Cr含有量の異なるフェライト系ステンレス鋼の時効および高温疲労挙動

抄録

The purpose of the present study is to evaluate aging and fatigue behaviour at elevated temperatures in ferritic stainless steels with different Cr contents. The materials used were ferritic stainless steels, Type 430 (16.3Cr), 444 (18.72Cr) and 447 (30.66Cr). In all the alloys, hardness was increased by aging around 500℃ due to the 475℃ embrittlement and the most remarkable increase was observed in Type 447 with the highest Cr contents. Fully reversed axial fatigue tests have been performed at ambient and elevated temperatures (480∼520℃). At both ambient and elevated temperatures, Type 430 and 444 had nearly the same fatigue strength and Type 447 showed higher fatigue strength than those alloys. When fatigue strength was characterized in terms of fatigue ratio (σ/σB), the difference of fatigue strength among three alloys became considerably small, but Type 447 still exhibited lower fatigue strength. Brittle fracture was seen at elevated temperature in Type 447, which could be responsible for the observed lower relative fatigue strengths. It was indicated by TEM analysis that the 475℃ embrittlement occurred due to the spinodal decomposition of Cr phase that the decomposition was suppressed by stress cycling.