The threshold stress intensity for hydrogen-induced crack growth
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THE stable growth of a crack at stress intensities well below K~r due to the action of hydrogen either within the metal or in the environment, has been studied widely. Attention has been focused particularly on high strength steels; 1-30generally speaking the phenomenon does not occur in low strength steels (less than 600 MPa tensile strength) although hydrogen may reduce ductility in other w a y s . 31-33 The observation that there appears to exist, for each metal/environment/constraint combination, a threshold stress intensity KTU (or K~,cc by analogy with stress corrosion cracking) below which hydrogen-induced slow crack growth does not occur, is of both practical and theoretical importance. A number of models of the embrittlement process have been p r o p o s e d 13-16A8-2~ which attempt to explain the existence of a threshold stress intensity, and its observed variation as environmental and material parameters are changed. Apart from the difficulty in establishing the basic embrittlement mechanism, such models usually require information about stresses ahead of a loaded crack tip and about the ways in which fracture proceeds. Knowledge of crack tip stress fields and micromechanisms of fracture is continually improving, so it is to be expected that the models by which hydrogeninduced cracking is described will be continually modified in the light of this new knowledge. The first part of the paper presents the results of an investigation of slow crack growth in a medium-high strength steel (tempered martensite microstructure) in gaseous hydrogen. While certain aspects of this steel's behavior appear relatively novel, and may throw new light on the processes of slow crack growth, in many ways the behavior is typical of that of other high strength steels which have been studied previously. In the second part of the paper a model of the processes which control the threshold stress intensity is proposed
K. N. AKHURST is Research Associate, C.E.R.L. Leatherhead, Surrey, U . K . T . J . BAKER is Lecturer, Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London, U.K. Manuscript submitted July 16, 1980. METALLURGICAL TRANSACTIONS A
based on current ideas of crack tip stress fields and micromechanisms of fracture. The predictions of this analysis are compared with the experimentally determined dependence of KTH on the external hydrogen pressure, and with published data for a variety of alloys.
MATERIAL In 1974 an end retaining ring (or end bell) on a 500 MW generator rotor at Nanticoke power station in Canada failed while the generator was operating. 35The end retaining ring is a hollow cylinder of high strength steel which retains the copper end-windings of the generator during operation. It is the most highly stressed, and so of necessity the highest strength, component in the generator. Since most modern generators are hydrogen cooled, there is the possiblility of hydrogen-induced fracture and the failure at Nanticoke was attributed to hydrogen-induced crack growth from a
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