A batch of shafts passes every dimensional check. Diameters are on size, runout is fine, the surface reads hard on the tester. Three months later, the customer calls: the shafts are bending under load, and two have snapped at the shoulder. Nothing was wrong with the dimensions. Something was wrong with the induction hardening depth.
Hardness is only half the story. How deep that hardness goes (the case depth) is what decides whether a shaft resists wear, survives shock loads, or quietly fails long before its time. This article explains what case depth means, how it is specified, and how a disciplined workshop controls it.
What Induction Hardening Actually Does
Induction hardening heats only the outer layer of a steel part using an alternating magnetic field from a copper coil. Within seconds, the surface reaches austenitizing temperature (the point where the steel’s crystal structure changes). It is then quenched immediately, usually with a water-polymer spray. The result is a hard martensitic shell around a core that stays tough and ductile.
That combination is the whole point. A fully hardened shaft would be wear resistant but brittle. A soft shaft would be tough but would wear quickly at bearing seats and seal surfaces. Induction hardening gives you both properties in one part, but only if the depth is right.
Effective Case Depth vs. Total Case Depth
Drawings often say something like “induction harden 50–55 HRC, depth 1.5–2.5 mm.” That number needs a definition, because there are two common ways to measure it:
- Effective case depth: the distance from the surface to the point where hardness drops to a defined value, often 50 HRC or a value tied to the steel’s carbon content.
- Total case depth: the distance to the point where the microstructure (or hardness) becomes the same as the core.
If the drawing does not say which one is meant, the supplier and the customer can both be “right” and still disagree by almost a millimeter. Good drawings specify the method. Good suppliers ask when it is missing.
Too Shallow, Too Deep: What Goes Wrong
| Condition | What happens in service | Typical cause |
|---|---|---|
| Case too shallow | Surface wears through quickly; subsurface fatigue cracks start under the hard layer | Scan speed too fast, power too low, coil gap too large |
| Case too deep | Part becomes brittle; snaps under shock or bending loads | Dwell time too long, power too high |
| Uneven case | One side wears faster; distortion and runout after hardening | Part not centered in the coil, inconsistent rotation |
| Case ending at a shoulder or fillet | Stress concentration; cracks at the transition zone | Hardened zone poorly placed relative to geometry |
The last row is the one most people miss. The transition between the hard case and the soft core carries residual stress. If that transition sits exactly at a fillet radius or a step in diameter, it stacks two stress raisers on top of each other. That is where the snapped shafts in our opening example failed.
How Case Depth Is Measured
Surface hardness can be checked on the part with a Rockwell tester. Case depth cannot. To measure it, a sample part is cut across the hardened zone, polished, and tested with a microhardness tester at small, regular steps from the surface inward. The readings form a hardness profile, and the depth is read from where the curve crosses the defined limit.
This is a destructive test, so it is done on setup pieces and at defined sampling intervals during a production run, not on every part. That is exactly why process stability matters: if the machine settings drift, nobody will see it on the surface.
What Controls Depth in Production
Four variables do most of the work:
- Frequency: higher frequency concentrates heat closer to the surface (shallower case); lower frequency penetrates deeper.
- Power and time: more energy, or a slower scan, pushes heat deeper.
- Coil-to-part gap: a small change in gap changes heating efficiency significantly.
- Material condition: carbon content and prior microstructure decide how the steel responds. A medium-carbon steel hardens very differently from a low-carbon bar.
Once a recipe produces the right profile on a cut sample, the job is to keep every one of those variables locked for the whole batch.
How We Handle Induction Hardening at Merdeka Precision
At Merdeka Precision, induction hardening is an in-house process, not a subcontracted one. Our precision component manufacturing line runs two fully automatic induction quenching and tempering units as part of a complete sequence: cutting, induction hardening, straightening, machining, thread rolling, and milling. Keeping the whole chain under one roof means the hardened zone is planned together with the machining steps that follow it, not guessed after the fact.
Verification is done with our own Microvickers and Rockwell hardness testers, supported by precision cutting and polishing machines for sectioning samples. That lets us build a real hardness profile instead of relying on a single surface reading.
The same understanding of hardened surfaces carries over to our piston and rod services, where worn hardened surfaces must be assessed before any repair decision is made. Much of this work supports customers in the automotive industry, where shafts and rods run millions of cycles and margins for error are small.
What to Put on Your Drawing
If you are specifying an induction-hardened part, include these four items and you will avoid most disputes:
- Surface hardness range (for example, 50–58 HRC)
- Case depth range, and whether it is effective or total depth
- The hardness value that defines the depth limit
- Where the hardened zone must start and stop relative to shoulders, grooves, and threads
If you are not sure what depth your part needs, the answer depends on the load type: wear, bending, torsion, or impact. That is a conversation worth having before the first sample is cut.
Do your current drawings specify case depth clearly, or are you relying on the supplier to interpret it? If you would like a second opinion on a hardened component, contact our engineering team and tell us how the part is loaded in service.