Case Depth in Induction Hardening
How to specify, control, and measure the hardened layer on gears, shafts, and crankshafts — with typical mm ranges and the parameters that drive them.
Case depth in induction hardening is the thickness of the hardened surface layer, measured from the surface inward to a defined hardness limit — usually 50 HRC. It decides how a gear or shaft resists wear and fatigue while the core stays tough. Get it right and parts last; get it wrong and they crack or wear out early.
What is case depth?
Case depth refers to the depth of the hardened layer (the case) produced on a steel component during induction hardening. During the process an electromagnetic coil rapidly heats the surface of the part to roughly 850–950°C, and an immediate quench transforms that heated skin into hard martensite while the core below stays soft and ductile.
Engineers distinguish two figures. Total case depth is the full distance from the surface to the unaffected core. Effective case depth — the value most drawings specify — is the perpendicular distance from the surface to the point where hardness drops to a defined limit, typically 50 HRC (about 513 HV) for steels. At Thakur Industries in Ludhiana, we work to the effective case depth and hardness band stated on your component drawing or OEM standard.
Typical case depth ranges by component
Case depth is always matched to the load case and tooth or section size. The table below gives the practical effective case depth ranges we see most often across automotive, agricultural, and industrial work in Punjab. Treat these as starting points — your drawing or application engineer has the final word.
| Component | Typical case depth | Surface hardness | Why this range |
|---|---|---|---|
| Spur & helical gears | 0.8 – 2.0 mm | 55 – 60 HRC | Tooth flank wear & pitting resistance |
| Transmission shafts | 1.5 – 3.0 mm | 54 – 58 HRC | Torsional fatigue strength |
| Axles & spline shafts | 2.0 – 4.0 mm | 54 – 58 HRC | Bending & contact fatigue |
| Crankshafts (journals) | 3.0 – 6.0 mm | 52 – 56 HRC | Heavy cyclic bearing loads |
| Pins, rollers & small parts | 0.5 – 1.5 mm | 58 – 62 HRC | Shallow, hard wear surface |
As a rule of thumb for gears, the effective case depth is kept below roughly one-quarter of the tooth thickness to avoid through-hardening the tooth, which would rob it of core toughness. For deeper-loaded shafts and induction-hardened shafts, lower frequencies carry heat further in for the thicker cases shown above.
How we control it: frequency, power, and time
Case depth is not set by one dial — it is the result of three coupled parameters working together. We tune them on PLC-controlled, medium-frequency induction systems so every batch repeats.
1. Frequency
Frequency sets how deep the eddy currents — and therefore the heat — penetrate. Lower frequencies (1–10 kHz) drive heat deep for thick cases on shafts, axles, and crankshafts; medium frequencies (10–100 kHz) suit gears and rollers; high frequencies (100–500 kHz) give shallow, hard cases on small precision parts.
2. Power density
Power density (kW per unit of surface area) controls how fast the surface reaches austenitising temperature. High power with a short dwell concentrates heat near the surface for a shallow case; moderate power allows heat to conduct inward for a deeper case before quenching.
3. Heating time
Heat time governs how long conduction carries heat below the skin. Longer cycles deepen the case; shorter cycles keep it shallow. Coil-to-part coupling, scan speed, and the quench rate then fine-tune the martensite profile so the case is both the right depth and the right hardness.
Measuring case depth
A specified case depth is only meaningful if it is verified. Our quality lab confirms case depth on sample parts using a repeatable, standards-based routine:
- Sectioning & mounting — the part is cut perpendicular to the hardened surface, then mounted and polished to a mirror finish.
- Microhardness traverse — a Vickers indenter (HV0.5 or HV1) takes a line of readings from the surface inward; the depth at which hardness falls to the specified limit, usually 50 HRC, is the effective case depth.
- Metallographic verification — the section is etched so the case profile and martensitic structure can be examined and photographed for the report.
- Surface hardness check — Rockwell C readings confirm the case meets the HRC band on the drawing.
Each component family ships with a hardness-and-case-depth record. For documented, traceable results see our quality certifications, which cover the test methods and standards we work to. For the wider workflow, our guide on the induction hardening process walks through heating, quenching, and tempering step by step.
Need a specific case depth on your components in Ludhiana?
Send us your component, material, and the effective case depth and hardness band you require. We will confirm the process window and a documented inspection plan.
Frequently asked questions
What is case depth in induction hardening?
Case depth in induction hardening is the thickness of the hardened surface layer measured from the surface inward to a defined hardness limit. Effective case depth is the perpendicular distance from the surface to the point where hardness drops to a specified value, usually 50 HRC (about 513 HV) for steels.
What case depth is typical for gears?
Most automotive and industrial gears use an effective case depth of 0.8 – 2.0 mm at 55 – 60 HRC. The exact value depends on tooth module, load, and material such as EN8, EN19, EN24 or 20MnCr5.
How is case depth controlled in induction hardening?
Case depth is controlled mainly by induction frequency, power density, and heating time. Lower frequencies and longer heat times push heat deeper for thicker cases, while higher frequencies and short cycles produce shallow, hard surfaces. Quench rate and coil-to-part coupling fine-tune the result.
How do you measure effective case depth?
We section the part, mount and polish it, then run a microhardness traverse (Vickers HV0.5 or HV1) from the surface inward. The depth at which hardness falls to the specified limit, typically 50 HRC, is reported as the effective case depth, supported by metallographic etching of the case profile.
Does deeper case depth always mean a better part?
No. Excessive case depth increases distortion, residual stress, and the risk of through-hardening the tooth core, which reduces toughness. The goal is a hardened case that is matched to the load with a tough, ductile core underneath.
Related services and reading
Explore the work behind these case depths: induction gear hardening and shaft hardening. Ready to discuss your part? Head to get a quote. To go deeper on metallurgy, see the induction hardening process explained. The ASM International handbooks remain the authoritative reference on effective case depth and hardness-traverse methods.