Effect of short-time low-temperature austenitizing on microstructure and mechanical properties of DT300 ultra-high strength steel fabricated by laser powder bed fusion

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion, different post-heat treatment schedules are performed. With the increase in austenitizing temperature and time, the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides, and the formation of nano-oxides and twin martensite is also inhibited accordingly. The rapid growth in the size of prior austenite grains and martensite laths, as well as the decrease in the content of nano-oxides and twin martensite, led to a rapid decrease in the strength (yield strength and ultimate tensile strength) from HT2 to HTF specimens. The HT1 specimens (austenitizing at 830 °C for 30 min, then oil quenching and tempering at 300 °C for 120 min and finally air cooling) display excellent mechanical properties of yield strength of 1572 MPa, ultimate tensile strength of 1847 MPa, elongation of 9.84%, and fracture toughness of 106 MPa m^1/2, which are counterparts to those of conventional DT300 steel forgings after heat treatment.