The influence of the strain-rate on the stress corrosion cracking of alloy 600 in high temperature primary water

Abstract

Alloy 600 used as a material for steam generator tubings in pressurized water reactors is susceptible to intcrgranular stress corrosion cracking. Laboratory tests have shown that on tensile smooth specimens alloy 600 stress corrosion cracking in the thickness (1.27 mm) involved first an initiation period depending mainly on electrochemical processes, then a slow-propagation stage with crack depth less than a critical size—highly influenced by stress and mechanical state of the surface—and finally a rapid-propagation stage leading to failure. Tests separating stress and strain-rate contributions have shown that strain-rate is the most suitable parameter to describe the different stages of propagation. Stress is only needed to generate a creep rate. Correlation between SCC susceptibility and creep strain-rate was investigated at 360 °C on six mill-annealed tubes showing different chemical compositions, mechanical properties and grain sizes. The tubes with high creep strain-rate exhibited the greatest susceptibility to stress corrosion cracking and several tubes with low creep strain-rate demonstrated a low susceptibility, but one slow-creeping tube evidenced a relatively easy cracking. Heat treatment at 700 °C, which has resulted in an increased creep rate, was found to improve stress corrosion cracking resistance. This lack of correlation between creep and stress corrosion cracking may be a consequence of differences of metallurgical structure contribution to creep and (or) of an influence of primary water on the creep rate of alloy 600. An assessment of the durations of the initiation and the slow-propagation stages of cracking was attempted for the uniaxial tensile tests, using the macroscopic strain-rate. A crack-tip strain-rate damage model is still in progress to predict these durations for constant-load tests from constant extension rate tests (CERTs).