was determined that all welded connections reached the critical cycle number at a load value of 0.2 kN. At this load value, the average fatigue strength cycles of normal welded connections were 327,116, while this number increased by 189% to 947,632 cycles in the RPH-treated samples. The application of the RPH process resulted in a significant increase in the fatigue strength of all welded connections. The reason for this is that the pre-heating process applied only to the HAZ before welding slowed down the cooling rate around the weld core and its surroundings, thus expanding the soft zone already present in the HAZ. As a result, the repeated loads spread over a wider area instead of a sharp point, leading to a significant increase in the cycle numbers of the welded connections. Declarations Funding This work was supported by the Karabük University within the scope of the FDK-2020-2132 project. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Kemal Aydın, Mehtap Hıdıroğlu and Nizamettin Kahraman. 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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-2952877","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":204540378,"identity":"6380f4d9-2cad-49bf-887b-7839c4d345eb","order_by":0,"name":"Kemal 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1","display":"","copyAsset":false,"role":"figure","size":164809,"visible":true,"origin":"","legend":"\u003cp\u003eDimensions of test samples (in mm): a cross-tensile, b tensile-shear.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/ee516f061c5ce22aaee4cd44.jpeg"},{"id":37695033,"identity":"6bc50b4d-8429-4e7d-9b71-523d6396aa71","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":109277,"visible":true,"origin":"","legend":"\u003cp\u003eElectrode designed and manufactured for RPH process.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/e094dd081d74b21f31a8a079.jpeg"},{"id":37695822,"identity":"fa3ef718-1d1d-4dba-9445-06bb378b0964","added_by":"auto","created_at":"2023-05-30 18:48:58","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":144130,"visible":true,"origin":"","legend":"\u003cp\u003e3D models and images of fixtures after manufacturing.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/bd60faee240fba907dd56a54.jpeg"},{"id":37695038,"identity":"8ef876a7-dd6e-4dfb-b468-470d0b7bf33d","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":198718,"visible":true,"origin":"","legend":"\u003cp\u003eWelding machine and test samples.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/ffb8af2526b1edbd03b6ec88.jpeg"},{"id":37695823,"identity":"2f6a55eb-93e8-41a3-962a-f0dce3fc3f49","added_by":"auto","created_at":"2023-05-30 18:48:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":479720,"visible":true,"origin":"","legend":"\u003cp\u003eMacro images of the welding cross-sections (dimensions in mm).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/e908d412e9c80068773bdac2.png"},{"id":37695034,"identity":"dbd4df5c-8568-4ad0-b55b-b49f428d4a62","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":234149,"visible":true,"origin":"","legend":"\u003cp\u003eMagnetic particle test results.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/6618aa6d46e302afd4c70d6e.jpeg"},{"id":37696141,"identity":"6a9b63bb-a257-41bf-9e19-0f129550ab20","added_by":"auto","created_at":"2023-05-30 18:56:58","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1191013,"visible":true,"origin":"","legend":"\u003cp\u003eMacro/microstructure images of normal specimens.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/3a3c5b97ea58257a8c208c38.jpeg"},{"id":37696143,"identity":"f41aad9f-777a-44f6-a725-6f5b6b106b2e","added_by":"auto","created_at":"2023-05-30 18:56:58","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1180779,"visible":true,"origin":"","legend":"\u003cp\u003eMacro/microstructure images of samples with RPH applied.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/bd899ef971e1dff3c2f704c1.jpeg"},{"id":37695051,"identity":"3c6607d6-4b22-464e-a779-d52183751b49","added_by":"auto","created_at":"2023-05-30 18:41:00","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":303930,"visible":true,"origin":"","legend":"\u003cp\u003eHardness value graph of conventionally welded joints.\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/be68a250a6e27f0775fbaef7.jpeg"},{"id":37696783,"identity":"8f8e81fb-bc8f-4832-8331-f637afaaad8a","added_by":"auto","created_at":"2023-05-30 19:04:58","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":280056,"visible":true,"origin":"","legend":"\u003cp\u003eHardness value graph of welded joints treated with RPH.\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/96552b4c2accc794f467abe2.jpeg"},{"id":37695035,"identity":"cdc90102-82c3-4a8b-afed-7b032aa71386","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":170587,"visible":true,"origin":"","legend":"\u003cp\u003eTensile-shear test results.\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/f2fc33e31424d4c05947f761.jpeg"},{"id":37695831,"identity":"8919dca4-c241-4bf3-9b14-d3ca604917a5","added_by":"auto","created_at":"2023-05-30 18:48:59","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":253035,"visible":true,"origin":"","legend":"\u003cp\u003eRupture images after tensile-shear testing.\u003c/p\u003e","description":"","filename":"floatimage12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/bb620cb2e2722627eae53411.jpeg"},{"id":37695825,"identity":"d383a0e5-9065-4006-a970-22e5cd06adfa","added_by":"auto","created_at":"2023-05-30 18:48:58","extension":"jpeg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":415671,"visible":true,"origin":"","legend":"\u003cp\u003eSEM Images of Tensile-Shear (Normal) Failure Modes.\u003c/p\u003e","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/0e643f2acd005a88cb36dd1c.jpeg"},{"id":37695826,"identity":"f76ddb21-8af5-4bf7-87f3-5099db018f7a","added_by":"auto","created_at":"2023-05-30 18:48:58","extension":"jpeg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":426680,"visible":true,"origin":"","legend":"\u003cp\u003eSEM Images of Tensile-Shear (RPH) Failure Modes.\u003c/p\u003e","description":"","filename":"floatimage14.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/6dbaa12f697c62e92d25c69c.jpeg"},{"id":37695050,"identity":"6e1a3176-6456-4cb5-819e-ff8b949127d3","added_by":"auto","created_at":"2023-05-30 18:40:59","extension":"jpeg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":160875,"visible":true,"origin":"","legend":"\u003cp\u003eThe cross-tension tests results.\u003c/p\u003e","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/28727c1ba7b64405fff6dca5.jpeg"},{"id":37695049,"identity":"a50de8ca-7699-4d4d-ad59-0559ffd8a7f1","added_by":"auto","created_at":"2023-05-30 18:40:59","extension":"jpeg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":564185,"visible":true,"origin":"","legend":"\u003cp\u003eRupture form of the welded sample after cross-tension test.\u003c/p\u003e","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/a2a488b85a16b3c23a1ddfd1.jpeg"},{"id":37695043,"identity":"d3573e13-b7f1-4bb7-b428-e6b578f7b645","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":320638,"visible":true,"origin":"","legend":"\u003cp\u003eResult graph of the fatigue test.\u003c/p\u003e","description":"","filename":"floatimage17.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/986df9c825c5461ba0f1da82.jpeg"},{"id":37695830,"identity":"411bb01a-0487-4c67-b9e2-9289863dce6e","added_by":"auto","created_at":"2023-05-30 18:48:59","extension":"jpeg","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":544733,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of the ruptured surface of samples subjected to normal treatment.\u003c/p\u003e","description":"","filename":"floatimage18.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/464b3e3f3526b3432ef9eb41.jpeg"},{"id":37695041,"identity":"ee2a7076-14eb-443a-8b9e-3c1f68c4dcb2","added_by":"auto","created_at":"2023-05-30 18:40:58","extension":"jpeg","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":507165,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of the ruptured surface of samples subjected to RPH treatment.\u003c/p\u003e","description":"","filename":"floatimage19.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/65e5c61b0e480ac3b30d787e.jpeg"},{"id":40085579,"identity":"e3d4ab49-88d9-4752-af9e-554f07f1be69","added_by":"auto","created_at":"2023-07-16 10:37:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3795708,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2952877/v1/a2ab751f-3dda-409c-9611-4560a3b3b691.pdf"}],"financialInterests":"","formattedTitle":"Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThird-generation advanced high-strength steels (3N-AHSS), developed for vehicle safety, fuel efficiency, and lightweight design, are becoming increasingly prevalent in automotive bodies [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The need for these types of steel is growing due to the increased crash resistance provided for passenger safety and the desire to reduce carbon emissions to address environmental concerns [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. DP steels, which belong to the high-strength steel group, consist of ferrite and martensite structures [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Dual-phase steels contain 20% martensite within the ferrite phase [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The martensite phase in the structure increases the necessary strength, while the ferrite phase ensures the desired ductility and improves the formability of the structure. Additionally, the strength-to-weight ratio is also important [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. With the introduction of advanced high-strength steels (AHSS), a weight reduction of approximately 25% in vehicles has been observed [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAdvanced high-strength steels (AHSS) also encompass high-strength (HSS) and ultra-high-strength (UHSS) steel groups [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. AHSS steels have higher yield, tensile, and elastic modulus compared to other steel groups. While the tensile strength of high-strength (HSS) steels ranges from 270\u0026ndash;700 MPa, the tensile strength of the newly developed high-strength AHSS steel groups exceeds 1000 MPa [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the automotive sector, DP 800 is used for A-B-C pillars, roof rails, reinforcements for bumpers and sills, while STRENX 700 CR is used for seat frames, seat rails, heavy-duty machinery, load-bearing structures, and is also considered for use in protecting electric vehicle batteries. When it comes to joining STRENX and DP steels, it can be seen that methods such as MIG-MAG, TIG, and to a lesser extent, RSW are used [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNumerous studies have been conducted on the weldability and mechanical properties of various types of AHSS steel using the RSW method. Rajarajan et al. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], joined 1.6 mm DP 800 steel using the RSW method and examined the micro/macro structures, hardness, cross-tension, and tensile-shear properties at different electrode pressures (3.75\u0026ndash;4.75 MPa) using a medium frequency current welding machine. It was found that the fusion zone widened as the electrode force increased, and the changes in mechanical properties were due to the martensitic structure observed in SEM examinations. Rajarajan et al. [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], also joined DP 800 dual-phase steel using the RSW method and examined the mechanical and microstructure properties of the welds. It was determined that the tensile-shear strength increased up to a certain point with increasing welding current and then decreased. Moreover, an increase in welding current led to an increase in the fusion zone and the highest hardness value was observed in the fusion zone according to microhardness measurements. Sun et al. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], examined the failure modes of welded joints when joining DP 800 and TRIP 800 high-strength steels using the RSW method and observed partial interface, interface, and nugget pullout damage modes. Yaghoobi et al. [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], joined high-strength DP dual-phase steel with fine-grain interstitial-free (IF) steel using the RSW method and found that the fusion zone diameter increased with increasing welding current. Aydın et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e], joined DP600 and DP 800 high-strength dual-phase steels using RSW method and reported that the strength increased with increasing welding current based on the tensile-shear and cross-tension test results of the welded joints.\u003c/p\u003e \u003cp\u003eThe increasing cost, the effect of energy losses, and, most importantly, the controllability of the welding current have led to the widespread use of MFDC welding machines [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. As a result, MFDC technology has started to be used in the automotive sector compared to AA technology, and significant improvements in welding quality have been observed [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, 1.2 mm thickness STRENX 700 CR and DP 800 steel sheet materials belonging to the group of advanced high strength steels (AHSS) were joined by the RSW method in accordance with the needs and demands of the developing automotive sector. Medium frequency direct current (MFDC) was used in the joining process and RPH process with a specific design and manufacturing was applied to the area under the heat influence (HAZ) of the parts to be joined prior to welding. Subsequently, the microstructure, mechanical properties, and fatigue behavior of the resistance spot welded joints between STRENX 700 CR and DP 800 steels were investigated and characterized.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eIn the experimental study, advanced high-strength steel types, which are preferred in the automotive sector due to factors such as strength, cost, weight, and CO\u003csub\u003e2\u003c/sub\u003e emissions, and produced by the SSAB company, namely DP 800 - STRENX 700 CR sheet materials, were used. The chemical properties of the high-strength steels used for welding are given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, and the mechanical properties are given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e [\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\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\u003eChemical content of STRENX 700 CR and DP800 steels (%)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"11\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\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\u003eC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNb\u0026thinsp;+\u0026thinsp;Ti\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eNi\u0026thinsp;+\u0026thinsp;Cu\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSTRENX 700 CR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0,02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0,01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0,015\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0,10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDP 800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0,012\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0,003\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0,040\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0,20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0,42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0,050\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eBalance\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=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMechanical properties of STRENX 700 CR and DP800 steels\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=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterials\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYield strength (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTensile strength (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eElongation\u003c/p\u003e \u003cp\u003e(min %)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSTRENX 700 CR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1000\u0026ndash;1200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDP 800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e620\u0026ndash;770\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e800\u0026ndash;950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10\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\u003eSTRENX and DP steel sheets with a thickness of 1.2 mm were prepared for welded joint with laser cutting method in the dimensions of 100x30x1.2 mm. The welding samples were prepared in accordance with EN ISO 14273 standards for tensile-shear and cross-tension testing. The dimensions of the tensile-shear and cross-tension specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eIn the conducted study, STRENX 700 CR and DP 800 steel sheet from the high-strength steel group were joined to each other using the RSW method. The fixture design was made to overcome the regional hardness after the welding of STRENX 700 CR and DP 800 dual-phase steels and to provide ductility for shaping, and it was adapted to the welding machine. In contrast to previous studies, no operation was performed on the welding core in this study, and only the preheat treatment was applied to the region under the heat effect. The most unique aspect of the study is the design and manufacturing of this system. The schematic and post-manufacturing images of the designed electrode (dimensions in mm) are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The 3D models and post-manufacturing images of the fixture designs are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. A second system was mounted on the fixture designed and manufactured to perform special tensile-shearing and cross-tensile tests.\u003c/p\u003e \u003cp\u003eA second head has been added to the RSW machine, which has a copper electrode fixed for a heating system. During the welding process, after the heating stage (0.2 seconds) is completed, the lower table moves suddenly within about 0.7 seconds, sending the part to the other welding head to complete the welding process. Various fixtures were designed and assembled for the welding machine, and the images of the test samples obtained after the welding process are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eOptimal welding parameters for joining STRENX and DP steel sheets have been determined through preliminary experiments (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). These parameters include welding current, electrode force, and welding time. The determined parameters have been integrated into the RSW machine using software. This integration has enabled precise and consistent results to be obtained by avoiding application errors with different parameter values. This minimizes the margin of error and ensures more accurate results.\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\u003eWelding parameters used in the experimental study\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAtmosphere\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003ePre-Heating Pulse\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003eWelding Pulse\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCurrent (kA)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eElectrode force(bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eWelding time(ms)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eWelding current (kA)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eElectrode force (bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003ePulse\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eWelding time (ms)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNormal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3,6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3,6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e500\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRPH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3,6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3,6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e500\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\u003eTensile-shear, cross-tension, and fatigue tests were performed to determine the mechanical properties of the welded samples, while microstructure analysis and hardness testing were carried out for the characterization of the weld zone. For microstructure analysis and hardness measurement, the samples were first subjected to grinding, polishing, and etching processes. The etching process was carried out in a 2% nitric acid solution for 5 seconds. After the samples were etched, the microstructure images of the weld zone were examined using a Nikon digital camera-connected optical microscope at different magnification ratios. Hardness measurements were performed in HV units using a Leco 247 AT device. Tensile-shear and cross-tension tests were conducted in accordance with DIN EN ISO 14272 standard using a SHIMADZU tensile testing machine with a capacity of 50 kN and a testing speed of 2 mm/min. The fatigue tests of the welded connections were performed by applying different load values with the help of an MTS modular hydraulic power and control unit. The morphological structure of the welded connections after rupture was examined using a Zeiss scanning electron microscope (SEM).\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Visual Inspection and Magnetic Particle Testing\u003c/h2\u003e \u003cp\u003eVisual inspection tests were performed on the joined samples of RSW to investigate whether there were any surface defects. Possible surface deformations, cracks, etc. around the welding area were examined, and the welding core diameter, height, electrode penetration depth, and spot diameter were measured in mm using calipers and micrometers. The images of the measurements are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, and the data in tabular form are given in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e represents a-normal and b-RPH processes.\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\u003eDimensions of the welding nugget and spot.\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=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNugget diameter\u003c/p\u003e \u003cp\u003e(mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNugget height\u003c/p\u003e \u003cp\u003e(mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eElectrode immersion depth\u003c/p\u003e \u003cp\u003e(mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSpot diameter\u003c/p\u003e \u003cp\u003e(mm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eNormal\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5,04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1,18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6,13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eRPH\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5,32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1,27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6,18\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\u003eThere are different standards for determining the minimum size of a weld in the automotive sector [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. In order for the welded joint to be able to withstand the desired load, it is required that the size of the weld nugget be as large as possible. When examining Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, it can be seen that while the electrode penetration depth is 0.52 mm, the nugget diameter is 5.04 mm, and similarly, when the electrode penetration depth is 0.58 mm, the nugget diameter is 5.32 mm. It can be observed that as the electrode penetration depth increases, the weld nugget diameters also increase.\u003c/p\u003e \u003cp\u003eWhen the welded joints were visually inspected, it was determined that there were no defects such as cracks, superficial deformations, or interface protrusions in the welding zone. In this context, it can be inferred that the parameter selections were appropriate in the DNA method that was performed. Of course, the fact that no welding errors were encountered is due to the adaptation of the welding machine to today's technology, allowing for software applications to minimize errors that could occur due to human factors.\u003c/p\u003e \u003cp\u003eAfter visual inspection, magnetic particle (MP) testing was performed to detect discontinuities that cannot be detected by the naked eye on the surface or near-surface internal structures of the test pieces joined by RSW. MP testing was performed on both sides of the STRENX and DP welded connections, and the obtained images are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Figures\u0026nbsp;\u0026lt;link rid=\"fig6\"\u0026gt;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u0026lt;/link\u0026gt;\u003c/span\u003e-a and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e-b represent the normal welded connections, while Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed represent the RPH-treated ones.\u003c/p\u003e \u003cp\u003eUpon examining Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, it was determined that liquid metal brittleness, which can occur due to the formation of intermetallics between the zinc coating and the steel, did not occur due to the residual austenite. As a result, it is believed that the absence of the austenite phase in the joined steels or the inability of residual austenite, which may exist in very small proportions, to diffuse with the melted liquid zinc, and the suHAZility of the selected parameters for the chemical and mechanical properties of the steels and welding processes support a flawless weld.\u003c/p\u003e \u003cp\u003eBaşer [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], joined galvanized beynitic ferrite-supported TRIP (TBF) steel using direct current medium frequency technology (MFDC) with the RSW method and applied magnetic particle (MP) testing to detect liquid metal embrittlement in the joints. As a result of the magnetic particle testing, it was reported that no liquid metal embrittlement was detected in the weld core and HAZ.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Microstructure Investigations\u003c/h2\u003e \u003cp\u003eIn all welding methods, microstructure studies performed in the welding zone are crucial in order to understand the changes that occur in the microstructure or the joining effects after the welding process. Therefore, macro and microstructure studies were performed on the welded specimens to determine the effects of the RPH process on the welding core and especially on the surrounding HAZ. Macro-microstructure images for normal welded connections are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, while welded connections obtained using the RPH process are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eIn the microstructure analyses of the welded samples, which were obtained using normal welding and RPH, in the cross-sections of the samples, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e represents the weld metal, 2 and 3 represent the transition zone between the weld core and the HAZ, and finally, 4 and 5 represent the transition zone from the HAZ to the base material. When the macro and microstructures of the welded samples subjected to RPH, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, were examined, it was observed that similar transition zones existed as in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. In addition, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (visual 3), the weld core-HAZ was divided into 4 different regions. On Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, a represents fine-grained HAZ, b represents coarse-grained HAZ, c represents the fusion boundary zone, and d represents the fusion zone.\u003c/p\u003e \u003cp\u003eWhen examining the microstructure images of the welded joint with RPH applied in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, which belongs to the transition area from HAZ to the base metal, relatively lighter-colored perlitic-ferritic structures are observed in these areas, while the formation of tempered martensitic structures around this region is noteworthy. Nikosohbat et al. [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], welded DP980 steel using the RSW method and examined the macro and microstructures of the welded specimens by taking sections. As a result of the examinations, they stated that the HAZ was composed of ferrite and martensite phases, there was softening in the HAZ as it went from the base metal to the welding zone, and this situation was tempered with the martensite phase due to the heat effect.\u003c/p\u003e \u003cp\u003eAlthough the chemical compositions of STRENX 700 CR and DP 800 high-strength steels are different, the cross-section images show similarities due to their similar resistances. It has been determined that the effect of STRENX 700 CR material on HAZ and the welding core is greater than that of DP800.\u003c/p\u003e \u003cp\u003eWhen the microstructure images of the welds produced in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e are examined, it is thought that the coarse and fine-grained structures observed in HAZ were transformed into austenite phase due to the peak temperature reached during the DNA process being above the Ac\u003csub\u003e3\u003c/sub\u003e temperature, and then coarse martensitic phases were formed due to cooling. It is also believed that this contributed to the formation of coarse grains in the welded joint due to the delayed cooling of the joint caused by the RPH process. The region where the highest temperature is reached in coarse-grained HAZ is above the Ac\u003csub\u003e3\u003c/sub\u003e value and is the area where there is enough time for the growth of austenite grains [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Hardness Test\u003c/h2\u003e \u003cp\u003eHardness measurements were taken along a line on the test pieces joined by the DNA method to investigate the effect of hardness changes in the welding area on mechanical properties. In this context, measurements were made from DP 800 and STRENX 700 CR materials and welding core regions to determine the hardness profiles of welded joints, and graphs were created. Graphs showing the changes in hardness strengths of RPH and normally joined DNA welds are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e, respectively, to examine the changes in hardness strengths of welded joints. These graphs, with and without RPH, were obtained at 120 \u0026micro;m intervals along the cross-sectional directions of the welded joints and from an average of 145 hardness measurement points.\u003c/p\u003e \u003cp\u003eThe two hardness graphs given below are very similar to each other regardless of the values. When the graphs are evaluated in general, it is seen that the highest hardness is in the normal welded joints of DP800 HAZ, followed by the welding metal. The lowest hardness values are found in the welded joints subjected to the RPH process, and it is determined that the lowest hardness is observed in STRENX 700 CR HAZ, and then hardness values are formed in the welding metal. Sanchez et al. [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], stated in their study that the hardness values of high-strength steels after DNA are caused by changes in ferrite and martensite phases in the structure, especially in HAZ. They also stated that the decrease in hardness in HAZ occurred as a result of the tempering of the martensite phase.\u003c/p\u003e \u003cp\u003eWhen Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e were examined, it was found that the highest hardness value in the weld metal in normally welded joints was 489 HV, while the highest hardness value in the weld metal after RPH application was found to be 465 HV. This result showed that RPH caused approximately a 5% reduction in hardness in the weld metal. Thus, it was seen that the reduction in hardness values in this region with the RPH process allowed for a more homogeneous structure by bringing it closer to the hardness of the base metal.\u003c/p\u003e \u003cp\u003eIt was observed that the lowest hardness of RSW-joined normal sources in STRENX 700 CR HAZ was 291 HV, while the highest hardness was 487 HV. Welded joints made with RPH showed that the lowest hardness in STRENX 700 CR HAZ was 287 HV, while the highest hardness was 454 HV, as seen from the hardness measurements taken and the graphs created. According to these results, there was almost no change in hardness values measured in the martensitic softening zone of HAZ in STRENX 700 CR (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e), while there was a 33 HV decrease in intercritical HAZ. As a result, a hardness decrease of approximately 6.8% occurred in STRENX 700 CR HAZ due to RPH prior to welding. Thus, it can be seen that the RPH process is effective in MFDC technology and reduces the upper critical hardness of HAZ, enabling more ductile ruptures to occur. At the same time, the changes in hardness values in test samples subjected to both normal and RPH treatments in critical sub-HAZ were negligible, and it is thought that the RPH treatment causes the forces applied to welded joints to be distributed over a wider area with the expansion of HAZ. It is believed that this prevents the formation of a sharper zone for the starting point of deformation when a mechanical force is applied to RSW joint, and thus makes welded joints more resistant under mechanical stress.\u003c/p\u003e \u003cp\u003eWhen the DP 800 HAZ was examined through the graphs, it was seen that the martensite softening observed in the STRENX 700 CR HAZ did not occur here. The highest hardness values observed in the DP 800 HAZ were 493 HV in normal welded joints and 458 HV in welded joints with RPH. The measurement results showed that a 7.8% decrease in hardness occurred in the DP HAZ with the application of the RPH process.\u003c/p\u003e \u003cp\u003eWhen the lowest hardness values observed in the welding metal were compared with each other through the graphs (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e), a 35 HV decrease in hardness values was observed. Similarly, in the measurements, it was observed that the difference between the highest hardness values observed in the welding metal was 24 HV. As a result, a decrease of approximately 8% in the lowest hardness and 5% in the highest hardness values of the welding metal was achieved as a result of the heat treatment applied in the joints.\u003c/p\u003e \u003cp\u003eChabok vd. [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], reported in their study that intercritical HAZ was present in the Ac\u003csub\u003e1\u003c/sub\u003e and Ac\u003csub\u003e3\u003c/sub\u003e range, which they considered as peak temperatures, and that the volume fraction of martensite in the weld zone was higher than in the base metal. In a similar study, Jonardhan et al. [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], stated that hardness values increased as they moved from the base metal towards the fusion zone, and that this was due to the presence of martensite and different zones formed in HAZ. Khan et al.[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] also found that hardness increased from the base metal towards the fusion zone, and attributed this to the chemical composition of the steels and the rapid cooling process.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Tensile-Shear Test\u003c/h2\u003e \u003cp\u003eThree welded samples were made for each parameter by joining the welded samples, and the graphs of all samples were created separately for each. As an example, the pulling-shearing graph obtained from the samples processed with PWHT is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e-a, and the pulling-shearing graph created to better understand the effects of welding parameters used during welding is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e-b.\u003c/p\u003e \u003cp\u003eIt can be seen that the pulling-shearing measurements obtained from all welded connections with both normal and PWHT treatments gave similar results in both force (kN) and extension (mm). When Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e-a is examined, the highest pulling-shearing strength in the welded joints performed with PWHT was measured as 15.58 kN. It is thought that the increase in strength was due to the expansion of HAZ as a result of the slow cooling of the welded connections after the PWHT process, and as a result of the wider area of reaction of the applied force against the pulling-shearing strength. As a result, it was understood that the RPH process contributed significantly to the pulling-shearing strength. Wang et al. [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], stated in their study that the pre-heating process increased the pulling-shearing strength, while Manladan et al. [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e] stated that the pre-heating process helped to clean the oxide layer and improved the contact resistance and welding quality. Lia et al. [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e] reported that pre-heating in spot welding promoted the formation of a fusion region and also increased the welding strength.\u003c/p\u003e \u003cp\u003eWhen Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e-b is examined, it is clearly seen that the application of RPH to the joined welded specimens led to an increase in their strengths compared to the welded specimens without RPH. Looking at the tensile-shear strengths in the graph, it can be observed that the strength increase in the RPH-applied welded specimens (15.45 kN) compared to the non-RPH applied joined specimen (14.51 kN) was approximately 6.7%.\u003c/p\u003e \u003cp\u003eWhen evaluated as a whole, the tensile-shear results showed parallelism with hardness studies and it was seen that the changes in hardness strengths had an effect on the tensile-shear strength capacities [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. At the same time, in order to determine the tensile-shear strengths, it is necessary to have a good knowledge of the morphology of the welded connections. High-strength dual-phase steels generally experience tensile-shear damage not from the base metal but from the HAZ, unlike other steels (low-carbon steels, high-strength low-alloy steels, etc.), and it is believed that this situation occurs due to HAZ softening, which is different from other steels [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn conclusion, when looking at the tensile-shear strength results, it is observed that RPH, by reducing the hardness in the upper critical HAZ and allowing it to expand, causes the mechanically applied tensile-shear force to spread over a wider area, thereby increasing the tensile-shear strength, in other words, the load-carrying capacity, with the application of RPH compared to normal welded connections.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1. Tensile-Shear Failure Modes\u003c/h2\u003e \u003cp\u003eThe rupture images, or in other words, the failure modes obtained after the tensile-shear tests on welded connections are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. In Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, a and b represent the RPH process, while c represents the normal welded connections. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, the post-rupture failure mode is buttoning, and the ruptures occur between the HAZ and the main material.\u003c/p\u003e \u003cp\u003eWhen the morphological structure of the damage modes was examined, it was concluded that the martensite phase, especially those present within the structure, was significant in thermal cycling, and that the softening of the tempered martensite that occured below the critical HAZ in microhardness measurements played a role in the buttoning-type ruptures. Additionally, the weakest area, the HAZ, was where the buttoning-type damage modes seen in Figs.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e-a, b, and c occured, and it was understood that the broader HAZ resulting from the RPH process not only led to a more homogeneous structure due to the reduction in upper critical HAZ hardness between Ac3 and Ac1 seen in hardness test results but also enabled the load stresses applied during the tensile-shear strength testing to be borne in a broader area. Hernandez [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] and Shojaee [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] reported that softening zones occured due to martensite tempering in the non-critical heat-affected zone (HAZ) close to the base metal and that the presence of these softening zones could affect the type of ruptures that occur during tensile-shear strength testing.\u003c/p\u003e \u003cp\u003eSEM images of the rupture damages obtained from tensile-shear tests conducted on STRENX 700 CR and DP 800 steels after different process treatments are given in Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e and \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e includes SEM images of the rupture damages of normal specimens, while Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e shows the SEM images of the rupture damages of specimens treated with RPH.\u003c/p\u003e\u003cp\u003eUpon examination of all the SEM images provided in Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e and \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e, it was seen that the ductile-brittle and brittle rupture modes occured due to the formation of shear rupture. When the structure morphology was examined, it was evaluated that the results obtained were one of the consequences of martensitic softening, which could vary depending on the martensite volume fraction, tempering degree, and severity in critical sub-HAZ in dual-phase steels. The region where martensitic softening occured was surrounded by the fusion boundary and the martensite phase that occured due to rapid cooling, which led to shear rupture in the form of failure after the tensile-shear strength test.\u003c/p\u003e \u003cp\u003eThe SEM images of resistance spot welded joints were further examined, and it was observed that cracks started from the lower critical HAZ of STRENX 700 CR, continued to the upper critical HAZ where high microhardness values were obtained, and then led to rupture by surrounding the fusion zone. It was also considered that the presence of heterogeneous structures in HAZ triggered internal stresses and led to crack formation. Similar studies support this finding [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Cross-Tension Test\u003c/h2\u003e \u003cp\u003eTensile-shear testing is one of the commonly used methods for determining the mechanical behavior of resistance spot welded joints [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Therefore, tensile-shear tests were conducted on all normal and PEO-treated welded joints, and the results were used to create a force (kN)- displacement (mm) graph (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e). For the tensile-shear testing, three welded joints were made for each parameter, and the normal graph was created as an example, which is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e-a.\u003c/p\u003e\u003cp\u003eWhen the averages of the cross-tensile strengths were examined, the lowest strength value was measured as 4.73 kN in the normal samples, while the highest strength value was obtained as 4.97 kN in the samples where the PWHT process was applied. When a general comparison was made between the welding operations, it was determined that an increase of approximately 5.3% was achieved in the cross-tensile strengths of the samples where the PWHT process was applied. A graph, composed of the triple averages of the welded samples (a total of 6) in both different parameters, is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e-b to be able to see the cross-tensile results together and for easy understanding. When the results are examined, it is clearly seen that the PWHT process contributes significantly to the increase in cross-tensile strength.\u003c/p\u003e \u003cp\u003eThe graph obtained in this way was actually an expected outcome. It was believed that the increase in cross-tensile strength was due to the changes that occured as a result of the effect of martensitic softening that occured after welding, critical sub-HAZ formation, upper critical HAZ, and also due to the lower heat input to the welding zone with the use of medium-frequency direct current technology. At the same time, the high cross-tensile strength values of the welded connections where the PWHT process was applied, as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e-b, were attributed to the fact that the force applied to the test samples was distributed over a wider area due to the expansion of the critical sub-HAZ zone (soft zone) observed in HAZ with the PWHT process. As a result, it was predicted that the tears spread over a wider area compared to normal welded connections, and thus, the cross-tensile load-bearing capacity increased. In addition, it was found that the cross-tensile strength values obtained from the welded samples were approximately three times lower than the tensile-shear strength values. It is known that the way the force was applied during testing was the reason for this situation in the joints made using the RSW method. Hernandez [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e], joined DP steels using the RSW method and stated that the cross-tensile values were lower than the tensile-shear test values. Chao [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e] applied tensile-shear and cross-tensile tests to welded connections by joining high-strength steels with the RSW method in his study. According to the results obtained, he stated that the cross-tensile strength was lower than the tensile-shear strength.\u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.5.1. Cross-Tension Failure Modes\u003c/h2\u003e \u003cp\u003eIn the automotive sector, the maximum load carrying capacity during collisions is determined using the cross-tension test, and the reliability of vehicles produced in this way is determined, and this method is frequently used. Therefore, the rupture surfaces of DNA samples obtained after the cross-tension test were examined macroscopically in detail, and images taken from the rupture regions of the samples after the test are given in Fig.\u0026nbsp;16, where a represents the RPH and b, c represent normal operations.\u003c/p\u003e \u003cp\u003eCross-tension tests were applied to all welded samples combined in two different parameters, and the rupture images of all cross-tension test samples (normal) made in a series are given in Fig.\u0026nbsp;16-a for an example. The rupture images obtained from the samples subjected to cross-tension tests under normal conditions are shown in Fig.\u0026nbsp;16-b, and the rupture images obtained from the samples subjected to RPH are shown in Fig.\u0026nbsp;16-c. Rupture damage modes are crucial in determining the toughness and load-carrying capacities of DNA connections. Looking at Fig.\u0026nbsp;16, it can be seen that all damage modes occured in the form of buttoning.\u003c/p\u003e \u003cp\u003eThe buttoning damage mode seen as a result of the cross-tension test indicates that it occurred around the weld or HAZ. It is believed that the formation of buttoning damage mode begins with cracking around the weld core due to the applied force, and it progresses as the force continues to load. Damage modes that occur in this way indicate that the welded joint will show the necessary strength when the structure is forced to separate under the static force application.\u003c/p\u003e \u003cp\u003eThe ruptures that occured as a result of the cross-tension test are seen to occur in a brittle and semi-brittle manner on the STRENX 700 CR side. When the structure morphology was examined, it was thought that factors such as martensite softening and upper critical HAZ triggered these types of ruptures. In this context, it was confirmed that the cross-tension results obtained were parallel to the tensile-shear results, and therefore, the optimum welding parameters determined by the tensile-shear test were also confirmed by the cross-tension test results. Tamizi et al. [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], stated that during the formation of cross-tension damage modes, the crack starts at the upper critical HAZ and then continues towards the lower critical HAZ.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.6. Fatigue Test Results\u003c/h2\u003e \u003cp\u003eThe effects of welding parameters on the fatigue life of DP and STRENX steels joined by RSW were investigated by subjecting them to fatigue tests. Fatigue tests were conducted at different load levels (0.2, 0.3, 0.5, 0.75 kN) and a constant frequency of 5 Hz to predict possible discontinuities (such as crack formation) that may occur as a result of continuous cyclic loading under future usage conditions. Force-life curves were created to understand the fatigue behavior of the materials using the data obtained from the fatigue tests.\u003c/p\u003e \u003cp\u003eThe result graph produced based on the numerical data obtained from the fatigue tests of welded connections (normal and RPH) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e17\u003c/span\u003e. When the force-cycle relationship of the RPH process was examined, it was determined that there were significant increases in fatigue cycle numbers due to load reductions.\u003c/p\u003e \u003cp\u003eWhen the welded joints were examined among themselves, it was observed that the average cycle numbers of the normal processed samples under 0.75 kN load were 4,850, while this rate increased by 109% to 10,179 cycle numbers in the fatigue strength of the welded joints with HSLA steel and DP steel joined by the RPH process. Similarly, while the average fatigue cycle numbers of normal processed joints under 0.2 kN load were 327,116, this rate increased by 189% to 947,632 cycle numbers in the welded joints joined by the RPH process.\u003c/p\u003e\u003cp\u003eAs a result, it was determined that there was a significant increase in the number of cycles due to the decrease in the applied force during the fatigue strength testing. The RPH process before welding was clearly effective in the formation of increased cycle numbers. Considering all these results, it is thought that the high fatigue strengths of welded connections with the RPH treatment compared to normal welded connections, which was previously observed during hardness measurements and explained in detail as the widening of the soft zone in HAZ, can be attributed to this soft zone. Ordonez et al. [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], joined DP980 dual-phase steel using RSW and applied fatigue testing to the resulting welded joints. They stated that a soft zone was formed due to the tempering of martensite as it could not reach the over-tempering temperature within the structure after welding, and that the soft zone and the decrease in hardness values due to softening could improve fatigue damage that could occur on HAZ. Banarjee et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e], examined the effects of weld size and geometry on the fatigue performance of DP590 steel welded joints using RSW. According to the results obtained from the fatigue tests, it was observed that the applied force and weld size had an effect on the fatigue strength, and insufficient fatigue strengths were obtained in cases of small weld sizes and high stress loads.\u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.6.1. Examination of Ruptured Surfaces after Fatigue Testing\u003c/h2\u003e \u003cp\u003eThe examination of rupture surfaces obtained after the fatigue strength tests revealed that ruptures occurred at the weld core and HAZ interface in all welded joints. It is known that in RSW studies in the automotive industry, fatigue strengths are important along with material performance, microstructure, and geometric properties [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. To analyze the detected ruptures after fatigue, one sample from each series was examined. It can be said that the crack initiation occurred around the weld core during fatigue testing, and due to the increase in dislocation density with the heat effect, the crack continued from the HAZ and then progressed along the material. Xu et al. [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e], investigated the fatigue performance of dual-phase steels using RSW and found that the fatigue crack initially started around the core.\u003c/p\u003e \u003cp\u003eThe rupture patterns observed in welded joints after fatigue testing appeared to be very similar and consistent. It is believed that all ruptures occured due to the softening of the weld nugget and the HAZ interface that occured after welding, followed by crack propagation. The softening caused by thermal transformations in the STRENX 700 CR region could be considered as a significant factor in crack initiation and rupture formation, as well as HAZ expansion and martensite softening.\u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e18\u003c/span\u003e and \u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e19\u003c/span\u003e show SEM images of ruptured surfaces of samples subjected to normal and RPH treatment, respectively.\u003c/p\u003e\u003cp\u003eUpon examination of the SEM images (Figs.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e19\u003c/span\u003e), it is believed that cracks formed during the fatigue test started from the tempered zone, i.e., STRENX 700 CR HAZ, and then continued along the cross-section, reaching the coarse-grained HAZ and surface coating, causing brittle ruptures. It is also highly likely that there are areas of ductile-brittle rupture formation. When a literature search was conducted, it was suggested that the decrease in fatigue strength after the RSW process could be due to the notch effect that occurred in the RSW and the microstructure that was created in the base metal, HAZ, and welding metal [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. Therefore, due to the heterogeneous microstructure present in the structure, the rupture images of the areas taken for SEM analysis may vary.\u003c/p\u003e \u003cp\u003eHolovenko et al. [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e], investigated the fatigue life of welded samples in their study and found that welds made by the RSW method showed a noticeable decrease in fatigue life compared to laser welding. The researchers explained that the cause of this decrease was the notch effect caused by the welding geometry, the different mechanical properties exhibited by the microstructures, and the presence of HAZ.\u003c/p\u003e \u003cp\u003eAccording to the SEM images, fatigue cracks are thought to occur as a result of the breaking of interatomic bonds in certain planes, and these crack progressions are called cleavage mechanisms. These progressions can be seen within grains or at grain boundaries. It is known that the yield strength of the ferrite phase in the structure is lower than that of the martensite phase. Therefore, while the martensite phase remains in the elastic state, the ferrite phase is subjected to plastic deformation. With this effect, the stress in the ferrite increases, and thus, regional deformations can be observed. As a result, ductile or cleavage-type brittle ruptures can be seen depending on different morphologies. In Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e18\u003c/span\u003e, the width of one measured crack was 380 \u0026micro;m, while the length of another crack was measured as 4,435 \u0026micro;m. In this context, it can be seen from SEM images that cleavage cracks occur in related fatigue ruptures, and it is also possible to observe cracks occurring at grain boundaries, as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e19\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, commonly used high-strength steels in the automotive sector, STRENX 700 CR and DP800, were joined by the RSW method. Only HAZ was subjected to a pre-heat treatment, and the results obtained are summarized below.\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eAccording to macro and microstructure examinations of all resistance spot welded samples, it was determined that the microstructures of the weld metal consisted of martensitic phase, but a small amount of regional bainitic structures also formed as a result of the applied pre-welding heat treatment process.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe chemical element contents and the inherent resistance differences of STRENX 700 CR steel and DP 800 steel was shown to have an effect on the weld nugget during the fusion-solidification process during welding.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eWhen the welded joints were examined, it was determined that the changes in HAZ were more pronounced on the STRENX 700 CR side compared to DP800, and HAZ expanded with the application of RPH.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAfter examining the hardness values of welded connections made on STRENX 700 CR by HAZ, it was observed that there was a reduction in hardness of approximately 7% in connections treated with RPH compared to normal welded connections. These results indicate that HAZ can transition to a more ductile structure, thereby reducing its brittleness.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAccording to the results of the hardness analysis, it was determined that the soft region, called critical zone 6 HAZ, did not undergo significant changes in hardness with the RPH treatment. However, the hardness of the upper critical HAZ in the range of Ac\u003csub\u003e1\u003c/sub\u003e and Ac\u003csub\u003e3\u003c/sub\u003e was found to decrease, contributing to the formation of a more homogeneous structure.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eWhen the tensile-shear strength results of the joined samples were analyzed, it was observed that a strength increase of approximately 6.7% was achieved when the RPH treatment was applied compared to normal welded connections. These results demonstrate that the RPH treatment is an effective method for increasing the strength of the connection.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eUpon examination of the cross-tensile strength of the welded connections, it was determined that the welded samples treated with RPH showed an increase of approximately 5.3% in cross-tensile strength compared to normal welded samples.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eLooking at the results of the tensile-shear and cross-tensile strength tests, it was determined that the application of the RPH treatment caused the HAZ to expand, resulting in a wider area of deformation after the application of force on the welded connection. As a result, an increase in both the tensile-shear and cross-tensile strength was observed.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eWhen the failure modes after the tensile-shear and cross-tensile tests were examined, it was determined that all welded connections failed in a button-pull mode, and the failures occurred between the weld nugget and STRENX 700 CR HAZ.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAfter the fatigue test, it was determined that all welded connections reached the critical cycle number at a load value of 0.2 kN. At this load value, the average fatigue strength cycles of normal welded connections were 327,116, while this number increased by 189% to 947,632 cycles in the RPH-treated samples.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe application of the RPH process resulted in a significant increase in the fatigue strength of all welded connections. The reason for this is that the pre-heating process applied only to the HAZ before welding slowed down the cooling rate around the weld core and its surroundings, thus expanding the soft zone already present in the HAZ. As a result, the repeated loads spread over a wider area instead of a sharp point, leading to a significant increase in the cycle numbers of the welded connections.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Karabük University within the scope of the FDK-2020-2132 project.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Kemal Aydın, Mehtap Hıdıroğlu and Nizamettin Kahraman. The first draft of the manuscript was written by Kemal Aydın and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMidawi ARH, Patel M, Shojaee M, et al (2023) Effect of Liquid Metal Embrittlement Indent Cracks on Zinc Coated 3rd Generation AHSS Mechanical Performance. Metals (Basel) 13:491. https://doi.org/10.3390/met13030491\u003c/li\u003e\n\u003cli\u003eRen S, Huang W, Ma N, et al (2023) Numerical modeling from process to residual stress induced in resistance spot welding of DP980 steel. International Journal of Advanced Manufacturing Technology 125:3563–3576. https://doi.org/10.1007/S00170-023-10845-Z/FIGURES/18\u003c/li\u003e\n\u003cli\u003eCh.Darabi A, Chamani HR, Kadkhodapour J, et al (2017) Micromechanical analysis of two heat-treated dual phase steels: DP800 and DP980. 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La Metallurgia Italiana 3:3–12\u003c/li\u003e\n \u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Resistance Spot Welding, MFDC, AHSS, STRENX 700 CR, DP 800, Dual-Phase Steel","lastPublishedDoi":"10.21203/rs.3.rs-2952877/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2952877/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSTRENX and DP steels are advanced high-strength dual-phase steel types used in the automotive industry for producing lightweight vehicles that have low cost, high fuel efficiency, and can carry more load volume. In this study, STRENX 700 CR and DP 800 steels were joined by resistance spot welding (RSW) using medium frequency direct current (MFDC). Some parts were joined under atmospheric conditions, while others were joined by applying regional pre-heating (RPH) to their heat-affected zones (HAZ) before welding. The design, manufacturing, and adaptation of the RPH system to the RSW machine were carried out as part of this study, considering it as a new system. After the joining process, non-destructive (visual inspection, magnetic particle) and destructive (microhardness, tensile-shear, cross-tension, fatigue) tests were performed on RSW connections between STRENX 700 CR and DP 800 steels. Hardness tests revealed that the RPH applied to the HAZ before welding reduced the hardness of the weld metal by approximately 8%, while the microstructure analysis showed that the RPH increased both the weld metal and the HAZ width. The tensile-shear strength increased by about 7%, and the cross-tension test strength increased by about 5% with the RPH applied to the HAZ before welding. In both the tensile-shear and cross-tension tests, all failures occurred in the form of button shearing from the HAZ. The RPH treated specimens showed the highest fatigue life, with an average of 947,632 cycles.\u003c/p\u003e","manuscriptTitle":"Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-05-30 18:40:53","doi":"10.21203/rs.3.rs-2952877/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1d28eda0-340b-4ec1-9056-862ad2813e1f","owner":[],"postedDate":"May 30th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2023-07-16T10:36:56+00:00","versionOfRecord":[],"versionCreatedAt":"2023-05-30 18:40:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2952877","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2952877","identity":"rs-2952877","version":["v1"]},"buildId":"-HB7Z8yhvgn0wM9Nzuekk","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}