Abstract: In a surface hardening process for metal parts by nitriding or carbonitriding, the parts to be treated are placed in a furnace and maintained at a temperature of between 490.degree. C. and 750.degree. C., in a nitrogen bearing atmosphere, such atmosphere formed by introducing into the furnace a gaseous mixture comprising in particular ammonia and an oxidizing gas for accelerating the catalytic dissociation of ammonia in contact with said parts. The oxidizing gas is nitrous oxide and the gaseous mixture contains about 0.1% to 10% by volume of nitrous oxide. The invention is well suited for the surface hardening of steel parts.