Evaluation of Creep Crack Growth Rate for Modified 9Cr-1Mo Steel Using the Q* Fracture Parameter

Abstract

The modified 9Cr-1Mo (ASME Grade 9Cr-1Mo, Gr. 91) steel is favoured structural materials for Generation-IV reactor systems, such as reactor pressure vessels in very high temperature reactor (VHTR) and steam generators, intermediate heat exchangers (IHX), and primary hot pipes in sodium-cooled fast reactor (SFR). The selection of the modified 9Cr-1Mo is based primarily on its low thermal expansion coefficient and high resistance to stress corrosion cracking in water-steam systems compared to austenitic stainless steels. In particular, it also has better mechanical properties at elevated temperatures compared to alternate 2.25 Cr-1Mo steel. Since their structural components are designed to last for up to 60 years at elevated temperature, above all, the creep and creep crack growth (CCG) behaviors should be evaluated for a design application. The material that is operated under high temperature and/or high stress during the long service life may generate localized creep damage, propagating cracks, and ultimately causing fracture. A significant portion of the component lifetime will be spent in crack propagation. Therefore, an accurate assessment of crack propagation owing to the creep crack growth rate (CCGR) is required for the design and safety considerations. To do so, each characteristics of the fracture parameters such as K, C*, and Q* should be investigated and compared to define the CCGR behavior. In this study, a series of creep and creep crack growth tests was carried out on the modified 9Cr-1Mo steel under various loads at 550oC and 600oC. The CCGR behavior of the modified 9Cr-1Mo steel was evaluated using the three fracture parameters of K, C*, and Q*, and their characteristics were compared and discussed. Results showed that the Q* parameter could reasonably evaluate the CCGR by a simple monotical linear function without a dual value due to nose in the early stage of creep crack growth curve, while the C* parameter revealed the dual value in the early stage. In the scattered data from early stage to acceleration stage, the Q* parameter was found to be superior to the C* parameter, while the K parameter was the largest scatter and there was no systematic trend among each series of tests. It was identified that the Q* parameter was regarded as an independent parameter, whereas the C* was regarded as a dependent parameter that depends on creep deformation rate, which is a function of stress, temperature, specimen shape, and activation energy of deformation, etc.