Subcritical Heat-affected Zone Research Articles

Influence of microstructural morphologies on liquid metal embrittlement of Zn-coated dual-phase steels

Dual-phase (DP, ferrite+martensite) steels are crucial for automotive components, predominantly spot-welded using thin Zn-coated cold-rolled sheets. However, the effect of martensite morphologies, namely fine-lamellar (LD) or coarse-globular (GD) shapes, on Zn-assisted liquid metal embrittlement (LME) remains underexplored. This study investigates LME mechanisms during resistance spot welding of Zn-coated DP steel sheets with LD or GD microstructures. Analysis focuses on cracks within the weld shoulder zone, categorized into sub-critical HAZ (SCHAZ), inter-critical HAZ (ICHAZ), and upper-critical HAZ (UCHAZ). The LME susceptibility index, calculated as total crack length divided by total contact length, increased in the order of LD-SCHAZ, LD-ICHAZ, GD-ICHAZ, and GD-SCHAZ. These results indicated that the microstructure of the GD sheet itself was very vulnerable to the cracking because the initial microstructure was almost retained in the SCHAZ. Cracking analysis in the LD-SCHAZ and GD-SCHAZ suggested that martensite grains near ferrite/ferrite boundaries impeded crack growth by deflecting propagation, and that the LD sheet showed higher crack resistance due to unfavorably oriented ferrite/martensite boundaries.

Microstructure and deformation behavior of rapid cooling friction stir welded Q&P1180 steel joint

In this work, Q&P1180 steels were butt welded by friction stir welded (FSW) and rapid cooling FSW (RCFSW), aiming to investigate the influence of rapid cooling on the microstructure and deformation behavior of joints. The results showed that for FSW and RCFSW joints, stir zone (SZ) composed of martensite phase, and inter-critical heat affected zone (ICHAZ) composed of ferrite/martensite dual-phase microstructure, and subcritical heat affected zone (SCHAZ) consisted of ferrite, martensite, carbide and residual austenite phases. Compared with FSW joint, the grain of HAZ was refined, the contents of residual austenite and ferrite increased, while the content of cementite decreased in RCFSW joint. The tensile strength and elongation of FSW joint were 1292 MPa and 7.5 %, respectively. Compared to FSW joint, the tensile strength of RCFSW joint slightly decreased (1237 MPa), while the elongation greatly increased to 13.5 %. During tensile deformation, the strain was mainly concentrated in HAZ of FSW joint, while the strain transferred from HAZ to basal material (BM) of RCFSW joints, and finally fractured in BM.

Crystallographic characteristics and mechanical behavior of laser welded joints of hot roll bending parts

In this work, laser welding was performed on the hot roll bending parts obtained by local-induction-heating bending forming. The influence of the crystallographic characteristics of the phase transformation products in different regions of the laser welded joint on the mechanical properties was systematically studied, and the reasons for the performance degradation were discussed in detail. The results show that upper-critical HAZ (UCHAZ) forms coarse prior austenite grain (PAG) and bainite, resulting in the lowest dislocation density of 2.5 × 1014 m−2 and the lowest impact toughness value of 46 J/cm2. Sub-critical HAZ (SCHAZ) forms tempered martensite, causing the joint to soften and the tensile strength to decrease by 18%. The number of variants in SCHAZ is the lowest, where short laths with smaller misorientations merge into long laths, and precipitates nucleation and growth occurs. The uniform orientation of the variants in SCHAZ forms low-angle grain boundaries (LAGBs), which reduces the hindrance to crack propagation, so the mechanical properties of SCHAZ decrease. In addition, the texture analysis found that UCHAZ has the highest 〈001〉//RD orientation volume fraction, indicating that UCHAZ is most prone to rapid expansion of brittle cracks on the {001} cleavage plane, so the toughness of UCHAZ is reduced. SCHAZ has the highest {001}〈110〉 texture volume fraction and the lowest proportion of high-angle grain boundaries (HAGB), indicating that the deformed grain structure with the {001} cleavage planes distributed along the LAGB after welding heat tempering is the main reasons for the delamination of SCHAZ.

Prediction of Mechanical Properties in the Sub-Critical Heat Affected Zone of AHSS Spot Welds Using Gleeble Thermal Simulator and Hollomon-Jaffe Model

Measuring the mechanical properties of weld Heat Affected Zone (HAZ) remains one of the main challenges in the failure analysis of spot-welded components. Due to the small size of the HAZ and variation in the temperature history, different peak temperatures and cooling rates impose a range of phase transformations across the resistance spot weld. Among the HAZ sub-regions, the sub-critical HAZ (SCHAZ), which experiences temperatures below AC1 (350–650 °C), usually shows a reduction in the hardness in most of the modern AHSS grades due to the martensite tempering phenomenon. SCHAZ softening may lead to strain localization during loading. Therefore, it is important to characterize the local properties of the SCHAZ region to accurately predict RSW failure. However, it is not feasible to extract standard mechanical test specimens out of the SCHAZ of the spot-welded structure due to its small size. In this work, the SCHAZ of the spot weld for two AHSS, 3G-980 and PHS-1500, was simulated using a Gleeble® (Dynamic Systems Inc., 323 NY-355, Poestenkill, NY 12140, USA) 3500 thermo-mechanical simulator. An in-situ high-speed IR thermal camera was used to measure the entire temperature field during the Gleeble heat-treatment process, which allowed for the visualization of the temperature distribution in the gauge area. The temperature and hardness data were fit to a Hollomon-Jaffe (HJ) model, which enables hardness prediction in the SCHAZ at any given temperature and time. Using the HJ model, a heat treatment schedule for each material was chosen to produce samples with hardness and microstructure matching the SCHAZ within actual spot weld coupons. Tensile specimens were machined from the coupons heat treated using simulated heat treatment schedules, and mechanical testing was performed. The results showed that the 3G-980 SCHAZ has a slight increase in yield strength and tensile strength, compared to the base metal, due to the formation of fine carbides within the microstructure. In contrast, the SCHAZ of PHS-1500 showed a significant reduction in the yield and tensile strength with yield point elongation behavior due to the reduction of the martensite phase and an increase in carbide formation due to the tempering process.

Investigations of Microstructure and Mechanical Properties of Post-Weld Heat-Treated DP780 Steel TIG Welds

This study investigates the microstructure and mechanical properties of DP780 steel that has been tungsten inert gas welded and post weld heat treated. Microscopy studies revealed that the weldment’s microstructure varied from martensite in the fusion zone to a mixture of martensite and ferrite in the heat affected zone (HAZ). This heterogeneity in the microstructure resulted in the formation of hardened and softened zones in the cross section of the weldment. The DP780 as-welded joint exhibited lower strength and ductility [yield strength (YS): 492 ± 5 MPa, ultimate tensile strength (UTS): 668 ± 8 MPa, and percent elongation (%El): 8 ± 1] compared to the base metal (BM) (YS: 538 ± 2 MPa, UTS: 794 ± 5 MPa, and %El: 27 ± 2) due to strain localization in the subcritical HAZ. The weldments subjected to post weld heat treatment (PWHT) at 500°C exhibited lower strength and higher ductility (YS: 471 ± 3 MPa, UTS: 624 ± 5 MPa, and %El: 13 ± 1) than the weldments subjected to PWHT at other conditions: 300°C (YS: 501 ± 7MPa, UTS: 658 ± 6 MPa, and %El: 9 ± 1) and 400°C (YS: 492 ± 3 MPa, UTS: 649 ± 5 MPa, and %El: 11 ± 1). The decrease in strength and ductility after PWHT can be attributed to the tempering of martensite present in the weldment. Erichsen cupping tests indicated a reduction in the formability of the as-welded joint due to the presence of a softened zone. While a significant increase in formability is observed in the weldments subjected to PWHT with an increase in temperature, the formability is still inferior to that of the BM due to the inhomogeneity in the microstructures across the weldment.

Physical Simulation and Numerical Simulation of Flash Butt Welding for Innovative Dual Phase Steel DP590: A Comparative Study.

In this study, the microstructure and performance of newly designed dual-phase steel (DP590) after joining by flash butt welding (FBW) for vehicle wheel rims was analysed and compared by two simulations, i.e., physical simulation and numerical simulation, due to the high acceptance of these two methodologies. Physical simulation is regarded as a thermal-mechanical solution conducted by the Gleeble 3500 simulator and which can distribute the heat-affected zone (HAZ) of the obtained weld joint into four typical HAZs. These are coarse-grained HAZ, fine-grained HAZ, inter-critical HAZ and sub-critical HAZ. A combination of ferrite and tempered martensite leads to the softening behaviour at the sub-critical HAZ of DP590, which is verified to be the weakest area, and influences the final performance due to ~9% reduction of hardness and tensile strength. The numerical simulation, relying on finite element method (FEM) analysis, can distinguish the temperature distribution, which helps us to understand the relationship between the temperature distribution and real microstructure/performance. Based on this study, the combination of physical and numerical simulations can be used to optimise the flash butt welding parameters (flash and butt processes) from the points of temperature distribution (varied areas), microstructure and performance, which are guidelines for the investigation of flash butt welding for innovative materials.

In-situ observation of microcrack initiation and damage nucleation modes on the HAZ of laser-welded DP1180 joint

In-situ SEM can intuitively observe the changes in microstructure and microcrack initiation during the deformation process of the specimen. In this paper, the microstructural evolution, mechanical properties, and damage nucleation modes of DP1180 laser welded joints were systematically discussed by the transmission electron microscope (TEM), focused ion beam (FIB), and in-situ tensile test. The precipitation of carbides and disappearance of dislocations tangles deteriorated the mechanical properties of tempered martensite in the sub-critical heat affected zone (SCHAZ), which caused its microhardness slightly decreased (∼27 HV) and also called the softened zone. The joints fractured in the SCHAZ during the tensile and Erichsen cupping tests and microcracks initiated from the tempered martensite, while the microcrack initiation of base material (BM) occurred at the ferrite/martensite interface. The dominant damage nucleation mode of the joint was tempered martensite cracking, and that of BM was ferrite/martensite interface decohesion during tests.

Residual stress and deformation in UHS quenched steel butt-welded joint

For the low-alloy ultra-high strength (UHS) quenched steels with a yield strength of 1500 MPa, the welded joints have two obvious characteristics, namely solid-state phase transformation (SSPT) in the heat-affected zone (HAZ) and softening effect in the subcritical heat-affected zone (SCHAZ). In the current study, a UHS single-pass butt-welded joint with filler wire of ER307Si was fabricated, and thermal cycles at specific points were measured by thermocouples. After welding, relevant measurements of residual stress, deformation, microstructure and hardness were carried out. In this paper, based on the experimental hardness on the cross-section of the joint, a novel softening model was proposed. Meanwhile, based on coding subroutines of ABAQUS software, an advanced computational approach considering the thermal-metallurgical-mechanical coupling behaviour including the strain hardening as well as the annealing effect in fusion zone, SSPT in HAZ, and softening in SCHAZ was developed to implement thermal, metallurgical and mechanical analyses of the UHS quenched steel butt-welded joint. Comparing numerical and experimental results of temperature field, residual stress distribution and welding deformation of the joint, the calculated results of welding residual stress, angular distortion and transverse shrinkage are in good agreement with the measured data when both SSPT and softening effect were considered in the finite element model. The computed results indicate that SSPT has a significant influence on both the magnitude and the distribution of longitudinal residual stress, but it has a small effect on transverse residual stress. Moreover, the softening effect dramatically decreases the peak value of longitudinal residual stress in the SCHAZ, while it has an insignificant influence on transverse residual stress. The simulation results suggest that SSPT has a small effect on both transverse shrinkage and angular distortion, while the softening effect has very limited influence on welding deformation.

Microstructural Evolution of a Hot-Stamped Boron Steel Automotive Part and Its Influence on Corrosion Properties and Tempering Behavior

Boron-manganese steel 22MnB5 is extensively used in structural automotive components. Knowledge about its microstructural evolution during hot stamping and resistance spot welding (RSW) is extremely relevant to guarantee compliance with application requirements. Particularly, corrosion properties are critical to the application of uncoated sheet steels. However, microstructural studies are usually simplified to top-hat geometries, which might not be fully representative of the complex thermomechanical cycles locally faced by a real component. Therefore, the present work brings an extensive characterization of a hot-stamped 22MnB5 automotive B-pillar in terms of microstructure, hardness and corrosion resistance, which were correlated with a reverse engineering of the process using numerical simulation. Physical simulations of the subcritical heat affected zone (SCHAZ) of RSW were done to assess the influence of microstructure on martensite tempering. Results showed that the component undergoes a complex strain distribution along its body during hot stamping. Most heavily strained regions presented higher amounts of ferrite, leading to poorer corrosion resistance, since ferrite behaves as an anode. Physical simulations of the SCHAZ showed that the softening degree due to martensite tempering is solely affected by peak temperature, while other microstructural features appear to exert negligible or no influence.

Application of a high-speed infrared thermographic camera to the study of HAZ softening in S960 welded joints

Hot strip rolling of low-carbon advanced high-strength (AHS) sheet steels is difficult because their products require specific chemical compositions to achieve excellent mechanical properties. The formation of a heat-affected zone (HAZ) during welding is therefore a challenge in terms of maintaining high strength values. This fact complicates the formation of the softening area when the hardness in the HAZ rapidly decreases. According to the continuous cooling transformation diagram (CCT), the chemical composition of the base metal is directly related to the fine-grained martensitic microstructure arising from very high cooling rates and microalloying up to 0.2 wt% by Nb, V, and Ti. The gas metal arc welding (GMAW) was used to perform the study, what presents a challenge for thin sheets because of very narrow HAZ formation, suscepible to HAZ softening phenomenon. The width of the HAZ was about 6 mm and determined by applying an infrared thermographic camera to measure the real-time temperature development. The significant reduction in microhardness, known as the softening phenomenon, occurred in the intercritical heat-affected zone or at the boundary between intercritical and subcritical HAZ, according to real-time diagnostics of the temperature development and the microhardness results. The amount of heat produced during welding plays an important role in this context and is directly connected to the heat input of the welding process. The softening area can be reduced and its impact on the mechanical properties can be lowered by appropriately modifying the welding conditions, e.g., optimising the heat input. The purpose of this paper is to point out the change in temperature during the welding process.

Investigation of a novel TIG-spot welding vis-à-vis resistance spot welding of dual-phase 590 (DP 590) steel: Processing-microstructure-mechanical properties correlation

Resistance spot welding (RSW) is the most widely used joining technique for thin sheets in automotive industries. However, this process requires access from both sides of the sheet metals, which is unavailable in many intricate applications. To address this gap, a novel one-sided tungsten inert gas spot welding (TIG-spot welding) is investigated for the joining of 1 mm thick dual phase (DP 590) steel. To establish the processing window for TIG-spot welding, weld current and arc time were varied between 100 and 180 A and 0.2–1.2 s respectively, considering one variable at a time approach. For RSW, to obtain a comparable nugget diameter with TIG-spot welds, the welding current was varied between 5 and 6.8 kA at a constant welding time of 0.2 s and an electrode force of 2.5 kN. For the investigation of the mechanical properties of the joints, tensile-shear, Coach-Peel, and Cross-tension tests were conducted. It was revealed that joints fabricated using both RSW and TIG-spot welding having a nugget diameter greater than 4 mm possessed load-bearing capacity greater than the threshold requirement. Similar to the RSW process, with an increase in welding current, nugget diameter increases, resulting in a significant rise in the load-bearing capacity of the joints. Further, a desirable pull-out mode of failure with fully ductile fracture could be achieved with the TIG-spot welding technique. Microstructure-hardness correlation is established for both RSW and TIG-spot welded joints. Softening of the sub-critical heat-affected zone in the TIG-spot welding process was associated with extensive tempering of martensite, which was not the case in the RSW process. Microstructural differences resulting in a relatively lower hardness in different zones of welding explain the slightly lower load-bearing capacity for the TIG-spot welds compared to RSW. However, the joint strength is adequate as per the standard and hence can meet the application requirements.

A novel technique to measure the local mechanical properties of third generation advanced high strength steel resistance spot welds

Distinct microstructures and mechanical properties are produced in resistance spot welds (RSW) of advanced high-strength steels (AHSS). The local mechanical properties within the fusion zone (FZ) and heat-affected zones (HAZ) govern the global failure response of RSW automotive structural assemblies in crash events. In this study, a novel experimental procedure was developed to characterize the local properties of RSW sub-regions using sub-size tensile and simple shear specimens extracted from the spot weld, with local strain measurement utilizing digital image correlation (DIC) techniques. The FZ properties were obtained using a simple shear test, while the HAZ properties were characterized using a tensile test containing a central RSW. A range of AHSS grades were considered to evaluate the proposed methodology, including 3rd Gen-980, 3rd Gen-1180, and press-hardened steel (PHS1500) with respective minimum ultimate tensile strengths of 980 MPa, 1180 MPa, and 1500 MPa. Strain localization occurred wherever the hardness was lowest such as in the subcritical heat-affected zone, with limited deformation in the harder regions, such as the upper-critical HAZ and FZ. The local stress-strain responses were then compared to a hardness scaling approach to estimate the tensile properties of RSW based on the local hardness in each zone. The hardness scaling method only appears to be a viable method if the microstructures across the weld have similar microconstituents in all sub-zones across the weld.

A comprehensive evaluation of tempering kinetics on 3rd generation advanced high strength steels

Emerging third-generation advanced high strength steels (3G AHSS) grades are being increasingly considered for autobody applications owing to their complex microstructures providing an excellent combination of strength and ductility. When welded with RSW, 3G AHSS such as 3G 1180 and 3G 980, exhibits a solid-state transformation in the sub-critical-heat-affected-zone (SCHAZ) due to the heat generated during the RSW process. The resulting changes in microstructure can affect the mechanical properties of the material within and around the spot welds and thereby influence the performance of welded components in case of a vehicle crash event. To study metallurgical transformations in the SCHAZ during the RSW process, 3G 1180 and 3G 980, and a reference martensitic press hardened steel, PHS 1500, were selected. These materials were subjected to isothermal tempering experiments at temperatures ranging between 350 °C and 650 °C and different tempering times from 0.2 s to 24 h. It was shown that the Hollomon-Jaffe model accurately describes the tempering kinetics of 3G 1180 and PHS 1500 but does not predict the transformation processes in the SCHAZ of 3G 980. The PHS 1500 and 3G 1180 showed softening (tempering) for all combinations of investigated tempering times and temperatures due to the decomposition of martensite. In contrast, the 3G 980 showed a combination of softening by the formation of martensite and secondary ferrite combined with secondary hardening due to the formation of fine M2C typed plate-like carbides. Furthermore, during long tempering, the M2C particles dissolve into the matrix and were replaced by cementite particles in 3G 980.

Effect of Post-Weld Heat Treatment on Microstructure and Fracture Toughness of X80 Pipeline Steel Welded Joint.

In the current study, post-weld heat treatment (PWHT 580 °C) was used for an X80 pipeline steel-welded joint, and the fracture toughness of the welded joint was investigated using a crack tip opening displacement (CTOD) test. The relationship between microstructure evolution and fracture toughness is also discussed in this study. The results showed that the weld center mainly consisted of acicular ferrite (AF). The subcritical heat-affected zone (SCHAZ) consisted of a large amount of fine polygonal ferrite and some AF, and it maintained the rolling state of the base metal. The microstructure of the coarse-grained heat-affected zone (CGHAZ) was composed of granular bainite (GB) and M/A constituents, the latter of which decreased after the PWHT. The CTOD values of the weld center were in the range of 0.18–0.27 mm, while those of the CGHAZ were in the range of 0.02–0.65 mm. A brittle fracture occurred in the CGHAZ for both the as-welded and PWHT samples; the CTOD values were 0.042 mm and 0.026 mm, respectively. The CTOD values of the SCHAZ’s location were in the range of 0.8–0.9 mm. The PWHT did not deteriorate the microstructure of the CGHAZ and had little influence on the fracture toughness of the X80 pipeline steel-welded joint; it ensured the fracture toughness of the welded joints and reduced the welding residual stress.

Local microstructure and mechanical characteristics of HAZ and tensile behavior of laser welded QP980 joints

Microstructural evolution of the heat-affected zone (HAZ) seriously affects the mechanical properties of the welded joints. In this paper, QP980 steels were butt-welded by fiber laser. The thermocouple and Gleeble1500 system were used to achieve the thermal simulated HAZ specimens of laser welded joints, which solved the problem that it was difficult to evaluate the mechanical properties due to the narrow HAZ of the laser welding. Large amounts of granular carbides were precipitated in the block martensite of the sub-critical HAZ (SCHAZ). Carbides were not found in lath martensite, but there were dislocation tangles in the lath martensite of the SCHAZ. In addition, the number of dislocations in the ferrite of the SCHAZ was significantly reduced. The SCHAZ had the lowest strength but the highest elongation owing to the tempering. The fine-grain strengthening resulted in the highest tensile strength in the fine-grained HAZ (FGHAZ). The tensile fracture morphologies indicated a brittle fracture mechanism in coarse-grained HAZ (CGHAZ) and FGHAZ, and exhibited a ductile fracture mechanism in the inter-critical HAZ (ICHAZ) and SCHAZ. According to the overview of the joint and the local mechanical characteristics of HAZ, the numerical model of the tensile test was performed by ABAQUS. And the simulation results showed good agreement with the experimental results, which both illuminated the constraint effect of SCHAZ and made the strain concentration transferred from SCHAZ to the BM with the increase of stress.

On the Measurability and Predictability of HAZ Softening in GMAW of Automotive DP980 Steel

Dual Phase (DP) steel, composed of a ferrite matrix with dispersed islands of martensite, has become popular in auto-body car construction due to its outstanding mechanical properties (i.e., high strength and good ductility). DP steel softens at the sub-critical heat-affected zone (SC-HAZ) when subjected to welding thermal cycles, owing to the tempering of the martensite phase. In this work, DP980 steel was subjected to varied thermal cycles: (a) furnace-tempering treatment, (b) gas metal arc welding (GMAW), and (c) resistance spot welding (RSW), in order to characterize the tempering of martensite below the Ac1 critical temperature and at the sub-critical heat-affected zone (SC-HAZ) in the case of the welded specimens. The coarsening stage of cementite phase was characterized through microstructure observations and hardness measurements. As expected, the comparative results indicated an advanced stage of the martensite tempering in the furnace heat-treated specimens, followed by the GMAW and the RSW specimens. Further, developed softening kinetic models have been suitably employed and adjusted in order to predict the extent of softening along the SC-HAZ of the GMAW specimen. Finally, as the advanced stage of cementite coarsening is due to the influence of the arc welded thermal cycle, a reasonable estimation of the hardness profile was obtained, particularly for tempering temperatures above 400 °C.

Effect of the welding technique on mechanical properties and metallurgical characteristics of the naval grade high strength low alloy steel joints produced by SMAW and GMAW

Naval grade, high strength low alloy (HSLA) steel is designed especially for shipbuilding applications due to its high strength to weight ratio. The conventional fusion welding processes of shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) are employed widely for structural applications. This work reports the influence of welding processes on mechanical properties and metallurgical characteristics of fusion welded DMR249A grade steel. The evaluating tensile, impact, microhardness properties were correlated with microstructures. The maximum recorded transverse tensile strength value of SMAW and GMAW joints were 558 MPa and 578 MPa. The lower microhardness had obtained in the subcritical heat-affected zone than other regions. The weld zone microstructure consists of fine, elongated, and columnar grain structures. From this investigation, it has been concluded that GMAW joints displayed marginally better properties than SMAW joints. But most weld failures has been observed in the subcritical heat-affected region; due to grain deformation and subsequent softening in the weldment.

Suppression of softening in heat affected zone by Mo addition in friction stir welded martensitic steel

The drastic softening in the heat affected zone (HAZ) of friction stir welded 0.2C–2Si–3Mn martensitic steel occurred due to the martensite tempering in the subcritical temperature range just below A 1 temperature in HAZ. It was revealed that the softening in the subcritical HAZ was successfully suppressed by Mo addition. The optimum condition was found to be 3 wt-% Mo addition with the solution temperature of 1120°C, where densely distributed nanometer-sized Mo2C were confirmed by transmission electron microscopy and atom probe tomography in the subcritical HAZ. These fine Mo2C precipitates gave rise to an increase in the hardness of approximately 110 HV compared with a no-Mo-added sample, which was in good agreement with the predicted precipitation strengthening value based on the Ashby–Orowan model.

Microstructural characterization of the physical simulated and welded heat-affected zone of 9% Ni steel pipe

The microstructure, Vickers microhardness, and prior austenite grain size (PAGS) of the heat-affected zone (HAZ) in a circumferential welded joint of a 9% Ni steel pipe were compared with those of a physically simulated (Gleeble machine) HAZ to validate a developed methodology for evaluating the HAZ via finite element method simulation. The welding of the cap pass was computationally simulated; subsequently, the thermal cycles of the HAZ subzones were reproduced (thermomechanical simulator), and the HAZ (physically simulated and of the welded joint) microstructures were compared. The HAZs in these conditions showed similar microstructures, microhardnesses, and PAGSs, thereby validating the developed methodology. The subcritical (SCHAZ), intercritical (ICHAZ), fine-grain (FGHAZ), and coarse-grain HAZs (CGHAZ) exhibited base metal unaltered microstructure, dual-phase microstructures (martensite and ferrite), refined martensite, and coarse martensite with coalesced bainite, respectively. The physically simulated HAZ was investigated using X-ray diffraction spectroscopy and transmission electron microscopy, and it exhibited globular and thin-film austenite morphologies, with minimum and maximum contents in the CGHAZ and ICHAZ, respectively. The developed computational and physical simulation methodology can be reliably reproduced and can evaluate the HAZ microstructure of the 9% Ni steel welded joint.

Microstructure, Mechanical Properties, and Corrosion Behavior of Laser‐Welded Dissimilar Joints Between DP980 and QP980 Steel

Herein, fiber laser welding experiments are performed to evaluate the microstructure, mechanical properties, and corrosion behavior of dissimilar/similar joints between DP980 and QP980 steel. Results show that the fusion zone (FZ) of all joints consists of lath martensite with high microhardness (≈480 HV); the retained austenite content in the subcritical heat‐affected zone (HAZ) in the QP980 side of the dissimilar weld joint (WJ) is less than base metals (BMs). Lower hardness in the subcritical HAZ is found compared with BMs in all joints. The microstructure of softening zones is tempered martensite, and the reduction in hardness is about 8.5−10.8% of BMs, which has a great influence on the tensile fracture position of WJs. All joints fracture at softening zones with a typical feature of ductile fracture, and the joint efficiencies are above 96%. The electrochemical corrosion test reveals that the welded samples exhibit a higher corrosion rate compared with the BMs due to the increase in martensite content and area fraction of grain boundary.