FACTORS DETERMINING GROWTH OF DIFFUSION LAYERS ON LOW CARBON STEEL AND IRON DURING VACUUM CHROMISING

Abstract

Layers produced on low carbon steel and Armco iron during vacuum chromising have been investigated. In particular, the influence of two fundamental factors, the surface area of the sublimating metallic chromium and the pressure within the furnace chamber, is discussed. Layer thickness and morphology, and the chromium and iron concentrations in the diffusion zone of chromised layers, have been studied. Layers consisting of solid solutions of chromium in iron resulted in all instances. Chromising conducted at low pressure, with a static rather than a dynamic vacuum, was found to be most effective for maximising the rate of growth of diffusion layers. Scanning electron micrographs of the surfaces of chromised low carbon iron alloys show the presence of microsteps characteristic of screw dislocations. Hence, it is concluded that the growth of diffusion layers during vacuum chromising depends on the generation and growth of dislocations according to Frank‘s spiral lnechanism model.