STRUCTURE OPTIMIZATION AND DAMAGE BEHAVIOR OF HEAT-RESISTANT CRMOV TUBE STEELS

The heat treatment of a heat-resistant tube steel 12 CrMoV 4 3 has bee n optimized in relation to its long-term creep resistance. The influen ce of initial structure and of stress, time and strain on the formatio n of creep cavities has been determined. Cooling rates from the austen itizing temperature and tempering conditions were varied during heat t reatment. It has been demonstrated that, with an increasing cooling ra te down from the austenitizing temperature, the structure tends toward s a more finely dispersed precipitation of VCN with formation of grain boundary carbides as well as a higher dislocation density. The optimu m tempering conditions are for 2 h at 725-degrees-C. Longer times or h igher temperatures lead to degradation of the structure. Creep rupture tests show that a clear influence of the structure on the secondary c reep rate and rupture time exists. Both pore size distribution and por e density in the specimen surface are strain- and time-dependent. An i nfluence of stress could not be detected. Pore density in the surface approaches, with increasing strain, a saturation value. This saturatio n value depends on the initial structure and is related to rupture str ain.