Study On Welding Process,Mechanical Properties And Mechanism Of Marine High-strength Steel By Laser-mig Hybrid Welding

The high strength steels are the key structural material in the marine engineering equipment.This steel has been widely applied in many manufacturing fields such as sea drilling platforms,submarine pipelines,and offshore wind installation vessels.The arc welding techniques are generally employed to join the high strength steels in practice.Though the conventional arc welding technique has many advantages,for instance,low production cost and proven welding technology,it also presents many drawbacks like high heat inputs and large welding-induced distortions.Those disadvantages limit its application on the joining of thick section high strength steels.As a newly developing welding technique,laser-arc hybrid welding(HLAW)can integrate arc welding and high-energy laser beam wending into one process and take advantage of both heat sources.This technique provides an effective solution method to the problems associated with the traditional welding techniques.The HLAW has significant superiorities in welding moderate and heavy plates.The HLAW has been supposed to be one of the important welding techniques for manufacturing marine high-strength steel structures.In this research,the HLAW was performed to join the 15 mm thick NV E690 steel plates.The influence of the welding parameters including filling pass,laser power,arc power,and welding speed was investigated to evaluate their effects on the shape and quality of the weld bead.Favorable welding process window was determined in terms of the double-pass HLAW.For example,the parameters for the first pass were laser power with 4 k W,arc current with 250 A,arc voltage with 28.5 V,and welding speed with 0.6 m/min.The parameters for the second pass were laser power with 1.5 k W,arc current with 300 A,arc voltage with 31 V,and welding speed with 0.6 m/min.The weld bead obtained by double-pass HLAW possessed high performance.Macroscopic welding defects including undercut,surface micro-cracks and lack of penetration,were not found in the weld bead.The evolution characteristics of the microstructure in the typical regions and subzones were analyzed.The result of hardness test depicts that the average hardness(368.46 HV)of fusion zone(FZ)is lower than that(456.69 HV)of heat affected zone(HAZ),but higher than that(280 HV)of base material(BM).Besides,Tensile tests,Charpy impact tests,and bending tests were carried out to further assess the mechanical properties of the as-welded specimens.Firstly,the standard tensile-tested specimen fractured at the BM with an ultimate tensile strength of 825.60 MPa,suggesting that the weld bead has higher strength than that of BM.The tensile strength of the non-standard tensile tested specimen was over 1100 MPa.Secondly,in the low temperature Charpy impact test,the average absorb energy was 34.67 J which is higher than the industrial standard(27 J).The fracture mode of the tested specimen was quasi-cleavage fracture.Thirdly,the bending strength of the weld bead reached approximate 1769.41 MPa.The strength of the weld bead was higher than that of BM(1668.94 MPa).The welded specimen for bending test fractured at the interface of HAZ and BM.However,no cracks were observed on the bending surface of BM.The ultrasonic peening treatment(UPT)technique was performed on the surface of aswelded specimen.As a mechanical process of post-weld treatment,the UIT was used to improve the mechanical properties and remove possible disadvantages of the weld bead.The microstructure morphology of the weld bead took place significant change.The severe plastic deformation layer with nano-crystalline structure was formed.Additionally,the hardness of the weld bead increased to 432.50 HV.The beneficial compressive stress was brought in the surface of the weld bead by UPT.The magnitude of longitudinal compressive stress was 426 MPa.The compressive stress in the surface of weld bead can enhance the ability of stress corrosion resistance.In this present research,the HLAW was performed on the high strength steel which had a minimum yield strength of 690 MPa.The welding process window,microstructure and mechanical properties were studied in detail.The welding property of the as-received NV E690 steel plates was evaluated.The research results provide the experimental data and theoretical basis for the application of HLAW technique on marine engineering equipment manufacturing field.