Wrap-up
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Carburizing and case hardening
Why you don't carburize tool steel, and where you'll actually run into carburized parts in a tool-and-die shop.
Tying it together
The mechanic
Place a low-carbon steel part in a carbon-rich environment at 800-1050°C (most commonly 900-920°C for gas carburizing). Carbon atoms diffuse into the surface. The carbon-enriched "case" — usually 0.5-2 mm deep — is then hardened by quenching, producing a hard surface over a tough, ductile core.
Three main methods
- Pack carburizing: The oldest method. Part is buried in charcoal powder mixed with carbonate accelerators, sealed in a box, heated to temperature. Slow, dirty, and largely replaced for commercial work, but still used occasionally for one-off parts.
- Gas carburizing: Modern standard. Endothermic gas (controlled mix of CO, H₂, N₂) combined with natural gas as a carbon source. Better control, faster than pack, runs at 850-950°C.
- Vacuum carburizing: Highest control. Part is exposed to propane (or similar) in a vacuum or low-pressure furnace at moderately lower temperatures. The carbon source breaks down cleanly and diffuses uniformly. Most expensive equipment but the cleanest, most predictable case.
Why you don't carburize tool steel
Tool steels already have plenty of carbon (D2 is 1.5%, H13 is 0.4%, A2 is 1.0%). Carburizing only makes sense on substrates that don't have enough carbon to harden on their own — typically the low-carbon and medium-carbon steels in the 0.1-0.25% C range. Common carburized parts: gears, shafts, bearings, automotive drivetrain components, fasteners.
What this means on the shop floor
- You probably won't carburize the tooling itself in a tool-and-die operation, but you'll handle plenty of carburized parts that drive your tooling: the gears in your press, the cam followers in your feed system, the shafts in your roll formers.
- If somebody on your team is comparing nitriding and carburizing for the same job, ask what the substrate is. If it's tool steel, the answer is almost always nitriding (or PVD), not carburizing. Carburizing-tool-steel is a sign of confusion between processes that have similar-sounding outcomes but different starting materials.
- Carbonitriding is a hybrid (carbon + nitrogen, intermediate temperatures) and shows up occasionally on medium-carbon parts. It's worth knowing the name; you can look up details when you encounter it.
Next: induction hardening, which uses no diffusion process at all and is often the right answer when you need localized hardness fast.