オーステンパー球状黒鉛鋳鉄の引張および疲労特性

抄録

The tensile and fatigue properties of spheroidal graphite cast irons with ferritic matrix structure and pearlitic one, being austenitized at 1173K and austempered for either 5min or 2h at 573K, were investigated. The amount of retained austenite in ferritic matrix was more than that in pearlitic matrix. Axial fatigue tests were conducted at a stess ratio of R=0.1.
The tensile strength of the both as-received materials were improved by austempering treatment. Separation at the graphite-matrix interface during tensile deformation occurred first and the initiation of cracks followed around each graphite. Finally these cracks were simultaneously linked-up. As the result of the coalescence of those cracks the fracture surface of the as-received materials exhibited brittle pattern. On the other hand, the fracture surface of the austempered specimens showed a mixed type of microvoid formation and quasi-cleavage associated with austempering structure pattern.
Although the fatigue strength of the material austempered for 2h was rather improved, the strength for the 5min one was weakend as compared with that of the as-received material. The fatigue crack growth rate da /dn of both austempered materials, was higher than that of the as-received at lower ΔK values. Fatigue crack path followed a mixed type of both transgranular and intergranular fracture.
Characteristic fracture surface in the vicinity of the fatigue limit was predominantly covered by intergranular fracture without straiation in the ferritic matrix, while considerable lamellar structural pattern was observed in the pearlitic matrix. Fracture surface of the austempered specimen showed no significant difference irrespective of austempering time and indicated the similar structural pattern as in the static tensile fracture.