Analysis of Fracture Toughness in the Transition Temperature Region of API X70 Pipeline Steels Rolled in Two-Phase Region

In this study, fracture toughness in the transition temperature region of three API X70 line pipe steels rolled in the two-phase (α + γ) region was analyzed in accordance with the ASTM E1921-05 standard test method. Elastic-plastic cleavage fracture toughness (KJc) was determined by three-point bend tests, using precracked Charpy V-notch specimens, and then the measured KJc values were interpreted by the three-parameter Weibull distribution. The fracture toughness test results indicated that the master curve and the 98 pct confidence curves explained the variation in the measured fracture toughness values well. Reference temperatures obtained from the fracture toughness tests as well as index temperatures obtained from the Charpy impact tests were lowest in the steel cooled at 500 °C, which did not contain brittle martensite and had a small effective grain size. In this steel, the absence of martensite led to the higher resistance to cleavage crack initiation, and the smaller effective grain size led to the higher possibility of crack arrest, thereby resulting in the best overall fracture properties.