Study of tensile behaviour of thermally aged alloy D9 wrapper tubes of fast breeder reactor using acoustic emission

Abstract

BACKGROUND: Hexagonal wrapper tubes in fast breeder reactor (FBR) contain smaller circular tubes called fuel pins made of austenitic 15Cr–15Ni–Ti modified stainless steel (alloy D9). This is used in 20% cold worked condition. At maximum service temperature of fuel clad (around 973 K), prolonged exposure to high temperature could remove the effect of cold work. Thermal ageing of cold worked alloy D9 at 823–1123 K results in carbide precipitation which affect the subsequent deformation behaviour. OBJECTIVE: The tensile behaviour of alloy D9 after ageing at 823–1123 K for 10–10,000 h has been studied using acoustic emission (AE). METHODS: Tensile tests along with AE monitoring were conducted at room temperature at nominal strain rate of 1.33 × 10 −3 s −1 . RESULTS: Strength decreased and elongation increased with ageing temperature. Till 1000 h ageing time, AE parameters (up to yield) decreased with ageing temperature. But for 5000 and 10,000 h, they increased first and then decreased with a peak at around 973 K. This coincides with a trough in elongation at around 973 K. CONCLUSIONS: The trough in elongation has been attributed to increasing difficulty of plastic deformation by pinning of dislocations. A correlation exists among tensile, Larson–Miller (LMP) parameter and AE parameters.