Carburized parts often see use in powertrain components for the automotive industry. These parts are commonly quenched and tempered after the carburizing process. The present study compared the austempering heat treatment to the traditional quench-and-temper process for carburized parts. Samples were produced from SAE 8620, 4320, and 8822 steels and heat treated across a range of conditions for austempering and for quench-and-tempering. Distortion was examined through the use of Navy C-Ring samples. Microstructure, hardness, and Charpy toughness were also examined. X-ray diffraction was used to compare the residual stress found in the case of the components after the quench-and-temper and the austempering heat treatments. Austempering samples showed less distortion and higher compressive residual stresses, while maintaining comparable hardness values in both case and core. Toughness measurements were also comparable between both processes.
Carbonitriding is a widely accepted method of heat treatment used by North American manufacturers for plain carbon steel. While the process imparts a hard, wear resistant case, it is also associated with both size and shape distortion, which can be problematic for components in tight fitting assemblies. This research compares the effects of the carbonitriding and nitrocarburising processes on SAE 1010 steel with respect to residual stress and distortion. Results indicate that the two processes develop residual compressive stresses and are associated with both size and shape distortion. Although the magnitude of the compressive stresses in nitrocarburised steels is lower, this process was noted to give rise to overall smaller dimensional changes than the carbonitriding process. The findings from this study are applied to a manufacturing application involving the surface treatment of a thin shelled automotive component in a light loading application.
Induction hardening is a key manufacturing process in the production of ductile iron automotive camshafts. Its desirable features include low-cost, a wearresistant surface, and ease of manufacturing. Due to the asymmetrical geometry of the camshaft, induction hardening generates non-uniform heating/cooling which leads to a variability in both the microstructure of the surface hardened case, principally the amount of retained austenite (RA), and the level of residual stress (RS). Both RA and RS can affect the contact fatigue performance of the camshaft. In the present study, automotive camshafts containing six lobes per camshaft were induction hardened to different schedules and the RA and RS (surface) variations were determined both between different lobes on the same camshaft and between camshafts that were induction hardened to different schedules. RA and RS measurements were made using x-ray diffraction methods by two independent, certified testing laboratories. Because of their low cost and ease of testing, we have also used optical metallographic methods to determine RA. A correlation was obtained between the RA values obtained by x-ray diffraction with those obtained by optical metallography. This data for ductile iron expands the database that was available for steels to higher carboncontent ferrous alloys. Finally, a correlation is made between RA content and RS level in order to define a robust process window.
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