（α＋γ）域からオーステンパ処理した球状黒鉛鋳鉄の疲労亀裂進展特性

抄録

  In order to clarify the effectiveness of austempering from (α+γ) phase region, tensile tests and fatigue crack propagation tests were carried out, and then the fracture surface and the cross section of fatigue cracks were examined. When the matrix is changed into the dual-phases of ferrite and bainite by the austempering, the tensile strength is improved and also the fatigue crack propagation resistance is more improved than those of as cast materials. The effect of stress ratio on the fatigue crack propagation in the case of the material austempered from (α+γ) phase region is smaller than that austempered from (γ) phase region. The improvement of the fatigue crack propagation resistance may be due to increase the roughness-induced and oxide-induced crack closure, because of the dual-phases of ferrite and bainite in the matrix. Consequently, it is concluded that the austempering from (α+γ) phase region is an effective heat treatment to improve both the tensile strength and fatigue crack propagation registance.