Investigation on Temper Embrittlement of TS1100 MPa Grade Ultra-High Strength Steel

Temper embrittlement is one of the major challenges encountered during the heat treatment of ultra-high strength steels. In this work, the microstructural evolution and mechanical properties during the austenite region quenching and tempering (Q-T) and the intercritical quenching and tempering (IQ-T) processes in a tensile strength (TS) 1100 MPa grade steel targeted for the manufacturing of a mooring chain were investigated. The temper embrittlement presented in the Q-T sample was attributed to the carbide network decorating the prior austenite grain and martensitic lath boundaries. In contrast, the IQ-T sample, where intercritical holding before quenching at the temperature of 12 °C lower than Ac3 was applied, exhibited a reasonable CVN impact energy of 188 J at − 20 °C with a slightly reduced strength. The ferrite formed during intercritical holding not only added extra interphase boundaries, but also refined the substructure of the quenched martensite. These increased boundary areas provided more nucleation sites for carbide precipitation during tempering. Meanwhile, the refined martensitic substructure promoted the diffusion of Cr, encouraging the fast coarsening of Cr-rich carbides. The formation of a carbide network was eventually prevented in the IQ-T sample. Thus, the IQ-T process was an effective approach to eliminate temper embrittlement caused by the carbide network.