Corrosion Resistance Enhancement of Type 420J2 Martensitic Stainless Steel by Laser Thermal Processing

A laser thermal processing method was developed to enhance the corrosion resistance of martensitic stainless steels, which typically offer high hardness but poor corrosion resistance. Specifically, type 420J2 martensitic stainless steel (420J2 SS) plates were subjected to laser irradiation, causing rapid heating and quenching that modified the microstructure. After processing, three distinct layers were observed from the surface downward: a remelted layer, a phase-transformed layer, and the substrate. The remelted layer was formed by localized melting from the laser heat, followed by rapid quenching and solidification. Corrosion-inducing inclusions within this layer melted into the matrix, and their reprecipitation was effectively suppressed. The microstructure of the remelted layer comprised fine needle-like features, along with martensitic and austenitic phases. Due to the dominance of the martensitic phase, the hardness of the remelted layer increased significantly. Consequently, the remelted surface exhibited improved corrosion resistance and hardness. In contrast, the microstructure and corrosion resistance of the inner layer remained similar to those of the untreated 420J2 SS, indicating that the laser thermal processing did not adversely affect the substrate. Overall, this method provides an effective means of improving both the corrosion resistance and surface hardness of martensitic stainless steels.