Detection of defects initiation in weld joints

Abstract

Welded joints on various pipelines, especially steam pipelines of fossil power plants, are exposed to high pressure and temperature of steam during operation. The applied stress and temperature together with the chemical composition and microstructure of the material have a major influence on the damage growth in these joints. Growth of defects as a time-dependent event, the sensitivity of the material to damage due to stress temperature and others plays a major role. In the case of steam pipes of fossil power plants, this is mainly creep damage. Early detection of these defects, especially at their initial stage, can help in managing the service life and thereby reducing the costs for operation, both by minimizing unplanned shutdowns as well as by planning any repairs in time. The work is focused on the detection of defect indications, especially creep damage occurring in weld joints and heat-affected areas. The aim is to distinguish manufacturing defects of welded joints from indications of early crack growth by ultrasonic testing. Indications from manufacturing defects may also be detected during testing and, if detected, will be evaluated in the same way as defects that are primarily targeted by the testing techniques. Several different techniques were tested on samples cut from the operated steam pipeline systems and compared with the results of metallographic analyses on selected parts.