Internal oxidation during carburizing and heat treating

The internal oxidation of the surface layers of ferrous engineering components during carburizing was known for many years, but post heat treating machining and grinding operations have ensured that this oxidation phenomenon has had no deleterious effects on their properties. Recently, however, economical considerations have demanded that where possible such finishing operations should be eliminated and hence there is now considerable interest in avoiding internal oxidation during carburizing—or heat treating steel in general. It is the purpose of the present paper to discuss the factors which in-fluence the depth and configuration of internal oxidation, in particular the effects of steel composition and the thermochemical processing conditions. Internal oxidation and the associated metallurgical difficulties cannot be sufficiently avoided during industrial car-burizing, and hence the factors necessary to minimize internal oxidation are discussed, including the use of oxygen free carburizing media and silicon reduced steels. This paper is based on a presentation made at a symposium on “Carburizing and Nitriding: Fundamentals, Processes and Properties” held at the Cincinnati Meeting of The Metallurgical Society of AIME, November 11 and 12,1975 under the sponsorship of the Heat Treatment Committee.