Coarse-grained Heat-affected Zone Research Articles

Synergistic effect of residual stress and phosphorus grain-boundary segregation on embrittlement of coarse-grained heat-affected zone in a novel Ni-Cr-Mo reactor pressure vessel steel

SA508Gr.4N steel is a novel Ni-Cr-Mo low-alloy steel for the new-generation reactor pressure vessel. Herein, a coarse-grained heat-affected zone (CGHAZ) in a P-doped SA508Gr.4N steel is simulated using Gleeble thermal cycling simulation with a peak temperature of 1320 ℃ under a heat input of 100 kJ cm−1. For the as-simulated specimen, due to the synergistic effect of high residual stress and P grain-boundary segregation, the fracture appearance changes in the order of ductile fracture to intergranular fracture, and then to cleavage fracture with decreasing impact temperature. With the residual stress relieved by annealing at 620 ℃, the specimen toughens to a certain extent even after ageing at 560 ℃, causing a decrease in ductile-to-brittle transition temperature (DBTT), although the boundary cohesion declines to a certain degree owing to the increase of P segregation. Simultaneously, the intergranular-to-cleavage fracture transition temperature (ICFTT) shifts to a lower temperature below –190 ℃, enabling the specimen to exhibit intergranular fracture even at −190 ℃. With the ageing temperature lowered to 500 ℃, the boundary segregation of P is raised to a high level, leading to a large increase in DBTT and a further decrease in ICFTT. Accordingly, the synergistic effect of residual stress and P boundary segregation affects the fracture nature of CGHAZ, thereby making fracture appearance have different characteristics. A mechanism diagram for the above effect is proposed based on the experimental results.

Effect of Ferritic Morphology on Yield Strength of CGHAZ in a Low Carbon Mo-V-N-Ti-B Steel

For investigating the impact of ferritic morphology on yield strength (YS) of the high-heat-input welding induced coarse-grained heat-affected zone (CGHAZ) of a low carbon Mo-V-N-Ti-B steel, a group of particular welding heat inputs were designed to obtain different ferritic microstructures in CGHAZ. The tensile properties were estimated from typical samples with ferritic microstructures. The mixed microstructures dominated by the intragranular polygonal ferrite (IGPF), the intragranular acicular ferrite (IGAF), and the granular bainite (GB) were obtained at the heat inputs of 35, 65, 85 and 120 kJ/cm, respectively. When the main microstructure changed from IGPF to IGAF and GB, YS increased first and then decreased. The microstructure consisting mainly of IGAF possessed the maximum YS. As the main microstructure changed from IGPF to IGAF and GB, the contribution of grain refinement strengthening to YS was estimated to be elevated remarkably. This means the strength of CGHAZ in a low-carbon steel subjected to the high-heat-input welding could be enhanced by promoting the fine-grained AF and GB formation.

Effect of Cooling Rate on The Microstructure And Cryogenic Impact Toughness of HAZ in 9% Ni Steel

The effects of cooling rate on the microstructure and cryogenic impact toughness of coarse-grained heat-affected zone (CGHAZ) and inter-critically reheated coarse-grained HAZ (IC CGHAZ) in 9% Ni steel were investigated. CGHAZ and IC CGHAZ specimens were prepared from 9% Ni steel by controlling the cooling rate of the simulated welding process. The microstructure of the CGHAZ specimens consisted of autotempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the autotempered martensite and the effective grain size decreased. A large amount of fine carbides was distributed inside the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. The microstructure of the IC CGHAZ specimens consisted of tempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the tempered martensite and effective grain size decreased. Finer carbides were distributed inside the tempered martensite than in the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. Cryogenic fracture revealed that ductile fracture occurred in the auto-tempered martensite and tempered martensite, and brittle fracture occurred in the lath martensite. The crack propagation path was zig-zag in the high angle grain boundaries of the lath martensite. The volume fraction of auto-tempered martensite and tempered martensite and the effective grain size in the HAZ specimens had a significant effect on cryogenic impact toughness. In the IC CGHAZ specimens, cryogenic impact toughness decreased and then became constant as the cooling rate increased, due to a decrease in the volume fraction of the tempered martensite and effective grain size.

Comparison of Impact Toughness in Simulated Coarse-Grained Heat-Affected Zone of Al-Deoxidized and Ti-Deoxidized Offshore Steels

The presence of acicular ferrite (AF) in the heat-affected zone (HAZ) of steels used offshore is generally seen as beneficial for toughness. In this study, the effects of varying fractions of AF (0–49 vol.%) were assessed in the simulated, unaltered and coarse-grained heat-affected zones (CGHAZ) of three experimental steels. Two steels were deoxidized using Ti and one using Al. The characterization was carried out by using electron microscopy, energy-dispersive X-ray spectrometry, electron backscatter diffraction and X-ray diffraction. The fraction of AF varied with the heat input and cooling time applied in the Gleeble thermomechanical simulator. AF was present in one of the Ti-deoxidized steels with all the applied cooling times, and its fraction increased with increasing cooling time. However, in other materials, only a small fraction (13–22%) of AF was present and only when the longest cooling time was applied. The impact toughness of the simulated specimens was evaluated using instrumented Charpy V-notch testing. Contrary to the assumption, the highest impact toughness was obtained in the conventional Al-deoxidized steel with little or no AF in the microstructure, while the variants with the highest fraction of AF had the lowest impact toughness. It was concluded that the coarser microstructural and inclusion features of the steels with AF and also the fraction of AF may not have been great enough to improve the CGHAZ toughness of the steels investigated.

Role of heat inputs on microstructure and mechanical properties in coarse-grained heat-affected zone of bainitic steel

This study aims to find out the effect of welding heat inputs on microstructure and mechanical properties relationship of naval high strength steel. The physical weld thermal cycles with four different heat inputs between 10 to 200 kJ/cm were experimented using the thermomechanical simulator Gleeble®3800. The weld coarse-grained heat-affected zone (CGHAZ) microstructure was characterized in detail by using optical microscopy, field emission scanning electron microscope (FESEM), and transmission scanning electron microscope(TEM) facilities and mechanical properties such as impact toughness, hardness and tensile strength were obtained using Charpy impact, microhardness, and tensile testing facilities. It was often noticed that the impact toughness decreased with increasing heat inputs attributed to the grain and bainite lath coarsening and the formation of new phases with different morphologies in CGHAZ regions. At lower heat input, the microstructure consisted of a fully martensite structure, while at higher heat inputs consisted of granular bainite and bainitic ferrite; however, in medium-range heat inputs, it consisted of bainite and martensite. The different phases were confirmed through TEM images. The mechanical properties were found to be decreased with an increase in heat input. It was due to the formation of coarse bainite lath, granular bainite, and hard M/A constituents. The weld structure properties were established in detail.

Effect of microstructure synergism on cryogenic toughness for CGHAZ of low-carbon martensitic steel containing nickel

Varying chemical composition are used to investigate the influence of martensite-austenite (M-A) constituents on the cryogenic toughness of coarse grain heat affected zone (CGHAZ) for low-temperature steel containing nickel. The tensile tests, Charpy impact tests and crack tip open distance (CTOD) tests are carried out at −196 °C using the thermal simulated samples to evaluate the cryogenic properties of CGHAZ. The microstructure feature is studied with Optical microscope (OM), Scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) detector, Electron backscatter diffraction (EBSD), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). Results show that the dislocation density and grain boundary density in martensite increase significantly by slightly increasing the carbon content. There is no obvious change on the content of M-A components caused by decreasing Ni content. With the refinement of martensite substructure, the occurrence probability of locally densely-distributed M-A constituents increase. In the case of small strain, the strengthened martensite reduces the stress concentration on M-A constituents. The synergism of martensite matrix and M-A constituents is helpful to reduce microcracks on their interfaces. With the strain increase, the microstructure synergism effect decreases and microcracks initialize. In the area of densely-distributed of M-A constituents, the coalescence of microcracks speed up the crack propagation rapidly. The resulting brittle instability accelerates the failure and reduces the cryogenic fracture toughness.

Effect of the interpass temperature on simulated heat-affected zone of gas metal arc welded API 5L X70 pipe joint

Welding costs associated with the laying of pipes for deepwater oil and gas extraction can be reduced using high interpass temperatures (ITs). However, a high IT can decrease the mechanical properties of the heat-affected zone (HAZ) of welded joints. With the use of high strength-toughness steels, this decrease may be an acceptable trade-off. Therefore, it is necessary to evaluate the influence of high ITs on the HAZ. The influence of the IT on the coarse-grain HAZ (CGHAZ) and intercritically reheated coarse-grain HAZ (ICCGHAZ) of an API 5L X70 pipe joint welded by gas metal arc welding was investigated. The welding was numerically simulated using finite element method software. The microstructure of the HAZ was predicted using thermodynamic simulation software. The CGHAZ and ICCGHAZ were also physically simulated and evaluated via optical microscopy and scanning electron microscopy, dilatometry, and Vickers microhardness and Charpy V-notch (CVN) impact tests. The increase in IT led to a decrease in CGHAZ microhardness but did not affect the ICCGHAZ. The CVN energies obtained for all ITs (CGHAZ and ICCGHAZ) were higher than that set by the DNVGL-ST-F101 standard (50 J). These results show that increasing the IT is an interesting and effective method to reduce welding costs. In addition, thermodynamic simulation proved to be a valid method for predicting the phases in the HAZ of API 5L X70 pipe welded joints.

Relationship of the Microstructure and Toughness of the Coarse Grain Heat-Affected Zone of TiNbV Microalloyed Steels Based on Electron Backscatter Diffraction Analysis

The microstructure evolution and impact toughness of the coarse grain heat-affected zone (CGHAZ) of TiNbV microalloyed steels were investigated by using a thermal simulation test. The samples were treated with various simulated welding thermal cycles. The phase constituents and grain sizes were analyzed by using electron backscatter diffraction analysis. The microstructure of the CGHAZ of the treated samples consisted of ferrite, acicular ferrite, pearlite, and bainite. The samples have a higher impact toughness under a lower welding heat input. This is because the microstructure of the CGHAZ is dominated by the higher volume fraction of the high-angle grain boundaries of acicular ferrites. The presence of bainite and coarsening grains are two key factors deteriorating the toughness of the CGHAZ of TiNbV microalloyed steels. The volume fraction of bainite sharply increased as the welding heat input increased, leading to a decrease in the impact toughness of the CGHAZ. For a higher welding heat input, both the severe coarsening of the grain size and a higher bainite content would result in poor impact toughness.

Study on Microstructure and Mechanical Properties of Laser Welded Dissimilar Joint of P91 Steel and INCOLOY 800HT Nickel Alloy.

This investigation attempts to explore the weld characteristics of a laser welded dissimilar joint of ferritic/martensitic 9Cr-1Mo-V-Nb (P91) steel and Incoloy 800HT austenitic nickel alloy. This dissimilar joint is essential in power generating nuclear and thermal plants operating at 600–650 °C. In such critical operating conditions, it is essential for a dissimilar joint to preserve its characteristics and be free from any kind of defect. The difference between the physical properties of P91 and Incoloy 800HT makes their weldability challenging. Thus, the need for detailed characterization of this dissimilar weld arises. The present work intends to explore the usage of an unconventional welding process (i.e., laser beam welding) and its effect on the joint’s characteristics. The single-pass laser welding technique was employed to obtain maximum penetration through the keyhole mode. The welded joint morphology and mechanical properties were studied in as-welded (AW) and post-weld heat treatment (PWHT) conditions. The macro-optical examination shows the complete penetrations with no inclusion and porosities in the weld. The microstructural study was done in order to observe the precipitation and segregation of elements in dendritic and interface regions. Solidification cracks were observed in the weld fusion zone, confirming the susceptibility of Incoloy 800HT to such cracks due to a mismatch between the melting point and thermal conductivity of the base metals. Failure from base metal was observed in tensile test results of standard AW specimen with a yield stress of 265 MPa, and after PWHT, the value increased to 297 MPa. The peak hardness of 391 HV was observed in the P91 coarse grain heat-affected zone (CGHAZ), and PWHT confirmed the reduction in hardness. The impact toughness results that were obtained were inadequate, as the maximum value of impact toughness was obtained for AW P91 heat-affected zone (HAZ) 108 J and the minimum for PWHT Incoloy 800HT HAZ 45 J. Thus, difficulty in obtaining a dissimilar joint with Incoloy 800HT using the laser beam welding technique was observed due to its susceptibility to solidification cracking.

Studying the influence of the interpass temperature on the heat-affected zone of an API 5L X65 steel welded pipe joint through computational and physical simulations

Welding costs can be reduced by increasing the interpass temperature (IT). However, this can adversely impact the mechanical properties of the heat-affected zone (HAZ) of the welded joint; therefore, it is necessary to evaluate the influence of higher ITs on the HAZ. The influence of the IT on coarse-grained HAZ (CGHAZ) and intercritically reheated coarse-grained HAZ (ICCGHAZ) of an API 5 L X65 steel pipe joint welded with gas metal arc was investigated. A microstructure-predicting method (MPM) was built from numerical and thermodynamic simulations to examine the CGHAZ and ICCGHAZ. The results were validated using optical and scanning electron microscopy as well as dilatometry. Vickers microhardness and Charpy V-notch (CVN) impact tests were performed on physically simulated specimens. For all ITs considered herein, granular and lath bainite were observed in the CGHAZ; and bainite, ferrite, and Martensite–Austenite–Carbides (M-A-C) were observed in the ICCGHAZ. Increasing the IT did not significantly change the hardness of the CGHAZ and ICCGHAZ, but it decreased the CVN impact energy of these zones owing to presence of granular bainite and massive M–A. Contrary to that of the ICCGHAZ, the impact energy of the CGHAZ was greater than the DNVGL-ST-F101 standard specified limit (45 J). Based on the MPM and experimental results, it can be concluded that increasing the IT leads to embrittlement of the HAZ.

Study on the mechanism of heat input on the grain boundary distribution and impact toughness in CGHAZ of X100 pipeline steel from the aspect of variant

The variant selection and pairing directly affect the grain boundary distribution in the coarse-grained heat-affected zone (CGHAZ) of pipeline steel and consequently impact toughness. In this study, we attempted to establish a direct correlation between heat input, variant, grain boundary distribution, and impact toughness in CGHAZ of pipeline steel by means of electron backscatter diffraction (EBSD) analysis. The results showed that the variant selection is strong at 4 kJ/cm, such that V1/V1 becomes the dominant variant pair in CGHAZ. As the heat input increases, the transformation strain decreases, such that the mean variant pair in CGHAZ is V1/V2 and V1/V4/V8 at heat inputs of 9 and 21 kJ/cm, respectively. Compared with V1/V1 and V1/V4/V8 variant pairs at 4 and 21 kJ/cm, respectively, V1/V2 variant pair promotes the formation of high-angle grain boundaries (HAGB) and decreases the residual stress. When the heat input is 9 kJ/cm, the microstructure of CGHAZ has maximum resistance to crack propagation and lower crack nucleation ability, leading to the highest impact energy.

Experimental investigation on microstructure, mechanical properties, and residual stresses of dissimilar welded joint of martensitic P92 and AISI 304L austenitic stainless steel

In this study, microstructural characteristics and mechanical properties of martensitic steel P92 and AISI 304L austenitic stainless steel (ASS) dissimilar metal weld (DMW) have been examined. The welding was performed using the multipass gas tungsten arc welding (GTAW) process. The ERNiCrMo-3 (Inconel 625) filler metal was used due to its excellent compatibility with the P92 and AISI 304L ASS metals. The microstructure characterization performed with an optical microscope and scanning electron microscope revealed the presence of an unmixed zone in the form of beach, peninsula, and island at the base metal and weld fusion zone interface. Secondary phases enriched with Nb and Mo, and migrated grain boundaries (MGBs) were also observed at the weld fusion zone. The microstructural characterization at the P92 side revealed the presence of a soft ferrite zone and delta ferrite at the weld and P92 interface. The different peak temperature experienced by the P92 metal during the welding process results into the formation of three distinct zones with diverse mechanical and microstructural properties namely coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), and inter-critical heat affected zone (ICHAZ). The post weld heat treatment (PWHT) at 760 °C for 2 h followed by air cooling was performed to homogenize this heterogeneous microstructure formed toward the P92 side. It was observed that this PWHT does not have any significant effect on weld fusion zone and 304L SS heat affected zone (HAZ) microstructure. But, PWHT modifies the microstructure of P92 side HAZ. The tensile test, Vickers micro-hardness test, and charpy impact test were carried out to determine the mechanical properties of the DMW. The tensile strength was found as 587.709 MPa in the as-welded condition and 594.515 MPa in PWHT condition. The tensile test results indicated that specimens were failed from 304L stainless steel (SS) base metal (BM) in as-welded and PWHT condition. The charpy impact toughness test results showed that the toughness of the weld joint was very low in as-welded and PWHT condition (47J) compared to the P92 (190J) and SS 304L (285J) BM. From the microhardness examination, it was observed that the ICHAZ was the weakest zone among all the other regions due to its lower micro-hardness value compared to the other regions. After welding process, residual stresses were measured in as-welded and after PWHT condition at the center of the weld and at the HAZ of P92 and SS 304L (through the thickness of the plate) using experimental-based deep hole-drilling (DHD) strain-gauge method. The results showed that both circumferential (hoop stress) and axial (longitudinal stress) welding residual stresses were decreased after PWHT.

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.

Microstructure-toughness relationship in the simulated CGHAZ of V-N microalloyed X80 pipeline steel

The microstructure evolution, hardness and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of a V-N microalloyed X80 pipeline steel were studied. The results indicated that the microstructure mainly consisted of bainitic ferrite, acicular ferrite and granular bainite. The area fraction and size of M/A constituents were increased with increasing heat input. As heat input increased from 15 to 60 kJ cm−1, the hardness gradually decreased from 301 to 243 HV, and impact energy decreased from 284 to 33 J. Hardness in the CGHAZ was not sensitive to heat input. When the heat input exceeded 30 kJ cm−1, the formation of coarse granular bainite and large-sized blocky M/A constituents resulted in the deterioration of impact toughness.

Precipitation behavior of carbides and cryogenic toughness in heat-affected zone of high-Mn twinning-induced plasticity steel welded joint

The newly designed cryogenic high-Mn steel plates (Fe-23Mn-1.5Al-0.45C) were joined by metal inert gas (MIG) multi-pass welding process. To evaluate the inhomogeneity of microstructure and cryogenic toughness in heat-affected zone (HAZ) of welded joint, the impact samples were prepared by zonal sampling method. The microstructure features and embrittlement mechanism in HAZ of welded joint were systematically investigated. The results reveal that grains obviously grew in the coarse-grain HAZ (CGHAZ). C element segregated and nano-sized lamellar and granular (Cr, Mn) 7 C 3 carbides precipitated at the grain boundary in HAZ, and the size of precipitate carbides in CGHAZ and fine-grain HAZ (FGHAZ) was about 100 nm and 40 nm, respectively. The interface between (Cr, Mn) 7 C 3 and γ-austenite matrix in CGHAZ was incoherent with the lattice misfit of 45.13%. Higher fraction of low angle grain boundaries (LAGBs) appeared in CGHAZ (25.89%) and FGHAZ (11.99%), compared with the base metal (4.33%). Such microstructural differences can degrade the impact toughness in CGHAZ. From cryogenic Charpy impact tests, the impact toughness in different zones of the joint at each test temperature follows this sequence: base metal > fine-grain HAZ > coarse-grain HAZ > fusion line. These results can provide a fundamental understanding and guidance on the brittle failure of cryogenic high-Mn steel welded joint during service.

Effect of step cooling process on microstructures and mechanical properties in thermal simulated CGHAZ of an ultra-high strength steel

Ultra-high strength steel (UHSS) has been widely applied in a variety of industries to satisfy lightweight requirements. For now, it is still a challenge to realize both high strength and excellent toughness in welded joints of UHSS, especially in the coarse grain heat affected zone (CGHAZ). In this research, an innovative step cooling process was proposed to control microstructures and mechanical properties in the thermal simulated CGHAZ of UHSS via thermal simulation test. The results show that the proposed step cooling process can realize the collaborative improvement of strength and toughness. With the cooling rate of 150 °C/s, the yield strength, the ultimate strength and the toughness are comparable to those of base metal (BM), even the total elongation exceeds that of BM. The combination of high strength and toughness of CGHAZ is primarily attributed to the synergetic effect from martensitic transformation, microstructure refinement and transformation-induced plasticity effect induced by the retained austenite.

N-induced microstructure refinement and toughness improvement in the coarse grain heat-affected zone of a low carbon Mo–V–Ti–B steel subjected to a high heat input welding thermal cycle

In the coarse grain heat-affected zone (CGHAZ) of a low carbon Mo–V–Ti–B steel subjected to a simulative welding thermal cycle with a heat input as high as 75 kJ/cm, the influence of an increase in nitrogen content from 0.0085 to 0.0144 wt% on the microstructures and impact properties of the steel was comparatively estimated using Gleeble-3800 simulation, microstructure characterization and impact testing. With a rise in the nitrogen content, the number of fine and coarse particles also increased. An increase in the number of fine precipitates can effectively pin the prior austenite grain (PAG) boundary and inhibit the grain growth, resulting in the refinement of PAG in the CGHAZ. The intragranular and intergranular V-containing coarse particles effectively promoted the nucleation of intragranular acicular ferrite (IGAF) and grain boundary polygonal ferrite (GBPF). Accordingly, the contents of heterogeneously-nucleated IGAF and polygonal ferrite (PF) increased; however, the bainite content decreased with a rise in N content, leading to a prominent microstructure refinement in this CGHAZ with a high heat input. Furthermore, the nitrogen addition led to the formation of finer and more dispersed martensite-austenite (M/A) constituents in the CGHAZ. Therefore, the impact toughness of this CGHAZ was enhanced. This can be attributed to the microstructure refinement of the CGHAZ, which in turn was ascribed to an increasing nitrogen.

Thermal induced residual stress and microstructural constituents of dissimilar S690QT high-strength steels and 316L austenitic stainless steel weld joints

The effect of thermal cycle on the residual stress, microstructural constituents, and alloying elements composition of dissimilar S690QT and 316L austenite stainless steel was studied. Finite element model (FEM) using ANSYS 19.1 software and an experimental investigation using gas metal arc welding (GMAW) process with fully austenite filler wire were applied to developed thermal cycle and evaluate residual stress in the heat-affected zone of both materials. The experimental data were recorded using a thermal-cycle sensor (TCS) and x-ray diffraction technique. A microstructural investigation was done using Scanning electron microscopy (SEM) and Energy-Dispersive x-ray Spectroscopy (EDS). The thermal cycle showed the maximum temperature (T max) in the HAZ of 316L side (850 °C) at a distance of 7 mm away from the centreline of the weld compare to S690QT side. The magnitude of tensile residual stresses in the 316L side decreased as welding heat input increased. The maximum residual stresses were observed on the S690QT side (700 MPa). Microstructural investigations revealed the formation of Bainite, and some retained of austenite at the temperature of 800 °C in the coarse grain heat-affected zone (CGHAZ) of S690QT. On 316L side, some grain boundary austenite (GBA), intragranular austenite (IGA), and carbides were observed in the CGHAZ. Compared to the initial microstructure of both materials, a slightly increase of Mn, Cr, and Si were observed at the respective values of 1.90%, 1.25%, and 0.40% on the S690QT side compared to the BM. For 316L side, it indicated an increase of Cr (26%), Mo (5.69%), and Ni (17%) in the alloying element composition compared to the BM. Applying 10 kJ cm−1 of heat input produced an excellent mechanical property and reduced the formation of carbide, inter-granular corrosion in the microstructure of 316L side.

Effect of Welding Heat Input on the Microstructure and Impact Toughness of HAZ in 420 MPa-Grade Offshore Engineering Steel

In the present work, the effect of the welding heat input on the microstructure, martensite–austenite (M–A) constituents, and impact toughness of the coarse-grained heat-affected zone (CGHAZ) in offshore engineering steel with Ca deoxidation is studied. With the heat input increased from 50 to 100 kJ/cm, the HAZ toughness decreased rapidly, while the measured microhardness decreases steadily. The grain sizes are increased from 52 to 132 μm, and the width of bainite lath increased from 0.4 to 2 μm. The area fraction of lath bainite (LB) decreased, while the area fraction of granular bainite (GB) increased. The average width of M–A constituents grows from 0.3 to 0.6 μm, and the average length grows from from 0.5 to 0.9 μm. Its area fraction is increased from 5.3 to 8.6% and then decreased to 6.1%, and its number density decreased from 0.7 to 0.2 μm−2. The morphologies of M–A constituents change from dot-like to slender and blocky, which are deleterious to impact toughness. The fracture mechanism changes from ductile to quasicleavage and cleavage as the heat input is increased. As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm.

Role of the Ni-based filler composition on microstructure and mechanical behavior of the dissimilar welded joint of P22 and P91 steel

The manuscript described the effect of Ni-based filler on microstructure and mechanical properties of the dissimilar welded ASTM A335 grade P22 and P91 alloy steel plate. The dissimilar welds joint (DWJ) was produced using the Gas Tungsten Arc Welding (GTAW) process. The DWJ was characterized for metallographic and mechanical testing for as-welded and post-weld heat treatment (PWHT) conditions. For mechanical properties, microhardness, tensile test and Charpy impact test were performed. The DWJ showed the heterogeneity in microstructure and mechanical properties (microhardness, impact toughness) along the welded joint. The weld metal showed the formation of the austenitic microstructure consisted of the equiaxed dendrite along with segregation of the alloying elements like Cr, Mo, and Ti at the inter-dendritic boundaries, while P91 and P22 HAZ showed the formation of coarse-grained, fine-grained, and inter-critical heat-affected region depends on the temperature experienced during the welding cycle. The region of P91 HAZ was characterized by untempered and tempered martensite, while P22 HAZ was characterized by coarse and fine bainitic microstructure. The PWHT showed a negligible effect on the microstructure of the weld metal, while a significant change in microstructure of P91 HAZ and P22 HAZ was observed. PWHT also led to the formation of the soft and hard zone along the interface of the P22 steel due to the carbon diffusion. At the interface of the weld metal and base material, a sharp gradient was observed for the elements Cr, Fe, Ni and Mo. The weld metal near the interface showed the columnar dendritic structure having segregation of the alloying elements Mo, Ti and Cr along the inter-dendrite boundaries. A variation in hardness was measured along the weldments for as-welded and PWHT conditions. The peak hardness was measured in P91 coarse-grained heat-affected zone (CGHAZ) for both as-welded and PWHT, while the poor hardness of 202 HV and 195 HV was measured for weld metal in as-welded and PWHT conditions. The strength of the welded joint was measured higher than the strength of P22 steel for both conditions; however, the fracture was observed in P22 base metal for the as-welded joint, while in PWHT, it was in P22 HAZ near the interface. A variation in Charpy impact toughness was observed along the weldment for both as-welded and PWHT conditions. In as-welded, the minimum Charpy impact toughness (CIT) of 95.8 J was measured in weld metal, while the maximum was 152 J in P91 HAZ. After the PWHT HAZ region of P22 and P91 steel showed a drastic increase in CIT, however reduction in CIT of the weld metal was measured.