Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Generation of creep-fatigue interaction diagram for an indigenous reduced activation ferritic martensitic steel (IN-RAFMS) at 823 K based on sequential tests Aritra Sarkar ⁎ , V.D. Vijayanand, R. Sandhya Materials Development and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102, Tamil Nadu, India ARTICLE INFO Keywords: RAFMS Fatigue Creep Degree of softening ABSTRACT A novel method of generating creep-fatigue interaction diagram is presented. Creep-fatigue interaction diagram for reduced activation ferritic–martensitic steel (RAFMS) is generated at 823 K by sequential tests decoupling fatigue and creep loading modes. Prior exposure to fatigue was found to be the life limiting factor compared to prior creep under creep-fatigue interaction, on account of the extensive cyclic softening occurring during the prior fatigue exposures. 1. Introduction Reduced Activation Ferritic Martensitic Steel (RAFMS) is a candi- date structural material for Test Blanket Module (TBM) in the fusion reactors [1]. Due to pulsed operation and/or maintenance periods, structural materials of in-vessel plasma facing components are sub- jected to low cycle fatigue (LCF) and creep-fatigue interaction condi- tions [2,3]. Creep-fatigue interaction diagram is basically evaluated according to the design codes well established for nuclear applications like ASME Boiler and Pressure Vessels Code, N-47 and the French RCC- MR code. A robust programme has been already initiated to generate the creep-fatigue design curve for RAFM steels [3]. Considering the fact that development of creep-fatigue design curve requires a large number of tests, several alternative approaches are being pursued like decou- pling fatigue and creep loads during creep-fatigue experiments [4–6]. Earlier researchers have carried out these type of approaches in pre- vious investigations for different materials to understand the effect of pre-existing damage on creep deformation and to develop a better un- derstanding towards life-limiting loading conditions [7–10]. In the present study, creep-fatigue interaction diagram has been generated based on two types of sequential tests where fatigue cycling is followed by creep and vice versa. Damage contributions from both fatigue and creep have been analyzed which show that cyclic softening under fa- tigue is a critical parameter affecting the remnant creep-life when the prior exposure was under fatigue whereas ductility exhaustion under creep and precipitation of laves phase influences the remnant fatigue life for tests with prior creep exposures. 2. Experimental The material used in the present study was an indigenous RAFM steel with the following chemical composition (in wt. %): C-0.126 Cr- 9.03 W-1.39 Ta-0.06 V-0.24 Mn-0.56 N-0.0297 Fe-balance. The steel was normalized at 1253 K for 30 min followed by tempering at 1036 K for 90 min. The normalizing and tempering treatment resulted in a tempered martensite structure in the material. Cylindrical low cycle fatigue (LCF) specimens of gauge length 25 mm and gauge diameter 10 mm were machined from the heat-treated blanks of 105 mm length and 22 mm × 22 mm square cross section. Identical specimen geometry was maintained for both fatigue and creep tests. All fatigue and creep tests were carried out at 823 K. LCF tests were conducted in air, under fully reversed, total axial strain control mode at a strain amplitude of ± 0.6% at a constant strain rate of 3 × 10 -3 s -1 in a closed loop servo hydraulic testing system equipped with a resistance heating furnace. Uniaxial creep tests were conducted at a constant load of 210 MPa at 823 K. The temperature of the furnace was maintained within ± 1 K during the test and the creep elongation was monitored continuously by a digimatic dial indicator attached to the extensometer and elongation values were continuously recorded using a data logger system. To carry out the creep-fatigue experiments, two types of sequential tests were performed. In the first testing mode, initial fatigue exposure was followed by creep tests and in the second testing mode, initial creep exposure was followed by fatigue tests. For fatigue followed by creep tests, fatigue tests were carried out at 823 K at a strain amplitude https://doi.org/10.1016/j.fusengdes.2018.10.028 Received 21 May 2018; Received in revised form 29 October 2018; Accepted 29 October 2018 ⁎ Corresponding author. E-mail address: aritra@igcar.gov (A. Sarkar). Fusion Engineering and Design 138 (2019) 27–31 Available online 05 November 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved. T