Temperature and Strain Rate Dependence of Low-Cycle Fatigue Properties of Heat Resisting Steel SUH 660

Abstract

Low-cycle fatigue tests were carried out on precipitation hardening, heat resisting steel, JIS SUH 660, at room temperature, 450, 600 and 700°C under a triangular strain waveform for which the strain rate was 40, 4 or 0.4%/min.
The results obtained are summarized as follows:
(1) The stress range decreased or increased first and then decreased with increasing strain cycles depending on the test condition. The strain rate dependence of the cyclic stress-strain relation was scarcely observed at temperatures up to 600°C. At 700°C stress was smaller for the strain rate of 0.4%/min than that for 40%/min.
(2) For the relation between the plastic strain range and the number of cycles to failure, the strain rate dependence was scarcely observed at room temperature and 450°C, but the number of cycles to failure decreased with decreasing strain rate at 600 and 700°C. The temperature dependence of the relation was clearly found at temperatures from room temperature to 600°C, but there was no difference between the relations at 600 and 700°C.
(3) Transgranular fracture surface covered with striations and small dimples were observed for the specimens tested at room temperature and 450°C. Intergranular fracture occurred at 600 and 700°C.
(4) For the temperature range at which transgranular fracture occurs, the temperature dependence of the Δε_p-N_f relation could be explained by the mechanical parameter Δε_p(Δσ/σ_u)² which was introduced by Tomkins.
(5) For the temperature range at which intergranular fracture occurs, the relation α=1/(1+an) was obtained, where a is a constant value of 4∼6 and α and n are exponents of Δε_p-N_f and Δε_p-Δσ relations, respectively.