EBSD Analysis of Alloy 690 Steam Generator Tubing

Examensarbete för masterexamen
Master Thesis
da Silva, Melina
Nickel-chromium-iron alloys, such as Alloy 600 and Alloy 690, are used in steam generator applications. While Alloy 600 steam generator tubes are prone to stress corrosion cracking (SCC), Alloy 690, which has a higher Cr-content, shows a superior corrosion resistance. So far it has not failed due to SCC, but the experience with this alloy is still limited to about 15 years of service. The objectives of this thesis work are to describe the texture and microstructure of Alloy 690 and to analyze the connection between the propagation/arrest of SCC in corrosion tested samples from steam generator tubes. The method of investigation was the Electron Backscattered Diffraction (EBSD) technique. Microtextures of single and double pilgered material are compared, as well as samples from tubes with different tube diameters. EBSD is a scanning electron microscopy (SEM)-based technique that measures crystallographic orientation (i.e. texture) of a small volume of material. The EBSD technique provides microtexture information, such as orientation distributions and grain boundary crystallography. The samples were investigated using a Leo 1550 Gemini SEM (with field emission gun) employing an EBSD-unit in combination with a Hjelen detector. For this study the tubes have been tested by reverse U-bending (RUB) in a caustic environment with traces of lead. Three different tubes were studied, a single pilgered tube with a tube diameter of 22 mm, a double pilgered tube with a tube diameter of 22 mm and a single pilgered tube with a tube diameter of 17.5 mm. Two samples were cut from each tube, one from an area far away from the cracks and the other from the cracked area. No apparent local texture (microtexture) differences were found between the studied materials. Manufacturing process and varying diameter of the tubes do not seem to cause major differences in microtexture. Microtexture measurements on Alloy 690 steam generator tubing show a weak {111}-fiber texture along the tube direction. In a caustic environment with low lead content, cracks are transgranular and the crack growth does not seem to be texture related. The grain size for the three different materials with varying tube diameter and manufacturing process is mainly the same.
Materialteknik , Materials Engineering
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