Heat treating sword blades.
I recommend learning and practicing this heat treatment technique with small pieces, either scrap or possibly dagger blades, before trying it on sword blades.
Read and understand all of this before you try it. The whole thing. There are very important safety rules to follow, and the procedure must be done properly to be successful. You on't want to waste the lot of work you put into making the blade.
Preparations for heat treatment:
Mount a magnet on a handle 10 or 20 cm. long. The handle could be wood or wire. The size of the magnet doesn't matter much, but if it's too small it won't work well and if it's too big it will be too heavy to hold conveniently. The magnet will be used for testing the tmperature of the heated blade. When the blade is heated to and above its "critical temperature" the magnet will not stick to the blade. The steel loses its magnetic properties when it is hot enough. It must be this hot in order for hardening to work.
Prepare a tank for the quenching fluid. It must be very close to where you heat the blade to red hot because the blade must be still at full hardening temperature when it goes into the quenching bath. You have to go very quickly from heat to quench. The quenching tank can easily be made from a large piece of pipe, 10 or 15 cm. in diameter and 120 to 150 cm. long. Weld a large, flat plate covering one end. The plate also forms a base to keep the tank from tipping over. It would be a good idea to drill and tap a drain plug hole close to the bottom of the pipe. Put in a plug or valve to use when you have to empty the tank. It would be a good idea to cover the inside bottom of the tank with a piece of wood to prevent damage to dropped blades, but it's hard to find a good way to hold down the wood and prevent it from floating to the top. There should also be a metal cover easily available to cover the tank and smother any burning oil. Putting a red hot blade into oil is likely to cause the oil to ignite and burn, and it will definitely make some smoke! IMPORTANT: when dunking a hot blade into the quenching fluid, be sure to hold the blade tang with long tongs or some other device to keep hands away from the possible firy flare-up from the oil.
The best quenching fluid for spring steel is oil. Nearly any type will do, from old, used motor oil to salad or cooking oil. Different oils do have different quenching properties, of course, and some types will be better than others. Water can also be used but will quench the heat of the blade much faster. This rapid quenching is likely to cause warping and possibly cracking.
Sword blade heat treatment procedure:
The blades must be heated very evenly over the whole blade.
Slowly, carefully, evenly heat the entire blade to about 1600 degrees Fahrenheit. Sorry, all temperature references here are in deg. F, rather than C. It will be dull cherry to cherry red. Hold the blade tang tightly and IMMEDIATELY lower the blade rapidly point first into the quenching fluid. The blade MUST still be at or above its "critical temperature" when it goes into the quenching medium or it will not harden. Be prepared for the fluid to boil vigorously. Keep moving the blade up and down a few centimeters while it cools. When it is cool enough to touch with your hand, it's done. Don't drop it - it may be fairly brittle, and break. Don't attempt to bend or straighten it.
Clean or polish the blade bright with fine sandpaper or some other convenient abrasive. It should be bright but doesn't have to be shiny. Then slowly and evenly heat the blade to about 600 degrees Fahrenheit (see table below). It must be a very even heat. It's possible to use a large steel box filled with sand and heated over burners, or to use molten lead or even a thick, fairly special oil that can withstand 550 to 600 degrees without catching fire. If using a tempering bath of oil or lead be extremely careful to ensure that there is no water or moisture on the blade before putting it into the bath. Otherwise a type of explosion may occur and spray hot liquid on anyone nearby. Another heating method with fairly good control is to mount a heavy cast iron or steel plate above burners, heat it red hot and rest the blade on it, turning or moving it as needed to keep the heat even.
Heat the blade to about 600 degrees F. As it heats, colors will appear on the polished surface. That's the reason for polishing the blade. Heat it until it turns dark blue and then immediately remove it from the heat source. You can then cool it either rapidly or slowly, whichever you prefer. However, if the blade warped during hardening, straighten it while it's hot from tempering.
For sword blades, a good hardness is about the same as a good leaf spring, a Rockwell C rating of about 52. That's what you should get from tempering at 600 degrees F. Test a few blades for strength and springiness. If they are too soft and not springy enough to return to straight from a 30 or 45 degree bend, reharden and then re-temper to a lower tempering temperature. If a blade is too hard, it will hold a cutting edge better but it will break. Broken blades are worse than bent ones!
After the hardening and tempering, you can do the final grinding on the blade. It's a good idea to grind off about 1/2 mm. or so because the outer part of the steel will lose some carbon due to oxidation while it's red hot. That means the thin steel at the cutting edge would be lower carbon and thus softer and less capable of holding a cutting edge. This means that you should leave the blade slightly oversized until grinding to final size after hardening and tempering are complete.
Approximate tempering temperatures for various Rockwell C hardnesses of leaf spring steel:
the following table lists a few of the colors in order of appearance, with their temperatures:
Color - - - - - - - - - - Degrees Fahrenheit
Very Pale yellow-------------430 degrees
Pale straw-yellow------------450 degrees
Dark yellow -----------------480 degrees
Brown yellow----------------500 degrees
Brown purple----------------520 degrees
Light purple------------------530 degrees
Full purple--------------------540 degrees
Dark purple-------------------550 degrees
Full blue-----------------------560 degrees
Dark blue----------------------570 degrees
Very dark blue---------------600 degrees
Light blue---------------------640 degrees
Red, visible in dark---------752 degrees
Red, visible in twilight----885 degrees
Red, visible in daylight----975 degrees
Red, visible in sunlight--1077 degrees
Dark red-----------------------1292 degrees
Dull cherry red--------------1472 degrees
Cherry red--------------------1652 degrees
Bright cherry red-----------1832 degrees
With a good eye and some practice you can judge temperatures in steel with surprising accuracy. It's very important to use a constant light source however, especially when judging the reds.
Another knifemaker's web site (Bob Engnath's) lists these:
RED HEAT TEMPERATURES
Faintest dull red - 900F
Dull red - 1200F
Cherry red - 1400F
Orange red - 1500F
Bright salmon orange - 1600F
Brilliant orange - 1640F
Here's how Ian Major does his heat treating:
I have been making weapons for about three years, and found that case-hardening is an
excellent method for getting a dark finish that is rust resistent. It
is quite easy to do, one just heats the blade and quenches in a high
carbon substance (I use sawdust) and then repeat the heating and cooling
process. You may know the technique by a different name, but I have
found it to make a durable blade.
I have found the best method is to fix the blade to a section of
more durable steel (such as a portion of a leafspring) that will take a
lot more heat to twist then the cold-rolled stock that I generally use.
I mainly drill a hole that I will eventually cover with the handle and
bolt this end to the 'guide steel'. The other end and the middle use a
couple of old C-clamps. It will take a lot of heat, as the guide will
adsorb a great deal of it, but the blade will stay true. Have a hollow
bar ready so you don't have to fiddle with the C-clamps (they don't have
much of a lifespan when you treat them like this.)
I hope this suggestion helps.
I have just finished a Tai-Chi sword in Stainless this week. It is
quite a different ballgame then what I'm use to. It took me over 3
hours drill and tap four holes for the handle.
There's a page with lots of heat-treating tables of information at a nice knife (and sword) making site by Bob Engnath at http://www.knives.com/heatreat.html .
Added December 21, 2000
More info, in response to an email question:
"Hardening" is quenching the steel from it's 'critical' temperature, when
it's no longer attracted to a magnet, cooling it as rapidly as possible down
to say 200 deg. F or so. Then the steel is as hard as it can get and is
also quite brittle. This is the condition of the steel that good files are
made out of.
Annealing is heating the steel to its critical temperature and then cooling
it very slowly, say 50 deg. F per hour. A common way is to heat it to very
red and then bury it in a light insulating material. Vermiculite, lime or
dry ashes are commonly used. Commercial heat treaters slowly decrease their
oven temperature to accomplish the same effect. The steel is then as soft
as it can get.
For blades, after hardening the next process is called tempering, or drawing
the temper. It is heating the steel again, but to a very limited
temperature. For a sword blade it's around 500 or 550 deg. F, for example.
Your choice of fuel for the fire is based strictly on cost and convenience.
The steel doesn't care. It takes a lot of charcoal, and once coal was
discovered, far less charcoal was used. Knifemakers now often use
gas-burning forges, either natural (from oil wells) gas or LP/propane gas
If you don't use power tools, all you need to do is saw and file. Lots and
lots of filing. In fact, with enough filing you wouldn't need to saw. Or
if you forge a blade to shape, then you don't need to saw it (to shape, like
You should be able to find lots of information about blacksmithing and
knifemaking. Both of those sources will give information on fires, fuels
and heat treating. There are many web sites about both activities and some
have great amounts of detailed information.
For more info online, see professional knifemaker Tim Lively's HEAT TREATING THE FORGED BLADE at http://220.127.116.11/heattreat.htm (he also has a video available)
-----home ---- back to Swordmaking Page ----e-mail me. ------ updated January 7, 2003