Experimental residual stress evaluation of hydraulic expansion transitions in Alloy 690 steam generator tubing
- Babcock and Wilcox International, Cambridge, Ontario (Canada)
- Lambda Research Inc., Cincinnati, OH (United States)
- McMaster Univ., Hamilton, Ontario (Canada)
Nuclear Steam Generator (SG) service reliability and longevity have been seriously affected worldwide by corrosion at the tube-to-tubesheet joint expansion. Current SG designs for new facilities and replacement projects enhance corrosion resistance through the use of advanced tubing materials and improved joint design and fabrication techniques. Here, transition zones of hydraulic expansions have undergone detailed experimental evaluation to define residual stress and cold-work distribution on and below the secondary-side surface. Using X-ray diffraction techniques, with supporting finite element analysis, variations are compared in tubing metallurgical condition, tube/pitch geometry, expansion pressure, and tube-to-hole clearance. Initial measurements to characterize the unexpanded tube reveal compressive stresses associated with a thin work-hardened layer on the outer surface of the tube. The gradient of cold-work was measured as 3% to 0% within .001 inch of the surface. The levels and character of residual stresses following hydraulic expansion are primarily dependent on this work-hardened surface layer and initial stress state that is unique to each tube fabrication process. Tensile stresses following expansion are less than 25% of the local yield stress and are found on the transition in a narrow circumferential band at the immediate tube surface (< .0002 inch/0.005 mm depth). The measurements otherwise indicate a predominance of compressive stresses on and below the secondary-side surface of the transition zone. Excellent resistance to SWSCC initiation is offered by the low levels of tensile stress and cold-work. Propagation of any possible cracking would be deterred by the compressive stress field that surrounds this small volume of tensile material.
- OSTI ID:
- 203828
- Report Number(s):
- CONF-950816-; ISBN 1-877914-95-9; TRN: 96:009794
- Resource Relation:
- Conference: 7. international symposium on environmental degradation of materials in nuclear power plants: water reactors, Breckenridge, CO (United States), 6-10 Aug 1995; Other Information: PBD: 1995; Related Information: Is Part Of Seventh international symposium on environmental degradation of materials in nuclear power systems -- Water reactors: Proceedings and symposium discussions. Volume 1; Airey, G.; Andresen, P.; Brown, J. [eds.] [and others]; PB: 664 p.
- Country of Publication:
- United States
- Language:
- English
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