Diffusion Chromising of Steel by CVD Using Chloride Compounds: 4 Influence of Steel Composition

Abstract

The effects of carbon content and low titanium additions on the structure and thickness of chemical vapour deposited chromium coatings produced at 930°C have been studied. Diffusion of chromium in these steels has been examined by calculation of the average diffusion coefficients and determination of the concentration dependence of the interdiffusion coefficient by the Boltzmann- Matano method. The structure and thickness of the coating obtained depend largely on the amount of soluble carbon in the steel. The steels that contained 0.002–0.004% soluble carbon were most suitablefor chromising since only a very thin surface carbide layer was formed. Those steels which contained >0·05% soluble carbon at 930°C produced coatings containing dispersions of (Cr,Fe)₂₃C₆ carbide. Such coatings remain thinner and are no longer pure diffusion coatings.