Effects of Grain Refinement and Predeformation Impact by Severe Plastic Deformation on Creep in P92 Martensitic Steel Vaclav Sklenicka,* Petr Kral, Jiri Dvorak, Yoichi Takizawa, Takahiro Masuda, Zenji Horita, Kveta Kucharova, Marie Kvapilova, and Marie Svobodova The paper is dedicated to Professor Terence G. Langdon on his 80th Birthday Creep testing is conducted on an advanced martensitic-ferritic creep-resistant P92 steel (ASME Grade 92) to evaluate the effects of grain refinement and the predeformation impact after processing by severe plastic deformation (SPD), namely, by high-pressure torsion and high-pressure sliding. Constant-load tensile creep tests are carried out in an argon atmosphere at 600  C and under an applied stress ranging from 50 to 200 MPa. The results show that under the same creep loading conditions, the ultrafine-grained (UFG) microstructure states after SPD processing exhibit higher minimum creep rate ˙ ε m and creep fracture plasticity ε f , but significantly shorter creep lives in comparison with the coarse-grained (as-received) state of the steel. These distinct differences between the coarse- grained and UFG states are explained by the different deformation mechanisms that operate; creep behavior in a coarse-grained state is controlled by the intragranular climb of dislocations, while creep in UFG states can be interpreted as the synergistic action of the dynamic recovery of free dislocations at high-angle grain boundaries and grain boundary-mediated deformation processes. 1. Introduction Advanced tungsten and boron-modified creep-resistant 9% Cr martensitic P92 steel (ASTM Grade P92) is currently the strongest commercially available steam pipe steel [1–4] for maximum applica- tion temperatures of about 620  C. It has been recognized as one of the main candidate materials for the next generation of coal-fired ultra-supercritical (USC) and advanced USC (A-USC) power plants, as it can improve thermal efficiency and reduce fuel consumption and the emission of environmentally hazardous gases. [5] Creep behavior, microstructure stability, and degra- dation of the creep properties of P92 steel are phenomena of major practical importance that often limit the performance of high- temperature components. This has moti- vated extensive experimental creep studies and theoretical modeling of the creep mech- anisms and microstructure evolution in P92 steel over the last two decades. [1–4,6–15] We are still far from a full understanding of the effects of grain size and shape on the creep resistance of P92 steel. From theories of diffusion creep, it follows that the creep rate ˙ ε ðb=dÞ p , where b is the length of the Burgers vector and d is the mean grain diameter. [16,17] The mechanisms of diffusion creep are well formulated, and it is known that the exponent p ¼ 2 if the vacancies diffuse via lattice [18,19] and p ¼ 3 if the vacancies diffuse via grain boundaries. [20] Although diffu- sion creep theories are advanced, the experimental evidence quoted in support of these theories is not fully convincing. A small effect of the grain size on creep deformation is assumed in the region of dislocation (power law) creep, [16,21] in which the Prof. V. Sklenicka, Dr. P. Kral, Dr. J. Dvorak, Dr. K. Kucharova, Dr. M. Kvapilova Department of Mechanical Properties Institute of Physics of Materials Academy of Sciences of the Czech Republic Brno 61662, Czech Republic E-mail: sklen@ipm.cz Dr. T. Masuda, Prof. Z. Horita Department of Materials Science and Engineering Faculty of Engineering Kyushu University Fukuoka 819-0395, Japan The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adem.201900448. DOI: 10.1002/adem.201900448 Dr. T. Masuda, Prof. Z. Horita WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) Kyushu University Fukuoka 819-0395, Japan Dr. Y. Takizawa Technology Department Nagano Forging Co. Nagano 380-0003, Japan Dr. M. Svobodova UJP PRAHA a.s. Praha- Zbraslav 15610, Czech Republic FULL PAPER www.aem-journal.com Adv. Eng. Mater. 2019, 1900448 1900448 (1 of 9) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim