Effect of heat treatment on microstructure and mechanical properties of directed energy deposition-Arc 300M steel

In this study, the influences of heat treatment on microstructure evolution and mechanical properties of directed energy deposition-Arc (DED-Arc) 300M steel are investigated. The results show that the original columnar grains are transformed into the equiaxed grains with the increased austenitizing temperature. The spherical cementite and rod-shaped ε-carbides in the as-deposited microstructure are completely dissolved into the matrix above 970 °C, which increases the carbon supersaturation in the martensite and facilitates the dispersive precipitation of fine ε-carbides. Meanwhile, the anisotropy of tensile properties resulting from the DED-Arc process is significantly weakened with the increased austenitizing temperature. The high microhardness and excellent tensile properties are obtained using the austenitizing temperature of 970 °C with the microhardness of 563 ± 12 HV, ultimate tensile strength of 1942 MPa, yield strength of 1707 MPa and elongation of 12.6%, which exceeds AMS6417K standard of 300M steel forgings. The main strengthening mechanism is ascribed to the reinforced solid solution strengthening effect caused by the carbon supersaturation and the coherent dispersion strengthening effect of fine ε-carbides. Based on the findings, the improved heat treatment procedure is proposed for DED-Arc 300M steel.