| As the newly developed 690MPa high strength and high toughness offshore platform 5%Mn steel,the large amount of Mn addition is not conducive to the welding properties of the steel There fore,in order to provide theoretical and experimental basis for the engineering application of 5%Mn steel,it is very important to study the welding material and welding process of this kind of steeL As the newly developed 690MPa high strength and high toughness offshore platform 5%Mn steel,the large amount of Mn addition is not conducive to the welding properties of the steel.Therefore,in order to provide theoretical and experimental basis for the engineering application of 5%Mn steel,it is very important to study the welding material and welding process of this kind of steeLIn order to determine the appropriate welding material for gas shielded welding,we used two kinds of homemade Mn-Ni-Mo series welding wire of mixed gas protection to weld the 5%Mn steel of 20mm thickness,and study the effect of different C and Ni components under the variation of welding joint micro structure and properties.through thermal simulation,study the effect of different C,the weld metal composition of Ni under continuous cooling transformation CCT diagram.The cold crack sensitivity of 30mm thick 5%Mn steel for industrial trial production was evaluated through the crack resistance test by oblique Y groove.and the effects of different heat input and Interlayer temperature on the microstructure and properties of 30mm thick 5%Mn steel welded joints were studied.The main experiment results can be summarized as follows:(1)Used two kinds of homemade Mn-Ni-Mo series welding wire to weld the 5%Mn steel of 20mm thickness,and study the effect of different C and Ni components under the variation of welded metal microstructure and properties.The results indicated that for the microstructure of high carbon and lower nickel weld metal,the end of weld microstructure mainly consisted of lath bainite and fewer lath martensite,and produce a large amount of granular banite in the HAZ of weld bead due to the influence of welding thermal cycle.The weld metal had high strength,poor toughness and obvious hardening.However,for the microstructure of low carbon and higher nickel weld metal,the end of weld microstructure mainly consisted of the proeutectoid ferrite and acicular ferrite.Though proeutectoid ferrite slight increased in the HAZ of weld head,still is given priority to with acicular ferrite,and weld metal had lower strength,excellent toughness.Meanwhile,the volume fraction of M-A constituents increase,the percentage of large angle grain boundaries reduce,and the.inclusions density increase,led to the lower toughness of higher carbon and lower nickel weld metal(2)The investigations on the CCT curve of continuous cooling transformation of weld metal with different C and Ni components.Experiment results show that when t8/3 is less than 1800s,with the increase of t8/3,the microstructure of high carbon low nickel weld metal microstructure changes from LM+LB to LM+LB+GB to GB,while the microstructure of low carbon nickel weld metal is AF+GB to GB to GB+F+P.(3)The study on the cold crack sensitivity of 30mm thick 5%Mn steel with low carbon high nickel wire through the crack resistance test by oblique Y groove.Experiment results show that when the preheating temperature is 0 ??60? and 100?,the root and the section crack rate is very high,and Irregular change.but when the preheating temperature is 120?.150? 200?,with the increase of temperature,the crack rate of root and section decreases.In especial,the crack rate of root and cross section decreased to 4.47%and 1%respectively at the temperature of 200?.(4)The investigations on the microstructure and properties of 30mm thick 5%Mn steelby using low carbon and high nickel wire with the heat input of 12k J/cm,15kJ/cm and 18kJ/cm.Results show that the microstructure of the weld metal under different heat input consists of proeutectoid ferrite,acicular ferrite and bainite.With the increasing of heat input,the proeutectoid ferrite content increased,the acicular ferrite size increases,the granular bainite content in weld bead heat affected zone increases,the number density and area percentage of inclusions in weld metal increases,the number density of large size inclusions can be used as crack sources increases.which lead to the tensile strength,yield strength,low temperature toughness and hardness of welded matal decreased with the increase of heat input.The microstructure in welded joint HAZ is composed of lath martensite.the prior austenite grain size is increased sharply,the lath is coarse and some "detached" oxide inclusion existence,which lead to low temperature toughness of welded joint heat affected zone poor.When the heat input is 18kJ/cm,the grain size of austenite in the CGHAZ is increased obviously,which lead to toughness is lower.(5)The investigations on the microstructure and properties of welded metal of 30mm thick 5%Mn steel by using low carbon and high nickel wire with the Interlayer temperature of 100? and 120?.Results show that with the increasing of Interlayer temperature,the proeutectoid ferrite content,the acicular ferrite size,the granular bainite content in weld bead heat affected zone,the number density and area percentage of inclusions in weld metal,the inclusion size,all increased slightly,Consequently,the yield strength and low temperature toughness of weld metal are reduced slightly.The low temperature impact energy ofjoint heat affected zone is 19J and 47J respectively,So decreasing the Interlayer temperature deteriorates the toughness of heat affected zone in joint. |
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