Near threshold fatigue behaviour of flake graphite cast irons microstructures

This paper describes the influence of material toughness degradation, through reversed temper embrittlement (RTE) and mean stress on the near threshold fatigue crack growth characteristics of a CrMoV turbine bolting steel at ambient and elevated temperatures. It was established at ambient temperatures that strong effects of R-ratio and material condition (toughness) were observed on near threshold fatigue crack growth characteristics. At elevated temperatures it was shown that for the non-embrittled material that only under low R-ratio conditions did increased temperature increase the level of threshold stress intensity ΔK_th, by some 20%. In the case of embrittled material, increasing the temperature increased ΔK_th levels by around 30% and decreased near threshold growth rates by an order of magnitude at low to intermediate R-ratio levels.The effects of R-ratio on ΔK_th for all material and mechanical testing conditions could be simply expressed by the difference between ΔK_th at R = O and a constant B multiplied by R.Quantitative fractographic observations indicated that, generally, the incidence of intergranular failure prevalent in embrittled and non-embrittled steels exhibited a maximum at some specific ΔK level. Also in embrittled steels large effects of environmental assisted crack (EAC) growth were observed at near threshold fatigue crack growth rates. It was suggested that this was the result of the much reduced material cohesive strength which was caused by the presence of both impurity and hydrogen atoms.