Ultrasonic study for detection of inner diameter cracking in pipeline girth welds using creeping waves

摘要

The assessment of pipeline girth welds in power plant equipment using a fitness for purpose approach relies heavily on accurate and estimation of defect size [1,2]. This is difficult to achieve with commonly used techniques employing standard ultrasonic transducers. Defects, particularly tight cracks located at the inner diameter (ID) of a girth weld and very close to the root of the weld, are difficult to detect and resolve from indications due to severe root undercut and other geometric defects. This could be overcome by the application of creeping wave probes. The secondary creeping wave, which is generated at the ID of the pipe and propagates on the ID surface, will respond to only ID cracking close to the root of the weld without generating any interfering echoes from the geometric defects. The high strength creep resistant martensitic steel X-20' used in power cycle piping is highly susceptible to cracking either due to the welding cycle or due to stress corrosion. Recently, the failure of girth welds in X-20 pipes revealed tight cracks at the ID very close to the root. The special ultrasonic techniques using the creeping wave probes revealed the problem of detection and sizing of the cracks. This paper describes our experience with the use of creeping wave probes.