The sintering of creep-induced cavities in a low alloy ferritic steel (1Cr1Mo0.75V)
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INTRODUCTION
MATERIALS used for high temperature components are subject to long term deterioration of integrity due to the accumulation of creep cavities on grain boundaries oriented normal to the principal stress axes. The nucleation and growth of such cavities at temperatures in excess of ~0.3 TmL2'3is a significant life-limiting parameter in many components, but the susceptibility of different materials to cavitation damage varies considerably. Even for materials of the same nominal composition the cavitational behavior under given conditions of stress and temperature can be strongly dependent on cast-to-cast variations in composition and heat treatment, and hence the distribution of minor alloying and impurity elements. 4'5'6 Such variations are a major contribution to the normal scatter in creep properties which exists within any nominal material type and for which reason life assessments based on post-service metallurgical examinations invariably suffer from a lack of knowledge of the original microstructure. Reheat treatment procedures to remove creep damage and hence effectively regenerate 'virgin' material would therefore be of value for life assessment purposes as well as providing obvious economic benefits if the overall life of high temperature components could be extended by this means. Any such procedure must obviously consider changes during service, and during subsequent reheat treatment, of microstructural properties such as fine scale solute distributions and the dispersion parameters of second phase particles. It must also be capable of either eliminating, or removing grain boundaries from, any creep cavities that have developed during the prior service exposure. 7 Although various analyses have been used to describe the sintering characteristics of powder compacts 8 and casting porosity, 9'1~in the case of creep cavitation the kinetics are enhanced by the ease of vacancy diffusion along grain boundaries. The growth of creep cavities occurs by the diffusion of vacancies along grain boundaries under a stressregulated chemical potential gradient. H Sintering may be described as a reversal of the growth process with the potential gradient determined simply by the surface energy of the cavity, or by the applied pressure when superimposed hydrostatic pressure is necessary to achieve sintering in acR. A. STEVENS and P. E. J. FLEWITT are both Research Officers with the Central Electricity Generating Board, South Eastern Region, Scientific Services Department, Canal Road, Gravesend, Kent DA12 2RS, England. Manuscript submitted July 13, 1982. METALLURGICALTRANSACTIONS A
ceptably short periods. 12'13 A considerable amount of data exists to support diffusional sintering of creep cavities in a range of materials including copper, t4 tungsten, 15 and nickel, ~3but for low alloy ferritic steels, although diffusional cavity growth under creep conditions has been demonstrated (e.g., for 2.25 CrlMo steel16), there has to date been little consideration of cavity sintering kinetics in such materials. In this
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