Electrothermal analysis for effectiveness of induction forge heating Análise Eletrotérmica para a eficácia do aquecimento por indução para forja

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1 Electrothermal analysis for effectiveness of induction forge heating Análise Eletrotérmica para a eficácia do aquecimento por indução para forja Alexander N. Ivanov (1) Vladimir A. Bukanin (1) José Carlos Zerbini (2) ABSTRACT An analysis of the existing induction forging heaters is carried out taking into account the problems of the economic crisis. Certain heating equipment does not satisfy the requirements of effectiveness for many reasons and needs to be modernized. It is shown that user-oriented programs ELTA (ELectro Thermal Analysis) and 2DELTA are ideally suited to investigation of many forge heating technologies and equipments. Both electrical and thermal analyses are essential to the success of forging. Electrical analysis can give information about optimal frequency of power source, design of induction coils and circuits for power delivery to increase electrical efficiency. Thermal analysis permits to obtain the predicted quality of heating and maximum of thermal efficiency. Examples of simulation and analysis are presented. Key words induction heating, hot forging, ELTA and 2DELTA programs. 1. INTRODUCTION The economic crisis, which affected many countries, including Brazil and Russia, makes it necessary to modernize many processes. Knowing these problems not randomly even at the end of 20-th century European Union TEMPUS-TACIS Project T_JEP was started and realized. Saint Petersburg Electrotechnical University, University of Padua, Italy (Department of Electrical Engineering) and University of Hannover, Germany (Institut für Elektrowärme) were the Universities partners of this project. One of the objectives of Joint European Project named "Economics in electroenergetics and energy saving by the rational use of electrotechnologies" was improving the academic education, within the curriculum "Electrotechnology", by inserting basic concepts of economic analysis and energy efficiency and by introducing the use of computers for the analysis and economic optimization of new technological applications [1]. The consequences of the above could be improving the competitiveness of the industry and the efficiency in the use of electricity. (1) Dr., Saint-Petersburg Electrotechnical University (LETI), Russia; anivanov@nsgsoft.com (1) Dr., Saint-Petersburg Electrotechnical University (LETI), Russia; VABukanin@nsgsoft.com (2) Eng., Synergetica Sistemas e Processos, zerbini@synergeticasp.com.br

2 A systematic approach based on an Energy Management System like standard ISO or similar can supply a pathway documentation to help companies in getting these tasks done. The forge heating by induction is sufficiently energy-consuming process, whose effectiveness can be increased. This circumstance is caused with the fact that at present the Russia government made the decision to bind the plants and the organizations, which expend many energy resources, yearly to economize on 3 % the electric power, the waters, gas, gasoline and other. Two ways in the increasing of effectiveness may be proposed. First way is education. It may be pointed out that forge heating by induction is very complex due to the hidden from eyes internal processes in these systems. Only experienced researchers and developers know particularities of heating processes, for example electromagnetic effects ( skin, proximity, edge, end, electrodynamics effects, etc.). These effects play a dramatic part in uniformity, productivity, efficiency and safety of induction heating. Care must be taken in choosing frequency of power source, parameters of induction coils and so on to meet the necessary requirements of heating technology. New induction installations cannot be created without detailed investigation and analysis of the induction system. However, the typical mistakes in designing process are met even at present. The developers and constructors must correctly choose power source, configuration, turn number, type and size of the coil tube, thickness of heat insulation, the electric or mechanical drive for moving of workpieces, cooling system, control system of temperature and other elements. Second way is development of convenient, intelligible and objectively oriented software to the nonprofessional users. There are several programs available, including 1D, 2D and 3D electromagnetic and thermal programs that can be used for investigation and design of induction heaters. Induction specialist has developed ELTA for induction people. The first variant of well-known ELTA was developed 20 years ago. To meet the changing needs of researchers and developers of induction systems, this program has evolved in powerful program with very wide possibilities. At present, a new version of ELTA 7.0 can simulate the heating of workpiece with cylindrical, square, rectangular cross section and even gear wheels. 2DELTA is the program that ideally suited to investigation of forge heating technologies and equipments for cylindrical workpieces [2, 3]. These programs open wide opportunities for the specialists employed in design and production of induction heating installations. They allow quick obtaining of rather accurate results for typical practical applications of induction heating. Advantages of programs consist in opportunity of their use without special training and programming skills, good for practical tasks accuracy and many additional options such as water cooling of the coils, convenient database, multiple modes of coil energizing, circuits for power delivery, etc. 2. FORGING INDUCTION HEATERS The large experience of an induction method application heating makes it possible considerably to more simply solve many problems of its introduction. At present, there is no need for convincing for critical for decision making in the advantages of induction heating over other traditional forms of heating. Many forging induction heaters have been developed by world known companies and they are working in many countries up to now. At the same time, several induction installations in the plants are old and need to be modernized. This circumstance is caused by technical and economic reasons, in particular by the tendency to increase effectiveness and productivity of forging process, to decrease the operational expenses, to save surface area, etc. The plants may purchase proposed new installation or by itself reconstruct it if there are experienced specialists. Russian plants use continuous or simultaneous forging induction heaters developed by Vologdin Institute of High Frequency Currents series KIN (Kuznechnyj Indukcionnyj Nagrevatel') for workpieces of round or square cross-section KIN-250, KIN-500, KIN-750, KIN-1500, KIN-2500 or INT-250, INT- 500, INT-750, INT-1500 and INT-2500 (power kw, frequency khz); company MOSINDUCTOR IKN-35 IKN-200 with transistor generator; IKNT-100 IKNT-8000 with thyristor generator; company RUSINDUCTOR IKN-25 IKN-250; company TESLINE INDUCTIVE TESLINE 50K-E (15 30 khz); company PROMINDUCTOR IKN 35 IKN 300 and other [4, 5]. Brazilian plants uses forging lines, old and new ones based on induction heating from local and abroad suppliers like, ABB, ABP, Ajax Tocco, AEG Elotherm, EFD Induction, Elphiac, GH Induction, Inductoheat, Jamo, Metaltrend, Newelco, SMS Elotherm, Pillar, Servifor, Servtherm, etc. with a range from few kw to MW and frequencies from 60 Hz to hundreds of khz.

3 The one of perspective work is investigation of new variant of inductors and technologies for effectiveness of induction forge heating trough the use of an electrothermal software. 3. PURPOSE AND RESULTS OF SIMULATIONS The main problems to be investigated for forge heating by induction, e.g. technological, technical and economical were described in previous publication [3]. This material is devoted to a new investigation. The main purpose of simulation in ELTA programs is obtaining integral and distributed parameters of heating process. Electrical analysis consists of obtaining the rational decision of power source frequency, design of induction coils and circuits for power delivery that can increase electrical efficiency. Thermal analysis includes a determination of required temperature distribution along the length and cross-section and maximum total efficiency of process. There are three or four main configurations of induction heating systems to realize forging depending on the desired part configuration and its size. Round, square or rectangular cross-section billets are the simplest and most commonly used shapes. Case I: Non uniform longitudinal temperature before forging for a round billet The first example described in [6] represents induction heating system for hot forging processes for automotive industry for heating with complex thermal profile. The final thermal profile must be as uniform as possible in the radial direction, insofar as such a heating addresses to forging. In the longitudinal direction, two hot zones are separated by a cold one. A hot zone is supposed to have a temperature of 1100 C, instead a cold one should have a final temperature that lays in the region C. Such temperature distribution may be achieved by using cylindrical coil (see figure 1) with non-uniform step of winding. Last version of 2DELTA supports current, voltage and power regimes of the inductor. Main parameters of the system Case I are presented below. Figure 1. Sketch of the cylindrical induction system Workpiece parameters: material carbon steel 42CrMo4, diameter is 30 mm, length 474 mm. Processing parameters: time of heating 80 sec, single frequency of power source khz (or 4 khz), output power 40 kw (or output current 1000 A). Inductor parameters: length 450 mm, internal diameter is 40 mm, turns number 30, (non-uniform winding according required thermal profile). Profile and dimension of copper tube round, diameter is 9 mm, thickness 0.2 mm. Thermal insulation: not applicable.

4 Key results of simulation are presented at figure 2.a. Comparison with experimental results presented in [6] and figure 2.b shows that result of simulation is rather close to experimental measurements. (a) (b) Figure 2. Final temperature distribution in the cross-section of workpiece (a) simulation in 2DELTA, (b) experimental results from [6]

5 Figure 3. Dynamics of integral parameters of the simulated system Case II: Channel coils for hot end heating in semi-continuous process The next example is devoted to simulation of induction system for hot forging of ends with channel coils (see figure 4). There are a lot of advantages of such systems for real technological processes like high productivity and simple construction of the loading/unloading devices. Otherwise such systems are rather complicated for simulation and design. The main reason of these difficulties is 3D nature of electromagnetic field in such systems and continuous movement of workpieces in such systems. But in the case of inductor with the length of the middle part much bigger than length of the end parts it is possible to replace such inductor to several cylindrical inductors with equivalent air gap connected in series. Differences between internal radius of cylindrical coil and external radius of the workpiece have to be fit to air gap of channel coil. Tube profile, number of turns and distance between turns of cylindrical coil have to be fit such parameters of channel coil. Type of processing in 2DELTA is semi-continuous. Final temperature in the volume of the half of workpiece has to be gathering than 950 C. It is possible to calculate parameters of equivalent semi-continuous process by using equations (1) and (2). t t t N Lch v heat push (1) heat (2) where t push time of discrete moving of workpieces in semi-continuous process, t heat time of staying workpiece in channel inductor during motion, N number of workpieces in channel inductor, L ch length of channel inductor, v linear velocity of worpkeses moving through channel inductor. Main parameters of the Case II system are presented below. Workpiece parameters: material 0.4 % C Steel 2T anneal, diameter is 30 mm, length 600 mm. Processing parameters: frequency of power source 10 khz, inductor voltage 600 V, speed of motion is equal mm/s.

6 Inductor parameters: coil type: channel, length 125 mm, internal width is 100 mm, turns number 9. Profile and dimension of copper tube rectangular, 20x30 mm, thickness 1,5 mm. Thermal insulation: not applicable. Parameters of equal inductors: coil type: cylindrical, coil number 4, length 300 mm, interior radius is 50 mm, turns number 9. Profile and dimension of copper tube rectangular, 20x30 mm, thickness 1,5 mm. Thermal insulation: not applicable. Processing parameters for semi-continuous mode: frequency of power source 10 khz, inductor voltage 500 V, rate of push is 60 s. Key results of simulation are presented at figure 5 Figure 4. Sketch of the channel-coil induction system Figure 5. Thermal profile at the end of the process Case III: Oval coils for hot forging of round ends billets The next example is devoted to simulation of induction system for hot forging of ends with oval coil (see figure 6a). In general such configuration requires 3D simulation. But it is possible to simulate such system in 2D model using 2DELTA with several assumptions. Oval inductor may be replaced by several cylindrical series-connected inductors. Number of cylindrical inductors is equal to the number of workpieces in the oval inductor. Such processes may be simulated in 2DELTA with semi-continuous type of model. Equivalent system is presented in the figure 6.b. Final temperature in the volume of the half of workpiece have to be gather than 1000 C.

7 Main parameters of the system Case III are presented below. Workpieces parameters: number of workpieces: 5, material Steel 0.4%C, 1%Mn, 1%Cr, diameter is 30 mm, length 110 mm. Processing parameters: frequency of power source 66 khz, inductor voltage 220 V, time of push is 90 sec. Inductor parameters: coil type: oval, length 70 mm, internal width is 200 mm, internal height is 40 mm, turns number 5. Profile and dimension of copper tube round, diameter 9 mm, thickness 2 mm. Thermal insulation: not applicable. Parameters of equal inductors: number of inductors 5, coil type: cylinder, length 70 mm, internal diameter is 40 mm, turns number 5. Profile and dimension of copper tube round, diameter 9 mm, thickness 2 mm. Thermal insulation: not applicable. Key results of simulation are presented at figure 7. (a) (b) Figure 6. Sketch of the oval induction system (a) and equal system with cylindrical inductors (b) Figure 7. Thermal profile at the end of the process

8 Case IV: Retangular coil for hot forging of slabs This example is devoted to simulation of induction system for hot forging of slabs in rectangular coil (see figure 8). In general such configuration requires 3D simulation. But it is possible to simulate such system in ELTA with several assumptions. 2D model in ELTA for rectangular system configuration allows to took into account edge effects of workpiece, but end effect of workpiece and inductor cannot be took into account. Final temperature in the cross-section of workpiece has to be gather than 1000 C. Main parameters of the Case IV system are presented below. Workpieces parameters: material Steel 0.4%C, 1%Mn, 1%Cr, dimensions: is WxH 250x250 mm, length 100 mm. Processing parameters: frequency of power source 1 khz, inductor voltage 400 V, time of heating is not more than 1500 sec. Inductor parameters: coil type: square, length 100 mm, internal width is 300 mm, internal height is 300 mm, turns number 9. Profile and dimension of copper tube rectangular, profile 10 mm x 10 mm, thickness 2 mm. Thermal insulation: not applicable. It is rather complicated task to achieve uniform temperature distribution in the volume of massive slab. One of the simplest ways to achieve uniform distribution of temperature is overheating of surface at the heating stage with subsequent alignment of temperature at the cooling inside the inductor. Key results of simulation are presented at figures 8 and 9. Figure 8. Sketch of the slab inductor

9 Figure 8. Thermal profile in the cross-section at the end of the process 37 th SENAFOR

10 Figure 9. Dynamics of the integral parameters during heating process CONCLUSIONS 2DELTA program allows to build rather complicated electrothermal models for preliminary simulation of induction heating processes for forging. Rather fast simulation procedure allows to use this software for process optimization procedure to increase efficiency of induction heating for forging technologies. ELTA program allows to simulate process of induction heating of bodies with square or rectangular cross-section taking into account edge effect of workpiece. Convenient interface and rather fast simulation procedure allows to use ELTA for optimization or revalidation of existing induction heating processes in the real industrial plants. References [1] Economics in Electroenergetics and Energy Saving by the Rational Use of Electrotechnologies Padua: SGEditorial, p. [2] José Carlos Zerbini, Alexander Ivanov, Vladimir Bukanin, Ademir Angelo de Vasconcelos. Use of the ELTA software for study of electromagnetic and thermal processes in induction heating steel forging lines. 35th SENAFOR 19th International Forging Conference. October 7-9, 2015, Porto Alegre, Brazil. [3] Valentin S. Nemkov, Vladimir A. Bukanin, Alexander N. Ivanov, José Carlos Zerbini. Perspective of using electrothermal software programs in investigation of forge heating by induction. 36th SENAFOR 20th International Forging Conference. October 5-7, 2016, Porto Alegre, Brazil. [4] Electrothermal equipment: The reference book. M.: Energy, 1980, 416 p. (in Russian) [5] Alexander N. Ivanov, Vladimir A. Bukanin, José Carlos Zerbini. History and perspectives of induction heating technologies for forging industry in Russia. 37th SENAFOR 21th International Forging Conference. October 4-6, 2017, Porto Alegre, Brazil. [6] M. Baldan, T. Steinberg, E. Baake. Numerical and practical investigation of tailored heating process for forging parts. XVIII International UIE-Congress Electrotechnologies for Material Processing Hannover (Germany), June 6-9, 2017, pp