EVALUATION OF CHARPY IMPACT TOUGHNESS FOR CRACK ARREST IN HIGH-GRADE GAS PIPELINE STEEL

Charpy impact experiments were conducted on high-strength low-alloy gas pipeline steel of grade API X100. Standard full-size V-notched specimens were used to measure the resistance to ductile axial fracture. All specimens were tested on an instrumented Charpy machine with 300J working capacity and a pendulum velocity of 5.5m/s. The load history was recorded in each test from which the hammer displacement, impact velocity and fracture energy were calculated. The results showed that there was a significant drop in hammer velocity during the fracture process resulting in a change of the fracture mode from dynamic to quasi-static. To resolve this problem either unconventional Charpy machines with higher impact capacity or sub-sized Charpy specimens should be used. This would allow the assessment of the conditions of dynamic crack propagation in the specimen ligament. From the instrumented test data the ratio of crack initiation energy to propagation energy was found to be around 30% for the X100 steel tested. It can be concluded that in impact testing of high-grade pipeline steel a significant portion of overall fracture energy is consumed in non-related fracture processes. The preliminary results of this research suggest that a correction factor of the order of 1.4 (as a minimum) might be necessary to extrapolate the current failure arrest/propagation models for assessing the fracture toughness of modern tough pipeline steels.

Keywords:

ductile fracture , gas pipeline , X100 steel , Charpy energy