Effects Of Heat Treatment On Microstructure And Mechanical Property Of Q406D Steel Weld Joint

Low-alloy high-strength steels(HSLA)have been widely used in machinery,bridges,and vehicles due to their high strength,high toughness,and low cost.However,high-strength steels are difficult to weld,and many defects in welds and heat-affected zones have greatly limited the application of high-strength steels in welded structural parts.Therefore,welding joints need post heat treatment to improves the microstructure in some special welding structural parts,thereby improving the comprehensive performance of the welded joint.The traditional heat treatment method of low-alloy high strength steel is mainly quenching tempering or normalizing-tempering process to obtain martensite structure,bainite structure or acicular ferrite structure,thereby ensuring the steel has good toughness.The study focused on the influence of different heat treatment processes,direct quenching,quenching tempering and quenching distribution heat treatment,on the microstructure and mechanical properties of Q460 D steel welded joints.Tests show that the quenching distribution process can improve the comprehensive performance by changing the room temperature structure and increasing the strength of the material under the condition of less plasticity and hardness loss.The microstructure and mechanical properties of welded joints were analyzed by means of optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),and the test means to mechanical properties such as room temperature tensile and Vickers hardness.The microstructure and mechanical properties were analyzed to investigate the effects of initial austenitizing temperature,quenching temperature,carbon distribution temperature,and carbon split time on the microstructure and properties of the welded joints.The microstructure and properties of the Q&P process on the welded joints were investigated by comparison with the conventional process.With the increase of austenitizing temperature,the residual ferrite structure in the room temperature decreases,and the tiny martensite/austenite islands structure increases and the grain grows with increasing temperature.The tensile strength of the welded joint decreases and the elongation rate increases.The tensile fracture has obvious toughness characteristics.After the austenitizing of the two-phase region,the martensite content and residual austenite decreases with the increase of the quenching temperature,and the granular martensite /austenite islands structure increased.The tensile strength decreases as the initial quenching temperature increases,and the elongation rate tends to change.In the process of partitioning,with the partitioning time prolonged,the content of retained austenite decreases,the tensile strength gradually decreases,and the elongation rate increases.After partitioning for 10 s,the carbides increase and distribute on the ferrite grain boundaries and the matrix.Under different Q&P process parameters,the maximum tensile strength is 850 MPa,the minimum is 630 MPa,the maximum elongation is 26%,and the minimum is 13%.The effects of quenching temperature and distribution temperature on the tensile strength are all greater than the elongation,and the partitioning time has little effect on the tensile strength.After the Q&P process,the weld joints and heat-affected zone of the welded joints have improved inhomogeneity,and the tensile strength of the weld zone is significantly improved.Comparing the three heat treatment processes,the tensile strength of the welded joint after the best Q&P treatment was increased by 50%,the elongation was not reduced significantly,and the softening zone in the heat-affected zone was improved.This indicates that the welded joint can be treated with Q&P process better The overall performance.The quenching-tempering process can effectively increase the tensile strength,but the plasticity improvement is not obvious.The direct quenching process can obtain higher strength,but the elongation is only 9% and cannot improve the overall performance.The three means of heat treatment process tensile fractures are ductile fracture morphology,direct quenching of the welded joints have a brittle fracture tendency,it should not be applied in the actual production.The best process for Q&P treatment of welded joints is austenitizing at 840 °C for 15 min,quenching at 300 °C,and distributing at 380 °C for 300 s.