Investigation Of Regulation Mechanism Of ZrO₂ Content In Fluxes For Weld Metal Under High Heat Input Submerged Arc Welding

The development trend of shipbuilding is continuing to improve with the continuous expansion of the scale of Chinese shipbuilding industry.The demand for lowering the cost,improving the efficiency and performance and extending service life of large-scale marine ships is increasing.The application of high heat input submerged arc welding technology in the shipbuilding process enables to improve the shipbuilding efficiency and lower the shipbuilding cost to meet the production demand.However,the results show that the microstructure of welded joint tends to coarsen and this causes the mechanical properties tend to be damaged after the process of high heat input submerged arc welding.Therefore,the composition of weld metal should be adjusted through applying appropriate flux ingredient,which will improve the weld metal microstructure and further optimize the mechanical properties of the weld metal.In this investigation,five recipe of agglomerated flux with different addition of ZrO2 were prepared for welding by using submerged arc welding with heat input of 60kJ/cm method,and the relationship between the composition,microstructure,inclusion characteristics,impact toughness and Vickers hardness of the weld metal and flux ingredient were studied.The results show that with the increase of ZrO2 content of flux from 5%to 25%,the Zr content in weld metal increases initially and then decreases,and reaches to the maximum of 120 ppm when the ZrO2 content is 15%.The size of inclusions distribution in weld metal is mainly less than 0.9 μm,and the smallest average diameter is 0.32 μm when 15%ZrO2 addition in the flux.The results revealed that the microstructures of weld metal mainly include polygonal ferrite,grain boundary ferrite and acicular ferrite.The acicular ferrite content reach to the highest when the ZrO2 content in flux is 15%.The low temperature impact toughness of each weld metal was tested.It was found that the variation of low temperature impact toughness of weld metal was related to the volume fraction of acicular ferrite in weld metal.When the content of ZrO2 was 15%,the average Charpy impact energy of weld metal at 40℃ reaches to the maximum of 80J.The Vickers microhardness of each weld metal was tested and the hardness increased from base metal,heat affected zone to weld metal.The temperature of 2000℃ and 2300℃ are selected as the equilibrium temperature for thermodynamic simulation by FactSage 6.4.The gas-slag-metal equilibrium model is established at 2300℃ to simulate.At 2300℃,the accuracy of prediction of the main elements content in the weld metal are better than that of 2000℃.However,some deviations still exist in the accuracy of prediction of the main elements content in the weld metal.