Description
This study concerns initiation and propagation of stress corrosion cracks in Alloy 600 in the primary water of PWRs. Constant elongation rate tests were carried out at 360°C on electropolished specimens without a superficial cold worked layer. It is shown that the majority of initiated cracks are stopped at the first triple junction between grains, and that, for the cracks having crossed this barrier, the crack propagation begins with a slow rate and, for the largest cracks, increases abruptly to reach a fast-rate propagation stage. Thus, a cold worked layer is not necessary for a slow to fast rate transition to occur. The transition between these two propagation stages would occur for a stress intensity factor of about 5 MPa·m^1/2. This value is lower that the one currently obtained with single crack specimens (9 MPa·m^1/2), such as CT specimens. The origin of this higher value could be the cold worked zone ahead of the crack induced by the fatigue mechanical sollicitation applied before testing the single crack specimens.

In order to investigate the influence of the cold worked layer depth in case of plastic straining during exposure to primary water, constant elongation rate tests and constant load tests were performed on shot peened specimens having the same surface stress level but different depths of cold worked layer. It is shown that a large depth of cold worked layer strongly increases the average propagation rate of the largest cracks. As for specimens without cold worked layer, the majority of the initiated cracks are stopped at the first triple line but for the shot peened specimens the slow rate to fast rate transition occurs for a stress intensity factor of about 9 MPa·m^1/2. This value is the same as the one obtained with single crack specimens, which could be due to the fact that this transition occurs inside the cold worked layer.