Improvement of Charpy impact properties in heat affected zones of API X80 pipeline steels containing complex oxides

The present study aimed at investigating the improvement of Charpy impact properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of steels were fabricated by adding Ti, Al and Mg to form complex oxides and their microstructures, tensile properties and Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al and Mg was included in the steels. After the HAZ simulation test, the steel having a number of oxides contained about 72 vol.-% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and granular bainite, whereas the HAZ microstructures of the conventional steels consisted of bainitic ferrite and granular bainite with a small amount of acicular ferrite. The volume fraction of acicular ferrite increased as the amount of complex oxides increased and the heat input decreased, and thus, Charpy impact properties of the oxide containing steel HAZs were greatly improved over those of the conventional steel HAZs.