Q345qD钢T形接头多层焊接接头组织转变

随着桥梁结构的焊接需求极大提升，钢种焊接技术随之飞速发展，Q345qD钢凭借良好的强度韧性以及优异的焊接性成为备受关注的桥梁钢种之一。采用熔化极气体保护焊对Q345qD钢T形接头进行焊接，观察显微组织转变并分析焊接温度场对其组织的影响。结果表明，焊缝成形良好，焊接接头的显微组织主要为铁素体和珠光体，打底层中珠光体含量较高，填充层相对较少，热影响区分布细小的铁素体与珠光体，并伴随细粒状贝氏体生成。结合有限元模拟热循环曲线，打底层冷却速度快，且受到两次后热处理，温度可达到467 ℃,生成上贝氏体；盖面层仅受前一层余温的影响迅速升温后冷却；填充层与热影响区受到其它焊层热作用温度超过AC3温度，经历近似正火过程。创新点： 采用有限元方法模拟Q345qD钢T形焊接接头多层焊时焊缝温度场变化，基于焊接温度场变化揭示了焊接接头的转变过程。

Investigation of microstructure transformation of multi-layer welded T-shape Q345qD steel

With the great increase in the welding demand of bridge structures, the welding technology of steel has developed rapidly.Q345qD steel, one of the bridge steels, has attracted much attention owing to its high strength, good toughness and excellent weldability.The T-joint of Q345qD steel was welded by MIG welding, the microstructure transformation was observed and the influence of welding temperature field on the structure was analyzed.The results show that the weld is well formed.The microstructure of the welded joint is mainly composed of ferrite and pearlite.The content of pearlite in the bottom layer is relatively high, but relatively lower in the filling layer.Fine ferrite and pearlite are observed in the heat-affected zone accompanied by the generation of fine bainite.According to the thermal cycle curve of the joint, obtained by finite element simulation, the cooling rate of the bottom layer is fast, and the temperature can reach 467 ℃ after two times post-heat treatments, which promotes the generation of upper bainite.The temperature of the capping layer rapidly heats up and then cools down owing to the effect of the residual temperature of the previous layer.The temperature of the filling layer and the heat-affected zone exceeds the AC3 temperature and undergoes a normalizing process under the heat effect of the previous welding process.Highlights: In this paper, the finite element method is used to simulate the temperature field of the T-shape Q345qD joint during multilayer welding.The phase transition process of the welded joint is revealed by microstructure analysis and welding temperature field change of the joint.