Advanced mechanical strength in post heat treated SLM 2507 at room and high temperature promoted by hard/ductile sigma precipitates

Abstract

Duplex stainless steel, 71 wt.% austenite, 13 wt.% ferrite and 16 wt.% sigma, was made upon heat treating of fully ferritic as-built selective laser melted (SLM) 2507 stainless steel at 1200◦C. Formation of sigma phase in the heat treated SLM 2507 was investigated using optical microscopy and scanning electron microscopy (SEM). The heat treated SLM 2507 demonstrated a yield strength of 686 MPa, ultimate tensile strength of 920 MPa and an elongation of 1.8% at room temperature with a brittle fracture morphology. Precipitation of sigma phase during heat treatment and slow cooling improved the mechanical and wear properties at high temperatures (1200◦C and 800◦C, respectively). The tensile strength and elongation of the heat treated SLM 2507 was measured 400 MPa and 20% as compared to casted duplex steel with 19 MPa and 30% elongation at 1200◦C. The 20 times higher mechanical strength as compared to casted duplex steel was attributed to sigma precipitates. Tribological behaviour of heat treated duplex SLM 2507 containing sigma at 800◦C showed very low wear rate of 4.5 × 10−5 mm3/mN compared to casted duplex steel with 1.6 × 10−4 mm3/mN.