利用Gleeble-1500模拟不同热输入的单道次焊接热循环过程, 通过光学显微镜(OM)、扫描电镜(SEM)及电子背散射衍射(EBSD)对高 Nb X80管线钢焊接热影响区中的粗晶区(CGHAZ)显微组织、马/奥组元(M/A)形态和分布以及原奥氏体晶粒粗化情况进行研究. 结果表明: 高Nb X80管线钢的焊接热输入应不高于30 kJ/cm, 虽然在高Nb钢中, 原奥氏体晶粒的平均尺寸不会随热输入增加过于粗大, 但当热输入高于40 kJ/cm时, 会使得原奥氏体晶粒内粒状贝氏体的晶体取向选择过于单一, 大角晶界(≥15o)密度明显降低, 有效晶粒尺寸较大, M/A组元也由于热输入过大而明显粗化. 上述原因使高Nb X80管线钢在大于40 kJ/cm热输入条件下热影响区粗晶区韧性明显恶化.
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.
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