Search Results (160)

This study systematically evaluates the influence of copper (Cu) addition in gas-shielded solid wires on the microstructure and cryogenic toughness of X80 pipeline steel welds. Welds were fabricated using solid wires with varying Cu contents (0.13–0.34 wt.%) under identical gas metal arc welding (GMAW) parameters. The mechanical capacities were assessed via tensile testing, Charpy V-notch impact tests at −20 °C and Vickers hardness measurements. Microstructural evolution was characterized through optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Key findings reveal that increasing the Cu content from 0.13 wt.% to 0.34 wt.% reduces the volume percentage of acicular ferrite (AF) in the weld metal by approximately 20%, accompanied by a significant decline in cryogenic toughness, with the average impact energy decreasing from 221.08 J to 151.59 J. Mechanistic analysis demonstrates that the trace increase in the Cu element. The phase transition temperature and inclusions is not significant but can refine the prior austenite grain size of the weld, so that the total surface area of the grain boundary increases, and the surface area of the inclusions within the grain is relatively small, resulting in the nucleation of acicular ferrite within the grain being weak. This microstructural transition lowers the critical crack size and diminishes the density for high-angle grain boundaries (HAGBs > 45°), which weakens crack deflection capability. Consequently, the crack propagation angle decreases from 54.73° to 45°, substantially reducing the energy required for stable crack growth and deteriorating low-temperature toughness. Full article

This study focuses on S32654 super austenitic stainless steel (SASS) and systematically characterizes the morphology of the sigma (σ) phase and the segregation behavior of alloying elements in its as-cast microstructure. High-temperature confocal scanning laser microscopy (HT-CSLM) was employed to investigate the effect of the rare earth element yttrium (Y) on the solidification microstructure and σ phase precipitation behavior of SASS. The results show that the microstructure of SASS consists of austenite dendrites and interdendritic eutectoid structures. The eutectoid structures mainly comprise the σ phase and the γ₂ phase, exhibiting lamellar or honeycomb-like morphologies. Regarding elemental distribution, molybdenum displays a “concave” distribution pattern within the dendrites, with lower concentrations at the center and higher concentrations at the sides; when Mo locally exceeds beyond a certain threshold, it easily induces the formation of eutectoid structures. Mo is the most significant segregating element, with a segregation ratio as high as 1.69. The formation mechanism of the σ phase is attributed to the solid-state phase transformation of austenite (γ → γ₂ + σ). In the late stages of solidification, the concentration of chromium and Mo in the residual liquid phase increases, and due to insufficient diffusion, there are significant compositional differences between the interdendritic regions and the matrix. The enriched Cr and Mo cause the interdendritic austenite to become supersaturated, leading to solid-state phase transformation during subsequent cooling, thereby promoting σ phase precipitation. The overall phase transformation process can be summarized as L → L + γ → γ → γ + γ₂ + σ. Y microalloying has a significant influence on the solidification process. The addition of Y increases the nucleation temperature of austenite, raises nucleation density, and refines the solidification microstructure. However, Y addition also leads to an increased amount of eutectoid structures. This is primarily because Y broadens the solidification temperature range of the alloy and prolongs grain growth perio, which aggravates the microsegregation of elements such as Cr and Mo. Moreover, Y raises the initial precipitation temperature of the σ phase and enhances atomic diffusion during solidification, further promoting σ phase precipitation during the subsequent eutectoid transformation. Full article

The prediction of microstructure evolution during thermal processing plays a crucial role in tailoring the mechanical properties of metallic components. Therefore, this work presents a comprehensive, multiscale modelling approach to simulating phase transformations in large-diameter steel rings during cooling. A finite-element-based thermal model was first used to simulate transient temperature distributions in a large-diameter ring under different cooling conditions, including air and water quenching. These thermal histories were subsequently employed in two complementary phase transformation models of different levels of complexity. The Avrami model provides a first-order approximation of the evolution of phase volume fractions, while a complex full-field cellular automata approach explicitly simulates the nucleation and growth of ferrite grains at the microstructural level, incorporating local kinetics and microstructural heterogeneities. The results highlight the sensitivity of final grain morphology to local cooling rates within the ring and initial austenite grain sizes. Simulations demonstrated the formation of heterogeneous microstructures, particularly pronounced in the ring’s surface region, due to sharp thermal gradients. This approach offers valuable insights for optimising heat treatment conditions to obtain high-quality large-diameter ring products. Full article

In this study, we have explored the thermomechanical processing on 0.1C3Mn steel to produce an ultrafine-grained (UFG) dual-phase (DP) microstructure. The composition was designed to allow a decrease in temperature for the warm deformation of austenite. It was found that the warm deformation of austenite induced a dramatic ferrite transformation, in contrast to the absence of the formation of ferrite in the well-annealed state. Compression by 60% at 650 °C resulted in the generation of a UFG-DP microstructure with a ferrite grain size of 1.4 μm and a ferrite volume fraction of 62%. The UFG-DP 0.1C3Mn steel presents a good combination of strength, ductility and fracture resistance, and the fracture strain of the UFG-DP is higher than the as-quenched low-carbon martensite. The high fracture strain of the UFG-DP could be attributed to delayed void nucleation and constrained void growth, as revealed by the quantitative X-ray tomography. Full article

The martensitic transformation mechanism in the heat-affected zone of DP1180 steel plays a decisive role in the strength of welded joints. In this work, the nucleation and growth kinetics of martensite laths in the coarse grain heat-affected zone (CGHAZ) are analyzed by a high-temperature laser scanning confocal microscope (LSCM). The grain distribution and stress distribution of the samples after in situ observation are analyzed by electron backscatter diffraction (EBSD). The results reveal that austenite grain growth is realized by continuous grain boundary annexation and grain boundary migration of small grains by large grains during the heating process. Seven growth modes of CGHAZ martensitic laths under laser welding conditions are proposed. Additionally, the end growth of martensitic laths is mostly attributed to the collision with grain boundaries or other laths to form a complex interlocking structure. The results of this study could provide important data support for the development of dual-phase steel materials and improvement of welding quality. Full article

To explore the impact of niobium (Nb) addition on the austenitization behavior of Fe-Mn-Al-C lightweight steels, the effects were examined through Thermo-Calc thermodynamic simulations, optical microscopy, transmission electron microscopy (TEM), and the development of austenite grain growth models. Three distinct Fe-Mn-Al-C steel compositions, each containing different Nb contents (0.38%, and 0.56%), were subjected to various austenitization temperatures and aging conditions, and a kinetic model for austenite grain growth was established. The results demonstrate that for heating temperatures below 950 °C, the austenite grain growth rate of the steels was similar. However, at temperatures above 950 °C, the grain growth rate of the steel without Nb (Steel No. 1) increased significantly compared to the niobium-containing alloys. Austenite grain size increased with higher heating temperatures. At constant heating temperatures, longer holding times resulted in larger grain sizes, though the rate of grain size growth diminished over time. Based on the experimental data and the kinetic theory of austenite grain growth, a grain growth model of No. 2 Steel (which contained 0.38% Nb) was established. The predicted grain size values derived from this model closely matched the experimental measurements, indicating a strong correlation and providing valuable insights for future studies. Full article

In this study, the phase transformation mechanism during the decomposition of undercooled austenite and its effect on the deformation behavior of a high-strength medium Mn steel were studied. The results indicate that the austenite formation during heating (α → γ) is a relatively fast reaction. However, the transformation of undercooled prior austenite above the martensite start (M_s) temperature (γ → α) is difficult due to its high thermal stability. Only martensite transformation occurred during the final air-cooling stage following a 120-h isothermal treatment at 360 °C (slightly above M_s). The growth of martensite laths was limited by the boundaries of prior austenite grains and martensite packets. High-strength tensile properties were achieved, with a yield strength of 955 MPa, ultimate tensile strength of 1228 MPa, and total elongation of 11.6%. These properties result from the synergistic hardening effects of grain refinement, high-density lattice distortion, and an increased boundary length per unit area. The composition design with medium Mn content increased the processing window for high-strength martensite transformation, providing a theoretical basis for an energy-saving approach that depends on the decomposition transformation of undercooled austenite. Full article

The mixture powders were designed by adding 0 wt.%~1.0 wt.% CeO₂ into the 2205 duplex stainless steel (DSS) powders. The 2205 DSS cladding layer was prepared on the surface of Q345 steel by plasma arc cladding technology. The effects of different CeO₂ contents on the macro-morphology, microstructure composition, and corrosion resistance of the cladding layer were studied. The action mechanism of CeO₂ in the cladding layer was also discussed. The results showed that the addition of CeO₂ modified the appearance and decreased the defect of the cladding layer. Also, the austenite grains were refined, and the austenite proportion was increased under the action of CeO₂. When the CeO₂ content was 0.5 wt.%, the appearance of the cladding layer was optimum; the austenite proportion in the upper cladding layer and the lower cladding layer reached up to 52.6% and 55.5%, respectively, and the crystal changed from columnar to equiaxed. CeO₂ decomposes into Ce element and O element under the action of the plasma arc, after which Ce element is easily absorbed at the grain boundary to reduce the surface tension and improve the fluidity of the liquid metal so as to modify the appearance of the cladding layer. Meanwhile, Ce element primarily reacts with O, S, Al, and Si elements to form low-melting-point oxygen sulfides and are then removed, which eliminates the defect of the cladding layer. Moreover, the high melting point of CeO₂ acts as heterogeneous nucleation sites during solidification, thus improving the value of nucleation rate/growth rate of the grain and promoting the transformation from ferrite to austenite. According to the electrochemical corrosion testing result, Ce element inhibited the enrichment of Cr element at grain boundaries and promoted the formation of Cr₂O₃, which improved the corrosion resistance of the 2205 DSS cladding layer. It was optimum with the CeO₂ content of 0.5 wt.%. Full article

This study used 347H heat-resistant steel as the base material and systematically investigated the microstructural evolution and second-phase precipitation in typical regions during welding and aging processes. The results showed that the weld metal consisted of austenitic dendrites and inter-dendritic ferrite in a lath-like form. In the welded samples, the HAZ (Heat-Affected Zone) and BM (Base Material) regions were composed of equiaxed crystals. The microhardness of the HAZ was lower, mainly due to the coarser grain size and fewer second-phase particles. After aging at 700 °C, the hardness of all regions of the welded joint increased significantly due to the precipitation of M₂₃C₆ and MX phases. When the aging temperature increased to above 800 °C, the stability of the M₂₃C₆ phase decreased, and the diffusion rate of Nb in the matrix accelerated, promoting the preferential growth and stable presence of the MX phase. As the MX phase competes with the M₂₃C₆ phase for carbon during its formation, its generation suppresses the further precipitation of the M₂₃C₆ phase. Under 800 °C aging conditions, the γ/δ interface exhibited high interfacial energy, and the Nb content in the ferrite was higher, which facilitated the formation of the MX phase along this interface. As the aging temperature continued to rise, the hardness of the HAZ and BM regions initially increased and then decreased. After aging at 800 °C, the hardness decreased because the M₂₃C₆ phase no longer precipitated. After aging at 900 °C, the hardness of the HAZ and BM regions significantly increased, mainly due to the large precipitation of the MX phase. The hardness of the W (Weld Zone) and FZ (Fusion Zone) regions gradually decreased with the increase in aging temperature, mainly due to the reduction of inter-dendritic ferrite content, coarsening of second-phase particles, weakening of the pinning effect, and grain growth. In the 900 °C aging samples, the MX phase particle size from largest to smallest was as follows: W > HAZ > BM. The Nb-enriched ferrite provided the chemical driving force for the precipitation of the MX phase, while the δ/γ interface provided favorable conditions for its nucleation and growth; thus, the MX phase particles were the largest in the W region. The HAZ region, due to residual stress and smaller grain boundary area, had MX phase particle size second only to the W region. Full article

Creep tests of Super304H austenitic steel were carried out at 700 °C under different stresses. The samples were characterized by an optical microscope (OM), scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that high-temperature creep promotes the precipitation of the M₂₃C₆, secondary MX carbide, σ phase, Cu-rich phase and Z phase. These fine precipitates improve both the matrix and grain boundary strength. Furthermore, the precipitation sequence of these second phases relates to the stress level during elevated temperature testing. The rapid precipitation of the σ phase is also observed at high stress levels, whereby fast growth at triangle boundaries notably deteriorates grain boundary strength. Conversely, the presence of dispersed fine MX precipitates under low-stress conditions during long-term creep should contribute significantly to microstructure stability and long-term creep strength. Despite the absence of homogeneous cavities observed on the grain boundary when subjected to creep for over 20,000 h, the decrease in grain boundary strength was explained from another aspect by analyzing the change in low angle grain boundary during creep. Full article

Fatigue-induced crack initiation and propagation are a major concern in pearlitic railway rails and wheels. Rails and wheels undergo significant plastic deformation on their near-surface layers during service, leading to the initiation and propagation of cracks within the deformed region. Existing models typically use finite element models (FEMs) to describe these kinds of fatigue phenomena. However, they fail to establish a strong connection between the microstructure of the undeformed and the deformed materials and their corresponding fatigue properties. Therefore, a model based on the soft-contact discrete element method (DEM) was developed that considers microstructural details such as prior austenite grains (PAGs), pearlite blocks, pearlite colonies, and lamellar orientation of the ferrite–cementite structure of the pearlite. The Voronoi Tessellation method was used to generate a hierarchical mesh to represent these microstructural details, considering the distribution of microstructural details. Crack propagation is simulated by applying damage laws on the microstructural interface level that degrade the stiffness of the fibers connecting the mesh elements. The model’s crack growth predictions are compared with experimental results from the literature to validate its accuracy for different deformation degrees. The developed model can be used in the designing and material selection phase in the railway industry to help select the material with optimum microstructural features. Also, it can be used for the selection of the optimum heat treatment process considering materials resistance to the fatigue crack growth. Full article

The development of new materials with low weight for the transport industry is required for the saving of natural resources and protection of the environment from carbon dioxide pollution. The microstructure and mechanical properties of the Fe-30Mn-10Al-3.3Si-1C steel in as-cast, quenched, aged, and hot-deformed states were investigated. Austenite, ferrite, and κ-carbides are present in the steel in an as-cast state. Hot deformation of steels was made using the thermal and mechanical simulation system Gleeble-3800 at temperatures of 900–1050 °C and strain rates of 0.1–10 s⁻¹. Mechanical properties in as-cast, annealed, aged, and hot-deformed states were determined by Vickers hardness and compression tests. A constitutive model of the hot deformation behavior of Fe-30Mn-10Al-3.3Si-1C steel with high accuracy (R² = 0.995) was constructed. The finite element analysis of the deformation behavior of the steel under the plane-strain scheme was performed. Compression tests at room temperature have shown an increase in strength and ductility after hot deformation. The strain hardening of ferrite and austenite grain refinement during dynamic recrystallization are the main reasons for the growth of steel’s plasticity and strength. A specific strength of the investigated material is in the range from 202,000 to 233,000 m²/s² which is higher than high-strength steels previously developed and used in the automotive industry. Full article

In this work, we aimed to study the austenite grain growth behavior of an ultra-high-strength stainless steel within the temperature range of 900–1150 °C and holding time range of 0–120 min, using a metallographic microscope and metallographic image analysis software to perform a statistical analysis of grain size variation. The undissolved phases of the steel were investigated using a field emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Within the temperature range of 900–950 °C, the grain growth rate of the steel was slow, while within the range of 1000–1150 °C, the grain growth rate was relatively fast. This is attributed to the precipitation of a large number of M₆C-type carbides during the forging and annealing processes. In the temperature range of 900–950 °C, the solid solubility of the M₆C phase was low and the pinning effect was significant, which hindered the growth of austenite grains. Above 950 °C, the carbides were dissolved extensively, weakening the pinning effect on the grain boundaries and accelerating the grain growth rate. A predictive mathematical model for the growth of the original austenite grains was established based on the Arrhenius equation, elucidating the effects of heating temperature, holding time, initial grain size, and number of carbides on the growth of austenite grains, providing a theoretical basis for heat treatment process design in actual production. Full article

This paper presents an experimental and modeling study to investigate and predict the microstructure evolution for single-phase austenite and ferrite steels with the chemistry of the corresponding phases in a duplex stainless steel SUS329J4L under hot forming conditions. For steels with the compositions corresponding to both austenite and ferrite phases, single-pass hot uniaxial compression tests, stress relaxation tests, and heat treatment tests have been conducted for a temperature range of 1000–1250 °C and strain rates ranging from 0.3 to 30 s⁻¹. The dynamic and static recrystallization mechanisms, as well as the grain growth behavior, were studied, and the material parameters of each mechanism were identified for a semi-empirical microstructure model called StrucSim. Double-compression tests in the same temperature and strain rate range were performed to validate the model, and a good correlation of the flow stress between the experiment and simulation was observed. Full article

The impact of prior austenite grain size (PAGS) on the kinetics of austenite formation with an initial martensite microstructure was investigated in a medium-carbon, low-alloy steel. Two distinct PAGS of 117 and 330 μm, representing the range of grain sizes encountered in industries, were considered. In this analysis, high-resolution dilatometry was used to study the formation of austenite during continuous heating experiments. The analysis of the dilatometry results revealed that grain refinement accelerated the rate of austenite formation without impacting its austenite formation temperature. Intermittent quenching tests were conducted to elucidate the nucleation and growth mechanisms of austenite formation using a combination of optical, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The differences in austenite formation kinetics as a function of prior austenite grain size were quantified and modeled in the framework of diffusion-controlled nucleation and growth theories using the genetic algorithm optimization. Full article