Heat treat with a torch?

If the blade is high carbon steel, and you know what you are doing, yes.
 
All know that I am a person who does not recommend the less complete path, but in fairness to those who can only travel that way:

HT requires two things- Reaching a desired temperature, and Holding it there evenly.

Reaching the desired temperature when using a torch is not a problem. Actually, keeping the blade low enough in temperature is usually the problem. The torch should have a large ,wide , and non-concentrated flame. A rosebud tip on an A/O torch will work. Another excellent torch for HT is a Presto-lite torch. It is a plumber's torch that runs a venturi torch on the end of a hose, and burns only acetylene. You change the tubular tips from a tiny flame you can do jewelry with to a huge tip that will thaw frozen pipes. A third choice is a hand held MAPP gas bottle torch with a wide flame nozzle. These are commonly used to sweat solder joints on copper pipe. All these will get carbon steel to 1500°F ( and beyond if you aren't careful).

Keeping the blade from getting too hot is usually the hard part. Use a magnet to detect when the steel reaches about 1350°F. At that point, the blade will stop being attracted to the magnet. It is called the Currie Point, and has to do with a lot of technical stuff about atoms and electrons that I won't go into. Just know that it happens every time on every steel at the same temp. Get a cheap welder's magnet from HF and stick it on any metal object near where you will be doing the HT. A vise or the anvil is fine. It just needs to be available within 12" of where you are heating the blade. As the blade heats up it will first become a dull red at around 900-1000°F. BTW, it is best to do HT in a more subdued lighting. Avoid bright light or direct sunlight. You can see the red shades better in shade or lower light. Next it will become a soft red at 1100-1200°F. Start touching the blade to the magnet briefly as the blade heats up. As it turns a bit brighter red color, it will suddenly stop sticking to the magnet. Heat it up a bit redder ( 100° hotter) to reach the desired range of 1450-1500°F, which is the norm for standard carbon steels used for blades. In all probability, you will heat it higher than you want, so try and pay attention to the color as it becomes non-magnetic, and shoot for just one shade brighter red. If possible ,try and keep the blade at just that temperature for a minute or two. Five and ten minute soak times as are recommended for 1095 and other steels are not really possible with a torch, so getting it there for a minute or two and quenching is the best you can hope for. 1084 steel needs no soak time at all - you just get it to right temperature and quench it.

Keeping the blade at an even temperature is the most important thing. The thin edge and the tiny tip will heat much faster than the spine......so pay attention to these areas mainly. If the spine isn't as hard as the edge, that isn't a problem. It would be a plus. Overheating the edge or the entire blade is the killer of a good knife. As the blade is heated, heat it from the spine side, and KEEP THE FLAME MOVING. Move it down the tang , along the spine, and almost to the tip - don't go quite to the tip. Try and keep the main flame off the edge, too. That will allow the edge and tip to have the heat radiate to them. This will give you much more control and evenness of heat. The other thing that is important is GO SLOW. Heating with a torch should use the minimum size flame that will get the blade up to 1450-1500°F. It is not needed or desired to have a flame that can melt steel. If it takes five or six minutes to heat up the blade, be thankful that you will get a much better hardening than if you brought it up to bright red in 30 seconds. The slower the heating, the more even it will be. The way to control the temperature in heat-up and trying to hold it there is to move the torch farther or nearer from the blade. A good trick is to put the torch in a vise and only hold the blade in the pliers/tongs. That way you don't have to deal with the torch while quenching. Another way is to have someone else hold the torch. You do not want to be trying to shut off a torch and quenching the blade all at the same time.

Quench the blade in 100-120°F Canola oil if you don't have a commercial quenchant. Do not use old motor oil, ATF, or any other fats or "found" oils. Quench the whole blade by plunging it straight into the tank point down. Move the blade up and down, and in a slicing motion from spine to edge, but never side to side. You want at least a gallon of oil for a small blade. Two gallons is better. The oil can be used many times if it is stored in closed bottles. The tank should be a metal tube of some sort, that is at least 6" deeper than the blade is long.

After the quench, it is OK to test the edge with a file to see if it skates down the hard steel. If it passes this quick test, wipe/wash off the oil and put the blade into the kitchen oven at 400°F for one to two hours. Take out and cool off in running water, then put back in the oven for another hour or two. Try and do this as soon after the quench as possible.
 
Why not use motor oil I have seen where many people use it. I am talking about making a knife from a file after the intial heating and shaping they put it in a fire again and heat until the magnet won't stick and then quench it in old motor oil. Is this not a good idea?
 
Good idea? Not reeally. It will work to some degree. But, the problems are an old file which is probably a steel with around 1% carbon, unless it was a cheep file that was only case hardened and will never make a good knife. If it is a steel like 1095, W1 or W2 the 1% carbon factor requires that you get it about 100f higher than non magnetic to get all the carbon into solution, less than 1475f all the carbon will not go into solution, much higher than that and it will suffer from grain growth and get more brittle. So temp is critical for optimum results. Then steels like this have a very short quench time that must be met to get the steel to form the maximum amount of martinsite. If either the quench speed or the temp is not met you will be left with a blade that has a lot of softer formations. Used motor oil does not have the ability to make this happen. You really need a much faster oil. A couple gallons of vegetable cooking oil like canola will work. Warm 130f oil will work better than cold oil by the way.

So, if you take a knife made from a good file and heat it to non magnetic and dunk it in motor oil it will harden, but not completely. You will end up with a substandard blade due to a poor heat treat. If you know what you are doing and take it up another 50f and then use a couple gallons of a better oil at 130f to quickly quench it and then temper it 2 times for 2 hours at say 425-450f you will end up with a MUCH better blade if the file was good steel. Much better off to buy some cheap 1080-1085 as it will go into solution at non magnetic and then use the Canola oil. Use the old motor oil to soak saw dust for fire starter or something. The only thing I use it for in my shop is feed it into the stove for heat.
 
Yes, many people use it. Many people also shoot themselves playing with guns, get STDs from their girl/boy friends because of unprotected sex, and skateboard down steep hills without a helmet. None of those are good ideas just because many people do them. A fire pit HT and motor oil quench is only one more to add to that list.

The same or less effort will give a fair to good blade with Canola and a torch.
 
I took your advice and ordered some 1080. After what yall said I think this gives me the best chance for success in my first attempt at knife making.
Thanks for your help.
 
Stacy has good info, listen to him on the light and keep moving the torch, really important. Make sure to check your knife with a magnet often. It is really easy to over heat your knife with the torch. I used a torch until I could buy an oven. I really depends on the type of steel and intended use of the knife. Good luck!
 
What about a Brine quench for 1095 and the like. I find it cools down the metal much faster than the canola oil I have used.
 
Brine works, but has a tendency to make smaller knives out of larger ones, compared to oil. Motor oil is too slow for 10xx steel like 1095. It will be harder, but not nearly as good as possible.
 
Stacy I have another question for you. I have some old files I want to practice on is a fire a ok way to heat them so I can shape them?
 
.;I have been heat treating knives for over 20 yrs. and have no problem using used motor oil for harding blades. the low lite is a very good idea.
you can judge color better in low lite. I have maybe two blades warp in the total time. Never lost a blade by having it crack. My only test for hardness is with a new file and i only hardenas far as the grind goes. from there into a 400 to 425 degree oven for an hour. I then let it cool in the oven. works good for me Leroyk
 
I don't think its a mater of can motor oil be used. It is a mater of, is the motor oil fast enough to make the "total" transition of martensite on a specific type of steel. Meaning getting the best out of the steel that needs a fast quenching medium. Really depends on how you choose to look at it.

For 1095, water, brine and parks 50 work. I have tried quenching 1095 in oil and it did not work properly, reason I know, is because I broke the blade and looked at it. Then I heat treated another and used water and looked at the differences. I wish I took pictures. Both done with a torch. I will have to do it again and add what soaking it in the oven and doing some normalizing to it looks like and then take pictures.
 
leroyk3 said:
.;I have been heat treating knives for over 20 yrs. and have no problem using used motor oil for harding blades. the low lite is a very good idea.
you can judge color better in low lite. I have maybe two blades warp in the total time. Never lost a blade by having it crack. My only test for hardness is with a new file and i only hardenas far as the grind goes. from there into a 400 to 425 degree oven for an hour. I then let it cool in the oven. works good for me Leroyk
Click to expand...

You may want to try something faster unless you are using O-1 you are leaving a lot of pearlite or retained austentite where you should have martensite. If your "test"that tells you your results are"ok" is that it skates a file, lamellar pearlite will skate a file just fine but you are probably coming up 5-10 Rockwell points softer than you should be

-Page
 
tbow,

A fire pit is about the worst way to do any heat function on a blade you plan on keeping. If you have one you don't like how it is coming out, toss it in the fire and let it destroy itself. If you want to use and keep a blade, do the HT some other way.

Seriously, I know that there are neo-tribal types who love to use a campfire or wood stove to do annealing, HT and even tempering....but there are so many better ways. Even a charcoal grill and a hair dryer is better.
 
I was actually going to use a charcoal grill. I planned to build a charcoal fire let it burn down and put the file in and then put some wood on it, all the while blowing on the fire with a fan or a hair dryer. Is this a viable plan? Can I use the torch you spoke of to soften the file to be able to shape it?
 
Do some searches on a "brake drum forge" and you will get some low cost forge ideas. You will also see how the fuel and air system works. A charcoal grill can be converted, but it takes more work than it is worth. Just building a wood fire in a grill isn't a good plan. A torch can be used to anneal a file, and then used to harden it after shaping with files and sandpaper. It isn't useful for forging.
 
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