Post-weld Heat Treatment Process Research Articles

Effect of post-weld heat treatment on microstructure and mechanical properties of twinning-induced plasticity (TWIP) steel joints

The effect of post-weld heat treatment on microstructure and mechanical properties of twinning-induced plasticity (TWIP) steel joints obtained by melting electrode inert gas-shielded welding (MIG welding) was systematic studied. A total of two post-weld heat treatment processes are proposed, i.e., post-weld annealing treatment (PWAT) with temperature/time of 1100 ℃/1h and subsequent furnace cooling and post-weld solution treatment (PWST) with temperature/time of 1100 ℃/1h and subsequent water quenching, which were compared with the case of post-weld non-treatment (PWNT). Following annealing treatment at 1100°C, the joint tensile strength dropped by 6.6% (from 845.46MPa to 789.85MPa), while its elongation at break increased by 40.87% (from 29.80% to 41.98%). The significant increase in plasticity was due to the significant elimination of weld stresses in the joints after annealing. Moreover, the stresses in the weld metal of the PWST joints were instead significantly elevated and the carbides were solidified, which resulted in the lower strength. When post-weld heat treatment was applied, the impact toughness of the joints was lower than when untreated. This was primarily due to the greater percentage of the low-angle grain boundaries and the precipitation of MC-type carbides. Consequently, post-weld annealing treatment was an effective method to further enhance the strong plasticity of new twinning-induced plastic steel joints.

EVALUATION OF WELDED JOINT (SMAW) AFTER POST WELD HEAT TREATMENT (PWHT)

Joining of two materials having similar physical, chemical, and thermal properties and to make the joint tough and stiff, special attention and investigation are required. In this research, the welding joint between two mild steel (MS) of IS 2062 grade E, was accomplished by using shielded metal arc welding (SMAW). To investigate the effect of electrode in the welding joint, MS electrode was exercised. For post-weld heat treatment (PWHT), temperature of 700 °C were investigated over the welded specimen. To explore the optimum welding process and PWHT process for this joint, both tensile and bending tests were carried out. Destructive tests have suggested that PWHT at 700 °C is better for the SMAW process with MS electrode. As the smallest hardness values were observed in the heat-affected zone (HAZ) which makes the sample susceptible to failure. The hardness values have decreased after PWHT which justified the achievement of higher ductility after heat treatment.

A comparison of microstructure and mechanical characteristics correlation of the joint specimens for duplex stainless steel UNS S32304 and super-duplex stainless steel UNS S32750: The role of post-weld heat treatment

The present investigation studies the impact of post-weld heat treatment (PWHT) on the microstructure and mechanical characteristics of joints made by gas tungsten arc welding between duplex stainless steel and super-duplex stainless steel specimens. The PWHT process was considered at 1100 °C for 10 minutes followed by water quenching on UNS S32304 and UNS S32750 materials. Scanning electron microscopy, optical microscopy, X-ray diffractometry (XRD), and energy dispersive X-ray analyses have been applied in microstructure evolution, including recognizing local chemical contents and elemental distribution. To analyze the mechanical characteristics of samples, tensile and hardness tests were performed. It was shown that after applying PWHT, the morphology of the austenite varied becoming mostly intergranular/spheroidal in shape, accompanied by an improvement in the percentages of austenite. This investigation indicates the PWHT results in restoring an equal percentage of both phases. The phase percentages, based on both ASTM E1245 and ASTM E562, show a good agreement between the austenite and ferrite phases of weld materials. According to XRD, the phases are mainly ferrite and austenite with different lattice parameters with no evidence of unwanted intermetallic phases. It was concluded that the PWHT process plays a crucial role in the ductility of joint specimens due to the phase balance between austenite and ferrite.

Stress relief simulation for post-weld heat treatment process of pressure equipment: Creep constitutive equation considering temperature and stress variations

Due to the limited testing methods and high costs, researches on the evolution law of residual stress in the post-weld heat treatment (PWHT) process of pressure equipment are mainly by numerical simulation, in which the selection of a reasonable and reliable creep constitutive model is crucial to ensure the accuracy of numerical predictions. It is worth noting that the temperature changes and residual stress variation during the heat treatment process result in the following creep characteristics: the creep occurs under different temperatures during the heat treatment processes of heating, holding, and cooling; the creep occurs under different residual stress level where the residual stress varies during the heat treatment stages. Thus, to accurately simulate the stress release behavior during the PWHT process, the proper creep constitutive model must involve the influences of the temperature and the stress state. Meanwhile, current heat treatment simulation usually adopts simplified qualitative approaches, and the creep constitutive models often ignore the physical mechanisms and characteristics of actual creep processes, such as the Norton, the Norton-Bailey, and the Omega models. As a result, the accuracy of heat treatment simulation is limited. In this paper, a new creep constitutive relation under varying temperature and stress is proposed. The hyperbolic-sinusoidal function is used to represent the complete three stages of creep, which compensate the inaccuracy prediction at the first and the third creep stages. Moreover, the Arrhenius function is introduced to characterize the creep response under varying temperature conditions, which enables the prediction of residual stress evolution throughout the entire heating-holding-cooling process of heat treatment. Then, a typical pressure vessel cylindrical circumferential weld is simulated. The stress release during the heat treatment is calculated, and the efficacy of the proposed model is validated through surface residual stress testing. It is found that the creep behavior is a crucial factor in numerical simulation of heat treatment, and the creep constitutive relation can significantly affect the accuracy of heat treatment simulation. The simulation utilizing the hyperbolic-sinusoidal temperature-dependent creep model is much more accurate compared to the classical simulation. The simulation results are much more consistent to the experimental results. Specifically, the traditional model often underestimates the stress state, the residual stress drops abruptly during the heating stage. The residual stress evolution mechanism is simulated using the new model: The majority of residual stress are released during the heating process which primarily relies on the creep strain transformation (contributes up to 85%). The stress state does not change during the holding stage and slight higher at the cooling stage.

Influence of post-weld heat treatment on welding residual stress in U-rib-to-deck joint

Welding residual stress (WRS) significantly influences the fatigue life and safety of welded steel structures. Post-weld heat treatment (PWHT) is a common method employed to mitigate WRS. In present study, the WRS evolution law and relaxation mechanism of U-rib-to-deck joints was investigated during PWHT. High-temperature heat treatment tests with varying holding times were conducted on U-rib-to-deck joints to reduce WRS. The WRS distributions were measured using the hole-drilling method before and after PWHT. ABAQUS software, incorporating the Norton-Bailey creep constitutive model, was employed to simulate the WRS evaluation during both welding and PWHT processes. The simulated values were validated against the measured data, and the effects of different holding times, holding temperatures, and heating and cooling rates during PWHT on WRS relaxation were discussed. The results demonstrate that PWHT effectively reduces WRS and leads to a more uniform distribution. Extending the PWHT holding time from 0.5 h to 1 h and 2 h results in a gradual decrease in WRS relaxation at 600 °C, primarily occurring in the high-value stress region of the weld zone with a small amplitude. Elevating the PWHT temperature from 500 °C to 550 °C and subsequently to 600 °C amplifies the significance of WRS relaxation. Variations in heating and cooling rates (200 °C/h and 80 °C/h, 100 °C/h and 40 °C/h, 50 °C/h and 20 °C/h) lead to a gradual decrease in WRS relaxation across the entire model. Furthermore, considering the creep effect, the stress relaxation rate in the weld is approximately 70 %–80 %, which found to be about 2–3 times higher than that without considering the creep effect. By understanding the evolution law and relaxation mechanism, this study provides valuable insights for designing PWHT processes and assessing their impact on structural safety and fatigue life.

Enhancement of the Microstructure and Mechanical Properties of 1020 Carbon Steel and AISI 304 Stainless Steel Dissimilar Weld Using Different Post-Weld Heat Treatments

In order to improve the microstructure and mechanical properties of gas metal arc dissimilar weldment of AISI 304 and 1020 carbon steel, different post-weld heat treatment (PWHT) processes including annealing, tempering and normalizing were performed. The post-tempered weldment exhibited improved grain refinement over the as-welded. The as-welded joint is characterized with the formation of hard martensitic phase and CrC precipitates while the post-weld heat treated (PWHTed) joints consist more of softer ferritic phase. The PWHTs resulted in the weldment hardness reduction with post-annealed demonstrating the least hardness. Only the post-tempered weldment demonstrated improved tensile strength (~5.2%) over the as-welded (421 MPa). All the PWHT processes resulted in improved elongation (i.e., ductility) and impact energies over the as-welded. While the entire PWHTed weldments demonstrated ductile fracture mode, the as-welded sample exhibited a combination of ductile and brittle fracture mode after the tensile test.

Microstructure Evolution at Ni/Fe Interface in Dissimilar Metal Weld between Ferritic Steel and Austenitic Stainless Steel.

The formation and evolution of microstructures at the Ni/Fe interface in dissimilar metal weld (DMW) between ferritic steel and austenitic stainless steel were investigated. Layered martensitic structures were noted at the nickel-based weld metal/12Cr2MoWVTiB steel interface after welding and post-weld heat treatment (PWHT). The formation of the interfacial martensite layer during welding was clarified and its evolution during PWHT was discussed by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), focused ion beam (FIB), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), transmission kikuchi diffraction (TKD), phase diagrams, and theoretical analysis. In as-welded DMW, the Ni/Fe interface structures consisted of the BCC quenched martensite layer and the FCC partially mixed zone (PMZ), which was the result of inhomogeneous solid phase transformation due to the chemical composition gradient. During the PWHT process, the BCC interfacial microstructure further evolved to a double-layered structure of tempered martensite and quenched martensite newly formed by local re-austenitization and austenite-martensite transformation. These types of martensitic structures induced inhomogeneous hardness distribution near the Ni/Fe interface, aggravating the mismatch of interfacial mechanical properties, which was a potential factor contributing to the degradation and failure of DMW.

Enhancement of mechanical properties of GTAW joints for ZC63 magnesium alloy by post-weld heat treatment

To further improve the microstructure and mechanical properties of gas tungsten arc welded (GTAW) welded joints for ZC63 magnesium alloy, post-weld heat treatment is carried out. It is found that the majority of the MgZnCu phase in the fusion zone (FZ) is dissolved in the α-Mg matrix under suitable heat treatment conditions. The remainder is diffusely distributed as rods or granules at the grain boundaries. The excessive heat treatment temperature (460 °C) leads to abnormal grain growth (AGG) in the FZ. The substructure gradient between the abnormally grown grains and the surrounding small grains provides the driving force for AGG. Meanwhile, the dissolution of the MgZnCu phase weakens the hindering effect of the second phase on grain boundary migration, setting the stage for AGG. In addition, the detrimental impact of the continuous MgZnCu phase on the mechanical properties of the welded joint is also lessened by its dissolution. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the welded joints are 255 MPa,119 MPa and 27.0%, respectively, under the post-weld heat treatment process of 440 °C × 12 h. The welding coefficient of the welded joint reaches 97.0%, satisfying the service criteria set forth by the mechanical properties of the welded joints.

Effect of post-weld heat treatment on the microstructure and properties of titanium/304 stainless steel laser welding dissimilar metal joints with copper as the transition layer

Low elongation has been an important factor limiting the application of titanium/304 stainless steel (304SS) laser welding of dissimilar metal joint products. By adopting the post-weld heat treatment (PWHT) process of holding the welded joint at 400 ℃, 550 ℃ and 700 ℃ for 30 min, this study increases the elongation of the welded joint from 0.3 % to 2.21 % in the welded state and further analyses and investigates the structural changes in the organization and the formation process of intermetallic compounds (IMC) within the weld seam. The results showed that the acicular structure in the weld could not be eliminated by holding at 400 ℃ for 30 min, but only part of the residual stress was released, and the elongation was increased to 0.85 %. After holding at 550 ℃ for 30 min, a large number of equiaxed crystals appeared to replace the acicular structure in the weld and released the residual stress. With the enhancement of the IMC layer thickness, grain gap and deformation coordination ability, the elongation increased to 2.21 %. After holding at 700 ℃ for half an hour, the IMC layer becomes thinner, the grain gap decreases, the deformation coordination ability deteriorates, and the tensile strength and elongation decrease. In the tensile test, the fracture position was all at the weld. After being held at 400 ℃ for 30 min, the average tensile strength of the tensile sample increased from 330 MPa in the welded state to 352 MPa; after being held at 550 ℃ for 30 min, the average tensile strength of the tensile sample was 340 MPa, and the fracture elongation was 7.4 times that of the welded state, reaching 331 µm.

Analysis of residual stress relief for Ti62A alloy welded joints by post weld heat treatment considering creep effect

In this paper, the Arrhenius-type constitutive equation based on strain compensation was developed, and the constants of the constitutive equation were determined by linear fitting to construct the material models of Ti62A alloy. Meanwhile, the creep parameters of Ti62A alloy at high temperature were corrected by existing experiments, and the Norton creep model during post weld heat treatment (PWHT) was established. The sequential coupling finite element method was used to calculate the residual stress distribution in the welding and PWHT processes of Ti62A alloy plates. The effects of PWHT parameters on the residual stress relief of welded joints were revealed, such as creep, heating rate, and heat treatment temperature. The results reveal that the creep has an important influence on residual stress relief during PWHT. Compared with longitudinal residual stress, the effects of heating rate and heat treatment temperature on transverse residual stress are more significant. A lower heating rate or higher heat treatment temperature will be beneficial for releasing residual stress. The results of this paper will contribute to a better understanding of PWHT mechanisms and provide a good theoretical guidance for optimizing process parameters.

Behavioural Investigation of Hardness Profile for Dissimilar TIG-Weldment between P22 and P91 Steels

The Gas Tungsten Arc Welding (GTAW) is now commonly being used to join condenser pipes of railway locomotives, beside boiler and super heater tubes, generally made from dissimilar steel-alloys like P22 (2.25Cr-1Mo) and P91 (9Cr-1Mo). After welding, excessive hardness values of the Heat Affected Zones (HAZs) of those dissimilar weld joints are certainly obtained due to uneven and uncontrolled cooling rates, which resulted in an eruption of abrupt martensitic transformations. This ultimately leads to cracking and failure while operating at elevated temperatures. Therefore, there is a dire need to establish an appropriately optimized post-weld heat treatment process to eliminate this hazardous nuisance. The present work statistically studies the hardness profile of these critical material zones, before and after GTAW.

Effect of post-weld heat treatment on the metallurgical studies of dissimilar weldments of SS 316L welded with DSS 2205

The present work discusses the macrostructure, microstructure, and microhardness of dissimilar metal weld joints between AISI 316L SS-DSS 2205 developed by gas tungsten arc welding (GTAW) welding. Post-weld heat treatments (PWHT) were carried out at temperatures of 750°C for 24 h and 850°C for 0.5 h. The macrostructure and microstructure of the welding joints at different welding conditions were observed by scanning electron microscope. The macrostructure study results indicated that all the welded joints were fully penetrated and free from defects. However, the microstructure investigation revealed a solid-state transformation of ferrite into grain boundary austenite, widmanstatten austenite, intergranular austenite, and partially transformed austenite. The post-weld heat treatment processes enhanced the precipitation of the sigma phase in the ferrite matrix. Secondly, the microhardness across and along the weld bead has been evaluated, and the results are compared. It was observed that the microhardness of the heat-affected zone of DSS 2205 is found to be higher as compared to a heat-affected zone of AISI 316L, the weld metal zone, and base metals at all welding currents (i.e. 90 A, 109 A, and 132 A). The microhardness values decrease when transverse from the face of the weld toward the root pass. Microhardness values at the fill (top) pass of the weld zone at 90 A, 109 A, and 132 A were 268.3 HV, 257 HV, and 255 HV respectively. It was found that the microhardness has improved significantly after post-weld heat treatments and was highest for 750°C/24 h. It may be due to higher holding time which led to the involvement of carbides.

Mechanical Properties Enhancement of Dissimilar AA6061-T6 and AA7075-T651 Friction Stir Welds Coupled with Deep Rolling Process.

The main purpose of this research was to enhance the mechanical properties of friction stir welds (FSW) in the dissimilar aluminum alloys 6061-T6 and 7075-T651. The welded workpiece has tensile residual stress due to the influence of the thermal conductivity of dissimilar materials, resulting in crack initiation and less fatigue strength. The experiment started from the FSW process using the 2k full factorial with the response surface methodology (RSM) and central composite design (CCD) to investigate three factors. The experiment found that the optimal rotation speed and feed rate values were 979 and 65 mm/min, respectively. Then, the post-weld heat treatment process (PWHT) was applied. Following this, the 2k full factorial was used to investigate four factors involved in the deep rolling process (DR). The experiment found that the optimal deep rolling pressure and deep rolling offset values were 300 bar and 0.2 mm, respectively. Moreover, mechanical property testing was performed with a sequence of four design types of workpieces: FSW, FSW-PWHT, FSW-DR, and FSW-PWHT-DR. It was found that the FSW-PWHT-DR workpiece had an increase in tensile strength of up to 26.29% and increase in fatigue life of up to 129.47% when compared with the FSW workpieces, as well as a maximum compressive residual stress of −414 MPa.

Enhancement of mechanical properties through modified post-weld heat treatment processes of T91 and Super304H dissimilar welded joint

The work is focused on examining the influence of heat treatment modification during post-weld heat treatment on microstructure and mechanical properties of dissimilar welds of T91 and Super304H. Tungsten inert gas (TIG) welding was employed with ERNiCr-3 as filler to prepare the dissimilar metal welds. Following that, two post-weld heat treatments (PWHTs) were performed, viz. post-weld normalizing and tempering (PWNT) and post-weld direct tempering (PWDT). The latter was performed at 760 °C for 1 h, while the former was carried at 1050 °C for 30 min (normalizing) and then tempered at 760 °C for 1 h. The microstructural examination using scanning electron microscopy and electron-backscattered diffraction suggested a uniform microstructure after PWNT compared to PWDT. For instance, the grain size range in heat-affected zone for T91 side and Super304H side was 5–80 μm and 10–140 μm, respectively, after PWDT, while it was respectively reduced to 5–35 μm and 10–60 μm after PWNT. Further, the PWDT heat treatment resulted in coarse Cr-carbides at grain boundaries (~ 671 nm), indicating sensitization on the Super304H side, whereas the PWNT heat treatment ensured small Cr-carbides (~ 504 nm), indicating that the sensitization phenomenon was delayed. Further, on the T91 side, the vulnerability of fine-grained heat-affected zone for type IV cracking has been identified and eliminated through PWNT treatment. The improved microstructure of PWNT against PWDT heat treatment correlated well with the enhanced mechanical properties, where 0.2% proof stress, tensile strength, % elongation and toughness for PWDT were 324 MPa, 649 MPa, 20%, and 109 MJ/m3, respectively, while after PWNT, these were improved to 384 MPa, 691 MPa, 22%, and 132 MJ/m3, respectively. For both treatments, failure was observed in the weld metal region due to coarse Nb/Ti intermetallic carbides.

Influence of post weld heat treatment on the tensile and microhardness characteristics of friction stir welded joints of AA6082/ZrO2/B4C composites

In this recent work, the influence of post weld heat treatment (PWHT) on the tensile strength and microhardness characteristics of friction stir (FS) welded joints of AA6082/ 3 wt% ZrO2/ 5 wt% B4C composites is investigated. PWHT is performed on FS welded AA6082/ 3 wt% ZrO2/ 5 wt% B4C composites in order to improve the microhardness (WNH) and ultimate tensile strength (UTS). The heat treatment employed is the solution treatment followed by ageing. PWHT process is performed by solutionizing FS welded AA6082/ 3 wt% ZrO2/ 5 wt% B4C composites at 450 °C for a soaking period of 1 h followed by water quenching and ageing at 120 °C for a soaking period of 24 h. PWHT is carried out on the FS welded composites at a welding condition during which a medium amount of heat input is considered and its effect on microstructure, UTS and microhardness of FS welded joints are analyzed. The experimental results confirm that the PWHT ominously influences the UTS and microhardness characteristics of the FSW joints. It is found that microhardness obtained after PWHT for the medium heat input of 57.11 kJ/mm is 112.6 HV and this is about 20% increase in the microhardness as compared to corresponding FS welded AA6082/ 3 wt% ZrO2/ 5 wt% B4C sample before PWHT. It is also found that UTS obtained after PWHT is 253 MPa and there is an increase of 0.8 % UTS as compared to corresponding FS welded AA6082/ 3 wt% ZrO2/ 5 wt% B4C sample before PWHT.

Optimization of tensile strength for heat treated micro-alloyed steel weldment

Welding and post weld heat treatment process variables play important roles in determining the quality of welds. Thus, to produce welded joints of desired microstructure with the requisite mechanical properties for optimal performance in service, the need for the analysis of the process variables requiring optimization to determine their influence on the response (output) is very vital. In this study, the optimal design approach was used to optimize the main influencing process variables of welding current, tempering temperature and holding time to determine their impact on the observable response (tensile strength). Through multiple regression analysis of the experimental data, the predicted response was expressed by the second-order polynomial equation in terms of coded values. The derived model was statistically analyzed to determine its significance and predictive capacity. The ANOVA results of the quadratic regression model used to fit the response showed that it is significant (P < 0.00001). The high coefficient of regression and adjusted regression coefficient values of 94.73% and 89.99% respectively, is an expression of the model degree of fitness. The model F-value of 19.99 greater than the P-value also attest to the model significant. Validation of the optimized predicted results revealed that they are in close consonant with the experimental results. This investigation has shown that the optimum values of welding current, tempering temperature and holding time for achieving optimal tensile strength of 258.141KN/m2 in the heat treated micro-alloyed steel weldment were 90.530 amp, 250.002 °C and 60.000 min respectively.

Small Diameter Tube Post-Weld Heat Treatment Process with Brinell Hardness and Microstructure Inspection

The local post-weld heat treatment (PWHT) process test of 9Cr-3W-3Co small-diameter pipe welded joints was carried out using rope-type resistance heaters with different parameters, and the Brinell hardness test and microstructure observation were carried out on the welded joints after heat treatment. The results show that when the heating width was 200mm, the constant temperature was 790°C-800°C, and the constant temperature time was 2 hours, the Brinell hardness of the weld was in the range of 246HBW-265HBW, which had good performance. After tempering at 760°C-800°C, the welds all showed a clear tempered lath martensite.

Holding Time pada Sifat Fisik Pengelasan SMAW Baja ASTM-A36 melalui Uji Penetran

In the industrialized world, low-carbon steel was often used as a construction material. One of the problems obtained in the low carbon steel welding process was its nature susceptible to shakes. The minimize the formation of residual voltage in the connection area (Heat Affected Zone) can be done by the Post Weld Heat Treatment (PWHT) process The purpose of this study was to find out the effect caused by variations in heat treatment on heating steel ASTM A36 (Holding Time) to penetrant tests and determine the suitable temperature given to ASTM A36 steel after the welding process. The PWHT process was carried out with several variations of Holding Time, namely 1 hour, 2 hours, and 3 hours at 400 °C. Based on penetrant testing showed that the best value in Holding Time was 3 hours with no appearance of porosity defects

Research Situation of Welding Performance for P92 Heat Resistant Steel

In this paper, the welding performance of P92 ferritic heat resistant steel in recent years was introduced. P92 steel was a ferritic/martensitic steel, which had been brought to commercial maturity during the last decade due to its excellent high temperature performances. However, the welding related problems of P92 steel was gradually exposed with the wide application, causing a serious impact on the production and operation of boiler unit. Therefore, Strengthening research and development of suitable welding process and post weld heat treatment process that can lead P92 heat-resistant steel to being better serve in the power industry for our country will be the focus of metal workers in the next step.

Phase transformation and the mechanical characteristics of heat-affected zones in austenitic Fe–Mn–Al–Cr–C lightweight steel during post-weld heat treatment

We identified the influence of post-weld heat treatment (PWHT) on the heat-affected zone (HAZ) in the Fe–29.7Mn–10.5Al–3.06Cr–0.93C (wt%) lightweight alloy based on transitions in its microstructure and mechanical properties. The investigation was performed for samples under various PWHT conditions including as-welded state. Gas tungsten arc welding (GTAW) and Gleeble simulator were used for analyzing HAZ characteristics, and the PWHT processes were carried out with a commercial furnace. For investigating detailed mechanical and microstructural characteristics, micro hardness, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were employed. According to the experimental results, it was revealed that the addition of Cr in the austenitic lightweight steel had various effects on HAZ characteristics. In the as-welded state, Cr stabilized the HAZ mechanical characteristics by suppressing the driving force of κ-carbide precipitation during the welding thermal cycle. However, in the PWHT process at 600 °C, Cr caused the phase transformation mechanism of M7C3 precipitation and allotriomorphic ferrite formation at grain boundaries both in the HAZ and the base steel. In addition, the analysis of fusion zone showed that the increased grain boundaries could accelerate the driving force of phase transformation mechanism and promote the harmful β‑manganese formation. Our results concluded that the addition of Cr might cause harmful effects in the HAZ of austenitic lightweight steels under the isothermal heat treatment such as PWHT.