Chengjia Shang

This study examines how a fast–slow–fast intermediate-cooling schedule redistributes bainitic transformation and hierarchical boundaries in an 800 MPa steel. Among intermediate temperatures of 630, 650, and 710 °C, 650 °C produced the highest tangent-derived first-interval contribution (79.44%). At T_h = 750 °C, a normalized bainite rate index reproduced the schedule ranking (Q_b,eff = 68.58 kJ mol⁻¹; root-mean-square error (RMSE) = 0.0043). Relative to continuous cooling, intermediate cooling increased the first-interval dilation share from 40.6% to 58.7% (+18.1%) and the processed hierarchical high-angle share from 46.9% to 52.2%. A common kernel average misorientation (KAM)–geometrically necessary dislocations (GND) conversion factor reproduced the reported medians within ±0.07%. Mean hole expansion ratio (HER) increased from 53% to 69% (+30.2%), whereas elongation changed by −0.5%. These results establish a process–kinetics–boundary–property pathway in which intermediate cooling shifts the transformation response toward the early interval, enriches high-angle boundary character, and improves local formability while maintaining the tensile-property balance.

This study comparatively investigates the coarse-grained heat-affected zone (CGHAZ) responses of two industrial X65 seamless pipeline steels with distinct composition–microstructure characteristics under simulated girth-welding thermal cycles. One steel exhibits a predominantly bainitic initial microstructure, whereas the other consists of a ferrite–bainite dual-phase microstructure. Low-temperature Charpy impact testing, microhardness measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), prior-austenite grain reconstruction, and JMatPro 13.0-based continuous cooling transformation (CCT) calculations were employed to evaluate their heat-input sensitivity and microstructural evolution. At heat inputs of 7–10 kJ/cm, both steels maintained high impact toughness at −20 °C, with average absorbed energies of approximately 250 J. A pronounced difference emerged at 15 kJ/cm the bainite-dominated steel retained relatively high impact toughness and higher crack-initiation and -propagation energies, whereas the ferrite–bainite steel exhibited a marked toughness reduction. At higher heat inputs of 20–30 kJ/cm, both steels showed substantial toughness deterioration associated with severe prior-austenite grain growth and coarsening of the bainitic transformation products. Microstructural and crystallographic analyses showed that the bainite-dominated steel generally retained finer prior-austenite grains and more refined crystallographic features under the investigated thermal cycles. Detailed characterization at 15 kJ/cm further revealed finer prior-austenite grain, packet, and block structures, together with more tortuous crack-propagation paths. JMatPro calculations predicted a lower bainitic transformation temperature for this steel, which is consistent with the experimentally observed tendency toward finer bainitic transformation products. The superior CGHAZ toughness retained by the bainite-dominated steel is therefore associated with the combined effects of alloy composition, initial metallurgical state, transformation behavior, and hierarchical crystallographic refinement rather than the initial microstructure alone. The results highlight the importance of coupled composition–transformation–microstructure effects in determining the welding heat-input tolerance of industrial X65 seamless pipeline steels.

Interstitial solute segregation at grain boundaries (GBs) plays a decisive role in determining the mechanical properties of ferritic steels, yet the underlying mechanisms behind the contrasting embrittling behaviors of light elements such as B, C, and N remain elusive. Through first‐principles calculations, the segregation and embrittling potency of these elements at two representative GBs are systematically investigated: the open Σ5(310) and the compact Σ3(112). A dual mechanism governing GB cohesion is revealed: at open boundaries, chemical bonding dominates, with B providing the strongest cohesion enhancement and C also acting as a strengthening solute, whereas N is consistently embrittling; at compact boundaries, mechanical distortion induced by solute size becomes critical, so that C remains an efficient cohesion enhancer while the size penalty reduces the beneficial effect of B; N again shows the strongest embrittling tendency. This work offers fundamental insights for grain‐boundary segregation engineering in the design of high‐performance ferritic steels.

This study investigates the effects of chromium (0.4~1.2) Cr content and temperature (35–80 °C) on the corrosion behavior and mechanisms of steels in a water-saturated supercritical CO₂ (S-CO₂) environment, aiming to provide theoretical foundations for material selection and corrosion management in S-CO₂ pipeline systems. Results indicate that increasing Cr content promotes the formation of granular bainite as the dominant microstructure, accompanied by refined martensite–austenite (MA) constituents with increased population and reduced dimensions, leading to enhanced strength at the expense of toughness. In the S-CO₂/H₂O environment, Cr reacts with CO₂ to form a dense Cr₂O₃ layer, significantly suppressing the corrosion rate. Temperature critically governs corrosion kinetics: at 35 °C, where S-CO₂ exhibits maximum density and CO₂ solubility in water peaks, electrochemical corrosion dominates, resulting in the highest corrosion rate. As temperature rises, the corrosion mechanism transitions to chemical corrosion, while accelerated formation of protective corrosion product films further reduces corrosion rates. Mechanistic analysis reveals that uniform corrosion arises from carbonic acid generated by water dissolution in S-CO₂, whereas localized corrosion intensifies upon direct contact between precipitated aqueous phases and the steel surface. These findings offer critical theoretical foundations for optimizing material design, operational parameters, and corrosion mitigation strategies in S-CO₂ transportation infrastructure.

In this study, we utilized Thermo-Calc software (2023a) to optimize the heat treatment process of martensitic stainless steel 90Cr18MoV through phase diagram calculations. The microhardness of 90Cr18MoV was characterized using a nanoindentation instrument. The microstructural morphology of the samples was analyzed using scanning electron microscopy (SEM). The composition of the samples was characterized through scanning electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Additionally, laser confocal microscopy (FIB) and transmission electron microscopy (TEM) were employed to characterize the precipitate phase composition and size before and after heat treatment, while also observing the dislocation structure within the samples. The relationship between the quenching temperature and the percentage of residual austenite content in the material was established. The influence of the dislocation structure and precipitate size on the hardness of the samples was investigated. The research findings confirm that the observed secondary hardening phenomenon in tempered samples is attributed to the co-precipitation of two types of carbides, M₂₃C₆ and MC, within the matrix. The study investigated the effects of the tempering temperature and duration on the size of secondary precipitates, indicating that M₂₃C₆ and MC particles with sizes less than or equal to 20 nm contribute to enhancing the matrix, while particles larger than 30 nm lead to a reduction in hardness after tempering. Notably, during the tempering process, M₂₃C₆ precipitated from the matrix nucleates on MC.

Cutting-edge wear is inevitable in the cutting process of the knife. Studying the relationship between the performance of the knife and the cutting-edge wear is conducive to optimizing the design of the knife and increasing its service life. The micro-morphology of the cutting edge during the cutting process was systematically characterized by optical microscopy (OM) and scanning electron microscopy (SEM). The strength of the material matrix was characterized by nanoindentation, and the microstructure of the material was characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The influence of different microstructures and geometry parameters of the cutting edge on edge wear was investigated. The experimental results show that the cutting knife longevity decreases with the increase in the edge angle. When the edge angle is 19°, the durability of knives 1# and 2# is 747.5 mm and 826.8 mm; when the edge angle is 29°, the durability of knives 1# and 2# is 377.8 mm and 486.8 mm. Under the same edge angle, the durability of knife 2# is higher, mainly due to its higher hardness and the presence of more micro-scale M23C6 carbides and nano-scale MC carbides. The edge wear process can be divided into two stages. In the initial wear stage, the edge curling phenomenon occurs, which is the plastic deformation of the edge. In the stable wear stage, plowing and stacking of worn materials are observed, which is the abrasive wear process.

The relationship between the stability of tool materials and their cutting performance is a critical technical challenge for the manufacturing industry, which is essential for selecting appropriate treatment processes to achieve superior treatment tool performance. In this paper, a standard cutting tool experiment was used to study the sharpness of the knife with different residual austenite content. The cutting edges of the knife were characterized by an optical microscope (OM), scanning electron microscope (SEM), electron back scattering diffraction (EBSD), and transmission electron microscope (TEM), to analyze the relationship between tool edge hardness and microstructure. The microstructure stability of the material was analyzed by a separated Hopkinson pressure bar (SHPB) experiment. The results show that the hardness and cutting performance of the knives are affected by the joint action of carbide and residual austenite, with an initial increase followed by decreases as the heat treatment quenching temperature increases. After the knife material is treated by cryogenic process, the hardness of the knife is increased by 3.89 HRC, the initial sharpness by 15.3%, and the sharpness and durability by 18.8%. The residual austenite in the knives was found to be unstable and easy to transformation during high-rate deformation processes. This study elucidates the effect of residual austenite content on the sharpness of the knives, providing a foundation for the reasonable control of residual austenite content in the actual production settings.

In this study, a novel hybrid heat source model was developed to simulate the welding temperature field in the heat-affected zone (HAZ) of X80 pipeline steel. This model replicates welding conditions with high accuracy and allows flexible three-dimensional adjustments to suit various scenarios. Its development involved the innovative integration of microstructural crystallography information with a multi-scale calibration and validation methodology. The methodology focused on three critical aspects: the weld interface morphology, the location of the Ac₁ temperature, and the size of prior austenite grains (PAG). The morphology of the weld interface was calibrated to align closely with experimental observations. The model’s prediction of the Ac₁ location in actual welded joints exhibited a deviation of less than ±0.3 mm. Furthermore, comparisons of reconstructed PAG sizes between thermal simulation samples and actual HAZ samples revealed minimal discrepancies (5 μm). Validation results confirmed that the calibrated model accurately describes the welding temperature field, with reconstructed PAG size differences between simulation and experimental results being less than 9 μm. These findings validate the accuracy of the calibrated model in predicting welding temperature fields. This research introduces a novel framework for the development of heat source models, offering a robust foundation for improving welding performance and controlling microstructure in different regions during the welding process of high-strength low-alloy (HSLA) steel.