Diffusion annealing of Fe-Ni alloy coatings on steel substrates

A two-stage surface treatment of steel is described. During the first stage , a steel surface is coated with an Fe-14% Ni electrodeposit having an init ial hardness of 300-400 HV. Subsequently, the microstructure and hardness o f the coatings are modified by thermal and thermochemical treatment. The an nealing at temperatures between 500 and 1.000 degrees C leads to the diffus ion of carbon from the substrate to the coating and an increase in coating hardness after cooling. In some cases, the en rich ment of coating in carbo n is enhanced by applying an external source of carbon and nitrogen. As an example, carburizing and carbonitriding in solid media are presented. Owing to a difference in the temperature of the alpha-gamma phase transformation between the steel substrate and the Fe-Ni coating, the thermal treatment i s conducted at a coexistence of alpha-gamma or gamma-gamma diffusion couple s. This allows us to obtain the various microstructure and depth-profiles o f hardness across the coating thickness and the adjacent region of the subs trate. Some benefits of the proposed surface treatment are discussed. (C) 1 998 Kluwer Academic Publishers.