Effect of Environment and Hold Time on the High Strain Fatigue Endurance of 1/2 Percent Molybdenum Steel

Abstract

Results are presented of high strain bending fatigue tests on 1/2 per cent molybdenum steel in air and in a vacuum in which the effect of introducing a hold period in the position of maximum strain was investigated. The environment exerts significant effects on high strain fatigue endurance both at room temperature, where the life in a vacuum was twice as great as in air, and at elevated temperature. The considerable reduction in life in continuous cycling in air at 500°C is due almost entirely to oxidation since tests in a vacuum restored the endurance to values comparable with those obtained in a vacuum at room temperature. The introduction of air into a continuous cycling test in vacuum at 500°C reduced endurance by a larger factor than when a 30-minute hold period per cycle was introduced into such a vacuum test. This suggests that under the conditions stated oxidation has a greater influence on fatigue resistance than creep. Significant reductions in endurance were found with hold time but not in continuous cycling when the temperature was raised from 500C to 550°C and this is consistent with the poorer creep rupture strength and ductility at the higher temperature.