Anisotropy and variability in thermal creep behaviour of Zr-2.5Nb pressure tube

The Zr-2.5Nb alloy pressure tubes of Indian Pressurized Heavy Water Reactors were manufactured from double melted ingots initially and subsequently from quadruple melted ingots to reduce impurities such as chlorine, phosphorus and carbon. In the current investigation, creep tests were carried out in the stress range of 0.7–0.9 times the yield strength and the temperature range of 350 °C–450 °C to characterize the thermal creep behaviour of Zr-2.5Nb alloy pressure tubes fabricated from double and quadruple melted ingots. The rupture time, minimum creep rate, stress exponent, activation energy and threshold stresses were obtained by carrying out the creep tests of the samples fabricated with their axis parallel to the axial and transverse directions of the pressure tubes. Experimental analysis revealed that the rupture time for double melted tube was 2–3 times lower than that of quadruple melted tubes, whereas no significant change in minimum creep rate was observed. Stress exponent values were obtained in the range of 3–5 signifying dislocation creep as the dominant creep mechanism. To predict the long-term creep properties, the master curves employing the Larson-Miller and Monkman-Grant methods were also evaluated. The effect of microstructural anisotropy, alloying elements and impurities on the thermal creep behaviour of Zr-2.5Nb pressure tube has been investigated. • The present study compares the thermal creep behaviour of the Zr-2.5Nb pressure tubes used in Indian Pressurized Heavy Water Reactors manufactured from double melted and quadruple melted ingots. Tubes manufactured from double melted ingot (DM-tube) contained more impurities such as Fe, C, Cl and H than that of tubes manufactured from quadruple melted ingot (QM-tube). • Both DM and QM tubes showed better tensile and creep strength along the transverse direction than along the longitudinal direction at 350 °C, but was vice-versa at 400 °C and 450 °C. It was due to the activation of additional slip system along direction and grain boundary weakening above 360 °C. The rupture time and rupture strain for DM tube was significantly lower than the QM tubes. The reason was attributed to the formation of low energy facets and fissures of trace impurities such as chlorine and carbon along the extrusion direction of pressure tubes, which is normal to the loading direction for transverse samples resulting in axial splitting of the samples. • Stress exponent and activation energy values revealed that the creep deformation mechanism was dislocation creep mechanism (dislocation climb – rate controlling mechanism, dislocation glide – strain producing mechanism).