Effect of Improvement in Matrix-structure on Fatigue Crack Propagation Resistance of Spheroidal Graphite Cast Iron

The fatigue crack propagation characteristics at the stage 2 in spheroidal graphite cast irons, whose matrix-structures were changed by heat treatments in order to improve the static strength, have been compared with those of the as-cast materials. Fatigue tests were carried out at stress ratio, R=0.1, and the results were investigated from the viewpoints of the crack closure and the fractography. In the case that the matrix-structure was changed into the single-phase of bainite by austempering, the static strength increased, while the fatigue crack propagation resistance decreased in low ΔK region. On the other hand, in the case that the matrix-structure was changed into the dual-phase of ferrite and martensite by quenching from the (α+γ) phase region, both the static strength and the fatigue crack propagation resistance increased. The difference in fatigue crack propagation resistance between the two heat-treated materials was explained from the crack closure behavior.