X80管线钢Cu-Ni含量及热输入对CGHAZ冲击离散性的影响

利用Gleeble-1500模拟实际焊接条件下双丝纵列焊接热循环过程，通过冲击试验、光学显微镜（OM）、扫描电镜（SEM）、透射电镜（TEM）以及电子背散射衍射（EBSD）对不同Cu-Ni含量的X80管线钢模拟焊接粗晶区（CGHAZ）的显微组织、马/奥组元（M/A）分布及形态、冲击韧性和室温组织粗化程度进行了研究，对低Cu-Ni含量的X80管线钢在不同焊接热输入下进行了显微组织、晶粒粗化程度和冲击韧性的表征. 结果表明，随着奥氏体稳定性元素含量的降低，CGHAZ平均晶粒尺寸无明显变化，但晶粒尺寸离散度增加；原奥氏体向贝氏体转变温度升高，晶界渗碳体含量增加，且粒状贝氏体的晶粒取向选择过于单一，大角度晶界（>15°）密度显著降低；M/A组元由块状向长条状转变且数量明显减少. 上述原因使X80管线钢模拟CGHAZ的冲击韧性离散性增加，但随着焊接热输入的降低，模拟CGHAZ晶粒尺寸离散度降低，大角度晶界（>15°）密度显著提高，这使X80管线钢模拟焊接接头的CGHAZ冲击吸收功得以稳定.

Effect of Cu-Ni components in X80 pipeline and heat input on discretization of toughness in CGHAZ

In recent years, the strength and toughness of pipeline steels were considered by aiming for setting alloy and branching out thickness. However, welding would worsen its microstructure and toughness in coarse grain heat affect zone (CGHAZ) on microalloyed pipeline steel with much lower cost, especially. In this paper, the microallyed X80 pipeline steels were chosen, and single welding thermal-cycles to simulating two-wires longitudinal submerged arc welding (SAW) were carried out by Gleeble-1 500 to study the correlation of microstructure and toughness in simulated CGHAZ. The values of toughness of the simulated CGHAZ in different Cu-Ni components and dissimilar welding heat input were tested, and corresponding evolution characteristics of microstructure in the same area were investigated by OM, SEM, TEM and EBSD. The results showed that the average grain size in simulated CGHAZ changed indistinctively with the decrease of austenite-stability alloys, but the discretization of grain size in CGHAZ increased. Moreover, the transition temperature from prior austenite to bainite was decreased. At the same time, the cementite was precipitated between grain boundaries during cooling process. Otherwise, the misorientation between granular bainite grains was small, i.e., it would decrease the density of high angle boundary (≥15°) in case of much lower Cu-Ni components. Furthermore, the shape of M/A has been changed from block-like structure to strip-like one during the decrease of austenite-stability alloys. Based on the mentioned results above, it has come true that discretization of toughness in stimulated CGHAZ of the X80 pipeline steel was increased. Oppositely, the discretization of grain size in stimulated CGHAZ decreased and the density of high angle boundary (≥15°) increased while the welding heat input was declining, which made the stimulated CGHAZ toughness of X80 pipeline steels stabilized.