Search Results (1,096)

Hydrogen-induced cracking (HIC) is a possible failure mode of high-strength pipeline steel welded joints. The HIC susceptibility of shielded metal arc-welded joint of X100 pipeline steel was investigated using the slow strain rate tensile test (SSRT) under current densities of 2 mA/cm², 5 mA/cm², 10 mA/cm², 20 mA/cm², and 30 mA/cm². Hydrogen diffusion behavior, distribution of microstructures, and inclusions in the welded joint were also surveyed and characterized. The SSRT results indicated that the welded joint exhibited a higher hydrogen embrittlement index than the base metal (BM) under the same current density, showing a higher HIC sensitivity than the BM. The weld metal (WM) showed a lower hydrogen diffusivity (5.38 × 10⁻⁵ cm²·s⁻¹) than the BM (6.23 × 10⁻⁵ cm²·s⁻¹). The martensite–austenite (M-A) constituent area fraction in the WM was higher than that in the coarse-grained heat-affected zone (CGHAZ), fine-grained HAZ (FGHAZ), and BM. The inclusion amount in the WM was larger than that in the BM. In addition, the high HIC vulnerability of the X100 steel welded joint was also associated with a coarse grain size in the WM. Full article

Fe-Mn-Al-C lightweight high-strength steels are promising for aerospace and defense applications, but selective laser melting (SLM) introduces steep thermal gradients and rapid solidification, causing directional grain growth, crystallographic texture, and the non-uniform precipitation of κ-carbides and B2 phase at grain boundaries. This results in pronounced mechanical anisotropy between XY and YZ planes, limiting engineering use. To eliminate this anisotropy, we investigate the post-SLM solution treatment of an SLM-fabricated Fe-Mn-Al-C steel at 1050–1150 °C for 0.5–1.5 h followed by oil quenching, and characterize microstructures and tensile properties on both planes. At 1050 °C, the XY plane remained equiaxed γ-austenite, while the YZ plane transformed to α and became equiaxed over time, causing strength–ductility anisotropy. At 1100 °C for 1 h, anisotropy was effectively removed: XY and YZ planes exhibited tensile strengths of ~1159 and 1154 MPa and elongations of ~40% and 41%. TEM revealed that uniform fine κ-carbides and coarsened B2 at grain boundaries suppressed direction-dependent strain. At 1150 °C, dissolved boundary phases and diffuse intragranular κ-carbides severely reduced ductility. The optimal treatment is 1100 °C for 1 h, yielding a homogeneous microstructure and excellent isotropic properties. Full article

The influence of different heat treatment processes on the microstructure and mechanical properties of 18Ni350 maraging steel manufactured by selective laser melting and the corrosion resistance of TiB₂ ceramic coatings Electro-Spark-Deposited on its surface when immersed in high-temperature molten aluminum have been investigated in the present study. The microstructures and mechanical properties of the differently heat-treated samples were analyzed using various precision instruments. The results reveal that the as-built sample exhibits a microstructure composed of cellular and columnar dendritic grains. After solution treatment, the microstructure fully transforms into lath-like martensite. After direct aging treatment, the cellular structures diminish, while precipitates at grain boundaries proliferate with increasing aging temperature. SAT- treatment achieves full microstructural homogenization, featuring fine-lath martensite and a small amount of randomly distributed austenite particles. DA- and SAT- significantly improve the strength, hardness and modulus of samples and were found to reduce the toughness and plasticity. After solution treatment at 800 °C for 1 h followed by aging treatment at 520 °C for 6 h (SAT 800-520), the specimen achieved an UTS of 2476 MPa while maintaining an EL of 4.6%. The TiB₂ coating and the Cr interlayer deposited via ESD form a continuous interfacial bond with the substrate, demonstrating favorable adhesion. After 4 h of static immersion in high-temperature molten aluminum, the coating remains intact without complete delamination, delivering effective protection to the substrate. Full article

The study systematically investigates the effects of tempering temperatures ranging from 610 °C to 690 °C on the multi-scale martensitic microstructure evolution and strength–toughness matching characteristics of 165 ksi grade Cr–Mo–V martensitic steel for ultra-high-strength and high-toughness oil drill pipes. The intrinsic strengthening and toughening mechanisms of the developed steel were further clarified. It was established that an increase in the tempering temperature resulted in a reduction in strength that was found to be monotonic. Concurrently, an enhancement in ductility and low-temperature toughness was observed. The yield strength (YS) and ultimate tensile strength (UTS) decrease monotonically from 1338 MPa and 1397 MPa to 819 MPa and 891 MPa, respectively. Meanwhile, the −20 °C low-temperature impact absorbed energy rises significantly from 32 J to 148 J, and the fracture elongation increases from 16% to 21%. Combined multi-scale microstructural characterization via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD) demonstrates that the prior austenite grain size remains nearly unchanged during tempering. In contrast, the hierarchical martensitic substructures, including packets, blocks and laths, undergo continuous coalescence and coarsening. The matrix dislocation density drops sharply from 3.02 × 10¹⁵ m⁻² to 1.20 × 10¹⁵ m⁻². The gradual relaxation of internal lattice strain reduces the kernel average misorientation (KAM) value from 0.32° to 0.21°, and the nano-scale rod-shaped precipitates gradually transform into coarsened spherical carbides. Quantitative analysis of various strengthening mechanisms reveals that grain refinement strengthening and dislocation strengthening serve as the dominant strengthening contributors to the superior strength of the steel, while solid solution strengthening and precipitation strengthening play auxiliary roles. The remarkable improvement in low-temperature impact toughness is primarily attributed to the substantial increase in crack propagation energy dissipation. Full article

Quenched and tempered (Q+T) bolt steels are widely used in key load-bearing structures such as bridges, wind power equipment, pressure vessels and engineering machinery, but they are susceptible to hydrogen-induced cracking under applied stress during service. In this study, a bolt steel was subjected to Q+T heat treatment, including quenching at 850, 900, 950, 1000 and 1050 °C, followed by tempering at 500 °C. Microstructural characterization, hydrogen permeation tests, and slow strain rate tensile tests were conducted to investigate the effects of quenching temperature on microstructural evolution, hydrogen diffusion behavior and resistance to hydrogen-assisted cracking. As the quenching temperature increased from 850 °C to 1050 °C, the prior austenite grains, packets and blocks were gradually coarsened, the fraction of high-angle grain boundaries decreased from 64.7% to 54.2%, and although partial dissolution of primary carbides may occur during austenitizing, the number/area fraction and size of carbides observed in the final tempered martensitic microstructure increased after the subsequent tempering treatment. Meanwhile, the Nb/Ti-rich precipitates changed only slightly, and the dislocation density increased. The effective hydrogen diffusion coefficient, D_eff, increased with increasing quenching temperature, mainly because grain coarsening significantly reduced the high-angle grain boundary area and weakened the hydrogen-trapping effect of grain boundaries. This dominant effect masked the diffusion-retarding effects caused by increased dislocation density and coarser carbides. With increasing quenching temperature, the strength loss ratio increased from 7.3% to 12.0%, and the plasticity loss ratio increased from 10.0% to 13.6%, indicating enhanced hydrogen-assisted cracking susceptibility. The fracture morphology gradually changed from deep dimples to flat dimples and flattened ductile–brittle mixed features, while the crack propagation path became straighter. A higher quenching temperature weakened the blocking effect of grain boundaries on crack propagation and reduced the resistance of the quenched and tempered bolt steel to hydrogen-assisted cracking. Full article

Laser-welded stainless steel joints usually suffer from strain localization and premature failure in the weld metal (WM) due to microstructural heterogeneity introduced during welding. In this work, surface mechanical rolling treatment (SMRT) was applied to laser-welded 304 stainless steel plates to enhance the mechanical performance of the welded joints. Laser welding introduced multiple heterogeneous features in the WM, including local Ni compositional fluctuations, nanoscale oxide particles and heterogeneous grain structures. Among them, the local fluctuation of Ni concentration is considered to play a dominant role by locally modifying the stability of γ-austenite and promoting strain-induced martensitic transformation during SMRT. As a result, the WM exhibited more severe grain refinement and a stronger gradient nanostructure than base metal (BM) under identical processing conditions. The near-surface hardness of the WM reached ~500 Hv, which was noticeably higher than that of the BM. Uniaxial tensile tests revealed that the yield strength increased from ~350 MPa to ~700 MPa, while the ultimate tensile strength reached ~1000 MPa with ~40% elongation. More importantly, the fracture location shifted from the WM to the BM after SMRT. The enhanced martensitic transformation and gradient nanostructure effectively suppressed strain localization and improved the mechanical reliability of the welded joint. Full article

Compression tests over a wide range of strain rates were performed on cold-rolled and annealed, as well as annealed-and-aged, Fe-26Mn-8Al-1.2C-3Mo steel to elucidate the roles of Mo₂C precipitation and κ/Mo₂C co-precipitation in microstructural evolution and compressive behavior. The annealed microstructure consists of partially deformed and equiaxed ultrafine recrystallized austenite grains, with Mo₂C carbides uniformly dispersed throughout the matrix. Aging at 550 °C induces nanoscale spherical κ carbides, while the size and spacing of Mo₂C particles remain essentially unchanged. Both conditions exhibit pronounced strain-rate strengthening, primarily attributed to intensified dislocation–carbide interactions. In the annealed state, deformation is dominated by dislocation bypassing of Mo₂C carbides, resulting in discontinuous slip microbands. After aging, κ-carbide precipitation facilitates slip-band propagation and promotes interactions among adjacent slip bands. As the strain rate increases from 10⁻³ to 10⁰ s⁻¹, dislocation density increases, slip-band propagation is hindered, and the strain-hardening rate decreases. At 10³ s⁻¹, adiabatic thermal softening becomes significant, leading to a further reduction in strain hardening. Overall, aging increases strength but reduces strain-rate sensitivity in the low strain-rate regime due to κ-carbide-induced slip-plane softening and an increased effective slip distance. Full article

This study examines the influence of powder characteristics and laser powder bed fusion (LPBF) processing parameters on the microstructure and mechanical performance of ASTM F139 stainless steel fabricated from powders supplied by two manufacturers. Feedstock powders were characterized with respect to chemical composition, particle size distribution, morphology, density, and flowability. Cubic and tensile specimens were produced using different combinations of laser power, scan speed, hatch spacing, and scanning strategy. The fabricated components were evaluated by density and porosity measurements, surface roughness analysis, optical and electron microscopy, hardness testing, and tensile characterization in both horizontal and vertical build orientations. Powder flowability and packing density were found to strongly influence consolidation behaviour, with improved flow characteristics promoting higher densification and reduced porosity. Scanning strategy also affected defect formation, and a 67° interlayer rotation produced lower porosity than the conventional 0°/90° pattern. An optimal processing window was identified at a laser power of 212 W, scan speed of 1600 mm s⁻¹, hatch spacing of 0.07 mm, and layer thickness of 30 μm, yielding components with ~1% porosity, surface roughness below 15 μm, and a density of 7.65 g cm⁻³ (>95% of the theoretical density). Under these conditions, horizontally built specimens exhibited an ultimate tensile strength of 612 ± 43 MPa and a yield strength of 544 ± 37 MPa, exceeding the corresponding values obtained for vertically built specimens. Microstructural characterization revealed a refined cellular austenitic structure associated with epitaxial grain growth during solidification, while fractographic analysis indicated predominantly ductile failure through microvoid coalescence. The results establish clear process–structure–property relationships in LPBF-fabricated ASTM F139 stainless steel and demonstrate that the combined optimization of powder quality, scan strategy, and energy input enables the production of near-full-density components. Full article

Temper embrittlement (TE) degrades the toughness of high-strength Ni-Cr-Mo steels, typically driven by competing mechanisms of impurity segregation and carbide precipitation. To decouple these effects, this study investigates the influence of prior austenite grain size (PAGS) on TE kinetics in a low-phosphorus (0.0038 wt.%) steel. Varying PAGS microstructures were subjected to isothermal aging and characterized using impact testing and atom probe tomography (APT). APT confirmed negligible phosphorus segregation, proving TE is driven primarily by M₂₃C₆ carbide precipitation. Compositional profiling revealed that carbide growth is kinetically governed by chromium (Cr) diffusion. Kinetic modeling via the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation demonstrated that coarse-grained steel exhibits a higher initial embrittlement rate due to continuous intergranular carbide networks that facilitate crack propagation. Conversely, fine-grained structures promote discontinuous precipitation, delaying early-stage embrittlement, although both microstructures reach comparable degradation after prolonged exposure. By isolating precipitation kinetics from impurity effects, this research demonstrates that PAGS critically dictates the rate of carbide-driven TE, providing predictive insights for the microstructural design and lifetime optimization of high-strength structural alloys. Full article

The optimization and the engineering development of additive manufacturing (AM) products both require accurate, non-destructive techniques to extract their mechanical performances. The Instrumented Indentation Test (IIT) has such a potential, although it currently lacks standard procedures that are suitable for analyzing materials which are affected by internal residual stress (RS). Additionally, nanoindentation testing suffers from the presence of indentation size effects (ISE), which hamper the possibility of correlating the measured mechanical performance at different indentation depths or peak loads using the standard indentation hardness (H_IT) and modulus (E_IT). This paper presents a novel IIT methodology that is based on new indentation parameters, namely the loading stiffness rate (LSR) and the rate-derived hardness (H_R), which are then used to assign the desired mechanical performances of an L-PBF 316L austenitic stainless-steel alloy obtained via multiload/multiscale IIT strategy. The mean values of LSR, H_R, H_IT, and E_IT on the macroscale were 57.3 ± 1.4 GPa, 2.33 ± 0.059 GPa, 2.41 ± 0.13 GPa, and 201 ± 7.8 GPa, respectively, whereas on the nanoscale they were 56.1 ± 5.1 GPa, 2.30 ± 0.21 GPa, 3.00 ± 0.36 GPa, and 219 ± 24 GPa, respectively. Unlike the standard H_IT, the new indentation parameters of the nano- and macro-IITs are within the standard deviation, proving their ISE-free property. The obtained E_IT was slightly higher than the reference Young’s modulus (~190 GPa) of the 316L stainless steel. The loading secant stiffness versus depth plot can be used to assess the susceptibility of RS to relax during indentation, which is an important performance factor for the engineering design of AM components. The successful correlation that has been found between electron backscatter diffraction (EBSD) analysis (in terms of crystal anisotropy, grain size, and dislocation density) and nanoindentation testing at three subregions of the core zone of the investigated deposit confirms the validity of the proposed methodology. The proposed methodology is a step towards the full determination of the three Ps, that is, process, properties, and performance of advanced AM products. Full article

In the present work, four grades of austenitic stainless steels, namely AISI 321, AISI 316Ti, AISI 309, and AISI 310S, are investigated in terms of electrochemical hydrogenation effect on their room-temperature impact toughness. All the materials were studied in their as-received (AR), i.e., industrially manufactured, material condition. LOM and SEM microstructural analyses combined with phase XRD and EBSD phase analyses revealed in all steels the polygonal-grain austenitic matrix and varying minor amounts of elongated δ-ferrite grains. Moreover, the metastable AISI 321 and AISI 316Ti steels exhibited noticeable occurrence (16% and 10%, respectively) of the BCC-structured phases (i.e., the strain-induced α′-martensite and non-equilibrium δ-ferrite) and little occurrence of primary TiN nitrides (below 1%). The AISI 321 and AISI 316Ti steels exhibited average amounts of 2.95% and 6.32% of δ-ferrite, respectively. The stable AISI 309 steel exhibited the occurrence of intergranular (Cr,Fe)₂₃(C,N)₆ precipitates (below 3%), indicative of prolonged (slow) cooling from the warm working temperature during the material manufacturing. The individual steel grades exhibited variable values of hardness and impact toughness depending strongly on their solid solution alloying and the amounts of individual minor phases in their microstructures. The AISI 316Ti steel exhibited the highest average hardness (273 HV) and lowest impact toughness (160 J/cm²) due to Mo-alloying and having the highest amount of δ-ferrite. The AISI 310S steel showed the highest impact toughness (210 J/cm²) and the second highest hardness (245 HV) thanks to having the most stable austenitic microstructure with the highest Ni- and Cr-alloying. The AISI 321 and AISI 309 steels show similarly low hardness (195 HV vs. 196 HV) and medium values of impact toughness (202 J/cm² vs. 193 J/cm²). More importantly, all the steels under investigation exhibited detectable hydrogen-induced toughening effects, indicated by the negative HEI values. The metastable steels showed the lowest toughening effects (HEI: −2.0% and −3.8% for AISI 321 and AISI 316Ti, respectively), likely due to the adverse effect of α′-martensite. In contrast, the stable steels exhibited much higher toughening (HEI: −5.2% and −7.6% for AISI 309 and AISI 310S, respectively). Microstructural observations indicated that such toughening behavior might be related to the hydrogen-enhanced deformation banding and hydrogen-enhanced deformation twinning mechanisms, dividing the grains into smaller deformation zones, increasing the overall dissipation of deformation energy and consequently the materials’ impact toughness. Full article

Fe-Cr-C-based coatings were fabricated on Q690 steel via ultrasonic vibration-assisted plasma cladding at varying ultrasonic powers (0–65 W) with a fixed frequency of 18.5 kHz. The coatings primarily consisted of martensite, retained austenite, and (Cr,Fe)₇C₃ carbides, along with (Cr,Fe,Mo)-B borides along grain boundaries. Increasing ultrasonic power promoted cavitation and acoustic streaming, which refined columnar dendrites, reduced elemental segregation (notably for B and Mo), and increased the fraction of fine equiaxed grains without altering phase composition. As a result, the average microhardness increased from 797.1 to 828.5 HV0.1. The friction coefficient decreased from 0.675 to 0.626, while the wear-track width, wear depth, and wear mass loss decreased from 4.0 mm to 2.5 mm, from 112.5 μm to 32.4 μm, and from 20.40 mg to 4.75 mg, respectively. The wear mechanism shifted from severe adhesive wear to mild abrasive wear. These results demonstrate that increasing ultrasonic vibration power effectively refines the solidification microstructure and significantly improves the hardness and wear resistance of plasma-clad Fe-Cr-C-based coatings. Full article

This study aims to investigate the effects of heterogeneous microstructure and strain rate on the microstructural evolution and mechanical properties of ultrafine-grained (UFG) austenitic stainless steel during elevated-temperature tension. In this research, 17Cr-10Ni austenitic stainless steel was rolled to a 60% reduction in thickness at room temperature and 200 °C, followed by annealing at 1000 °C and 500 °C, respectively. The microstructural evolution of the annealed samples and high-temperature tensile specimens was characterized using optical microscopy, transmission electron microscopy, scanning electron microscopy equipped with electron backscatter diffraction, and X-ray diffraction. Results show that at room temperature, the heterogeneous twinned UFG (TW-UFG) sample, influenced by hetero-deformation-induced stress strengthening, maintains good ductility while exhibiting higher strength than the uniform UFG sample. During tensile deformation at 600 °C, grain refinement still contributes to strengthening, and the dominant deformation mechanism in the uniform UFG sample is dislocation dynamic recovery, whereas in the TW-UFG sample is detwinning combined with dynamic dislocation recovery. At low strain rates (10⁻⁴ s⁻¹), sufficient dynamic recovery and detwinning in the TW-UFG sample delay plastic instability and improve elongation. Full article

Hot cracking in welding is a very complex phenomenon. It can happen in the weld metal zone during solidification but also in the heat-affected zone (HAZ). Hot cracking defects are material decohesion that occur at high temperatures along grain boundaries when the strain and strain rate exceed a certain level. The cracks can be internal or open to the surface in the weld bead. During a welding operation, different types of hot cracks can appear, such as hot cracking due to solidification, hot cracking due to liquation, hot cracking due to loss of ductility. The main factors favoring hot solidification cracking include the presence of residual elements and impurities, leading to the formation of a low-melting eutectic; the solidification mode; and mechanical restraints. This review paper gives an introduction to solidification cracking in stainless-steel welds, the weldability of the austenite grades, and the causes of solidification cracking occurrence. The main methods with which to detect and inspect cracks are investigated. Particular focus is placed on TIG (tungsten inert gas), also known as Gas Tungsten Arc Welding (GTAW). A review of the literature reveals that considerable progress has been made in terms of the improvement in the properties of the weld joint through the application of mitigation means and strategies. The effort made by researchers in understanding solidification cracking phenomena has been key to enhancing cracking resistance and ensuring the integrity of structures. Full article

High-temperature low-pressure carburizing significantly reduces the time required to produce carburized layers. However, its application promotes austenite grain growth and, consequently, the formation of coarse acicular martensite. In this study, the possibility of limiting this phenomenon in AMS 6265 and 18CrNiMo7-6 steels using the PreNitLPC^® technology was evaluated. The process was carried out at 1050 °C, with pre-nitriding applied during charge heating. In both steels, comparable carbon concentration profiles and carburized layers with an effective case depth of approximately 1.0 mm were obtained. The introduction of nitrogen into the surface layer resulted in a local reduction in austenite grain growth compared with the core. The average grain size in the surface layer was approximately 14.5 µm for AMS 6265 steel and 12.5 µm for 18CrNiMo7-6 steel, whereas in the core it increased to approximately 25.1 µm and 24.1 µm, respectively. At the same time, AMS 6265 steel exhibited a higher fraction of retained austenite, approximately 20%, compared with approximately 15% for 18CrNiMo7-6 steel. This resulted in a lower near-surface hardness of AMS 6265 steel, approximately 750 HV0.1, compared with approximately 800 HV0.1 for 18CrNiMo7-6 steel, corresponding to a hardness difference of about 50 HV0.1. TEM/NBD/EDS investigations showed that nanoscale AlN precipitates formed in both steels and acted as the main factor inhibiting austenite grain-boundary migration. The results confirm that the PreNitLPC^® technology enables high-temperature low-pressure carburizing without detrimental grain growth in the surface layer. Full article