2025 Volume 66 Issue 9 Pages 1239-1245
Iron-powder pack (IPP) treatment, which has been proposed as a novel surface modification technique, was applied to a hard chromium film plated on low-carbon steel to further enhance its hardness. As the first step, a piece of pure chromium was used to examine the effectiveness of IPP treatment, since chromium is known to be covered with a stable oxide film. It was embedded in mixtures of iron, graphite and alumina powders and subsequently heat-treated at 1273 K for 3.6 ks in a nitrogen flow. The volume ratio of iron to graphite was varied from 0:5 to 4:1, and that of iron + graphite to alumina was fixed at 5:3. When mixtures with the iron powder was used, the diffusion of carbon and nitrogen into pure chromium occurred. The specimen heat-treated using a 3:2:3 mixture of iron, graphite and alumina powders showed an average surface hardness of HV = 1940 because of the formation of reaction products like M7C3 (M = Fe, Cr). However, the microstructure near the chromium surface was inhomogeneous. This would be caused by contact and a reaction with an iron powder used in mixtures. To avoid such a situation during heating, low-carbon steel coated with a hard chromium film was wrapped with a weighing paper and then was subjected to the above IPP treatment. There was no influence of the paper on the diffusion of carbon and nitrogen, and a relatively homogeneous microstructure was obtained. In addition, the hardness of the modified film was more than HV = 1500 and was about two times higher than that of as-plated state, which had a hardness of approximately 710.
