Residual Stresses in Alloy 800 Steam Generator Tubing

Abstract

The final operations in the production of Alloy 800 steam generator tubing for CANDU reactors are: anneal (about 1263k), straighten, surface grind/polish and, finally, non-destructive examination. The surface grinding operation results in a work hardened surface layer which can sustain high surface residual stresses in U-bends and in U-bend to straight leg transitions. Two techniques were explored to reduce the level of surface residual stress in bent tubing. One technique was to electropolish the production tubes to remove approximately 0.05 mm of tube wall prior to bending. The second technique was to modify the production sequence so the final operations prior to non-destructive examination were to anneal and straighten the tubes. These techniques resulted in a reduction of peak longitudinal surface stresses in bent tubing from measured values as high as 800 MPa to about 200 MPa. Details of the programme and of the technique used to measure the residual surface stresses are provided.