Search Results (1,125)

The main aim of this study is to investigate the microstructure and microhardness of Wire Arc Additive Manufactured (WAAM) samples produced under different layer deposition strategies and corresponding interlayer temperature conditions. Experimental samples were produced using the WAAM process with X45CrSi9-3 steel. During the experiments, both the number of layers and the thermal conditions (heating and cooling) were systematically varied. This was achieved by fabricating samples consisting of five layers with three beads per layer. The layer deposition procedure was implemented in two different ways: (i) with a waiting period after each layer to allow cooling to room temperature, and (ii) without such a waiting period. Thermal cycles at selected locations within the samples were calculated using simulation modeling. By combining these thermal cycles with the continuous cooling transformation (CCT) diagram, the expected microstructures in the vicinity of these locations were determined. These predictions were supplemented by microstructural analysis and hardness measurements. Particular emphasis was placed on the influence of interlayer temperature and repeated heating and cooling cycles. The analyses enabled the identification of process parameters that facilitate control over microstructure, microhardness, and property gradients. It can be concluded that the interlayer holding time provides an effective means of controlling the microstructure of the workpiece, ranging from predominantly austenitic to predominantly martensitic. Depending on the thermal cycles, the measured microhardness varied within the range of 360–900 HV. Metallographic examination revealed a wide spectrum of non-equilibrium microstructures, including martensite with varying degrees of tempering, retained austenite, pearlite, and bainite. The application of a thermal model to the conducted experiments, combined with the CCT diagram, indicated that the expected microstructures consist predominantly of martensite with varying degrees of tempering, retained austenite, carbides, and, in some cases, up to 5% pearlite. Full article

In the present work, eleven self-shielded flux-cored wires with nickel (Ni) contents ranging from 1.42 wt.% to 4.02 wt.% were designed for the semi-automatic welding of X80 pipeline steel. The effects of Ni on the microstructural evolution and mechanical properties of the weld metal were investigated. The results indicate that when the Ni content is below 2.06 wt.%, the microstructures of both the solidification zone and the inter-pass reheating zone are dominated by coarse granular bainite and martensite/austenite (M/A) constituents. As the Ni content increases from 2.06 wt.% to 3.73 wt.%, the microstructure transforms to fine lath bainite with M/A constituents characterized by low content, small size, and uniform distribution. When the Ni content reaches 3.73 wt.%, the microstructure becomes almost fully bainite. Furthermore, with increasing the Ni content, both the yield strength and tensile strength of the weld metal increase from ~600 MPa to ~700 MPa and from ~660 MPa to ~730 MPa, respectively. However, the impact energy at −20 °C of the weld metal initially increases and then decreases, reaching a peak of ~110 J with the lowest degree of dispersion at a Ni content of approximately 3.73 wt.%. When the Ni content exceeds 3.73 wt.%, the ductility decreases slightly. Further analyses indicate that the synergistic effects of Ni in refining the microstructure and reducing the activity coefficient and solubility of nitrogen (N) jointly contribute to the impact toughness of the weld metal. Full article

This study employs molecular dynamics simulations to investigate hydrogen diffusion and deformation mechanisms in FeCrNi-based austenitic stainless steels, with a focus on the effects of alloying composition, temperature, and hydrogen concentration. Arrhenius analysis reveals that Cr increases, while Ni decreases, the activation energy for hydrogen migration. Alloys with low Cr and Ni contents (6 wt.%) promote FCC→BCC→HCP martensitic transformations, accompanied by stress drops, whereas high Cr or Ni levels (24 wt.%) suppress these transformations and favour dislocation plasticity dominated by cross-slip. High hydrogen concentrations reduce stacking-fault energy, activating dense Shockley partial dislocations in agreement with hydrogen-enhanced localised plasticity. Elevated temperatures and high hydrogen concentrations synergistically promote dislocation-mediated plasticity and facilitate vacancy formation, which can cluster into hydrogen–vacancy complexes and proto-nanovoids, accelerating material failure. These findings advance our understanding of the coupled effects of composition, hydrogen, and temperature on degradation in austenitic stainless steels and provide guidance for tailoring Cr/Ni ratios, controlling hydrogen content, and optimising service temperatures in the design of hydrogen-related structural alloys. Full article

The exploitation of advanced high-strength steels in cold climates requires a deep understanding of their structural stability and mechanical reliability. This study investigates the mechanical response of a 0.45C-1.57Si-2.61Mn (wt.%) steel with nanobainite microstructure to moderate sub-zero exposure (SZE) followed by stress-relief tempering. It was found that SZE at –25 °C and –50 °C induces a progressive degradation of tensile properties and impact toughness that persists after rewarming to room temperature. This deterioration is primarily driven by the accumulation of residual stresses due to thermal expansion mismatch between phases, with only minor contributions from athermal martensitic transformation of retained austenite. Notably, stress-relief tempering at 220 °C effectively restores the tensile performance and doubles the impact toughness compared to the as-austempered condition. Despite Mn and Si segregation which caused a scatter in retained austenite content (13.6–21.5 vol.%), the austenite remains stable throughout the SZE and tempering cycles. These results identify a critical threshold (between 0 °C and –25 °C) for SZE-induced degradation in nanobainitic steels and demonstrate that stress-relief tempering is essential for enhancing their performance in cold-climate applications. Full article

Hot deformation effectively refines the microstructure and homogenizes the composition of high-nitrogen martensitic stainless steel (HNMSS), but its influence on austenite stability during subsequent cooling remains unclear. In this study, the effect of the hot deformation strain on austenite stability in HNMSS 30Cr15Mo1N0.37 was investigated by means of a Gleeble thermomechanical simulator, X-ray diffraction (XRD), electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM). The austenite stability is evaluated by the austenite fraction measured via XRD at room temperature. The results show that the austenite content in HNMSS 30Cr15Mo1N0.37 gradually increases with the strain range from 0 to 0.8. The austenite fractions are 69.5%, 73.1%, and 80.7% when the strains are 0, 0.4, and 0.8, respectively. At a strain of 0.14, dislocation accumulation leads to the formation of dislocation cells and sub-grains within austenite, which enhances its stability. When the strain exceeds 0.36, the austenite grains are significantly refined, the austenite stability is attributed to the synergistic effects of dislocation accumulation and grain refinement, which collectively increase the resistance to martensitic transformation. Furthermore, both recrystallized grains and dislocation cells influence the morphology and size of martensite laths. The martensite laths are significantly refined from 100 nm at a strain of 0 to 35 nm as the strain reaches 0.8, and their morphology changes from straight to curved. Full article

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

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

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

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

In this work, the effect of partial to complete Al substitution of Si on the microstructure, retained austenite (RA) stability, and mechanical properties of cold-rolled TRIP-aided steels was investigated. Four experimental TRIP-aided steels (Fe-0.2C-1.5Mn-1.5/1.0/0.5/0Si-0/0.5/1.0/1.5Al-0.025Nb, wt.%) were designed. The results indicate that replacing Si with Al significantly increases the volume fraction of soft polygonal ferrite (from 52% to 73%) and decreases that of bainite. Although the volume fraction of RA decreases (from 15.6% to 12.4%), its average carbon content and, consequently, its mechanical stability are enhanced, which suppresses the strain-induced martensitic transformation. In terms of mechanical properties, the substitution leads to a monotonic decrease in both yield strength (from 573 MPa to 536 MPa) and ultimate tensile strength (UTS) (from 839 MPa to 648 MPa), primarily due to reduced solid-solution strengthening, coarsened ferrite grains, and a weakened TRIP effect. Conversely, the total elongation (TEL) increases from 28.3% to 32.4%, attributed to the higher fraction of ductile ferrite. Consequently, the product of tensile strength and total elongation (PSE) exhibits a slight decline. The 1.5Si-TRIP steel exhibited the most balanced mechanical properties, achieving the highest PSE of 23.7 GPa·%. Full article

A microalloyed high-carbon low-alloy steel was designed to clarify the combined effects of austenitizing temperature and deep cryogenic treatment (DCT) on microstructural evolution and mechanical performance. Specimens were austenitized at 770–900 °C, water-quenched, subjected to DCT at −196 °C, and subsequently tempered at 180 °C. Microstructural characterization by XRD, EBSD, and TEM indicates that the quenched microstructure is dominated by martensite and cementite, with retained austenite below 1% at moderate austenitizing temperatures. DCT does not fundamentally alter the martensitic morphology but promotes the transformation of retained austenite and induces substructure fragmentation, dislocation reorganization, and a more homogeneous lattice strain distribution. Concurrently, carbon redistribution during cryogenic exposure facilitates the formation of finely dispersed carbides. After tempering, partial recovery and stabilization of the martensitic substructure lead to reduced lattice distortion while maintaining a high density of effective strengthening features. Mechanical testing shows that DCT combined with appropriate austenitizing (770–790 °C) improves hardness and ultimate tensile strength with acceptable ductility, whereas excessive austenitizing at 900 °C results in severe grain coarsening and intergranular brittle fracture. The results demonstrate that optimized integration of microalloying and DCT enables a favorable strength–toughness balance in high-carbon tool steels. Full article

The behavior of shape memory alloy (SMA) materials is more complex than linear isotropic metals because of their nonlinear thermomechanical coupling. When an SMA material is mechanically stressed or joule-heated, phase transformation happens in the material, and accordingly some material properties dramatically change. In any loading or unloading scenario, the initial state of the material should be known because it significantly affects its behavior. Stress and strain alone are not enough to describe such materials. Temperature and martensitic fraction are also required to simulate SMA materials accurately. This paper presents a MATLAB application, called “SMA Simulator,” that was developed to simulate the nonlinear behavior of SMA wires under mechanical or thermal loads. This tool is very effective in helping users understand the shape memory and pseudoelastic effects in such smart materials, as it allows for plotting the loading path in the 3D stress–strain–temperature space while monitoring the evolution of the martensitic fraction. Any load–unload scenario can be studied, including multiple consecutive partial loading cycles. Since the application is not based on any numerical method that would require extensive meshing, the computational time is minimal, allowing users to perform more simulations and acquire results instantaneously. Full article