Effect of deformation on deformation-induced carbides and spheroidization in bearing steel

Abstract

Three different rolling processes were performed on bearing steel to study the effect of deformation-induced carbides (DIC) on spheroidization. The results indicated that the effect of DIC on spheroidizing process was twofold. On the one hand, a small number of DIC precipitated at the grain boundaries and deformation bands were beneficial to the spheroidizing process. On the other hand, substantial precipitation of network of deformation-induced carbides hindered the spheroidizing process. Based on the state of DIC, the rolling temperature above A_rcm was divided into three parts: traditional rolling region (TRR), critical temperature region (CTR) and induced dual-phase region (IDR). There was near absence of DIC precipitated under the conditions of TRR, and the number of carbides increased with the decrease of rolling temperature. The degree of spheroidization increased when the specimens were rolled in the CTR with the same annealing time, whereas decreased in IDR. And the range of CTR decreased with the increase of pass reduction and decrease of rolling passes. The result of selected-area electron diffraction showed that the nature of DIC was M₇C₃, which was different from the carbide particles formed during the annealing treatment. The formation of DIC led to a decrease in the C and Cr content of the austenite, which was beneficial for spheroidization. In addition, the existence of DIC also provided nucleation sites for newly formed carbide particles and resulted in a carbon poor area which transformed to ferrite. Finally, ferrite matrix and spheroidized carbides would form directly instead of forming the lamellar pearlite by traditional cooperative mechanism.