Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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I have had a couple recent emails about quenchants and quenching as well as straightening. I thought I would put some basic and simple info on how all this works together. The numbers given are approximate and not intended to be exact for every steel type. I used 1095 as my example. This information may help new makers understand what is going on in a quench and how to control various parts of that quench as well as straightening out of the quench.
For more detailed info on each steel type and the TTT/ITT charts, I highly recommend everyone get Larrin's book Knife Engineering.
Why do they list quenchants in "seconds" speed?
The "speed" given in seconds for a quench is a rating developed for industry by cooling a 1/2" nickel ball from 1620°F tp 670°F in 200ml of the quenchant. Obviously, it isn't comparable to any knife related quench times. We just use the number of seconds as a reference to get an idea of quench speed.
Water/Brine speed - approx. 200-250 °/sec = 2-4 seconds. (For this short article we will discuss oil quenchants. Brine/water quenching is a whole different world.)
Fast oil= 165 °/sec = 7-9 seconds
Medium oil= 115-165 °/sec = 10-14 seconds
Slow oil = 90-125 °/sec. = 15-20 seconds
The "seconds" time given is not an actual rating of how fast a knife blade will cool or needs to be in the oil but is a guide to the cooling rate.
A 1500°F 1095 blade will need to drop 500-600 degrees in less than one second. That is why it requires a fast oil. The usual quenchant for that is called a 7-9 second oil. Parks #50 is the standard for this speed of oil.
Some basic metallurgy:
Heated above 1350°F, steel becomes austenite. Austenite is a soft and easily bent structure. For those who care, the structure is Face Centered Cubic ... FCC. If you are not gentle with a blade during HT and quench you can easily bend the blade. many folks blame this on warp. It is just a mechanical issue and nothing to do with changes of structure causing warp. Blades should be rested spine down in the kiln or forge and held the same way when taken to the quench tank. Holding a blade sideways above 1400°F can easily bend the blade by gravity. Plunging it into a too shallow tank and hitting the bottom can easily bend the tip or bend the whole blade.
As the steel cools in the quench, it has two directions to go. The end result depends on the cooling rate.
One direction is to cool slowly and drop into the pearlite range and the structure converts to pearlite. This is what annealing does. Pearlite is a softer structure composed of layers of ferrite and cementite. It is an FCC structure.
The other direction the steel can go is to cool rapidly enough to miss the pearlite nose and stay austenite. The nose is around 1000°F. Once past the nose and staying to the right in the time-temperature-transformation graph the steel remains as super-cooled austenite. This is still very soft, pliable, and easily bent ... It can be bent by improper procedures, or rough handling in the austenitization and quench. It can also easily be straightened if a warp or twist is seen. You have from 20 second to 100 seconds, depending on the steel and other circumstances, to make these adjustments before the steel changes structures.
Once the super-cooled austenite reached around 400°F is hits the martensite start point (Ms), the structure starts to convert to martensite. Martensite is very hard and very brittle. For the nerds, the structure is Body-Centered-Cubic, or BCC. By 200°F the conversion is mainly over, and by room temperature, it is all converted for most simple carbon steels like 1095. We call this spot on the graph the martensite finish point (Mf). On high alloy and stainless steels the Mf may not happen until the blade is -100°F. This is why these steels need cryo or at least a dry ice bath. The charts show the rate at which this conversion happens (see TTT chart below). Below around 50% conversion the steel will start to stiffen rapidly. This is the sign to stop any straightening attempts. Any attempt to straighten after the blade is fully martensite will snap it like glass. A cooled but untampered blade will shatter if dropped on a concrete floor and may even crack sitting quietly on the bench overnight. This is why you should do the first temper immediately after the blades cool to room temp.
I said the steel "fully converted to martensite" but in actuality there is a small amount of austenite left. We call this Retained Austenite (RA). The amount of RA varies depending on alloying.
Once tempered by heating for one to two hours at a temperature between 350°F and 500°F the blade is cooled to room temp and is now tempered martensite. The first temper also creates a tad of new brittle martensite from the retained austenite. The second temper tempers this new martensite and slightly further tempers the initial martensite. Once done with the two tempers the blade will be hard and tough.
Recap - The first temper softens the brittle martensite into tempered martensite, and also converts some of the RA into new martensite. This new martensite is tempered in the second tempering. The second tempering also assures the first martensite is fully tempered. Skipping a second temper is not wise.
So, how does this play out in blade steel.
For most fast quench steels like 1095, the drop from the austenitization point (around 1500°F) past the pearlite nose at 1000°F has to be done in one second or less. You pick a quenchant like Parks #50 because it can cool the blade that fast. The charts for other steels will show that alloys like O-1 can use a medium speed quenchant.
Once past the pearlite nose, the chart says it has about 400 seconds for 1095 to go from 1000°F to 400°F. Not sure how those Ms numbers are measured? Seems much shorter in my experience. Maybe that is a maximum with slowed cooling. It takes even more time to get down to 50% martensite.
Perhaps Larrin will tell us how they calculate cooling times that long.
The TTT charts are great for figuring quench speed needed ... it has to be fast enough to get past the pearlite nose but should slow down once past it. They are also a good guide for figuring the time you have for straightening. I always advise that you have from 20 seconds to a minute to straighten a blade before it stiffens, depending on size and ambient temp.
I also advise that the blade needs to be handled gently when too hot to hold in your hand ... and once at room temp, very gently until tempered.
Two things that influence the cooling rate after the pearlite nose are the quenchant temperature and how long you keep the blade in the tank. Most oils are at their sweet spot between 120° and 130°F. Parks #50 is an exception and used at room temp for knives. You could quench a blade and leave it in the tank for 10 minutes if you wanted to. It would be fine, except you can't do any straightening until after the tempers.
The time in the quenchant is important if doing straightening right out of the quench- Most carbon steel blades can be removed from the tank after three or four seconds. There are dozens of personal quench procedures makers have worked out. Mine is IN-2-3-4 - OUT 2-3-4-... and a quick check for straightness. If none is seen, I wipe it off quickly with a rag (the rag may catch fire occasionally) and wearing leather gloves I check again for a warp. As long as the blade is still too hot to hold bare handed, I straighten as needed, and then let it finish cooling hanging in the air. I use a magnet strip to hang the blades. If a blade does not need any adjustment, I immediately hang it gently on the magnets. Let them hang untouched until at room temperature.
It is a bad idea to set a hot blade on the anvil to cool. As the structures change it may warp the blade because the side touching the anvil will convert sooner than the other side. Hanging in still air is the best method.
As for the blade temperature and how fast it drops - The time after the nose and before the blade is 50% martensite should give you lots of time to straighten a blade as long as you don't speed up the cooling rate once taken from the oil. It is what affects the cooling after it leaves the oil where you gain or lose time. Air is a lousy coolant, so you can easily check the blade for straightness/twist. A blade sitting on an anvil and hammered with a metal hammer will cool rapidly. A blade sitting on a wooden plank and hammered with a wooden hammer will cool much slower. A blade only touching a small area on a wooden board will cool slowest. Where the blade is after checking for straightness affects the time you have for straightening.
Continued next post......
For more detailed info on each steel type and the TTT/ITT charts, I highly recommend everyone get Larrin's book Knife Engineering.
Why do they list quenchants in "seconds" speed?
The "speed" given in seconds for a quench is a rating developed for industry by cooling a 1/2" nickel ball from 1620°F tp 670°F in 200ml of the quenchant. Obviously, it isn't comparable to any knife related quench times. We just use the number of seconds as a reference to get an idea of quench speed.
Water/Brine speed - approx. 200-250 °/sec = 2-4 seconds. (For this short article we will discuss oil quenchants. Brine/water quenching is a whole different world.)
Fast oil= 165 °/sec = 7-9 seconds
Medium oil= 115-165 °/sec = 10-14 seconds
Slow oil = 90-125 °/sec. = 15-20 seconds
The "seconds" time given is not an actual rating of how fast a knife blade will cool or needs to be in the oil but is a guide to the cooling rate.
A 1500°F 1095 blade will need to drop 500-600 degrees in less than one second. That is why it requires a fast oil. The usual quenchant for that is called a 7-9 second oil. Parks #50 is the standard for this speed of oil.
Some basic metallurgy:
Heated above 1350°F, steel becomes austenite. Austenite is a soft and easily bent structure. For those who care, the structure is Face Centered Cubic ... FCC. If you are not gentle with a blade during HT and quench you can easily bend the blade. many folks blame this on warp. It is just a mechanical issue and nothing to do with changes of structure causing warp. Blades should be rested spine down in the kiln or forge and held the same way when taken to the quench tank. Holding a blade sideways above 1400°F can easily bend the blade by gravity. Plunging it into a too shallow tank and hitting the bottom can easily bend the tip or bend the whole blade.
As the steel cools in the quench, it has two directions to go. The end result depends on the cooling rate.
One direction is to cool slowly and drop into the pearlite range and the structure converts to pearlite. This is what annealing does. Pearlite is a softer structure composed of layers of ferrite and cementite. It is an FCC structure.
The other direction the steel can go is to cool rapidly enough to miss the pearlite nose and stay austenite. The nose is around 1000°F. Once past the nose and staying to the right in the time-temperature-transformation graph the steel remains as super-cooled austenite. This is still very soft, pliable, and easily bent ... It can be bent by improper procedures, or rough handling in the austenitization and quench. It can also easily be straightened if a warp or twist is seen. You have from 20 second to 100 seconds, depending on the steel and other circumstances, to make these adjustments before the steel changes structures.
Once the super-cooled austenite reached around 400°F is hits the martensite start point (Ms), the structure starts to convert to martensite. Martensite is very hard and very brittle. For the nerds, the structure is Body-Centered-Cubic, or BCC. By 200°F the conversion is mainly over, and by room temperature, it is all converted for most simple carbon steels like 1095. We call this spot on the graph the martensite finish point (Mf). On high alloy and stainless steels the Mf may not happen until the blade is -100°F. This is why these steels need cryo or at least a dry ice bath. The charts show the rate at which this conversion happens (see TTT chart below). Below around 50% conversion the steel will start to stiffen rapidly. This is the sign to stop any straightening attempts. Any attempt to straighten after the blade is fully martensite will snap it like glass. A cooled but untampered blade will shatter if dropped on a concrete floor and may even crack sitting quietly on the bench overnight. This is why you should do the first temper immediately after the blades cool to room temp.
I said the steel "fully converted to martensite" but in actuality there is a small amount of austenite left. We call this Retained Austenite (RA). The amount of RA varies depending on alloying.
Once tempered by heating for one to two hours at a temperature between 350°F and 500°F the blade is cooled to room temp and is now tempered martensite. The first temper also creates a tad of new brittle martensite from the retained austenite. The second temper tempers this new martensite and slightly further tempers the initial martensite. Once done with the two tempers the blade will be hard and tough.
Recap - The first temper softens the brittle martensite into tempered martensite, and also converts some of the RA into new martensite. This new martensite is tempered in the second tempering. The second tempering also assures the first martensite is fully tempered. Skipping a second temper is not wise.
So, how does this play out in blade steel.
For most fast quench steels like 1095, the drop from the austenitization point (around 1500°F) past the pearlite nose at 1000°F has to be done in one second or less. You pick a quenchant like Parks #50 because it can cool the blade that fast. The charts for other steels will show that alloys like O-1 can use a medium speed quenchant.
Once past the pearlite nose, the chart says it has about 400 seconds for 1095 to go from 1000°F to 400°F. Not sure how those Ms numbers are measured? Seems much shorter in my experience. Maybe that is a maximum with slowed cooling. It takes even more time to get down to 50% martensite.
Perhaps Larrin will tell us how they calculate cooling times that long.
The TTT charts are great for figuring quench speed needed ... it has to be fast enough to get past the pearlite nose but should slow down once past it. They are also a good guide for figuring the time you have for straightening. I always advise that you have from 20 seconds to a minute to straighten a blade before it stiffens, depending on size and ambient temp.
I also advise that the blade needs to be handled gently when too hot to hold in your hand ... and once at room temp, very gently until tempered.
Two things that influence the cooling rate after the pearlite nose are the quenchant temperature and how long you keep the blade in the tank. Most oils are at their sweet spot between 120° and 130°F. Parks #50 is an exception and used at room temp for knives. You could quench a blade and leave it in the tank for 10 minutes if you wanted to. It would be fine, except you can't do any straightening until after the tempers.
The time in the quenchant is important if doing straightening right out of the quench- Most carbon steel blades can be removed from the tank after three or four seconds. There are dozens of personal quench procedures makers have worked out. Mine is IN-2-3-4 - OUT 2-3-4-... and a quick check for straightness. If none is seen, I wipe it off quickly with a rag (the rag may catch fire occasionally) and wearing leather gloves I check again for a warp. As long as the blade is still too hot to hold bare handed, I straighten as needed, and then let it finish cooling hanging in the air. I use a magnet strip to hang the blades. If a blade does not need any adjustment, I immediately hang it gently on the magnets. Let them hang untouched until at room temperature.
It is a bad idea to set a hot blade on the anvil to cool. As the structures change it may warp the blade because the side touching the anvil will convert sooner than the other side. Hanging in still air is the best method.
As for the blade temperature and how fast it drops - The time after the nose and before the blade is 50% martensite should give you lots of time to straighten a blade as long as you don't speed up the cooling rate once taken from the oil. It is what affects the cooling after it leaves the oil where you gain or lose time. Air is a lousy coolant, so you can easily check the blade for straightness/twist. A blade sitting on an anvil and hammered with a metal hammer will cool rapidly. A blade sitting on a wooden plank and hammered with a wooden hammer will cool much slower. A blade only touching a small area on a wooden board will cool slowest. Where the blade is after checking for straightness affects the time you have for straightening.
Continued next post......
