Selective hardening gives a precision tool a hard working surface on a tough body: case hardening packs the part in charcoal at about 900°C for 2-6 hours so carbon soaks in 0.5-1.5 mm and then quenches it to over 60 HRC at the surface, while flame hardening heats only the working surface and quenches it. Flame hardening works only on steel with enough carbon to harden, and any grinding afterward must stay well inside the thin case.
Part of Precision Tool Making
Two ways to harden only the surface of a tool, and how deep the hard layer goes and what it costs.
A Hard Shell On A Soft Core Resists Wear And Shock Together
A fully hardened part is brittle: a dropped micrometer frame or a pivot pin under a shock can crack. A part that is hard only at its surface wears slowly where it touches and stays tough in the middle. The carrier names two methods (Precision Tool Making).
Case hardening. Pack the tool in charcoal powder, heat to 900°C for 2-6 hours, and carbon penetrates the surface 0.5-1.5 mm deep. Quench. The surface reaches 60 HRC or more while the core stays soft and tough. Carburization adds the principle: wrought iron holds almost no carbon and will not harden, and carburizing adds carbon at the surface so that it can. The same article says a thin case of 0.5-1 mm on bearings, pins and pivot points dramatically reduces wear.
Flame hardening. Heat only the working surface with a torch to cherry red and quench immediately; the bulk remains unhardened. This is the method that needs the right steel. Steel needs at least 0.4% carbon to harden significantly (Hardening), so flame-hardening a low-carbon part simply does nothing, and it is the reason to use case hardening on mild steel and flame hardening on medium- or high-carbon steel.
What You Need
- For case hardening: a sealed iron box, charcoal powder and a furnace or forge that holds 900°C for hours
- For flame hardening: a torch able to bring a surface to cherry red quickly and a quench tank
- A tempering means (Tempering)
- A file or hardness check, and stock allowance for finishing
Method: Case Harden A Part
Step 1: Machine the part close to final size with a small grinding allowance, and consider the case depth you need.
Step 2: Pack the part in charcoal powder in the box, seal it, and heat to 900°C.
Step 3: Hold for 2-6 hours depending on the case depth. The carrier gives 0.5-1.5 mm over that range; the Carburization article gives about 1-2 mm for 2-4 hours on thin pieces and 2-4 mm for 6-10 hours on chisels, so the two sources overlap but do not agree. Test on a scrap piece before committing a finished part.
Step 4: Quench from the heat.
Step 5: Temper at the colour the tool needs.
Step 6: Finish by grinding and lapping, removing less than the case depth.
Worked Example
A pin 10 mm in diameter with a 0.5 mm case: the hard annulus has outer diameter 10 and inner diameter 9 mm. Hardened area = pi / 4 x (10 squared - 9 squared) = pi / 4 x 19 = 14.9 square millimetres, out of pi / 4 x 100 = 78.5, which is 19%. With a 1.0 mm case the inner diameter is 8 mm and the hardened share is (100 - 64) / 100 = 36%.
The case is thin compared with the pin and any grinding must take less than that. If you grind 0.2 mm off a 0.5 mm case, 40% of the case is gone and the hard layer is down to 0.3 mm. Lapping removes about 0.001 mm per hour (the carrier’s figure), so lapping is gentle on a case, but a careless grind is not.
Common Mistakes
- Flame-hardening mild steel. It will not harden.
- Grinding through the case. The soft core shows and the tool wears fast.
- Leaving out the temper. The surface is brittle.
- Case-hardening and expecting the whole part to be hard. The core stays soft by design.
Safety
Furnace gas, torch and quench
Packing charcoal in a sealed box and heating it for hours produces carbon monoxide; work outdoors or with strong ventilation and never in an enclosed room. Flame hardening uses an open torch near quench oil, which can ignite: use a lid or damp cloth, keep the area clear of rags and solvents, and wear eye protection. Do not test for any gas with a flame. Quenching a tool can crack it and throw fragments.
What’s Next
- Learn next: Lapping for Ultra-Precision - finishing the hard surface
- Combine with: Hardening and Finishing Tools - the order of operations