Fatigue Crack Propagation Characteristics of Ductile Cast Iron Austempered from (α+γ) Phase Region

Abstract

In the case that the matrix-microstructure of ductile cast iron was changed into the single-phase of bainite by austempering it from γ phase region (full-austempering), the fatigue crack propagation resistance decreased considerably in the low ΔK region as compared with that of the as-cast material. In this paper, the effect of austempering from (α+γ) phase region (partial-austempering) was investigated to improve the fatigue crack propagation resistance of ductile cast iron. Fatigue tests were carried out at a stress ratio, R=0.1.
The matrix-microstructure consisting of the dual-phase of ferrite and pearlite in the as-cast material was changed into the dual-phase of ferrite and bainite by partial-austempering, and the fatigre crack propagation resistance of the partial-austempered material was higher than that of the as-cast material in the entire ΔK region. The improvement of fatigue crack propagation resistance was due to the increase of crack closure which was induced by the roughness of the fracture surface and the formation of the oxide deposits. Furthermore, it was found that the static tensile properties might be improved by partial-austempering.