Formation of Fine and Coarse Grain Layer by Atmosphere Control in Case Hardening*

Abstract

Abstract Grain coarsening can cause decrease of the fatigue strength of carburized steel components. This artefact is considered to derive from a change in crack-initiation behaviour at the surface. However, grain coarsening can increase the hardenability of steel and thereby avoid the addition of alloying elements bringing about such hardenability. Thus, it is expected that steel with a dual-layer structure of a fine grain layer at the surface and a coarse grain layer in the inside achieves good fatigue strength and high hardenability while saving alloy elements. In this study, low pressure carburizing of JIS SCR420-Low N steels were carried out at 1323 K, both with and without the use of NH3 gas as a source of N to precipitate AlN. After carburizing without NH3 gas, only coarse grains were observed from the surface to the inside of the specimens. In contrast, after carburizing with NH3 gas, a dual layer structure with only fine grains at the surface and only coarse grains at the inside was observed. These results show that a dual-layer structure can be successfully obtained through the control of the AlN precipitation behavior by optimizing NH3 gas conditions during low pressure carburizing.