DT300 ultra-high strength steel fabricated using selective laser melting: Densification, microstructure, and mechanical properties

Abstract

DT300 ultra-high strength steel has been widely used due to its combination of high mechanical properties and a relative low cost. However, the traditional casting and forging process can hardly meet the rising needs of producing parts with complicated shapes and high precision. In this study, selective laser melting was utilized to fabricate DT300 steel for the first time. Fully dense (>99% relative density) parts were successfully fabricated and the types and formation mechanism of defects under different conditions are discussed in detail. The average equivalent diameter of martensite laths is only 0.64 μm, and the retained austenite content of horizontal and vertical section is 5.6% and 7.1% in volume fraction, respectively. The fine prior austenite grain boundaries are continuously and uniformly distributed both in horizontal and vertical sections. Both martensite and austenite show strong texture orientation, which is closely related to the laser scanning direction of each layer. The specimens fabricated by the optimized parameter M8 (250 W laser power, 800 mm/s laser scanning speed, 90 μm hatch spacing) possess outstanding mechanical properties (yield strength up to 1309 MPa, ultimate tensile strength up to 1636 MPa, 19.79% elongation to fracture) as well as outstanding surface quality (Ra of 14.37 ± 2.15 μm). The mechanical strength of SLM fabricated DT300 steel is higher than that of as-cast with heat treatment condition but 10% lower than the wrought one with post heat treatment.