O1
O1 is the steel that taught a generation of knifemakers to heat treat. It is sold as precision ground flat stock — arriving flat, square and ready to profile — it hardens in ordinary oil at temperatures a modest kiln reaches easily, and it rewards a careful thermal cycle with a fine, keen edge at 59–61 HRC. What it does not offer is toughness. O1 is a slicer’s steel, not a chopper’s, and the makers who are happiest with it know that going in. Get the austenitizing temperature right and it is one of the most satisfying steels in a small shop.
O1 FAQ
Is O1 a good knife steel?
For kitchen knives, EDC slicers, hunters and woodworking tools, yes — very good. Its fine carbide structure takes a keen edge and holds it noticeably better than a simple carbon steel like 1084. Its weakness is impact toughness: at 60 HRC it is well behind 5160, 80CrV2 or A2. Do not build a chopper out of it.
What temperature should O1 be austenitized at?
1475°F with a 10–15 minute soak. This is the detail most O1 tutorials get wrong. Larrin Thomas’s testing at Knife Steel Nerds found that pushing the austenitizing temperature up to 1550°F measurably reduces toughness without buying any useful hardness in return — the higher soak coarsens grain and retains more austenite for nothing. Hotter is not harder here. Stay at 1475°F.
What oil should I quench O1 in?
A medium-speed oil — Parks AAA or another oil in the 11-second class. O1 is deep hardening thanks to its manganese, chromium and tungsten, so it does not need a fast oil to make full hardness, and using one (Parks 50) mostly buys extra cracking and warping risk. Canola works in a pinch at temperature. Full breakdown in Knife Quench Oils Explained.
What hardness should O1 be tempered to?
Temper twice for two hours at 350–450°F. Around 400°F lands near 59–61 HRC, which is the sweet spot for most knives; 350°F holds 61–62 for thin kitchen work; 450°F drops toward 57–58 if you want a little more forgiveness in a larger blade. See Why Hardness Matters for the tradeoff behind that choice.
Is O1 stainless?
No. The roughly 0.5% chromium in O1 is there for hardenability, not rust resistance — you need about 13% before a steel is stainless. O1 will flash rust on a fresh grind and spot overnight in a humid shop. Oil it, dry it after use, and consider a forced patina as a working finish.
O1 or 1084 for a first knife?
1084 if you are heat treating in a forge with a magnet, O1 if you have a kiln or a controlled oven. 1084 forgives temperature error; O1 wants a real soak at a real number. The payoff for that extra control is a finer, longer-lasting edge — see our O1 vs 1084 comparison.
Also known as: AISI O1, DIN 1.2510, ~SKS-3, commonly sold as precision ground flat stock (PGFS)
Composition
Carbon 0.85–1.00%, Manganese 1.00–1.40%, Chromium 0.40–0.60%, Tungsten 0.40–0.60%, Vanadium 0.30% max, Silicon ~0.50%
Properties
| Property | Rating | Notes |
|---|---|---|
| Toughness | ★★★☆☆ | O1's honest weak point. The same high carbon and fine carbide content that give it a keen, wear-resistant edge cap its impact toughness at moderate — better than 1095 at equal hardness, but clearly behind 5160, 80CrV2, 15N20 and A2. Suited to slicing blades; a poor choice for choppers, machetes or big camp knives. |
| Edge Retention | ★★★☆☆ | Strong for a low-alloy steel. Chromium and tungsten form fine, evenly distributed carbides that out-slice the simple carbon steels noticeably, and the fine structure means the apex itself is very keen rather than merely hard. Still well below D2 and the powder-metallurgy steels in raw wear resistance. |
| Corrosion Resistance | ★☆☆☆☆ | Essentially none. The ~0.5% chromium serves hardenability, not stain resistance. O1 rusts readily — it will spot overnight in a humid shop and flash rust on a fresh grind. Oil it, dry it, and treat a patina as a feature rather than a flaw. |
| Machinability / Grinding | ★★★★☆ | One of the friendliest steels in the shop. Annealed O1 files, drills and grinds cleanly, and because it is sold as precision ground flat stock it arrives flat and square — no surface grinding needed before layout. Hand-sands to a clean finish without fighting back. |
Heat Treatment
Normalize and thermal cycle first if the stock has been forged. Austenitize at 1475°F with a 10–15 minute soak — do not go to 1550°F, which testing shows reduces toughness with no hardness benefit. Quench in a medium-speed oil (Parks AAA or an 11-second class oil); O1 is deep hardening and does not need a fast oil. Temper twice for two hours at 350–450°F: ~400°F gives 59–61 HRC, 350°F holds 61–62 for thin blades, 450°F drops to 57–58 for larger ones.
Best Uses
Kitchen knives, EDC and gentleman's folders, hunting and skinning blades, straight razors, woodworking tools and plane irons — anywhere a fine, keen, long-lasting slicing edge matters more than impact resistance. Also a superb first stock-removal steel for anyone with a kiln, since it arrives flat and true.
Sources: Knife Steel Nerds — O1 steel history, properties and heat treating (knifesteelnerds.com); Hudson Tool Steel — O1 technical data