Austempering Process for Carburized Low Alloy Steels 2013-01-0949

There is a continual need to apply heat treatment processes in innovative ways to optimize material performance. One such application studied in this research is carburizing followed by austempering of low carbon alloy steels, AISI 8620, AISI 8822 and AISI 4320, to produce components with high strength and toughness. This heat treatment process was applied in two steps; first, carburization of the surface of the parts, second, the samples were quenched from austenitic temperature at a rate fast enough to avoid the formation of ferrite or pearlite and then held at a temperature just above the martensite starting temperature to partially or fully form bainite. Any austenite which was not transformed during austempering, upon further cooling formed martensite or was present as retained austenite. The final result was a high-carbon surface layer of bainite or martensite which is expected to have good wear and fatigue resistance and a low carbon bainite and/or martensite core to provide toughness.

The effect of the austempering temperature and time was studied for the three types of steel. A combination of three austempering temperatures, 260, 288 and 304°C and three times 1, 2 and 4 hours provided seven different heat treatments that were applied to the three types of alloy steel selected for the tests. Charpy tests were done on all the heat-treated steels to determine the differences in toughness due to the material and heat treatment conditions. The surface hardness and the geometric distortion were measured as well. The results obtained for the carburize/austemper process were compared to results obtained for the more traditional carburize/quench/temper process. One of the important conclusions was that the distortion produced by the carburize/austemper process was significantly less than the distortion produced by the carburize/quench/temper process. The results can be used to help select the best heat treatment conditions and materials for applications that include transmission shafts, output shafts, pump shafts, differential pinions, splines and parking pawls.