Cyclic oxidation and crack growth during high strain fatigue of low alloy steel

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.Several effects contribute to the fatigue failure of low alloy steels at high temperature. As well as the fatigue strain cycle there may be a component of creep, and environmental attack also becomes more pronounced than at low temperatures. Therefore it is important to be able to separate the contributions of 'oxidation' and 'creep' damage in high temperature fatigue. For example, when creep occurred during stress relaxation in the tension cycle in fatigue of !Mo steel at 500°C, endurances were reduced.! This applied both for air and vacuum tests. Vacuum testing eliminates the gross oxidation component and, for !CrMo-V steel at 550°C, increases the endurance? Recent experiments on Cr-Mo-V steels at 550°C have also shown that cracks advance faster in air than in vacuum, both in high strain fatigue 3 and in tests at a given stress intensity.4The exceptions are at low stress or strain amplitudes in air, where the crack becomes'sealed with oxide.Oxide behaviour at grain boundaries is also known to influence fatigue-crack initiation and propagation. 5 Clearly, an oxide scale which is firmly adherent to the deforming underlying metal will improve fatigue life.Oxidation rates in iron are increased by surface cold work,6 and, in 2!Cr-IMo steel, by prestraining. 7 Enhanced 64 Metal Science February 1978 oxidation with deformation acting concurrently has been reported during high strain fatigue of A-286 alloy at 510°C,8 and has been measured after high frequency fatigue of En2 steel in air at 500 and 570°C. 9 It is not surprising, therefore, that after high strain fatigue testing of low alloy steel specimens at 550°C in air, surfaces and cracks were wellendowed with oxide.!' 3The purpose of the present work is to assess the contribution of such oxidation to fatigue crack growth, during high strain fatigue. It was realized that stress-free oxidation data...

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