Study On Microstructure And Properties Of Friction Stir Welded Low-C Medium-Mn Steel Joints

Low C medium-Mn steel(C content<0.1 wt.%,Mn content 4～12 wt.%)is characterized by high strength,high elongation and excellent toughness due to its unique microstructure,including tempered martensite,reversion austenite and fine carbide.At present,the application of medium-Mn steel in the fields of construction steel,pressure vessel,bridge,shipbuilding and marine platform has become more and more widely concerned.For medium-Mn steel,the welding process is a very important part.For all kinds of welding methods commonly used in engineering,including submerged arc welding,metal active gas welding and shielded melting arc welding(SMAW),because of the high heat input,there are many problems in joints.The peak temperature of joints is so high,that the heat affected zone(HAZ)is easy to produce coarse austenite,and serious segregation of Mn element in medium-Mn greatly improve the hardenability of the material,so that cold cracking tendency is obvious during the process of welding.It is easy to formate brittle martensite and coarse widmanstatten in HAZ.And serious segregation of Mn happens in joints,seriously affect the strength and toughness of welded joints.Friction stir welding(FSW)is a new type of solid-state bonding technology.The welding heat input of FSW is low,so it can avoid overheating and coarsening of HAZ,reducing the hardening tendency of joint,and obtaining high strength and toughness welding joints.In this paper,the comparative study of SMAW and FSW for medium-Mn steel was carried out respectively.The influence of different welding methods and process parameters on micro structure evolution and mechanical properties of welded joints of medium-Mn steel and the effect of post weld heat treatment on FSW joints are analyzed.The specific results of this study are as follows:The welded joints of SMAW medium-Mn steel are divided into nugget zone(NZ),heat affected zone(HAZ),inter-critical heat affected zone(ICHAZ)and base material(BM).During the welding process,the NZ metal is completely melted and cooled rapidly after welding.A large number of coarse martensitic formed.The segregation of Mn resulted in the formation of a small amount of retained austenite between martensitic lath.Because of the influence of NZ,the peak temperature of HAZ is slightly lower than that of NZ,so there is large content of martensite in HAZ.With the increase of the distance from NZ,the effect of peak temperature gradually weakened,and the content of martensite decreased as the content of ferrite increased.The welded joints of FSW medium-Mn steel are divided into NZ,HAZ,ICHAZ and BM.In 400 rpm the welding heat input is low,NZ peak temperature is slightly higher than that of Ac3,NZ and HAZ is composed of martensite and bainite dual phases structure;in the case of 600 rpm which has medium heat input,plastic deformation strength and martensite grain is significant refine in NZ and it is equal to BM.There isn’t obviously difference of peak temperature in NZ,so there isn’t obviously difference in the organization and the grain size of two parameters HAZ.In high heat input of 800 rpm,the peak temperature of NZ increased significantly,there is coarse lath martensite in NZ and HAZ;there is martensite and ferrite in ICHAZ of different heat input.With the increasing distance to NZ,the content of martensite decreased gradually.After heat treatment,the content of martensite in HAZ and ICHAZ was reduced,and the grain size did not change obviously.The impact toughness of 400,600 and 800 rpm joints are respectively 200,204 and 75J.Because of the plastic deformation of 600 rpm joint is greater than 400 rpm,it has more refined organization,and there is no difference between the peak temperature,the impact toughness of the 600 rpm is higher.The tensile strength is 930,930 and 940 MPa respectively,and the elongation is 10.9,10.8 and 10.9%respectively.During the welding process,the NZ microstructure is martensite,and the tensile strength and elongation rate are not significantly different.The impact toughness of 600 rpm joints is 161,132 and 145 J at 200,250 and 300℃,respectively.Because of the temper brittleness at this temperature,the impact toughness of joints after heat treatment is not increased.The tensile strength is 930,870 and 850 MPa,the elongation is 10.8,8.9 and 6.5%respectively.After tempering,the precipitation of C decreases the content of martensite.Therefore,the tensile strength decreases with the increase of tempering temperature,and the segregation of C at martensite grain boundary reduces the elongation of welded joints.