This study examines the effects of Mo, V, and Nb additions on the metallurgical and mechanical behavior of austenitic Fe-Mn-Al-C lightweight steels and their heat-affected zone (HAZ), with a particular focus on κ-carbide precipitation behavior. Microstructural characteristics and mechanical properties, including tensile behavior and Charpy V-notched impact toughness, were evaluated for both the base metal and simulated HAZ specimens, produced using a Gleeble thermal simulator. Metallurgical analyses revealed that the addition of Mo, V, and Nb effectively suppresses κ-carbide precipitation during welding thermal cycles by increasing the thermodynamic barrier to κ-carbide formation and/or by consuming carbon through competitive carbide precipitation, thereby mitigating the degradation of impact toughness in the HAZ compared to the base metal. However, since a small amount of κ-carbide still formed during the welding thermal cycle, tensile fracture occurred in the base metal rather than in the HAZ, indicating localized strengthening of the HAZ as each HAZ was locally strengthened by κ-carbide precipitation. These findings demonstrate that controlled alloying and steelmaking strategies can enhance the weldability and mechanical reliability of austenitic lightweight steels, while highlighting the importance of balanced alloy design to achieve optimal performance under welding conditions.

The effect of the base metal (BM) microstructure on the hot cracking susceptibilities of Alloy 718 during welding was investigated. A forged ring of Alloy 718 was subjected to a series of heat treatments to produce distinct initial BM microstructures with variations in grain size and precipitate distribution. The cracking susceptibilities in the fusion zone (FZ) and heat-affected zone (HAZ) were quantitatively assessed using Varestraint tests. The results revealed that the FZ cracking susceptibility increased with increasing BM grain size, owing to epitaxial grain growth during solidification that promoted the formation of coarser grains in the FZ. In the HAZ, cracking susceptibility was strongly influenced by Nb-rich precipitates formed during solution treatment, such as Laves and grain boundary delta phases, which promoted constitutional liquation and segregation-induced liquation along grain boundaries, respectively.

Resistance spot weld (RSW) joints between medium-Mn transformation-induced plasticity (TRIP) and dual-phase (DP) steels often exhibit premature interfacial fracture due to the brittle heat-affected zone (HAZ). Although boron (B) segregation is well known to influence the microstructure of press-hardened steels, its direct role in controlling the local ductility and cross-tension strength (CTS) of dissimilar TRIP/DP RSWs remains unclear. This study aims to elucidate how boron grain-boundary segregation controls the ductility and fracture mode of the coarse-grained heat-affected zone (CGHAZ), and consequently the joint-level CTS, thereby providing a quantitative basis for composition-controlled interfacial behavior. To isolate local properties, CGHAZ specimens were produced by Gleeble thermo-mechanical simulation and tested in tension. Boron was added up to 50 wt. ppm. CTS increased with boron and reached a maximum at similar to 20 wt. ppm. The improvement correlated with higher ductility of the CGHAZ and a transition from intergranular brittle fracture to mixed ductile fracture. Atomistic- to microscale characterization indicated that boron segregated to prior-austenite grain boundaries, which reduced boundary decohesion. At >= 30 wt. ppm, the benefit diminished. The reduction in CTS is attributed to boundary liquation, accelerated grain coarsening, and excessive segregation that promotes intergranular cracking. These results establish a compositional window in which boundary segregation is toughening, while over-addition reverses the effect. The combined use of dissimilar RSW testing and simulated CGHAZ tensile testing provides a mechanistic link between segregation, local tensile behavior, and joint-level CTS.

This study examines the microstructural evolution and mechanical properties of medium-thickness DP980 advanced high-strength steel subjected to simulated welding thermal cycles. Gleeble simulations reproduced the thermal histories of coarse-grained (CG), fine-grained (FG), intercritical (IC), and subcritical heat-affected zone (SCHAZ) subregions. Microstructures were analyzed by scanning electron microscopy (SEM), and X-ray diffraction (XRD), while Vickers microhardness and Charpy impact tests were performed at room temperature and -40 ℃. Results showed that CGHAZ and FGHAZ developed polygonal prior-austenite grains and lath martensite, yielding higher hardness and toughness than the base metal. In contrast, ICHAZ contained a heterogeneous mixture of ferrite, martensite, retained austenite, and martensite-austenite (M-A) constituents, which led to significant toughness loss at both temperatures examined. The SCHAZ contained tempered martensite within ferrite and showed only moderate changes in impact energy. Although overall trends were similar, impact toughness decreased further at -40 ℃. These findings highlight the critical role of intercritical microstructures in governing toughness and emphasize the need to control welding parameters to suppress detrimental M-A formation in DP980 steel.

Conventional heat treatment standards for Alloy 718 are primarily based on cast and wrought processes and may not be suitable for Wire Arc Additive Manufacturing (WAAM), which involves rapid solidification and repeated thermal cycling. As such, there is an increasing need to develop tailored post-processing strategies for additively manufactured Alloy 718. This study investigates the homogenization heat treatment response of WAAM-fabricated Alloy 718, with a focus on the dissolution behavior of Laves phases formed due to elemental segregation. In addition, a review of the conventional Aerospace Material Specifications (AMS) heat treatment standards applied to cast and wrought processes is conducted to provide context and contrast with the WAAM process. Specimens were homogenized at 950 °C, 1100 °C, and 1170 °C for 12 hours. In-depth electron microscopy analyses revealed that Laves phases remained at 950 °C, partially dissolved at 1100 °C, and were completely eliminated at 1170 °C. At 1170 °C, a dominant <101> grain orientation was observed, along with significant grain coarsening. These findings suggest that a homogenization temperature above 1100 °C is required to fully dissolve Laves phases in WAAM Alloy 718, and that optimizing the heat treatment time is essential to minimize grain growth. This study provides insights into the heat treatment behavior of WAAM Alloy 718 from the perspective of Laves phase dissolution and offers design considerations for appropriate post-processing strategies.

This study investigates the effects of chromium addition on phase transformation behavior and mechanical properties in the heat-affected zone (HAZ) of austenitic lightweight steels under various welding heat input conditions. Fe-30.0Mn-10.4Al-0.9C-xCr alloys were fabricated and subjected to simulated welding thermal cycles using a Gleeble simulator with heat inputs ranging from 10 to 300 kJ/cm. Microstructural analyses were conducted via OM, SEM, XRD, and TEM, while Vickers hardness test was conducted to understand κ-carbide precipitation behavior. The results confirmed that the addition of Cr promoted ferrite formation, suppressed austenite grain coarsening in the HAZ, and inhibited κ-carbide precipitation in HAZ regardless of welding heat input. TEM analysis and microhardness measurements also confirmed that Cr effectively suppressed κ-carbide precipitation in the HAZ, and this suppression is attributed to the consumption of κ-carbide forming elements during ferrite stabilization. These findings indicate that Cr addition is an effective approach for enhancing the weldability and mechanical reliability of Fe-Mn-Al-C austenitic lightweight steels, providing valuable insights for the design of advanced structural materials with superior performance.

Wire arc-based metal additive manufacturing (AM) commonly results columnar grain structures aligned with the build direction, causing anisotropic mechanical properties and performance degradation. To mitigate the formation of columnar grain structures, ultrasonic vibration (UV) at 20 kHz was applied during the continuous heat input directed energy deposition (DED) process, targeting microstrucrual refinement in high-purity Ni alloy. The application of UV disrupted thermal gradients and facilitated dynamic recrystallization, transitioning columnar grains into equiaxed grains near the substrate. Electron backscatter diffraction (EBSD) analysis revealed a marked reduction in crystallographic texture, showing randomized grain orientations in UV-treated samples, contrasting with the strong anisotropy in untreated samples. High-speed imaging revealed localized vibrations at the interface between the metal bead and molten metal, forming band-like structures that progressively refined dendrites during solidification. These findings demonstrate the potential of UV to refine grain structures, enhance isotropy, and improve the mechanical properties of AM-produced materials.

This study investigates the influence of chromium (Cr) addition on the metallurgical characteristics of Fe-Mn-Al-C austenitic lightweight steels, with a focus on the fusion zone and heat-affected zone (HAZ) of actual welds and simulated samples. Lightweight steel samples, with and without Cr addition, were fabricated using a vacuum induction furnace. Welding by autogenous gas tungsten arc welding method and HAZ simulation by Gleeble simulator were performed. Microstructural and mechanical properties were analyzed in detail using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that Cr addition significantly affects the hardness transition in the HAZ by suppressing κ-carbide precipitation during the welding thermal cycle, while tensile properties remain unchanged. It was also observed that carbide precipitates contributed to increased hardness in the fusion zone of the Cr-added sample. These findings suggest that controlling alloy chemistry and understanding the precipitation behavior in austenitic lightweight steels can enhance their practical applicability in industrial settings. Further investigation into the role of additional alloying elements during welding is recommended.

In this study, the effect of welding heat input on the microstructure and mechanical properties of reduced-activation ferritic/martensitic steel weld metal was investigated to provide a basis for developing welding technology for this steel, which is considered a structural material for fusion reactor blankets. Autogenous bead-on-plate gas tungsten arc welding was performed with heat inputs of 0.57, 1.38, and 2.32 kJ/mm, and the microstructural evolution and mechanical properties of the weld metal were analyzed. The fraction of residual delta-ferrite in the weld metal varied depending on the welding heat input, which acted as a primary factor contributing to the reduction in weld metal strength, although it remained higher than that of the base metal. In addition, the effect of post-weld heat treatment (PWHT) at 730 degrees C for 1 h was evaluated. Before PWHT, the weld metal exhibited significantly higher hardness compared with the base metal. However, after PWHT, its hardness was substantially reduced, thereby minimizing the differences in hardness of the weld and the base metal.

This study examined the effects of long-term thermal aging on the mechanical properties and microstructure of 17-4 PH stainless steel (SS) at temperatures from 300 degrees C to 400 degrees C for up to 12,000 h. Mechanical tests, including hardness, strength, and impact toughness tests, were conducted, along with microstructural analysis using transmission electron microscopy. The results indicated that aging at 400 degrees C leads to early embrittlement and a decrease in mechanical strength after 10,000 h of exposure, due mainly to spinodal decomposition and G-phase formation. At 350 degrees C, the formation of a G-phase was observed at the boundary between Cu precipitates and martensite matrix after 5,000 h, contributing significantly to the rapid decrease in toughness, but the hardness and mechanical strength were only minimally affected. In contrast, at 300 degrees C, the mechanical strength increased more gradually, with only spinodal decomposition influencing the mechanical behavior. In particular, slight softening was observed during the first 1,000 h at 300 degrees C and 350 degrees C because of carbon diffusion that promoted the growth of niobium and chromium carbides, weakening the martensitic matrix. This study highlights the significant role of microstructural evolution, particularly the relationship between the formation of the G-phase and impact toughness, in determining the long-term mechanical properties of 17-4 PH SS under prolonged thermal aging under simulated thermal conditions for nuclear applications.

In this study, the solidification cracking behavior was investigated in Fe-30Mn-10.5Al-xC (x = 0.7, 0.9, 1.1 wt.%) austenitic lightweight steels. To evaluate the solidification cracking susceptibility of each alloy, a longitudinal Varestraint test was conducted with an applied strain ranging from 1 to 4%. The experimental results indicate that all the austenitic lightweight steels exhibit great resistance to solidification cracking due to the beneficial effects of high Al content, as evidenced by maximum crack length (MCL) values below 380 μm, despite their high alloying element content. However, the MCL increased directly as a function of carbon content. Microstructural analysis confirmed that the increased carbon content destabilized δ-ferrite in the fusion zone during solidification, consequently accelerating the generation and propagation of cracks. In addition, excessive carbon content also led to the formation of a eutectic (Fe, Mn)3C phase along the grain boundaries during the terminal stage of solidification, resulting in a drastic increase of solidification crack susceptibility due to its low melting point. The high content of Al and C also promoted κ-carbide precipitation in the fusion zone. However, it was confirmed that κ-carbide formation had an insignificant influence in the viewpoint of solidification crack susceptibility due to its narrow precipitation temperature range. Hence, we suggest further investigation through Varestraint test with various austenitic lightweight steels to confirm the influence of some alloying elements, for understanding the phase transition behavior in welds and improving the usability of austenitic lightweight steel in the industrial fields.

Wire arc-based directed energy deposition (DED) is a highly productive additive manufacturing (AM) technique; however, excessive heat input often results in distortion and irregular bead geometry, leading to increased surface waviness and necessitating extensive post-processing. To address these challenges, this study introduces a novel 3D weaving path aimed at enhancing wetting behavior and minimizing micro-scale waviness in wire arcbased DED. The weaving motion promotes metal spreading by adjusting the wetting area, thereby reducing the contact angle and improving surface smoothness. High-speed imaging and computational fluid dynamics (CFD) simulations were utilized to investigate molten metal behavior during deposition. Experimental results revealed that the 3D weaving path reduces surface waviness by more than 70 % compared to conventional stringer paths, significantly lowering the required machining depth. Additionally, mechanical property evaluations confirmed that the proposed approach maintains consistent hardness and tensile strength, ensuring structural integrity. These findings demonstrate the potential of 3D weaving path technology to enhance the efficiency and precision of large-scale metal AM, reducing post-processing demands and improving manufacturability.

This study presents a comprehensive investigation of liquation cracking resistance in Borated Stainless Steels (BSSs) within the compositional range of ASTM A887 grade. Thermomechanical simulation was utilized to evaluate their hot cracking characteristics, providing valuable insights into the microstructural aspects. Cylindrical specimens were prepared from the three types of hot-rolled BSS sheets with varying boron content. The Gleeble simulator was employed to conduct on-heating and on-cooling hot ductility tests, specifically replicating the thermal cycles of the weld heat-affected zone, with a focus on the partially melted zone. The reduction in area was quantified as a measure of ductility, and in-depth microstructural analysis was conducted to unveil the underlying mechanisms contributing to liquation cracking susceptibility. BSSs corresponding to 304B3, B4, and B5 generally exhibited a lower brittle temperature range, and an increase in boron content was found to slightly reduce cracking susceptibility. This research clarifies the issues of low liquation cracking susceptibility in BSSs, offering valuable insights that significantly enhance their industrial performance as neutron absorbers in disposal of used nuclear fuel and nuclear power plants.

This study focuses on the microstructural evolution of Inconel 718 superalloy, a precipitation-hardened superalloy, subjected to various manufacturing processes and subsequent thermal exposures. Conventional manufacturing methods such as casting and forging were compared with modern 3D printing techniques, notably wire-arc additive manufacturing. Subsequent treatments, including solution treatment and two-stage aging, as well as bead-on-plate welding, were performed to assess their effects on the microstructure of Inconel 718. The transformations of strengthening phases such as γ′(Ni3(Al,Ti)), γ″(Ni3Nb) and intermetallic δ(Ni3Nb) phases were observed to vary significantly under different thermal cycles, and these variations in phase transformation are anticipated to lead to a degradation in mechanical performance post-welding. Additionally, thermodynamic calculations using commercial Calphad software were utilized to investigate phase transformations in the welds, providing critical insights into how manufacturing processes and thermal exposures affect the stability and distribution of microstructural features, thereby highlighting the complexities of phase dynamics in this high-performance alloy.

In this study, we evaluated the high-temperature corrosion characteristics of copper(Cu), which is considered a candidate material for disposal containers in deep geological repositories for spent nuclear fuel, through electrochemical methods. Deep geological repositories serve to isolate spent nuclear fuel from humans and the environment, employing a multi-barrier concept consisting of engineered and natural barriers. Among these, engineered barriers consist of canisters, bentonite buffer materials, and others. Various countries are progressing with conceptual designs, with Cu, known for its corrosion resistance, being considered a key material for canister. While Cu is predicted to experience minimal corrosion in groundwater environments where deep geological repositories are typically situated, recent studies suggest that exposure to temperatures exceeding 100oC may lead to some corrosion. In previous studies by the authors, Cu corrosion evaluations were conducted in environments with various compositions at temperatures ranging from 60 to 70oC. The results confirmed that corrosion of copper could occur in the presence of corrosive species such as Cl-. Expanding upon these findings, the current study aims to assess various base metal and weld of Cu under high-temperature and high-pressure conditions using electrochemical methods.

This study aims to investigate the influence of the carburizing layer on the microstructure of laser-welded joints in 316L stainless steel, aiming to enhance the properties of stainless steel components through the incorporation of carburization. Through a comprehensive experimental approach, including nanoindentation and Vickers hardness measurements, the microstructural analysis of the laser-welded specimens was conducted. The nanoindentation results reveal a significant increase in hardness, with the carburizing layer exhibiting a remarkable 215% improvement compared to the base material. In order to effectively harness the wear resistance for various applications, the examination of macroscopic characteristics in laser-welded joints reveals the necessity of regulating heat input and minimizing defects to attain welds of superior quality. Additionally, the phase transformation behavior of stainless steel during welding was found to be altered due to dilution in the fusion zone and the presence of high carbon content in the heat-affected zone of the carburizing layer. Overall, this research provides valuable insights into the microstructural changes induced by carburization and its implications for the welding performance and mechanical properties of stainless steel.

The role of retained austenite in the mechanical property changes in the medium Mn transformation-induced plasticity (TRIP) steel heat-affected zone (HAZ) was elucidated throug a comparison with conventional TRIP steel. The two steel grades had similar microstructures consisting of ferritic phases with a high fraction of retained austenite in the as-received state; however, the difference in properties after welding was significant. The microstructures of both steels were dramatically changed to a martensitic matrix in the coarse-grained HAZ region. The medium Mn TRIP steel HAZ can have a higher retained austenite fraction than the conventional one; hence, an excellent combination of tensile strength and elongation can be obtained. The decisive difference between the two steels was determined in terms of austenite stability and the initial grain size.

The aim of this study is to develop a frame for exterior facades of architectural curtain walls using numerical and experimental analyses of stainless steel T-joint fillet laser welds. The main characteristics required for this frame are that it should have an aesthetic surface from the narrow bead width and exhibit improved tensile strength at the joints. In this study, a method for optimizing the welding conditions was developed to simulateneously improve two mutually exclusive properties. It is essential to achieve more than 80% penetration of the weld zone to guarantee tensile properties. Laser welding parameters such as power, welding speed and focus height are the main variables. In addition, when high-strength stainless steel is applied as a curtain wall frame, its structural performance improves compared to conventional stainless steel.

This study investigates the relationship between the mechanical characteristics and microstructural transitions in heat-affected zone (HAZ) of various austenitic Fe–Mn–Al–C lightweight steels. The analysis was conducted on the simulated HAZ samples with various post-weld heat treatment (PWHT) conditions. The base steel was prepared via vacuum induction melting, hot rolling, and solution treatment prior to water quenching. Gleeble simulator was used to manufacture the simulated HAZs, and the samples were subjected to PWHT with various durations by electric furnace. To understand the effects of thermal history on the mechanical characteristics, ultimate tensile tests, micro-hardness tests, and cryogenic Charpy impact toughness tests were conducted, and microstructural analyses were conducted by optical microscopy, scanning electron microscopy, X-ray diffraction analysis, and transmission electron microscopy. The experimental results confirmed the influence of Al content on the austenitic Fe–Mn–Al–C lightweight steels, which promotes κ-carbide precipitation, both in the austenite matrix and grain boundaries, thereby increasing the strength and hardness while decreasing the ductility and toughness. Each mechanical property showed a linear relationship with the growth kinetics of the κ-carbide particles. However, abnormal coarsening of the inter-granular κ-carbide during welding and PWHT caused severe embrittlement, regardless of thermal history, by acting as inter-granular crack propagation channels. The experimental results suggest that Fe–Mn–Al–C lightweight steels should be developed with controlling the alloying elements to overcome HAZ discontinuity and mechanical property degradation occurring by intra- and inter-granular κ-carbide precipitation.