Analysis of Fracture Toughness in Transition Temperature Region of an API X70 Linepipe Steel

摘要

In this study, fracture toughness the transition temperature region of an API X70 linepipe steel was analyzed according to ASTM E1921 standard test method. Two kinds of steel specimens were fabricated by varying finish rolling temperatures, and their microstructures and mechanical properties were examined. Elastic-plastic cleavage fracture toughness (K_(Jc)) was determined by 3-point bend tests of precracked Charpy V-notch specimens, and then the measured K_(Jc) values were interpreted by the 3-parameter Weibull distribution. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing critical J value. The higher volume fraction of martensite-austenite (MA) constituent led to the lower resistance to cleavage crack initiation, and the larger effective grain size led to the lower resistance to cleavage crack propagation. Thus, the steel specimen rolled in the two-phase region had the higher fracture toughness in the transition region and the lower reference temperature and energy transition temperature than the steel specimen rolled in the single phase region because of the lower volume fraction of MA and the smaller effective grain size.