Induction heating hardening utilizes the principle of electromagnetic induction, causing parts to cut magnetic field lines in an alternating magnetic field and thereby generating induced currents on the surface. A quenching method that utilizes the Skin Effect of alternating current to concentrate the induced current on the surface of the workpiece, rapidly heating the surface of the part in the form of eddy currents, and then performing rapid cooling through water quenching or oil quenching.

Hereby we will introduce you the professional technical introduction about induction heating hardening process, especially high-frequency hardening and medium-frequency hardening.

Classification of induction hardening

Induction hardening is mainly classified into high-frequency hardening, medium-frequency hardening and power frequency hardening based on the difference in current frequency. In actual industrial production, high-frequency quenching and medium-frequency quenching are the most widely used.

1. what is high-frequency induction hardening?

• Current frequency: ranges from 100khz to 500kHz, the most common the induction heating hardening machine is usually operating around 200kHz~300kHz.
• Depth of hardening zone: Shallow depth, generally between 0.5mm and 2.5mm
• Hi-frequency hardening process features: When the frequency is super high, the alternating magnetic field changes very rapidly, and the skin effect is extremely significant. Heat is highly concentrated on an extremely shallow surface layer. In practical production, it only takes a few seconds to heat the surface of the workpiece to the quenching temperature (such as 850C~1000C)
• Typical applicable metal-works: It is suitable for small-module gears, small shafts (such as micro motor shafts), pins and other parts with small dimensions, shallow hardened layers and extremely strict deformation requirements.

2. what is medium-frequency induction hardening?

• Current frequency: Typically ranges from 500Hz to 10 kHz, the most popular induction hardening machinery operates around 2.5kHz~8kHz.
• Depth of hardening zone: The frequency of medium-frequency hardening is relatively low, and the current penetration depth is large. The depth of the hardened layer is generally between 2~10mm.
• Mid-frequency hardening process features: The heating speed of medium-frequency hardening is relatively slower, but it is still much faster than the ordinary heating method. Usually, it takes tens of seconds to several tens of seconds to reach the quenching temperature, and the internal heat conduction is obvious.
• Typical applicable metal-works: In terms of applicable materials, medium-frequency hardening is more suitable for large metal parts and materials that require a deeper hardened layer, such as large shafts and large-module gears. For example, for large shafts made of 45 # steel, medium-frequency induction hardening can achieve the exact hardening depth and balance mechanical properties required for heavy-duty service.

3. Compare with HF induction hardening and MF induction hardening

Selection guide for Induction hardening process: Medium frequency or High frequency

High-frequency induction hardening heats metal at an very fast speed—almost in the blink of an eye, the surface is heated up. And It can make the metal surface rapidly reach a very high hardness, effectively encasing the metal in a layer of ultra-hard armor. High frequency induction hardening is ideal for complex-shaped parts that’s requiring localized hardness, such as small gears.

Medium-frequency induction hardening, on the other hand, heats more gradually but penetrates deeper into the metal. Think of it as a deep-tissue massage that ensures uniform internal heating. It is well-suited for processing work-pieces requiring a balance of hardness and toughness, such as shafts.

Metal treatment with high-frequency induction hardening achieves extreme surface hardness, offering exceptional wear resistance. In contrast, medium-frequency induction hardening process yields a work-piece that combines good hardness with excellent internal toughness, making it less likely to break.

Regarding equipment, high-frequency induction hardening/quenching systems operate at higher frequencies and are relatively compact, whereas medium-frequency systems operate at intermediate frequencies and are somewhat larger.

In practical applications, the selection should be based on the specific conditions of the parts. If only a hard surface is required, then high-frequency quenching should be chosen. If both internal and external aspects are to be taken into account, medium-frequency quenching is more suitable. Just like choosing the right "useless manual" for different tools, we can deliver the greatest advantages based on different needs.

In general

•  Choose HF Hardening if the work-piece is small, the primary requirement is surface wear resistance, and dimensional distortion must be strictly controlled.
•  Choose MF Hardening if the work-piece is large, requires a deep hardened layer, and must withstand heavy impact, high torque, or severe contact stress.

Optimal Capacitor placement for induction heating treatment 

The medium frequency hardening machine has a great significance to improve the technology of heat treatment. Induction hardening machine often contain induction coil and induction heating power system. Next, we will introduce capacitor in power system. 

In medium-frequency induction heating systems, the induction coil presents a highly inductive load with a remarkably low natural power factor. To achieve rapid heating rate and maximize energy efficiency, Medium-Frequency Water-Cooled Resonant Capacitors must be integrated into the tank circuit to provide reactive power compensation.

When sourcing or replacing capacitors for your MF induction hardening or melting furnaces, prioritizing the following three technical benchmarks ensures system reliability and prevents catastrophic field failures:

1. Advanced All-Film Dielectric & Non-PCB Impregnates

Modern high-performance Power Capacitors utilize specialized All-Film structure, extend aluminum foil as the dielectric, impregnate with eco-friendly insulation oils (such as PEPE or M/DBT/vegetable oil).

Benefit of All-film power capacitor: This configuration guarantees an exceptionally low dissipation factor, minimizing internal dielectric losses and mitigating localized thermal runaway under high-current continuous operation. It hold superior performance than metallized film type capacitor.

2. Built-in Water Cooling Systems

Medium-frequency resonant capacitors handle immense reactive currents. Effective heat dissipation is mandatory. Premium Water-Cooled Capacitors utilize high-purity oxygen-free copper tubing welded directly to the internal electrode foils.

Benefit of water-cooled capacitors: Ensures a low temperature rise across the entire internal structure. For continuous MF hardening lines, a highly efficient water-cooling circuit prevents capacitance drift and substantially prolongs the service life of the capacitor bank.

3. High Current Carrying & Low ESR/ESL

Induction hardening operations involve frequent, rapid switching cycles that subject components to severe current surges.

Benefit: Capacitors engineered with heavy-duty internal copper busbars and multiple discharge paths feature ultra-low Equivalent Series Resistance (ESR) and low Equivalent Series Inductance (ESL). This robust build eliminates internal connection overheating and reliably withstands the rigorous pulse stresses inherent to multi-stage induction quenching.

Partner With Flair for optimal capacitor placement solutions

We specialize in engineering and manufacturing premium-tier Water-Cooled Power Capacitors for Medium-Frequency Furnaces and Induction Hardening power Systems. Our products are optimized to deliver stable capacitance, ultra-low losses, and extended field service life under the most demanding induction heating system.

Whether you are upgrading an existing MF induction line or manufacturing new induction heating equipment, our engineering team is ready to provide the optimal capacitor placement matching to optimize your system's power factor and heating efficiency.

Contact our technical sales department today for detailed product catalogs, drawings, and custom inquiries.