Effect of Heat Treatment on Corrosion and Mechanical Properties of M789 Alloy Fabricated Using DED

The directed energy deposition (DED) process offers potential advantages, such as a large building space, limited dilutions, narrow heat-affected zones (HAZ) and potentially improved surface properties. Moreover, heat treatments have been reported to significantly improve the properties of the as-built sample by modifying the microstructure. In this study, the influences of various combinations of heating and cryogenic treatments on the mechanical performance and corrosion resistance of DED M789 steel have been critically investigated. The microstructure and hardness were examined to discuss the characteristics of the M789 parts in the as-printed and heat-treated states. The corrosion rate was determined from the weight loss monitoring based on the seawater immersion condition. The microstructural results revealed the distortion of martensite lattice and the formation of nano-carbide precipitates after the cryogenic treatment. Moreover, the microhardness of the cryogenically treated M789 steel was found to be significantly higher which was attributed to the precipitate strengthening and elimination of retained austenite, resulting from the increased volume fraction of carbides due to cryogenic treatment. The corrosion characteristics were also modified by the heating/cryogenic treatments, and the substrate-to-deposit ratio of the corrosion sample also substantially affected the overall corrosion rate.