ESSC & DUPLEX 2019

Due to the high cooling rates and layer by layer building characteristics of the additive manufacturing (AM) process Selective Laser Melting (SLM), the as-fabricated parts can contain strongly inhomogeneous microstructures, porosities, rough surfaces and residual stresses. For example, the as-fabricated microstructure of SLM 316L typically consists of elongated austenite grains with a cellular substructure of approximately 1µm cell width, wherein the dislocation rich cell boundaries are enriched in Cr and Mo. Therefore, post-processing by heat treatment or surface finishing is often necessary to optimize the materials properties and performance. For the austenitic stainless steel AISI 316L heat treatment has always been essential in order to obtain the desired corrosion performance. Conventionally, AISI 316L is austenitized in a hydrogen atmosphere or in high vacuum at temperatures in the range 1040 -1120$^{\circ}$C, to ensure a fully austenitic structure with optimal corrosion resistance. In AM some producers prescribe nitrogen gas as the “inert” atmosphere for SLM of AISI316L. Nevertheless, nitrogen pick-up in the solid state occurs for a temperature above, say, 900$^{\circ}$C. Such uncontrolled nitrogen dissolution in the stainless steel can have a positive or a negative effect on the electrochemical and mechanical properties. Hitherto, the effect of “nitrogen alloying” in printed parts has received little attention in the literature. Dissolution of nitrogen in austenitic stainless steels is also be carried out as the *deliberate* surface treatment referred to as high temperature solution nitriding (HTSN). HTSN improves the corrosion resistance and slightly enhances the hardness by the addition of up to 0.7 wt% nitrogen, depending on the alloy composition. Low temperature surface nitriding (LTSN) dissolves much higher nitrogen contents in the surface of the stainless steel (even more than 8 wt%) and results in the formation of expanded austenite, which significantly increases the surface hardness (up to 1200 HV). With the increasing popularity of SLM 316L, there is a need to investigate the corresponding corrosion properties of the inhomogeneous microstructures formed by SLM and the effect of subsequent heat treatment and thermochemical surface treatment on the microstructure-property relations. The present work investigates how SLM 316L with a heterogeneous microstructure, responds to different heat- and thermochemical surface treatments, and how such processes affect the corrosion properties. Specifically, conventional austenitization in hydrogen, resulting in loss/removal of nitrogen from the printed part, and a new process termed “active austenitization”, where a controlled nitrogen pressure adjusts the amount of nitrogen in the printed part, were investigated. Additionally, the influence of the thermochemical surface treatments HTSN and LTSN on microstructure, corrosion and wear properties was investigated. The microstructure of the specimens was investigated with X-ray diffraction, reflected light optical microscopy and scanning electron microscopy. Micro-hardness measurements and wear testing (pin-on-disc) were used to characterize the mechanical properties, while the corrosion properties were evaluated with potentiodynamic polarization testing.