Abstract: The powdered material contains oxygen in the oxide and/or adsorbed form and the oxygen present is reduced in a first pre-sintering stage and the cohesion of the material is ensured in a second sintering stage. The pre-sintering stage is carried out under a reducing atmosphere based on hydrogen and neutral gas whose flow rate F.sub.G is higher than or equal to: ##EQU1## in which relation: S.sub.P =section of the layer of powder to be sintered in sq.mD.sub.P =voluminal mass of the powder in kg/cu mX(O.sub.2)i=percentage of oxygen mass in the powder before the pre-sintering stage, in the oxide and/or adsorbed form,P(H.sub.2)i=voluminal percentage of hydrogen in the gas introduced into the furnace,P(H.sub.2)f=the smallest voluminal percentage of hydrogen in the atmosphere in the furnace at a point where the oxides have been completely reduced,v.sub.S =speed of feed of the material in the furnace expressed in m/hr,.alpha. is a constantF.sub.G being expressed in cu.m/hr.

Abstract: A heat process for producing corrosion resistant steel articles, comprising a first step during which the articles placed in a furnace are nitrided in a gaseous atmosphere to produce on their surface a first layer consisting essentially of epsilon type iron nitride, followed by a second stage during which the articles are exposed to an oxidizing atmosphere to form a second, oxide-rich layer on the surface of the articles. According to the invention, during the first step, an atmosphere comprising 20 to 70% by volume of ammonia NH.sub.3, 1 to 5% by volume of nitrous oxide N.sub.2 O and 25 to 79% by volume of nitrogen N.sub.2 is introduced into the furnace, the concentration ratio of ammonia to nitrous oxide [NH.sub.3 ]/[N.sub.2 O] in the introduced atmosphere being greater than 5 so that the furnace atmosphere is not oxidizing, and during the second step, an atmosphere comprising 0 to 25% by volume NH.sub.3, 0 to 5% by volume N.sub.2 O and 70 to 100% by volume N.sub.

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.