Effect of microstructure on drop weight tear properties and inverse fracture occurring in hammer impacted region of high toughness X70 pipeline steels

Abstract

In the present study, the effect of microstructure on drop weight tear test (DWTT) properties and inverse fracture occurring in the hammer impacted region of high toughness API X70 pipeline steels was investigated. Six kinds of steel specimens were fabricated under varying hot rolling conditions. The DWTT was conducted on the hot rolled steel specimens and the results were discussed in comparison with the Charpy V-notch (CVN) impact test data. The DWTT results indicated that all the specimens showed excellent DWTT properties as the per cent shear area well exceeded 85% at –20°C or lower, irrespective of microstructures. The specimens rolled in the single phase region had the lower 85% shear appearance transition temperature (SATT) than the specimens rolled in the two phase region, which was similar to the CVN test results. This 85%SATT had a good correlation with the energy transition temperature (ETT) obtained from CVN test. Inverse fracture started to occur at –20°C and its size increased with decreasing test temperature. The strain hardened hammer impacted region after DWTT became thicker and harder with decreasing temperature, thereby deteriorating the toughness of the specimens.