Effect of oxidation on fatigue crack growth threshold of alloy-and carbon-steels at elevated temperature

At elevated temperature which is still below the creep operating region in air or water environment, characteristics of fatigue crack growth threshold for alloy-and carbon-steels are not well clarified. For the structural integrity operating at those temperature ranges, such as nuclear reactors, it is an urgent task to make clear them up. In the previous study, the fatigue crack growth rates and the crack closure were measured for a low alloy steel, JIS SFVV3 (A508-3), used for the pressure vessel of the nuclear reactor. The temperature range examined was from room temperature up to 350degC which covers the operating temperature at 288degC of the nuclear reactors. Further detailed studies were attempted by accumulating the threshold data for various alloy-and carbon-steels. Influencing factors are made clear and a simple method for predicting thresholds is developed by accounting for the above effects. The temperature dependence on the growth rate and the crack closure and the stress ratio effect are studied. Especially near the fatigue threshold where the temperature effect is significant, it is found that the oxidation produced on the crack surfaces at elevated temperature have an important role. Their contribution is examined quantitatively. At room temperature, the fretting oxide debris (Fe2O3) cause the crack closure. Higher the temperature, the oxidation (Fe3O4) protects the crack surfaces from the occurrence of fretting oxide. Thus the fatigue threshold decreases as increasing temperature up to around 100degC. Hereafter, it increases due to the increasing in thickness of oxidation which promotes the occurrence of crack closure. The dependencies of the yield stress and the cyclic frequency on the growth rates can be explained from the same view point. (author)