Stress Corrosion Cracking Behavior of Selective Laser-Melted M300 Maraging Steel in 3.5 wt.% NaCl Solution

M300 maraging steel was fabricated by selective laser melting (SLM), and the effect of heat treatments on the corrosion and stress corrosion resistance of the alloy was evaluated in 3.5% NaCl solution. The microstructure of the alloy in the solution-treated and aged condition is typical of lath martensitic structure along with metallurgical defects typical of SLM-processed material. The results indicated that M300 steel processed by SLM is highly susceptible to stress corrosion cracking (SCC). The fracture morphology of the samples exhibited brittle intergranular cracking along with ductile cracking features in NaCl. HIP treatment after SLM and before solution treatment and aging improved the SCC resistance of the alloy in terms of marginal increase in elongation in NaCl solution. However, this treatment could not completely eliminate the SCC susceptibility of the maraging steel in NaCl solution. The SCC susceptibility of M300 maraging steel processed by SLM can be attributed to the presence of defects such as pores which assisted the early initiation of cracks and the resultant intergranular (IGC) failure. The defects were considerably healed by subjecting the SLM-processed alloy to HIP treatment, thus improving the SCC resistance. In addition to the reduction in pore density, the increased austenite content present in the alloy after heat treatment played significant role toward improving the corrosion resistance of the as-printed M300 steel.