Performance of creep crack growth on W strengthened 9-12Cr steel

This paper describes the properties of the creep crack growth in W strengthened Cr steel, newly developed for turbine rotors in ultra super critical coal fired power plant (USC). This material shows longer creep lives when compared to conventional heat resistant materials, such as SUS304 or 2.25Cr-1Mo steels. Thermal stresses induced by temperature fluctuations due to start-and-stop operation in high temperature machines and structures could be reduced due to its high thermal conductivity and low thermal expansion rate. However, since its creep ductility at rupture has been recognized as relatively low, a series of creep crack growth tests have been systemically conducted to assess the material performance controlling the initial stress intensity factor K with the use of the CT type specimen. The time rate of creep crack growth was evaluated as three parameters: stress intensity factor K, the net stress σ_net and C* / creep J-integral. The minimum rate was achieved at the early stage of creep life. Although the time rate behavior changed with the test temperature and initial load, it was expressed as a function of power law during the acceleration period for each test. However, when the data were arranged using the parameters of K and σ_net, the coefficient in the power law depended on the temperature and the initial value of K at loading. In contrast to these, the correlation between the creep crack growth rate and C* integral had almost the same coefficient, even when the temperature and the initial load were changed, and their relationship was plotted within a factor of two. It was also found that the creep crack growth rate was about ten times larger than that of 2.25Cr-1Mo steel at the same value of C* integral.