低合金鋼の高温高サイクル疲労におけるフィッシュアイ破壊

抄録

Internal fractures with the fish-eye were recognized in high cycle fatigue for Cr-Mo low alloy steel at elevated temperatures. To understand this phenomenon, the fish-eye patterns were examined in detail.
Stepwise S-N curves were obtained for rotating bending and uniaxial loading fatigue tests and internal fractures with the fish-eye were observed at low stress and high cycle region. Stress amplitudes to obtain S-N curves were bigger than 0.2% proof stress for both types of fatigue tests and it was recognized that specimens hardened during the fatigue tests.
Ligaments were observed on the fracture surface between the fish-eye and the specimen surface. In the relationship between the width of the ligament and the plastic zone size at tip of the fish-eye estimated by using cyclic 0.2% proof stress as yield strength, there is no good correlation between them on the whole. But the data plots could be classified into three types, that is, one where good correlation was recognized between the two and the others where the plastic zone sizes were estimated bigger or smaller comparing with the width of the ligament.
A range of histogram of hardness measured at a depth of 40μm from the specimen surface at the cross section of a fatigued specimen expanded to higher levels of hardness than that at a depth of 150μm. The hardened region at the surface layer of the fatigued specimen was considered to be a crystal grain where slip deformation occurs easily depending on the relationship between crystallographic orientations and loading axis, and hardening due to dynamic strain aging occurs effectively.
Crystal grains at the specimen surface where fatigue crack should form basically hardened during the fatigue test, and then, it became difficult for fatigue crack to initiate from the grains. During this period fatigue crack initiated at an internal defect such as a nonmetallic inclusion and internal fracture with the fish-eye occurred.