A Complete Guide of Induction Hardening

The Two Stages of Induction Hardening

Induction hardening is divided into two stages. The first is induction heating, which uses an electromagnet to heat electrically conductive metals. The second is quenching, which changes the surface structure of the material.

Stage 1: Induction Heating

The materials are commonly put within a water-cooled copper coil and subjected to an alternating magnetic field. They go through electromagnetic induction through an electromagnet and an electronic oscillator. This oscillator transmits alternating currents through the electromagnet, which causes alternating magnetic fields that flow through the material. The outcome is eddy currents (loops of electrical current) that heat the metal within the coil over the transformation temperature. Induction hardening is surface hardening that may reach depths of up to 8 mm. The higher the frequency of the alternating magnetic fields, the deeper the currents must penetrate.

Stage 2: Quenching

The metal must be quenched immediately after the induction heating process, which means it must be cooled down incredibly rapidly. The part is normally placed in an oil or water tank; cold air is sometimes used. Quenching guarantees that the material’s surface is hardened and that heat does not seep into the material’s core, preventing phase transformations. Furthermore, fast cooling produces a martensitic or ferritic-martensitic structure on the surface layer. These structures have greater tensile strength and a lower initial yielding stress than pure ferritic structures. Quenching also reduces grain size, which is very important for enhancing material hardness.

Typical Induction Hardening Materials

Medium to high carbon and alloy steels, cast irons, and powder metal are often used in induction hardening. Additionally, the applications of Induction hardening can be found in some stainless steels in several sectors.

Because the induction hardening process does not generally include diffusion of carbon or other elements, the material must possess enough carbon and other elements to give hardenability, enabling martensitic transformation to the appropriate hardness level. This usually signifies a carbon content above 0.40%, resulting in a 56 – 65 HRC hardness. Lower carbon materials like 8620 may be employed, causing a lower attainable hardness (40-45 HRC). Steels like 1008, 1010, 12L14, and 1117 are hardly utilized due to the minimal improvement in hardness that may be achieved.

1045 is the most common steel used for the induction hardening of parts. It is easily machinable, inexpensive, and maybe induction hardened to 58 HRC+ because of its nominal carbon content of 0.45%. Moreover, it has a reduced chance of cracking during treatment. 1141/1144, 4140, 4340, ETD150, and different cast irons are also widely used for induction hardening.

The Advantages and Disadvantages of Induction Hardening

Induction hardening is rapidly heating a material to a high temperature and fast cooling it to form hardened surfaces. The process creates exceptionally durable and wear-resistant parts. However, induction hardening is connected with both advantages and disadvantages. Let’s look at them more closely.

Advantages of Induction Hardening

High Efficiency and Speed

Speed is one of the greatest benefits of induction hardening. The process may be done far more quickly than other heat-treating processes, such as case hardening or nitriding, making it excellent for high-volume manufacturing runs. 

Save Time and Money

Other heat-treating processes may require extra materials, like salt baths or gas tanks, whereas induction hardening does not. This saves time and money. In addition, induction hardening gives an even finish over the whole part, eliminating the need to be concerned about inconsistencies in the finish of your parts.

Adjust Hardness Level

Parts may be tempered after induction hardening to get the required hardness level. Like any other process that results in a martensitic structure, tempering reduces hardness while lowering brittleness.

Enhance the Hardness, Durability, and Wear Resistance

The induction hardening improves the metal’s hardness by heating it with an electromagnetic field. This process enables a more consistent heat treatment, leading to a harder metal.

Meanwhile, the enhanced hardness makes the metal more durable and resistant to wear and tear. Induction hardening is therefore suited for parts subjected to high stress or friction.

Improve Fatigue Resistance

Induction hardening also enhances the metal’s fatigue resistance, making it less susceptible to failure under repeated stresses. This is because the process produces a more homogenous microstructure less prone to failure.

Reduce Distortion

Using induction hardening can also reduce distortion. This is because the process can be more accurately controlled, resulting in less heat delivered to the metal. Consequently, the metal is less likely to warp or distort throughout the cooling process.

High Accuracy

High accuracy is also an advantage of induction hardening. Unlike other heat-treating methods, induction hardening does not require any operator interactions; instead, computer controls precisely control the temperature and length of time. This guarantees that each product receives the same level of heat treatment each time it is manufactured.

Cost-effective

Because induction hardening is relatively rapid and easy, it is also cost-effective. In addition, there is no requirement for expensive equipment or specialized skills, making it a viable choice for many companies.

Deep Case with Tough Core

The typical case depth is between .030″ and .120″, which is deeper on average than operations like carburizing, carbonitriding, and nitriding at sub-critical temperatures. For particular projects, such as axels or parts still serviceable after a significant amount of material has worn away, the case depth may be as much as ½ inch or greater.

Disadvantages of Induction Hardening

Restricted to Ferrous Metals

Induction hardening is often confined to ferrous metals like iron and steel. This is because induction hardening uses magnetic fields to create heat, and the only materials affected by magnetic fields are ferrous metals. Nonferrous metals like aluminum and copper cannot be induction hardened.

High Running Cost

The most significant downside of induction hardening is its high cost. While this method is less expensive regarding raw materials and labor than heat-treating methods, it still requires specialized equipment to function correctly. Furthermore, because this process utilizes electricity rather than fuel sources such as gas or oil, the long-term operating costs might be fairly high owing to energy consumption fees.

Require Induction Coil that Corresponds to the Part’s Geometry 

Due to the importance of part-to-coil coupling distance for heating efficiency, the size and shape of the coil must be carefully chosen. While most treaters have a stock of basic coils for heating round objects like shafts, pins, rollers, and so on, certain jobs may need a bespoke coil. 

Require Specialized Equipment

Induction hardening also has the problem of necessitating the use of specialized equipment, which is not always readily accessible. Induction hardening equipment is costly and not always accessible from all heat treatment suppliers. Furthermore, some types of induction hardening equipment may need further training or certification for correct operation.

Thickness Limitation

Unlike other heat treatment methods, induction hardening is often confined to thinner materials. This is because induction hardening heats the surface of a material rather than the entire material. Consequently, it is more difficult to uniformly heat thicker materials, which might result in uneven hardness or even cracks.

The Application of Induction Hardening

Induction hardening is used in various sectors, including transportation, aerospace, and energy. Several applications can benefit from this process. Induction heating is used in the following applications:

• Surface heating, melting, soldering, and so on.
• The heating of liquid and gaseous conductors.
• Plastic injection molding machines use this process.
• Used for Tamper-resistant cap sealing on bottles and medications.
• It may be used to weld metals and sometimes plastics when doped with ferromagnetic ceramics.
• Inductive heating is used in the semiconductor industry to heat silicon.
• Inductive heating is used in inductive furnaces to heat metals to the melting point.
• The induction stoves used in the kitchen operate on the inductive heating principle.
• As induction heating is a non-contact heating method. Therefore, induction heating is utilized in vacuum furnaces to produce specialized steels and alloys that oxidize if heated in oxygen.

Induction Hardening vs. Case Hardening: What are Their Differences?

Case hardening, like induction hardening, aims to harden the outer layer of workpieces. Two approaches are suitable for mass-production workpiece hardening. A comparison of these two approaches reveals their differences and respective benefits.

Parts Handling

The first apparent distinction is the handling of parts. While case hardening works many workpieces simultaneously, induction hardening concentrates on individual workpieces. Components are hardened workpiece by workpiece using induction hardening. Case hardening might be best described as “batch by batch.”

Manufacturing Method

Case hardening requires logistics to transport parts between a production line and the hardening. However, induction hardening may be incorporated directly into the production line with an appropriate hardening machine and become part of the cycle.

Processing Differences 

Case hardening technology combines heat and chemistry to generate a thermochemical reaction. In contrast, an induction-hardened metal part utilizes pure electromagnetic energy to “induce” an alternating current within the part. This non–contact heating process uses electromagnetically to produce heat on the surface of the worked part.

Application

As a localized solution, induction hardening works best on crankshafts and gear teeth. On the other hand, case hardening operates well on bigger, less geometrically intricate components, particularly when they require batch processing speed.