Investigations have been carried out on steel/chromium diffusion couples, which were prepared by electrodepositing chromium on various plain carbon steels. Influences of carbon contents in the steels used as materials, of heat treatment temperature, and of holding time on the structure, composition, hardness etc., of produced diffusion layer were studied.
The results are summarized as follows:
1) The diffusion layer is formed even after heat treatment at a comparatively low temperature such as 600°C. On all the steels, the produced diffusion layers are of about 85% in Cr concentration.
2) When heat-treated at a higher temperature than 800°C, the steel has a diffusion layer of the composition different from that in (1), the layer being of about 75% in Cr concentration. In this system, as the material contains carbon, the produced polyphase diffusion layer is different from that observed in the Fe-Cr system and changes according to the temperature range.
3) Formation of the diffusion layer hardly depends on carbon content of the material and the thickness of the layer is almost constant for all the carbon contents.
4) In the diffusion layer of a high Cr concentration, carbon is locally saturated and the layers of constant-carbon cencentration are formed which contain more carbon than in the steel. It appears that, chromium carbide and double carbide of iron and chromium may be formed but the existence of those carbides could not be proved by X-ray diffraction, perhaps because of their too small amounts.
5) The produced diffusion layer has a comparatively good resistance against hydrochloric acid, picric acid etc., and its hardness, as one of mechanical properties, is as high as 1 000 or more in HK, which may be typical of carbide.