Author(s)

C. Nan, D. O. Northwood, R. J. Bowers, X. Sun & P. Bauerle

Abstract

Ferritic nitrocarburizing is being proposed as an alternative to gas carbonitriding to improve the surface characteristics of SAE 1010 plain carbon steel automotive components without producing unacceptable part distortion. Navy C-rings are specially designed specimens for the evaluation of the distortion (size and shape) that result from any heat treatment process. In this study, gas ferritic nitrocarburizing and nitrogen cooling was used to heat treat Navy C-ring specimens of varying thicknesses (2.8 mm–19.05 mm) at temperatures ranging from 510°C to 595°C. For each combination of part thickness and heat treatment temperature, the following parameters were evaluated: size and shape distortion; XRD and OM (optical microscopy) to characterize the nature of the nitride layers formed at the surface; X-ray determination of the residual stresses in the nitride surface layers. The various combinations of nitrocarburizing temperature and time resulted in an expansion of the OD (Outside Diameter) dimension and a small deterioration in flatness. The ID (Inside Diameter) changed from a small expansion (+0.02%) for the thickest specimens (19.05 mm) to a small contraction (-0.02%) for the thinnest specimens (2.8 mm). The gap tends to close up as the thickness decreases from the thickest to the thinnest specimens. Microstructural differences were found in the nitrided layers formed at the different heat treatment temperatures. The residual stresses in the outer ε-nitride layers (Fe3N) were typically tensile. On the basis of these results, preliminary recommendations are made as to heat treatment parameters to lower distortion. Keywords: navy C-ring, ferritic nitrocarburizing, carbonitriding, dimensional distortion, residual stress.

Keywords

navy C-ring, ferritic nitrocarburizing, carbonitriding, dimensional distortion, residual stress