The Residual Stresses in and the Stress Corrosion Performance of Roller Expansion Transitions in Alloy 600 Steam Generator Tubing

Abstract

The residual stresses induced by roller expansion were studied in two overexpanded and two underexpanded Alloy 600,3/4 inch O.D. tubes. Each pair were intended to be duplicates and were expanded using procedures similar to those applied in nuclear power plant installations wherein the steam generator tubing is expanded into a hole in the tubesheet. The stresses were measured using a combination of strain gage and stress relief and X-ray diffraction (XRD) with most being performed by XRD. Over 1000 measurements were repeated at least three times on each tube sample. Both hoop and axial direction stresses were measured along transverses through the expanded zone of the tubes on the outside diameter (O.D.) and inside diameter (I.D.) surfaces. Also, subsurface measurements were performed on the inside diameters. The profiles of the XRD obtained residual stress distributions agreed reasonably well with previously published results, but the magnitudes were much higher. The profiles and magnitudes of the stresses, when compared with in-service ad stress- corrosion tests, confirmed the correlation between cracking and residual stress patterns in the overexpanded (340 MPa hoop and 245 MPa axial) and underexpanded (436 MPa hoop and 508 MPa axial) tubes confirmed the greater susceptibility of the latter for stress corrosion cracking.