Effect of decarburization on microstructure of DC-plasma nitrided H 13 tool steel

บทคัดย่อ

Decarburization may occur during heat treatment of H13 steel resulting in carbon deficiency near the specimen surface. Microstructures and properties of H13 tool steel after surface treatment such as plasma nitriding can be affected by decarburization. This study is focused on the microstructure of plasma nitrided H13 steel. Decarburized and non-decarburized H13 steel was plasma nitrided for 36-72 ks at temperatures varying from 773 to 823 K. The microstructure of the plasma nitrided layer was analyzed by an optical microscope and Electron Probe Micro Analysis (EPMA). The thickness of the nitriding layer is measured. The microstructures of the decarburized and the non-decarburized and plasma nitrided samples are different. The non-decarburized sample shows only one surface layer which consists of Fe₃N (ε), Fe₄N (γ′), and CrN phases. In contrast, the nitriding layer of the decarburized sample consists of two layers. The top layer adjacent to the surface contains only Fe₃N (ε) and Fe₄N (γ′) phases, while the beneath layer consists of Fe₃N (ε), Fe₄N (γ′), and CrN phases. The difference of microstructures also affects surface hardness and hardness depth. Surface hardness and hardness depth of the decarburized samples is lower than that of the non-decarburized samples.