Research on the influence of heat treatment on the microstructure and properties of bearing steel

This study investigates the influence of austenitizing and isothermal heat treatment conditions on the microstructure, mechanical properties, and wear behavior of GCr15SiMn bearing steel. This Chinese-designated steel is approximately equivalent to AISI 52100 (SAE 52100) but contains higher Si (0.52%) and Mn (1.02%) to improve hardenability and tempering resistance. The experimental matrix included austenitizing at 800, 840, 860, and 880 °C for 30 minutes, followed by isothermal transformation in a salt bath at either 195 °C or 235 °C for 180 minutes. A conventional quenching and tempering process (860 °C quenching + 250 °C tempering) was also conducted for comparison. The purpose of these tests was to evaluate hardness, impact toughness, and wear resistance—properties critical for bearing steel performance under cyclic loading conditions. The results show that increasing the austenitizing temperature from 800 °C to 880 °C leads to: (i) a progressive decrease in the volume fraction of undissolved cementite from approximately 8% to 3%; (ii) an increase in average bainitic lath thickness from approximately 0.2 μm to approximately 0.4 μm; and (iii) a rise in retained austenite (RA) content from approximately 6% to approximately 19%, with corresponding increases in its carbon content from 0.99% to 1.22%. Hardness was improved by approximately 18%, and wear resistance was significantly enhanced as indicated by a 36% reduction in wear cross-sectional area. An increase in salt bath temperature from 195 °C to 235 °C resulted in cementite coarsening and accelerated bainite transformation kinetics, but reduced hardness by 3% and impact energy by 7%. The optimal properties were achieved with 860 °C austenitization followed by isothermal treatment at 195 °C. Compared to conventional tempering, this optimized treatment increased impact energy by approximately 22% and improved wear resistance by approximately 16%.