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INDUCTION HEATING

 

Induction Heating. Principles and Application

The phenomenon of heating a metallic material by applying high frequency Eddy currents is not something new and the physical principles and their possible applications have been known for decades. There is documentation of practical application of this principle in the 1920s for hardening of metals in the manufacture of engines.

Induction Heating is a Non-Contact method that uses conductive electrical materials that heat materials sensitive to this phenomenon that are arranged so that the induced electrical current is efficient in raising the temperature to the desired levels. Using high frequency, the alternating current in the coils of the conductive material creates a very fast alternating magnetic field. This magnetic field passes through the material to be heated creating a current flow (Eddy currents) in that material; heat is generated due to the resistance of the material to the flow of the eddy current (loss of I 2 R).

Calentamiento por inducción

More recently, with the need to improve productivity and costs as well as the quality of the products, technology has produced devices based on these rediscovered principles for industrial and domestic applications that contribute to the development that we have been experiencing for a long time. What makes this heating method so effective is that instead of using an energy source that heats the component from its outer surface, like more traditional methods, flame or incandescent resistor elements, it is the component itself, the one that is actually “induced” when the electromagnetic current applied to it circulates through, with which said component “self-heats”.

When exposing the operation of heating equipment based on the physical principle of Eddy Current Induction, one still thinks of the more traditional processes that require the physical contact of the hot elements with the materials to be heated. On the contrary, the intrinsic differences of both processes are those that suppose the great advantages that in welding are obtained when applying this principle of Induction Heating.

Description of Necessary Equipment

The basic equipment is, today, relatively simple. Basically, a source of high-frequency alternating current is required and elements for conducting said current that can be arranged on the component that needs to be heated or treated effectively.

It should be taken into account that a wrong configuration or an inappropriate choice of the elements that make up the necessary equipment, can result in an undesired temperature distribution and, therefore, a significant or even catastrophic detriment to mechanical or dimensional properties of the component in question.

For the application of the process, it is necessary to have the necessary knowledge of the effects of temperatures on the materials and experience on the most appropriate configuration for each application, or otherwise, it will result in a very risky practice.

In industrial applications, perhaps due to the means available at all times, users are not shocked by practices that would be unthinkable in domestic applications, although the effects are equally comparable. Nobody would think of roasting a food by putting it directly over a flame or over a red resistance: its surface would be completely scorched while the inside would be completely raw. If this is so obvious, how do we heat metallic materials, knowing the effects that temperatures and heat treatments cause on their metallographic structures and consequently on their properties, using techniques that we would never use with food?

A possible way of classifying induction heating equipment is according to the desired temperature of the components to be heated and, therefore, the type of heat treatment that is carried out. Thus, we can have:

 

Preheating Equipment:

  • For temperatures in the components to be heated up to 204 ° C (400 ° F)
  • Air cooled systems

Heat Treatment Equipment:

  • For temperatures in the components to be heated up to 788 ° C (1450 ° F)
  • Water-cooled systems, or also air-cooled, providing adequate insulation to the cables
  • They can also be used for preheating at lower temperatures
  • For dehydrogenation treatments
  • For Stress Relieve and PWHT treatments
  • For fitting / unlocking components in assembly by means of homogeneous expansion
  • And many other applications
Calentamiento tubo en carga

An entire system can be programmed to do several types of jobs:

  • Preheating:

Program a target temperature and holding time.

  • Dehydrogenation:

Program the dehydrogenation temperature, the holding time, the temperature and the cooling speed in degrees / hr.

  • PWHT:

Program the temperature rise ramp in degrees / hr

Treatment temperature and holding time

(for cooling, if not programmed otherwise, the equipment will automatically use the same ramp)

  • Free Program:

For scheduling treatments other than the above: pipe thawing, experimental treatments, etc.

Curvas calentamiento por inducción

Treatment example showing the temperature variations indicated by 4 thermocouples during several working hours. It is virtually impossible to achieve such equal graphs of each thermocouple at different positions on a component with traditional techniques.

Clamp for rotating components

Precalentamiento virola en rotación

The fact that induction heating does not require contact of the heating elements with the component that is being heated, allows designing configurations so that the transmission of induction currents is carried out in moving components.

The most common and useful application in the manufacture of metal components of a cylindrical nature, such as vessels or pipes, which can heat up as the joints are welded in the most favorable positions.

Advantages of Induction Heating

The key benefits that are obtained immediately compared to traditional heating methods such as Flame, Furnace or Electric Resistance are:

  • The energy hardly reflects on the surface that is heated. This supposes a minimum loss of energy and heat and therefore a maximum effectiveness of the energy that is applied in heating
  • Concentration of energy in the induced zone. With this, the heat is applied in a controlled and local way exclusively to the area that is desired. The zone affected by said heat and therefore susceptible to transformation of unwanted metallographic structures is minimal. Another advantage associated with this phenomenon is that components that contain flammable elements or that are not desired to be heated to said temperatures do not reach hot or risky temperatures, if they are not susceptible of being induced with the applied current.
  • Welders and operators can be in direct contact and touch the cooled cables through which the heating is carried out, which is immediately reflected in:
    • An increase in the productivity, comfort and safety of the welders,
    • The use of combustible gases and their effects are eliminated,
    • The risk of electrocution and fire is substantially reduced, and, fundamentally,
    • Uniform, controlled and extremely rapid heating is achieved throughout the thickness of the material being heated.
    • When a proper setup is made and the right equipment is chosen, the process is highly repetitive and controllable.
    • Adaptability: It can be used on elements with a multitude of geometries.
  • Homogeneity of the heat map: the temperature differences throughout the material thickness and along the surface that is heated is minimal from the beginning of the treatment. Using other techniques such as flame, resistance, furnace, it must be expected that the heat diffuses through the thickness of the material, with which the most superficial areas are subjected to high temperatures for much longer until the interior also reaches the desired temperature .
  • Quickness in achieving the desired temperature, choosing the equipment or combination of equipment that provides the necessary energy according to the dimensions of the component to be heated and its thermal insulation.

Application examples

Precalentamiento y postratamiento por inducción

Preheating of the joint to be welded in a large diameter reactor vessel (5 m) and wall thickness (250 mm)

Precalentamiento soldadura de tubo

Preheating of joint to be welded in gas / oil pipeline manufacturing

Post tratamiento térmico local con inducción

Local and controlled Thermal Post-Treatment, without affecting welds that do not require re-treatment

Precalentamiento local con inducción

Heating of components with complicated geometries for heating joints to be welded without affecting the welder

Preheating of circumferential and longitudinal fillet welds for joining tiles, way-tees or pipe conduit accessories during full service operation

Permanent full operation pipeline repair
Permanent pipeline repair during full operation
Permanent repair welding on pipeline

Local heat treatment of welds

 

Preheating and dehydrogenation of thick plates of carbon and low-alloy steels with a high yield strenght

Calentamiento por inducción para soldadura

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