Case hardening and surface engineering

A gear tooth has two incompatible jobs. Its flank must resist contact wear, which wants hard material. Its root must survive shock loading, which wants tough material. Through-hardening gives you the first and loses the second.

Case hardening solves it by making one part metallurgically two materials.

An induction hardening machine at work on a steel gear, a copper induction coil glowing around the rim of the rotating gear with a bright heated band on the tooth flanks and a quench spray ring below, in a dark workshop. — Only the teeth are heated, so only the teeth harden. The core comes through the process unchanged — still tough enough to absorb the shock that would break the hard surface on its own.

The four routes

A comparison of four case hardening processes. For each of carburising, nitriding, induction hardening and flame hardening the diagram shows a sectioned part with the hardened case shaded, along with the process temperature, whether a quench is required, the typical case depth and the expected distortion. A hardness against depth plot below overlays the four profiles on common axes, showing the deep gradual case from carburising, the very thin hard case from nitriding, and the sharply bounded case from induction hardening. — Four ways to get a hard skin on a tough core. Depth, temperature and distortion are the three things that separate them, and distortion is usually what decides.

Process	Temp	Quench	Case depth	Distortion	Suits
Carburising	900 – 950 °C	Yes	0.5 – 2.0 mm	High	Gears, heavy contact loads
Nitriding	~500 °C	No	0.1 – 0.6 mm	Very low	Crankshafts, dies, finished parts
Induction	Local, seconds	Yes	0.5 – 5.0 mm	Moderate, local	Journals, shafts, selected areas
Flame	Local	Yes	1.0 – 6.0 mm	Moderate	Large parts, low volume, ways

Carburising adds carbon. The part starts as a low carbon steel that cannot harden much on its own; holding it hot in a carbon-rich atmosphere enriches the outer layer, and the quench then hardens that layer while the low-carbon core stays tough.

Nitriding forms hard nitrides at around 500 °C — below the transformation temperature, so there is no phase change and no quench. The part barely moves.

Induction and flame hardening add no chemistry at all. They heat a surface layer fast and quench it before the heat reaches the core, so the steel must already contain enough carbon to harden by itself.

The part nobody mentions — compressive residual stress

Hardness is only half of why a case works.

The case expands as it transforms, and the core restrains it. The surface ends up locked in compression. Fatigue cracks start at the surface and need tension to open — so the applied load has to first cancel that built-in compression before a crack can even begin.

On a gear root or a spring, that residual stress contributes as much fatigue life as the hardness does. It is also why shot peening works: each impact stretches the surface, the surrounding material resists, and compression is left behind. A purely mechanical route to the same benefit, which is why it goes exactly where cracks start.

Case depth means a specific thing

Hardness does not stop at an edge; it falls away gradually. So a case has no natural boundary and one has to be defined by a number.

Effective case depth — the depth at which hardness falls to a stated limit, commonly 550 HV. This is what drawings normally mean.
Total case depth — where the hardness merges into the core. A larger, different number.

Two ways to destroy a case you already paid for

Plating and coatings

Plating adds corrosion resistance or wear resistance, and brings one serious hazard with it.

What to take away

Case hardening makes one part two materials — hard skin, tough core.
Carburising adds carbon to a low-carbon steel, then quenches. Deep case, high distortion.
Nitriding needs no quench, so it suits finished parts. Thin case, long cycle.
Induction and flame add no chemistry — the steel must already have the carbon.
The case ends up in compressive residual stress, and that is half of why it resists fatigue.
Shot peening gets the same compression mechanically.
Effective case depth is measured to a hardness limit, usually 550 HV. Say which you mean.
Grinding burn can reverse the residual stress; over-grinding removes the case entirely.
Bake high strength parts after plating or risk delayed hydrogen embrittlement fracture.