Compound Layer Characteristics Resulting from Plasma Nitrocarburising in Atmospheres Containing Carbon Dioxide Gas Additions

Abstract

Plasma ferritic nitrocarburising of iron-carbon alloys and low alloy steels has been performed at 570°C in atmospheres containing carbon dioxide gas additions. Variations in compound layer structure, thickness, porosity, and composition have been investigated. It has been found that the formation of a predominantly & phase compound layer structure is favoured by the use of high nitrogen atmospheres, and cementite formation isfound to be delayed by the presence of carbon dioxide in the system. Initiation of cementite is found to be strongly dependent on substrate composition, and the formation of this cementite phase is not necessarily via pore formation or a denitriding lnechanism, as is commonly found with gaseous nitrocarburising. The compound layer thickness was found to increase with increasing carbon dioxide content, except where the substrate contained carbides, when a continuous reduction in compound layer thickness was observed. Some degree of porosity in the & subregion may develop during plasma nitrocarburising with high nitrogen additions. The pores are of an intergranular type, and the pore walls can dissolve oxygen when in direct contact with an open atmosphere. Proper process control and the addition of carbon dioxide to the atmosphere results in a reasonably thick compound layer composed predominantly of & phase.