CHARACTERIZATION OF CARBONITRIDED LAYERS FORMED ON STAINLESS-STEEL BYCONVERSION ELECTRON MOSSBAUER SPECTROMETRY

Austenitic stainless steel was carbonitrided by the tufftride process, and the hardened layers formed on the surface were investigated by co nversion electron Mossbauer spectrometry (GEMS) and grazing angle X-ra y diffractometry (GXRD). It was found that carbides such as M(7)C(3) ( M = Fe, Cr), chromium nitride (CrN), epsilon-nitride (M(2)N, M = Fe, C r), and epsilon-carbonitride {M(2+x)(C,N), M Fe, Ni} were precipitated on the outermost surface at the initial stages of carbonitriding. By the increase of treatment time up to 20 and 30 minutes, epsilon M(2+x) (C,N) became a main component, while M(7)C(3) and CrN disappeared in t he outermost surface. After 60 minutes, M(7)C(3) and CrN were observed again, and the gamma' nitride, the oxide of iron and chromium (FeCr2O 4), was formed on the outermost surface for the first time. Cross-sect ional micrographs of surface layers using a scanning electron microsco pe (SEM) after etching the hardened layers with Marble reagent reveale d the presence of black and white layers. The former layer mainly cons isted of epsilon M(2+x)C,N), epsilon M(2)N, CrN, and M(7)C(3), and the latter layer did not contain nitrogen, although carbon was detected i n both layers. The Vickers hardnesses of the black and white layers we re HmV(O.1) 1000 to 1200 and HmV(0.1) 500 to 600, respectively. It was said that both layers were harder compared with HmV(0.1)200 of bulk. The white layer was far superior to the black one in the corrosion res istance proved by anodic polarization curve measurements in 5 vol pet H2SO4 solution. The white layer formed on carbonitrided stainless stee l beneath the black layer has possibilities as an excellent corrosion and wear resistive layer.