Effects of cooling rate on isothermal spheroidizing annealing of hot-rolled GCr15 bearing steel

The effects of cooling rate on isothermal spheroidizing annealing (SA) of hot-rolled GCr15 bearing steel have been investigated. The results show that controlling the cooling rate between austenitizing and secondary annealing within a specific range is crucial to prevent the formation of lamellar carbide (LC). With a suitable cooling rate, more uniform carbide distribution, lower dislocation density and hardness can be achieved. An increase in the cooling rate results in a decrease in Ar1 and an increase in undercooling. However, an excessively high cooling rate suppresses the divorced eutectoid transformation (DET), leading to an increase in the amount of LC, dislocation density and hardness in rapidly cooled samples. Conversely, in the sample cooled too slowly, the appearance of LC may be attributed to the sufficient precipitation and coarsening of carbides, resulting in a high density of carbides before phase transformation. LCs can be eliminated by extending the secondary annealing time appropriately. This work demonstrates that for isothermal SA, using an appropriate cooling rate can effectively reduce the secondary annealing time, which provides a new idea for the rapid and complete spheroidization of carbides in GCr15 bearing steel.