Effects of specimen thickness and notch shape on drop weight tear test and charpy impact test properties of API X70 and X80 line-pipe steels

Effects of specimen thickness and notch shape on drop weight tear test (DWTT) and Charpy V-notch (CVN) impact test properties of API X70 and X80 line-pipe steels fabricated by varying hot-rolling conditions were investigated. The DWTT results revealed that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstrucrures consisted of acicular ferrite, which was similar to the CVN test results. The DWTT energy transition temperature (ETT) of the steel rolled in the two phase region was the lowest because it had finer effective grain size and lowest volume fraction of hard secondary phases. The ETT increased with increasing specimen thickness because of the increase in the constraint state, and the ETT of the chevron-notch DWTT was slightly higher than that of pressed-notch DWTT. In the ductile fracture zone above transition temperature, the DWTT total energy density was higher than the CVN energy density, and was higher in the pressed-notch DWTT than in the chevronnotch DWTT. When the total DWTT energy density was divided into initiation and propagation energy densities, the fraction of propagation energy vs total DWTT energy was higher than that of initiation energy, and the fraction of initiation energy of the pressed-notch DWTT specimens was about 0.40, higher than that of the chevron-notch DWTT specimens.