·Table of Contents
·Materials Characterization and testing
Study of Defects Detected in Thin-Walled Zircaloy Tubes by Ultrasonic Testing in High Speed Rota- 25R.K.Mistry, R.K.Srivastava, N.Saratchandran
Nuclear Fuel Complex, Hyderabad
The soundness of the tubes is being checked by using high speed computerized, fast response, probe rotation ROTA-25, Ultrasonic tube testing system. So far, around 4,200 km. PHWR fuel tubes have been tested with the above UT system. During testing it was found some of the tubes are having defects like laminations, cracks, pits, porosities, rough surface etc. which causes rejections during ultrasonic testing and low recovery at finished stage. In order to improve the recovery a study was made to find out the origin of the above defects related with different fabrication steps.
Thus paper brings out clearly the studies and investigation regarding the origin of the defects and necessary corrections needed.
Key Words: Thin walled tubes, ultrasonic testing, Zr-4, soundness, integrity, probe rotation technique, origin, defects, laminations, cracks, porosities, recovery, investigation.
Sound beam configuration is the shape of the transducer sound beam within the tube wall during ultrasonic testing. A uniform acoustic power is essential for best testing result, for which the requirement is careful selection of transducer size, its lens, shape, water path and refracted angle to match the outside convex surface. Zircaloy thin wall tubes have fine-grained microstructure which require high sensitivity and resolution to detect smaller defects and hence, high frequency (10 - 15MHz) & small beam length focused transducer is suitable for ultrasonic testing of tubes. Also, reflection of sound from a real flaw is not specular, as inherently assumed in the use of smooth planer or smooth cylindrical artificial defects. Natural flaws differ in several ways like - most of the natural defects are:
These factors, together with specific sound attenuating characteristics of bulk material affect the height and shape of echo signals. Echo height is a direct function of the area of the reflecting interface.
Most of the non-destructive testing acceptance criteria are concerned with flaw depth, but ultrasonic technique does not measure the actual flaw depth. Rather, the flaws test response amplitude is compared to the reference notch depth amplitude. Generally, V type notch 60 ± 1° depth 10% of wall thickness or 0.037mm (maximum) in longitudinal and transverse direction on outer surface and inner surface is used as reference standard for calibration of UT systems. 10 & 15 MHz line focused transducers were used for bi- directional, 4 - level UT testing. Also, a special UT standard with different notch lengths was also prepared and used to compare the defect detected during testing. Advanced computerized data processing systems provided control cards and control runs to check the stability and sensitivity of calibration and most sophisticated electronic set up were used for correct recording of defects.
A total 375 nos., (from 25 lots representing various melts) PHWR fuel tubes flaw marked pieces exceeded the flaw alarm threshold of reference notch depth 10% of wall thickness (0.038mm) were selected for defect analysis. These defective portions of the tubes were cleaned properly and bright pickled to see the type of defects and their locations on outer or inner surface of the tube. Metallurgical and visual evaluation of defective portion of tubes was carried out to correlate the causes of rejections with type of defects and probable origin of defects related with various processing steps. Expertise in metallography and ultrasonic inspections made possible to identify the deleterious defects.
|Cracks & Laminations on Outer surface||Cracks & lamination on Inner surface|
|PHOTOGRAPHS OF SOME TYPICAL DEFECTS DETECTED DURING ULTRASONIC TESTING|
|Melting:||Porosity and piping.|
|Extrusion:||Deep-scoring lines on ID of blanks due to failure of copper jacket, cracks in billet, necking of mandrel and improper soaking time and temperature.|
|Pilgering:||Ratio of wall to diameter was too low, use of wrong size of mandrel; excessive amount of feed and deep scoring lines in blanks ID|
|Grinding defects:||Uneven size of SiC particles of grinding belt.|
|Cracks:||Deep-scoring lines on extruded blanks, improper heat treatment |
before pilgering,use of wrong size of die, excessive amount of feed,
improper setting of mandrel and incorrect wall to diameter reduction
|Handling defects:||Wrong handling of thin wall tubes on shop floor.|
|Pilgering defects:|| Rough surface of die and mandrel, improper cooling and filtering of|
lubricants, wrong calculation of feed and side relief, non-uniform
rotation of blanks during pilgering and improper heat treatment of
|Burning marks:|| Non-uniform rotation of tubes in grinding. Also jumping of tubes if |
tension of grinding belts is not proper.
The authors express their sincere gratitude to Dr. C. Ganguly, Chief Executive, NFC for permission to present this paper at 15th World Conference on Non - Destructive Testing (15 -21 Oct. 2000), Rome, Italy. The authors are also thankful to Shri Harinder Singh, SO/E & Shri C.V.Bhaskara Rao, GM, (ZF&T) for cooperation and support in providing information on PHWR fuel tubes.
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