Development Of Computational Approach To Simulate Microstructure And Hardness Of High Strength Low Alloy Steel Welded Joint Used In Vehicle

Automobile has become the main source of energy consumption and pollutant emission in the world,and lightweight is one of the most effective measures to cut energy consumption and emission.Low alloy high strength steel is widely used in automobile manufacturing because of its low carbon content,less other alloy elements,lighter weight and longer life than ordinary carbon steel under the same strength,good weldability,lower cold brittleness tendency and better cold and hot working formability.Fusion welding is the most commonly used connection method of low alloy high strength steel.It is easy to produce the chemical composition and microstructure inhomogeneity in the welding process.It is not difficult to develop brittle hard martensite structure in the HAZ.Under the influence of multiple factors,it may also lead to defects such as cold crack in welding.Therefore,it is extremely important to study the microstructure evolution of the welded joint of LAHS steel for the quality and safety evaluation of the welded structure.In order to observe the microstructure and test the hardness of the welded joint,the conventional test method will destroy the weldment permanently and irreversibly,but the numerical calculation method to simulate and characterize the microstructure of the joint does not need to cause any damage to the structure,so the finite element method to predict the microstructure and hardness of the welded joint is favored by the academia and engineering.In this study,the conventional finite element software ABAQUS is used as the calculation platform,and FORTRAN language is used to program.The thermal metallurgical sequential coupling finite element method is considering solid phase transformation is developed to simulate the microstructure and hardness distribution of LAHS steel welded joints.In this paper,firstly,the TIG remitting welded joint of Q345 steel is taken as the research object,and the Microstructure and Hardness Calculation Model Based on a single SH-CCT₁₃₀₀ diagram(the CCT diagram with a peak temperature of 1300)is established.The temperature field distribution,structure distribution and hardness distribution of the welded joint with a heat input condition of 1.4k J/mm are simulated by the developed finite element calculation method.The results show that the volume fraction of martensite in welding seam(WM),coarse-grained of HAZ(CGHAZ)and fine-grained HAZ(FGHAZ)is higher,the highest is 76%,the volume fraction of ferrite in HAZ is lower than 5%,and the hardness of WM,CGHAZ and FGHAZ is higher than350HV.At the same time,the temperature thermal cycle of the welded joint was calculated by means of experiment,the microstructure of the joint was observed,and the hardness distribution of the joint was measured.The microstructure of WM and CGHAZ is mainly martensite,the microstructure of FGHAZ and part transformation heat affected zone(ICHAZ)is mainly ferrite,the hardness of WM and CGHAZ is the highest,the average hardness is 380HV,the hardness of FGHAZ and ICHAZ is relatively low.Comparing the calculation results with the test results,it can be seen that the calculation model based on single SH-CCT₁₃₀₀ diagram can effectively predict the microstructure and hardness of WM and CGHAZ,but the calculation results of FGHAZ and ICHAZ are quite different from the experimental results.In order to improve the calculation accuracy,two calculation models of microstructure and hardness based on single SH-CCT₉₀₀ diagram and single SH-CCT₁₁₀₀ diagram and one calculation model of microstructure and hardness based on SH-CCT₉₀₀ diagram,SH-CCT₁₁₀₀ diagram and SH-CCT₁₃₀₀ diagram are set up in this paper.The three models are used to simulate the microstructure distribution and hardness distribution of the joint with the heat input of 1.4 k J/mm.Based on the single SH-CCT₉₀₀ diagram calculation model,the microstructure and hardness at the junction of FGHAZ and ICHAZ are in good agreement with the experimental results,but the deviation of other areas is large;based on the single SH-CCT₁₁₀₀ diagram calculation model,the microstructure and hardness at the junction of FGHAZ and CGHAZ are in good agreement with the experimental results,but the deviation of other areas is large;based on three SH-CCT diagrams calculation model,the microstructure and hardness of each region in WM and HAZ are in good agreement with the experimental results.The comparison of numerical simulation results and experimental results shows that the calculation model based on three SH-CCT diagrams has higher accuracy than that based on a single SH-CCT diagram.Then,based on the three SH-CCT diagrams,this paper simulates the joint structure distribution and hardness distribution under the condition of heat input of 0.7k J/mm(small heat input).The microstructure and hardness distribution of WM and HAZ obtained by numerical simulation are also in good agreement with the experimental results,which shows that the calculation model based on the three SH-CCT diagrams is not only suitable for the welding conditions with large heat input,but also suitable for the welding conditions with small heat input.As mentioned above further proves the validity and appropriateness of the method developed in this paper to simulate the microstructure and hardness of the welded joint of low alloy high strength steel.Finally,based on the calculation models of three SH-CCT diagrams,a model for the calculation of microstructure and hardness of multi-layer and multi pass welded joint is developed.The temperature field,microstructure distribution and hardness distribution of the two-layer and two passes welded joint of Q345 steel are calculated by this method.The results show that the CGHAZ of the first weld pass is affected by the thermal cycle of the second weld pass,and the microstructure and hardness of the SCCGHAZ and ICCGHAZ vary greatly during the welding process.In this paper,the prediction of microstructure and hardness of low alloy high strength steel welded joint is studied systematically based on the finite element method.The research on the calculation model of microstructure and hardness based on three SH-CCT diagrams has certain scientific significance.The research on the calculation model of microstructure and hardness of multi-layer and multi-channel welded joint promoted on the basis of the calculation model of three SH-CCT diagrams has great importance Engineering application value.