表面硬化処理したSCM420H 鋼の水素添加による遅れ破壊寿命特性評価

ガス浸炭焼入れを施したJIS SCM420Hの応力集中係数（Kt）が異なる環状切欠き丸棒に対して、遅れ破壊寿命特性を実験力学的に評価した。定常域での変形速度および破断寿命はともにKtに依存しなかったが、遅れ破壊限度は、Ktが大きいほど低下した。そこで、破面解析の知見から水素がトラップされる切欠き近傍の浸炭領域と、比較的に水素の影響を受けない内部の非浸炭領域に区分し、それぞれの領域での静水圧応力σmの平均値（σmCおよびσmNC）で遅れ破壊限度および破断寿命を評価することができた。

The delayed fracture properties were evaluated by conducting constant load test using notched round bar specimens with different stress concentration factor （Kt） of gas carburized JIS SCM420H. The amount of hydrogen added to specimens after gas carburizing were almost same regardless of Kt value. The testing method employed was a weight-lever method with constant load application, and the displacement of the specimen during the test was measured with a laser displacement meter. Both the deformation rate and the fracture time did not depend on Kt, but the delayed fracture limit decreased as Kt increased. Based on the morphology of fracture surface analysis and the distribution of hydrostatic stress（ σm），this study was conducted in two cases for the carburizing region near the notch where hydrogen accumulated and for the internal non-carburizing region obtaining the averaged σm in each area,σmC and σmNC. The presented stresses could evaluate the delayed fracture limit and the fracture time.

Delayed fracture, Life evaluation, Hydrogen, SCM420H, Carburizing