The role of microstructure modifications on electrochemical and plasma-nitriding behaviour of 316L steel produced by laser powder bed fusion

Abstract

ABSTRACT The study investigates the impact of microstructure modifications on the corrosion, passivation, and plasma-nitriding behaviour of 316L steel. Microstructure modifications are achieved through the laser powder bed fusion (L-PBF) process and surface mechanical attrition treatment (SMAT). Three scanning strategies (concerning the orientation of the sample surface with scanning directions) are used in the L-PBF process, and the corresponding samples are labelled as HNS (0°), INS (45°), and VNS (90°). The scanning strategies have altered the average grain size (maximum for HNS and minimum for VNS) and porosity (HNS has the highest). Porosity disappears after SMAT. The surface of the SMATed specimen (VS) has equiaxed austenite nanograins (∼32 nm) with a fine distribution of α’-martensite, nanotwins, and high dislocation density. Microstructure affects the passivation, corrosion, and nitriding behaviour of the steel. The VNS has the lowest corrosion rate, decreasing further after SMAT. The SMATed sample exhibits the lowest nitrided layer thickness (∼65 μm). SMAT followed by nitriding causes a gradient-structured layer (with improved hardness) consisting of a nitrided layer, SMATed layer, and core. The nitrided HNS sample (∼78 μm thick nitrided layer) is dominated by γ’-Fe4N, while the nitrided VNS and VS samples have a relatively higher proportion of expanded austenite.