A STUDY OF MECHANICAL, FRACTURE AND FATIGUE BEHAVIOR OF MODIFIED 9Cr-1Mo STEEL

Abstract

Generation-IV nuclear reactors have been proposed in the twenty first century by International Atomic Energy Agency (IAEA) as per the fast-growing demand for energy all over the World. Generation-IV nuclear reactors include Supercritical Water Reactor (SCWR-water cooled), Very High Temperature Reactor (VHTR-gas cooled), Sodium Fast Reactor (Na-LMR-liquid metal cooled) and Lead Fast Reactor (Pb-LMR-liquid metal cooled) etc. Total six designs have been selected for research and development of this new generation (Generation-IV) nuclear reactor systems. Among these, core outlet temperature of supercritical water-cooled reactor is targeted (350-620) °C. To meet the challenges of new Generation nuclear reactor, ferritic/martensitic steels such as P91, P92, NF12 and SAVE12 etc. are under development to increase the working temperature up to 600 °C and a maximum operating temperature up to 650 °C. Additionally ferritic/martensitic steels must be capable to sustain high radiation dose (~ 100 displacement per atom), cyclic variation of thermal stresses and resistant to corrosion and oxidation. Mod. 9Cr-1Mo steel is contemplated as possible structural material for secondary piping, core outlet, sub-assembly of wrapper tubes and steam generator in the Generation IV nuclear reactors. In the past, various newly developed ferritic/martensitic steels tested at 650 ºC experienced sudden drop in mechanical strength. These steels get high temperature strength from precipitation strengthening. Normalizing and tempering temperature strongly influences the shape, size and area fraction of second phase precipitate particles (i.e., M23C6, MX). Additionally, severe plastic deformation techniques such as hot rolling, equal channel angular extrusion, cryo rolling, etc. cause refinement of grain size up to nano-scale, and increase in number of precipitates with several order of magnitudes. On the other hand, these severe plastic deformation techniques drastically increases/shift the ductile to brittle transformation temperature and reduces toughness. Therefore, optimum combination of strength and toughness is required in unirradiated and irradiated condition to reduce the risk of premature brittle failure. Several factors such as prior austenite grain size, effective grain size, lath size, precipitates, crystallite size, micro-strain, crystallographic texture and distribution of grain boundary mis-orientation angle (low/high angle grain boundary) affect the ductile to brittle transition temperature, upper shelf energy, mechanical strength of ferritic/martensitic steel. For structural integrity and damage assessment of components, fatigue crack growth rate, fatigue threshold and fatigue strength are important at different operating temperature ranges. There are various parameters, which influence threshold and Paris regimes.