Development of Creep Life Assessment of Martensitic Heat Resistant Steels

高铬耐热钢用于火电厂的主要部件。它们具有回火马氏体板条组织，沉淀硬化是提高其蠕变强度的有效强化技术。在日本，运行了20多年的火电厂在不断增加。因此，对耐热钢的蠕变寿命进行估算在工业上具有重要意义。对蠕变寿命的评估提出了多种方法。Omega （Ω）方法是一种有用的技术。为了建立耐热钢的蠕变寿命评价方法，对马氏体钢进行了显微组织观察和蠕变试验。在9Cr-1Mo钢中，SEM/EBSP技术表明，在蠕变断裂之前，块状和包状边界减少了很多。用Ω方法分析了含V、Nb的10Cr钢的蠕变曲线。结果表明，蠕变试验得到的蠕变寿命与Ω方法估算的蠕变寿命相吻合。Ω方法对于估计耐热钢的蠕变寿命是非常有用的。此外，本研究还考察了添加V和Nb对Ω的影响。随着V含量的增加，蠕变强度有所提高，随着Nb含量的增加，蠕变强度略有降低。含V或Nb钢的Q值均大于无V和Nb基本钢的Q值。Ω对添加量的依赖性在V和Nb之间是不同的。在含V钢中，Ω随V含量的增加而增大。在含Nb的钢中，Ω变小了。结果表明，Ω与蠕变强度相对应，表明Ω的值可以通过添加元素的强化效应来估算。

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.