Case hardening gives a low-carbon steel part a hard, wear-resistant skin and a tough core: pack the part in charcoal, optionally with bone meal, in a sealed box at about 900 °C for 2-8 hours so carbon diffuses into the surface to a depth of 0.5-2 mm, then quench and temper. Surface hardness reaches 60 or more HRC over a core of 20-30 HRC. Depth grows with the square root of time, so doubling the case depth costs four times as long, and molten cyanide salt baths, a historical industrial method, are not described here because they are not safe to reproduce.
Part of Heat Treatment
Case hardening is a surface-hardening process in which carbon is diffused into the outer layer of low-carbon steel at high temperature, so that quenching hardens only that layer.
Why The Core Stays Soft And The Skin Hardens
Hardening needs carbon: Heat Treatment lists steel of 0.05-0.15% carbon as not hardenable and 0.60-0.90% as hardenable. A part made of mild steel or wrought iron will not harden by quenching, however it is heated. Pack carburising raises the carbon only in the outer layer. Quenching then forms hard martensite in the carbon-rich skin and leaves the low-carbon core in its tough form. The Advanced Materials article gives a surface hardness of 60 or more HRC over a core of 20-30 HRC. The result is a part that resists wear on the outside and shock on the inside, which is the Advanced Materials article’s description of the process. The structural reason is in Crystal Structure: hot face-centred iron dissolves carbon readily, and carbon cannot diffuse in until the iron is in that form.
Method: Pack Carburising
Carburization gives the full pack process. In outline, from it and from the Advanced Materials article:
- Make the part slightly oversize, about 1 mm per face, as that article says, to allow for scale and grinding.
- Pack it with at least 3 cm of carburising compound all round, in a clay pot or iron box. The Advanced Materials article’s compound is charcoal powder or a mixture of charcoal and bone meal. Leather scraps are a further option in the corpus. The part must not touch the walls.
- Seal the box with clay lute, but leave a small vent for gas, because a fully sealed box can burst as it heats.
- Heat to about 900 °C and hold for 2-8 hours. Longer gives a deeper case. A thin iron indicator wire packed in with the part can be pulled out, quenched and snapped to show how much carbon has been taken up.
- Quench. The Advanced Materials article quenches directly from the box into water or oil. A better but longer route is to cool in the box, reheat to the hardening temperature given in Hardening (760-800 °C for 1% carbon steel, to the loss of magnetism) and quench from there, which gives a finer structure than quenching straight from the long hot soak.
- Temper. The Advanced Materials article warns that even 200 °C for 1 hour dramatically improves toughness. See Tempering, which is the steel article, not the clay one.
Time Costs More Than Depth
Carbon moves in by diffusion, and the depth grows with the square root of time, not in proportion to it.
Worked example (assumed inputs): suppose your furnace gives a 0.6 mm case in 2 hours. For 1.2 mm, twice as deep, you need (1.2 / 0.6)² = 4 times as long, or 8 hours. Eight hours is the top of the Advanced Materials article’s 2-8 hour range, so a case that deep has already used most of the time a pack run normally takes. The corpus Carburization article gives larger cases (1-2 mm in 2-4 hours); the Advanced Materials article’s figure of 0.5-2 mm over 2-8 hours is the more cautious range, so trial a test piece and cut it open.
Check The Result
- File test. A file should skate over the case. See Hardness Testing.
- Break test. Break a sacrificial test bar processed with the part: the case shows a fine, bright, crystalline fracture and the core a duller grey fibrous one.
- Etched section. Metallography shows the case depth directly.
Historical Cyanide Baths Are Not An Option
Industrial case hardening also used molten baths of cyanide salts, which carburise quickly and evenly. A settlement should not use them and this article gives no procedure. Cyanide kills by skin contact and by the fume, its waste is a poison that cannot be dumped, and a drop of water falling into a molten bath throws it out violently. Proprietary “case hardening compounds” sold in tins may contain cyanide-bearing salts, so treat any unknown salvaged compound as poison and do not heat it. The pack method above reaches the same result more slowly and safely.
Carbon monoxide, hot boxes and quench fire
Heating a charcoal pack produces carbon monoxide, which is colourless and odourless and fatal in an enclosed space, so work outdoors only. Burning leather and bone gives off harmful smoke. Never open a hot pack box in air, because the part oxidises and the charcoal flares. Let it cool or quench at once. A 900 °C part dropped into oil can ignite the oil, so use a deep container with a lid ready, as in Heat Treatment, and steam from a water quench scalds.
Common Mistakes
- Using steel that already hardens — the core goes brittle too.
- Skipping temper — the case cracks or chips.
- Too strong an air blast during carburising — it burns carbon out, as the Carburization article warns.
What’s Next
- Learn next: Precipitation Hardening — a different way to harden by heat
- Combine with: Failure Modes — why a hard skin also resists fatigue