Flaw Quantification in Colmonoy Overlays Using Immersion Ultrasonic Testing

Corrosion, wear and galling resistance under high temperature service conditions are highly essential in nuclear components such as grid plate and diverse safety rod drive mechanism. In order to protect the components from these worst service conditions, hard facing alloys like colmonoy are overlaid on the nuclear components using plasma transfer arc and Gas Tungsten Arc Deposition techniques. Typical defects that occurred during deposition include lack of bonding between substrate and overlay, cracks and porosities. Hence the characterization of such defects is highly essential before using it for the intended application. Conventionally radiography, liquid penetrant testing and ultrasonic testing are used. Radiography cannot detect debonds which are parallel to the surface of the specimen. Though liquid penetrant testing is used for detecting surface cracks, ultrasonic testing could be effectively used in order to characterize highly planar defects which are parallel to the surface. This paper highlights the successful application of immersion ultrasonic testing for characterization of colmonoy overlays. The sample consisted of Colmonoy coating (4-5 mm) on a 316 LN substrate. Ultrasonic examination was carried out using the Immersion Ultrasonic flaw detector. Typical A scans of good region and debonded regions were considered and compared in order to predict the actual location of the defect. In addition B and C scans were also considered for exact characterisation of the defects. Results clearly indicated the superior defect detectability by the use of immersion ultrasonic testing on colmonoy overlays.