| High strength weathering steel has been widely used in the field of railway locomotive manufacture because of their excellent mechanical properties and corrosion resistance.Compared with ordinary steel,the influence of it on the stress concentration sensitivity,mismatching of toughness and strength and the deterioration of heat affected zone is particularly important because of the strength increased.In this paper,the problem of poor toughness of welded joints was researched.By using the method of welding simulation,the microstructure transition curve under the influence of different thermal cycles on the microstructure and mechanical properties of the HAZ was studied.The effect of different heat input on the microstructure and mechanical properties of welded joints was analyzed by means of gas metal arc welding(GMAW)method,the relationship between heat input and microstructure and properties was analyzed.It provides the theoretical basis and data support for the selection of welding method and process formulation of experimental steels.When the cooling rate is between the 0.5 ℃/s-2 ℃/s,the microstructure of CGHAZ is bainite.When the cooling rate is between 2 ℃/s-10 ℃/s,the microstructure is bainite and martensite.When the cooling rate is greater than 10 ℃/s,the microstructure is martensite.Through the change of the microstructure in different cooling rates is known that advantageous to the formation of the non diffusion type of phase change tissue when the cooling speed is greater than 10 ℃/s.With the increase of cooling rate,the hardness of CGHAZ gradually increases first and then tending to stable when the cooling rate after 10 ℃/s.When the cooling rate of high strength weathering steel is greater than 10 ℃/s,the CGHAZ is easily hardened and the cooling rate should be avoided excessive cooling rate after welding.If the cooling time is constant,when the peak temperature is increased from 800 ℃ to 1100 ℃,the microstructure of HAZ change from ferrite and pearlite to the mixture of ferrite and bainite.When the peak temperature increases to 1350 ℃,the microstructure is made of martensite,and the grains grow significantly with the increasing of peak temperature.When the peak temperature is 1350 ℃ and the cooling time is less than 30 s,the microstructure of CGHAZ is martensite;While the cooling time is more than 30 s,the microstructure of CGHAZ is martensite and bainite.With the increasing of the peak temperature,the impact energy of HAZ has the tendency to increase first and then decrease,and the highest impact energy 266 J is obtained at 950 ℃,The best toughness area appears in the FGHAZ,and its impact toughness decreases obviously in the CGHAZ.The hardness of the HAZ increases first and then decreases,the highest hardness value 284 HV is obtained at 1100 ℃.With the cooling time increase,the impact energy is improved when the cooling time is more than 50s and the hardness of HAZ has the tendency to increase first and then decrease.So the cooling time should be controlled more than 50 s in the actual welding process.Using the gas metal arc welding method,welding of experimental steel was conducted under the conditions of 10 kJ/cm,13 kJ/cm and 15 kJ/cm.Under the three heat input conditions,the micro structure of fusion zone(FZ)is composed of ferrite and small amount of bainite.With the increase of heat input,acicular ferrite content increased first and then decreased and the proeutectoid ferrite grain size of FZ is gradually coarsening.The microstructure of the CGHAZ is martensite.The microstructure in FGHAZ and incomplete recrystallization heat affected zone(ICHAZ)is ferrite and a small amount of pearlite.However,the grain size is small and homogeneous in FGHAZ,and the grain size is different in ICHAZ.There is no softening zone in the welded joint under the three heat input condition.The highest hardness of HAZ is 300 HV,and the highest hardness of weld area is 260 HV.The influence of the three heat input on the tensile properties of the joints is not large,and the tensile fracture appears in the base material,showing good tensile properties.The impact energy of FZ under three heats input conditions at-20 ℃ is increased first and then decreased with the increase of heat input,the optimum impact energy is 103 J of FZ was obtained with about 13 kJ/cm heat input.The impact energy of FZ at-40 ℃ is decreases gradually with the increase of heat input,lower amplitude of 10 kJ/cm and 13 kJ/cm.The fundamental reason for the difference of impact toughness in weld area is that the increase of heat input causes the acicular ferrite content to increase first and then decrease.The impact energy of HAZ under three heat input conditions at-20 ℃ and-40 ℃ are increased first and then decreased with the increase of heat input,the optimum impact energy was obtained with about 13 kJ/cm heat input.When the heat input is about 13 kJ/cm,the gas metal arc welding method is used to weld high strength weathering steel,the welding joint with excellent mechanical properties can be obtained. |
