Technical Note: Transverse Flux Induction Heat Treating

Transcription

1 J. Heat Treat. (1988) 6: Springer-Verlag New York Inc. Technical Note: Transverse Flux Induction Heat Treating G.F. Bobart Abstract. Transverse flux is an efficient method of induction heating thin strip for various heat treating processes. This article will cover methods of interfacing power, handling, and control parameters for high speed in-line strip heating applications. Process considerations and results of applications when annealing steel strip and solution heat treating of aluminum strip will be addressed. INTRODUCTION Induction heating has been used extensively for heat treating processes in a variety of metalworking applications. Some of these applications are associated with discrete parts such as shafts, bearings, gears, and hubs. Here, the most common processes are hardening and tempering of carbon and alloy steel parts to improve strength and wear characteristics. However, many induction heating applications are associated with in-line heat treating processes to permit high speed production of both ferrous and nonferrous plate and strip products. Here, often a wider range of heat treat processes are required to permit subsequent manufacturing operations such as rolling, forming, and joining, as well as to improve the mechanical properties of the final product. These in-line processes include partial and full annealing, normalizing, hardening, tempering, and even solution heat treating (in the case of some nonferrous products). Induction heating provides a very efficient method of heating magnetic strip. Here, heating is readily applied by using longitudinal flux that is created by a solenoid type inductor coil that encircles the strip to be heated. The solenoid coil magnetically couples to the strip, creating a circulating current that flows across the width of the strip and returns on itself on the opposite side as shown in Figure 1. Even for relatively thin magnetic strip, good efficient heating can be maintained since the depth of current penetration is small, and, therefore, it does not cancel itself as it returns on the opposite side of the strip. This method George F. Bobart is a consultant with Pillar Industries, Inc., N92 W15800 Megal Drive, Menomonee Falls, WI of heating is ideally suited for magnetic strip and even for nonmagnetic strip that has a higher electrical resistivity and part thickness. However, as the strip gets thinner, higher frequencies must be used to maintain a good transfer of energy from the coil to the strip as shown in Table 1. And, for low resistivity materials, such as aluminum, copper, and brass, both the efficiency and power factor are at significantly lower levels than for ferrous products. As a result, an alternative method known as transverse flux induction heating offers considerable benefits for in-line strip heating applications. This method of heating has been known for many years; however, it requires specially designed iron cored laminated inductor coils and tighter control of the power, strip handling, and process parameters. It has been in recent years that advancements in solid-state power sources and closed-loop process control have provided the parameters for productizing the transverse flux technology. While there is no basic improvement in the heating efficiency of magnetic strip, there can be benefits in capital equipment costs, since lower frequencies can be used on the thinner strip. However, lower frequencies do impart more magnetic force and can produce undesirable vibrations in the material as it passes through the inductor coil pole faces. Transverse flux develops a current pattern in the strip as shown in Figure 2. Here, it is across the width of the strip and returns a pole-pitch distance down the length of the strip. This eliminates the current cancellation problem associated with longitudinal flux heating, providing optimum heating efficiency and coil power factor. It also permits use of lower frequency power sources as shown in Table 2. And, with this type of heating, further latitude in frequency selection J. Heat Treating, Voi. 6, No. 1,

2 _% #,~STRIP v ~ CURRENT FLOW _. STRIP. TRANSVERSEFLUX 9 STRIP CTOR COIL R CE,. J (, ~ COPPERWINDINGS~_._~/ ~OLE STRUCTURE (LANINATED) ~ "--L~ /CURRENT FLOW i fl ~1 POLE FACE Fig. 1. Longitudinal flux heating.,, J 9 II I is possible, since other parameters can be varied to provide good coil efficiency and power factor. The following identifies optimum frequency for transverse flux: where 24.5 p g l06 f= tl2o f = frequency, Hz p = resistivity, Ohm cm g = air gap, in. t = strip thickness, in. lo = pole pitch, in. DESIGN/APPLICATION CONSIDERATIONS Power Source Power source considerations are important and critical to meeting process requirements. These include not only the power rating, but the choice of frequency and type of control to provide necessary process temperatures, heating uniformity, and production rate (tons/h). The resistivity and thickness of the material are key factors that dictate the power source frequency. While line frequency (50/60 Hz) can provide efficient heating for some of the thicker and/or lower resistivity materials as shown in Table 2, it develops strong magnetic fields to provide the necessary strip heating Table 1. Frequency Selection--Longitudinal Flux Steel Freq. B.C. A.C. Brass Alum. 60 Hz >1.5 in. 7.0 in. 2.0 in, 1.5 in. 200 Hz >0.5 in. 4.0 in. 1.1 in, 0.85 in. 1 Kl-Iz >0.2 in. 1.7 in in in. 3 KHz >0.09 in. 1.0 in in. 0.2 in. 10 KHz >0.04 in. 0.5 in in in. Fig. 2. Transverse flux. as it passes through the coil. As a result, it has a greater tendency to produce a vibration in the strip, particularly where materials with magnetic properties (like steel up to 1300 ~ F) are involved. Therefore, the applications ideally suited to transverse flux are thin nonferrous materials that utilize frequencies in the 150 Hz to 10 KHz spectrum. This is a good frequency range that is well covered by solid-state power sources. A variable frequency solid-state power source is desirable where a range of product thicknesses are involved. This permits shifting frequency to obtain nearoptimum coil efficiency and provides flexibility in load matching the variations of load impedance to the power source. Either current-fed or voltage-fed power sources can provide this variable frequency to the load; however, both require proper rating and special power control to cover the variations in load impedance under normal line operating conditions. The coil efficiency and power factor are obtained by operating at the optimum frequency. However, the curve is reasonably flat and over'a relatively wide range provides efficiencies in the range of 75 to 85% and coil power factors of 30 to 40% (Q of 2.5 to 3.5). Inductor Coil Inductor coil design for transverse flux heating involves a number of parameters that affect the power source, the system efficiency, and the uniformity of heating9 Coil design, along with the power source and Table 2. Frequency SelectionwTransverse Flux Steel Brass Alum. 60 I-Iz in. 2.0 in. 0.4 in. 0.2 in. 200 Hz in. 0.5 in. 0.1 in in. 1 KHz in. 0.1 in in in. 3 Kl-lz in in in in. 10 KHz in in in in J. Heat Treating, Voi. 6, No. 1, 1988

3 process controls, are, therefore, the key considerations of a transverse flux heating system. The speed and uniformity of heating not only affects the mechanical properties of the strip (strength, formability, ductility, etc.) but can have an effect on the shape and surface conditions of the strip as it exits the heating portion of the process line. Since energy savings is a major consideration when evaluating the use of transverse flux heating, care must be given to evaluating the coil design parameters and how they are effected by the strip width, material, and line speed requirements. Let's look at each individually to see how they affect the overall power requirements for a given process line. Strip Width. This affects the coil design parameters such as pole pitch, slot width, and air gap considerations. Generally, inductor coils for wider strip provide more flexibility in coil design without having a major effect on coil efficiency and, therefore, energy consumption. Where various strip widths are involved in a given application, care must be taken to maintain uniform heating across the strip width. Generally, a uniformity within - 2% of the temperature rise in the strip can be maintained with a fixed line speed. However, when narrower width products are involved, special arrangements are required to adjust the iron laminations and/or coil winding contour to maintain temperature uniformity across the strip. Material. The type of material affects not only the choice of frequency, but also the coil length and pole shaping techniques to maintain temperature uniformity. Depending on the type and thickness of the material, longer line lengths may be required to prevent canoeing or shaping distortion across the strip width. Line Speed. As the speed increases, generally the power and coil length increase. This can be easily accomplished in the coil design by adding additional poles to the winding and core structures, so that the temperature rise per foot of length is not excessive to create thermal shock and heat wrinkles in the strip. While the line length is usually extremely short compared to a typical gas or electric radiant type of furnace, it still must provide ample time at temperature for the process requirements, as well as enough line length to eliminate thermal shock or wrinkling in the strip. Fortunately, most of the process requirements (except solution heat treating with aluminum) can be met by using higher processing temperatures with correspondingly shorter heat times and, therefore, shorter line lengths. However, line lengths can generally be reduced by a factor of 10 from that required in a ra- diant furnace. For example, a process line running at 300 ft/min may require a radiant furnace with a 100 ft length; however, similar process results (at a slightly higher process temperature) can often be obtained with only about 10 ft of an induction furnace line. There is the other factor to consider; namely, thermal wrinkling or uneven growth during heating that could cause buckling or canoeing of the product. Since this is a function of power density and process temperature requirements, it is only indirectly affected by line speed considerations. However, proper coil design parameters can generally support a temperature rise of 200 ~ F or more per foot of line length. Therefore, even for higher temperature annealing processes on steel and stainless steel, the 10 ft line length is normally adequate to prevent thermal wrinkling from affecting the finished product. Application Application areas cover a broad range of materials and processes. While ferrous materials can be readily heated with transverse flux, they can often be efficiently and uniformly heated using conventional longitudinal flux with relatively simple solenoid type inductor coils. However, where very thin ferrous materials (such as in. or less) are involved, or where the process requires heating to temperatures above the curie point, transverse flux offers a good alternative heating approach. Otherwise, vacuum tube types of power sources are required to provide radio frequencies (200 to 500 KHz) for conventional longitudinal flux inductors. Transverse flux is ideally suited for heating nonferrous materials, since relatively low coil efficiency and low coil power factor are encountered with longitudinal flux. Transverse flux can significantly improve both of these factors, as weu as permit the use of lower frequency power sources. This reduces the capital equipment costs, and where it shifts from requiring RF frequencies, the power source conversion efficiency is also significantly improved. Aluminum, brass, copper, and austenitic stainless steel strip lines are ideally suited for transverse flux heating. Each of these materials often requires in-line processes like partial or full annealing or solution heat treating to provide necessary mechanical properties for subsequent finishing operations. Power Requirements Power requirements for heating the various materials are dictated by the production rates, the specific heat, and the processing temperatures involved with each of the materials. The thermal energy required can easily be determined by the following: Pth,~ = M C T J. Heat Treating, Vol. 6, No. 1,

4 where P = power, kw M = production rate, lb/min C = specific heat, Btu/lb. ~ T = temperature rise, ~ Using transverse flux, the heating is fast, and, therefore, the radiation losses are relatively low compared to heating in a conventional furnace. Since the iron cored inductor provides a high heating efficiency, the power source requirements are basically determined by the production tonnage and process requirements. These are shown in Figures 3-5 for the common types of processes used on aluminum, brass, steel, and stainless steel strip. It can be noted that the curves increase with tonnage and process temperature. While the curves are shown for a specific thickness and width, power source requirements can be ratioed up or down for other thickness and width parameters, so long as proper power source frequency and coil control considerations are provided. Energy Consumption Energy consumption is a key consideration, since it can be a reasonable portion of the overall operating cost. Generally, transverse flux uses less energy since the coil efficiency is in the 85% range. Since the other system losses are quite low, the energy consumption in kw-hr/ton is generally less than one-half of that required in an electric fired batch or in-line strip heating furnace. These are shown in Figure 6 for aluminum, brass, steel, and stainless steel over the temperature range required for the various heat treat processes. Typical power source ratings and energy consumption levels are summarized in Table 3 for the key 1500 POWER SOURCE K.W. A - SOLDTION HEAT TREAT "F, j A, B - FULL ANNEAL "F, 1200 C - PARTIAL ANNEAL " F, ~ ~00 BASEDON ~ ~ B ' 600 F~ 300 WIDTH = 50" J "NESS LoTo ~ / jc o 2o' 4'o 6'o ~0 io'o LINE SPEED - FT,/MIN. Fig. 3. Transverse flux heating--aluminum strip. POWER SOURCE K.W / ' A ' A - 70/30 CARTRIDGE BRASS - FULL ANNEAL - IROO'F,~ B - 60/40 NAVAL BRASS FULL ANNEAL IIO0"F C - CARTRIDGE/NAVAL BRASS - PARTIAL ANNEAL 80/~'F'~. B. 90~ BASED O~ ~ ~ C. W DTN 40" ~?RNESS / 300. O ~ D L I ~ L, LINE SPEED - FT/MIN. Fig. 4. Transverse flux heating--brass strip. strip heating materials and processes. As a rule, both the coil efficiency and power factor are significantly higher for the nonferrous strip (such as aluminum or brass) using transverse flux versus longitudinal flux. This means lower operating energy costs in kw. hr/ ton. Since higher frequencies are generally required with longitudinal flux, the capital costs are normally higher as well. PROCESS CAPABILITIES Different materials require a variety of processes to provide necessary yield and ultimate tensile strength, ductility, and grain size requirements for subsequent operations. While solution heat treat processes require POWER KW 2500 A STAINLESS STEEL "F /~ B - ELECTRICAL STEEL "F. A, / C 430 STAINLESS STEEL "F. D LOW CARBON STEEL *F, ~ B BASEO :;T?s.,o o. / / / /, J,, i00 LINE SPEED - FT/MIN. Fig. 5. Transverse flux heating--annealing steel/stainless strip J. Heat Treating, Vol. 6, No. 1, 1988

5 PROCESS TEMP "F, BRASS 1000 / ~ A L U M I N U M 500 KW BRS/TON STEEL Fig. 6. Transverse flux heating--typical energy consumption. holding at temperature for a finite period, most heat treat processes of both ferrous and nonferrous products require very little time at temperature to produce similar characteristics to those obtained in long heat time batch furnace operations. However, similar to inductor tempering, most induction annealing operations require heating to a higher temperature to obtain comparable properties with extremely short heat times. In the case of steel, considerable work was conducted by Battelle under a program sponsored by the Electric Power Research Institute (EPRI). Here, transverse flux annealing was conducted on various grades of aluminum killed steel, nonoriented-grain electrical steels, and 304 grades of stainless steel. The results for the low carbon aluminum killed steels (heated to 1550~ indicated that the high speed transverse flux heating yielded similar results to conventional furnace heating but provided a finer grain size. On medium grade aluminum killed steels heated to the same temperature, the smaller grain size was accompanied by different mechanical properties. It therefore, required a brief soak time to permit grain growth and formation of course carbides. This could probably be accomplished by delaying any forced cooling of the strip and hot coiling to lengthen the overall cooling cycle. Both the electrical and nonmagnetic stainless steel responded well to short cycle transverse flux annealing. The electrical steel testing was conducted in the 1800 to 1900~ F range, and the 304 stainless tests at 1950 to 2250 ~ F. In both cases, the mechanical properties developed were at least as good as those obtained in conventional processing lines. Independent tests were conducted on type 430 (magnetic grades) stainless steel and type 301 (nonmagnetic) stainless steel. This testing was performed on 24 in. wide strip with thicknesses from in. to in. These tests were conducted to determine process sensitivities to heating rates, strip shape, and metallurgical properties. In all cases, comparable metallurgical properties to conventional heating could be obtained at 2000 to 2200 ~ F without any extended time at temperature requirements. In addition, the strip shape and temperature uniformity could be properly controlled so long as excessive power densities with foreshortened coil lines were not employed. Tests conducted on 70/30 brass strip indicated that good annealing could be obtained by heating to about 1400~ F and then allowing slow cooling to ambient temperature. Similar results could be obtained for partially or fully annealing aluminum strip with transverse flux. Since aluminum has a number of alloys, various heat treatment processes are used depending on the alloy and the subsequent processing operation. Different levels of annealing may be applied to various grades of aluminum alloy. These include partial annealing that may be accomplished in the 600 ~ F range to full annealing that requires a temperature as high Table 3. Transverse Flux Strip Heating--Typical Power Requirements Based on 10 Tons/h Power Source Material Process Temp. Rating lb/kw 9 h Aluminum Sol. heat treat 1100 ~ F 2200 kw 215 Full anneal 800~ 1500 kw 150 Partial anneal 600 ~ F 1200 kw 118 Brass 70/30--Full anneal 1400 ~ F 1200 kw /40--Full anneal 1100 ~ F 900 kw 86 Partial anneal 800 ~ F 700 kw 62 Steel Low carbon anneal 1600 ~ F 2100 kw 204 Elec. steei anneal 1900 ~ F 2500 kw 245 Stainless steel 430 anneal 1800 ~ F 2400 kw anneal 2200 ~ F 3000 kw 285 J. Heat Treating, Vol. 6, No. 1,

6 as 850 ~ F to make the material dead soft. In most cases, the grain size is smaller due to the shorter heat times involved; however, comparable levels of strength and ductility are being obtained. Solution heat treating or precipitation hardening of certain types of aluminum alloys is possible; however, depending on the type of alloy, a holding time is generally required before the strip is quenched to ambient temperatures. In most cases, the required heat treating temperature is in the range of 1100~ followed by up to 2 min holding period prior to quenching. Since transverse flux can rapidly heat the strip to 1100 ~ F, it is ideally suited for a preheat operation, combining either an existing or new radiant fired furnace to hold the strip at temperature. Since a typical existing gas or electric fired furnace could have the capability of holding at temperature for a minute or more, it offers the potential of broader processing (an- nealing and solution heat treating) with a single heating line. SUMMARY Transverse flux induction heating of strip is an efficient approach to in-line process heating of ferrous as well as nonferrous products. Generally, comparable metallurgical results can be obtained by increasing the strip temperature to compensate for the shorter heat times involved in this process. Significant advantages can be realized due to much shorter line lengths, reduced energy and overall operating costs, and lower up-front capital equipment expenditures. The selection of the right type of power source, coupled with proper inductor design and precision process controls are key factors in providing the right installation for strip heat treating applications J. Heat Treating, Vol. 6, No. 1, 1988