Research On Cold Crack Criterion Of 10Ni5CrMoV Steel Welding Structure With Large Restraint And Development Of Austenitic Welding Wire

Problem of 10Ni5CrMoV steel cold cracking is very serious when welding thick tube-sheet structure with high restraint.Engineering examples show that cold cracking is still not avioded under the strict preheating-postheating process conditions even if the ultra-low hydrogen welding consumable is used.Many existing criteria represented by critical fracture stress for evaluating cold cracks can not accurately determine the crack susceptibility of 10Ni5CrMoV steel welded structures.It is necessary to do basic research on the law of cold cracks in 10Ni5CrMoV steel restrained structures.It is hoped that a reasonable criterion for large restrained structures will be proposed.And based on this criterion,effective measures are proposed through welding consumable and process.Therefore,this paper takes10Ni5CrMoV steel tube-sheet structure with large restraint as the research object,deeply studies the law of cold crack of welded joint,develops austenitic welding wire with high strength,high toughness and excellent weldability,and restrains the cold crack of 10Ni5CrMoV steel tube-sheet structure with large restraint comprehensively and effectively.Firstly,the influencing factors and generation law of cold crack in 10Ni5CrMoV steel welded joints were studied by means of theoretical analysis and experiment.In view of the fact that the existing criteria for cold cracks are not suitable for the welded joints of 10Ni5CrMoV steel,the three elements of forming cold cracks in the welded joints of 10Ni5CrMoV steel are analyzed in depth.The results of three-dimensional atomic probe test show that the hydrogen content in weld metal is much higher than that in HAZ.The diffusion hydrogen content in weld metal under different working conditions is measured by chromatographic method.The CCT diagram of weld metal drawn by water-cooled copper mold method shows that under certain conditions the weld microstructure is changed little,so the weld microstructure is not the main reason for the formation of cold cracks.The restraint degree of typical welded structure of 10Ni5CrMoV steel and the maximum strain of weld metal are calculated quantitatively by numerical simulation method.The results show that the restraint degree of tube-sheet structure is the largest,and the strain of weld metal increases with the increase of restraint degree.Therefore,the formation of cold cracks in10Ni5CrMoV steel tube sheet structure needs to focus on the hydrogen in weld metal and the restraint of welded joints.The mechanical properties of weld metal with different hydrogen content were obtained by hydrogen-charged tensile test with cathodic electrolytic pre-charging method.The results show that hydrogen content has little effect on the strength index of weld metal,but has great influence on the plastic index represented by elongation.Therefore,it is more reasonable to use the plastic index of different hydrogen content as the criterion of cold crack.The relationship between elongation and hydrogen content in electrolytic hydrogen-charged tensile test was fitted by Gaussian fitting.The equation of[H]content and elongation in weld metal was obtained.The critical strain was taken as the main basis for forming cold crack in weld metal of 10Ni5CrMoV steel.If the maximum strain of the welded joint is greater than the critical strain,cold crack will be occur.Considering that the restraint degree of structure is related to the strain of weld metal,the crack resistance tests of unrestrained butting,window restraint butting,Y-groove restraint butting and tube sheet butting are carried out to verify the feasibility of the criterion.Then,according to the above research,the austenitic welding consumable with large elongation must be used in 10Ni5CrMoV steel tube plate structure.For this purpose,high Cr-Ni welding wire with high strength,high toughness and excellent weldability has been developed.The chemical composition of welding wire is optimized and determined by theoretical analysis and experiments.The weldability and corrosion resistance of the welding wire were tested.Tensile strength,elongation and impact toughness of the weld metal were above 740 MPa,30%and 107J respectively.The results show that the weld metal meets the performance requirements.In order to determine the adaptability of high Cr-Ni austenitic wire to thick plate multi-pass welding,the optimization of GMAW shielding gas and crack resistance of the developed austenitic wire were further studied.The arc stiffness,droplet transition stability and mechanical properties of weld metal under unitary/binary/ternary shielding gases of Ar/He/CO₂/N₂ were studied by means of high-speed photography and phase analysis.45%Ar+50%He+5%N₂ ternary gas was determined as the best shielding gas for high Cr-Ni austenitic wire welding.The addition of N₂ in shielding gas ensures the content of N elements in weld metal.The obtained multi-layer and multi-pass welded joints are well formed,and the strength,impact toughness and elongation of the weld metal meet the engineering requirements.The results of non-destructive test and cross-section inspection for crack resistance of high Cr-Ni austenitic welding wire with 45%Ar+50%He+5%N₂ ternary shielding gas show that the welded joints of Y-groove,window restraint and cup-shaped tube joints under non-preheating conditions have no cold cracks,which meets the crack resistance requirements of large rigid structures.Finally,application of developed high Cr-Ni austenitic welding wire for welding10Ni5CrMoV steel has been verified by test-plate level and typical tube-sheet large restraint simulation structure.The weldability test results show that tensile strength of welded joints are all above 839 MPa,bending test surface has no defects which shows good cold bending resistance,and the impact energy of V-notch at-50?is above 92J.The NDT results of welded joints with three kinds of tube sheet simulation structures are qualified.Macroscopic photos of the welded joints show that multi-layer multi-pass welding is well formed and no macroscopic defects such as cracks,incomplete fusion and porosity are found.The strength and plasticity of weld metal meet the requirements of engineering specifications.