Regulation Mechanism Of TiO₂ Content In Submerged Arc Fluxes On The Acicular Ferrite In Weld Metal Of EH36 Shipbuilding Steel

With the rapid development of the world economy and maritime trade,the demand for large ships,represented by crude oil tankers and container ships,has been increased year by year.And now global ocean transportation is also moving towards large-sized patterns.It is imminent to improve the efficiency of shipbuilding in order to meet the trend of large-scale shipping.Welding is an important process and takes up 40%of working hours during shipbuilding.Therefore,high heat input welding methods have gained a wide range of engineering applications due to enhanced construction efficiency and reduced cost.Improving the mechanical properties of welded joints by oxide metallurgy is the key to solving this thorny problem.Since Ti has strong affinity with O in weld metal and is easy to form fine,dispersed and stable inclusion particles,it is considered to be able to transfer from the flux to the weld pool,which is usually applied for oxide metallurgy technology to promote acicular ferrite nucleation.In this study,four groups of sintered flux with 1%,6%,11%and 16%TiO2 content were utilized to study the effect of TiO2 content in flux on the microstructure in weld metal.The type of CHW-S10 submerged arc welding wire was used as the cladding metal material and the EH36 grade shipbuilding steel plate was welded by submerged arc welding with the heat inputs of 39.2 kJ/cm and 58.8 kJ/cm.It is found that when the content of TiO2 is constant in the flux,high heat input welding usually leads to significant coarsening of austenite grains and formation of brittle microstructures such as ferrite side plate and upper bainite,resulting in the reduced toughness of welded joint and heat affected zone(HAZ).However,it is found that the TiO2 content in the flux has a certain influence on the volume fraction of acicular ferrite in the weld metal.The results indicated that the number and the volume fraction of metallic oxide inclusions per unit volume were controlled by titanium bearing,as well as the inclusion size distribution.The increase of acicular ferrite volume fraction greatly improves the impact toughness of the weld metal,and the increase of Ti content strengthens the strength and hardness of weld metal.As a result,the change of TiO2 content in flux is the key factor for the change of acicular ferrite volume fraction and mechanical properties of weld metal.Finally,the optimum TiO2 content for the fluxes in this investigation was 6%,a middle level in this study.The result indicated that adding TiO2 to the flux has a positive effect on the amount of Ti from the flux to the weld metal and disperse inclusions.And fine inclusions are benefical to increase the volume fraction of acicular ferrite and improve mechanical properties of the weld metal.However,the amount of Ti from the flux to the weld metal will be suppressed when the TiO2 content in the flux is more than 11%,which will adversely affect the volume fraction of acicular ferrite and mechanical properties.