Introduction to Induction Hardening

Induction hardening is one of the popular methods of heat treatment, which customize the required physical, mechanical, and chemical properties of metal. It uses electromagnetic induction to heat the part.

The induction hardening process involves heating work parts inside the helical shape coil (Induction coil) by using electromagnetic induction and immediate cooling. The power source (AC) creates the eddy current, which generates the heat required to raise the temperature for the heat treatment process. The heated component is quickly quenched (cooled) by a water-spraying mechanism coupled to the heating system. However, the coolant agent might be water, air, or oil.

Induction Hardening of Steel

Steel hardening involves metallurgical changes to improve its mechanical properties. The induction hardening process heats the steel at 880 to 1050 degrees Celsius, followed by a speedy cooling process. The extreme temperature rises in steel lead to changes in the microstructure, increasing hardness. The quick quenching process stabilizes the microstructure changes.

Induction heating offers greater uniformity throughout the part for effective hardening. The accurate heating pattern helps to form a relatively cold & stable core.

How the steel changes during induction hardening

Induction hardening of steel involves three distinct phenomena: electromagnetic, heat transmission, and metallurgical changes. While the steel part is placed into an induction coil and provides the alternating current (AC), the alternating magnetic field developed on the surface produces the eddy current. The eddy current is responsible for heating of steel part. It changes its properties significantly once it reaches critical temperature (7240 C).

Let’s discuss two crucial transformations that happen during the induction hardening of steel:

  1. Austenite

The changing of crystallographic orientation within the limit of original grains results from the austenite transformation of steel. The austenite contains an FCC crystal structure, which exhibits a denser packing pattern. The denser packing patterns result in open-up spaces in the crystal structure, allowing the interstitial carbon atom to fit. The higher solubility of carbon

  1. Martensite

The martensitic formation is related to the immediate cooling of super-hot steel (<10000 C) (Guterres, 2022). The carbon atoms in the space between iron atoms get trapped because instant cooling prevents diffusion.

Through the chronological transformation to austenite and Martensite, steel changes its crystalline structure. In addition, it stabiles into a dense packing pattern with more carbon content, increasing the original hardness. The achieved hardness value depends on the content of carbon.

Induction hardening effect in hardness of steel

Let’s understand some important terminology before moving into how hardness is affected by induction hardening.

Terms Description
Crystal and grains Crystal refers to the repetitive arrangement of atoms in a periodic pattern, where grains are the collection of crystals separated by the respective grain boundaries.
BCC The body center crystal structure refers to the arrangement of atoms in the cube’s center and at eight corners, where each corner is shared with a corner of another cube.
FCC In FCC, eight atoms are arranged in each corner of the cube, and one remains in the center of each face, shared with an adjacent face.

When force is applied, the grains transfer the energy from one to another. If the force is too high, the grain boundaries tend to slip while passing the force, which causes deformation. Since hardness is the capability to resist deformation, the hardening process adds more strength to resist deformation.

The heating changes the crystal structure of steel from BCC to FCC, where the iron atoms at the center move out and creates the interstitial space between them. The carbon atoms fit between the particles, which results in a more packed & dense pattern.

The instant cooling of the induction hardening machine stabilizes the structure change, making it much more capable of absorbing energy. In addition, compact grains will require more energy to slip at the boundary position.

Benefits of Induction Hardening

Induction hardening of steel improves its mechanical properties, making it suitable for a wide range of applications, from simple tools & kitchenware to advance automotive parts.

  1. Excellent wear resistance

As induction heating increases the hardness, it automatically undergoes improvement in wear resistance capability. Due to the tightness of the inner structure, the hard steel prevents wear and tear from the material surface.

  1. Minimum warpage

Suppose you compare induction hardening with another conventional hardening approach. The uniform heating changes the warpage scenario dramatically. There will be a very low warpage, and the effect on the functionality of the properties will be negligible.

  1. The specific level of hardness

It is easy to obtain a particular level of hardness with induction hardening. For example, a wheel needs a specific hardness level to satisfy its functionality. In this case, steel can be pre-heated to get the required hardness level.

  1. Strength

Since the hardened part contributes more volume than the core, the compressive strength of steel increase significantly, and the hardness also improves the fatigue life of steel because of the high increment in mechanical strength.

Still, increasing the hardness of steel is the most significant benefit of induction hardening, which makes it superior for several industries. Rock crushers, axles, gears, shafts, building beams, and many other applications require hard steel.

Conclusion

Induction heating is the ideal way to improve the hardness. It refers to heat treatment using an induction coil, followed by a quick quenching process.

The advanced induction hardening equipment allows customizing the hardness and other various mechanical properties of steel very precisely. It is also possible to automate the process to optimize productivity.

FAQ’s

How does induction hardening improve the hardness of steel?

Steel heating with an induction coil goes under two transformations, austenite & martensite. Internal structural modifications brought about by these transformations make steel harder.

What are the advantages of steel Induction hardening?

The prime advantage is an increment in the hardness and mechanical strength. It prevents wear and tears from enhancing the part’s life, along with other functional benefits in various applications.

What factors affect the result of induction hardening?

There are three factors: the required hardness level, the operator’s skill, and the induction hardening technology. Utilizing automated and cutting-edge induction hardening equipment will produce excellent results.