ORIGINAL CONTRIBUTION The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25 Roman Petráš 1,2 | Ivo Šulák 1 | Jaroslav Polák 1,2 1 Department of Mechanical Properties, Institute of Physics of Materials CAS, Brno, Czech Republic 2 CEITEC, Institute of Physics of Materials ASCR, Brno, Czech Republic Correspondence J. Polák, CEITEC, Institute of Physics of Materials ASCR, Žižkova 22, 616 62, Brno, Czech Republic. Email: polak@ipm.cz Funding information Czech Grant Agency, Czechia, Grant/ Award Number: 18-03615S; Czech Academy of Sciences, Grant/Award Number: RVO 6808173; Ministry of Education, Youth and Sports of the Czech Republic, Grant/Award Number: LQ1601; Large Infrastructures for Research, Experimental Development and Innovation, Grant/Award Number: LM2015069 Abstract Superaustenitic steel Sanicro 25 has been subjected to out-of-phase thermomechanical fatigue cycles with 10-min dwell at peak temperature in the temperature range from 250  C to 700  C. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated. The internal structure and nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy disper- sive X-ray spectroscopy in scanning transmission electron microscope. Scan- ning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for fatigue crack nucleation and propagation. Effect of dwells in in-phase and in out-of-phase cycling on the fatigue behaviour, on the modifica- tion of internal structure and damage mechanisms, were analysed and discussed. KEYWORDS damage mechanisms, effect of dwells, precipitates, Sanicro 25, thermomechanical fatigue 1 | INTRODUCTION The production of energy with the lowest impact on the environment has become of topical concern. Even though the participation of electricity generation such as renewable energy resources increases, the power plants do still represent a dominant contribution regarding electricity production all over the world. A new generation of thermal power plants, advanced ultra-supercritical (A-USC) power plants working at temperatures up to 700  C, has been designed to assure enhanced efficiency and to achieve the reduction of emissions as well. The materials utilized in boiler con- struction have to be chosen according to both service and economy criteria. To minimalize the expenses con- cerning superheat and reheat tubing, highly alloyed advanced austenitic stainless steels such as NF709, H3RC and Sanicro 25 should be used. 1 Sanicro 25 exhibits very good resistance to steam oxidation and high temperature corrosion and has high creep rupture strength, higher than the other austenitic stainless steels available today. 2,3 Materials used in the production of energy are subjected to high variable stresses at elevated tempera- tures, and fatigue and creep can lead to premature fracture. Therefore, high temperature materials used in power generation industries, that is, Cr-Mo steels, aus- tenitic stainless steels and superalloys, were subjected to creep-fatigue-oxidation studies at elevated tempera- tures. 4 The most severe loading conditions are those where stress or strain and temperature vary simulta- neously, that is, in the case of thermomechanical fatigue. Numbers of thermomechanical studies were devoted to superalloy single and polycrystals 5–13 and to Received: 3 August 2020 Revised: 12 October 2020 Accepted: 1 November 2020 DOI: 10.1111/ffe.13385 Fatigue Fract Eng Mater Struct. 2020;1–16. wileyonlinelibrary.com/journal/ffe © 2020 John Wiley & Sons Ltd 1