At a glance
- Most carbon steels are hardened from around 780–830 °C, close to where they stop being magnetic.
- The quench must be fast enough to harden the steel, but not so harsh that it cracks the blade.
- Fast commercial quench oils suit most knife steels; water is risky.
- Always temper straight after quenching.
This page is background reading. Hardening uses steel at around 800 °C and hot oil that can catch fire. Learn it with a smith, with the right equipment, before trying it yourself.
What happens when steel is hardened?
At room temperature, steel's structure holds carbon in a form that's relatively soft. Heated past its critical temperature, the structure changes (to austenite) and dissolves carbon. Cool it slowly and it goes back to soft. Cool it very fast and the carbon is trapped in a new, highly strained structure called martensite: very hard, and very brittle.
The steps
- Normalising. After forging, the blade is heated above its critical temperature and cooled in still air, usually a few times, to relieve stress and refine the grain.
- Heating to hardening temperature. For most carbon knife steels that's around 780–830 °C, depending on the steel. A magnet is a rough guide: carbon steel stops being magnetic just below the right temperature. Accurate kilns and thermocouples give better results, and are essential for stainless and alloy steels.
- Quenching. The blade goes edge-first and straight down into the quenchant, and is moved gently to keep fresh liquid against it.
- Tempering. Straight away, the blade is reheated to a lower temperature, often around 180–220 °C, to trade a little hardness for toughness. See temper colours.
Which quenchant?
| Quenchant | Speed | Notes |
|---|---|---|
| Fast commercial quench oil | Fast | Made for the job; suits 1084, 80CrV2, 15N20 |
| Warm canola (rapeseed) oil | Medium-fast | Common for beginners; slower than fast oils |
| Water or brine | Very fast | For water-hardening steels such as W1 and W2 (some use it for 1095); cracks blades easily |
| Aluminium plates or air | Slow | For air-hardening steels such as many stainless steels |
The steel decides the quench. Plain carbon steels such as 1084 need a fast oil; stainless steels like AEB-L are hardened in a kiln and often quenched between plates.
Common mistakes
- Overheating: grows the grain and makes the blade weaker.
- Underheating: the steel doesn't fully harden.
- Uneven heating: causes warping.
- Too harsh a quench: cracks, especially near holes and sharp corners.
- Waiting to temper: a hardened, untempered blade can crack sitting on the bench.
How do you know it worked?
A fully hardened blade skates a file across its edge without biting. Makers who need precision test with a Rockwell hardness tester. See best steel for knife making for typical hardness ranges.
Questions people ask
What temperature do you harden a knife at?
For most carbon knife steels, around 780–830 °C, depending on the steel. Stainless and alloy steels need higher, precisely controlled temperatures in a kiln.
What oil is best for quenching knives?
A fast commercial quench oil suits most carbon knife steels. Warm canola oil is a common, slower alternative for beginners. Never use motor oil.
Why do knife blades warp when quenched?
Uneven heating, uneven grinding, or entering the quench at an angle make one side cool or shrink faster than the other.
Do you have to temper after quenching?
Yes. A blade straight from the quench is too brittle to use and can crack. Temper it as soon as it is cool enough to handle.
Learn to make a knife like this
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A D Price Metalcraft (ADPriceMetalcraft)
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B Prothero & Co Blacksmiths (Forging Ahead)
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Related pages
Sources
- Machinery's Handbook (Industrial Press): heat treatment of steel
- Knife Steel Nerds (Larrin Thomas): hardening, quenching and tempering articles
- Thomas, L. (2020) Knife Engineering: Steel, Heat Treating, and Geometry