Design of a spheroidization processing for ultrahigh carbon steels containing Al

A spheroidization processing has been designed for the forged ultrahigh carbon steels (UHCSs) containing Al, in which the mechanism of the divorced eutectoid transformation is applied. Guided by the CALPHAD calculations, the processing is carried out by two periods consisting of austenization at an intermediate temperature in the three-phase(γ + α + cementite)-zone and then isothermal annealing at a temperature a little lower than the critical temperature at which the austenite decomposes completely. The desired microstructure with the fine spheroidal carbides dispersed on the ferrite matrix has been observed, and the well combined mechanical properties, such as the tensile strength Sb = 1050 MPa, the yield strength Ss = 740 MPa, the elongation ratio δ5 = 14% and the reduction of cross-section ψ = 22%, are achieved.