ガス窒化処理したクロム含有低合金鋼における拡散層硬さ分布予測の実用モデル

摘要

ガス窒化処理は，Fe-N系の共析変態点以下の温度で処理されることから熱ひずみが小さく，また変態ひずみを伴わないため，浸炭や高周波焼入れといった他の表面硬化処理と比ベ熱処理ひずみが小さい表面硬化処理である.ガス窒化処理された鋼の表面には，化合物層と呼ばれるγ′相(Fe_4N)やε相(Fe_(2-3)N)を主体とする鉄窒化物層と，その下地に母相に固溶した窒素拡散層が形成する.また，ガス窒化処理に適用するJIS-SCM435鋼やJIS-SACM645鋼のようなCr含有低合金鋼の多くは，拡散層中に合金窒化物が析出し，拡散層をさらに硬化させる.化合物層は耐摩耗性や耐食性，また拡散層は耐疲労特性の向上に寄与することから，化合物層の厚さや相構造，また拡散層の硬さ分布といったそれぞれの層を代表とする窒化特性を事前に予測可能であれば多くの試作低減に寄与する.%A model to predict the diffusion layer's hardness profile in gas-nitrided low alloy steels such as JIS-SCM435, DIN-31CrMoV9 and JIS-SACM645 was successfully developed taking into account the following considerations. The diffusion of dissolved nitrogen in the surface boundary was given as the nitrogen flux, which can be determined by either the nitriding potential or thermo-equilibrium between α-Fe and γ′-nitride after the formation of the compound layer. The kinetics of CrN and AlN precipitations was characterized as the K_P concerned with nucleation and diffusion of Cr or Al and the parabolic rate constant x concerned with the growth of disc-shaped CrN and AlN. K_H was defined as a proportional constant between the increase in hardness and the nitrogen concentration. K_P and the solubility product K_(CrN) for CrN were fitted well using the numerical model and the hardness profiles. K_H was experimentally determined by using the hardness and nitrogen concentration in the diffusion layer. Then, the numerical results showed good agreement with the experimental ones. In SACM645especially, a more accurate calculation was demonstrated taking into account immobile excess nitrogen in the CrN and AlN.