The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water
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INTRODUCTION
S T R E S S corrosion cracking (SCC), defined as "Cracking caused by the simultaneous presence of tensile stress and a specific corrosive medium, "m has long been known to cause cracking in certain metal-environment systems. In spite of the long recognized need for a tensile stress, the role of the applied stress in crack growth has not been fully explained. It appears to depend upon the mechanism of cracking, and as such it is not expected to be the same for all systems which undergo SCC. For the system under investigation, high strength steels cracking in water, there are two proposed mechanisms of crack growth: Hydrogen Embrittlement (HE) and Anodic Dissolution (AD). The role of stress and the predicted effect of strain rate will be discussed for both. HE models of SCC are based on the well-documented cracking of high strength steels in gaseous hydrogen and hydrogen sulfide. In this cracking mechanism, the role of stress is to concentrate hydrogen in front of the crack and to fracture the embrittled material at the crack tip. The effect of strain on cracking would be through an apparent increase in the diffusion of hydrogen due to hydrogen transport on dislocations. Recent research implies that dislocation motion does not increase the apparent diffusivity of hydrogen and may actually decrease the effective diffusivity due to trapping effects, t21 It is also doubtful that strain could have any effect on the threshold of cracking. In work on 4340 cracking in hydrogen, Oriani and Josephic t31 have shown that the fracture stress is an equilibrium stress, independent of time. Thus, it appears that strain, p e r se, has no role in HE. Crack growth by AD involves the dissolution of metal along a preferred path which for high strength steels in water is the prior austenite grain boundaries. The role of stress in this mechanism is to produce a crack tip strain rate which continuously breaks the passive film which tends to form at the crack tipJ 4] Without this strain, the R.M. RIECK, Senior Tutor, and A. ATRENS, Senior Lecturer, are with the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Queensland, 4067, Australia. I.O. SMITH, formerly Associate Professor with the Department of Mining and Metallurgical Engineering, University of Queensland, is General Manager at CRA-Advanced Technical Development, P.O. Box 347, Cannington, WA 6107, Australia. Manuscript submitted November 20, 1987. METALLURGICAL TRANSACTIONS A
presence of the passive film will restrict corrosion to such an extent that SCC will not occur. This importance of the passive film in controlling corrosion has some important implications for SCC by HE. When cracking in water, the only source of hydrogen will be through the cathodic reduction of H + ions balancing the anodic corrosion reaction. As the corrosion reactions are under crack tip strain control, it follows that H E will be controlled by strain. This makes it impossible to differentiate between AD and H E through the effect of strain rate. However
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