Root Cause of Degradation in the Creep Strength of Martensitic Steel

Abstract

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. 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 because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q, V, and C even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

The Degradation in Creep Strength of ASME Grade 122 Steel

“…These large particles lead to larger or newly formed HRHD zones. This vicious cycle degrades the long-term rupture strength of high-strength martensitic steel consuming strengthening particles of MX, the details of which are shown in the previous studies (Tamura & Abe, 2021a;2021b;Tamura, 2022). The simultaneous decreases in 𝑄𝑉𝐶 with an increase in strain during the stress and strain concentrations and the microstructural degradations in the HRHD zone can be simulated using Equation 1or 3 (Tamura & Abe, 2021a).…”

“…The calculation method has been described in detail in previous works (Tamura & Abe, 2021a;2021b;Tamura, 2022) and is briefly explained here. The time to rupture (𝑡 ) or time to a specific strain (𝑡 ), where 𝜀 is strain in %, can be expressed as…”

“…In early stages of creep of high-strength martensitic steel, 𝑄𝑉𝐶 increases simultaneously with an increase in strain, which is caused by a combination of SBSD, DFRP of M 23 C 6 carbides, and the precipitation of Laves phase (Tamura & Abe, 2021a;2021b;Tamura, 2022). Frequently we can also observe simultaneous decreases in 𝑄𝑉𝐶 with an increase in strain even during the early stages of transient creep (Tamura & Abe, 2021a;2021b;Tamura, 2022). However, it may be impossible to successfully explain the simultaneous decreases in 𝑄𝑉𝐶 from a general metallurgical sense of perspective.…”

“…Table 1 shows the product form, thermal history, prior austenite grain-size number (PAGSn), hardness (Rockwell hardness C scale; HRC), and chemical composition (percentage by mass (%)) of the steels analyzed: T122/RHU, P122/RHQ, pl-122/RhA, and DTB/RHT, where RHU, RHQ, RhA, and RHT are NIMS reference code names given for a single heat. In this study the results for Grades 91 and 92 (Tamura & Abe, 2021b;Tamura, 2022) are also quoted as references. To conform to the ASME standard and NIMS code name, the convention of T91/MGC for a tube, pl-91/MgC for a plate, T92/MJT for a tube, and P92/MJP for a pipe were used as steel names for Grades 91 and 92 steel in addition to the steel names mentioned above.…”

“…The rupture strength breakdown in Gr.IIIa is the most significant; that is, the slope ∆𝜎 ∆log𝑡 ⁄ is the steepest. The degradation of 𝑡 for Gr.IIIa is primarily caused by the formation of the Z-phase which consumes the strengthening particles of MX (Tamura & Abe, 2021b;Tamura, 2022). The strain rate was calculated using Equation 3.…”