Research On Narrow Gap Laser Filled Wire Welding Process And Joint Microstructure And Properties Of D36 Steel For Offshore Platform

As a low-alloy high-strength steel,D36 steel has good comprehensive mechanical properties and good corrosion resistance,so it is widely used in offshore platform manufacturing.Narrow-gap laser wire-filled welding has small welding deformation,low welding material consumption,low assembly accuracy,high welding efficiency,and can realize low-power welding of large thick plates.As a new type of welding method,it is widely used in thick plate connection.In this paper,the 40 mm thick D36 low-alloy high-strength steel is welded by narrow-gap laser wire-filled welding.The entire weld is completed by 1layer laser self-fusion bottom welding and 7layers laser filler wire welding.In this paper,the microstructure of each area of the joint is analyzed,and the mechanical properties of the joint are analyzed by room temperature tensile,microhardness,bending and low temperature impact.First,the single factor test method is used to study the influence of defocus,laser power,wire feed speed and welding speed on weld penetration and penetration width.The research results show that reducing the welding speed,or increasing the laser power,the weld penetration depth increases significantly;increasing the defocus amount,or increasing the wire feed speed,the weld penetration width increases significantly.Then,through orthogonal experiments,the welding process parameters are determined as follows: laser power is 4600 W,welding speed is 5 mm/s,wire feeding speed is 3.2 m/min,and defocus is +20 mm,and high-quality welded joints can be obtained.The single factor test method is used to study the influence of laser power,welding speed,wire feed speed and defocusing amount on weld penetration.Studies have shown that reducing the welding speed or increasing the laser power will increase the weld penetration significantly;increasing the defocus,reducing the welding speed or increasing the wire feed speed will increase the weld penetration significantly.Orthogonal experiments are used to find the best process parameters for weld formation.The experiments show that welding speed and defocus have a greater influence on the aspect ratio of welds;laser power and welding speed are the most significant influences on the tensile strength of welds.The test results show that due to the fast cooling rate of laser welding,a coarse equiaxed crystal structure appears in the weld,showing typical metal solidification structure characteristics,and its main structure is proeutectoid ferrite and pearlite Obviously coarse-grained and fine-grained regions appear in the heat-affected zone.The heat input of welding is large,and the heat loss of the thick plate is fast,and the coarse-grained area is close to the center of the weld,and the temperature rises and falls sharply,which causes the austenite homogenization time to be too short,and the austenite does not have time to undergo diffusive phase transformation.Shear-type phase transformation occurs directly,and lath martensite penetrating the grains is generated;The fine-grained area undergoes heating and cooling twice to recrystallize phase change,so that the crystal grains are significantly refined.After cooling,fine-grained pearlite and ferrite are obtained.The overlapping area of the weld is subjected to a secondary thermal cycle,and is melted again by the energy of the next weld.At this time,the remelted part is cooled by the base material of the two side walls and the previous weld.Compared with the center of the molten pool,the degree of supercooling is obvious Increase,resulting in disordered orientation of the interlayer area of the weld and the resulting crystal grains are smaller.The tensile,bending and low-temperature impact test results at room temperature all meet the requirements of the standard.The tensile fracture locations are all in the base material.After measurement,the average tensile strength of the sample is 534 MPa,and the average elongation after fracture is 15.6%,indicating that the strength of the weld joint is higher than that of the base material.The average impact energy of the low-temperature impact of the weld is 59 J,which meets the national standard.Due to the large amount of martensite structure produced in the heat-affected zone,the heat-affected hardness is significantly increased,reaching a maximum of 470.3 HV.Through the comparison of polarization curves,it is found that the corrosion resistance of the welded joint is better than that of the base metal.The main reason is that the welding wire contains corrosion-resistant elements such as Cu and Cr.The addition of these elements improves the corrosion resistance of the welded joint.