Evaluation of a Residual Stress Level in the Expansion Transition Region of a Steam Generator Tube by Corrosion Tests

Residual stress level in the expansion transition region of Alloy 600 steam generator tube was determined by a corrosion test method. The expansion tube specimens were fabricated from sensitized Alloy 600 tubes by the explosive and the hydraulic expansion methods. The expansion procedures were identical to those used for the steam generators of nuclear power plants. C-ring stress corrosion test specimens were prepared from the same sensitized Alloy 600 tubing material and tested in a 0.1 M Na2S4O6 solution at room temperature under various stresses in a range of 90∼150% of the relevant yield stress level. The cracking time increased with a decreasing stress level. A relationship between the applied stress and the cracking time was obtained. Stress corrosion cracks were observed at eighteen hours of the test time for the specimens expanded by either the explosive or the hydraulic techniques. This is interpreted, in that the residual stress in the expansion region is 220 MPa (90% of the yield stress) at a maximum. It appeared that the level of the maximum residual stress formed in the expansion transition region was the same regardless of the expansion techniques or the expansion conditions applied