Research On Electromagnetic Heating Characteristics In High-frequency Welding And Medium-frequency Induction Heating Of Longitudinal Welded Pipe

One of the key areas of expansion for the global petroleum sector and a frontier for scientific and technological advancement is the development of deep-water oil and gas reserves.Higher standards for the dependability,service life,economic viability,and production of deep-water pipelines have been imposed by the challenging marine environment of low temperature,high pressure,strong corrosion,and fast-rising energy demand.High-frequency welded pipe is frequently used as oil casing due to its benefits of clean environmental protection,high welding speed,and low production cost.China is a big country of welded pipes and is now moving towards a strong country of welded pipes.The bottleneck of high-frequency welded pipe development is how to achieve the improvement from quantitative change to qualitative change.The core of high-frequency welded pipe quality improvement is the key performance improvement of the weld.After high-frequency welding,the toughness of the weld is low and the mechanical properties are poor,which needs to be improved with the medium-frequency heat treatment.The existing mediumfrequency heat treatment method makes it difficult to match the high-frequency welding heat source.The proximity effect will create a large temperature gradient in the direction of wall thickness,resulting in the heating blind zone or overheating zone.To a certain extent,the heat sources matching problem of high-frequency welding and medium-frequency heat treatment affects the production quality of high-frequency welded pipes.In actual production,domestic factories rely more on the experience of long-term practice to match the heating parameters in high-frequency welding and medium-frequency heat treatment,and the cost of trial and error can be effectively reduced using numerical simulation.One of the key points to solve the above problem is to obtain an accurate highfrequency welding heat source and further design a matching medium-frequency heating method.The electromagnetic heating characteristics in the process of high-frequency welding and heat treatment are researched in the study.The main research contents are as follows.Firstly,a method of load shifting is proposed to obtain the heat source and electromagnetic distribution of dynamic high-frequency electric resistance welding that is suitable for the actual working conditions.A three-dimensional model of electric-magneticthermal coupling is established for the mobile heating of welded pipes.The microstructure distribution characteristics of the samples in different areas of the welded pipe and the corresponding samples after thermal simulation are compared,indirectly verifying the correctness of the welding heating model.The distribution characteristics of the electromagnetic field and temperature field in the process of electromagnetic mobile heating are analyzed.The evolution law of the moving heating temperature field of welded pipe under the action of multiple parameters such as typical process elements and geometric elements that cause the change in heat input is defined.Then,to improve the matching between the temperature field during intermediate frequency heating and the high-frequency welding heat source,two methods are proposed:dual heat sources induction heating and frequency conversion heating.Two typical electromagnetic heating topics are studied in the process: the interference of multi-heat source electromagnetic heating and the electric-magnetic-thermal characteristics of multifrequency heating.In terms of the interference of multi-heat source electromagnetic heating,the temperature field distribution of dual-heat source heating is explored by analyzing the effect of dual-heat sources on magnetic field distribution under moving conditions.By researching the mode of heat transfer inside the welds with heat sources under motion conditions,the temperature field evolution,spatial magnetic field distribution,and spatial matching relationship in cases of dual heat sources interfering with each other,tangential to each other,and not interfering with each other are revealed.In terms of multi-frequency heating,an electric-magnetic-thermal coupling model for frequency conversion heating of welded pipes is established and verified by temperature measurement experiments,thereby revealing the distribution characteristics of the induced current,temperature,and magnetic flux intensity.A new concept-the energy surface of electromagnetic heating-is proposed,to visually characterize the energy concentration zone of variable frequency heating.The process parameters of induction heating suitable for thick-walled pipes are formulated,and the temperature gradient in the direction of the wall thickness of the welded pipe is reduced.The problem of large heating blind zones in the heat-affected zone of the inner wall of the welded pipe caused by traditional single heat source heating is solved.Finally,the comparison also demonstrates how electric resistance welding and induction welding differ in terms of heating rate and efficiency.Quantitative differences between the two heating methods in terms of heating rate,inter-critically heat-affected zone,and electric arc stepping position are discovered using electric arc step position as the observation section,heating rate,and critical heat-affected zone as the evaluation parameters.Further evidence is provided for the impact of electromagnetic and geometric characteristics on the electric arc step position.The research provides a new idea and theoretical basis for improving the electromagnetic heating quality of the high-frequency longitudinal welded pipe and accurately controlling the electromagnetic heating process.