| Duplex stainless steel is named"duplex"because of the ferrite and austenite two phase structures.It has been developed for nearly a hundred years since its emergence in 1927.Compared with ferritic stainless steel and austenitic stainless steel,duplex stainless steel not only has good toughness,high thermal conductivity,high yield strength,no brittleness at room temperature,but also has strong corrosion resistance,so it is widely used in petroleum,chemical,energy,shipbuilding and other fields.China is the largest welding material consumption country in the world,but it is not a strong welding material production country.Special alloy welding materials such as stainless steel,heat-resisting steel,low-temperature steel and high-strength steel are mainly purchased from developed countries abroad.The development of“corrosion-resistant steel materials with complex stress states and environments”is one of the goals proposed by“Made in China 2025”.The proposal of this goal will accelerate the in-depth development and application of duplex stainless steel and promote the development of duplex stainless steel welding materials.Based on the current status of the welding consumables market and the development goals of"Made in China 2025"for new materials for high-end equipment,the research team conducted the development and research of duplex stainless steel flux-cored welding wires.In this paper,carbon dioxide arc welding flux-cored welding wire with Cr,Ni,Mo as the main alloying elements and Mn,Cu,N as a few elements have been developed by means of fixed variable method and orthogonal method.The optimized compositions of flux-cored wire were obtained by comparing the microstructure,corrosion properties and mechanical properties.Then,the optimized flux-cored welding wires were welded with four groups of different heat input of 8.32k J/cm,11.02 k J/cm,14.04 k J/cm and 17.39 k J/cm,and the optimal welding process parameters were finally determined.Next,the optimized flux-cored wires were welded with the optimal welding process parameters,and the effects of heat treatment temperature on the microstructure and properties of the welded joint were studied,and the sensitive temperature range of the precipitated phase and the corresponding heat treatment temperature point of the welded joint with the best performance were obtained.Finally,the corrosion resistance of the weld metal of flux-cored wire was compared with that of the base metal and the corrosion resistance mechanism was analyzed.Through the above studies,the conclusions are as follows:（1）By using TCC software to calculate the composition of the flux-cored welding wire of2205 duplex stainless steel,the metal phase composition diagrams of the flux-cored welding wire with different ratios of Cr,Ni,Mo,Mn,Cu and N are obtained,revealing the influences of alloying element variation on phase composition and proportion.The results show that there are precipitated phases such asσ,M23C6,chromium nitride,χ,R in weld metal during aging and holding at 500-1000°C.The addition of different elements with different contents in the weld metal will not only lead to different types of precipitated phases in the weld metal,but also cause different temperatures corresponding to phase equilibrium.（2）The influences of Cr,Ni and Mo on the properties of the coated metal in 2205 duplex stainless steel flux cored wire were obtained.According to the principle of giving priority to strength,impact toughness and corrosion resistance,the optimal composition points of the main elements of Cr,Ni,and Mo are determined to be 22wt.%,5wt.%and 3wt.%through the range analysis.（3）The effects of different contents of Mn,Cu and N elements on the microstructure,mechanical properties and pitting corrosion properties of deposited metal are studied.With the increase of Mn content,the tensile strength increases but the impact energy decreases.The pitting corrosion resistance is the best when the Cu content is 0.6%.The elongation and reduction of area values are the highest when the N content is 0.2wt.%.（4）The influences of welding heat input on the performance of flux-cored wire welded joint after optimization are revealed.There are four different forms of austenite structure in the weld,namely grain boundary austenite（GBA）,weistenite austenite（WA）,intra grain boundary austenite（IGA）and secondary austenite（γ2）.The greater the heat input,the greater theσcontent.With the increase of heat input,α/αgrain boundary andγ/αphase boundary are coarsed,and theγcontent is increased.When the heat input is 11.02 k J/cm,the mechanical properties and pitting resistance are better.The tensile strength of the weld metal can reach 782 MPa,the impact energy（-40℃）is 42J,and the spot corrosion rate is 0.782 g/（m2·h）after using the optimized flux cored wire.The corrosion resistance of the weld metal of flux cored wire in 3.5wt.%Na Cl solution is slightly lower than that of the base metal,and the shape of the polarization curve is consistent with that of the base metal.（5）The effects of post-weld heat treatment on the properties of welded joints of optimized flux-cored wire are studied.With the increase of heat treatment temperature（600-800℃）,theσphase in welded joints increases,and the impact toughness and corrosion resistance decrease.When the heat treatment temperature is 900℃,the content ofσphase decreases.A small amount ofσphase and secondary austenite（γ2）are found in the welded joint in the as-welded state.After the heat treatment,the width of the fusion zone is reduced,the base metal and the weld are well fused,and the microstructure is more uniform.The ferrite content increases with the increase of the heat treatment temperature and the grain size also increases,theσphase disappears.The best heat treatment temperature is 1050℃for 1 hour. |
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