Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel
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TRODUCTION
SINCE modified 9Cr-1Mo steel was first industrialized,[1] many high-Cr heat-resistant steels with high creep strength have been developed.[2] Some of these steels have been registered by the American Society for Testing & Materials (ASTM) and other authoritative public organizations. These developments have allowed power stations to employ steam conditions higher than 873 K (600 C) and 30 MPa, thus realizing ultra-super critical power generation and increasing the efficiency of power generation.[2] With this background, there has been less activity in developing new martensitic steels of high strength, but the long-term rupture testing of highstrength martensitic steels is still necessary. Indeed, it has become more important to estimate the long-term rupture strengths of high-strength martensitic steels because such steels are manufactured day by day, and the allowable tensile stresses of the steels are periodically reviewed.[3] Many kinds of parameters combining temperature and creep time have been proposed for estimating long-term strength, where fitting curves for estimation were obtained by combining the parameters and stress. Among these parameters, the Larson–Miller parameter (LMP)[4] has been widely used to estimate long-term rupture strengths of carbon steel and low-alloy steel. Rupture data for periods longer than 100,000 hours have already been published in the data sheet for 0.2 pct carbon steel tubes by the National Institute for Materials Science
MANABU TAMURA, formerly with the National Defense Academy, Yokosuka, Japan, is now retired. Contact e-mail: mtamura. [email protected] Manuscript submitted November 2, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(NIMS) (Tsukuba, Japan), and the regression curves obtained using the LMP related to a polynomial of the logarithmic stress fully follow the observed times to rupture.[5] This successful result is attributed to the fact that the microstructure of carbon steel is stable over a long period.[6–8] However, some high-Cr martensitic steels with high strength for use in ultra-super critical power stations have been reported to deteriorate unexpectedly in creep testing longer than several tens of thousands of hours.[9] The deterioration is attributed to localized regions near prior austenitic grain boundaries (PAGB) being preferentially recovered through the formation of Z phase at PAGB accompanying the disappearance of one of the main strengthening factors of the finely dispersed carbonitride (MX, where M denotes metallic elements preferentially forming carbide and/or nitride, and X denotes carbon and/or nitrogen) particles,[10,11] besides the scavenging effect of solute elements of W and Mo owing to the grain boundary precipitation of Laves phase.[12,13] It is thus easy to overestimate the long-term strength of these high-strength steels. Kimura et al.[14] proposed for this problem that rupture data below 50 pct of proof stress (0.5PS) at test temperatures are used in the analysis. However, there are still differences between the calculated and obser
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