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Lesson 12.2TreatChapter 12 · Heat treatment and surface hardening

Case hardening and nitriding

Carbon or nitrogen diffused into the surface

Case hardening makes only the outer skin of a steel part hard, while the inside stays tough. Carbon (carburizing) or nitrogen (nitriding) atoms are soaked into the surface in a hot furnace. Gear teeth, camshafts and crankshafts need exactly this: a skin that resists wear over a core that does not crack.

Case hardening and nitriding: a real photo
Photo: 吉岡正三、加藤, Public domain, via Wikimedia Commons
Carbon or nitrogen soaks into the surface, making a hard skin over a tough coreTwo routes side by side (schematic; case thickness exaggerated)Carburizingcarbon, at about 900–950 °Ccarbon-rich gas around the part123quench in oil,then temperNitridingnitrogen, at about 500–570 °Cammonia gas or plasma around the part123no quench: cools slowly,so it barely distorts4Cut through the surface afterwardshard case, about 0.5–2 mmtough corethin hard case, about 0.1–0.6 mmtough corecarburizednitridedSteel part (darker = hard case)Carbon or nitrogen atomsFurnaceHeat
Carburizing and nitriding side by side, with the resulting hard caseBoth panels share circled numbers 1 to 3; number 4 is the cut-through view of the finished surface.

How it works

  1. Load the parts into the furnace. Clean parts go into a sealed furnace filled with a carbon-rich gas (carburizing, typically about 900–950 °C) or with ammonia gas or plasma (nitriding, typically about 500–570 °C). Areas that must stay soft are masked, for example with a copper coating or a paste.

  2. Let the atoms diffuse in. Carbon or nitrogen atoms enter the surface and creep inward by diffusion (atoms slowly wandering into the metal). The longer the soak, the deeper the case (the hardened skin), typically over several hours.

  3. Finish the cycle. Carburized parts are quenched in oil and then tempered, as in lesson 12.1, so the carbon-rich skin turns very hard. Nitrided parts are already hard and simply cool slowly, so they barely distort.

  4. Check the case. A test piece is cut, polished and measured for hardness from the surface inward to confirm the case depth.

Key facts

Materials
Carburizing: low-carbon steels; nitriding: alloy steels with chromium, molybdenum or aluminum, tool steels, some stainless steels
Typical parts
Gears, camshafts, crankshafts, bearing races, pins, injection-mold parts, extrusion screws
Part size
From small pins to large gears over a meter across, limited by the furnace
Precision
Carburizing distorts parts slightly, so key faces are often ground after; nitriding keeps sizes within a few hundredths of a millimeter
Surface finish
Carburized parts are often dark and need cleaning or grinding; nitrided parts are matte gray
Batch size it suits
Batches of hundreds to thousands of parts per furnace load
Tooling and machine cost
Industrial furnaces from about a hundred thousand to millions of dollars; most firms send parts to specialist heat-treat shops
Speed
Carburizing: several hours per load; nitriding: often 10–60 hours or more, depending on the case depth

Strengths and limits

Strengths

  • Very hard, wear-resistant skin over a tough core that resists shock
  • Raises fatigue strength, so parts last longer under repeated loads
  • Nitriding needs no quench, so distortion is minimal
  • Many parts treated at once in one furnace load

Limits

  • The case is thin; deep damage or heavy grinding can remove it
  • Carburizing plus quench can warp parts
  • Nitriding is slow, and only suits certain alloy steels
  • Specialist furnaces and gas control; usually outsourced

Where it's used

  • AutomotiveGearbox gears, camshafts, crankshafts, piston pins
  • AerospaceTransmission gears, bearings, landing-gear parts
  • ToolingInjection-mold parts, extrusion dies, plastic extruder screws
  • Industrial machineryGears, shafts, chain parts, hydraulic components
  • FirearmsSlides and barrels, often salt-bath nitrided

How to recognize it

  • Matte gray or black surfaces on gears and shafts, with ground bearing faces.
  • A file skates off the surface, even though the part may bend rather than snap.
  • On a cut and etched sample, a darker or lighter rim of uniform depth around the edge.

Compare with

Heat treatment12.1hardens the whole part through its thickness instead of only the skin.Shot peening12.3strengthens the surface by hammering it, without changing its chemistry or heating it.PVD coating11.8adds a very thin hard layer on top of the surface instead of changing the metal itself.

Watch it

Artisan Makes · A simple explanation, then workshop case hardening

THE FACILITY · Pack carburizing, quench and a file test before and after

ION HEAT · How the two nitriding methods differ

Ionitech Ltd. · Parts glowing inside a plasma nitriding chamber

See real photos

Remember

  • Case hardening puts a hard skin over a tough core.
  • Carburizing adds carbon at high heat, then quenches; nitriding adds nitrogen at lower heat, with no quench.
  • Perfect for gears and shafts that must resist both wear and shock.

More in Heat treatment and surface hardening

All 100 processes
12.1Heat treatment (hardening, tempering, annealing)Heat, quench and temper (or slow-cool) the whole partOpen the lesson12.3Shot peeningSurface hammered by tiny balls to leave it compressedOpen the lesson

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