The Effect of Degree of Spheroidization on the Cold Forgeability of Steel

Abstract

The cold forgeability has been studied using a medium carbon steel, (0.48%C), spheroidizeannealed to give a wide variation of degree of spheroidization.
The degree of spheroidization varies with the temperatures and insufficient degrees are brought about with both higher and lower temperatures than the optimum. A spheroidization treatment with lower temperatures gives the structure containing the unresolved pearlite, whereas the treatment with higher temperatures reforms well-defined pearlite.
The degree of spheroidization has a minor effect on the deformation resistance (strength), it, however, has a pronounced effect on the formability (ductility). Although the unresolved pearlite deteriolates the ductility to a less extent, the reformed pearlite impairs it noticeably.
The microscopic observation has been carried out by means of scanning electron microscopy on the ductile fracture process to interpret the results mentioned above. The observation substantiates that voids associated with fully or partially spheroidized carbides tend to grow separately along the tensile axis and to consume a considerable amount of the strain to fracture after void initiation. On the other hand, voids with reformed pearlite are ready to grow to the size as large as pearlite nodules and to propagate in the transverse direction.
The difference in ductile fracture behavior is expected to account for the effect of degree of spheroidization on ductility.