Effect of isothermal quenching on microstructure and hardness of GCr15 steel

Abstract

Through hardness measurement and microstructure observation, the effect of isothermal quenching on the bainite transformation, the size and area fraction of carbides in GCr15 steel was studied. Results show that, at the solid solution temperature of 860 °C, as the isothermal quenching temperature increases from 200 °C to 230 °C, the bainite transformation rate in GCr15 steel increases. At the initial stage of isothermal quenching, there is relatively less bainite transformation in GCr15 steel, but the increasing rate is relatively larger. With the extension of the isothermal time, the bainite content increases, and the increasing rate gradually becomes slower. The above phenomena are related to the existence of the incubation period of bainite transformation, the driving force of bainite ferrite nucleation, and the stability of undercooled austenite. As isothermal time increases, the carbide content decreases, while the carbide size generally increases, which is related to the dissolution of some undissolved carbides due to the change of the surrounding carbon concentration and the Ostwald ripening mechanism. The hardness of GCr15 steel is obviously reduced at the stage of the first 1 h, while between 2 and 4 h the hardness decrease of GCr15 steel tends to be gentle, which is related to the increase of bainite content, the weakening of dispersion strengthening effect resulted from the increase of carbide size, and the self-inhibiting behavior of the bainite transformation.