Effects of post-weld heat treatment on the HAZ of VLBW-fabricated Ti6321 joints: elemental redistribution, microstructural evolution, and mechanical property

The present study centres on the effect of post-weld heat treatment (PWHT) on microstructure and mechanical properties of the deep penetration keyhole Tungsten Inert Gas (K-TIG) welded dissimilar joint between creep strength enhanced ferritic (CSEF) steel and austenitic stainless steel (ASS). The as-received normalized and tempered CSEF steel was joined with ASS in a single pass without using any filler materials and edge preparation. Detailed characterization across the welded joint was conducted using stereomicroscope, electron microscopy, energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), hardness test, tensile test and Charpy impact test. Results showed that PWHT had significant effect on the microstructure and mechanical properties of both the weld metal and CSEF steel heat-affected zone (HAZ), while it had little influence on the ASS side. By using proper PWHT, the hardness gradient across the welded joint could be mitigated and toughness in both the weld metal and the CSEF steel HAZ could be restored. 760 °C was considered the most appropriate PWHT temperature for such dissimilar joint in terms of the overall mechanical properties. The tensile properties of K-TIG welded joint after PWHT were comparable to both friction stir welded joint and laser and/or electron beam welded joint, indicating that deep penetration TIG welding technology may provide a good alternative for the nuclear industry. The correlation among welding thermal cycle, various heat treatment, microstructure evolution and mechanical properties was also analysed in detail.

Effect of post-weld heat treatment on microstructure evolution and pitting corrosion behavior of UNS S31803 duplex stainless steel welds

A research on the microstructure and mechanical properties of the dissimilar joint was carried out so as to understand the effects of post-weld heat treatment (PWHT) on microstructural evolution, microhardness, tensile and flexural properties of the dissimilar friction stir welded (FSWed) joint. The results showed that there is a sufficient and rather complicated material mixing in the nugget zone (NZ). After PWHT, the grains in the joint area except for the NZ demonstrated a grain refinement with more homogeneous and equiaxed grains. Fine and clustered SiC particles in the NZ are confirmed by scanning electron microscope (SEM). In addition, the microhardness values of nugget zones both in as-welded and PWHT condition exhibited a higher Vickers compared to that of AA6061-O. Furthermore, a mean tensile strength of 183.30 MPa that demonstrates a 52.22% increase in tensile strength is also observed in the transverse tensile tests subsequent to PWHT. Considering the three-point bending test results, it is explicit that an increase of 121.96% in the bending extension is obtained as there is no significant increase in maximum bending force after the PWHT.

Effect of post-weld heat treatment on microstructure evolution and pitting corrosion resistance of electron beam-welded duplex stainless steel

Microstructure and corrosion behavior of A390-10 wt% SiC composite-AA2024-T6 aluminum alloy dissimilar joint: Effect of post-weld heat treatment

As a solid-state process, friction stir welding (FSW) can avoid the formation of solidification cracking and porosity associated with fusion welding processes and significantly improve the weld properties of aluminum alloys [1]. However, many studies on the mechanical properties of FSW joints of heat-treatable aluminum alloys such as 2017-T351 [2], 2024-T6 [3], 2195-T8 [4], 2519-T87 [5], 6061-T6 [6, 7], 6063-T5 [8] and 7075-T651 [9] have indicated that FSW gives rise to softening of the joints and results in the significant degradation of the mechanical properties. In order to restore the mechanical properties of the FSW joints, a post-weld ageing heat treatment to 6082 aluminum alloy joints has been performed [10]. When the base material was in T6 condition, the microhardness in the weld zone increased, but the one in the heat-affected zone decreased; while the base material is in T4 condition, the micro-hardness in the entire joints increased. Another post-weld solution and ageing heat treatment to 6061-O aluminum alloy joints indicated that the micro-hardness values across the joint significantly increased, while the toughness of the joints deteriorated [11]. These results imply that the effects of the post-weld heat treatment (PWHT) on the mechanical properties of the joints are related not only to the base material conditions, but also to the PWHT processes. The present letter demonstrates the effect of the post-weld solution-ageing heat treatment on the mechanical properties of 2219O aluminum alloy FSW joints. The emphasis is placed on the tensile properties and fracture locations of the joints. The base material used in this study was a 5-mm-thick 2219-O aluminum alloy plate, with the chemical compositions and mechanical properties listed in Table I. The welding samples, 260 mm by 50 mm, were longitudinally butt-welded using an FSW machine. The welding tool size and welding parameters are listed in Table II. After welding, the samples were cut into two parts, one part for the PWHT and the other part for the as-welded examinations. The PWHT process includes solid solution at 535 ◦C for 32 min, quenching in water at 25 ◦C, and ageing at 165 ◦C ∗Author to whom all correspondence should be addressed. for 18 hr. After the heat treatment, all the joints, including as-welded joints, were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests. The cross-sections of the metallographic specimens were polished with a diamond paste, etched with Tucker’s reagent and observed by optical microscopy. Vickers hardness distributions in the joints were measured along the centerlines of the cross sections under a load of 4.9 N for 10 s, and the distance between the adjacent measured points was 1 mm. The room-temperature tensile test was carried out at a crosshead speed of 1 mm/min using an Instron-1186 testing machine. The marked length of each specimen was 50 mm, and the tensile properties of each joint were evaluated using three tensile specimens cut from the same joint. Fig. 1 shows the tensile properties of the joints welded at different welding speeds before and after the heat treatment. The tensile properties of the as-welded joints almost do not change with the welding speed (see Fig. 1a). The tensile strength is equivalent to that of the base material, and the elongation is changed between 10 and 12%. However, the tensile properties of the heat-treated joints increase with increasing welding speed (see Fig. 1b). The tensile strength is up to 385 MPa, equivalent to 2.4 times that of the base material, and the maximum elongation is 3.2%. These results indicate that the heat-treated joints possess a higher tensile strength and a lower elongation than the as-welded joints. Fig. 2 shows the fracture locations of the joints before and after the heat treatment. As seen from this figure, the as-welded joints fracture in the base material zone (BMZ) (see Fig. 2a), while the heat-treated joints fracture in the weld zone (WZ) (see Fig. 2b). This implies that the PWHT process has a significant effect on the fracture locations of the joints. That is to say, the BMZ is a weak part of the joints before the heat treatment, while the WZ becomes the weak part of the joints after the heat treatment is performed. In most cases, the tensile properties and fracture locations of the joints are related to the hardness

Multipass welding procedures are common methods for 13Cr4Ni steels' fabrication and repairs. Compared to a single-pass weld procedure, the weld microstructure in a multipass weld is more heterogeneous due to the complex local thermal cycles imposed by adjacent weld passes. Furthermore, the final microstructure and mechanical properties of these steels are very sensitive to their thermal history which increases the microstructure heterogeneities. Thus, post-weld heat treatments are performed to reduce heterogeneities and produce a relatively homogenous weld. It has been found that the best option to improve mechanical properties of 13Cr4Ni steels is forming a “room temperature stable austenite” phase by heat treatments. This study focuses on the effects of these post-weld heat treatments on the austenite phase and carbide formations and the related evolutions of microhardness distribution. The study shows that nanometer-size carbides form at martensite lath interfaces and sub-block boundaries, and then at higher temperatures austenite lamellae appear at these locations. Results also show that the highest percentage of stable austenite achievable by a single-stage tempering was obtained at 610 °C. When the heat treatment temperature is lower than 610 °C, longer holding time produces softer steel while longer heat treatments at temperatures higher than 610 °C, produces harder steel. Still, double-stage heat treatments are more effective and produce the highest percentage of austenite and the lowest hardness of all heat treatments.

Heat treatment can sensibly change the properties and microstructure of the material. Nevertheless, few studies have addressed the effect of postweld heat treatment (PWHT) on the mechanical properties and microstructure change of the friction stir welded joints of AA7075-T6. The purpose of this study is to investigate the effect of several post-weld heat treatments. Using different solution heat treatment, followed by quenching in ambient water and different ageing treatment for a different ageing time to improve the mechanical properties of 3 mm aluminium alloy 7075-T6 plate welded by FSW processes. Hardness profile is obtained across the weld using destructive test 'Vickers's hardness test' for both as-welded and post-weld heat treatment. Microstructure is characterised by the optical microscope to compare both treated and non-treated welded plate. The experimental results prove that friction stir welding process resulted in a loss of hardness and change in microstructure on the joined zone, dependent mostly on welding parameters including: tool shape, rotation speed, velocity or travel speed and tool tilt. The post-weld heat treatments lead to a significant change of microstructure in the joint area and provide a remedy for the loss of hardness of the welded bead.

The martensitic creep-resistant steel designated as ASTM A335 for plate and as P91 for pipe is primarily used for high-temperature and high-pressure applications in steam power plants due to its excellent high-temperature properties such as high creep strength, high thermal conductivity, low thermal expansion, and so on. However, in the case of welded joints of such steels, the presence of an inter-critical heat-affected zone (IC-HAZ) can cause the joint to have lower creep strength than the base metal. In the present study, the effect of post-welding heat treatment (PWHT) and weld groove designs on the overall microstructure and mechanical properties of P91 steel pipe welds produced by the gas tungsten arc welding process was studied. Various regions of welded joints were characterized in detail for hardness and metallographic and tensile properties. Sub-size tensile samples were also tested to evaluate the mechanical properties of the weld metal and heat-affected zone (HAZ) with respect to PWHT. After PWHT, a homogenous microstructure was observed in the HAZ and tensile test fracture samples revealed shifting of the fracture location from the IC-HAZ to the fine-grained heat-affected zone. Before PWHT, the conventional V-grooved welded joints exhibited higher tensile strength compared to the narrow-grooved joints. However, after PWHT, both narrow- and V-grooved joints exhibited similar strength. Fractography of the samples indicates the presence of carbide precipitates such as Cr23C6, VC, and NbC on the fracture surface.

Effect of post-weld heat treatment on the mechanical behavior and dislocation density of friction stir welded Al6061

The spray forming 7055 aluminum alloy is welded by electron beam welding (EBW). After welding, the single solution treatments under different temperatures and then aging treatment (STA) and the stepped solution treatment and then aging treatment (SSTA) are carried out for the welded joints respectively. The effect of different post-weld heat treatment (PWHT) procedures on the microstructure and mechanical properties of welded joints is systematically investigated. Results show that, in as-welded condition, the fusion zone mainly consists of α-Al phase, and there are some precipitated phases such as Mg32(Al,Zn)49, Al7Cu2Fe, Al2CuMg, and a small amount of phase MgZn2 in weld metal. After PWHT, the continuous network structures at grain boundary disappear. There are only a few isolated particle phases at grain boundary, and many ellipsoidal η′ phases and rod-like η phases are precipitated within grains. The mechanical property of welded joint is improved after PWHT. Compared with that of in STA condition, the strengthening effect to welded joint is more obvious after SSTA. In SSTA condition, the hardness of weld zone is close to that of base metal (BM), and the tensile strength of welded joint reaches 486.2 MPa, which is 85.6% of that of BM. There are many equiaxed dimples on the tensile fracture surface, and the joint mainly presents the characteristic of ductile fracture.

The effect of different post-weld heat treatment (PWHT) on microstructure, low temperature impact toughness fracture morphology and hardness, of 309L/E5015 fusion zone of bimetal lined pipe weld was investigated by using optical microscopy (OM), electron probe microscope analysis (EPMA), scanning electron microscopy (SEM) and hardness test. The results showed that the microstructure of the fusion zone of the 309L/E5015 under non thermal treatment was martensite, and the lath was fine acicular. At 550 °C, the lath of martensite was bulky, and the feature was obvious; at 650 °C, the microstructure was tempered martensite, and the microstructure was obviously refined; at 750 °C, the microstructure was further refined and became multilateral; at 850 °C, the martensite microstructure was gradually analyzed, and the carbides were dispersed. With the increase of heat treatment temperature after welding, the element distribution gradient of fusion zone slowed down and the hardness first decreased and then increased. At 750 °C, the weld had the best toughness, because the lath of martensite was multilateral, and the hardness reached the qualified range, and the low temperature impact energy was maximum than other temperatures.

In this study, ultra-high-strength steels, namely, cold-hardened austenitic stainless steel AISI 301 and martensitic abrasion-resistant steel AR600, as base metals (BMs) were butt-welded using a disk laser to evaluate the microstructure, mechanical properties, and effect of post-weld heat treatment (PWHT) at 250 °C of the dissimilar joints. The welding processes were conducted at different energy inputs (EIs; 50–320 J/mm). The microstructural evolution of the fusion zones (FZ) in the welded joints was examined using electron backscattering diffraction (EBSD) and laser scanning confocal microscopy. The hardness profiles across the weldments and tensile properties of the as-welded joints and the corresponding PWHT joints were measured using a microhardness tester and universal material testing equipment. The EBSD results showed that the microstructures of the welded joints were relatively similar since the microstructure of the FZ was composed of a lath martensite matrix with a small fraction of austenite. The welded structure exhibited significantly higher microhardness at the lower EIs of 50 and 100 J/mm (640 HV). However, tempered martensite was promoted at the high EI of 320 J/mm, significantly reducing the hardness of the FZ to 520 HV. The mechanical tensile properties were considerably affected by the EI of the as-welded joints. Moreover, the PWHT enhanced the tensile properties by increasing the deformation capacity due to promoting the tempered martensite in the FZ.

In this study, the effects of post-weld heat treatment (PWHT) on residual stress distribution and fatigue crack propagation (FCP) behavior in linear friction welded (LFW) Ti-6Al-4V joints were investigated. Microstructural evolution in the weld center zone (WCZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM) was characterized using scanning electron microscropy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties were evaluated via Vickers hardness testing and digital image correlation (DIC)-based tensile testing. Residual stresses before and after PWHT were measured using the contour method. The LFW process introduced significant residual stresses, with tensile stresses up to 709.2 MPa in the WCZ, resulting in non-uniform fatigue crack growth behavior. PWHT at 650 °C and 750 °C effectively reduced these stresses. After PWHT, fatigue cracks propagated uniformly across the weld region, enabling reliable determination of crack growth rates. The average crack growth rates of the heat-treated specimens were comparable to those of the base metal, confirming that PWHT, particularly at 750 °C, stabilizes the fatigue crack path and relieves internal stress.

The effect of various post-weld heat treatment (PWHT) cycles on the as welded heat affected zone (HAZ) microstructures of C–Mn steels microalloyed with niobium, or niobium plus vanadium, has been studied. Single pass welds were produced at an arc energy of 3·5 kJ mm−1; examination was carried out using optical and electron microscopy, along with hardness and crack tip opening displacement testing. As welded, the C–Mn–Nb HAZ contained a significant proportion of auto tempered martensite. After PWHT at 550°C, isolated hard regions remained, but at 600°C all hard regions had been removed, with a concomitant increase in cleavage resistance. In contrast, ferrite with aligned second phase with lower hardness was found mainly in the as welded HAZ of the C–Mn–Nb–V steel. When the PWHT temperature was raised, HAZ hardness increased to a maximum at 600°C; overaging would be required to obtain improved toughness, although this would soften the parent plate. The results indicate that the current practice of specifying a common heat treatment procedure for steels to a given specification is not satisfactory; allowance should be made for the particular composition and as welded HAZ microstructure.MST/1190