Analysis of Abnormal Fracture Occurring during Full Scale Tension Increasing to Failure Test of High Strength Low Alloy Steel Casing

Abstract

The full scale evaluation test of a set of API Q125 high strength low alloy steel casing specimens was conducted. With the tension increasing to failure, the abnormal fracture occurred on each specimen. A series of testing and analysis including fracture observation, chemical composition analysis, mechanical properties testing, metallographic analysis and SEM analysis were made for the specimen with abnormal spiral fracture. The geometry measurement, visual examination, mechanical properties testing and metallographic analysis were conducted for a new specimen. The mechanical properties of the two specimens are in compliance with the specified material specification. The new specimen has lower external diameter ovality and higher wall thickness eccentricity. The smaller wall thickness is distributed in a spiral shape, similar with the abnormal fracture. The fracture originated in the middle of casing wall, and the fracture position had obvious necking. According to fractography and fracture mechanics analysis, the abnormal spiral fracture was attributed to the metallurgical and geometry imperfection grown out of tubing processing process.