Mo-rich layer formation on maraging steel and surface modification by large-area electron beam irradiation

Maraging steels have been typically applied to precise metal mold and products in aerospace industry, since they have excellent properties such as high strength and toughness. However, the maraging steels are inferior to other steels in some surface functions such as corrosion resistance. Hence, it is necessary to improve surface functions of maraging steel. On the other hand, large-area electron beam (EB) of 60 mm in diameter can instantly melt metal surface. Our previous study clarified that the surface roughness of alloy tool steels such as SKD11 in JIS specification could be reduced efficiently by the EB irradiation. Furthermore, the surface characteristics were also improved by formation of thin re-solidified layer with a fine microstructure. In this study, formation of Mo-rich layer in re-solidified layer on maraging steel surface by large-area EB irradiation was investigated. Mechanism for formation of Mo-rich layer was discussed by unsteady heat conduction analysis. In addition, surface finishing and improvement of surface characteristics of maraging steels by the EB irradiation were also examined. The experimental results show that surface roughness decreases to approximately 1.0 µmRz in a few minutes under low energy densities conditions, and Mo-rich layer with thickness of a few micrometers is self-formed in re-solidified layer of EB irradiated surface. Moreover, water repellency, corrosion resistance and wear performance are improved by the formation of Mo-rich layer in re-solidified layer. Therefore, maraging steel surface can be finished and modified simultaneously by large-area EB irradiation method.