The detection of hydrogen induced cracking in welded and seamless steel pipes using acoustic emission and ultrasonic techniques

Abstract

The detection and location of hydrogen induced cracks In steel pipes is unreliable and time consuming because of the unpredictable nature of the defect and the lack of sensitivity of conventional non destructive testing techniques. Determination of the susceptibility of steels to the formation of hydrogen induced cracking by attack from a sour gas environment has always been based on laboratory testing of small samples where the samples are subjected to attack from all sides. This is unrealistic compared to the in-service situation and a single sided exposure test is more realistic. In this thesis the subject literature is reviewed. Experiments on small samples show that seamless steel is the least susceptible to hydrogen induced cracking, whereas electric welded un directionally formed pipe is the most susceptible. The susceptibility of submerged arc welded pipe depends on the metallurgical form of the pipe but is always less susceptible than the electric welded pipe. Ultrasonic techniques have been used to detect the location of hydrogen induced cracks but manual techniques are labour intensive and unreliable. Four complete pipes were subjected to a sour gas environment from one side, one of which was seamless and one electric welded. These pipes were monitored using a passive non destructive testing technique, acoustic emission. A mechanised ultrasonic scanner was used to examine the last two pipes found to be susceptible to cracking using a specially selected ultrasonic transducer. The acoustic emission data collected was used to detect and locate areas of high acoustic activity produced by the formation of hydrogen Induced cracking. These areas were examined metallographically and shown to include several forms of hydrogen induced cracking. The mechanised ultrasonic technique failed to detect near surface (<1 mm) cracks, but was able to resolve mid wall affected areas. The seamless steel pipe was unaffected, whereas the electric welded pipe was severely affected by the sour gas environment. The automatic welded pipes suffered varying degrees of attack. This supports laboratory based experiments on small samples reported previously by other workers.