Deep-hole carburization in a vacuum furnace by forced-convection gas flow method

Carburization in the deep hole with high aspect ratio is a great challenge for the metallurgist. In this paper, the deep-hole carburization of JIS SCM415 as well as SNCM 220 steel with an aspect ratio of beyond 30 has been developed in a two-chamber vacuum furnace under low-pressure acetylene atmosphere by virtue from the forced-convection gas flow carburizing method. The proposed experiment was performed at 950–1050 °C for 15 or 30 min to evaluate the specimens on the uniformity of the carburized layers for the inner surface of the holes through the hardness distribution profiles and the effective case depth (ECD) of the hardened layers. Additionally, the exterior ECD of the specimens was also determined for comparison. With the experimental results, it is shown that a uniform carburized layer was formed in the surface of the deep hole. Furthermore, the diffusion coefficient of carbon atoms was then determined through this study. This paper surely contributes the approach to carburize the deep hole with high degree of uniformity regardless of the aspect ratio.