Effect of deterioration on high temperature low cycle fatigue properties of low alloy steels

Low cycle fatigue (LCF) tests were carried out at 839 K on Cr-Mo→V forgings and castings taken from various power plant components after long-term service. Δepsilonsub(e)-Nsub(c) and Δepsilonsub(p)-Nsub(c) properties of the used and artificially deteriorated materials were compared with the virgin material. The effect of deterioration on mechanical properties and its correlation with LCF properties were examined. The results obtained are summarized as follows. (1) Cr-Mo→V forging and casting became soft and/or brittle through long-term heat. Material deterioration (softening and embrittlement) changed LCF properties of these steels as follows. i) Δepsilonsub(e)-Nsub(c) relationship shifted downward by softening. ii) Δepsilonsub(p)-Nsub(c) relationship shifted downward when embrittlement became dominant. (2) Vickers hardness had good correlation with Δepsilonsub(e)-Nsub(c) coefficient C1, as long as hardness took the value from 200 to 260, while Δepsilonsub(e)-Nsub(c) expenent α1, Δepsilonsub(p)-Nsub(c) coefficient C2 and exponent α2 were insensitive to the change of hardness. (3) True fracture strain had no clear correlation with C1, α1, C2 and α2. (4) FATT (50 % fracture appearence transition temperature) had good correlation with C2 of casting. Embrittled casting had relatively high FATT and low C2. De-embrittled casting decreased FATT remarkably and increased C2 to some extent. By fractographic observation the former showed intergranular cracking, and the latter showed ductile striation pattern. The values of α2, C1 and α1 were insensitive to the change of FATT. As the effect of deterioration on LCF properties is complicated, further investigation is necessary. (author)