Abstract
This paper attempts to assess the contribution of oxidation to crack growth during high strain fatigue of a Cr-Mo-V steel at 550°C from three separate oxidation studies. Stress-free data are clearly inapplicable to the highly strained crack tip and so it is first shown that internal stresses, as in bainitic material, promote enhanced oxidation. Weight gain experiments on oxide-free fracture surfaces are next described. It is shown that the stored energy of fatigue fracture likewise causes an increase in oxidation rate but that it is difficult to simulate the process at the tip of an advancing crack. Similarly, metallography of oxide in fatigue cracks does not reveal propagation. history. Finally, a dynamically worked surface, provided. by high-strain fatigue specimens deforming at 550°C, causes increasing oxidation with increasing total strain above a threshold value of oxide strain. It is shown that this is due to layering producing at least a tenfold increase compared with stress-free specimens. Therefore the cyclic surface growth data are used together with observed crack propagation rates in air and vacuum to estimate the oxidation rate in the highly-strained crack tip.
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