Study On The Mechanical Properties And Welding Thermal Effect Of Q690CFD High Strength Steel

High strength steel is widely used in structural engineering because of its advantages of light weight,high strength,convenient for construction and environmental friendly.The use of high strength steel can effectively reduce the size of member’s cross section,reduce the weight of structure and save the consumption of materials.At present,strength of high-strength steel(HSS)normally enhanced by adding alloying elements in the manufacturing process and the mechanical properties are further improved by quenched tempered process(QT)or thermal-mechanical control process(TMCP).The mechanical properties of QT HSS and TMCP HSS are differ under different working conditions due to their different manufacturing processes and chemical composition,especially for welded structures.The current research on the properties of HSS materials is mainly focused on QT HSS,and lack of systematic research on TMCP HSS.Above reasons restrict the application of TMCP high strength steel in engineering.Hence,in order to solve this problem,the mechanical properties under elevated temperatures,post-fire mechanical properties,high-temperature creep properties and the thermal effect of butt welding of Q690 CFD HSS are experimentally studied in this paper.The test results analyzed :(1)the effect of welding heat input,cooling rate and temperature on mechanical properties of Q690 CFD under elevated temperatures and post-fire mechanical properties;(2)the effect of temperature and stress level on high temperature creep properties of Q690 CFD steel;(3)the effect of welding heat input,strength ratio of the weld material,specimen constraint conditions and heat treatment(including preheating and post-weld heat treatment)on residual stress,microstructure and hardness of butt welded joints.The main conclusions are as follows :(1)GB/T51249-2017 prediction of elastic modulus of Q690 CFD at high temperature is the most accurate,while EC3 and AISC360-16 provide the lower limit for prediction of elastic modulus.the Q690 CFD steel produced by TMCP process has better high-temperature mechanical properties than other high-strength steels produced by other QT processes,such as S690 steel and Q690 steel;(2)welding will significantly reduce Q690 CFD mechanical properties under elevated temperatures and post-fire mechanical properties.it is recommended that the welding heat input be limited to 2 k J/mm when welding the Q690CFD;(3)when the temperature exceeds 800℃,water-cooling will increase the yield strength and ultimate strength under post-fire condition.Moreover,the elastic modulus of Q690 CFD under post-fire condition is not sensitive to the change of welding heat input;(4)the basic law of high temperature creep of Q690 CFD steel is obtained by high temperature creep test.the proposed creep model is in good agreement with the test data,which can provide model data for Q690 CFD numerical simulation of post-weld heat treatment considering creep effect;(5)the increase of welding heat input will prolong the cooling time of the welding piece,resulting in larger temperature gradient inside the welding piece,which leads to the increase of welding residual stress;(6)The peak temperature during welding determines the microstructure of the welded joints,and when the peak temperature is similar,it will produce similar microstructure.