Abstract
To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.
Highlights
Grades 91 and 92 steels, which were originally developed by Oak Ridge National Laboratory and Combustion Engineering Corporation (Sikka, Cowgill, & Roberts, 1983) and Nippon Steel Corporation (Hasegawa, 2014), respectively, exhibit greatly improved efficiencies of power generation, reducing the contribution to global warming (Masuyama, 2001; Kimura, Sato, Bergins, Imano, & Saito, 2011; Muroki, 2017)
The long-term creep curves of Grades T91, T92, and P92 along with the laboratory-prepared 9Cr-1W and 9Cr-4W steels were analyzed by applying an exponential law to the test temperature (T), stress (σ), and time to rupture (t ) or time to a specific strain ( t )
The variations in Q, V, and C as functions of creep strain are discussed metallurgically and the following conclusions are obtained: 1) Sub-grain boundary strengthening by the swept out dislocations (SBSD) is an essential process at the initial stage of the creep of martensitic steel
Summary
Grades 91 and 92 steels, which were originally developed by Oak Ridge National Laboratory and Combustion Engineering Corporation (Sikka, Cowgill, & Roberts, 1983) and Nippon Steel Corporation (Hasegawa, 2014), respectively, exhibit greatly improved efficiencies of power generation, reducing the contribution to global warming (Masuyama, 2001; Kimura, Sato, Bergins, Imano, & Saito, 2011; Muroki, 2017). 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). The negative effect of Ni contained in the specification range of Grade 91 (Ni ≤ 0.40 mass%, hereinafter %) on the long-term rupture strength during the formation of the Z-phase particles was reported by Kimura et al (2013) and Sawada, Kushima, Hara, Tabuchi, and Kimura, (2014a). Sawada et al (2019a) recently demonstrated the jmsr.ccsenet.org
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