Effect of tempering conditions on the final properties of ball bearings

The primary assignment of bearing is that the machine element we can encounter in every field. It provides minimum friction between raceways of inner and outer ring with relative rotary motion and ensure smooth load transformation. The bearing must have high hardness and strength properties for resistance to external forces. The hardness of steel is dependent on carbon contents in chemical composition. For this reason, 100Cr6 bearing steel, which is defined as high carbon low alloy steel, is commonly used by bearing manufacturers. The through-hardening process consists of quenching and tempering conditions. Austenitization temperature is used at 850°C. Moreover, the quenching condition that provides high hardness, includes stress. Tempering was carried out between 150-300°C by applying 10°C intervals. Also, two different tempering durations were selected; 90 and 240 minutes. The tempering condition removes residual stress and retained austenite. Retained austenite can be decomposed into the martensite phase depending on tempering temperatures and durations. However, this case causes dimensional distortion for rings. Balls are trapped between inner and outer rings. This situation causes failure of bearing. A dimensional stabilization test should be applied to the bearing rings to observe dimensional distortion. Accordingly, tempering temperatures are determined. Since there is no international dimensional stabilization test standard, the bearing manufacturers develop their own dimensional stabilization test parameters. Three different dimensional stabilization test conditions were applied to rings according to heat classes. In this study, 6208 types of bearing rings are used. Effects of tempering conditions on the final properties of bearing rings are examined. For outer rings; hardness values were observed between 790 HV1 to 643 HV1 for 90 minutes and 775 HV1 to 658 HV1 for 240 minutes duration at 150 and 300°C tempering temperature. The amount of retained austenite was completely decomposed into martensite at 230°C for 90 minutes and 220°C for 240 minutes duration. For inner rings under the same tempering temperatures; hardness values were observed between 792 HV1 to 643 HV1 for 90 minutes and 773 HV1 and 632 HV1 for 240 minutes duration. The amount of retained austenite was completely decomposed into martensite at 230°C for 90 minutes and 210°C for 240 minutes duration. The amount of retained austenite caused the dimensional distortion on rings according to dimensional stabilization test. The 240 minutes tempering duration is more suitable than 90 minutes for the phase stabilization of rings.