抄録
Low cycle fatigue (LCF) tests were carried out at 839K on Cr-Mo-V forgings and castings taken from various power plant components after long-term service.
Δεe-Nc and Δεp-Nc 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 examine.
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) Δεe-Nc relationship shifted downward by softening.
ii) Δεp-Nc relationship shifted downward when embrittlement became dominant.
(2) Vickers hardness had good correlation with Δεe-Nc coefficient C1, as long as hardness took the value from 200 to 260, while Δεe-Nc expenent α1, Δεp-Nc 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 α1were insensitive to the change of FATT.
As the effect of deterioration on LCF properties is complicated, further investigation is necessary.
