Kazuyoshi Kurosawa

Characterization by CEMS, XRD and XPS of oxidized layers formed on the surface of carbonitrided low-carbon steel

Abstract The surface structure and oxidation characteristics of carbonitrided low-carbon steel, prepared by oxidizing the surfaces in salt-baths after the carbonitriding process, have been studied using conversion electron Mossbauer spectrometry, X-ray diffractometry, and X-ray photoelectron spectrometry. The formation of an iron oxide layer in the carbonitrided surface improved the corrosion resistance significantly. The steel surface after carbonitriding for 3600 s consisted of e- and γ′-nitride, e-carbonitride, and cementite. After the oxidation, two sextets and one doublet were observed, attributable to the presence of magnetite, Fe3O4, and of fine crystals of ferromagnetic iron oxides such as α-Fe2O3, γ-Fe2O3, and Fe3O4. Crosssectional analysis or depth profiling of the carbonitrided layer revealed the presence of iron oxides not only in the outermost oxide layer but also in the carbonitrided layer. Oxygen diffused deep into the carbonitrided layer and formed fine crystals of iron oxides there, distorting the crystal lattices of e-carbonitride and γ′-nitride.

Characterization of carbonitrided layers formed on stainless steel by conversion electron Mössbauer spectrometry

Austenitic stainless steel was carbonitrided by the tufftride process, and the hardened layers formed on the surface were investigated by conversion electron Mössbauer spectrometry (CEMS) and grazing angle X-ray diffractometry (GXRD). It was found that carbides such as M7C3 (M = Fe, Cr), chromium nitride (CrN),ε-nitride (M2N, M = Fe, Cr), andε-carbonitride º2+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,∈ M2+x(C,N) became a main component, while M7C3 and CrN disappeared in the outermost surface. After 60 minutes, M7C3 and CrN were observed again, and theγ nitride, the oxide of iron and chromium (FeCr2O4), was formed on the outermost surface for the first time. Cross-sectional micrographs of surface layers using a scanning electron microscope (SEM) after etching the hardened layers with Marble reagent revealed the presence of black and white layers. The former layer mainly consisted of∈ M2+x(C,N),∈ M2N, CrN, and M7C3, and the latter layer did not contain nitrogen, although carbon was detected in both layers. The Vickers hardnesses of the black and white layers were HmV(0.l) 1000 to 1200 and HmV(0.l) 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 resistance proved by anodic polarization curve measurements in 5 vol pct H2SO4 solution. The white layer formed on carbonitrided stainless steel beneath the black layer has possibilities as an excellent corrosion and wear resistive layer.