High Temperature Uniaxial Compression and Stress–Relaxation Behavior of India-Specific RAFM Steel

India-specific reduced activity ferritic martensitic steel (INRAFM), a modified 9Cr-1Mo grade, has been developed by India as its own structural material for fabrication of the Indian Test Blanket Module (TBM) to be installed in the International Thermonuclear Energy Reactor (ITER). The extensive study on mechanical and physical properties of this material has been currently going on for appraisal of this material before being put to use in the ITER. High temperature compression, stress–relaxation, and strain-rate change behavior of the INRAFM steel have been investigated. The optical microscopic and scanning electron microscopic characterizations were carried out to observe the microstructural changes that occur during uniaxial compressive deformation test. Comparable true plastic stress values at 300 °C and 500 °C and a high drop in true plastic stress at 600 °C were observed during the compression test. Stress–relaxation behaviors were investigated at 500 °C, 550 °C, and 600 °C at a strain rate of 10⁻³ s⁻¹. The creep properties of the steel at different temperatures were predicted from the stress–relaxation test. The Norton’s stress exponent (n) was found to decrease with the increasing temperature. Using Bird–Mukherjee–Dorn relationship, the temperature-compensated normalized strain rate vs stress was plotted. The stress exponent (n) value of 10.05 was obtained from the normalized plot. The increasing nature of the strain rate sensitivity (m) with the test temperature was found from strain-rate change test. The low plastic stability with m ~ 0.06 was observed at 600 °C. The activation volume (V^*) values were obtained in the range of 100 to 300 b³. By comparing the experimental values with the literature, the rate-controlling mechanisms at the thermally activated region of high temperature were found to be the nonconservative movement of jogged screw dislocations and thermal breaking of attractive junctions.