Effect of Heat Treatment on Corrosion Properties of Selective Laser Melted Stainless Steel Parts

Abstract

One of the most popular additive manufacturing processes among today’s manufacturing industries is Selective Laser Melting (SLM) in which very intricate shapes can be fabricated directly from its three dimensional digital design data by melting metal powders using laser. Layer by layer deposition of material about different build axes make SLM parts anisotropic in nature. Also, non-uniformity in thermal loading at top and bottom surfaces of a SLM part leads to inhomogeneous microstructure and may change electro-chemical properties across the part. Suitable heat treatment as a post processing technique can reduce this anisotropy and produce homogeneous microstructure leading to reproducible mechanical and electrochemical properties. Depending upon the application in actual industrial scenarios, SLM parts may be subjected to corrosive media and thus may affect service life of the part. In the present study, effect of different heat treatment namely solution annealing, ageing, overaging on corrosion properties of SLM 15-5 Precipitation-Hardened (PH) stainless steel have been studied. Various metallurgical characterizations have been carried out wherever required to support experimental observations. As-built specimens have approximately six times higher pitting potential which may be attributed to higher nitrogen content present in as-built specimens but corrode more over time than solution annealed (SA) specimens. Relatively bigger size pits and non-uniformity in their distributions can be attributed to residual stresses and inhomogeneous microstructure associated with as-built SLM specimens respectively. Specimen undergone standard ageing condition (H900) corrodes least over time among all the heat treatment conditions considered in the present study. However, in this case, a large number of shallow pits can be observed from the corroded surface. Overaged (H1150) specimens corrode more than H900 specimens but pitting starts late in case of H1150 specimens since pitting potential is almost ten times higher in the former case. Increased ageing temperature and soaking time (Mod H900 (SA)) increases formation of higher Cr23C6 precipitates than that of H900 condition and hence corrode more over time. Ageing without solution annealing (Mod H900 (AB)) leads to higher corrosion and larger pit size non-uniformly distributed over the corroded surface than that of Mod H900 (SA) condition which may be attributed to presence of residual stresses and non-uniform precipitation throughout the matrix. Present study will be useful for selecting suitable heat treatment yielding desired corrosion resistance for SLM stainless steel parts.