Microstructure, corrosion behavior, and fatigue resistance of laser powder bed fusion-produced precipitation-hardening martensitic M789 stainless steel

Abstract

Additive manufacturing has facilitated the fabrication of stainless steel components with intricate designs, providing exceptional strength and corrosion resistance. This advancement, however, poses challenges in accurately predicting fatigue resistance under high-cycle loading, especially when surface features resulting from the printing process and corrosion are overlooked. This study investigates the microstructure of laser powder bed fused Böhler M789 AMPO stainless steel aiming to elucidate its relationship with corrosion resistance and fatigue properties.Through a comprehensive examination of the microstructural features using El-Haddad’s model and extreme value extrapolations, this research reveals the predominant influence of surface roughness, resulting from the printing process, on the fatigue strength of net-shaped specimens. For a stress ratio of 0.1, the estimated fatigue strength ranges between 69 MPa and 162 MPa. Upon mechanically polishing the surface to eliminate the surface roughness effects, the significance of other features is highlighted. Particularly, the effect of corrosion pits, shallow pores and oxides, and internal discontinuities follow in importance.Moreover, this study evaluates the impact of the as-received-oxide layer on corrosion resistance. Upon removal of this layer via surface etching or polishing, an improvement in the stability of the re-formed passive layer was observed. This enhancement is evident in the expanded potential range between the pitting and corrosion potentials, increasing from 0.2 V for the net-shaped surface to 1.05 V for the polished one.This work emphasizes the significance of characterizing microstructural characteristics to predict both fatigue and corrosion resistances. It also highlights potential research areas in additively-manufactured stainless steels.