Carbon Mass Transfer in Multimetallic Systems Containing Potassium

Carbon usually occurs in alkali metal containment materials as an impurity or alloying addition, and is a major constituent of carbide cermets used for bearings and valve seating and sliding components. Under certain conditions, carbon can transfer and result in deleterious changes associated with carburization and decarburization. Experiments were conducted to examine carbon transfer between pertinent materials, and demonstrate that it is reduced significantly by utilizing carbides of greater thermodynamic stability than those typically found in commercial materials. To determine whether carbon transfer from ferrous alloys would be impeded by alloying with titanium, which forms a relatively stable carbide, companion capsules of Type 316 (Fe-17Cr-12Ni-2Mo- 0.04C) and Type 321 (Fe-17Cr-12Ni-0.07C-0.46Ti) stainless steels containing potassium were heated for 1000 h at 1400°F. Both capsules contained Cb-lZr alloy specimens which are readily carburized when carbon is present in potassium. The Cb-lZr specimens were carburized significantly in Type 316SS capsules but not in Type 321SS capsules, demonstrating that the titanium addition impeded carbon depletion, and that bimetallic systems of Type 321SS and Cb-lZr are more attractive for reducing carbon transfer effects. (Nitrogen transfer was similarly impeded by titanium in Type 321SS.) The effects of carburization and nitriding on the mechanical properties of Cb-lZr are described. Carbon transfer from carbide cermet bearing materials was investigated with isothermal capsules for 1000 h with potassium at 800°, 1200° and 1600°F. Decarburization of the cermets was determined by weight change, X-ray diffraction and chemical analysis of the Cb-lZr capsule material for evidence of carburization. With commercial, cobalt-bonded WC and other materials containing WC to achieve high hardness, decarburization was observed at 1200° and 1600°F, but not 800°F. This was accompanied by carburization of the Cb-IZr capsule material and Mo-TZM specimens located near the cermets. X-ray diffraction of the cermets identified new phases, especially elemental tungsten in the surface region, which would alter friction, wear and diffusion bonding characteristics. Conversely, TiC cermets bonded with columbium, tungsten or molybdenum were not decarburized. These results are consistent with expectations based on the relative thermodynamic stabilities of the carbides involved. (author)