Austenitic Microstructure Research Articles

The Effect of the Heating Process on the Warm Deformation Behavior and Microstructure of Medium Mn Steel

This study investigates the effects of heating processes on the microstructure and deformation behavior of medium Mn steel during warm deformation. The samples treated by austenitization maintain an austenitic microstructure during deformation, which transforms into martensite upon cooling to room temperature. In contrast, the room‐temperature microstructure of samples deformed by direct heating to the deformation temperature is dependent on the deformation temperature: deformation above Ac3 results in a martensitic microstructure; deformation within the intercritical range (Ac1–Ac3) produces a mixture of ferrite, martensite, and austenite; and deformation below Ac1 yields a microstructure of ferrite and carbides at room temperature. When the microstructure only contains austenite during deformation, medium Mn steel exhibits excellent work‐hardening capability, characterized by a gradual increase in work hardening with strain until a dynamic balance between hardening and softening. Conversely, when ferrite is present in the microstructure during deformation, the flow behavior is dominated by ferrite, resulting in poor work‐hardening capability, with peak stress reached at low strain, followed by dynamic softening. Microstructure and stress–strain curve analyses reveal that, below 640 °C, ferrite exhibits higher strength than austenite. At 640 °C, the strengths of ferrite and austenite are approximately equal; above this temperature, austenite surpasses ferrite in strength.

Powder Metallurgy and Additive Manufacturing of High‐Nitrogen Alloyed FeCr(Si)N Stainless Steel

Nitrogen, as an alloying element in stainless steels, is valued for its ability to enhance strength, corrosion resistance, and its strong austenite‐stabilizing effect. Compared to other elements like nickel and manganese, nitrogen is more accessible, biocompatible, and cost‐effective, making it ideal for sustainable and economical austenitic stainless steel production. This study explores a powder metallurgical approach to produce an austenitic stainless steel based on the FeCr(Si)N alloy system. A powder mixture of Fe20Cr and Si3N4 is hot isostatically pressed (HIP), dissolving Si3N4 and enriching the matrix with nitrogen. While a primarily austenitic microstructure is formed, small martensitic regions appear due to localized silicon segregation (lower austenite stability). The same powder mixture was used in the laser powder bed fusion (PBF‐LB/M) process to manufacture shell–core samples. In this method, a partially powdered core is encased by a dense shell, with subsequent HIP ensuring full compaction and dissolution of the remaining Si3N4 particles in the powdered regions. However, an Si3N4 decomposition reaction during PBF‐LB/M results in nitrogen loss, leading to a martensitic–ferritic microstructure instead of an austenitic one. Optimization strategies to achieve an austenitic FeCr(Si)N microstructure also in the PBF‐LB/M process, which is particularly relevant for medical technology, are presented.

Nickel-Free Austenitic Stainless Steel Manufactured by Laser Powder-Bed Fusion from Martensitic Powder Mixed with Interstitial Compounds

Abstract Using laser powder bed fusion (L-PBF), nickel-free austenitic stainless steel was manufactured from mixing AISI 420S martensitic stainless-steel powder with austenite-stabilizing components. Chromium nitride (Cr2N), chromium carbide (Cr3C2), chromium (Cr) and graphite (C) powder were admixed in different quantities. The resulting microstructures were investigated using light- and electron microscopy, X-ray diffraction, and hardness indentations. Nitrogen, carbon, and chromium from the admixed powders were dissolved in solid solution; no remnants of nitrides/carbides were identified. The as-built specimens had a lower nitrogen content than the mixed powders. Insufficient additions of austenite-stabilizing elements resulted in a dual-phase microstructure of austenite and martensite, which experienced in-situ tempering of martensite during fusion of consecutive layer(s) in the L-PBF process. Relatively high contents of austenite-stabilizing elements resulted in a fully austenitic microstructure with a hardness of 380–500 HV5, depending on Cr and interstitial content. The tendency for forming hot cracks was found to correlate with the solidification interval as calculated using a modified version of the Scheil-Gulliver model.

Effects of Cr on Microstructure and Pitting Corrosion Resistance of Femnalc Lightweight Steels

Lightweight steels, characterized by a significant addition of Al, have lower density compared to conventional steels. These steels are attracting attention due to their excellent mechanical properties and the anticipated weight reduction effect that would result from replacing conventional steels with low-density alternatives. FeMnAlC lightweight steel was initially developed in the 1950s to create inexpensive corrosion-resistant steel, but it exhibits significantly lower corrosion resistance compared to FeCrNi stainless steel. In this study, the effects of Cr content on the microstructure and pitting corrosion resistance of Fe-19Mn-12Al-1.5C-xCr (wt%) lightweight steel were investigated. Microstructural analysis indicated that the optimal Cr content for a homogeneous austenitic microstructure was 5 wt%. Cr content higher than 5 wt% resulted in a multi-phase microstructure composed of austenite, ferrite, and Cr7C3. Passive film analysis showed that alloying Cr up to 5 wt% in the matrix increased both the Cr and Al content in the passive film. Thus, Fe-19Mn-12Al-1.5C-5Cr exhibited the highest resistance to pitting corrosion among the investigated alloys due to the homogeneous microstructure and a (Cr,Al)-enriched passive film.

High Temperature Deformation of Austenite: Texture and Anisotropy Effects

The deformation of low carbon steels at high temperature in austenite is technologically important but its study is complicated by the decomposition of austenite upon cooling which does not allow for direct characterization of the microstructure. In this study, the grain size and crystallographic texture of austenite were assessed by reconstruction of the prior austenite grains from the ambient temperature bainite. The austenite microstructure was found to consist of nearly equiaxed grains with annealing twins and a texture similar to copper deformed in plane strain. This texture was rationalized by visco-plastic self-consistent (VPSC) simulations. The presence of a non-random texture was predicted to lead to an anisotropic plastic response which was confirmed by experiments. A constitutive model for the high temperature flow stress was established in the Kocks-Mecking framework for work hardening and fit to experiments. The model described the experiments well and was validated by independent compression tests. Finally, the work hardening behaviour was assessed in comparison to other FCC metals. Its behaviour was found to fall between copper and silver, closer to copper.

Influence of annealing on microstructures and mechanical properties of laser powder bed fusion and wire arc directed energy deposition additively manufactured 316L

The annealing response of 316L stainless steel manufactured with both laser powder bed fusion (PBF-LB) and wire-arc directed energy deposition (DED-Arc) was investigated in the context of microstructural evolution and mechanical properties. Because additive manufacturing (AM) comprises several technologies that differ in heat source, feedstock, and scan strategy, among other build variables, the final products can experience a range of thermal histories and defect (dislocation) populations. These unique thermal histories can subsequently influence as-built properties and annealing response of AM materials, most notably those manufactured with different AM deposition processes. The PBF-LB process (with higher cooling rates) yielded finer austenite microstructures having more lattice strain, higher dislocation densities, and higher yield strength values than the DED-Arc 316L process (with lower cooling rates). Electron backscattered diffraction (EBSD) data of the kernel average misorientation (KAM), boundary density, and geometrically necessary dislocation (GND) density support the differences in lattice strain. As a result, the PBF-LB 316L exhibited more recovery and recrystallization after annealing at elevated temperatures above 873 K based on changes to yield strength, work hardening behavior, and microstructure evolution. The DED-Arc 316L exhibited a δ-ferrite/austenite microstructure with microsegregation that influenced deformation mechanisms active after annealing. Tensile data, deformed microstructures and misorientation histograms from EBSD showed a decreasing amount of deformation twinning when comparing the as-built condition to annealed conditions (up to 1473 K/1h) of PBF-LB 316L. The opposite trend was noted in DED-Arc 316L. The behavior was interpreted to be due to the differences in chemical segregation during solidification and the effects of heat treatment on chemical homogenization and local stacking fault energy.

Influence of Ageing Treatment on Microstructure and Mechanical Properties of GH4169 Alloy Prepared Using Wire Arc Additive Manufacturing

The effects of solid-solution and ageing treatments on the microstructure and mechanical properties of a GH4169 alloy made by wire arc additive manufacturing, micro-casting, and forging were researched. The microstructure, along with the size and type of precipitated phase, were analysed using an optical microscope, scanning electron microscope, and transmission electron microscope. The strength and toughness were tested using a tensile testing machine. The results show that a polygonal austenite microstructure was obtained for the GH4169 alloy prepared through wire arc additive manufacturing, micro-casting, and forging, followed by solid-solution and double-ageing treatments at different times. There were a few twins in the austenite matrix. A large number of nano-sized γ″- and γ′-precipitated phases and a small number of Laves phases and MX phases were found in the matrix. The tensile strength and yield strength of the GH4169 alloy increased first and then decreased with the ageing time. After ageing for 16 h, the maximum yield strength was 1287 ± 22 MPa, the maximum tensile strength was 1447 ± 19 MPa, and the elongation was about 19.5%. The main strength mechanism is precipitated phase strength and solid-solution strength. The fracture exhibited obvious ductile fracture characteristics.

Adapting the Mn content within a Fe-Mn-Si-based shape memory alloy by in situ parameter variations in laser-based additive manufacturing

Shape memory alloys exhibit unique properties ideal for functionally integrated components, such as actuators. While commonly used Ni-Ti alloys are well established, especially in biomedicine and aerospace, their high cost limits wider applications. Fe-based shape memory alloys present an affordable alternative, suitable for diverse applications, with a larger thermal hysteresis but lower recovery strain. Nonetheless, their functional properties can be enhanced through optimized processing methods like laser powder bed fusion and adjustments to their alloy composition. In the present study, laser powder bed fusion was used for modifying the composition and microstructure of a Fe-30Mn-6Si-5Cr alloy. For this purpose, laser parameters were varied to evaporate up to 47.3% of the initial Mn in the manufacturing process. In this context, the amount of Mn evaporated was dependent on both the line energy and more considerably on the volume energy density introduced in the manufacturing process. Subsequently, the impact of the resulting compositional changes on the microstructure was analyzed, and the functional properties were visualized using a demonstrator. Lowering the Mn content below 24.1% led to a higher degree of ferrite in the otherwise austenitic microstructure. This also affected the functional properties. These findings highlight the potential of laser-based additive manufacturing for modifying the composition and microstructure of shape memory alloys in situ and, thus, tailoring their functional properties.

Clarifying the effect of irradiation and thermal treatment on the austenitic microstructure and austenitic hardening in austenitic stainless steel weld metal

The weld cladding on the inner surface of nuclear pressure vessels due to irradiation damage and thermal effect presents to a safety issue. Unfortunately, the effect of irradiation and long-term thermal treatment on the austenitic microstructure and austenitic hardening in austenitic stainless steel weld metals (ASSWMs) remains poorly understood. In this study, the effects of irradiation and thermal treatment on the austenitic microstructures and austenitic hardening of 308L ASSWMs were investigated using nanoindentation, atom probe tomography and transmission electron microscopy. The results suggested that irradiation resulted in the formation of Ni/Si-rich clusters, voids, and Frank loops in austenite, thereby inducing austenitic hardening. Subsequently, thermal treatment decreased the size and the number of Frank loops in irradiated austenite, which had a minor effect on austenitic hardening. However, thermal treatment promoted the growth of Ni/Si-rich clusters and void formation, which have a primary effect on austenitic hardening, thereby enhancing the hardening of irradiated austenite. Furthermore, thermal treatment has little effect on the microstructure and hardening of austenite. Then, irradiation promoted the formation of Ni/Si-rich clusters, voids, and Frank loops in thermally treated austenite, resulting in austenitic hardening. The interaction of irradiation and thermal treatment can promote the formation of voids. The austenitic hardening was mainly due to the contribution of Frank loops, voids, and Ni/Si-rich clusters, which acted as short-range barriers by pin-moving dislocations.

253MA heat-resistant stainless steel by wire arc additive manufacturing: microstructure and mechanical properties

ABSTRACT The 253MA stainless steel fabricated via wire arc additive manufacturing displays a mixed austenite and ferrite microstructure. It features many dislocations, minimal inclusions, and carbides, observed through TEM. Notably, CeO2 spherical inclusions enhance its high–temperature and oxidation resistance. Microhardness test 2 shows average values of 187.9 HV0.2 and 183.9 HV0.2 in transverse and longitudinal directions, respectively. Mechanical properties at room temperature include a yield strength of 388 MPa, a tensile strength of 681 MPa, 18.89% elongation, and 34% area reduction post–fracture. Impact toughness measured 89.7J and 133.7J in transverse and longitudinal directions, respectively. At 600°C, the steel's yield strength and tensile strength drop to 208 MPa and 412 MPa, with elongation increasing to 32.06% and area reduction to 53%. The stress–strain curve at 600°C shows serrated flow, suggesting dynamic strain ageing. SEM analysis indicates ductile fracture with larger dimples at elevated temperatures.

A prediction model for austenite grain size distribution of casting slab in thermal rebound process: Considering α-ferrite precipitation and re-austenization

The precipitation and re-austenitization behavior of α-ferrite during thermal rebound process of casting slab are crucial to the austenite grain size distribution. Accurately obtaining austenite grain size distribution plays an important guiding role for avoiding the formation of slab cracks and effectively improving the quality of steel. In this work, a quantitative model to calculate α-ferrite precipitation at different temperatures has been constructed by employing high-temperature expansion test and peak area method. On this basis, a prediction model for the austenite grain size distribution of casting slab in the thermal rebound process has been theoretically derived with consideration of the α-ferrite precipitation and re-austenitization. Then, the accuracy of the quantitative model for α-ferrite precipitation and the prediction model for austenite grain size distribution have been evaluated by high-temperature quenching experiments. The results show that the maximum error between the calculation values of the prediction model and the experimental values is only 3.67 %, which can reliably predict the austenite grain size distribution of slab under different phase fractions of precipitated α-ferrite and initial austenite grain sizes. When the phase fraction of precipitated α-ferrite reaches about 70 %, the average grain size after re-austenitization comes to nearly 50 % of the initial size, which achieves the best refining effect. The reduction in initial austenite grain size is beneficial to the refinement of austenite grain after thermal rebound. Reasonable control of thermal rebound temperature and multiple cycles of phase transformation are effective means of obtaining austenite microstructure with smaller average grain size and more uniform distribution.

Zinc-Induced Liquid Metal Embrittlement in Austenitic Microstructures

The problem of liquid metal embrittlement (LME) is one of the main concerns to be addressed when developing high-strength automotive steels. Although LME has been extensively investigated over the past decade, the role of LME-induced cracking on the failure mechanism of steel substrates has not been adequately explored. This study investigates the influence of LME cracking on the tensile properties and failure mechanism of an austenitic microstructure. An in-depth analysis of the LME crack propagation path revealed that the crack propagated predominantly along highangle, random grain boundaries. The detailed failure analysis showed that the Zn-coated austenitic steel failed in an intergranular mechanism without any plastic strain being applied to the grains during deformation. The results of the study indicate that stress-assisted grain boundary diffusion is responsible for the premature failure of Zn-coated austenitic microstructures at much lower tensile stresses than that of the uncoated specimen. It was found that the application of thermomechanical conditions characterized by low stress and low temperature, combined with microstructures featuring a low Zn grain boundary diffusion rate, can effectively reduce LME cracking.

Effects of retained austenite and martensite microstructure on fatigue crack propagation in quenched and tempered high carbon steels

Commercial high-carbon 52,100 steel was subjected to varying quenching and tempering heat treatments to develop plate martensite and retained austenite (RA). The mode I fatigue crack growth (FCG) behavior of quenched and tempered microstructures with varying amount of RA and degree of martensite tempering was investigated. The interaction between the fatigue cracks and the surrounding microstructure was characterized using electron microscopy/backscatter diffraction and quantitative fractography. Higher initial amounts of RA and greater degree of martensite tempering improved FCG resistance of high-carbon plate-martensite austenite microstructures. Stress-assisted RA to martensite transformation was observed in the vicinity of the fatigue cracks in the microstructure heat treated to have an elevated amount of RA. The fatigue fracture surfaces of the different microstructures exhibited varying combinations of intergranular (IG) fracture, mixed-ductile brittle (MDB) and transgranular (TG: cleavage, quasi-cleavage, and ductile striations) fracture as a function of stress intensity range at the crack tip. Microstructure conditions showing greater fractions of MDB and TG fatigue fracture had better FCG resistance. The presence of a higher initial RA content suppressed brittle IG fracture and promoted TG fracture, potentially due to transformation–toughening associated with RA to martensite transformation or transformation induced micro-crack coalescence near the fatigue crack.

Effect of cooling rate in the mid-temperature region on fatigue crack propagation rate of bainitic steel

Controlling continuous cooling is an important means to control the most important mechanical properties of bainitic steel for frogs in railroad applications. In this paper, a bainitic steel for railway frogs is taken as the research object. By designing different cooling rates in the bainitic transformation medium temperature region, the effect of cooling rate on fatigue crack propagation rate was studied. The results show the sample of 4 °C/s cooling rate exhibits higher strength and impact toughness, while the crack propagation rate near the threshold is significantly lower and the threshold is relatively higher at 0.1 °C/s cooling rate. This is related to the fact that more massive blocky retained austenite and coarse microstructure in the sample at 0.1 °C/s cooling rate are more favorable to the occurrence of crack closure effect, especially in the near-threshold region. The fatigue crack propagation rate of the tested steel in the stable propagation region at 4 °C/s cooling rate is slightly higher than that at 0.1 °C/s. This is related to its complex interface distribution characteristics. However, with the increase of Δ K, the tested steel at 0.1 °C/s cooling rate tends to preferentially fail, which is related to its poor toughness.

Development of a Flow Rule Based on a Unified Plasticity Model for 13Cr-4Ni Low-Carbon Martensitic Stainless Steel Subject to Post-Weld Heat Treatment

This study aims to develop a flow rule for evaluating the relaxation and redistribution of residual stresses during the post-weld heat treatment (PWHT) of hydroelectric runners made from low-carbon martensitic stainless steel (13Cr-4Ni composition). During the PWHT, austenite reforms in the filler metal and surrounding areas of the base metal near welded joints. The evolving inelastic strain rate with reformed austenite led to defining two distinct flow rules in the pure martensitic (α′) and austenitic (γ) phases. A linear rule of mixture was then applied to assess global effective stress based on the inelastic strain rate and current austenite fraction during the PWHT. A unified constitutive model incorporating drag stress and back stress, evolving with creep and plastic deformation mechanisms during the PWHT, described the stress–strain behavior. To validate this analysis, a third flow rule was determined in the 18% tempered austenitic microstructure, compared with the rule of mixture’s effective stress contribution from each phase on the inelastic strain rate. Isothermal constant strain rate tests in stabilized crystalline microstructures evaluated constants specific to their respective flow rules. This study demonstrates the stability of reformed austenite at elevated temperatures during slow cooling and its significant influence on the mechanical properties of 13Cr-4Ni steels. The effectiveness of estimating yield stress using the rule of mixture based on individual phase behaviors is also confirmed.

Modeling the interaction of carbon segregation to defects and carbon partitioning in multiphase steels

Carbon segregation to defects in martensite is a phenomenon known for its occurrence and interference with mechanisms such as carbon partitioning in multiphase steels. Especially in martensite–austenite partitioning processes, carbon trapping at/de-trapping from martensite defects plays an important role since it interacts with the austenite enrichment. In this work, we develop a physics-based model in which we incorporate the concurrent evolution of carbon partitioning and trapping at/de-trapping from martensite defects. The model describes the global and local, time-dependent distribution of carbon between three lattice types, namely martensite defects, martensite solid solution, and austenite. We implement the model in mean-field and full-field descriptions, and discuss the interaction between carbon enrichment in austenite and segregation to martensite defects, on the basis of global equilibrium as well as on the carbon kinetics. We apply the model in several martensite — austenite microstructures and discuss the dependence of the interaction between carbon partitioning and trapping at/de-trapping from defects on specific microstructural features, i.e. phase fractions and microstructural banding.

Quasi-static tensile and fatigue behaviour of a metastable austenitic stainless Cr-Ni-Cu-N steel

The tensile as well as the fatigue behaviour of an experimental copper-alloyed metastable austenitic stainless steel has been investigated. Three batches were studied: (i) cast material as reference state and two states were recrystallised after (ii) rotary swaging and (iii) forward impact extrusion. Deformation-induced α’-martensite was formed during both rotary swaging and forward impact extrusion with volume fractions of 48 vol% and 4 vol%, respectively. The material was then heat treated to form a fine-grained, fully austenitic microstructure. These fine-grained material states exhibited a higher strength compared to the cast state. However, strain-hardening was highest in the cast state. The effect of the initial microstructure on the cyclic deformation behaviour and fatigue life was investigated using strain-controlled tests. During cyclic deformation all three states exhibited a martensitic phase transformation and concurrently a pronounced cyclic hardening with the formation of up to approximately 70 vol% α’-martensite. The fine-grained state produced by rotary swaging and reversion annealing showed a significantly higher fatigue life of about factor 4 compared to the coarse-grained cast state. Relative to forward impact extrusion, rotary swaging provided a higher fatigue life and fatigue strength.

The Effect of Static Recrystallisation of Parent Austenite on the Lath Martensite Intervariant Boundary Network

The current study investigated the effect of static recrystallisation (SRX) of parent austenite microstructure on the lath martensite intervariant boundary network. In general, the misorientation angle distribution of lath martensite exhibited a bimodal distribution in the range of 10–22 deg and 47–60 deg for all thermomechanical conditions, closely matching the misorientations expected from the ideal Kurdjumov-Sachs orientation relationship. Nevertheless, the population of 60 deg misorientation angle initially reduced with the extent of softening up to 50 pct beyond which it progressively increased to a maximum in the fully recrystallised condition. This was closely consistent with the trend noticed for the 60 deg/[110] intervariant boundaries having twist character. This was explained through the distinct variant selection mechanism occurring due to the change in the parent austenite grain characteristics (i.e., size and deformed state) upon static recrystallisation. The recrystallisation initially led to the formation of fine dislocation-free parent austenite grains up to 50 pct softening, promoting a 4-variant (V1V2V3V4) clustering arrangement to decrease the strain related to the martensite transformation. In turn, this promoted the formation of symmetric tilt 60 deg/111¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {11\\overline{1}} \\right]$$\\end{document} intervariant boundaries, which ultimately reduced the 60 deg/110\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {110} \\right]$$\\end{document} boundaries. Further softening, accompanied with the growth and/or coalescence of parent austenite grains, altered the variant clustering to a 3-variant arrangement (V1V3V5) and the increase in 60 deg/110\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {110} \\right]$$\\end{document} twist intervariant boundaries. Indeed, the martensite boundary network was largely controlled by the SRX parent austenite characteristics (i.e., size), which can be controlled to enhance the materials performance for a given application.

Investigation on microstructural and mechanical integrity of GTAW dissimilar welded joint of IN 718/ASS 304L using Ni-based filler IN 625 using EBSD and DHD techniques

The current study investigates the relationship between the microstructural and structural characteristics of dissimilar gas tungsten arc welding (GTAW) between Inconel 718 (IN 718) and austenitic stainless steel 304L (ASS 304L) using Inconel 625 (IN 625) as a filler material. The microstructural characterization of the dissimilar weld includes the application of an optical microscope (OM), field emission scanning electron microscopy (FE-SEM) including energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The optical and FESEM analysis of the base metals (BM) indicates the austenitic nature of the IN 718 BM matrix, which incorporates strengthening precipitates γ′ and γ'' distributed throughout the Nickel matrix. In contrast, the ASS 304L BM exhibits an austenitic microstructure along with twins. Moreover, IN 625 weld metal exhibits austenitic microstructure in the form of cellular, columnar, and equiaxed dendrites. Elemental segregation demonstrates the existence of Nb, and Mo-rich carbides, including the laves phases at the interdendritic regions of the IN 625 weld zone and IN 718 Heat-Affected Zone (HAZ). Ferrite stringers were noticed in the HAZ of ASS 304L. EBSD analysis shows the improved texture of the weld zone exhibiting fine equiaxed recrystallized microstructures. EBSD/Kernel average misorientation (KAM) micrographs reveal the existence of high strain at the weld metal grain boundaries. Vickers microhardness assessment of the dissimilar weld demonstrates the high hardness of 257.5 HV for the IN 625 weld zone because of the occurred NbC, laves phases, and Mo-rich carbides. Room temperature tensile characterization shows the failure of tensile specimens from ASS 304L BM reflecting the high strength of the IN 625 weld metal (662 MPa). Tensile tests at high temperatures were performed at 550 °C, 600 °C, and 650 °C. High-temperature tensile result exhibits a higher tensile strength of IN 625 weld zone than the ASS 304L BM, as the tensile specimens fails from 304L BM. The specimen exposed at 550 °C exhibits the highest tensile strength of 388 MPa. The Charpy impact toughness test was conducted, with notches at the HAZ of ASS 304L, HAZ of IN 718, and the weld center being evaluated. The toughness value of the weld metal decreases (99 J) due to occurrence of the secondary precipitates. To analyze residual stress, the deep hole drilling (DHD) technique was used, which revealed that the peak residual stress (230 MPa) is induced at 6 mm from the weld surface and falls into the tensile stress category.

Enhancing load-bearing capacity of martensitic stainless steel resistance spot welds: Microstructural vs. geometrical approaches

The load-bearing capacity of resistance spot welds is determined by a combination of weld geometrical attributes, such as the size of the weld nugget (WN), and weld metallurgical features. When resistance spot welds are made on martensitic stainless steels (MSSs), their load-bearing capacity is reduced due to the formation of a brittle martensitic microstructure within the WN. In this study, we investigated two distinct approaches to enhance the mechanical properties of AISI420 MSS spot welds. The results indicated that manipulating the WN size did not effectively produce strong welds in both tensile-shear (TS) and cross-tension (CT) tests. Therefore, we focused on examining the impact of the weld microstructure by employing two different interlayers: low carbon steel (LCS) and pure nickel. The analysis using electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA), along with equilibrium and non-equilibrium phase diagrams, revealed that the microstructure of the WN with low carbon steel interlayer of various thicknesses exhibited similarities with spot welds made without an interlayer, and the solidification mode was still δ-ferrite. This microstructure primarily consisted of martensite (M) and likely residual delta ferrite (δ) phases, resulting in no significant improvements in mechanical properties. In contrast, the utilization of the nickel interlayer led to remarkable enhancements in mechanical properties. This improvement can be attributed to a shift in the solidification mode from primary delta (δ) to austenite (γ) within an optimal interlayer thickness. This transition facilitated the development of a predominantly austenitic (γ) microstructure, contributing to the formation of an exceptionally tough microstructure within the weld nugget. Consequently, the load-bearing capacity was significantly improved.