The effect of build direction and heat treatment on atmospheric stress corrosion cracking of laser powder bed fusion 316L austenitic stainless steel

Abstract

Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) when exposed to chloride ions and tensile stresses. For laser powder bed fusion (LPBF) components, high residual tensile stresses exist from the large thermal gradients during solidification. While post fabrication heat treatments can relieve residual stresses, their effectiveness in improving SCC resistance of LPBF material is undetermined. The effects of different heat treatments (i.e. temperature and cooling method) and build orientations on SCC susceptibility of LPBF 316L were explored. Samples were exposed to MgCl 2 and maintained at 75 °C and 70% RH for the test duration. Cracks were highly branched and propagated perpendicularly to build direction even in the presence of an applied perpendicular 60 MPa stress. Vertical as-built specimens exhibited higher cracking densities than horizontal ones, due to higher surface residual stresses. SCC and corrosion favoured one side for horizontal specimens while the distribution was random for vertical specimens. After 750 °C (2 h, furnace cooled) heat treatment, SCC susceptibility reduced for vertical 316L but increased for horizontal 316L. This was attributed to competing effects: reduction in residual surface tensile stresses (beneficial) and loss of non-equilibrium microstructure (detrimental). Encapsulation to prevent oxidation during heat treatment negatively impacted SCC resistance of LPBF 316L, likely due to changes to surface oxide condition. No SCC was observed after 900 °C (0.5 h) heat treatments within the test duration, however, pitting corrosion persisted.