induction heating of different shaped tubes - ATE Applicazioni ...

INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSMauro Schiavon, ATE – Applicazioni Termo Elettroniche [Vicenza]Michele Forzan – Studio di Ingegneria [Padova]ATE, Applicazioni Termo Elettroniche, is a qualified manufacturer of industrial equipment forinduction heating of metals, including process automation, control and supervisory systems, basedin Vicenza, Italy.The collaboration of Studio di Ingegneria, Italian FLUX partner, is devoted to the design ofinductors to be used in several plants. Here, we would like to point out that the design of theinductor calls for a complete investigation of the heating process and not just a mere study tocompute the right frequency or number of turns.Tubes are pressed to countersink their extremities, like shown in figure 1. The material is subjectedto a cold-working process: the tube must be heated up to 520 °C to relieve internal stresses.The working frequency is 500 Hz supplied by an 80 kW converter.5Fig. 1 The extremities of a tube must be uniformly heated.Tube manufacturer requires a constant thermal profile along 160 mm: maximum variation shouldnot exceed ±15 °C.The same inductor must be able to heat both the extremity shapes and different tubes dimensions:diameters vary from 251 mm up to 340 mm and tube depth starts from 8 mm up to 20 mm.Tubes are made of ferromagnetic steel, low and medium carbon alloy steel. Material properties ofmetals vary as temperature varies, as is well known, mainly ferromagnetic materials modify theirbehavior dramatically as temperature rises. FLUX2D is a unique tool for this specific application:magneto-thermal module allows the user to automatically perform a complete coupled magnetic andthermal analysis, taking into account the temperature dependence of all material properties. Nonlinearrelative permeability and its temperature dependence are taken into account as well. Thevarying of the material properties causes different distribution of the induced power densities.FLUX2D allows the designer to choose the inductor shape that produces the required thermalprofile. The system can be simulated by means of an axial-symmetric model: one has to model justone half of the entire system.The initial inductor, which has been designed taking into account that the heating sources areconcentrated in the middle of a long coil, has been made by subdiving it into three regions,characterized by different turn densities, like shown in figure 3.

INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSThe inductor has been modified several times in order to reach the required performance: forexample, the inductor, shown in fig. 6, has two different internal radiuses.Finally, the most efficient inductor is the one shown in figure 9.Zone 3:10 turnsZone 2:8 turnsZone 1:11 turnsFig. 3 – The initial inductor. Infigure, the model representsjust one half of entire system.Zone 3:9 turnsFig. 4 - Equiflux isovalues.Fig. 5 – Thermal profile on thesurface. Temperature differencesare too high [420 °C – 485 °C].Line path starts from the bottomof the tube.5Zone 3:7 turnsZone 1:11 turnsFig. 6 – Another inductorsimulated.Fig. 7 - Equiflux isovalues.Fig. 8 – Thermal profile on thesurface. Temperature distributionhas been improved, butdifferences are still too high [420°C – 470 °C]. This solution hasbeen dropped becausetemperature profile has beenimproved only when the narrowextremity is heated.22 / 6

INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSZone 3:10 turnsZone 2:no turnZone 1:13 turnsFig. 9 – The final inductor.Fig. 10 - Equiflux isovalues.Fig. 11 – Thermal profile on thesurface. Temperature differencesare now very low [447 – 475°C].The same inductor designed for the narrow extremity must be used to heat the other wider end.Simulations have been devoted to find out the best position of the tube inside the inductor.Initial position of the workpiece is shown in figure 12, where the tube is positioned completelyinside the inductor. The consequently heating process leads to a non uniform thermal profile.One can improve the temperature the temperature distribution by moving the tube upwards, seen infig. 15 – 18.533 / 6

INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSFig. 12 – The inductor designedfor the narrow end now heats theexpanding extremity.Fig. 13 – Equiflux isovalues.Fig. 14 – Thermal profile on thesurface. Temperaturedifferences are too high [420 °C– 500 °C].5Fig. 17 – Temperaturedifferences are significantlyreduced [410 – 460 °C].Fig. 15 – The tube position hasbeen modified to improve thetemperature uniformity.Fig. 16 – Equiflux isovalues.44 / 6

INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSFig. 20 – Temperaturedifferences are now acceptable[424 – 458 °C].Fig. 18 – The final position of theexpanding tube.Fig. 19 – Equiflux isovalues.Measurements have been performed on both the extremities; demonstrating that the requiredtemperature is achieved with an error even lower than the one computed (+/- 10 °C).5fig. 21 – ATE induction equipment for stress relieving of steel tubes, installed in Dalmine works -Bergamo.55 / 6

6 / 6INDUCTION HEATING OF DIFFERENT SHAPED TUBES : DESIGN OF ANINDUCTOR FOR STRESS RELIEVING APPLICATIONSFLUX2D magneto-thermal module is a powerful tool aiding in the design of induction-heatingequipment.It reduces the cost for the design, because one does not need to build several different inductors todiscover which is the optimum solution. A deeper knowledge of the process also allows thedesigner to carry out modifies required to improve the characteristics of the system.Finally, we would like to point out that a two dimensional model is necessary to describe an ‘edge’problem similar to the one previously discussed and the fact that the model must consider non linearproperties of materials.56