Effect of Multiple Arc Melting of Ingots on Thermal Creep Behaviour of Zr–2.5Nb Pressure Tube

Tubes made of Zr–2.5Nb alloy in cold-worked and stress relived condition are used as pressure boundary component called pressure tube or coolant tube in Pressurized Heavy Water Reactors (PHWR). Earlier the pressure tubes were manufactured from the double-melted ingots, but later it was changed to quadruple-melted ingots to achieve high initial fracture toughness attributed to impurities, such as chlorine, phosphorus and carbon. In the present study, thermal creep behaviour of Zr–2.5Nb alloy pressure tube material produced from double and quadruple-melted ingots was characterised in the temperature range of 350–450 °C and a stress ranging from 0.7 to 0.9 times of yield strength. The rupture time, minimum creep rate, stress exponent and activation energy were determined using creep specimens with its axis parallel to the axial and transverse direction of the tubes. An attempt has been made to understand the role of alloying elements and impurities on thermal creep behaviour of Zr–2.5Nb pressure tube alloy.