Effect of tempering temperature on the precipitation behavior of B-containing carbides and mechanical properties of heat-resistant steel

The precipitation behavior of B-containing carbides during tempering at 300-700 degrees C has here been studied. With an increase in the tempering temperature, the variation of mechanical properties of the heat-resistant steel was then revealed. It was found that the BN inclusion, which had been formed during solidification, disappeared and a fine (Nb,V)(C,N) precipitation was observed by SEM analysis after isothermal quenching. The carbides were acicular M7(C,B)3, with a dominant brittle fracture morphology, when the tempering temperature was lower than, or equal to, 550 degrees C. Also, the carbides were block M23(C,B)6, with a dominant ductile fracture morphology, when the temperature was higher than 550 degrees C. It was also found that the lath boundaries of the martensite and the phase boundaries of M7(C,B)3 were favorable conditions for a preferential nucleation of M23(C,B)6.