利用示波冲击试验机对X100管线钢直缝埋弧焊实际焊接接头热影响区不同位置处的冲击韧性进行了测试. 结果表明, 当缺口穿过不完全重结晶粗晶区(ICCGHAZ)时冲击韧性很低(平均为51 J), 而当缺口不穿过ICCGHAZ时冲击韧性则高达183 J. 还利用Gleeble热模拟试验机对热影响区不同区域的组织进行了模拟, 得到均一组织的粗晶区(CGHAZ), 细晶区(FGHAZ)和不完全重结晶区(ICHAZ)的冲击韧性较高, 平均分别为244, 164和196 J, 而ICCGHAZ的冲击韧性只有32 J. 因此, ICCGHAZ是导致冲击韧性骤降的主要原因. ICCGHAZ由粗大的原奥氏体晶粒及沿晶界呈链状分布的马氏体-奥氏体(M-A)组元构成, 晶粒内部为粗大的粒状贝氏体或者上贝氏体. 断口分析表明, ICCGHAZ是整个断面的起裂源, 且裂纹扩展过程中M-A组元易成为解理刻面的起裂源. 示波冲击结果显示, ICCGHAZ的存在使得起裂功显著降低. 对断口下方二次裂纹的研究表明, CGHAZ处的断裂机制为形核控制型, 而在ICCGHAZ处则为扩展控制型. 因此, ICCGHAZ中链状M-A组元的存在是导致热影响区韧性恶化的根本原因, 并且使得断裂行为和断裂机制发生显著变化.

After decades of development, mechanical properties of pipeline steels have a good combination of strength and toughness. But after welding, in the heat affected zone (HAZ), microstructure of the base plate was erased by the welding thermal cycle. Several subzones with different microstructures were formed in the HAZ due to different thermal histories they went through. Toughness of the HAZ varies due to the heterogeneous microstructure. In this work, toughness of the HAZ of X100 pipeline steel was examined with two notch locations. Low toughness of 51 J was obtained when the notch encountered intercritically reheated coarsen-grained (ICCG) HAZ and high toughness of 183 J when the notch did not contain ICCGHAZ. Meanwhile, different sub-zones in the HAZ were simulated using Gleeble thermal simulation machine. Simulated coarsen-grained (CG) HAZ, fine-grained (FG) HAZ and intercritically reheated (IC) HAZ with uniform microstructure had good toughness of 244, 164 and 196 J, respectively. In contrast, toughness of simulated ICCGHAZ was only 32 J. Therefore, ICCGHAZ consisting of coarse granular/upper bainite and necklace-type martensite-austenite (M-A) constituent along grain boundaries was proved to be the primary reason for low toughness. Instrumented Charpy impact test results showed that ICCGHAZ could notably embrittle the sample and lower the crack initiation energy. Characterization on the fracture surfaces of the as-fractured Charpy impact specimens showed that ICCGHAZ was found to be the crack initiation site of the whole fracture, and M-A constituent in the ICCGHAZ was characterized as cleavage facet initiation. Fracture mechanisms in the CGHAZ and ICCGHAZ were separately investigated using EBSD. The results showed that necklace-type M-A constituent in the ICCGHAZ notably increased the frequency of cleavage microcracks nucleation. Fracture mechanism changed from nucleation controlled in the CGHAZ to propagation controlled in the ICCGHAZ due to the existence of necklace-type M-A constituent. Therefore, the formation of necklace-type M-A constituent in the ICCGHAZ could not only cause notable drop of toughness in the HAZ, but also change the fracture behavior/mechanism. Hence, research on how to control the distribution status of M-A constituent in the ICCGHAZ is the key to improve the toughness of a weld joint.