ANALYSIS OF FRACTURE TOUGHNESS IN TRANSITION TEMPERATURE REGION OF APIX70 AND X80 LINEPIPE STEELS

Abstract

This study is concerned with the analysis of fracture toughness in the transition temperature region of three API X70 and X80 linepipe steels fabricated by varying hot-rolling conditions in accordance with the ASTM E1921-05 standard test method. Elastic-plastic cleavage fracture toughness (K-Jc) was determined by 3-point bend tests, using precracked Charpy V-notch specimens, and then the measured K-Jc values were interpreted by the 3-parameter Weibull distribution. The fracture toughness test results indicated that the master curve and the 98% 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 X70 steel rolled in the two phase region because it had smaller effective grain size and lowest volume fraction of hard secondary phases. In this steel, few hard phases 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. Measured reference temperatures were then comparatively analyzed with index temperatures obtained from the Charpy impact tests, and the effects of microstructures on these temperatures were discussed.