NEAR-THRESHOLD FATIGUE-CRACK PROPAGATION BEHAVIOR OF CRMOV TURBINE STEEL

This paper describes the influence of material toughness degradation, through reversed temper embrittlement (RTE) and mean stress on the nea r threshold fatigue crack growth characteristics of a CrMoV turbine bo lting steel at ambient and elevated temperatures. It was established a t ambient temperatures that strong effects of R-ratio and material con dition (toughness) were observed on near threshold fatigue crack growt h characteristics. At elevated temperatures it was shown that for the non-embrittled material that only under low R-ratio conditions did inc reased temperature increase the level of threshold stress intensity De lta K-th, by some 20%. In the case of embrittled material, increasing the temperature increased Delta K-th levels by around 30% and decrease d near threshold growth rates by an order of magnitude at low to inter mediate R-ratio levels. The effects of R-ratio on Delta K-th for all m aterial and mechanical testing conditions could be simply expressed by the difference between Delta K-th at R = O and a constant B multiplie d by R. Quantitative fractographic observations indicated that, genera lly, the incidence of intergranular failure prevalent in embrittled an d non-embrittled steels exhibited a maximum at some specific Delta K l evel. Also in embrittled steels large effects of environmental assiste d crack (EAC) growth were observed at near threshold fatigue crack gro wth rates. It was suggested that this was the result of the much reduc ed material cohesive strength which was caused by the presence of both impurity and hydrogen atoms.