Kingo Kiyonaga and Toshio Okuno
The gas-carburizing capacity of some case-hardened steels in common use (JIS-S15CK, SCM22 SNC22, SNCM21 and SNCM23) was studied, using a batch-type gas furnace. Namely, the effect of dew point, carburizing temperature and holding time on the C distribution curve was determined by means of a turning method. From these results, the behavior of surface C content, the case depth and the diffusion coefficient of C due to various carburizing conditions was clarified concerning these kinds of steels. The main results obtained were as follows: (1) Cr-Mo steel showed a higher surface C content than Ni-Cr or Ni-Cr-Mo steels under the same gas conditions, and especially a much increased surface C content was observed in the former due to production of free cementite when the C potential of reaction gas rose over thes aturation content of C in austenite. On the other hand Ni-containing steels did not show such a tendency. (2) The case depth varied with changes in carburizing temperature, time, dew point and kinds of steels, but it will be noteworthy that the effect of dew point was particularly remarkable. (3) The diffusion coefficient of C at 925°C of these steels was 1300~1700 × 10-5 cm2/day; within this range,that of Ni-containing steel was larger than that of Cr-Mo steel. (4) As the conclusion, the gas-carburizing capacity of case hardened steels are determined by C diffusivity and the behavior of surface C content from the view-point of kind of steels. That is, the steel with a larger C diffusivity and a comparatively high surface C content is effeciently carburized and the case depth becomes larger, under the same gas condition.
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