How does induction hardening frequency affect case depth?

The reference (skin) depth δ in induction heating follows δ (mm) ≈ 500 / √(f × μr × σ), where f is frequency (Hz), μr is relative permeability, and σ is electrical conductivity. In practice: 1–10 kHz gives case depths of 3–10 mm (gears, crankshafts); 10–100 kHz gives 1–3 mm (shafts, cams); 100–500 kHz gives 0.3–1 mm (small parts, needles); above 1 MHz gives thin surface heating for solder reflow and small medical components. For steel at austenitizing temperature (800–900°C), μr drops to 1 (non-magnetic), which increases the reference depth significantly.

What steel grades are suitable for induction hardening?

Induction hardening requires sufficient carbon to form hard martensite — typically 0.35–0.65% C. Common grades: EN8 (0.40% C), EN43 (0.50% C) for medium duty; EN24 (Ni-Cr-Mo) for high-strength applications; 42CrMo4 for automotive applications. Case hardening steels (EN36, EN39) are used for gears where a tough core is also needed. Avoid steels with >0.65% C for complex shapes due to quench cracking risk. Stainless steels are generally not suitable for induction hardening.

What power density is required for induction hardening?

Power density at the coil/workpiece gap determines heating rate and ultimately case depth uniformity. For contour hardening of gears and complex profiles: 0.5–2 kW/cm² at the coil. For scan hardening of shafts: 1–4 kW/cm². For through hardening of small parts: 5–20 kW/cm². Higher power density heats faster, reducing the thermal conduction depth and giving shallower case depths. Lower power density allows more heat diffusion, giving deeper cases but risks heating below the critical temperature on the surface before the centre reaches temperature.

How is the case depth measured after induction hardening?

Effective case depth (ECD) after induction hardening is defined as the depth at which hardness drops to a specified value — typically 50 HRC or 550 HV, per ISO 18203 / SAE J423. Measurement is done on a metallographic cross-section using micro-hardness testing (Vickers, 200–500 g load) at intervals from the surface to the core. Total case depth (TCD) is the full depth of visibly altered microstructure. It is important to specify which definition is being used in engineering drawings and purchase orders.

Induction Hardening Calculator & Process Guide — Depth, Frequency & Power

Induction Frequency vs Case Depth Reference

Frequency Range	Skin Depth (mm)	Typical Case Depth	Applications
1–3 kHz	8–15 mm	5–12 mm	Large gears, rolls, heavy shafts
3–10 kHz	5–8 mm	3–8 mm	Crankshafts, large gears
10–30 kHz	3–5 mm	2–5 mm	Shafts, cams, medium gears
30–100 kHz	1.5–3 mm	1–3 mm	Small shafts, spindles
100–500 kHz	0.5–1.5 mm	0.3–1.5 mm	Needles, small pins
>1 MHz	<0.5 mm	<0.5 mm	Surface coating, small parts
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Calculator Features

Reference depth calculation by frequency and material
Required power density from target heating time
Case depth prediction from frequency and power
Steel grade suitability check for induction hardening
Quench medium selection (water spray, oil, polymer)
Tempering temperature to relieve induction stresses

Common Applications

Automotive crankshafts and camshafts
Transmission gears — tooth root and flank hardening
Steering racks and driveshafts
Bearing races and rolling elements
Agricultural and construction machinery parts
Railway axles and wheel seats

Induction Hardening — Process Principles

Induction hardening uses electromagnetic induction to rapidly heat the steel surface to austenitizing temperature, followed by immediate quenching. The process is extremely fast (seconds rather than hours) and highly repeatable, making it the dominant surface hardening method for high-volume production. Unlike case carburising or nitriding, induction hardening is applicable to medium-carbon steels without changing the bulk composition.

The key process variables are frequency (which controls where the energy is deposited), power density (which controls heating rate and temperature), and heating time (which, combined with thermal diffusivity, determines case depth). Interaction between these variables is complex — the Bloor Engineering induction hardening calculator uses validated models to predict case depth and required power for your specific part geometry and steel grade.

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Induction Hardening Calculator & Process Guide

Calculator Features

Common Applications

Induction Hardening — Process Principles

Calculate Case Depth & Power Requirements

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