Research Status of Large-Size Carbides in High-Carbon Chromium Bearing Steel

Metallurgical quality of bearing steel is an important foundation and guarantees for determining the performance, accuracy, service life, and reliability of bearings. Nowadays, advances in refining technology have led to a significant increase in the cleanliness of bearing steel, and the control of large-size carbides (primary, banded, and network carbides) has become particularly important. In this article, the recent research on large-size carbides regarding morphology, precipitation mechanism, and control methods is reviewed. Firstly, the morphological characteristics of carbides are summarized. Primary carbides appear as blocky, lamellar, or irregular shapes, while banded carbides are distributed in bands. Network carbides mainly precipitate along the austenitic grain boundaries and aggregate to form networks. Secondly, the precipitation mechanisms of carbides are summarized. Dendritic segregation is the fundamental cause of the formation of primary and banded carbides, whereas network carbides are the supersaturation precipitation of carbon at austenite grain boundaries. Finally, methods for controlling large-size carbides are summarized, including improving dendritic segregation, high-temperature diffusion annealing, improving rolling and cooling processes and using magnesium or rare-earth treatments. No single method offers sufficient carbide control, and mechanism of rare-earth refining carbides is unknown. Thus, a comprehensive approach should be used in production to effectively regulate carbides in steel.