The Effect of Core Microstructural Stability associated with the transformation occurred in the Carburized Surface on Residual Stress Evolution during Tempering

The tempering effect on as-quenched compressive residual stress of commercial carburized samples is the main objective of this work. The compressive residual stress developed during the quenching process of carburized parts is fairly well known. The study was performed using SAE 4120 RH steel samples with two different diameters of 15 and 38 mm, which resulted in different cooling rates, with a direct effect on the core microstructure. The carburized surface microstructures were considered similar. The smaller diameter core microstructures were predominantly martensitic however bainitic in the larger bars samples. The as-quenched surface compressive residual stress was lower in the smaller diameter samples. The 15 mm diameter samples showed two different types of behavior, i.e., the compressive stress for lower temperatures during tempering process was reduced, whereas the values were increased at higher temperatures. The compressive residual stress reduction as a result of increasing the temperature of the tempering process was continuous for pieces with the 38 mm diameter.