Research On Welding Process And Performance Of 690MPa High-strength Steel Double-wire CMT

The low alloy high strength steel Q690 is widely used in the building materials of ocean platform due to its good mechanical properties.However,the complex chloride environment tends to invalidate ahead of lifetime in welding part,so it is significance to study the corrosion behavior of welded parts.In addition,the leg is in a low temperature environment for a long time,which requires that the materials have excellent low temperature toughness.Due to the influence of current,impact property is also an important index of materials.It is said that the low temperature toughness and corrosion resistance of welded joint are poor.It requires to improve the properties of the welded joint by means of process or welding method to meet the requirements.The study contents are presented as follows.This experiment mainly was adopted the welding method of CMT Twin with low heat input,which can obtain good welded joint by changing different welding parameters.The main research contents and results are as follows:1.Optimization of welding process parameters.Through the study of single factor to weld forming,the optimized process parameters are obtained: The final optimized process parameters:pulse current is 250 ~ 290 A,CMT current of 160 ~ 220 A,air flow for 15 ~ 20 l/min,the welding speed of 2 m/min,wire extension of 12 mm,arc length correction coefficient is 0%.2.The analysis of mechanical properties on welded joints: the welding joints fracture in the parent material,and the tensile properties of welded joints are significantly better than that of the parent material(792MPa).The low temperature impact performance(92J)of weld recrystallization zone at-20 ? is better than not part of the remelting(67.7 J),and is much larger than the standard(27J);The low temperature impact performance(99.7J)of weld recrystallization zone at-40 ? is better than not part of the remelting(67.7J),and is much larger than the standard(47J),this is mainly affected by the M-A groups;The hardness of the welded zone is 285 HV,which is larger than that of the base metal(270HV).3.The analysis of corrosion performance on welded joints: the anode current density value in the overheated zone is about 6uA,while the anode current density in the base metal is about 5uA,and the base metal and the overheat zone have a large corrosion rate.The anode current density is about 4uA and the corrosion rate is small.Due to the high nickel content in the wire,the corrosion resistance of the weld area is obviously superior.The corrosion degree of microstructure is mainly:heat affected zone> base metal >welded zone.The results shows that the structure of multi-layer welding was significantly improved,and thehardness,tensile properties and low temperature toughness were obviously increased.The M-A formation of the special welding heat cycle in the CMT Twin method has great influence on the low temperature toughness.In particular,the uniform distribution,long strip and large number of M-A groups and the low temperature impact properties of the welded joints are obtained.Because of the high nickel element in the wire,the corrosion resistance of the weld area is greatly improved.The research results of this paper can provide experimental basis for welding of pile legs.