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Is it possible to harden knife blades using a coal forge with any luck?
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Newbie question
- Thread starter theduffies
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Did not try it yet but ...
From what I know YES, it's possible. You should use a simple carbon steel for the blade, keep the blade on coal until it's non-magnetic and quench it in oil (or water depends on the steel). Then clean all the oil from the blade, put it in the kitchen oven at 250°C (not sure but must be something like 450°F) for an hour or so then let it cool in the oven. Repeat the oven stuff one more time and that should do.
I will do a little blade very soon, don't have a coal forge but I got a propane torch that goes up to 1750°C (i'll need only 900°C so it's OK) and costed $15. I'll do the heattreating using that torch, so I will know more about this after I try it.
Hope it helps.
Hey, welcome to the forums!
From what I know YES, it's possible. You should use a simple carbon steel for the blade, keep the blade on coal until it's non-magnetic and quench it in oil (or water depends on the steel). Then clean all the oil from the blade, put it in the kitchen oven at 250°C (not sure but must be something like 450°F) for an hour or so then let it cool in the oven. Repeat the oven stuff one more time and that should do.
I will do a little blade very soon, don't have a coal forge but I got a propane torch that goes up to 1750°C (i'll need only 900°C so it's OK) and costed $15. I'll do the heattreating using that torch, so I will know more about this after I try it.
Hope it helps.
Hey, welcome to the forums!
- Joined
- Nov 26, 2001
- Messages
- 1,375
Not only it is possible, but it's been made successfully for thousand of years!
What said below is valid for most carbon steels.
You need to know the temperatures at which you must heat treat the steel and quenching medium, though.
Using the non-magnetic test is simple and works greatly, but for tempering you must know the exact temp and soaking time or you risk spoiling whatever work you did during hardening.
You may also do it this way:
take a scrap piece of the same steel you use for the knife, harden it, file it clean on a face and heat it over the same forge you used to heat it for hardening.
DO NOT put it IN the forge. Keep it over to heat it slowly.
Watch carefully for color change.
When reaching a certain heat the steel will change color, sign that its internal crystalline structure is somewhat changing.
The more you'll heat the piece, the more hardness you'll lose in favour of toughness. Roughly speaking.
The colors are, from harder to tougher
mild straw
straw
burnt siena
ochra
purple
bluish purple
blue
grey blue
If you have a short, thick blade that you want to be sharp and remain sharp (example: skinner) a mild straw to straw temper is enough.
For longer blades, which may have to endure impact (example: fighting knives or swords) you may want to go up to blue.
As soon as the blade reaches the desired color, immediately quench it in water to stop the tempering process.
be also very careful to SLOWLY heat the blade. Once the blade begins changing color, going over the desired color is extremely easy.
The slower will be the heating, the more control you'll have on the tempering process.
About any moron can make up a decent hardening in a piece of steel.
It's tempering, and the right harden/temper ratio that makes the difference between a really good blade and a useless piece of steel that won't hold an edge even if you cry or that will miserably shatter the first real impact it takes.
Make various tests with scrap metal before you attempt it on your knife.
IMPORTANT:
When heating for hardening, keep the blade at the critical temperature for the minimum time necessary to make sure the whole blade is uniformly heated.
Non uniformly heated blades may warp during quench.
Excessive permanence at critical temp will cause excessive grain growth which in turn will make your blade brittle.
Quench the blade in the proper medium. Oil quench steels will crack or shatter if quenched in water.
Quenching in oil is the safest way, as for blade. It may not be the safest for you, if you don't know what to expect, as oil will flare up when a red-hot piece of steel it's immersed in it.
Don't worry, use long thongs to quench, continue quenching and don't worry about the flames. They'll go off by themselves or you may extinguish them with a CO2 fire extinguisher once you are finished quenching.
Just keep away from them and don't do it near anything flammable.
Before hardening it's always better to do some annealing.
Heat the blade to its critical point, then let it cool slowly.
The slower the cooling, the better the annealing will be.
Repeat at least a couple of times, three is better.
The air in the room should be motionless, even better, put the blade in a bucket of hot sand and leave it there for many hours.
A NOTE ON STAINLESS STEEL.
Many stainless steels are air-hardening. This means that air is a good quenching medium for them.
Annealing is impossible with normal methods and should be carried out in electronically controlled electric ovens which follow a precise curve of cooling.
So, at least as a beginner, stay away from stainless or have it heat treated by a heat treating firm.
A FINAL NOTE:
"The Complete Bladesmith" by Jim Hrisoulas (Paladin Press), contains all that you need to know about forges, their use and safety precautions.
Have fun!
What said below is valid for most carbon steels.
You need to know the temperatures at which you must heat treat the steel and quenching medium, though.
Using the non-magnetic test is simple and works greatly, but for tempering you must know the exact temp and soaking time or you risk spoiling whatever work you did during hardening.
You may also do it this way:
take a scrap piece of the same steel you use for the knife, harden it, file it clean on a face and heat it over the same forge you used to heat it for hardening.
DO NOT put it IN the forge. Keep it over to heat it slowly.
Watch carefully for color change.
When reaching a certain heat the steel will change color, sign that its internal crystalline structure is somewhat changing.
The more you'll heat the piece, the more hardness you'll lose in favour of toughness. Roughly speaking.
The colors are, from harder to tougher
mild straw
straw
burnt siena
ochra
purple
bluish purple
blue
grey blue
If you have a short, thick blade that you want to be sharp and remain sharp (example: skinner) a mild straw to straw temper is enough.
For longer blades, which may have to endure impact (example: fighting knives or swords) you may want to go up to blue.
As soon as the blade reaches the desired color, immediately quench it in water to stop the tempering process.
be also very careful to SLOWLY heat the blade. Once the blade begins changing color, going over the desired color is extremely easy.
The slower will be the heating, the more control you'll have on the tempering process.
About any moron can make up a decent hardening in a piece of steel.
It's tempering, and the right harden/temper ratio that makes the difference between a really good blade and a useless piece of steel that won't hold an edge even if you cry or that will miserably shatter the first real impact it takes.
Make various tests with scrap metal before you attempt it on your knife.
IMPORTANT:
When heating for hardening, keep the blade at the critical temperature for the minimum time necessary to make sure the whole blade is uniformly heated.
Non uniformly heated blades may warp during quench.
Excessive permanence at critical temp will cause excessive grain growth which in turn will make your blade brittle.
Quench the blade in the proper medium. Oil quench steels will crack or shatter if quenched in water.
Quenching in oil is the safest way, as for blade. It may not be the safest for you, if you don't know what to expect, as oil will flare up when a red-hot piece of steel it's immersed in it.
Don't worry, use long thongs to quench, continue quenching and don't worry about the flames. They'll go off by themselves or you may extinguish them with a CO2 fire extinguisher once you are finished quenching.
Just keep away from them and don't do it near anything flammable.
Before hardening it's always better to do some annealing.
Heat the blade to its critical point, then let it cool slowly.
The slower the cooling, the better the annealing will be.
Repeat at least a couple of times, three is better.
The air in the room should be motionless, even better, put the blade in a bucket of hot sand and leave it there for many hours.
A NOTE ON STAINLESS STEEL.
Many stainless steels are air-hardening. This means that air is a good quenching medium for them.
Annealing is impossible with normal methods and should be carried out in electronically controlled electric ovens which follow a precise curve of cooling.
So, at least as a beginner, stay away from stainless or have it heat treated by a heat treating firm.
A FINAL NOTE:
"The Complete Bladesmith" by Jim Hrisoulas (Paladin Press), contains all that you need to know about forges, their use and safety precautions.
Have fun!