History of Metallurgy and Induction Heating

Transcription

1 Practical Induction Heat Treating, Second Edition Copyright 2015 ASM International R.E. Haimbaugh All rights reserved Chapter 1 History of Metallurgy and Induction Heating THIS CHAPTER includes a brief history of metallurgy, followed by a discussion of the development of scientific theories involving the electrical nature of induction heating. The author incorporates personal experiences and memories throughout. History of Metallurgy The Egyptians are believed to have worked copper for centuries before 3500 b.c. A piece of heat- treated steel was found in one of the pyramids, and it is thought to date from 3000 b.c. Early metal workers found certain metals and ores could be refined, processed, and made into tools and weapons, but it was not until the Iron Age and the Hittites that metallurgical processes were developed that would consistently produce strong steel weapons. Although the art of metallurgy developed as early smiths found that heating and cooling iron in different ways could make iron either softer or harder, metallurgical theory lagged behind until relatively modern times. In 1864 Henry Clifton Sorby first used a microscope to study metals. This was followed by Albert Sauveur trying to convince American steelmakers that something practical was to be gained from microscopic examination. However, it has only been since about 1930, when x- ray diffraction with wave mechanics was applied to metals, that the science of metallurgy was born. The first induction phenomenon was observed by Michael Faraday in the middle 1800s when the effect that caused the heating of transformer and motor windings was considered to be undesirable. The first constructive use of induction occurred in 1916 when it was used to melt metals. Induction heat treating came into prominence in the 1930s, when high-

2 2 / Practical Induction Heat Treating, Second Edition frequency motor- generator sets were developed and used for the induction hardening of crankshaft journals and bearings. In 1938, Caterpillar installed a power supply for induction hardening track links, and by 1943 they had 16 induction- hardening units in production (Ref 1). In 1941 Vaugn, Farlow, and Meyer presented a paper titled Metallurgical Control of Induction Hardening at the convention of the American Society for Metals, which provided proof that alloy elements such as nickel and chromium were wholly unnecessary for maximum surface hardness and that carbon steels could be used in place of alloy steels. Caterpillar subsequently purchased a 500 kw, 9.6 khz motor generator set for induction hardening their final drive gear with a 642 mm (25.7 in.) diameter by a 125 mm (5 in.) wide face. In an article in the July 1943 issue of Metal Progress, the Caterpillar process for contour hardening this gear was presented. Figure 1.1 shows the contour pattern produced at that time by Caterpillar. Caterpillar must be considered the early pioneer in the contour hardening of gear teeth. Progress in research in metallurgical principles of induction hardening continued, and at the 26th annual meeting of the American Society for Metals in 1944, D.L. Marten and F. E. Wiley presented a paper that reported investigation of temperature, composition, and previous structure upon induction- hardening characteristics of plain carbon steel (Ref 2). The basic metallurgical theory as presented at that time is still being taught today. Fig. 1.1 Hardness survey (Rockwell C scale) of hardened tooth, sectioned on center. Magnified 2¾ diameters. Source: Ref 1

3 Chapter 1: History of Metallurgy and Induction Heating / 3 Induction Heating after World War II In 1946, Edwin Cady listed the basic types of induction equipment (Ref 3) with frequencies ranging from 25 Hz to 50 MHz: Electronic circuit (vacuum tube, 300 to 530 khz, and greater than 1 MHz) Spark gap (15 to 60 khz and 125 to 450 khz) Rotary converter (motor generator, 1 to 10 khz) Mercury arc (400 Hz to 3 khz) Standard power cycle (line frequency of 60 Hz, or 25 Hz as generated by some steel mills) While other types of power supplies and converters have been used over the years, the intent of this book is to discuss those commonly used for induction heat treating. The first induction heaters sold by General Electric during World War II had rectifier tubes for the conversion of the alternating current (ac) to direct current (dc). Output control and tuning were accomplished through a combination of different taps on the output of the tank coil and a control knob that tapped the plate transformer for different output voltages. The power was turned on and off through the use of a main solenoid- activated contactor. Cycle times were controlled through use of a mechanical, camdriven timer. The output was the high voltage obtained directly from the tank voltage, and low- turn work coils could not be used. Coil designs to heat small areas were developed in many creative ways. For example, to get around overloads, a shunt coil was used. This was a coil made of copper tubing that was placed directly across the high- voltage output from the tank circuit. The coils were typically wound to a 102 mm (4 in.) diameter; the number of turn shunts varied from 5 to 13. The shunts were water cooled, with the water coming from T connections on both sides of the shunt. While it seemed that all of the power would be lost in the shunts, they actually worked quite well. If power reduction to prevent a slight overload at the heat on position was needed, a 13- turn shunt could be installed and used. If the overload was severe, a 5- turn shunt could reduce the power substantially. Setup instructions would indicate what shunt was to be used. The shunts, when used properly, actually leveled the power output so that the plate amperage was held more constant. As the workpiece on heating passed through the Curie temperature, the plate amperage with the use of a shunt did not drop as much. The use and development of induction heat treating practices continued to grow after World War II, and output transformers were developed to help the power supplies and load match when using low- turn work coils. Around 1948, General Electric performed research on the optimum design

4 4 / Practical Induction Heat Treating, Second Edition for output transformers for radio- frequency (RF) induction heaters. They ran tests on both the size of the transformer (settling on 152 mm, or 6 in.) and the stepdown ratio (11:1 found to be most desirable). The primary was sealed in beeswax, and the secondary was one turn, water- cooled, similar to current output transformers. The first transformer tops for sealing and mounting the outputs were Bakelite, which was later replaced by Micolex. In the 1950s General Electric went to a 279 mm (11 in.) diameter transformer with a 7:1 stepdown ratio. Because of the output transformers single turn secondary, low turn work coils could now be used. In addition, the coils could be grounded on one side, so arcs that occurred with highvoltage coils were practically eliminated. From the 1940s through the 1950s, the use and application of large motor generators and RF oscillator induction power supplies continued. In the mid- 1950s General Electric introduced a new RF induction heater. It featured an aluminum- enclosed oscillator section; the internal bus components were silver plated. Rectifier tubes were still used for the dc conversion, but two of the models that were available had three triode rectifier tubes so that thyratron power controls could be used for stepless power control and for power turn- on. Two of the models featured an internal output transformer that was rigidly attached and had an air core. The output power ratings were proven through running a water load in a work coil. The induction heaters featured what was called a filament regulated transformer- capacitor network that provided about 3% regulation through swings of line voltage. Motor- driven voltage regulators could be furnished on request. The induction hardening of air- to- air missile fuse bodies was developed during the Korean War. These were the striking end of the air- to- air missiles. The bodies required a soft nose to allow collapse on impact, to explode the missile. The requirement was for the body to be hard but the nose totally soft. At that time induction tempering did not produce parts in specification. A 20 kw RF induction heater was used for austenitizing with direct tank- loaded coils (high voltage). The coils themselves were contoured to the shape of the fuse body, starting with 6.35 mm (0.25 in.) copper tubing at the top for about four turns, moving into 4.18 mm ( 3 /16 in.) copper tubing around the bottom to provide higher current concentration. The nose of the fuse body was placed onto a brass, water- cooled nest to prevent the nose from heating and hardening. Nitrogen atmosphere was used to prevent scale, with an austenitizing cycle of 9 s. The parts were oil quenched in mineral oil and then furnace tempered. The final surface finish was good enough that the fuse bodies could be plated afterward with any finish machining or polishing. About 1959, International Harvester (IH) found that customers who purchased new tractors were replacing the track shoe bolts with Caterpillar bolts before use. The Caterpillar bolts were induction hardened and did not wear. The IH bolts were overall hardened and tempered but did

5 Chapter 1: History of Metallurgy and Induction Heating / 5 not have contour- hardened heads. The heads of the bolts wore in use, and the bolts had to be chiseled out for replacement. A track shoe bolt fixture was designed and built to handle production for IH. A 50 kw RF induction heater could run 2,500 bolts/h for a 14.3 mm ( 9 /16 in.) diameter, 3,500 bolts/h for 12 mm ( 1 /2 in.), and 5,000 bolts/h for 11 mm ( 7 /16 in.). Case hardening of full length bars for a steel company was developed in the early 1960s. A horizontal scanning system for case hardening of 38 mm (1.5 in.) diameter, 3.8 m (12 ft, 6 in.) long, AISI 1045 steel bars was developed. The scanner had fixed nylon rollers for a distance of about 4 m (14 ft) on each side of the induction coil. A pusher was used to move the bars through the coil, with pneumatically activated restraint wheels on each side of the coil. When one bar was done, the restraint rollers would elevate. The hardened bar was lifted up by pneumatic lifts and moved to the side so that another new bar could be loaded. Then, pusher direction was reversed, and the quench solenoids were switched so that the quench was applied on the exit side of the bar from the coil. The bar was austenitized and quenched in the opposite direction. The two- direction hardening produced high productivity. Starter ring gear teeth were hardened for a locomotive manufacturer in the mid- 1960s. A fixture was built to induction harden the 256 teeth of a starter ring gear that was made by a local gear company. A 20 kw induction heater was used with a remotely mounted transformer. The transformer was indexed into the tooth for tooth- by- tooth hardening. After austenitizing when the transformer and coil quickly moved back, the gear was indexed into an oil spray quench. An entire tooth was skipped during indexing each heating and quenching cycle. In order to harden all the teeth, the gear rotated around twice. After the teeth were all hardened, the gear was cleaned and moved over to a 50 kw, 10 kw motor generator set and single- shot induction tempered. About five gears per day were run over a period of about five years (the production of locomotives at that time). The production stopped when the manufacturing was moved to Canada. Automotive wheel hubs were induction case hardened for a large bearing company. About 1990 a large bearing company that had been producing the hubs in Japan provided sample hubs for test. Tooling was developed, and tests were run using 10 khz. The pattern with 10 khz was better than the customer was producing in Japan. A decision was made to build a production line. At that time more research on frequency was done, with the decision made that 25 khz would be a better frequency. Automatic fixturing was built using a 150 kw, 25 khz power supply. At that time little research had been done on induction tempering, so the decision was made to furnace temper after induction hardening. This line was in 8 to 16 h production for about 20 years. About 1999, a design of parking pawls needed to have the opposing faces case hardened. The parts were to be made of AISI 5160 steel that were Carbo- Austempered prior to induction hardening. Because the pawl

6 6 / Practical Induction Heat Treating, Second Edition was considered a safety item, there were strict requirements on case depth at both faces of the pawl. The pawls quenched to a hardness of 66 HRC. This was followed by a 160 C (325 F) furnace temper. One nick on these parts could cause the transmission to jam. The tooling was built to work using a four- position dial fixture, heating both faces simultaneously, with automatic activation of position holddown and subsequent unloading after quenching. Since there were four nests, one part from each nest was cut every 4 h. This job ran for about 10 years until there was a redesign of the pawl by the automotive company using the pawl. In the 1960s solid- state power supplies were invented for the conversion of line frequency into medium- frequency induction heating. Because of their higher efficiency and increased versatility as the reliability of the solid- state power supplies increased, solid- state power supplies started to replace motor- generator sets in the 1970s. The continued development trend of solid- state power supplies has been into higher frequencies as the solid- state devices have continued to increase in current- carrying and voltage- blocking characteristics. Today, while there is still a market for RF oscillators, most of the induction heating equipment sold is solid- state. Solid- state has made even the RF oscillators more efficient through replacement of the tube rectifiers by solid- state diodes. Development of better transformers for load matching will increase the potential use of solid- state RF power supplies. If the past is used as a basis for projection into the future, the probability is that transistors will continue to improve and at some time will convert the frequency for all RF power supplies. Advantages of Induction Heating Induction heating has the ability to rapidly heat specific areas of a part, such as the teeth of a gear or the bearing area of a shaft. Not only can superior mechanical properties be produced in such an area, but also the entire part does not have to be heated as is done with furnace heat treating. Significant benefits are produced, such as: Superior mechanical properties: A hard case and a soft core provide a good blend of strength and toughness not attainable with furnace through heating. Furthermore, because the hardness of as- quenched steels depends only on carbon content, carbon steels can be used instead of alloy steels for most applications. Induction- hardened tractor axles have a significant increase in bending fatigue over axles that are conventionally furnace hardened. Axles and shafts are also induction case hardened to produce high torsional strength, and many parts such as gears are selectively induction hardened to provide wear resistance on the gear teeth. Lower manufacturing costs: Total energy costs can be reduced because the entire part does not have to be heated. The costs of other

7 Chapter 1: History of Metallurgy and Induction Heating / 7 processes that are necessary for furnace- hardened parts are reduced because the lower distortion reduces the need for grinding and finishing for final net shape. Straightening can sometimes be eliminated. Manufacturing compatibility: Induction heat- treating systems can be automated for high production requirements and can be incorporated into manufacturing cells. Floor space requirements are reduced, and the workplace operating environment is improved. The development and use of solid- state power supplies for induction heating continue today in all frequency ranges. There are many different applications for induction heating outside the heat- treating area, but only heat treating of steel is discussed in this book. There are many other historic specific terms used for induction heating power supplies, such as converter, inverter, motor generator, vacuum tube oscillator, spark gap generator, and frequency tripler. These frequency converters change the 50/60 Hz line frequency to higher frequency. This book is concerned with the application of power supplies that are most commonly used in induction heat- treating practice that change or convert three- phase, 50/60 Hz line frequency into single- phase high frequencies above 3 khz. While there are installations and systems that use frequencies below 3 khz for heat treating, they relate to a smaller number of specific installations rather than wide and varied commercial use and deal with dedicated, through- heating type applications such as the heat treating of pipe. Induction is also used widely for forging, melting, and a good number of individual applications. For purposes of this book, the terms induction heater and power supply will essentially mean the same thing: An induction heater is a power supply that produces the high frequency for induction heating. References 1. G.C. Riegel, Casehardening Large Gears with High Frequency, Met. Prog., July 1943, p D.L. Marten and F.E. Wiley, Induction Hardening of Plain Carbon Steels: A Study of the Effect of Temperature, Composition, and Prior Structure on the Harden and Structure after Hardening, Transactions of the ASM, Vol 34, 1945, p E. Cady, Induction Heating, Materials and Methods, Aug 1946, p 401