ガス窒化処理したSCM435鋼の回転曲げ疲労強度に及ぼす表面化合物相の影響

摘要

The bending fatigue strength (10~5-10~7 cycle) of gas nitrided or gas nitrocarburized SCM435 steels was evaluated. The steel formed compound layer on the surface exhibited higher fatigue strength than that with less compound layer. When the compound layer was mainly with γ-Fe_4N phase, the strength was remarkably high. The yield stress and residual compressive stress of the compounds were higher than those of the diffusion layer, respectively. The compound layer may prevent initial crack generation. In both of ε-Fe_3N and γ compounds, multiple microcrackings on the specimen surface may reduce the stress concentration at the main crack tip and make the steel toughed. The γ compound showed the higher residual compressive near the surface stress than ε one, was plastically deformed, and was of fine grains so that the microcrack growth in the γ compound may be prevented. Therefore, γ compound showed the higher fatigue crack initiation resistance than that of the ε compound showing brittle crack growth, and then the nitrided steel with γ compound layer exhibited higher fatigue strength.%ガス窒化·軟窒化したSCM435鋼の回転曲げ疲労強度を評価した。化合物層形成材は化合物層無し材ょりも高い疲労強度を示し，/相の場合において顕著であった。化合物層は，拡散層より高い降伏強さと大きな圧縮残留応力を示すことから，初期き裂抑制効果がある。£相，/相いずれにおいても複数の微小き裂形成により，主き裂先端の応力集中を緩和し，疲労強度を高める。さらに，/化合物は，£化合物よりも大きな表面圧縮残留'お力に加え，塑性変形を示し，かつ微細結晶粒であることから，微小き裂成長の抑制に有効である。したがって，γ化合物は，脆性的き裂成長を示す£化合物よりも表面き裂発生抵抗が大きく，高い疲労強度を示す。