X70大变形管线钢焊接热影响区组织和性能研究-材料加工工程专业论文.docxVIP

万方数据 万方数据 摘 要 大变形管线钢是基于应变设计的管线钢，可有效地抵抗地质灾害中产生的大 量变形。为了研究其焊接后的力学性能变化及原因，本文采用焊接热模拟技术与 热膨胀测试技术相结合的手段，建立了粗晶区奥氏体长大动力学模型以及贝氏体 连续转变动力学模型。借助于光学显微镜、扫描电子显微镜、透射电子显微镜、 显微维氏硬度计和低温冲击试验机研究了各亚区的组织特征及力学性能变化规 律。研究结果显示：随着热输入的增加，粗晶区原奥氏体晶粒尺寸增大，板条贝 氏体含量减少，粒状贝氏体含量增加；临界区内相变重结晶的程度增加；临界粗 晶区中的 M/A 组元含量减少。时效脆化区比较特殊没有发生相变，随着时效峰 值温度的增加，M/A 组元与铁素体界面处开始有块状 Fe3C 析出并逐步长大，且 位错密度减小。当热输入增大时，除界面处析出的块状 Fe3C 外，另有针状 Fe3C 于 M/A 组元内部析出。随着热输入的增加，针状 Fe3C 尺寸增大，块状 Fe3C 尺 寸则先增加后减小且位错密度减小。位错的缠结钉扎现象由峰值温度、热输入和 位错密度共同决定。力学性能测试发现除粗晶区和临界粗晶区外，其他各区均有 软化现象，其中临界区软化现象最为严重；仅粗晶区和临界粗晶区发生脆化，其 中临界粗晶区脆化最为严重。 关键词： 大变形管线钢； 热影响区； 相变动力学；显微组织；力学性能 ABSTRACT Pipeline steel with excellent deformability is a kind of pipeline steel based on strain design and can stand up lager deformability generated during geological disasters. In order to understand the microstructure and mechanical properties of pipeline steel after welding, the grain growth kinetics and phase transformation kinetics of coarse grained heat affected zone (CGHAZ) were investigated using a Gleeble-3500 thermal-mechanical simulator and a dilatometer. Optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the change of microstructure. The results of microstructure indicate that, with the increasing of heat input, the grains of CGHAZ grow very quickly, the volume of lath bainite decreases while the volume of granular bainite increases. In the intercritical heat affected zone (ICHAZ), the austenitizing volume increases with increasing heat input which leads to the change of microstructure. The amount of M/A constituent of the intercritical coarse grained heat affected zone (ICCGHAZ) decreases with increasing heat input. The subcritical heat affected zone (SCHAZ) is the special sub-region which has no phase transformation. However, with the increasing of peak temperature, blocky Fe3C begins to precipitate and grows from the interface of M/A constituent and ferrite. With the increasing of heat input, the si