Research On The Weldability And Welding Procedures Of JG785DB Steel

As an important steel material, ultra low carbon bainitic steel is characterized by fine grain, super-metullrgical purification and homogenization, high strength and toughness, and it is widely many industries such as mine-machines, pressure container, pipeline of boiler, vehicle and shipping, bridge and building. JG785DB steel is 800MPa grade ultra low carbon bainitic steel developed by JINAN Iron & Steel Group Corporation. It has a high strength level and is dificult to weld because its alloy system is so complex. So the problem that needs to be solved instantly is the weldability of the steel, which is so significant for the engineering application of the steel. Meanwhile it is significant for enriching the theoretics of welding of bainitic steel.This paper generally introduces the new ideology of the design of the component, alloying principle and mechanics of strengthening and toughening, analyses the weldability of the steel from different aspects, and evaluates the cold crack susceptivity of the steel using Y -slit type cracking test and HAZ maximum hardness evaluation test. Then, CO₂ gas metal arc welding is used to weld the steel under different heat inputs; mechanical test (tensile test, bending test and impact test) are carried out, microstructures of weld metal, HAZ and fractography are investigated by means of light microscopy and SEM; the effect of heat input on microstructures and mechanical property of welding joint, the effect of processing factors and microstructures on strength and toughness of welding joint are studied and analyzed systemly. Finally the mechanical property of CO₂ gas metal arc welding and CO₂+Ar mixed gas metal arc welding joint obtained using MKGHS80 wire are tested and evaluated.It is shown that the steel had a low hardenability and cold crack susceptivity with suitable welding parameters. Using MK·GHS80 wire, the welded joint of the steel obtains high tensile strength that is over 90% of the base metal tensile strength under middle and small heat inputs and with the heat input increases, the tensile strength of the welded joint decreases gradually. At the same times, the impact toughness of weld metal is higher also, initially increases and then decreases, but the impact toughness of HAZ is obvious higher than that of weld metal and decreases gradually with the heat input increases. The microstructures types of weld metal are proeutectoid ferrite,FSP,acicular ferrite,granular bainite under different heat input, the formation and numerical measure of microstructures are varied partly with heat input; The size of prior austenite grain for CGHAZ and the width of the lath-like bainitic ferrite are increase with the increase of heat input, meanwhile the amount of lath bainite decreases gradually, but the amount of granular bainite increases. So the impact toughness of CGHAZ is reduced.Lower energy input is suitable for the steel, the CO₂ gas metal arc welding joint obtained using MKGHS80 wire as the filler metal and 17 KJ/cm or so as the heat input energy has excellent mechanical properties. While the steel is welded by using 80%Ar+20%CO₂ mixed gas metal arc welding, MK·GHS80 wire and 21 KJ/cm or so as the heat input energy, formation of weld surface is modified obviously, spatter decreases greatly and the weld joint has good comprehensive mechanical properties.