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

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.

Root Cause of Degradation in the Creep Strength of Martensitic Steel

“…Further, the linear relationship between the minimum creep rate (MCR) and 𝑡 in a double logarithmic diagram should be confirmed based on the Monkman-Grant relationship. All data studied were checked, and the typical examples were shown in previous work (Tamura & Abe, 2021a;2021b). The validity of applying Equation 1 to the creep data of martensitic steel with high strength was already confirmed although the scattering for times to low strains are rather large.…”

“…It is established that the value of 𝑄 in the high-temperature creep of materials is almost equivalent to the activation energies for self-diffusion (Sherby et al, 1953;Tamura, 2017); therefore, the lower limit of 𝑄 approximately approaches that for the self-diffusion of the alpha-iron, i.e., 267 kJmol −1 (Oikawa, 1982). The magnitude of the back stress can be also roughly estimated using an exponential law; the details are discussed in a previous paper (Tamura & Abe, 2021b).…”

“…where 𝜌, 𝑏, 𝐶 , 𝜆, 𝜈 , and Δ𝑆 denote the dislocation density that contributes creep strain, length of Burgers vector, Monkman-Grant constant, maximum distance that a dislocation can move from a starting point to the next stable position through the activation process, effective attempt frequency per unit time to overcome the obstacles, and entropy change involved in the activation process, respectively (Tamura et al, 2013). Though the 1 st term of Equation 5 depends on 𝜌, the value of the 1 st term of Equation 5 is approximately 13 for typical heat-resistant steels (Tamura & Abe, 2021a;2021b). Therefore, the 2 nd term of the entropy term is roughly 7 for 𝐶 ≅ 20, which corresponds to the entropy change related to the formation and migration of vacancies during creep controlled by self-diffusion.…”

“…This is because Equations 1 and 2 indicate that 𝑡 is the product of a very large value, i.e., exp (𝑄 − 𝜎𝑉) 𝑅𝑇 ⁄ and a very small value of 10 . Since 𝑄 is large compared to 𝜎𝑉, 𝑄 and 𝐶 should have a strong positive correlation (Tamura & Abe, 2021a;2021b). However, note that 𝑄 and 𝐶 have different meanings.…”

“…Tamura and Abe (2021a) highlighted that the occurrence of HRHD at a later stage of transient creep is the root cause for the degradation in the creep strength of martensitic steel. Therefore, Tamura and Abe (2021b) applied this idea of HRHD for the unexpected degradation in the long-term creep strength of Grade 91 and 92 steels and concluded the following: i) SBSD is an essential process at the initial stage of the creep of martensitic steel with high strength. ii) Hardening by the DFRP of fine M 23 C 6 particles and the precipitation of the Laves phase are added to the SBSD for high-Cr martensitic steel with high strength, which increases 𝑄 with increasing strain.…”