Study On Microstructure Transformation And Mechanical Behavior Of Welded Joints Of Q345C Steel

Nowadays,the typical material of train bogie is imported high-strength weathering steel material S355J2W(H),SMA490BW,and domestic Q345 series steel.Due to the good fatigue property and its superior low temperature impact property of imported steel,China's high-speed train bogie is more dependent on imported steel.In the field of rail transit,the domestic Q345 series steel was applied more often.But in the city where the service environment is more severe,imported steel was preferred.From this perspective,the localization of the bogie steel didn't spread widely.In the process of promoting the localization of bogie steel,it is very important to study the weldability of domestic steel.By studying the microstructure and mechanical behavior of welded joints via certain methods,it can set up the basic database for the microstructure and mechanical performance of the material itself.What's more,by matching different welding materials and selecting the appropriate welding process parameters,the goal of improving the performance of welded joints can be achieved.This study can facilitate the application and promotion of domestic steel.In this paper,the continuous cooling transformation test on Q345C steel and the deposited metal welded by ER50Q and ER80S-G was carried out,in which the changes of microstructure and hardness of the materials at different cooling rates were analyzed.Through the analysis of the microstructure and mechanical properties of Q345C welding heat affected zone obtained via thermal simulation and the welded joints,the conclusions were obtained as follows:With the decrease of temperature,the SHCCT curve of Q345C could be divided into 4 zones-ferrite,pearlite,bainite and martensite.Among the 4 zones the range of bainite transformation was wider.In the condition of cooling rate of 0.2?/s?100?/s,the upper bainite,granular bainite and bamboo leaf-shaped lower bainite could be observed in the microstructure.By using the color metallographic technique,the bulk polygon ferrite structure was different in colors caused by different orientations and under the optical microscope,the retained austenite between the bainitic ferrite lath could be observed.In addition,when distinguishing bainite and martensite,the bamboo leaf-shaped lower bainite and the low carbon martensite with regular arrangement at high cooling rates could be clearly observed via color metallographic technique.So the goal of identifying and quantitative analyzing the microstructure could be achieved.The SHCCT curve was accurate and was suitable for new welding methods such as laser-MAG hybrid welding process as well.The two kinds of SWCCT curves of deposited metal could be divided into three zones.They were ferrite,pearlite and bainite.The transformation range of bainite was wider,which contained acicular ferrite,granular bainite,lath bainite and carbon free bainite.The content of acicular ferrite in the ER80S-G deposited metal was higher than that of the ER50Q deposited metal.The 1st and 2nd weld seams of the ER50Q and the ER80S-G weld in welded joints were composed of bulk ferrite,acicular ferrite and pearlite.The 3rd and the 4th weld seams were composed of lath-shaped pro-eutectoid ferrite along grain boundaries,acicular ferrite,bainite and a small amount of pearlite.In ER80S-G weld seam,the acicular ferrite was more and the ferrite was less than ER50Qweld seam.The hardness,tensile strength and yielding strength of the ER80S-G weld seam was higher than those of the ER50Q weld,and those of both ER80S-G and ER50Q weld seams were higher than base material.The FATT50 of ER80S-G and ER50Q weld seams were-5.6? and-1.1?,respectively.The ductile-brittle transition temperature of ER80S-G weld seam was lower,so ER80S-G welding wire can be selected to improve the strength and toughness of weld seam.Using Vwer empirical formula with better accuracy could achieve the conversion between the welding process parameters and t8/5.The 4th welding process parameters were used to calculate the t8/5 of the 4th weld seam of ER50Q and ER80S-G weld seams.Based on the structure observation and hardness comparison of the 4th weld seam of the ER50Q and the ER80S-G weld,the hardness was the same as SWCCT curve in the same t8/5.It was further verified that the measured SWCCT curve was accurate.