| The development of large-scale marine ships is facing the challenges of wide cruising range,high shipbuilding cost and low efficiency.At present,the application of high heat input welding technology in the shipbuilding process can effectively improve the shipbuilding efficiency and save the shipbuilding cost.However,the higher heat input makes the microstructure of welding joint coarser and the toughness decreases sharply.Therefore,how to optimize and improve the weld metal structure of large heat input submerged arc welding and obtain the weld metal with good mechanical properties is an urgent problem to be solved.In this research,the weld metal obtained by submerged arc welding with high heat input of 60 kJ/cm for four kinds of flux with different SiO2 content was studied.The effects of alloying elements content,austenite grain size and inclusions on microstructure of weld metal were analyzed.The investigation shows that with the increase of SiO2 content in the flux,the Si content in weld metal increases,the average width of austenite grain increases from 100μm to 250 μm,the size of inclusions in weld metal is concentrated in the range of 0.3～0.7 μm,and the volume fraction of inclusions increases with the increase of Si content in flux.The microstructure of weld metal changes from proeutectoid ferrite to acicular ferrite.EBSD was used to measure the distribution of large angle grain boundary and grain size of weld metal.It was found that SiO2 in flux could refine the microstructure of weld metal,increase the number of large angle grain boundaries and decrease the average grain size.The results show that the average Charpy impact energy of weld metal at-40℃increases with the increase of SiO2 content in flux,and the maximum impact energy is 67.5 J.The change trend of low-temperature impact toughness of weld metal is related to acicular ferrite content in weld metal.The higher acicular ferrite content is,the higher low-temperature impact toughness of weld metal obtains. |
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