Cr-Mo-V鋼の高温疲労変形とそれに及ぼす繰返速度の影響

Abstract

The present investigation was planned as one of our successive studies concerning the fatigue strength and the fatigue deformation of materials at elevated temperatures to find out several features of Cr-Mo-V steel, a kind of heat resistant steel, under such conditions-some experiments were made at room temperature for comparison on the same material. A particular emphasis was put on the effect of the frequency of alternating stress on the fatigue characteristics at elevated temperature, since there had been few experimental results on this subject.The frequencies of 800 and 120cpm were selected at 490°C, and only 1800cpm at room temperature. Most of the specimens were tested under axial alternating stress combined with mean stress, and some of them were tested under static creep stress at 490°C for comparison.The main results obtained from the experiments are as follows: At room temperature the strain developed under fatigue stress is larger than that under static stress, and the yield point for fatigue is lower than that for static test. These features somewhat resemble to those of brass rather than of mild steel.At 490°C, when the frequency and the stress amplitude are high enough, no local contraction or necking takes place at the test section of specimen, and the final permanent strain is as low as 1%. On the other hand, under the low stress amplitude or low frequency large local contraction is observed and the final permanent strain is so large. These phenomena seem to be due to the different effects of alternating stress and mean stress; the former acceralates the development of local fatigue crack and the latter causes the increase of creep strain. At 490°C, after a short period of loading, after loading of one hour, for instance, any influence of the frequency on the creep strain can not be recognized, while after a longer period the difference depending on the frequency becomes large, and as a rule, a higher frequency or a higher stress amplitude has a tendency to prevent the increase of strain. In respect of the stress range causing failure, both results of the two frequencies show a good agreement in the range of large mean stress and small stress amplitude, but in the range of high stress amplitude and relatively small mean stress, another agreement is obtained based on the number of stress cycles rather than the loading time.