Abstract
This paper describes part of an investigation into differences in cavitation characteristics in commercial and high-purity Cr-Mo-V steels of the types used in rotors and pipes in steam power plant. Large differences were observed depending on heat-treatment, microstructure, and ductility. Creep tests were carried out at 550°C at stresses giving lives up to 2000 h, and the cavitation characteristics were examined by optical microscopy in specimens tested to various fractions of the creep life. Heat-treatments were applied to give fully bainitic microstructures in steels of rotor composition 1Cr-1Mo--¼%V, and ferrite-bainite microstructures in steels of steam pipe composition ½Cr-½Mo-¼%V. These conditions gave considerably different ductility at rupture depending on the heat-treatment and purity, and this was reflected in the cavitation behaviour. In the high-purity steels the time and strain for onset of cavitation were found to be greater than in the commercial steels, and the rate of formation of cavities was less by one to two orders of magnitude for similar creep strengths. The accumulation of cavities was related to the combined strain/time characteristics of creep behaviour but not to either time or strain in a simple linear fashion. Quantitative information of the type presented here might conceivably form the basis of a method for predicting the life of components in service, but particular conditions applicable to the behaviour of each material would have to be ascertained before general application.
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