Evaluation of Fracture Initiation Energy in API X65 Pipeline Steel

In this paper, energy absorption characteristics of spiral welded gas pipeline steel are investigated under impact loading. Emphasise is given to energy consuming processes before fracture propagation in tested linepipe steel. The API X65 grade pipe was produced (by Sadid Pipe and Equipment Company) from thermo-mechanical controlled process (TMCP) coils supplied by a Korean steel mill. To measure material impact toughness, an instrumented Charpy machine was used. Experiments were conducted at room temperature on different sets of standard full size Charpy V-notched specimens taken from the pipe material, seam weld and heat affected zone. The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The numerical results showed good agreement between the instrumentation data and those read from dial indicator. From fracture energy plots it was found that the maximum and minimum fracture energy was associated with the pipe material and seam weld, respectively. In all test samples, a significant amount of energy was consumed in non-fracture related processes including indentation at the support anvils and at the impact point, bending of test specimen and crack initiation. From this finding, correction factors were suggested to account for considerable energy level of non-fracture related processes. This energy had been ignored apparently in conventional pipeline failure models calibrated in the past on low toughness pipe materials in which fracture initiation energy was negligible. The paper concluded with a comparison of suggested correction factors with those obtained by full-scale burst experiments on tough pipeline steels.