IMPACT TOUGHNESS PROPERTIES OF API X65 HSLA PIPELINE STEEL

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components, i.e. fracture and non-related fracture energy, are of great importance for pipeline engineers. This paper presents the results of instrumented Charpy impact experiments on high-strength pipeline steel of grade API X65 (manufactured by Sadid Pipe and Equipment Company). First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the datarecording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are presented and discussed. These include the test data from full size Charpy V-notched specimens, taken from base metal (BM), weld metal(WM), and heat affect zone (HAZ). 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 results showed that there was a significant drop in hammer velocity during the impact event. This resulted in a change in the fracture mode from dynamic to quasi-static Accurate analysis of the instrumented impact test data showed that the ratio of crack initiation energy to propagation energy was more than 20% for the X65 steel. 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. This high fracture initiation energy should be accounted for if the current failure models are going to be used for toughness assessment of highstrength low-alloy gas pipeline steels.

کلیدواژه ها:

Instrumented Charpy impact test- API X65 steel ، ductile fracture ، crack propagation ، faliure model