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.
Gaseous ferritic nitrocarburizing was investigated as a replacement for the carbonitriding process currently used to improve the surface properties of thin-shelled, plain carbon steel automotive powertrain components. Nitrocarburizing can impart a hard, wear-resistant case suitable for light load applications at lower processing temperatures than carbonitriding, thereby potentially reducing size and shape distortions and the associated need for finish grinding. Carbonitriding and gaseous ferritic nitrocarburizing were compared with respect to size and shape distortion, retained austenite, and residual stress values. Testing was performed on a torque converter piston, an automotive powertrain component stamped from a sheet of 3-mm cold-worked SAE 1010 steel. A total of 53 pistons were used. While the heat treatment conditions for the carbonitrided pistons were similar to the current production schedule, conditions for the nitrocarburized pistons incorporated a range of processing times from 2–14 hours and temperatures from 510–605°C (950–1125°F). The results of this work were generally consistent with those from an earlier study involving the use of Navy C-ring specimens. For both the Navy C-rings and torque converter pistons, the nitrocarburizing process gave rise to lower part distortion, but resulted in overall lower compressive residual stress values than in the carbonitrided steel.
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