Influence of Prior Warm Deformation on Cementite Spheroidization Process in a Low-Alloy Medium Carbon Steel

Warm deformations have been applied to a low-alloy medium carbon steel (AISI 5140) to promote faster spheroidization during soft annealing treatments. The application of warm deformation leads to the fragmentation of cementite lamellae and the formation of defects on both cementite and the matrix. This induces faster lamellae break-up according to a boundary splitting mechanism, which is responsible for the improved spheroidization after annealing. The substructure developed in the matrix enhances pipe diffusion through the sub-boundaries, which helps the lamellae terminations to coarsen and causes lamellae fast splitting and finally yields a coarse cementite particle distribution. When deforming up to ε = 0.3, almost fully spheroidized microstructures are obtained after annealing at 993 K (720 °C), independently of the initial pearlite features. By means of the EBSD technique, it has been observed that the applied warm deformation, in addition to enhancing the degree of spheroidization, allows a much finer microstructure to be formed after annealing. Grain refinement takes place as a consequence of a continuous recrystallization process, which is directly related to cementite spheroidization in the long term.