The Effect of Inclusions on the Fracture Toughness of Local Brittle Zones in the HAZ of Girth Welded Line Pipe

A programme of testing and research has been carried out to deliberately identify the causes of, and measure the fracture toughness of, potential local brittle zones (LBZs) in the heat affected zone (HAZ) of girth welds in a 20mm thick API 5L X70 grade pipe. This involved determining the most susceptible test temperature and notch location within the HAZ of two different welds by Charpy testing to define the T_40J. Subsequent fracture toughness testing of a large set of fracture mechanics specimens notched into this region and tested at a low temperature based on T_40J was able to achieve occasional low fracture toughness results. Post-test metallography identified that the inclusions in the parent metal affected the risk of brittle fracture initiation in two ways; firstly large and clustered inclusions acted as local brittle zones and/or fracture initiation sites; secondly, the large inclusions had reduced the effect of grain boundary pinning, as would be expected from well-dispersed smaller inclusions. This then resulted in significant grain coarsening in the HAZ fusion line, which could also act as a low toughness fracture initiation region. The inclusion types were chemically analysed, and recommendations were made to the steel producer in order to further improve the fracture toughness of the X70 pipe at increasingly lower temperatures.