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).
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.
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:
Heat Treatment Equipment:
An entire system can be programmed to do several types of jobs:
Program a target temperature and holding time.
Program the dehydrogenation temperature, the holding time, the temperature and the cooling speed in degrees / hr.
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)
For scheduling treatments other than the above: pipe thawing, experimental treatments, etc.
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.
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.
The key benefits that are obtained immediately compared to traditional heating methods such as Flame, Furnace or Electric Resistance are:
Preheating of the joint to be welded in a large diameter reactor vessel (5 m) and wall thickness (250 mm)
Preheating of joint to be welded in gas / oil pipeline manufacturing
Local and controlled Thermal Post-Treatment, without affecting welds that do not require re-treatment
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
Local heat treatment of welds
Preheating and dehydrogenation of thick plates of carbon and low-alloy steels with a high yield strenght