Preparation of an ε-carbon nitride surface layer on ferrous metal parts

Claim 1

The carbon nitriding process of producing an ε-carbon nitride layer on the surface of ferrous metal articles in a gaseous atmosphere contained in a furnace chamber, comprising contacting at a temperature between 500° and 650° C said ferrous metal articles with said gaseous atmosphere, said gaseous atmosphere comprising (i) partially dissociated ammonia, and (ii) carbon monoxide and carbon dioxide in a total of between 2 and 20 volume %, and maintaining said gaseous atmosphere during the nitriding process to have an average nitriding potential, p NH .sbsb.3 /p H .sbsb.2 1 .5 , of between 0.5 and 5, and a partial pressure relationship, p CO /p CO .sbsb.2, of between 1 and 10; wherein, p NH .sbsb.3 is the partial pressure of ammonia in the furnace chamber; p H .sbsb.2 is the partial pressure of the hydrogen in the furnace chamber; p CO is the partial pressure of carbon monoxide in the furnace chamber; p CO .sbsb.2 is the partial pressure of the carbon dioxide in the furnace chamber said gaseous atmosphere having been formed by introducing (a) an ammonia-containing gas, and (b) a combusted gas which is a mixture of carbon monoxide, carbon dioxide and nitrogen, into said furnace chamber, said ammonia-containing gas (a) being in a ratio to said combusted gas (b) of between 1:0.3 and 1:3.