Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

Abstract

Long-term rupture data of 11Cr-2W-0.4Mo-1Cu-Nb-V steel were analyzed using an exponential equation for stress regarding time to rupture as a thermal activation process. The fitness was compared with the usually employed method assuming power-law creep. In the exponential method, rupture data are classified into several groups according to the thermal activation process; the activation energy, Q; the activation volume, V; then, the Larson-Miller constant, C, values are calculated, and a regression equation is obtained for each data group. The fitness level of the equation was satisfactorily high for each group. The values of Q, V, and C were unusually small for a data group where an unexpected drop in rupture strength was observed. The critical issue is how to comprehend signs of degradation within the short term. We can observe several signs at a creep time of approximately one-tenth of the times of the degradation events. The small values of Q and V indicate that completely softened regions form and creep locally, which is consistent with previous observations. From both metallurgical considerations and the variations of Q and V, it is suggested that the rate of the unexpected drop in strength is mitigated after further long-term creep.

Changes in Estimated Dislocation Density during Creep in Martensitic Heat-Resistant Steel

“…According to an experimental rule that the time to rupture is inversely proportional to the MCR (Monkman & Grant, 1956), an equation similar to Equation 29 may be obtained for the time to rupture. Therefore, the linear semi-logarithmic relationship for the time to rupture is also confirmed in Figure 1 as is explained previously (Larson & Miller, 1952;Tamura et al, 1999;Tamura et al, 2003;Tamura et al, 2013;Tamura, 2015). , and for the MCR were obtained using Equation 35 and individual minimum-creep-rate data, and the results are plotted at a strain for the MCR of the synthesized strain rate versus strain curve, shown in Figure 9.…”

“…From the results of these studies, representative highstrength martensitic steels have been developed using sophisticated control technology for precipitation to delay recovery (Sikka, Ward, & Thomas, 1983;Naoi et al, 1995;Abe, Tabuchi, Tsukamoto, & Shirane, 2010). However, more than 10 years passed following the development of high-strength steels before it was clarified that the longterm rupture strengths of some of the steels tended to lower unexpectedly (Kushima, Kimura, & Abe, 1999;Tamura, 2015). Moreover, it requires a very long time and much effort to investigate metallurgically the recovery behavior during the creep of these steels.…”

“…The allowable stresses for ASME Gr.122 (America Society of Mechanical Engineers) were revised because it was found that the long-term rupture strengths of the steel were considerably less the strength expected from shortterm data (Masuyama, 2007). It was also clarified for this steel that a sign of degradation can be detected even at the time to 0.5% strain, which is before the time to the MCR (Tamura, 2015). It is thus considered important to analyze creep behavior in the transition creep range in detail.…”

Relation between Sub-grain Size and Dislocation Density During Steady-State Dislocation Creep of Polycrystalline Cubic Metals

“…Steel products composed of high-Cr martensitic steel with high strength are widely used for power plants, which has contributed to reducing greenhouse gas emissions from fossil power plants (Masuyama, 2001). However, the unexpected decrease in strength of these steels after long-term service remains a critical issue (Kushima, Kushima, Yagi, & Tanaka, 1991;Abe, 2006;Tamura, 2015), and the maximum allowable tensile stresses for these highstrength steels has been revised (Masuyama, 2007). However, sufficient rupture data at 100,000 h for these steels needed to determine the allowable stresses have not yet been obtained.…”

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

“…The most significant issues regarding these steels are their low rupture strength of the welded joints (Abson & Rothwell, 2013) and the unexpected degradation of the base metal in long term rupture strength after creep test longer than several tens of thousands of hours (Kushima, Kimura, & Abe, 1999;Sawada, Kushima, Kimura, & Tabuchi, 2007). Regarding this unexpected reduction in strength of base metal, numerous studies have been undertaken with the aim of developing new alloys (Abe, 2006a;2011;Hashizume, et al, 2009;Dudova, Plotnikova, Molodov, Belyyakov, & Kaibyshev, 2012), estimating methods for long-term rupture strength (Tamura, 2015a;Maruyama, 2019), and investigating metallurgically the unexpected decline in strength which is overviewed by Abe (2006b). The formation of coarse Z-phase particles at the expense of the finely dispersed MX particles and recovery zones in the vicinity of the primary austenite grain boundaries (PAGBs) are considered as the dominant sources of the unexpected decline in strength (Kushima et al, 1999;Suzuki, Kumai, Kushima, Kimura, & Abe, 2003;Sawada, Kushima, & Kimura, 2006;Danielsen, 2007;Hald, 2008;Kimura, Sawada, Kushima, & Toda, 2013).…”