Development of Creep Life Assessment of Martensitic Heat Resistant Steels

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High Cr heat resistant steels are used for main components in the thermal power plants. They have tempered martensitic lath structure and precipitation hardening is an effective strengthening technique for the improvement of their creep strength. In Japan, the thermal power plants which have been operated during over 20 years increase. Therefore, it is important that the creep life time of heat resistant steels is estimated in industry. Many techniques are proposed for the creep life time assessment. An Omega (Ω) method is one of the useful techniques. In this study, in order to develop the creep life assessment of the heat resistant steels, microstructures of Martensitic steels were observed and the creep tests were carried out. In 9Cr-1Mo steels, it was clarified that much amount of block and packet boundaries was reduced just before creep rupture by SEM/EBSP technique. In various 10Cr steels bearing V and Nb, many creep curves were analyzed by Ω method. As the result, it was found that the creep life time which obtained creep tests corresponded to the estimated creep life time by Ω method. The Ω method is very useful to estimate the creep life time for heat resistant steels. In addition, in this study, the effects of addition of V and Nb on Ω were investigated. Creep strength was improved with increasing the V content and was slightly lowered by increasing the Nb content. Q in all the steels bearing V or Nb was larger than that in fundamental steel (no V and Nb). The dependence of Ω on the amount of additions was different between V and Nb. In steels bearing V, Ω became larger with increasing V content. In steels bearing Nb, Ω became smaller. These results revealed that Ω corresponds to the creep strength and indicated that the value of Ω can be estimated using the strengthening effects of the additive elements.