Effect of approach to test temperature on DHC behavior of Zr-2.5Nb pressure tube material

Zr-alloy components are susceptible to failure by Delayed Hydride cracking (DHC), which occurs in the presence of hydrogen more than a critical value, a flaw and load imposing a stress intensity factor (K_I) more than the threshold value. DHC is characterized by mean crack growth rate called V_DHC and threshold K_I called K_IH, which is used for safety assessment of these components. The V_DHC is observed to initially increase exponentially with increase in temperature and after reaching a peak value it decreases rapidly with further increase in temperature to a negligible value. The temperature corresponding to peak V_DHC is called inversion temperature and the temperature corresponding to negligible V_DHC is called crack arrest temperature. Both inversion and crack arrest temperature strongly depends on whether the test temperature is attained by cooling from a peak temperature (cooling approach) or by heating from the ambient temperature (heating approach). For the first time the inversion and crack arrest temperatures for the DHC in Zr-2.5Nb pressure tube (PT) material used in 220 MWe Indian pressurized heavy water reactor has been determined by cooling and heating approaches using curved compact tension (CCT) specimens machined from a Zr-2.5Nb PT spool charged with 100wppm hydrogen. Metallographic examination revealed circumferential orientation of hydrides. The inversion temperatures for cooling and heating approach were found to be 283 and 200 °C, respectively. The crack arrest temperatures were found to be 325 and 212 °C, respectively for cooling and heating approach. The activation energy for cooling and heating temperature approach was 55.1 and 32.6kJ/mol, respectively. (author)