HEAT TREATMENT OF STEEL AND METALS; COATING OF METALS AND METAL ALLOYS AND CERAMICS IN VACUUM INSTALLATIONS AND PLASMA VACUUM INSTALLATIONS; SURFACE TREATMENT OF METALS, METAL ALLOYS AND CERAMICS; HARDENING OF METALS AND ALLOYS IN A VACUUM; SURFACE TREATMENT OF METALS, METAL ALLOYS AND CERAMICS THROUGH DIFFUSION, IN PARTICULAR NITRATING