The Pros and Cons of Induction Hardening

Induction hardening is a type of heat treating method in which you selectively and rapidly harden a metal surface. In the process, you place a copper coil that carries an alternating current near the part, but not directly touching it. Heat is generated close to the surface with eddy current and hysteresis losses. Then, a water-based quench (often with a polymer addition) gets directed at the part or submerged, resulting in the structure being transformed into martensite, a significantly harder structure than what previously existed.

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Modern induction hardening in Gastonia, NC is most frequently performed with a scanner. In this method, the part is held between centers, rotated and then sent through a progressive coil, which generates heat and quench alike. The quench is directed below the coil, meaning any area of the part is being rapidly cooled right after it&#;s heated.

Benefits of induction hardening

Here&#;s a quick overview of some of the benefits of this process:

• Increased strength and fatigue life: The softer core and residual compressive stress at the surface creates increased overall strength and fatigue life. This is a result of the hardened structure at the surface taking up more space than the core and the previous structure.
• Tempering: Parts can be tempered after induction hardening, which allows you to customize the hardness level according to your needs. The more you temper, the lower the hardness will be, and the lower the brittleness will be as well.
• Minimal distortion: The induction hardening process results in relatively minimal distortion to a piece of metal. Other hardening processes tend to feature a lot more warpage than what you get in the induction hardening process.
• Low cost: With induction hardening in Gastonia, NC, you can use lower-cost steels than you can in other types of hardening methods. This allows you to save money on the overall process.
• Wear resistance: The wear resistance of the parts you work on will increase significantly with the induction hardening process, assuming the material had either already been annealed or treated so it was in a softer condition.

Drawbacks of induction hardening

There are a few disadvantages of the induction hardening process to consider as well:

• Distortion: Distortion levels do tend to be larger in induction hardening than they are in processes such as ion or gas nitriding as a result of the rapid heating and quenching process and the martensitic transformation that occurs as a result. However, this distortion is still less than what you&#;d get from conventional heat treatments.
• Cracking: There is a greater likelihood of cracking in induction hardening than there is in other types of heat treatments.
• Material limitations: Not all types of materials can be put through the induction hardening process, so it&#;s not quite as versatile as some other types of heat treatments.

For more information about the various pros and cons associated with induction hardening in Gastonia, NC, we encourage you to contact J.F. Heat Treating Inc. today with any questions you have about the services we provide.

What is Induction Hardening?

Induction hardening is a method of quickly and selectively hardening the surface of a metal part. A copper coil carrying a significant level of alternating current is placed near (not touching) the part. Heat is generated at, and near the surface by eddy current and hysteresis losses. Quench, usually water-based with an addition such as a polymer, is directed at the part or it is submerged. This transforms the structure to martensite, which is much harder than the prior structure.

A popular, modern type of induction hardening equipment is called a scanner. The part is held between centers, rotated, and passed through a progressive coil which provides both heat and quench. The quench is directed below the coil, so any given area of the part is rapidly cooled immediately following heating. Power level, dwell time, scan (feed) rate and other process variables are precisely controlled by a computer.

Typical Induction Hardening Materials

Typical materials include:

• ETD150
• Cast Irons

Benefits of Induction Hardening

Increased Wear Resistance

There is a direct correlation between hardness and wear resistance. The wear resistance of a part increases significantly with induction hardening, assuming the initial state of the material was either annealed, or treated to a softer condition.

Increased Strength & Fatigue Life due to the Soft Core & Residual Compressive Stress at the Surface

The compressive stress (usually considered a positive attribute) is a result of the hardened structure near the surface occupying slightly more volume than the core and prior structure.

Parts may be Tempered after Induction Hardening to Adjust Hardness Level, as desired

As with any process producing a martensitic structure, tempering will lower hardness while decreasing brittleness.

Deep Case with Tough Core

Typical case depth is .030&#; - .120&#; which is deeper on average than processes such as carburizing, carbonitriding, and various forms of nitriding performed at sub-critical temperatures. For certain projects such as axels, or parts which are still useful even after much material has worn away, case depth may be up to ½ inch or greater.

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Selective Hardening Process with No Masking Required

Areas with post-welding or post-machining stay soft - very few other heat treat processes are able to achieve this.

Relatively Minimal Distortion

Example: a shaft 1&#; Ø x 40&#; long, which has two evenly spaced journals, each 2&#; long requiring support of a load and wear resistance. Induction hardening is performed on just these surfaces, a total of 4&#; length. With a conventional method (or if we induction hardened the entire length for that matter), there would be significantly more warpage.

Allows use of Low Cost Steels such as

The most popular steel utilized for parts to be induction hardened is . It is readily machinable, low cost, and due to a carbon content of 0.45% nominal, it may be induction hardened to 58 HRC +. It also has a relatively low risk of cracking during treatment. Other popular materials for this process are /, , , ETD150, and various cast irons.

Limitations of Induction Hardening

Requires an Induction Coil and Tooling which relates to the Part&#;s Geometry

Since the part-to-coil coupling distance is critical to heating efficiency, the coil&#;s size and contour must be carefully selected. While most treaters have an arsenal of basic coils to heat round shapes such as shafts, pins, rollers etc., some projects may require a custom coil, sometimes costing thousands of dollars. On medium to high volume projects, the benefit of reduced treatment cost per part may easily offset coil cost. In other cases, the engineering benefits of the process may outweigh cost concerns. Otherwise, for low volume projects the coil and tooling cost usually makes the process impractical if a new coil must be built. The part must also be supported in some manner during the treatment. Running between centers is a popular method for shaft type parts, but in many other cases custom tooling must be utilized.

Greater Likelihood of Cracking Compared to most Heat Treatment Processes

This is due to the rapid heating and quenching, also the tendency to create hot spots at features/edges such as: keyways, grooves, cross holes, threads. (Please talk to an AHT representative if you have concerns.) 

Distortion with Induction Hardening

Distortion levels do tend to be greater than processes such as ion or gas nitriding, due to the rapid heat/quench and resultant martensitic transformation. That being said, induction hardening may produce less distortion than conventional heat treat, particularly when it&#;s only applied to a selected area.

Material Limitations with Induction Hardening

Since the induction hardening process does not normally involve diffusion of carbon or other elements, the material must contain enough carbon along with other elements to provide hardenability supporting martensitic transformation to the level of hardness desired. This typically means carbon is in the 0.40%+ range, producing hardness of 56 &#; 65 HRC. Lower carbon materials such as may be used with a resultant reduction in achievable hardness (40-45 HRC in this case). Steels such as , , 12L14, are typically not used due to the limited increase in hardness achievable.

* Blog was updated in July to reflect our Cullman, Alabama location now having induction hardening. 

About the Author

Scot Clay is the Sales Team Manager at Advanced Heat Treat Corp., where he&#;s been an integral part of the Sales and Marketing Team for over 30 years. He&#;s managed numerous induction hardening projects, solving wear and performance issues for customers throughout the United States. Contact him for technical advice on your next project at 319-291- or for more information.

The company is the world’s best Induction Hardening Machine supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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