Effects of Complex Oxides on Charpy Impact Properties of Heat Affected Zones of Two API X70 Linepipe Steels

Abstract

This study is concerned with effects of oxides on Charpy impact properties of heat affected zone (HAZ) of API X70 linepipe steels. Various oxides were formed by adding alloying elements of Ti, Al, and Mg to the API X70 steels, and effects of these oxides on HAZ microstructures and Charpy impact properties were investigated. Oxides present in the steels were sized about 2 mu m, and had characteristics of complex oxides composed of various elements. The steel containing more Al, Ti, and Mg showed higher volume fraction of oxides than the conventional steel. After the HAZ simulation test, the oxide-containing steel HAZs were composed mainly of acicular ferrite, together with small amount of bainite, whereas the conventional steel HAZ were composed mainly of bainite with a small amount of acicular ferrite. This formation of acicular ferrite in the oxide-containing steel HAZs was associated with the active nucleation of acicular ferrite at oxides, thereby leading to the great (five times or more) improvement of Charpy impact properties of the HAZs over the conventional steel HAZs.