This study systematically investigates the inheritance effect of the initial microstructure of advanced high-strength bainitic steel on its microstructure and properties during continuous annealing. Three representative hot-rolled microstructures were designed and combined with continuous annealing. Multi-scale microscopic characterization and mechanical property testing were employed to reveal how the initial microstructure synergistically regulates the final bainite lamellar structure and characteristics through an inheritance mechanism. The study found that the initial microstructure type significantly influences microstructure evolution during continuous annealing. HR650, which contains a higher volume of RA, exhibits poorer stability after annealing. RT, characterised by fine lamellar structures and a high dislocation density, promotes the formation of fine, highly stable, film-like RA. The microstructural inheritance effect weakens as overaging temperatures increase. During low-temperature overaging, the HR650 steel exhibits superior elongation due to its high RA content. During high-temperature overaging, the RT steel demonstrates a better balance of strength and plasticity due to its finer bainite structure and stable RA. This study establishes a systematic research framework for the design of initial microstructure, the regulation of their heredity effects, and the customization of their final properties, elucidating the microstructural mechanisms that synergistically enhance the strength and plasticity of bainitic steels through pre-process design. This provides crucial theoretical foundations for the industrial production of high-performance bainitic steels.
Reducing the sintering temperature and electrical resistivity of silver nanoparticles (Ag NPs) remains a key challenge for advanced flexible electronics. Herein, two types of quasi-spherical silver nanoparticles (Ag NPs), namely polycrystalline Sample P and single-crystalline Sample S, were synthesized via the polyol approach to investigate structure-dependent low-temperature sintering behavior. Structural characterization verifies that Sample P is principally composed of particles with abundant twin boundaries, terminated by (111), (200), and (220) crystal planes and containing Ag+ species, whereas Sample S is dominated by single-crystalline particles predominantly terminated by (111) facets. Relative to their multi-domain polycrystalline counterparts, single-crystalline particles exhibit a smaller crystallite size and lower microstrain. After screen-printing and sintering at 140 °C for 1 h, the single-crystalline Ag NP film on polyimide (PI) substrate delivers a resistivity of 2.1 × 10-5 Ω·cm, remarkably lower than the value of 2.2 × 10-4 Ω·cm for Sample P. Mechanistically, sintering of single-crystalline Ag NPs proceeds via bulk and surface diffusion pathways, while polycrystalline particles undergo twin boundary diffusion, giving rise to substantial microstrain elevation. Intense electron scattering induced by twin boundaries, defects, and pores is identified as the predominant origin of the elevated resistivity of sintered films derived from Sample P. After sintering at 200 °C for 1 h, the Ag NP film from Sample S achieves a porosity of 0.9% and a resistivity of 9.0 × 10-6 Ω·cm, approaching that of bulk silver (1.59 × 10-6 Ω·cm). In sharp contrast, the film from Sample P shows a porosity of 7% and a resistivity of 8.4 × 10-5 Ω·cm.
Aiming at the heat accumulation problem in the laser directed energy deposition (LDED) of CoCrNi alloy, although active cooling can alleviate thermal effects, an excessively high cooling rate may introduce residual stresses and aggravate the deformation of thin-walled parts. To address this issue, laser power, scanning speed, powder feeding rate, and the temperature of the active cooling device were taken as input variables, while the layer thickness, layer width, and melt pool temperature served as response indicators. Twenty-five sets of orthogonal experiments were carried out based on the Box–Behnken design. A support vector machine prediction model optimized by the stellar oscillation optimization algorithm (SOO-SVM) was established, and the NSGA-II algorithm was employed for multi-objective global optimization. The results show that the optimal combination of process parameters is a laser power of 1492.84 W, a scanning speed of 11.92 mm s−1, a powder feeding rate of 14.53 g min−1, and a cooling temperature of 8.83 °C. After optimization, the layer width and thickness increased by 22.1% and 85.4%, respectively, while surface defects were significantly reduced., and the microstructure exhibited uniform equiaxed dendrites, directional dendrites/cellular crystals, and sound columnar-crystal metallurgical bonding. The average microhardness was enhanced by 10.4%, and the longitudinal and transverse tensile strengths of the thin-walled parts were improved by 13.5% and 19.2%, respectively. This study verifies the effectiveness of incorporating cooling temperature into process parameter optimization and provides a theoretical basis for active-cooling-assisted LDED manufacturing of high-performance CoCrNi alloy.
B has been widely acknowledged to improve the hardenability of steels. However, its role on the resistance of steels for oil country tubular goods to sulfide stress cracking (SSC) has not been well understood. Considering that B protection from N is important in retaining free B in steel, in the current work, two B-added high-strength steels with various Ti/N ratios were prepared to investigate the effect of Ti/N ratio and tempering time on the microstructure and SSC resistance. The results showed that, a low Ti/N ratio did not obtain full martensite but gave rise to a small amount of upper bainite, obtaining a refined effective grain size. The introduction of upper bainite decreased the dislocation density, with little impact on the precipitation behavior of cementites. Moreover, the differences in hydrogen diffusion and trapping behaviors during electrochemical hydrogen permeation and thermal desorption spectroscopy tests were primarily associated with variations in dislocation density. After tempering for a same time, the steel with upper bainite exhibited decreased yield strength and impact toughness compared with the steel without upper bainite, and it was assumed that upper bainite also impaired the SSC resistance, given that the strength often inversely correlates with SSC resistance in most practical applications. Increasing the tempering time could greatly increase the SSC resistance without greatly sacrificing the yield strength, which was due to the decrease in reversible hydrogen traps resulting from decreasing dislocation density. This work offers both technical guidance and theoretical basis for the composition design, microstructural control and servicing performance optimization of high-quality OCTG materials.
During the fabrication of bimetallic heterostructures via Wire Arc Additive Manufacturing (WAAM), the preheating temperature of the deposited layer directly governs the elemental diffusion kinetics, phase transformation pathways, and microstructural evolution at the material interface, which is critical to the metallurgical bonding quality of the bimetallic interface. This study elucidates the effect of deposited-layer preheating using thermoelectric ceramic plates on the interfacial bonding quality and mechanical performance of the bimetallic heterostructures, along with the underlying mechanisms.The results indicate that preheating temperature induces the evolution of crystallographic texture and recrystallization behavior in the interfacial transition zone (TZ). With increasing preheating temperature, the microhardness of the interfacial TZ exhibits a non-monotonic trend of first decreasing and then increasing, whereas the ultimate tensile strength (UTS) and uniform ductility show a consistent upward trend. At a preheating temperature of 100 °C, strong Goss {011}<100> and Brass {011}<211> textures effectively suppress deformation-induced martensitic transformation (DIMT), and the corresponding component exhibits the optimal combination of mechanical properties. Specifically, the TZ width reaches its maximum at this temperature, with a microhardness of 297.1 HV, an ultimate tensile strength of 545 MPa, and an elongation after fracture increased to 28.9% - representing a 35.1% improvement compared to the 21.4% elongation of the room-temperature (unpreheated) component. When the preheating temperature exceeds 100 °C, a large number of Fe-Cr intermetallic compounds (IMCs) precipitate at the interface, leading to local stress concentration. While this precipitation increases the microhardness of the TZ, it deteriorates the tensile performance.
