退火处理对异种钢激光焊接头组织及性能的影响

Effect of Annealing Treatment on Microstructure and Properties of Laser Welded Joint of Dissimilar Steel

目的通过研究相同时间不同退火温度下304/Q345激光焊接头的组织和性能,得到优化后的退火处理制度,为制定异种钢激光焊退火工艺提供一定的理论指导和技术支撑。方法基于前期大量试验,在激光功率和离焦量不变的情况下,调节焊接速度由15 mm/s到35 mm/s,通过改变焊接速度获得性能优异的焊接接头。由于异种钢物理化学性能差异较大,接头组织不均匀,因此还需对激光焊接头进行退火处理。结果焊缝区显微组织由板条马氏体与残余奥氏体组成,Q345低碳钢热影响区显微组织为多面体状铁素体与黑色珠光体,其中铁素体含量更多,304奥氏体不锈钢热影响区在奥氏体晶界上析出了铁素体。不同退火温度下碳元素均由Q345低碳钢侧向焊缝迁移,在Q345低碳钢侧形成脱碳层,在焊缝中形成黑色条带状增碳层。随着退火处理温度的升高,峰值温度升高,高温停留时间变长,碳迁移强度随之增加,黑色条带状区域增碳层宽度增大。拉伸性能测试结果表明,试样均断裂于Q345母材一侧,且断口存在韧窝,属于典型韧性断裂。此外,焊缝区硬度(390HV左右)最高,热影响区硬度(300HV)次之,母材的硬度(130HV左右)最低。结论在给定试验参数范围内,优化后的最佳退火工艺参数为550℃-1.5 h。

The work aims to study the microstructure and mechanical properties of 304/Q345 laser welding joints at differ-ent annealing temperature,and obtain the optimized annealing treatment system,to provide certain theoretical guidance and technical support for the development of laser welding annealing process for dissimilar steel.Based on a large number of ex-periments,the laser power and defocusing amount were kept unchanged,and the welding speed was varied from 15 mm/s to 35 mm/s so as to obtain the sound welded joint with excellent mechanical performance.However,for the difference of the physical and chemical properties of dissimilar steel,and the weld seam was not uniform,annealing treatment was needed to improve the microstructure uniformity.The results showed that the microstructure of the weld zone was composed of lath martensite and re-sidual austenite.The microstructure of the heat affected zone of Q345 was polyhedral ferrite and black pearlite in which ferrite was dominant,and the ferrite was deposited at the austenite grain boundary in the heat affected zone of 304.At different an-nealing temperature,carbon elements migrated from the fusion zone of Q345 low alloy steel to the side weld seam,forming a decarburization layer on the side of Q345 low alloy steel,and forming a black strip carburization layer on the side of weld seam.With the increase of annealing treatment temperature,the peak temperature increased,the high temperature residence time was longer,the carbon migration intensity increased,and the width of the black strip carburizing layer increased.The tensile prop-erty test showed that all the specimens were fractured on one side of Q345 base metal,and there were a lot of dimples in the fracture,which was a typical ductile fracture.In addition,the hardness of the weld zone was the highest(about 390HV),the hardness of the heat affected zone took the second place(300HV),and the hardness of the base material was the lowest(130HV).Combined with the results of microstructure analysis and mechanical properties,it is concluded that the optimized annealing process is 550℃-1.5 h within the given range of annealing parameters.