微粒子ショットピーニングによるガス浸炭材の表面改質挙動に及ぼす投射材硬さの影響

摘要

Effects of the hardness of fine media used in shot peening on the surface modification behavior of workpiece with high hardness were investigated. Gas carburized JIS-SCM822 steels with the surface of 720 HV in Vickers hardness were shot peened by fine media with the range from 390 to 1550 HV in Vickers hardness, followed by the examination of their surface modification behavior. Shot peening was carried out by using suction-type air peening machine with 0.6 MPa in air pressure,10 s in peening time and 50 mm in peening distance. Various gas atomized powders sieved from 45 to 125 μm in particle diameter were used as fine peening media. On the peened surface by the fine media with remarkably higher hardness than that of workpiece, deep nanocrystalline layer with around 1 μm in depth and less than 15 nm in grain size was generated. In these nanocrystalline layers, the thickness increased and grain size decreased with increasing Vickers hardness of used peening media. In addition, Vickers hardness on peened surface also increased with increasing Vickers hardness of used peening media. Especially, the hardening effect on peened surface was outstanding, in the case of using fine peening media with more than 1020 HV in Vickers hardness. It seemed that this hardening effect was caused by grain refinement strengthening in nanocrystalline layer.%ショットピーニングは，投射材と呼ばれる金属やセラミックスの粒子を被処理材に噴射•衝突させることにより被処理材の表面に圧縮残留応力を導入し，疲労特性を改善する処理であり，使用する投射材の粒径により2種類に分類される.直径0.6〜1.0mm程度の投射材を用いる通常のショットピーニングに対し，直径0.1mm以下程度の投射材を用いる処理は微粒子ショットピーニングと呼ばれる.微粒子ショットピーニングは通常のショットピーニングと比較し，圧縮残留応力が被処理材の表面近傍に導入されるとともに被処理材の表面粗さが過度に増加しないことから，疲労強度改善効果に優れる.さらに被処理材表面にナノ結晶粒層を形成することも知られており，この層によりさらなる疲労強度の増加が見られることも報告されている.このような大きな疲労強度改善効果から，微粒子ショットピーニングの適用範囲は今後さらに拡大すると見込まれている.