Research On Microstructural Control And Mechanical Behavior Of Q390/Q690 Dissimilar Welded Joints

Since dissimilar steel welded structures are widely used in engineering structures,the strength mismatch of dissimilar welded joints has been a research hotspot in the field of steel welding.Due to the obvious differences in microstructure and mechanical properties of the welded base metals,the joint is easy to form unstable zone of mechanical properties.In the service process,this zone is susceptible to lose efficacy and cause catastrophic accident.Therefore,the reasonable welding system design for dissimilar welded joints is an effective approach to ensure its structural integrity.Two kinds of Q390/Q690 dissimilar welded joints were prepared with different strength grade welding wires(ER50-6 welding wire and ER76-1 welding wire),the reasonable strength matching mode and corresponding micro structural characteristics of dissimilar welded joints were determined,based on a series of results of mechanical properties and fine characterization of microstructure.The nucleation mechanism of acicular ferrite was revealed by high temperature laser confocal microscope and the control ranges of welding cooling rates to obtain the target micro structure were analyzed by static phase change instrument.The conclusions are briefly summarized as follows:(1)The microstructural evolution of Q390/Q690 dissimilar welded joint was studied.When used ER50-6 welding wire,the microstructure of the weld metal(WM)was mainly polygonal ferrite and degenerated pearlite,and the volume fraction of polygonal ferrite was about 84%;the microstructure of WM was mainly acicular ferrite and granular bainite when used ER76-1 welding wire,the volume fraction of acicular ferrite could reach about 82%.(2)The results of mechanical properties tests showed that the dissimilar welded structure prepared with ER76-1 welding wire had a better combination of strength and toughness,and the weak zone has transferred from the WM to the Q390-side HAZ,which indicated that the WM with acicular ferrite as the main microstructure guarantees the uniform gradient distribution of the yield strengths of the welded joint(σQ390<σWM<σQ690),the intensity gradient difference of the adjacent region was about 170 MPa.In other words,the volume fraction of acicular ferrite in the WM reaches more than 80%,which could obtain the excellent combination of strength and toughness of the welded joints.(3)The sizes distribution of non-metal inclusions in the WM and the nucleation position of acicular ferrite were statistical analyzed.The results showed that the inclusion size greater than 0.5 μm could be the effective nucleation of acicular ferrite and the greater sizes of inclusions could provide the larger inter interface for acicular ferrite nucleation,which have a strong ability to promote the nucleation of acicular ferrite.(4)Based on the phase transition laws of the WM,a new method for optimal design of welding system was proposed,i.e.,taking the volume fraction of acicular ferrite as the invariant as an optimization object to obtain the control ranges of welding cooling rates.When alloy content in the WM is low,the welding cooling rate is controlled near 30℃/s,the volume fraction of acicular ferrite in the WM could reach more than 80%.When alloy content in the WM is high,the cooling rate is controlled in the range of 3～15℃/s,which could also make the WM microstructure mainly composed of acicular ferrite(more than 80%)and the dissimilar welded joints satisfy the optimal strength matching mode.