Improvement in Properties of Martensitic Stainless Steel SUS420J2 Fabricated by LPBF Method Through Heat Treatment

In this study, martensitic stainless steel SUS420J2 (AM material) was fabricated by laser powder bed fusion (LPBF), one of the additive manufacturing (AM) methods, using the laser scanning pattern that was selected in our previous study as a suitable one to reduce residual stress. AM material was heat-treated under two conditions, that is, conventional quenching and tempering (H1 treatment) and only tempering at low temperature for a long time (H2 treatment), and their effects were systematically investigated. The results showed that the amount of the retained γ phase in the microstructure of AM material significantly reduced with the H1 treatment and the microstructure became almost the same to that of the H1-treated wrought material (W material). Hardness, static strength and wear resistance of the H1-treated AM material were comparable to those of the H1-treated W material. Moreover, the H1 treatment gave the effect to markedly reduce the directional dependence and the absolute value of residual stress generated in AM material. On the other hand, the microstructure of AM material was unchanged by the H2 treatment. Further, its hardness and static strength were lower than those of the untreated AM material, besides the directional dependence and the absolute value of residual stress remained almost unchanged. The fatigue strength of W material was greatly improved by the H1 treatment, but the improvement of the fatigue strength of the H1-treated AM material was smaller than that of the H1-treated W material because the fatigue strength of AM material was significantly affected by molding defects, and the fatigue strength of the H2-treated AM material was not improved.