The Analysis and Control of Distortion in Carbonitrided and Nitrocarburized Plain Carbon Steels

The main objective of this study was to compare the carbonitriding and gaseous ferritic nitrocarburizing processes. The driving force behind this research was to be able to use these results to determine the potential of ferritic nitrocarburizing as a suitable replacement to the carbonitriding process currently being used to impart a hard, wear resistant case on the surface of SAE 1010 steel torque converter pistons. The carbonitriding and ferritic nitrocarburizing processes were evaluated with respect to physical, mechanical, and metallurgical properties. The processes were compared quantitatively using distortion, retained austenite, and residual stress values, and qualitatively through optical and scanning electron microscopy. All test specimens were machined from low carbon steel and heat treated according to specified schedules. While the carbonitrided conditions were similar to those of the current production schedule, conditions for the nitrocarburized test specimens incorporated a range of processing times and temperatures. The results from this study support the use of gaseous ferritic nitrocarburizing as a means of reducing size and shape distortion in the torque converter pistons. Not only were distortion values lower after heat treatment, but the lack of retained austenite within the steel decreases the likelihood of further distortion associated with the delayed transformation of austenite to martensite. Although these findings do support the use of the nitrocarburizing process, the presence of tensile stresses measured at the surface of the pistons warrant the need for additional wear testing. Until such tests are performed, a change in process to gaseous ferritic nitrocarburizing cannot be endorsed