Effect of quenching-tempering processes on the microstructure and properties of X80 pipe fittings steel

The effects of quenching-tempering heat treatment processes on the microstructure and properties of X80 pipe fittings steel were studied by scanning electron microscopy(SEM), electron backscatter diffraction(EBSD), transmission electron microscopy(TEM) and other experimental methods.The results show that after the first quenching at 930 ℃, the yield strength first increases and then declines with the tempering temperature increasing, reaching the maximum 588 MPa at 630 ℃, but the tensile strength decreases gradually and reaches the lowest 630 MPa at 680 ℃.After the second two-phase quenching and the tempering at 630 ℃, X80 pipe fittings steel has the optimum comprehensive mechanical properties, while its impact strength significantly reaches 210 J at -50 ℃.This can be attributed to the fine-grain austenite microstructure after the secondary quenching at 860 ℃, the combined effect of recovery and softening of the dislocation sub-structure, the blunting mechanism along strip boundaries, and the precipitation strengthening from decomposition of massive M-A after tempering at 630 ℃.