MICROSTRUCTURE AND TOUGHNESS OF HAZ IN X80 PIPELINE STEEL WITH HIGH Nb CONTENT

High Nb-bearing X80 pipeline steel has low cost and excellent combined properties. However, welding will worsen its microstructure and toughness. In this paper, X80 pipeline steel with 0.1%Nb (mass fraction) was chosen, and single welding thermal-cycles with different heat inputs (16, 20, 30, 40, 50, 58 kJ/cm) were simulated by Gleeble-1500 to study the correlation of toughness and microstructure in heat affect zone (HAZ) of actual welding pipe. The evolution characteristics of microstructure of the coarse grain zone in welding heat affect zone (CGHAZ) were investigated by OM, SEM and EBSD. The results indicated heat input in single welding should be less than 30 kJ/cm to ensure good Charpy impact toughness. Although the strong dragging effect of solute Nb can suppress serious coarsening of average grain size in high Nb steel, the uniformity of prior austenite grains is worsened as increasing the heat input. Moreover, the characteristics of high angle boundaries (HABs) and M/A constituents also are influenced by heat input. In the case of low heat put, higher density of HABs, disperse and fine M/A constiuents were observed, and HABs can form at two sites, one is prior austenite grain boundary, the other is between the lower bainites belong to different Bain groups in the same austenite grain. Otherwise, in the case of high heat input (>= 40 kJ/cm), the misorientation between granular bainites in idential austenite grain is small, i.e., the effective grain size is almost the diameter of prior austenite grain, and it will decrease the density of high angle boundary largely, moreover, coarse M/A constituents which are benefit for crack initiation will be generated, consequently, the impact toughness of the coarse grain zone will be worsened obviously in welding heat affect zone.