Metal treatment – Compositions – Heat treating
Patent
1986-10-07
1987-10-27
Silverberg, Sam
Metal treatment
Compositions
Heat treating
148 635, 148 165, C23C 824
Patent
active
047027797
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.2 is introduced into the furnace, the concentration ratio of ammonia to nitrous oxide in the atmosphere introduced in the second step being such that 0.ltoreq.[NH.sub.3 ]/[N.sub.2 O].ltoreq.5, the furnace atmosphere being renewed at a rate of 2 to 5 such that the atmosphere gradually becomes oxidizing. The steel articles thus produced feature a base layer of epsilon type iron nitride over the core, an intermediate layer of iron oxinitride having a thickness of 2 to 12 microns and an outermost surface layer of iron oxide having a thickness of 1 to 7 microns.
REFERENCES:
patent: 4563223 (1986-01-01), Dawes
patent: 4596611 (1986-06-01), Dawes
Madsac Michel
Mauvoisin Rene
L'Air Liquide
Robinson, Jr. Lee C.
Silverberg Sam
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