熱間ダイス鋼における軟窒化拡散層のＴＥＭ組織観察

Abstract

Nitriding, which includes nitrocarburizing, is the most popular surface treatment in order to prolong durability of hot working die steels. However, it has not been elucidated that either precipitation hardening or solid solution hardening caused the hardening mechanism of the nitriding. In this work, the morphology of nitrogen in a diffusion layer of Fe-C-Si-Mn-Cr in a practical nitrocarburizing condition was investigated by transmission electron microscopy and X-ray diffraction in order to elucidate the influence of Cr on the hardening mechanism in the nitrocarburizing. In the diffusion layer of the nitrocarburized specimens, precipitation of CrN was identified regardless of the amount of Cr contents. The crystal structure of CrN was confirmed as a cubic B1 type and has Baker-Nutting orientation relationship with martensite. Besides, the lattice constant of martensite in the diffusion layer measured by X-ray diffraction was still similar to that of the unnitrided zone. Those results show that the major contribution of nitrogen to the hardening in the diffusion layer is precipitation hardening as CrN, and the contribution to the solid solution hardening is small. And increasing Cr contents enhanced the CrN precipitation and the hardness of the diffusion layer, although the diffusion layer shrank.