Masahiro Okumiya1, Yoshiki Tsunekawa1, Itaru Niimi1 and Hiroyuki Kameda2
1Faculty of Engineering, Toyota Technological Institute, Nagoya
Pure iron and low-carbon steel (S15CK) foils were used as a specimen. Carbon and nitrogen contents in the furnace atmosphere of gas-carbonitriding were controlled by the dew point and retained ammonia, and the influence of temperature on the critical carbon and nitrogen contents for the formation of specific microstructures was studied. The specific microstructures were determined to be void, cementite, and graphite. The effects of the carbon and nitrogen contents on the mechanism of formation of the specific microstructures were discussed. In both kinds of specimen, the critical nitrogen content for formation of voids decreased with the treating temperature, and time. The critical carbon content for the formation of cementite or graphite increased with the treating temperature, but it was constant regardless of the treating time. The critical carbon content for the formation of cementite was practically unaffected by the nitrogen content. Under the same treating conditions, the critical carbon for the formation of cementite or graphite in pure iron and S15CK was nearly equal to each other. The critical nitrogen for the formation of void in pure iron was always lower than that in S15CK, because there were nonmetallic inclusions in S15CK and the austenite grain size was finer. In both specimens, graphite was formed only in the case of carbortitriding in which the nitrogen content was over about 0.1 mass%. Cementite existed whenever graphite was formed, and graphite existed mainly in and near cementite but sometimes at a distance. It appeared that nitrogen added to steels would afford the situation for formation of graphite in the solid phase and assist graphite formation.
pure iron, S15CK, gas-carbonitriding, critical carbon content, critical nitrogen content, specific microstructure, void, cementite, graphite
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