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- Nov 26, 2001
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- 1,375
I've read a lot of questions about heat treat from beginner knifemakers. Why not make a sticky post from the experts that explains the basics?
I'll start here with some advice. Not as an expert, but as a dummy that has started understandig some things, has low tech equipment and just wants to write "How I did it"
1) As a beginner, avoid fancy steels. What you want to use are plain carbon steels. To harden, steel has to have at least .40 carbon content. Depending on carbon content, and other small quantities of alloying elements, steels will be easier or more difficult to harden.
2) You need a forge. A simple burner can't obtain sufficient heat, nor get the steel to an even heat.
You may make a forge out of a soft refractory brick by hollowing it out with a lengthwise oval cavity, smoothing the inside with a half-round file. Make 1 or 2 holes on the side and put 1 or 2 Bernzomatic, Rothenberger or Walkover burners in, at a tangent with the cavity, to get a swirl of hot gases that will help avoid hot spots.
3) You need a magnet. Plain carbon steel is ready to quench when it becomes non-magnetic. Steel is a poor heat conductor, so you'll need to let the heat soak in for some more time before quenching, when it gets non magnetic. The point is to avoid rising in temperature.
Keep the steel at too high a temperature for too much time, and you'll have crystal growth that will weaken the blade.
4) Use 5 W 30 or, even better, hydraulic fluid, to quench steel (best would be an appropriate quenching oil, but they may be difficult to find)
PROCEDURE:
The forge must not be at its maximum output. The steel must heat slowly and must not go very much past non magnetic.
For a gas forge (say a one brick) keep the burners down on gas. For a coal forge, have a nice, uniform bed of coals, orange-yellow, and turn the blower down to a mere breath.
Heat up the finished piece to be heat treated slowly, and check with magnet. When it becomes non-magnetic, take note of the color the steel has. Let the piece soak heat some more (say 1 minute), but it must not become brighter than it is.
Get it out from the forge, and let it cool slowly in the hot gases coming from the forge, then put it on the top of the forge and let it cool to room temp. This is called normalizing. It refines the grain in the steel and relaxes any internal stresses caused by working that may make the blade warp during quench.
Do it twice.
Then, heat again slowly with the hot gases, until the blade changes color, then put it in the forge (this is to avoid putting a cold blade directly in the raging inferno of the forge).
Periodically check with the magnet. If the steel fails to reach non-magnetic, turn up the heat a little.
As the steel becomes non magnetic, let it soak heat some more (time varies with stock size and thickness: for a medium sized knife with 5 mm blade I soak it for a minute or so). You must have the quenching medium ready. I use hydraulic fluid.
Take the blade out and IMMEDIATELY plunge it in the quenching medium tip down, slowly moving it in an "8" pattern for the first 10 seconds or so. Then leave it stationary, and don't take it out until the oil has stopped rippling due to convective motion. This way the blade will be cold enough to handle.
CAUTION: thin oil and hydraulic fluid tend to flare up badly during quench if the blade is not fully sumberged (as in differential hardening, described below). Use long tongs to hold the blade, and quench in open air.
Avoid breathing the fumes.
When finished, cover the can and the fire will extinguish itself.
Clean away any residue oil, and grind away the scale. The blade now is as hard and as fragile as glass. DON'T tap it on something to see if it rings or such! You will easily break it. Just trust me: the blade is hard. If your faith is weak, take a file and try it on the edge: the file should skate on the edge, and it shouldn't be able to bite in the steel. You have just dulled the teeth of a perfectly serviceable file. That's the reward for those who doubt!
Grind away enough scale to see the sides of the blade.
It must be done delicately and quickly.
Return to the forge. Use barely a breath of gas or air now.
Expose the knife to the hot gases of the forge to temper it.
Tempering will reduce hardness somewhat, but will greatly increase toughness.
Here is where things get difficult and you need the utmost patience. Tip and edge will heat more quickly than the spine and tang of the knife. So, try to keep the blade spine down and the tip out of the direct flux of hot gases.
Keep the blade distant from the forge, initially. It must heat up slowly.
A drop of water on the blade may tell you when you reach boiling temperature. From this point, in some time the blade should begin to change color. From pale yellow, to straw yellow, to brown, to purple, to blue, to bright blue and then, finally, to gray again.
Colors tell you how hard and tough the steel is. The more you proceed along this colors, the softer and tougher the steel will become. For most knives a straw color is good temper. Thinner and longer blades may be tempered at brown.
Short, stubby, thick blades may go no farther than bright yellow.
The trick is to heat veeeery slowly, and keep the blade at that color for several minutes (no less than 15-20).
This way the temper will go through the entire blade. Rotate the blade every now and then, and keep a close eye on it for color change. When color change will start, if you keep the blade too close it will start and go on very quickly.
For this reason you must keep the blade off the heat and let it heat very slowly.
When you have the proper color and enough time is passed, let the blade cool in air.
When it's cold, you can finish the knife.
Tempering requires PATIENCE.
If you get the blade near the fire to heat it more, you'll quickly run through yellow and brown into purple and blue.
If you see the blade get brown, quickly quench it in water. Clean it, and start again until you get a proper straw color, going slower this time.
If the blade reaches blue, you spoiled the hardening. You must start heat treating all over again.
So, go SLOWLY.
Heat SLOWLY.
Let the blade soak heat.
When you get near the color you want, back off a little, and let the blade come to the exact color you need very slowly. Back off a little more, and keep it there for several minutes, to make sure the tempering goes well through the whole thickness of the blade.
Turn the bladee very now and then to change the face exposed to heat.
Be careful of the edge and tip, because they easily overheat. Try to keep them out of direct heat.
You may also make a differential heat treat of your knife in two ways:
DIFFERENTIAL HARDENING
Quench the blade at a diagonal so that just the edge and point go into the quenching liquid.
Keep it there for 10-15 seconds or so, then quench the entire knife.You'll get a very hard edge, and softer spine.
Temper to pale straw, no more, since the knife's spine is already soft (but check against the steel properties. Some steels harden and become fragile anyway).
You may also try to quench the edge, then take out the knife and let it cool in air for some time: clean off the scale from the edge.
The heat from the incandescent spine will go into the edge, tempering it. When it reaches proper color, quench the entire knife.
You may also obtain differential hardening by heating the blade edge down on the hot coals, and have it heat up from the edge and tip first (they heat more easily).
When the edge is non magnetic, quench the whole blade, then temper to straw.
The spine will not have reached a sufficient heat to harden.
Again, hard edge and soft spine.
Finally, there's refractory clay tempering (japanese style). One could write a book on this subject alone (and indeed many have been written) so I'll leave it at this stage, just mentioning it exists.
DIFFERENTIAL TEMPER
Temper to a pale straw yellow the whole knife.
Take a block of steel, folded over rebar, or copper block, and heat it to incandescence (don't put copper into your forge if you want to forge weld in it!).
Put the spine, and the spine only, of the blade, up to an inch or so from the point, and put it in contact with the hot mass, until it gets blue. You may also use an oxy-gas torch keeping the edge in water to avoid heating it.
This way you'll get a hard edge with a tough blade.
You may also temper spine down on the hot coals.
Now, the real experts could step in and correct any stupid things I've said (there will be many of them) and add their tips
I'll start here with some advice. Not as an expert, but as a dummy that has started understandig some things, has low tech equipment and just wants to write "How I did it"
1) As a beginner, avoid fancy steels. What you want to use are plain carbon steels. To harden, steel has to have at least .40 carbon content. Depending on carbon content, and other small quantities of alloying elements, steels will be easier or more difficult to harden.
2) You need a forge. A simple burner can't obtain sufficient heat, nor get the steel to an even heat.
You may make a forge out of a soft refractory brick by hollowing it out with a lengthwise oval cavity, smoothing the inside with a half-round file. Make 1 or 2 holes on the side and put 1 or 2 Bernzomatic, Rothenberger or Walkover burners in, at a tangent with the cavity, to get a swirl of hot gases that will help avoid hot spots.
3) You need a magnet. Plain carbon steel is ready to quench when it becomes non-magnetic. Steel is a poor heat conductor, so you'll need to let the heat soak in for some more time before quenching, when it gets non magnetic. The point is to avoid rising in temperature.
Keep the steel at too high a temperature for too much time, and you'll have crystal growth that will weaken the blade.
4) Use 5 W 30 or, even better, hydraulic fluid, to quench steel (best would be an appropriate quenching oil, but they may be difficult to find)
PROCEDURE:
The forge must not be at its maximum output. The steel must heat slowly and must not go very much past non magnetic.
For a gas forge (say a one brick) keep the burners down on gas. For a coal forge, have a nice, uniform bed of coals, orange-yellow, and turn the blower down to a mere breath.
Heat up the finished piece to be heat treated slowly, and check with magnet. When it becomes non-magnetic, take note of the color the steel has. Let the piece soak heat some more (say 1 minute), but it must not become brighter than it is.
Get it out from the forge, and let it cool slowly in the hot gases coming from the forge, then put it on the top of the forge and let it cool to room temp. This is called normalizing. It refines the grain in the steel and relaxes any internal stresses caused by working that may make the blade warp during quench.
Do it twice.
Then, heat again slowly with the hot gases, until the blade changes color, then put it in the forge (this is to avoid putting a cold blade directly in the raging inferno of the forge).
Periodically check with the magnet. If the steel fails to reach non-magnetic, turn up the heat a little.
As the steel becomes non magnetic, let it soak heat some more (time varies with stock size and thickness: for a medium sized knife with 5 mm blade I soak it for a minute or so). You must have the quenching medium ready. I use hydraulic fluid.
Take the blade out and IMMEDIATELY plunge it in the quenching medium tip down, slowly moving it in an "8" pattern for the first 10 seconds or so. Then leave it stationary, and don't take it out until the oil has stopped rippling due to convective motion. This way the blade will be cold enough to handle.
CAUTION: thin oil and hydraulic fluid tend to flare up badly during quench if the blade is not fully sumberged (as in differential hardening, described below). Use long tongs to hold the blade, and quench in open air.
Avoid breathing the fumes.
When finished, cover the can and the fire will extinguish itself.
Clean away any residue oil, and grind away the scale. The blade now is as hard and as fragile as glass. DON'T tap it on something to see if it rings or such! You will easily break it. Just trust me: the blade is hard. If your faith is weak, take a file and try it on the edge: the file should skate on the edge, and it shouldn't be able to bite in the steel. You have just dulled the teeth of a perfectly serviceable file. That's the reward for those who doubt!
Grind away enough scale to see the sides of the blade.
It must be done delicately and quickly.
Return to the forge. Use barely a breath of gas or air now.
Expose the knife to the hot gases of the forge to temper it.
Tempering will reduce hardness somewhat, but will greatly increase toughness.
Here is where things get difficult and you need the utmost patience. Tip and edge will heat more quickly than the spine and tang of the knife. So, try to keep the blade spine down and the tip out of the direct flux of hot gases.
Keep the blade distant from the forge, initially. It must heat up slowly.
A drop of water on the blade may tell you when you reach boiling temperature. From this point, in some time the blade should begin to change color. From pale yellow, to straw yellow, to brown, to purple, to blue, to bright blue and then, finally, to gray again.
Colors tell you how hard and tough the steel is. The more you proceed along this colors, the softer and tougher the steel will become. For most knives a straw color is good temper. Thinner and longer blades may be tempered at brown.
Short, stubby, thick blades may go no farther than bright yellow.
The trick is to heat veeeery slowly, and keep the blade at that color for several minutes (no less than 15-20).
This way the temper will go through the entire blade. Rotate the blade every now and then, and keep a close eye on it for color change. When color change will start, if you keep the blade too close it will start and go on very quickly.
For this reason you must keep the blade off the heat and let it heat very slowly.
When you have the proper color and enough time is passed, let the blade cool in air.
When it's cold, you can finish the knife.
Tempering requires PATIENCE.
If you get the blade near the fire to heat it more, you'll quickly run through yellow and brown into purple and blue.
If you see the blade get brown, quickly quench it in water. Clean it, and start again until you get a proper straw color, going slower this time.
If the blade reaches blue, you spoiled the hardening. You must start heat treating all over again.
So, go SLOWLY.
Heat SLOWLY.
Let the blade soak heat.
When you get near the color you want, back off a little, and let the blade come to the exact color you need very slowly. Back off a little more, and keep it there for several minutes, to make sure the tempering goes well through the whole thickness of the blade.
Turn the bladee very now and then to change the face exposed to heat.
Be careful of the edge and tip, because they easily overheat. Try to keep them out of direct heat.
You may also make a differential heat treat of your knife in two ways:
DIFFERENTIAL HARDENING
Quench the blade at a diagonal so that just the edge and point go into the quenching liquid.
Keep it there for 10-15 seconds or so, then quench the entire knife.You'll get a very hard edge, and softer spine.
Temper to pale straw, no more, since the knife's spine is already soft (but check against the steel properties. Some steels harden and become fragile anyway).
You may also try to quench the edge, then take out the knife and let it cool in air for some time: clean off the scale from the edge.
The heat from the incandescent spine will go into the edge, tempering it. When it reaches proper color, quench the entire knife.
You may also obtain differential hardening by heating the blade edge down on the hot coals, and have it heat up from the edge and tip first (they heat more easily).
When the edge is non magnetic, quench the whole blade, then temper to straw.
The spine will not have reached a sufficient heat to harden.
Again, hard edge and soft spine.
Finally, there's refractory clay tempering (japanese style). One could write a book on this subject alone (and indeed many have been written) so I'll leave it at this stage, just mentioning it exists.
DIFFERENTIAL TEMPER
Temper to a pale straw yellow the whole knife.
Take a block of steel, folded over rebar, or copper block, and heat it to incandescence (don't put copper into your forge if you want to forge weld in it!).
Put the spine, and the spine only, of the blade, up to an inch or so from the point, and put it in contact with the hot mass, until it gets blue. You may also use an oxy-gas torch keeping the edge in water to avoid heating it.
This way you'll get a hard edge with a tough blade.
You may also temper spine down on the hot coals.
Now, the real experts could step in and correct any stupid things I've said (there will be many of them) and add their tips
