Template:MECH370

Introduction

^W uses a magnetic field generated by an electromagnet with an AC current passing through it. The fluctuating magnetic field induces a current in the magnetically conductive specimen, which increases temperature due to ^W from the specimen's internal resistance. The temperatures reached can be precisely controlled by limiting several variables.

^W is a form of thermal processing commonly used to produce the ^W desired. The motivation for producing a unique microstructure is to relieve mechanical stresses due to plastic deformations in the microstructure which increases softness, ductility and toughness of the specimen ^[1]. Annealing is a common metallic processing operation, and can be accomplished quickly, easily, with a high degree of repeatability and cost effectively using an induction heating operation.

Background Information

Annealing

There are three steps involved in any annealing process:

• Heat up: increase temperature at a prescribed rate to the annealing temperature.
• Soak: Specimen is left at annealing temperature until desired microstructure is obtained.
• Cool down: Usually back to room temperature.

The rates and duration of each step can be varied to produce vastly different structures and physical properties from the same material specimen. All variables (i.e. time, temperature, pressure, atmosphere, etc.) must be carefully controlled for the entire process to achieve the desired final product. Also, there is no single best combination of these variables; they must be optimized for each specific process.

The elevated temperatures of the annealing process increase atomic mobility and relax the crystal lattice. This permits the diffusion of atoms and defects through the lattice increasing grain size which reduces toughness and ductility while increasing softness, and reducing total defects while combining those that remain in their lowest energy configurations. The resulting microstructure is one that is easier to work on from a production or manufacturing perspective (i.e. easier to machine, stamp or roll; less tool wear, etc.)

Induction Heating

There are three components to an induction heating apparatus:

• High-frequency AC source: Typical frequencies are often between 5-30kHz for thick materials or deep penetration, while 100-400kHz are used for thinner parts or shallow penetration (i.e. surface treatments; case hardening) ^[2].
• Work Coil: Also known as the induction coil, a magnetic field is generated in its center when AC current is passed through the conductive work coil.
• Work Piece: Placed in the center of the work coil, the work piece must be electrically conductive or else no heat will be generated.

When a high-frequency alternating current is passed through the working coil of the apparatus, a magnetic field is generated, flowing through the center of the working coil. This magnetic field in turn induces a current to flow through the electrically conductive work piece which is to be heated. The simplicity of this procedure contributes to its wide applicability, including: surface heat treatments; sterilization of medical and other equipment; hot forming; brazing; curing; shrink fitting.

Heat Generation

The combined work coil and work piece essentially compose a typical ^W. ^W are generated in the work piece (secondary coil) by the magnetic field from the work coil (primary coil), but since the work piece is short-circuited these currents are dissipated by joule heating, resulting in a rapid temperature increase.

Typical in any electrical conductor, current is forced to flow through a thin layer at the surface of the part, creating the ‘skin effect’ which is responsible for some heat generated in the specimen ^[3]. Since the pathway through which the current is permitted to flow is restricted, the resistance is greatly increased, leading to greater joule heating and higher temperatures.

For ferrous materials, the alternating eddy currents continuously magnetize and de-magnetize iron atoms in the crystal lattice. This constant switching generates relatively large quantities of heat due to the friction losses associated with these switching motions. Heating by this mechanism is called hysteresis heating as the energy lost (irreversibly) is due to loading and unloading of the magnetic properties of individual atoms. This process is present up to approximately 700ᵒC (the Currie temperature) in steels, beyond which steel looses its magnetic properties ^[3]. Beyond this temperature, only eddy currents and the skin effect are responsible for heat generation.

It is worth noting that while there are several similarities between an induction heating apparatus and a typical transformer, the distinct difference between the two is what gives an induction heater its function. Numerous steps are taken in transformer design to reduce the heat generated in the core(s) which represents lost electrical power, or wasted energy. Compare this to an induction heater where the thermal energy is used for joule heating and any remaining voltage at the end of the circuit is essentially wasted energy (i.e. not dissipated as heat), from the thermal processing perspective.

Susceptor Heating

One of the requirements for induction heating is that the specimen to be heated is electrically conductive, which excludes many materials such as polymers and ceramics. To combat this fact, a susceptor, or metallic object, may be added to this non-conductive object to permit this type of heating. For example, a polymer to be melted could be placed in a metallic container, and the temperature of the metallic container be raised to the melting point of the polymer. The container is heated by induction while heat is transferred to the polymer by conduction. Typical susceptor materials include graphite, molybdenum, silicon carbide, stainless steels, niobium, aluminum and virtually any other conductive materials. An appropriate susceptor material is selected for each application, and is decided as a function of cost, thermal requirements, reusability and other factors ^[2].

Electrical Components and Considerations

The electrical control and power-delivery systems involved in an induction heater can be rather complicated, due to the current carrying capacities of various components. An inductor-capacitor circuit (LC-circuit), or resonant circuit, is used to ensure a sinusoidal function of a specific frequency is connected to the work coil, improving the electrical efficiency of the inverter. This circuit may be connected in either of two ways to the inductor coil:

• Series Connection: Connecting the LC-circuit in series with the working coil produces the desired sinusoidal output signal at low currents. This means that many coils of the working coil are required to achieve a strong magnetic field and thus high induced currents and temperatures. The heating power builds due to the resonant characteristics of the circuit which increase voltage over time. This requires a close proximity between the inverter and working coil to keep losses in the remaining electrical components to a minimum. This circuit is shown in Figure 1.
• Parallel Connection: Connecting the LC-circuit in parallel with the working coil also produces the desired sinusoidal output signal, but at higher currents for the same amount of losses (i.e. same electrical efficiency). In this configuration, shown in Figure 2, the capacitor takes some of the current while leaving a large voltage drop over the inductor, reducing the current through the inductor while keeping the same inducting power. This also greatly reduces losses in the inductor, permitting the inductor and signal source to be farther away from each other.

There are several other properties of the electrical system which must be optimized, and are done so based on laws which govern electrical systems. These include: impedance matching of components between inductor and the work coil to evoke the highest possible current in the work piece while maintaining a low current (and high voltage) in the inductor; built-in safety and back-flow current protection to protect other sensitive electrical equipment; methods of power control. All of these electrical production and control elements of the system are beyond the scope of this paper.

Increasing Efficiency

Impeder

Korean researchers conducted a three-dimensional analysis of high-frequency induction welding (HFIW) processes which included electromagnetic activities and the resulting eddy current distributions, joule heating, thermal conductivity and temperature distribution ^[4]. The analysis concluded that by including an impeder – a solid plug with low conductivity inserted into the hollow centre of the shaft to be welded – eddy currents can be focused at the surface of the work piece, increasing local temperatures. A typical impeder material is ferrite, due to its low conductivity and relatively low cost and high availability. The lifespan of the impeder is another important concern; how many times can it be repeatedly used? If it contacts molten metal (reasonably likely in many of these high-temperature applications), the low conductivity can be ruined, the strength of the impeder is compromised in the direction of the molten metal resulting in a lower local eddy current density and temperature, defecting the work piece and warranting the discard of the impeder.

This research further yielded the following relationships between the various parameters and the consequential effects on the work piece ^[4]:

• Both eddy current density and surface temperatures increased significantly with the inclusion of an impeder. Also, larger impeders had greater effects on eddy current density at the surface of the material, however if the impeder was too large and contacted molten metal it was to be discarded.
• Using a higher frequency AC source increased temperature, noted by a greater production of molten metal.
• Higher temperatures were reached for lower rates of motion of induction coil relative to work piece.
• Eddy current density and temperature were found to be maximized in thinner-walled tubes (i.e. smaller specimens) and peaked specifically at the point on the work piece coinciding with the centre of the work coil.

Testing Apparatus

It is often desirable to test the effect of multiple inputs to an object which operates under complex conditions. For example, creep-fatigue testing is often used on turbine blades which operate at high temperatures and often under high mechanical loads due to pressure gradients, high angular velocities and hydrodynamic forces. An accurate, repeatable way to thermally affect the test specimen is through induction heating ^[5]. Low energy requirements, the avoidance of a complex conduction apparatus and an increase in heating efficiency of specific regions of the blade are several of the advantages inherent in induction heating which thermal testing capitalizes on. The unobtrusive apparatus is easily installed and leaves plenty of viewing room to observe the affect of heat and other forces on the specimen as they occur (as opposed to being tested in an insulated oven, etc.).

Induction Heating with Semiconductors

As recently as 1990, semiconductors have revolutionized the induction heating industry ^[6]. Instead of the magnetic field alternating as in a typical induction heating system (caused by AC current flowing through inductor coil), the work piece is rotated in a static magnetic field developed when DC current passes through a semiconductor cooled to approximately 30 K. Eddy currents are only generated in the work piece when the magnetic field varies relative to the orientation of the work piece so for a static field, the part must be rotated. The advantages to this technology are smaller thermal gradients, greater penetration by eddy currents means more consistent cooling from surface to core, very reproducible thermal profiles and point temperatures from one work piece to the next, increased durability of the magnetic field source since it is not moving and is not exposed to high temperatures generated; all while reducing the adjustment required from one work piece to the next and using less energy. Some of the negatives associated with this technology is a lower maximum temperature, since it is limited by the rotational speed of the work piece; balancing of the rotating part must be done; more difficult to work on two (or more) rotating pieces for processes like welding or brazing.

In conclusion, using semiconductors for induction heating processes increases operational efficiency to ~80% (from ~50% for typical AC induction heating processes), but limits the flexibility of the operation. Also, savings at initial investment are sacrificed for lower operating costs over the lifespan of the equipment.

Potential Problems in Annealing and Induction Heating

Thermal Stress or Thermal Shock

Researchers at the Innovative Nuclear Fuel Division of the Korea Atomic Energy Research Institute discovered that heating and cooling rates as well as physical dimensions of a specimen contribute to the strength of the material against thermal loadings. As the rate of temperature increase increased, larger differences in temperature between the surface and bulk or core of the specimen were noted. In extreme cases (heating rates of 442 K/min), cracks were observed to penetrate clear through a uranium dioxide (UO₂) specimen due to the large thermal gradients established by high heating rates. Again, the skin effect is a significant contributor to the high local temperatures at the surface which establish these thermal gradients.

Oxide Layers

Metallic oxides and oxide layers which cover many specimens serve as an electrical insulator at room temperatures. For many specimens, this means that currents are difficult to induce in the skin layer and induction heating takes a great amount of time and energy to initiate. To combat this, the specimen may be heated through a mechanism known as ‘skull melting’ involving a direct connection between the oxide layer and a high temperature source. The high-frequency electric field is present externally from this apparatus, and assumes heating responsibilities once the temperature of the specimen is high enough that the oxide layer can be penetrated by a high-frequency magnetic field.

Heat Affected Zone

Often accompanying a welding procedure is analysis of the heat affected zone (HAZ), or region surrounding that to which heat was added. A similar process occurs in induction heating. The potential issue associated with the HAZ is the uncertainty associated with the resulting microstructure. Because of the accuracy achievable with induction heating, a specific area can be targeted for heat addition for a brazing, welding, annealing or other such process requiring heat. The remaining microstructure (taken a sufficient distance from the region of direct heat addition) remains unchanged, but the region immediately around the zone of heat addition is slightly altered, and represents microstructural uncertainty. The factors affecting the size and effect of the HAZ include ^[5]:

• Rate of heat input and cooling (faster rates of heating and cooling decrease size of HAZ)
• Maximum temperature reached in zone
• Metallurgical factors such as pre-existing dopants or alloying elements, grain size, plastic deformation of microstructure, etc.
• Physical properties including thermal conductivity and heat capacity of metal.

Since induction heating focuses the thermal energy over such a small volume of material for a relatively short period of time (usually on the order of seconds, but may be as long as a few minutes), the HAZ is slow compared to other processes. This permits a specimen to achieve very different properties at different locations, with little to no affect to the other regions.

Applications of Technology

Because of the relatively simple apparatus required, few stipulations on properties the work piece must possess and nearly infinite range of operating temperatures and heating/cooling rates, the list of applications for this technology is very long. Several of these processes are listed:

• Annealing: Using induction heating to increase the specimen to the annealing temperature can be done quickly and efficiently.
• Shrink fitting: Using induction heating to soften one component and forcing the other into the hole provides a secure, tight fit between components.
• Thermal Sealing: Heating low-temperature glue on the back of a foil seal in the lid of a medicine bottle melts the glue, sealing the bottle.
• Sintering Ceramics: Placing a ceramic powder in a susceptor to be heated via induction is an efficient way to transform a green compact to a sintered specimen ^[7].
• Increase Specific Energy Delivery: Higher energy densities can be achieved using induction heating compared to gas-fired furnaces or conduction, improving yield in processes like silicon production ^[8].
• Zone Purification: In semiconductor industry, silicon is purified by moving section of molten metal through the bulk. This can be done by moving bulk through inductor coil.

Contact details

This article is a work-in-progress, and will be continually updated as I develop the topic and scope further. If you have any information on this subject, or can recommend any resources please leave a note on the discussion page of this article. Thank you and I hope this article is of great use to you and your process.

References

<layout name="Project" />