Research On Crack And Mechanical Properties Of Q345/Q690 Welded Joint Of Offshore Platform's Leg

The requirements of the steel strength for the legs of offshore platform are getting higher and higher due to abominable working environment.However,cold cracks,joint toughness reduction and brittle fracture will occur when welding the legs of offshore platforms.Therefore,the welding of legs have become one of the technical difficulties in the construction of offshore platform's legs.In this paper,the welded joint of high strength steel Q345/Q690 for the legs of offshore platform is taken as the research object.Combined with numerical simulation analysis,welding test and metallographic analysis,the crack and mechanical properties of Q345/Q690 welded joint were studied.By analyzing the influence of different welding materials and welding parameters on the strength,toughness and microstructure of the welded joint,the reasonable welding parameters were determined.(1)The elastic-plastic finite element method was used to analyze the crack resistance of the welds in the different high strength steels.The ANSYS software was used to simulate the welding joint of the Q345/Q690 inclined Y-groove,then the formation mechanism of welding crack was studied,the influence of the preheating temperature and the heat input on the welding residual stress were analyzed.The results showed that the welding residual stress can be effectively reduced by preheating before welding;the greater the heat input,the greater the residual stress existed at the welding joint;the maximum stress of the welded joint was not more than 30% of the Q690's yield strength and the cold crack rate was low when the welding heat input was 12~20kJ/cm.(2)Three kinds of strength grade welding wire JQ.MG50-3,JQ.TH550-NQ-II,JQ.MG70-G-2 were used to test the weld crack of Q345/Q690 inclined Y-groove,the mechanism of welding crack formation was studied,and the effect of preheating temperature,welding heat input and welding material on the crack rate of joint was studied.The test results showed that the crack rate of welding section decreased with the increase of welding wire strength grade;the crack rate of welding section increased with the increase of welding heat input when the welding heat input range was 12~20kJ/cm;the crack rate of welding section would be significantly reduced by preheating;the crack rate of welding joint with welding wire JQ.MG70-G-2 was the lowest;through preheating,the crack rate of the welding section of the three kinds of welding wire can meet the production requirements;through the metallographic analysis,when the microstructure was fine in the welded joint and the dislocation density was large,the crack was not easy to expand,and vice-versa.(3)The mechanical properties of welded joints of three kinds of strength grade wires JQ.MG50-3,JQ.TH550-NQ-II and JQ.MG70-G-2 were tested.The influence of welding heat input and welding material on Q345/Q690 welded joint were studied.The test results showed that the fracture locations of tensile specimens were located on the Q345 side and far away from the welding seam;the elongation of three kinds of welding joints increased with the increase of welding wire strength,and decreased with the increase of welding heat input;the impact energy of the three kinds of welding joints decreased with the increase of welding heat input;and under the same conditions,the best impact toughness can be obtained by using JQ.MG70-G-2 welded joint.(4)The Q345/Q690 welded joints were observed by laser scanning confocal microscope and scanning electron microscope(SEM).The effect of microstructure on the strength and toughness of Q345/Q690 welded joints was studied.The results showed that with the increase of wire strength grade,the content of acicular ferrite in weld seam increased and the content of first eutectoid ferrite decreased,which was beneficial to improve the strength and toughness of welding joint.With the use of JQ.MG70-G-2 wire,the ferrite content decreased with the increase of welding heat input,the increased content of bainite and martensite and coarsened austenitein the fusion zone and the heat affected zone would reduce the strength and toughness of the welded joint.It was found that when the preheating temperature was 150? and the welding heat input was 12.4kJ/cm,and the JQ.MG70-G-2 welding wire was used to weld the Q345/Q690 material,the crack rate of the welded joint was the lowest and the mechanica properties were the best.