Forging Knives

Cold Forging

Updated 6/3/09

The term 'cold working' has a couple meanings:

The first, as it's applied to the steels in this tutorial, means tool steels which don't hold up when exposed to red heat during service.

The second, means altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading, it is carried out below the recrystallisation point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered.

What this means Bucky, is that you can forge a knife from tool steel at room temperature! Forget all the torches and muffles, forget all the metallurgical talk, forget all the temperatures and charts and stuff. Just pound it out on the anvil! Can you really do this? Yes, you can really do this AND it has advantages.

What we're talking about here is the same thing as cold rolled steel, where steel is reduced in thickness at room temperature. Strength and hardness increase, as the steel is strain hardened. Hardness and tensile strength are essentially different measures of the same thing. If a steel is hard, it's also strong and vice versa. For values around HRC 59 (+/- 3) multiply the HRC number by 5200 to convert to tensile strength in psi. Thus, a steel hardened to HRC 60 has a tensile strength of about 312,000 psi. So, most knives with sufficient hardness have a tensile strength of around 300,000 psi. Conversely, any steel with a tensile strength of around 300,000 will be fairly hard.

What's the upshot of this? Carbon steels (1080-1095) can be work-hardened to strengths in excess of 300,000 psi without undue difficulty. Which means they're hard enough for service as knives. In fact, in small sections like piano wire, tensile strength goes as high as 600,000 psi--making it some of the strongest material on the planet. Taking this conversation to its conclusion, the maximum theoretical strength for steel is about 3 million psi--about one magnitude greater than is achieved in knives.

Most metals strain harden at room temperature. Grain size diminishes as crystals flatten out and lengthen. It is not necessary to heat steel into the Critical range in order to anneal from cold working. The recrystallization temperature of pure iron is in the region of 930F, consequently, a higher temperature of 1200-1250F brings about rapid recrystallization of the distorted ferrite. Heating steel until color appears is usually enough to achieve a sub-critical anneal.

Cut a piece of 1/8" O1 into the shape of the Forging Blank and begin hammering--cold, meaning room temperature. O1 usually comes annealed and it's not much more difficult to cold forge than 1018. O1 can be stretched a considerable amount before it work hardens to the extent forging ceases. Since with 1/8" you're just going to taper the edge (and maybe part of the spine) this works out almost ideally. Working hardening won't set in as a limiting factor until you've got the shape you want. And believe it or not, you can create a minimally decent edge just by work hardening alone. Just keep pounding the edge until the metal will no longer move, sharpen and you're done! No heat, no heat treatment. With O1 you can achieve an edge hardness in the fifties just by cold forging.

You can do considerably better finish work while cold forging than working with hot steel. Since there is no heat applied, no decarburization or scaling takes place. Decarburization is the removal of carbon from the outer surface of the steel, usually by heating in an oxidizing or reducing atmosphere. Water vapor, oxygen and carbon dioxide are strong decarburizers. Reheating with adhering scale is also strongly decarburizing in action. At room temperature, no diffusion, migration or chemical action of any kind takes place. You'll end up with a clean, fairly shiny surface.

There's no grain growth as there's no opportunity for grain growth. Just the opposite occurs--grain reduction. An 80% cold work reduction make the grains five times smaller. And even after a sub-critical anneal the grains remain more than twice as small.

If a fine-grained edge is important to you, cold forging is the way to go.

Another advantage of cold forging is that since the steel temperature never exceeds Ac1 (1350F in the case of O1) there is no need for annealing or normalization prior to heat treatment because no Austenite was ever produced.

Cold forging is often where a peening hammer comes into play because as the metal hardens you'll want to reduce the area of the hammer face.

Ordinarily, after cold forging we'll heat treat as normal and be on our way. Cold forge at least one knife, either 1018 or O1, just to get the hang of the process. Cold forging takes some effort but you'll end up with a superior metal structure and very fine grain size.

Percent Cold Work

If the edge of 1/8" steel (2 sixteenths) is pounded down to #16 wire gauge thickness that's a 60% reduction. #22 is an 80% reduction.

After cold forging do any necessary grinding/filing. If you intend to heat treat then get the blade finish down to 220/320 grit otherwise go ahead and finish the knife.

Heat treating will change the steel's structure from Ferritic to Martenisitic. Cold forging substantially reduces grain size. The greater the cold work percentage the smaller the grain. This works out ideally for knives since the greatest reduction is at the edge and that's where you want the smallest grain size.