Raising corrosion resistance of surface layer of stainless steel - with low carbon content by diffusion of nitrogen, useful for treatment of tools for food

摘要

A heat treatment process for raising the corrosion resistance hardened surface layers of units of stainless martensitic steel with less than 0.4% of C involves diffusion of 0.2-0.8% of N into the surface layer at a temp. above Ac1, followed by direct hardening at drawing at 150-250 deg.C. Nitrification is in a gas atmos. comprising N2 and/or NH3 and opt. contg. H2; the atmos. may contain CH4, propane or other hydrocarbons in amt. such that the C content in the surface layer does not exceed the N content. The nitrification may be supported by a plasma as in plasma carbonisation. USE - Treatment of tools for food and polymer processing, of pump units and valves for fluids carrying particles, of roller bearings and of gear parts is claimed. Other applications are to tools and valves.