Study On Preheat-free Welding Process Of 390MPa Grade Steel For Construction

Welding is the main way to connect steel for building structures.preheat-free welding is an effective measure to save energy,improve production effciency and ensure the performance of welded joints.In this paper,the thermal simulation method and practical welding application were used to research transition laws of structure and properties of 390MPa grade building steels with different carbon content(0.15 wt.%and 0.07 wt.%)at different welding simulation processes.The influences of preheating temperature and inter-layer tenperature on the hardening tendency,softening and impact toughness of Heat Affected Zone(HAZ)are discussed.Final determine the optimum-welding processes and preheating temperature for the two experimental plates of steel.The main results are as follows:(1)The SH-CCT(Simulated HAZ Continuous Cooling Transformation)curves of the two plates of steel were determined by the continuous cooling transformation experiment in the heat affected zone.The results show that the 0.07%C steel reduces the tendency to form martensite structure,and mainly forms bainite and ferrite structure under different t8/5,reduces the tendency of structure hardening.The starting phase transition temperature of the fine grain heat affected zone(FGHAZ)is higher than that of the coarse grain heat affect zone(CGHAZ),which tends to be the high-temperature transition,and the grain size is smaller than that of the coarse grain zone significantly.The hardness decreases with the extension of t8/5 time of both plates of steel regardless CGHAZ or FGHAZ.The hardness of 0.15%C steel is more than 350HV when t8/5 is less than the 20s(the corresponding heat input is 16kJ/cm),which has obvious microstructure hardening phenomenon,and the maximum hardness does not exceed 350HV of 0.07%C steel.(2)The experiment of different heat input on the microstructure and properties of HAZ was carried out by the method of thermal simulation experiment.The 0.15%C steel is easy to form hardened martensite when the heat input is less than 20kJ/cm.The 0.07%C steel is bainite when the heat input below 40kJ/cm.When the heat input is larger than 40kJ/cm,the pro-eutectoid ferrite structure surrounding the grains appears in both plates of steel,and the impact strength of the coarse grain zone of the 0.07%C steel is better than that of the 0.15%C steel.The micro structures of the two FGHAZ are relatively smaller,mainly bainite and ferrite,and the impact properties are higher than 150J.The impact properties of 0.07%C steel are relatively good.(3)The theoretical calculation of the preheating temperature and the experiment about the influence of different preheating temperatures on the microstructure properties of HAZ demonstrate that preheating can improve the tendency of hardened martensite in 0.15%C steel and improve the impact performance of 0.15%C steel FGHAZ obviously.Preheating temperatures can increase the ratio of the large angle grain boundary and inprove the impact performance,but there is no obvious improvement effect on the CGHAZ with large grain size.The impact performance of 0.07%C steel at different preheating temperatures is greater than 47J,which is significantly better than 0.15%C steel(4)A secondary thermal cycle experiment was carried out using a thermal simulation machine to study the effects of different secondary peak tenperatures on the micro structure of the CGHAZ.The results show that when the secondary peak temperature is 1350℃,the microstructure is same as the CGHAZ under the first thermal cycle,which is the weak zone of the welded joint;When the secondary peak temperature is 700℃,a pearlite structure distributed along the grain boundary is formed,which deteriorates the welding heat affected zone performance.(5)Through simulating the evolution law of the second thermal cycle with different primary preheating temperature s and inter-layer temperature,it is found that the primary preheating temperature can improve the microstructure of the CCGHAZ and relieve the tendency of hardening.The enrichment of carbon in ICCGHAZ increases with the increasing inter-lay er-temp erature,forming large pearlite,carbide bulk or agglomerate structure which is distributed on the grain boundary or inside the grain.The carbon enrichment phenomenon of 0.07%C steel is not obvious,which reduces the formation of hard phase structure.