Effect of tensile test conditions on the fracture behaviour of 20Cr-25Ni-Nb stabilised stainless steel after high-temperature irradiation

Abstract

Tensile specimens were cut from 20 Cr-25 Ni-Nb stabilised austenitic steel tubes irradiated at 650–700 °C, in contact with UO 2 fuel at their inner surfaces and CO 2 at their outer surfaces, to a thermal neutron dose between 1 and 10 × 10 20 nvt. Tensile tests were made at temperatures between 450 °C and 800 °C using three strain-rates in thp range 1.4 × 10 −3 to 1.4 × 10 −6/ sec. The results were correlated using the expression σt n = K ε enp ( Q/ RT), where σt is the flow stress and n is an index which proves to be close to 7. Under test conditions giving full recovery-creep an activation energy Q of ∼ 68.5 kcal/ mole was obtained for the irradiated specimens and ∼ 80 kcal/mole for unirradiated thermal control specimens. The tensile elongation and rupture mode of the specimens were also related to the Zener-Holloman parameter Z = log 10 [ ε exp ( Q/ RT)]. The intergranular fracture mode of the irradiated specimens varied from development of micro-cracks at the higher values of Z to cavitation at lower values. The reduced ductility observed on the irradiated tubing is discussed in terms of the effect of helium from the 10B(n, < α) 7Li reaction on intergranular crack and cavity development. The fission-fragment-bombarded zone at the tube bore was ∼ 10 μm thick and contained numerous gas bubbles of 20–100 Å dia. This zone apparently had little overall effect on the ductility of the tubing.