Correlation Study of Microstructure, Hardness, and Charpy Impact Properties in Heat Affected Zones of Three API X80 Linepipe Steels Containing Complex Oxides

Abstract

This study investigated the correlation between the microstructure, hardness, and Charpy impact properties in heat affected zones (HAZs) of API X80 linepipe steels containing complex oxides. Three types of steels were fabricated by adding Ti, Al, and Mg to form complex oxides; their microstructures, hardness, 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. The simulated HAZs containing a number of oxides showed a high volume fraction of intra-granular transformation microstructure (IGT) region because the oxides acted as nucleation sites for the ferrites. According to the correlation study between the heat input, volume fraction of the IGT region, and Charpy impact properties, it was found that ductile fractures predominantly occurred when the volume fraction of the IGT region was 65 % or higher, and the Charpy absorbed energy was excellent over 200 J. When the volume fraction of the IGT region was 45 % or lower, the Charpy absorbed energy was poor below 50 J as brittle cleavage fractures prevailed. These findings suggest that the active nucleation of the ferrites in the oxide-containing steel HAZs is related to the improvement of the Charpy impact properties of the HAZs.