Influence of last stage heat treatment on microstructure and mechanical properties of laser additive manufactured AF1410 steel

In order to optimize the mechanical properties of laser additive manufactured (LAM) AF1410 steels, two temperatures of 505 °C and 515 °C were selected for the last stage tempering treatment. Microstructure and compositions were observed by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The mechanical properties were examined by tensile tests and fracture toughness. Results showed that after heat treatment, the microstructure of LAMed AF1410 steels mainly consisted of fine dispersion carbides, lath martensite and film-like reversed austenite. The increase of tempering temperatures from 505 °C to 515 °C caused the coarsening of carbides and increasing amount of reversed austenite. Consequently, the ultimate tensile strength (UTS) and yield strength (YS) dramatically decreased about 9% and 8% respectively. While the elongation (EL) and the reduction in area (RA) slightly increased from 13.3% and 66.3% to 15.0% and 67% respectively. What's more, the change of tempering temperatures only had small impact on fracture toughness of steels due to the opposite effect of the coarsened carbides and increase number of reversed austenite. In consideration of tensile properties and fracture toughness, the optimum comprehensive mechanical properties were obtained when tempering temperature was 505 °C.