Fracture toughness analysis in transition temperature region of API X70 pipeline steels

This study is concerned with the analysis of fracture toughness in the transition temperature region of API X70 pipeline steels according to ASTM E1921 standard test method. Two kinds of steels 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 three-point bend tests of precracked Charpy V-notch specimens, and then the measured K_Jc values were interpreted by the three-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. Effects of martensite-austenite constituent (MA) and effective grain size on fracture toughness in the transition region were analyzed from the standpoint of cleavage crack initiation and propagation. The higher volume fraction of MA 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 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 rolled in the single-phase region because of the lower volume fraction of MA and the smaller effective grain size.