Search Results (2,604)

Martensitic chromium-molybdenum steels such as 15Kh5M are widely used in high-temperature oil and gas equipment, but their weldability is limited by high hardenability and susceptibility to cold cracking, which usually necessitate energy-intensive preheating. This study evaluates an alternative route based on the combination of root-pass mechanical vibration (50 Hz, ~1 mm amplitude) and post-pass water-air jet cooling during mechanized GMAW. Three welding variants were compared: conventional preheated welding, vibration-assisted welding without preheating, and hybrid thermo-vibrational welding with active cooling. Among the tested conditions, the hybrid route produced the narrowest heat-affected zone, reducing its width from about 7 mm to about 3 mm, which is consistent with a compressed thermal cycle. Microhardness in the heat-affected zone decreased from 380 to 440 HV in the preheated condition to 330–370 HV in the hybrid condition. Optical microscopy further indicated a finer and more homogeneous transformed microstructure in the hybrid case. Results indicate that simultaneous vibro-treatment and controlled cooling effectively mitigate harmful metallurgical effects typically induced by rapid cooling, enabling preheat-free fabrication of thick-walled components. The proposed hybrid approach may offer energy savings, shorter production cycles, and improved automation compatibility in field welding applications. Full article

A three-factor, four-level orthogonal design was employed to optimize the overall forming quality of powder bed fusion with a laser beam (PBF-LB)-fabricated TC4 alloy containing 0.3 wt.% YH₂. Sixteen process-parameter combinations were established, and two specimens were fabricated for each combination. Laser power, scanning speed, and hatch spacing were selected as the investigated variables. Relative density, surface roughness, and Vickers hardness were evaluated using the entropy weight method combined with the weighted-sum method. On this basis, the microstructure of the specimens produced under the optimal process parameters was systematically characterized. The results showed that the influence of the investigated factors on overall forming quality followed the order: hatch spacing > laser power > scanning speed. The optimal process parameters were a laser power of 200 W, a scanning speed of 1100 mm/s, and a hatch spacing of 0.10 mm, under which the specimens exhibited superior overall forming quality. The addition of 0.3 wt.% YH₂ did not significantly alter the dominant phase constitution of the alloy, but promoted α′ martensite refinement and weakened the texture through the in situ formation of Y₂O₃ nano-oxide particles. Full article

This study investigated the relationship between Mn segregation, damping capacity, and mechanical properties of a Mn–Cu damping alloy after aging at different temperatures. The results showed that after aging, the alloy underwent spinodal decomposition, forming Mn-segregated regions, while α-Mn precipitates appeared at the grain boundaries. The microstructure resulting from spinodal decomposition promoted martensitic transformation, created twin boundaries, and enhanced damping capacity. As the aging temperature increased, the Mn content in the Mn-rich regions gradually rose, thereby raising the martensitic transformation temperature. The twin density first increased and then decreased, which may be attributed to the precipitation and broadening of the α-Mn phase along the grain boundaries of the Mn-rich regions when the aging temperature was too high. At an aging temperature of 425 °C, the tanδ reaches a maximum of 0.05, and the martensitic transformation temperature reaches 100 °C, at which point the tanδ remains 0.04. After aging at 425 °C, a preferred orientation along <001> develops. The [001] orientation has the largest Schmid factor, which is most favorable for the reversible motion of twin boundaries under external stress, thus achieving the highest energy dissipation. To summarize, by promoting the creation of fine {011} twins by means of spinodal decomposition and by increasing the [001] oriented grain fraction through texture development, aging enhances the damping properties of the Mn–Cu alloy. In particular, the aging at 425 °C can provide the best combination of the microstructure and texture conditions, providing the highest damping performance in a wide temperature range. Full article

The present study aims to investigate the interaction between different martensite variants (MVs) activated in an AISI 304 austenite steel subjected to tension. Particular attention is paid to the abnormal morphologies of martensite–martensite interaction (MMI) and their possible formation mechanisms during deformation-induced martensitic transformation. The abnormal morphologies of martensite–martensite interaction (MMI) were characterized. It was revealed that MMI was accompanied by the formation of extremely incoherent interfaces. MVs can continue to grow upon impinging on each other, resulting in the morphology where one MV is crossed or totally surrounded by another. The present findings provide new insight into martensite growth behavior and variant interaction and may contribute to a better understanding of the microstructural origin of the excellent strain-hardening capability and mechanical performance of metastable austenitic steels. Full article

This study investigated the influence of Mn content (70 wt.%, 75 wt.%, and 80 wt.%) on the microstructure, mechanical properties and damping capacity of Mn-Cu alloys using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical testing and dynamic mechanical analysis (DMA). The results indicate that during cooling after aging, the Mn-Cu alloy undergoes martensitic transformation, resulting in a dual-phase structure of fcc and fct. The 70 wt.% Mn alloy exhibits a mixed-grain structure with mostly long, straight twin bands, while the 75 wt.% and 80 wt.% Mn alloys consist of fine equiaxed grains with mostly intersecting twin bands. The microstructure determines the properties of the alloy. As the Mn content increases, the mechanical properties initially increase and then decrease, and the 75 wt.% Mn alloy has the best mechanical performance (UTS = 534 MPa, YS = 263 MPa). In contrast, the damping capacity shows a decreasing trend, and the 70 wt.% Mn alloy exhibits the best damping capacity (tanδ = 0.064). The main damping peak of tanδ in Mn-Cu alloys is derived from the relaxation of the twin boundaries, and the less obvious secondary peak is the internal friction peak of martensitic transformation. Full article

Additive manufacturing (AM) using powder bed fusion (PBF) has been the predominant printing method used over the last decade. The capability of this approach to produce complex parts with high precision has attracted the attention of major industries as a potential tool for replacing traditional manufacturing technologies. 15-5 PH stainless steel is one of the alloys being studied as a candidate for PBF processes. Its superior strength and corrosion resistance have made it a highly attractive option in numerous industries, including the automotive, nuclear, and petrochemical industries. To enhance the properties of 15-5 PH stainless-steel AM parts following printing, one can use a thermal treatment such as age hardening. However, very little research exists regarding the functional properties of AM parts made from this alloy after heat treatment. This study aims to evaluate the effect of H1150M age hardening heat treatment following printing on the properties of 15-5 PH steel, particularly regarding its mechanical properties and environmental behavior. The microstructure was studied using both optical and electron microscopy, along with X-ray diffraction (XRD) analysis. The mechanical properties were examined by tensile testing and fracture toughness assessment. Corrosion behavior was analyzed in terms of potentiodynamic polarization and using impedance spectroscopy. The results obtained have shown that over-aging caused by H1150M heat treatment has a detrimental effect on the mechanical and environmental behavior of the tested alloy. This was primarily attributed to the formation of an austenitic phase within the inherent martensitic matrix, the generation of brittle phases (mainly carbonitrides of Cr and Nb) and a reduction in grain size. Full article

Biomedical Ti–Nb–Zr–Si alloys containing 12 and 18 wt.% Nb were fabricated by electron beam melting and subjected to thermomechanical processing, including forging, cross-helical rolling, and subsequent cooling or quenching. The effects of Nb content and processing route on phase composition, microstructure, and mechanical properties were systematically investigated using X-ray diffraction, scanning electron microscopy, and tensile testing. The results indicate that increasing Nb content promotes stabilization of the metastable α″ phase, leading to a significant reduction in elastic modulus. The Ti–18Nb–4Zr–1Si alloy exhibited a modulus of ~60 GPa after rolling, which further decreased to ~40 GPa after additional quenching. In contrast, the Ti–12Nb–4Zr–1Si alloy showed higher values of 76–94 GPa due to the predominance of the α′ phase. Both alloys demonstrated a favorable combination of strength and ductility. Microstructural analysis revealed the formation of silicides, whose type and morphology depend on Nb content and processing conditions. The Ti–12Nb–4Zr–1Si alloy predominantly contains (Ti,Zr)₅Si₃, whereas the Ti–18Nb–4Zr–1Si alloy exhibits complex silicides composed of (Ti,Zr)₅Si₃ and (Ti,Zr)₃Si phases. These results highlight the potential of controlling phase composition and silicide evolution to tailor mechanical properties, particularly the elastic modulus, for biomedical applications. Full article

This study investigates the effects of tempering temperature on the microstructural evolution and mechanical properties of Cr-Ni-Mo-V steel designed for pressure vessel applications. The microstructure was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM, Thermo Fisher Talos F200X), electron backscatter diffraction (EBSD), and physicochemical phase analysis. Mechanical performance was evaluated through tensile and impact tests, followed by a detailed discussion of the underlying strengthening mechanisms. The results demonstrate that the microstructure after tempering is fully tempered martensite. Samples tempered between 425 °C and 525 °C exhibit significant tempering resistance, maintaining a tensile strength of approximately 1300 MPa. This is primarily attributed to the synergistic effect of dislocation strengthening and the precipitation of MC-type carbides. As the tempering temperature increases to 625 °C, the dislocation density decreases sharply from 3.71 × 10¹¹ cm⁻² to 1.18 × 10¹¹ cm⁻², leading to a decline in strength. Concurrently, the impact energy increases significantly from 71 J to 132 J. The improvement in toughness is mainly attributed to the significant elevation of the crack initiation threshold, which is dominated by the reduction in matrix dislocation density, the coarsening and spheroidization of carbides, and the alleviation of local stress concentration. The relative proportion of high-angle grain boundaries (HAGBs, misorientation > 15°) increases from 51.9% to 57.7% during tempering, which is a result of the massive elimination of low-angle grain boundaries rather than an increase in the absolute length per unit area of HAGBs. Full article

To solve the problems of poor weld formation, difficult slag removal, and inferior joint microstructure and hardness in conventional narrow-gap submerged arc welding (NG-SAW), a swing arc NG-SAW process assisted by pre-embedding cold wires was proposed. Synergistically optimizing the welding energy parameters and additional cold wires ensured sound weld formation and enhanced slag detachability, while the efficiency of multilayer welding was improved by reducing the number of weld layers by 33.3%. The slag adhesion mechanism is clarified as follows: a high welding heat input facilitates elemental diffusion at the weld–slag interface, leading to the formation of a continuous and thick interlayer composed of (Fe,Mn)O and MgO-Al₂O₃-CaO phases. This interlayer strengthens the chemical bonding between slag and weld, thereby impeding slag removal. Microstructure evolution analysis of the multilayer welded joint revealed that the variable-angle design increases the groove volume and, combined with the heat-absorbing effect of the additional wires, accelerates molten pool cooling, thereby refining grains in both the weld metal zone and reheat-affected zone. Meanwhile, the tempering exerted by the heat-affected zone (HAZ) of the subsequent weld layer on the previous layer is attenuated. This promotes the gradual transformation of hard-brittle lath martensite in the coarse-grained heat-affected zone (CGHAZ) of the bottom layer into tougher tempered martensite/bainite in the CGHAZ of the upper layers. As a result, the hardness uniformity within the HAZ, the critical weak region of the joint, was enhanced by 54%, enabling synchronous improvement in microstructural homogeneity, hardness distribution, and overall welding efficiency. Full article

This study investigated the correlation between mechanical strengthening and electrochemical corrosion behavior in 18Ni300 maraging steel fabricated via laser powder bed fusion (LPBF). To evaluate the impact of post-processing, specimens were analyzed under four conditions: solution treated (S), solution peened (SP), solution aged (SA), and solution aged peened (SAP). The aging treatment (490 °C for 6 h) effectively enhanced the corrosion resistance by homogenizing the martensitic matrix and promoting the formation of a stable passive film, resulting in the lowest corrosion current density ($i_{corr}$ of 1.716 × 10⁻⁶ A/cm²). In contrast, the application of shot peening after aging (SAP) significantly degraded the corrosion resistance, characterized by the most negative corrosion potential ($E_{corr}$ of −0.374 V and a 2.4 times increase in $i_{corr}$ compared to the SA condition. Quantitative analysis revealed that the 1250 MPa of compressive residual stress induced by peening increased the thermodynamic instability of the surface through extreme lattice distortion, thereby lowering the activation energy for anodic dissolution. Furthermore, the increased surface roughness (60.68 µm) expanded the effective electrochemical reaction area, acting as a kinetic accelerator for corrosion. The results demonstrate that while the SA process provides an optimal balance between microstructural stability and corrosion resistance, additional shot peening (SAP) imposes a significant corrosion penalty despite its mechanical benefits. This study concludes that for 18Ni300 maraging steel, the trade-off between mechanical reinforcement and electrochemical stability must be carefully managed, emphasizing the need for surface stabilization when high-intensity peening is applied in corrosive environments. Full article

The hole expansion ratio (HER) is a critical formability metric for advanced high-strength steels (AHSS) in automotive applications; however, its experimental determination is costly and time-consuming. This study presents a machine learning framework for HER prediction using physics-informed synthetic data generation to address data scarcity challenges. A dataset of 300 AHSS conditions was generated based on validated empirical relationships from the literature, incorporating chemical composition, microstructure fractions, and mechanical properties. Multiple machine learning algorithms were evaluated, with the optimized Gradient Boosting model achieving excellent predictive performance on an independent test set (R² = 0.80, RMSE = 5.81%, MAE = 4.93%). The feature importance analysis revealed physically meaningful rankings, with the ultimate tensile strength dominating (40.9%), followed by the bainite volume fraction (15.1%), martensite volume fraction (14.7%), and strain hardening exponent (12.4%). These rankings align with the established metallurgical understanding, thereby validating our synthetic data approach. The results demonstrate that machine learning models trained on physics-informed synthetic data can accurately predict the HER values with errors comparable to the experimental variability, providing a practical tool for accelerated AHSS design and optimization in automotive applications. Full article

Pulse repetition frequency (PRF) controls pulse off-time and, therefore, the extent of thermal accumulation, melt expulsion, and dielectric recovery in finishing electrical discharge machining (EDM). This study clarifies how PRF modifies crater evolution and surface integrity in finishing EDM of 4Cr13 martensitic stainless steel, a corrosion-resistant mold steel used in precision dies and molds. A 2D axisymmetric electro-thermo-fluid model was established in COMSOL, where Gaussian current density, heat-flux, and plasma pressure were periodically imposed at PRFs of 25–100 kHz, while pulse-on time (6 μs) and peak current (8 A) were kept constant. The simulations tracked the transient pressure, heat-flux, velocity, and temperature fields over a common elapsed time of 25 μs. Finishing experiments were then carried out on flat 4Cr13 coupons at 50, 75, and 100 kHz using a copper electrode and deionized water, followed by characterization by laser confocal microscopy, SEM/EDS, and X-ray diffraction using the cosα method. Increasing PRF localized the coupled pressure-heat-flow fields near the crater rim, but shortened off-time and intensified inter-pulse heat accumulation. Accordingly, the surface roughness decreased from Ra = 1.18 μm at 50 kHz to 0.63 μm at 75 kHz, and then slightly increased to 0.71 μm at 100 kHz because of crater overlap, re-melting, and incomplete gap recovery. SEM observations confirmed large irregular craters with cracks at 50 kHz, more uniform fine craters at 75 kHz, and overlapping re-solidified traces at 100 kHz. The residual stress remained compressive for all tested conditions (−341 to −409 MPa). Overall, 75 kHz offers the best compromise between crater uniformity, roughness, and compressive stress for finishing EDM of 4Cr13 steel. Full article

The demand for miniaturized high-temperature components necessitates advanced additive manufacturing techniques, yet the microstructural and mechanical consequences of scaling down the laser powder bed fusion (LPBF) process remain poorly understood. In this study, we systematically investigate the scaling effects of micro laser powder bed fusion (μ-LPBF) versus conventional LPBF on the phase transformation kinetics and performance of the near-α Ti65 alloy. Results demonstrate that μ-LPBF significantly enhances surface integrity, reducing the arithmetic mean roughness (Ra) by 59.5%. Microstructural characterization reveals that the extreme cooling rates intrinsic to the microscale melt pool induce a massive refinement of hierarchical α′ martensite and promote a highly randomized variant selection. Consequently, the strong building-direction crystallographic texture typical of LPBF is substantially weakened, and the proportion of high-angle grain boundaries increases to 91.6%. This microstructural homogenization effectively mitigates mechanical anisotropy, reducing the directional variance in the Schmid factor by 35%. In terms of mechanical properties, μ-LPBF demonstrates exceptional strengthening at both room temperature and 600 °C, achieving a room-temperature yield strength of 1297 MPa and an ultimate tensile strength of 1514 MPa, which represent increases of 16.5% and 8.6%, respectively, compared to those of conventional LPBF. These findings provide critical insights into defect suppression and multiscale microstructural control under extreme thermal gradients, paving the way for the fabrication of isotropic, high-strength micro devices. Full article

The dense passivation film (DPF) formed on the surface of martensitic stainless steel effectively improves corrosion resistance, but it also hinders the adsorption and diffusion of active nitrogen atoms during gas nitriding. In this work, the influence of the DPF of 1Cr11Ni2W2MoV stainless steel on gas nitriding was overcome by controlling the cooling rate during stainless steel solution treatment, thereby enabling the successful formation of a nitrided layer. The effects of nitrogen potential on the microstructure, phase constitution, and tribological performance of the nitrided layer were systematically investigated. A dense passivation film formed at a solid-solution cooling rate of 110 ± 5 °C/s effectively inhibited nitrogen diffusion, resulting in the absence of a nitrided layer. However, when the cooling rate during solid solution was reduced to 80 ± 5 °C/s, the precipitation of chromium carbide along the grain boundaries damaged the density and integrity of the DPF, thereby enabling the formation of a nitrided layer during gas nitriding. A high nitrogen potential enhanced nitrogen diffusion and increased the nitrided layer thickness. However, an excessively high nitrogen potential led to nitrogen enrichment along grain boundaries, resulting in microcracking and reduced mechanical integrity of the compound layer. When the nitrogen potential was 1.0, a uniform and crack-free nitrided layer with a surface hardness exceeding 1000 HV_0.1 was obtained. Tribological tests combined with SEM observations of the worn surfaces showed that gas nitriding significantly reduced the friction coefficient and wear rate compared with the matrix sample. Among the nitrided samples, H-10 exhibited the lowest friction coefficient and wear rate, whereas H-23 showed relatively inferior wear resistance due to microcrack-related brittleness. The dominant wear mechanism changed from severe abrasive–adhesive wear in the matrix sample to mild abrasive wear in the nitrided samples. These results indicate that regulating passivation film integrity through heat treatment, together with optimizing nitrogen potential, is an effective strategy for achieving high-quality gas nitriding and improved tribological performance in martensitic stainless steel. Full article