The Effect of Atmosphere on Fatigue Strength of Carbon Steels at Elevated Temperature

Fatigue tests of carbon steels were carried out at 550°C under rotating bending and also under repeated torsion in town gas to reduce air oxidation, as a first step to study the effects of atmosphere on fatigue strengths at elevated temperatures. For long lives under low stresses, the strength was higher in gas than in air, and the influence of corrosion fatigue due to oxidation was found in air at high temperature. Meanwhile the strength was lower in gas than in air for short lives under high stresses, and the S-N curves in air and in gas cross each other. This may be attributed to the hydrogen embrittlement resulting from the decomposition of water vapor. The S-N curve at elevated temperatures may take a considerably different shape if the effects of oxidation by air are removed, and its slope becomes more gentle after a long time.