How Does an Induction Furnace Work?

An induction furnace is an energy-efficient and clean-melting system designed to process a wide range of metals, including steel, iron, copper, zinc, and aluminum.  

Unlike arc-based furnaces, induction furnaces allow precise heat regulation, making them ideal for preserving valuable alloying elements. Additionally, they produce minimal waste and pollutants, helping to reduce the overall carbon footprint.  

Refractory materials play a crucial role in the operation of an induction furnace, forming a protective barrier between the molten metal and the furnace's induction system. Saint-Gobain Performance Ceramics & Refractories provides high-performance refractory materials that safeguard the furnace coil from exposure to ferrous and non-ferrous molten metals at extreme temperatures. This protective lining is essential for resisting intense physical, thermal, and chemical stresses during operation, ensuring long-lasting furnace performance.

Principles of an Induction Furnace  

Faraday's Law explains how a magnetic field interacts with an electric circuit to produce electromotive force, a process known as electromagnetic induction.  

In an induction furnace, when a conductive material, such as the charge or load, is placed within an alternating magnetic field, an electric current is induced. As this current flows through the material, it encounters resistance, resulting in the dissipation of energy in the form of heat through the Joule Effect.

Types of Induction Furnaces

There are two types of induction furnace, each fulfilling particular requirements:

Core-Type Induction Furnace  

A core-type (or channel) induction furnace operates on the transformer principle, using mains frequency to transfer electrical energy between two alternating circuits. In this setup, an alternating current flows through a primary coil that surrounds an iron core.  

The molten metal, acting as the secondary coil, surrounds both the core and the primary coil. As current flows through the primary coil, it induces a larger current in the secondary, generating heat through the Joule Effect.  

To ensure maximum power transfer between the primary and secondary coils, the magnetic flux is concentrated in the metal loop, achieving up to 98% power efficiency. However, a drawback of the channel furnace is the need to continuously maintain the metal loop, preventing it from cooling.  

Core-type induction furnaces are well-suited for high-throughput, high-production operations that require minimal alloy changes and continuous casting schedules. Due to stable operating conditions and temperatures, the refractory linings can last for years, enduring thousands of operations. These furnaces are commonly used to melt single alloys such as iron, aluminum, zinc, and copper.

Coreless-Type Induction Furnace  

A coreless-type induction furnace operates using transformer action but does not rely on a secondary coil. Instead, it features a water-cooled copper primary coil that surrounds a crucible or refractory lining containing the metal charge. The primary coil induces a current directly into the metal charge, generating heat.

One of the major advantages of the coreless induction furnace is its ability to be shut down and restarted cold as needed, making alloy changes much simpler. It also offers versatility, as it can operate across a wide frequency range, from 50 Hz to 10 kHz, providing unmatched flexibility.

However, without a flux-concentrating core, the power efficiency of a coreless furnace is lower compared to a channel furnace, with an approximate efficiency of 75%. Smaller coreless furnaces, however, can operate at higher frequencies, maintaining efficiency and promoting a strong stirring action within the molten metal.

While the frequent alloy changes and the cooling and heating cycles can reduce the refractory lining's lifespan, coreless furnace linings are much cheaper and quicker to replace, making them a cost-effective solution in many applications.

Refractory Lining in an Induction Furnace  

Refractory linings are crucial to the efficient operation of an induction furnace, ensuring optimal metallurgical performance. As a consumable material, the refractory lining gradually degrades throughout the furnace's operational life, known as its lining life. This lifespan is determined by the number of heat cycles the lining can endure before requiring repair or replacement.  

A stable refractory lining is expected to maintain several key properties, including:  

- Low thermal conductivity  

- Resistance to corrosion  

- Resistance to thermal shock  

- Ease of installation  

- Ease of maintenance  

The suitability of a refractory lining depends on factors such as slag formation, operating temperature, and furnace capacity. The type of slag produced during operation significantly influences the choice of refractory material. For example, silica is ideal for acidic slags, magnesia is suited for basic slags, and alumina works well with neutral slags.  

Other factors that influence refractory material selection include:  

- Scrap quality, size, and shape  

- Choice of alloying agents and additives  

- Holding time  

- Melting temperature  

Applications of an Induction Furnace  

Induction furnaces are widely used in foundries, metalworks, and various other industries for a range of applications, including:

- Alloy Manufacturing: Induction stirring ensures optimal uniformity and flexibility in the production of alloys.  

- Aluminum Melting: Induction melting reduces energy consumption in the process of converting alumina into aluminum and its alloys.  

- Investment Casting: Both air and vacuum furnaces are employed in investment casting to produce precise metal components.  

- Shrink-fitting, Annealing, and Brazing: Induction furnaces are used for processes like shrink-fitting, annealing, and brazing, offering efficient heating and high control over temperature.