What is the best material for induction hardening?

What materials are best suited for induction hardening?

For induction hardening to be effective, the selected materials must meet certain criteria. Primarily, they should possess high thermal and electrical conductivity. However, only a limited range of materials can be successfully hardened through induction techniques due to specific microstructural and metallurgical properties needed to endure the hardening process.

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Materials intended for induction hardening must exhibit a suitable microstructure that facilitates effective hardening through a phase transformation process. This typically requires an appropriate carbon content. Furthermore, the materials must maintain metallurgical characteristics that enable them to endure high levels of hardness post-cooling. For instance, austenitic steels are generally unsuitable for this process due to their low hardenability.

The following materials are most commonly utilized in induction hardening projects:

• Cast iron (>2% carbon content)
• Medium carbon steel (0.3% to 0.6% carbon content)
• High carbon steel (0.6% to 1.4% carbon content)
• Alloy steels
• Tool steel
• Stainless steel

It is worth noting that aluminum and titanium alloys can also be treated using induction hardening, although these non-ferrous options are less frequently used compared to other materials.

Induction hardening stands out as the most widely employed method among various energy processing techniques. This process harnesses alternating current to generate a magnetic field around the workpiece, which induces heating at a specific depth below its surface. Following the heating phase, the workpiece undergoes quenching, significantly enhancing its hardness in the areas subjected to heat. Importantly, this entire process is completed in a relatively short timeframe. The performance attributes of the finished workpiece are influenced by the hardness profile and internal stresses, which depend on the steel grade (its chemical composition) and the microstructure of the original material.

The resulting hardness pattern from induction heating (%%_MU_1%%) depends on several factors including the type and shape of the inductor and the method of heating. Rapid cooling or quenching of the workpiece is typically achieved using spray or submerged methods, commonly employing water or water-based polymers. The severity of the quenching process is regulated by the concentration of the polymer. Cooling rates generally fall between the extremes achieved with pure water and oil. In specialized circumstances, compressed air may be utilized for the quenching process. Certain applications may require through hardening, ensuring that the entire workpiece is hardened and tempered.

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Material Considerations

Plain carbon steels, categorized under the AISI system, can be classified into four distinct series:

• The first series consists of plain-carbon steels with a manganese content no greater than 1.00%.
• The second series involves resulfurized carbon steels, enhanced for improved machinability through added sulfur.
• The third series features steels that are both resulfurized and rephosphorized for machinability benefits.
• The fourth series contains steels with manganese content ranging from 1.00 to 1.65%.

Carbon content is crucial in determining the hardening capability of steel. Higher carbon levels correlate with increased hardness potential (%%_MU_2%%). Conversely, lower carbon content restricts the maximum achievable hardness level.

Commonly used steel grades for induction hardening include , , , , , , , and . Some of these steels may undergo cold drawing with significant reductions, leading to high tensile and yield strengths. To optimize hardness and minimize the case-transition zone, alloy steels should have a properly quenched and tempered prior microstructure. While higher carbon content can complicate machining, and grades are ideal where both hardenability and machinability are needed, as resulfurized steels that produce sulfide inclusions enhance machinability.

A guiding principle in choosing steel grades is to select one that meets the required hardness and mechanical properties for its specific application. For instance, achieving a hardness of 40 HRC typically necessitates a carbon level below 0.30%, while achieving hardness over 60 HRC demands a carbon content exceeding 0.50%. Opting for lower carbon levels enhances ductility and diminishes the likelihood of quench cracking.

Induction-hardened plain carbon steels are characterized by shallower case depths, while alloy steels can achieve higher hardness at deeper cases. Introducing even a small quantity of boron (0.001%) into low and medium carbon steels improves hardenability (%%_MU_3%%). Literature contains charts that illustrate various alloys in relation to hardness, machinability, and grinding ease, facilitating assessment of manufacturing processes. Steel suppliers typically provide grade-specific pricing and product forms to assist in these evaluations.

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References

1. D.H. Herring, F.J. Otto, and F.R. Specht, Gear Materials and Their Heat Treatment, Industrial Heating, September, .
2. R.E. Haimbaugh, Practical Induction Heat Treating, ASM International, .
3. Properties and Selection: Irons, Steels, and High-performance Alloys, Vol 1, ASM Handbook, ASM International, .