Limited Induction Heating Glossary
Induction heating
Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater (for any process) consists of an electromagnet, through which a high-frequency alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses in materials that have significant relative permeability.
IGBT
The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners, and even stereo systems with digital amplifiers. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters.
The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amps with blocking voltages of 6,000 V.
Skin effect
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.
Forging
Forging is the term for shaping metal by using localized compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is done at a high temperature, which makes metal easier to shape and less likely to fracture. Warm forging is done at intermediate temperature between room temperature and hot forging temperatures. Forged parts can range in weight from less than a kilogram to 170 metric tons.Forged parts usually require further processing to achieve a finished part.
Annealing
In metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the re-crystallization temperature and maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.
In the cases of copper, steel, silver, and brass this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool slowly. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming. It also presents no problem with decarburization.
Quenching
In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.
Melting
Melting (sometimes called fusion) is a process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased (typically by the application of heat or pressure) to a specific temperature (called the melting point) at which it changes to the liquid phase. An object that has melted completely is molten.
Ordinarily, the melting point of a substance is a characteristic property. The melting point is equal to the freezing point. However, under carefully created conditions supercooling or superheating past the melting or freezing point can occur. Water on a very clean glass surface will often supercool several degrees below the melting point without freezing. Fine emulsions of pure water have been cooled to -38 degrees celsius without the nucleation of ice taking place.[citation needed]. The change is due to fluctuations in the properties of the material. If the material is kept very still there is often nothing to triger this change, such a physical vibration and supercooling (or superheating) can occur. Thermodynamically, the supercooled material is unstable with respect to the frozen phase. This phenomena is similar to hysterisis in permanent magnets, as they are heated and cooled near the Curie point.
The thermodynamics of melting
From a thermodynamics point of view, at the melting point the change in Gibbs free energy (ΔG) of the Material is zero, but the enthalpy (H) and the entropy (S) of the material are increasing (ΔH,ΔS > 0). Melting phenomenon happens when the Gibbs free energy of the liquid becomes lower than the solid for that material. At various pressures this happens at a specific temperature. It can also be shown that:
The "T","ΔS", and "ΔH" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting.
Curing
Induction heating can be used to cure organic coatings on electrically conductive materials by delivering rapid heat into the substrate. This technique is especially beneficial in that the curing occurs from within (induction heating induces current directly into a substrate), which tends to minimize coating defects created by the bubbling effect of heat flowing from the outside into the substrate. Because induction heating can heat as fast as required, it can be used in continuous production environments such as curing paint on sheet metal.
Bonding
Induction heating can deliver rapid localized heat to the area that is to be bonded. A typical bonding application makes use of thermosetting adhesives to create the bond between metal and any number of materials. Induction bonding is used heavily in the automotive industry. Other types of bonds can include; plastic to metal, plastic to plastic (using metal gaskets), glass to metal and rubber to metal are also common. Induction bonding processes are limited only by ones own imagination.
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of ferrite and cementite. Precipitation hardening alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal. Tempering is accomplished by a controlled reheating of the work piece to a temperature below its lower critical temperature.
The brittle martensite becomes strong and ductile after it is tempered. Carbon atoms were trapped in the austenite when it was rapidly cooled, typically by oil or water quenching, forming the martensite. The martensite becomes strong after being tempered because when reheated, the microstructure can rearrange and the carbon atoms can diffuse out of the distorted BCT structure. After the carbon diffuses, the result is nearly pure ferrite.
In metallurgy, there is always a tradeoff between strength and ductility. This delicate balance highlights many of the subtleties inherent to the tempering process. Precise control of time and temperature during the tempering process are critical to achieve a metal with well balanced mechanical properties.
BRAZING & SOLDERING
Induction brazing and soldering are processes that take advantage of induction heating's ability to deliver rapid localized heating to a particular region of a part. It is not often necessary or desired to heat the entire part when brazing or soldering. Unlike furnace soldering or brazing, induction heating allows the user to deliver the heat only where it is needed within close proximity to the joint. During the soldering or brazing process, on like materials, the narrow region on each of the two parts to be joined are brought to temperature at the same time. Induction heating delivers rapid localized heat, which can minimize oxidation and reduce cleaning after joining, especially when rapid cooling is utilized. Because induction heating can be localized, it often eliminates distortion and other undesirable metallurgical changes in other regions of the part -- especially in higher temperature brazing applications. The ability to deliver localized heat helps induction heating to produce neat and clean joints by keeping the alloy from flowing in areas that it shouldn't. This ability to create clean and controllable joints is one of the reasons that induction soldering has been applied extensively in the electronics and electrical industries. Since most of the variables are controllable (i.e., parts handling and fixturing, amount of alloy, time cycle, etc.), induction brazing and soldering can often eliminate the need for skilled operators as well as produce high quality repeatable parts.
Repeatability is maintained if the following parameters are followed:
All parts are positioned in the induction coil in a consistent manner.
The amount of filler metal is controlled
Insuring the integrity of the part to be joined is not altered; induction heating will produce consistent reliable parts time and time again.
For the most part, induction brazing and soldering is done in an open-air environment but it can also be done in a controlled atmosphere when necessary to keep the parts completely clean and free of oxidation. Dissimilar metals can also be joined by induction heating but they do require special attention and techniques to bring the parts to temperature at similar rates due to differences in the materials resistivity, relative magnetic permeability and coefficients of thermal expansion. Induction brazing and soldering is especially beneficial from both an economic and practicality standpoint when a scenario exists whereby there is medium to high production runs of same or similar parts. In this scenario, it would be typical but not mandatory, to utilize some sort of semi or fully automated parts handling system to maneuver and place the parts into the induction heating coil in a rapid and consistent fashion. Although automation of the process is often desirable, it is more common to have an operator running the soldering/brazing station. While most induction brazing and soldering processes utilize the power supply's internal timer to control cycle times, temperature control feedback may be utilized as well by the use of thermocouples, IR pyrometers or visual temperature sensors.
Cooling towers
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling.
INDUCTION HEAT TREATING
Heat-treating is basically the controlled heating and cooling of a metal or alloy in order to obtain a set of desired metallurgical properties. Induction heating has many advantages in heat-treating applications over methods that involve heating devices such as furnaces and ovens. Induction heating delivers rapid heat from within the part versus the slow heat delivered from the outside of the part inward, via radiation and convectional heat transfer in alternative heating methods such as ovens and furnaces. The higher speeds of induction heating can lead to higher productivity as well as a reduction in surface oxidation and scaling. Induction heating can often improve component quality and reduce scrap due to the real-time nature of an induction heating process. By having parts come off in a continuous fashion versus a batch method, quality issues can be caught and corrected before a complete batch is manufactured. Economic benefits can be gained with induction heating due to reduced floor space, energy efficiency, high reliability, instant on characteristics (no warm up time), and the previously mentioned reduction in scrap. The localized heating benefits of induction heating often come in to play with heat-treating as well. Typical Induction heat-treating applications include; Hardening, Annealing, Tempering, and Stress relieving.
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