液体Ｈｅ温度での軸ひずみ制御による３０４Ｌステンレス鋼の低サイクル疲れ特性評価

Abstract

A cryostat has been developed for this study, which enables to conduct axial-strain controlled fatigue tests of SUS 304L stainless steel at 4K with the strain range between 1.04 and 2.52%, the strain rate of 0.4%/sec and the strain ratio of -1. The loading in this cryostat is achieved by inner and outer tubes so that the requirement for high structural rigidity to suppress the specimen buckling and that for the low cross section area to keep the heat leak as low as the liquid He consumption rate of 1.5l/H or less are both satisfied. The fatigue tests at 300 and 77K were also conducted for comparison.On the curves showing the changes of maximum and minimum stresses with the number of cycles as well as the stress-strain curve, the serration-like stress change was observed at 4K, while the smooth stress change was observed at 300 and 77K. The cyclic stress-strain curves at 4 and 77K intersected each other, showing that the stress is lower at 4K than at 77K in a small strain amplitude. Those curves at 4 and 77K lied far away above the one at 300K.The fatigue life curves or the total strain range vs. fatigue life curves at 4, 77 and 300K also intersected each other, having a trend that the fatigue strength at 4K is the highest in the high cycle range but becomes the lowest in the low cycle range. The fatigue life curves of SUS 304L obtained here at 4, 77 and 300K gave almost the same fatigue strength as that of AISI 304L obtained under the diameter-strain control condition. When compared with other austenitic stainless steeels such as AISI 304, AISI 316 and AISI 310, the fatigue strength of SUS 304L was lowest at all temperatures.