Effects Of Post-weld Heat Treatment On The Microstructure And Mechanical Properties Of Fe-29Mn-9Al-0.9C Low-density Steel Argon Tungsten Arc Welded Joints

With the number of cars has increased year by year,and the consumption of oil and carbon dioxide emissions have also increased.Therefore,energy conservation,emission reduction and environmental protection are urgent,and the lightweight of automobiles is a powerful measure to reduce oil consumption and carbon emissions.The density of the Fe-29Mn-9Al-0.9C low-density steel involved in this paper is 6.82g/cm³,which is 13.2%lower than pure iron(7.86g/cm³).At the same time,the tensile strength reached 883 MPa and the elongation is 58%.In order to study the welding performance of the low-density steel and optimize the post-weld mechanical properties,tungsten argon arc welding is used in this paper,and solution treatment and annealing treatment are performed on the welded joints.The mechanical properties are characterized by tensile test at room temperature,microhardness test and nano-indentation.The microstructure of weld is observed by optical microscope and scanning electron microscope.The properties of weld grain orientation,dislocation,recrystallization and texture are characterized by EBSD(Electron Backscattered Diffraction).Provide theoretical basis for welding applications of low density steel.The study on the weld crack sensitivity of Fe-29Mn-9Al-0.9C low-density steel shows that the weld hot crack is not easy to produce,but the preheating treatment is helpful to improve the mechanical properties of welded joints.The weld microstructure is composed of austenite matrix and granular ferrite,and the solidification process is L?L+F?L+F+A?F+A.The tensile strength of the welded joint reach 814.94MPa and the elongation is 34%,which is lower than that of the base metal.Through nano-indentation characterization,the average austenitic phase hardness and Young's modulus are 3.86 GPa and 166.79 GPa,and the ferrite phase hardness and Young's modulus average are 3.27 GPa and 159.45 GPa.The results of EBSD show that there is great residual stress,obvious dislocation plug and obvious Cubic texture.In order to improve the mechanical properties of welded joint,this paper firstly adopt solution treatment for welded joint.After 30min to 60min at1050°C,the ferrite particles in the weld are transformed into austenite.The tensile test shows that the tensile strength reach 845.55MPa when the holding time is 30min,and decrease with the increase of solution time,and the elongation increase with the extension of solution time.The results of microhardness test show that the hardness of weld is higher after solution treatment,and the hardness of heat affected zone is the lowest.EBSD results show that with the increase of solution time,the degree of recrystallization increases,the grains are equiaxed,dislocation density decreased,the texture orientation of weld gradually changed from uniform distribution to very obvious Goss texture.This paper also used the annealing treatment on welded joint,at 680°C and 730°C for 0.5 h,1 h and 2 h.The weld precipitates DO₃ type Fe₃Al phase at the ferrite and austenite phase boundaries.Tensile experiments show that the tensile properties are best at 680°C for 1 h and at 730°C for 2 h,reach 992.1 MPa and 978.59 MPa,respectively,and the elongations are 32.7%and 42.8%,respectively.The nano-indentation show that the average hardness of Fe3Al phase is above 4.6 GPa,the average hardness of austenite is from 4.0 GPa to 4.4 GPa,and the average hardness of ferrite is less than3.4GPa.The EBSD results show that the dislocation density is greatly reduced,and some grains are recrystallized.The texture is generally dominated by Goss texture and Cubic texture.