This study aims to comprehensively assess the suitability of post-processing annealing (at 900–1200 °C) for enhancing the key properties of 316L steel fabricated via laser powder bed fusion (LPBF). It adopts a holistic approach to investigate the annealing-driven evolution of microstructure–property relationships, focusing on tensile properties, nanoindentation hardness and modulus, impact toughness at ambient and cryogenic temperatures (−196 °C), and the corrosion resistance of LPBF 316L. Annealing at 900–1050 °C reduced tensile strength and hardness, followed by a moderate increase at 1200 °C. Conversely, ductility and impact toughness peaked at 900 °C but declined with the increasing annealing temperature. Regardless of the annealing temperature and testing conditions, LPBF 316L steel fractured through a mixed transgranular/intergranular mechanism involving dimple formation. The corrosion resistance of annealed steel was significantly lower than that in the as-built state, with the least detrimental effect being observed at 1050 °C. These changes resulted from the complex interplay of annealing-induced structural transformations, including elimination of the cellular structure and Cr/Mo segregations, reduced dislocation density, the formation of recrystallized grains, and the precipitation of nano-sized (MnCrSiAl)O₃ inclusions. At 1200 °C, an abundant oxide formation strengthened the steel; however, particle coarsening, combined with the transition of (MnCrSiAl)O₃ into Mo-rich oxide, further degraded the passive film, leading to a sharp decrease in corrosion resistance. Overall, post-processing annealing at 900–1200 °C did not comprehensively improve the combination of LPBF 316L steel properties, suggesting that the as-built microstructure offers a favorable balance of properties. High-temperature annealing can enhance a particular property while potentially compromising other performance characteristics.

Martensitic stainless steel is a commonly used stainless steel, and is widely used in daily production and life, but its high content of alloy elements increases its cost. This study aimed to develop low-cost martensitic stainless steel with excellent seawater corrosion resistance by using Zr-Ti combined deoxidation with molten iron from low-priced laterite nickel ore as raw material, taking advantage of the corrosion-resistant elements Cr and Ni, abundant in laterite nickel ore. The characteristics of corrosion-active inclusions in steel, such as their density and electrostatic potential saturation-current density, were observed and studied using scanning electron microscopy and electrochemical testing methods. The intentionally added composite deoxidizing elements (Zr, Ti) form highly stable oxide particles at high temperatures. During the solidification of the molten steel and subsequent solid-state phase transformation, the highly corrosion-active MnS nucleates and disperses on the oxide particles already formed in the liquid phase, while TiN continues to precipitate and coat the MnS particles. This significantly reduces the saturation-current density of locally corrosion-active inclusions, resulting in a marked improvement in seawater corrosion resistance.

Non-metallic inclusions are preferential sites for pitting corrosion initiation in austenitic stainless steels. This study investigates the effect of magnesium addition on inclusion evolution and localized corrosion behavior of austenitic stainless steel using SEM–EDS, immersion testing, scanning Kelvin probe force microscopy (SKPFM), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Magnesium addition modifies the inclusion population from coarse (Mn,Al)-O into finer MgO·Al₂O₃ spinel-based inclusions with improved distribution. Immersion testing in FeCl₃–HCl solution reveals delayed inclusion dissolution and reduced localized pitting attack in the Mg-treated steel. SKPFM maps, together with immersion observations, provide qualitative evidence of changes in the local electrochemical contrast between inclusions and the surrounding matrix. Electrochemical measurements in 3.5 wt.% NaCl further demonstrate improved corrosion resistance after Mg addition, as evidenced by an increase in pitting potential from 0.28 V to 0.52 V vs. SCE and charge-transfer resistance. The improved corrosion resistance is associated with inclusion refinement and modification of the local surface-potential distribution, which may influence inclusion-related localized corrosion processes. Overall, Mg microalloying modifies the inclusion characteristics of austenitic stainless steel and enhances its resistance to chloride-induced localized corrosion, demonstrating the potential of inclusion engineering as a strategy for improving stainless-steel corrosion performance.

Overcoming the strength–ductility trade-off at cryogenic temperatures is a key bottleneck for austenitic stainless steels. This study investigated the effects of Nb additions (0, 0.03, and 0.06 wt.%) on the mechanical properties and fracture behavior of 304 steel from room temperature (RT) to −196 °C. Within the investigated range, the 0.06 wt.% Nb steel produced the largest measured response. Compared with the Nb-free steel, the RT yield strength and elongation after fracture increased by ~5% and 2.4%, respectively; at −196 °C, they increased by 11.1% and 4.1%, and impact toughness by 31.8%, mitigating the low-temperature strength–ductility trade-off. Fractography revealed a transition from brittle cleavage/mixed fracture to dimple-dominated ductile fracture. The improvement in mechanical properties is consistent with the refined microstructure and the modified inclusion distribution, with the underlying mechanisms likely involving grain refinement and the solid-solution strengthening of the austenite matrix by Nb. It should be noted that, owing to experimental limitations, the deformation-induced α′-martensite was not quantitatively characterized in the present study, and no rigorous quantitative correlation has yet been established between inclusion characteristics and low-temperature impact toughness. Consequently, the relative contribution of each contributing factor to the observed performance enhancement remains unresolved. Nevertheless, this work provides experimental support for the microalloying design of ultra-low-temperature austenitic stainless steels.