Tensile and Creep Behaviour of Modified 9Cr-1Mo Steel Cladding Tube for Fast Reactor Using Metallic Fuel

Abstract

Modified 9Cr- 1Mo ferritic martensitic steel has been considered as cladding tube material in metallic fuel fast reactors in view of the relatively lower operating temperatures. In this study, tensile and creep behaviour of modified 9Cr-1Mo steel cladding tube have been investigated. Microstructure of the normalized and tempered steel consisted of tempered lath martensite with precipitates at the prior austenite grain boundaries and sub-boundaries. Tensile tests on the cladding tube were carried out at a strain rate of 3x10-3 s-1 over the temperature range of 300 - 923K. The variations of 0.2% yield stress, ultimate tensile strength and elongation of the steel with temperature have been studied. Yield stress and ultimate strength of the cladding tube exhibited plateau in the intermediate temperature range of 523 - 673K, where the elongation exhibited a broad minimum. The tensile strength and ductility of the steel cladding tube were comparable with those reported for different products forms of the steel.Creep properties of modified 9Cr-1Mo steel cladding tube were studied at 823K at various stress levels. Creep curves of the steel generally consisted of primary and tertiary stages with no secondary stage of deformation. The variation of minimum creep rate with stress obeyed a power law. The stress exponent ‘n’ was around 25, which is the characteristic of precipitation hardened alloys. Rupture life was found to decrease with increase in stress. The Monkman-Grant relationship relating minimum creep rate with rupture life was found to be obeyed by the steel. Creep rupture strength of the modified 9Cr-1Mo steel cladding tube was in accordance with the reported values for other product forms.