Clarification of micromechanism on Brittle Fracture Initiation Condition of TMCP Steel with MA as the trigger point

Abstract

Brittle fracture of carbon steel has a serious effect on the safety of steel structures. In case of bainitic microstructure it is well known that MA works as a triggering point of brittle fracture. However, the brittle fracture prediction procedure has not been fixed yet because of its complexity of the microstructure.The target of this series of research is to clarify the selection rule of the MA which causes brittle fracture from a number of MAs in the bainitic steel manufactured by TMCP process. Double-notch tests was carried out to identify the true trigger point of brittle fracture comparing with a simple whole cleavage fracture surface observation. Then, with respect to the nucleus of fracture occurrence, orientation information data of crystals acquired by EBSD was investigated after surface processing using FIB polishing. As a result, it was revealed that brittle crack occurred from the MA located in the much more coarse crystal grain than the surrounding grains. Furthermore, according to the GROD (Grain Reference Orientation Deviation) map, a large amount of piled-up dislocations is observed around MA which worked as the trigger of brittle crack initiation. This observation will help an establishment of quantitative model which can explain the behavior of brittle fracture in TMCP steel. Authors also established numerical simulation model of DWTT test used in quality assurance in line pipe field.