Abstract
The protection of crack formation by a functionally graded layer at the interface of surface-hardened iron alloys has been investigated using a modified reactive diffusion process which consists of the two-stage diffusion heat-treatment for diffusion species. From the stress analysis on thermal shock test and the microstructure observation, it was known that cracks formed on the thermal shock test were onion cracks arising from a radial stress σr and radial cracks due to a circumferential stress σθ. In the specimens subjected to the modified reactive diffusion heat-treatment, a functionally graded layer was formed at the interface between the surface carbide layer/iron substrate, which effectively suppressed the generation of cracks by shifting a location of the maximum tensile stress toward the substrate and relaxing a stress concentration at the interface.
