Effect of heattreatment variations on strength and ductility of Cr-Mo-V steels during creep at 550°C

Abstract

The performance of Cr-Mo-V steels, as widely used for steam pipe work, turbine rotors and casings, and associated bolts in steam power generating plant, may be limited by a susceptibility to failure at low ductility, especially when heat treated to high strength levels. To investigate the ductility in creep conditions of such steels, an extensive study of the interrelation of heat treatment, microstructure, creep strength, rupture strength, rupture ductility, and fracture characteristics has been made in commercially produced 0·5Cr-0·5Mo-0·25V and 1Cr-1 Mo-0·25V steels tested at 550°C for times up to >20000h. The results show that the rupture strength and ductility can be varied widely by changing the austenitizing temperature and the rate of cooling therefrom, and that a heat treatment at a high temperature (1300°C), given to simulate the microstructure in the vicinity of a weld, can produce very low creep-rupture ductility (<1%). Low ductility in these steels is associated with an intergranular fracture process resulting from nucleation and growth of cavities on grain boundaries, and detailed optical metallography of cavitation characteristics has been carried out on creep-tested specimens of the two steels in conditions giving different creep behaviour. Examination of these specimens at various proportions of their life to rupture has shown that the strain at which creep damage, as evidenced by grain boundary cavitation, is first observed may be as low as 0·1% for microstructures simulating those in the heat-affected-zone of a weld, and this strain level remains the same irrespective of applied stress. For the entire range of conditions studied, the creep strain accumulated before cavities were first observed increased as the ductility at rupture increased, but this strain level was lower for the 0·5Cr-0·5Mo-0·25V steel than for the 1Cr-1Mo-0·25V steel. For the limit of detection adopted, the proportion of the creep life during which cavities were present was found to decrease as the elongation at rupture increased. This connexion between rupture ductility and propensity to cavitation emphasizes the importance of ductility as a criterion for the resistance of a material to intergranular creep failure. As differences in creep cavitation characteristics and rupture ductility depend on metallurgical condition, the possibility arises of controlling these properties by suitable heat treatment. In this connexion, observations are made on the possible significance of some microstructural features to cavitation.