Abstract
The substructural change during low-cycle fatigue of SUS316 stainless steel at elevated temperature has been investigated by X-ray diffraction and transmission electron microscopy, and its correlation with the fatigue hardening behavior has also been shown. Fatigue test was performed under a total strain range of 1.0% with a strain rate of 4×10-4 sec-1 at 600, 700 and 800°C.
The main results obtained are as follows:
(1) The saturation hardening stage at tained after around 100 cycles at 600°C, 12 cycles at 700 °C and a few cycles at 800°C.
(2) At 600°C, the particle size (<D>e) decreased markedly with increasing cycles up to 10 cycles and varied little afterwards. At 700°C, <D>e decreased in the hardening stage, became constant in the earlier stage of the saturation hardening stage but increased at the later stage. At 800°C, <D>e became 3000040000Å at the earlier cycle and varied little afterwards.
(3) At the saturation hardening stage, <D>e changed little but microstrains varied markedly within one cycle. Storage, relief, storage and relief of strain energy are repeated in one stable hysteresis loop. As a result, it seems that there is little net' change in stored energy within one cycle.
(4) It seems that the stress range (Δσ) at the hardening stage depends on the dislocation density (ρ) and the relation between them is given by Δσ/2bG=1.05×105+0.96√ρ where G is the shear modulus.
