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View Full Version : A few questions about heat treating



PetrifiedWood
07-22-2011, 10:42 AM
I want to be reasonably sure that I am doing a correct job of heat treating. I spend several hours here a couple of days ago reading through the HT section and it seems like the more I read, the more confused I became.

I have a 27" Evenheat kiln with a Rampmaster controller. It's new, and has only had a break-in run, and one live run with a 1/8" A2 knife.

What I want to know is, can I trust that if I set a temperature and time program in the kiln that corresponds to the steel manufacturer's instructions, will the steel be properly hardened?

I have some O1 on hand and that's going to be the medium for my next several knives. I have read all kinds of arguments for and against a range of different oils, and different oil temperatures.

I also want to know why it's so important to temper immediately. I've heard that internal stresses in the steel can cause it to crack, and that quickly tempering can prevent this from happening. But it seems to me that if a knife is going to crack it will happen in the first minutes after quenching. Has anyone had a blade crack on them several hours after quenching that was fine for a few hours after?

The reason I ask is that in order to achieve some of the tempering temperatures I'd like to use, I might need to use my kiln. It would need to cool to a suitable temperature first, and that takes time. Enough time that I don't think it qualifies as "immediately". So I want to know what happens to the steel after it's been quenched and before tempering that can be avoided by tempering immediately.

Also, I read a really old post by a guy who has an evenheat and was measuring a different temperature at the top and bottom of his oven opening. The only way I have to measure temperature in my kiln is by trusting the thermocouple and controller readout.

I had hoped to eliminate the guesswork and experimentation by getting a kiln. If guys with a coal forge and a magnet can get good consistent results, surely a temperature controlled kiln should be able to produce the proper results?

For example, my A2 program involved placing the blade in the oven cold. Then ramp up to 1500 and hold for 30 minutes. Then ramp up to 1800 and hold for 30 minutes, then I removed it from the kiln and hung it up to cool in my shop. I then tempered at 550 degrees when it was "hand warm" for 2 hours. Let it cool to "hand warm" and tempered again for 2 hours. I was shooting for a rockwell C of 58 to 60. I have no way of measuring hardness.

So I have to trust that the steel manufacturer's instructions will allow me to achieve that hardness if I follow them exactly. But my reading has shaken my confidence that my equipment is accurate enough to be trusted.

Any thoughts?

Doug Lester
07-22-2011, 11:43 AM
It would be best to avoid tempering at temperatures between 500-600 degrees F. This might actually lead to embrittling of the steel. That also seems to be a little high but admittledly my references aren't the best. As far as testing the hardness goes, without a hardness tester you are going to have to religh on performance testing to see if the edge chips out or rolls over. Even with a hardness tester it might be difficult to check the hardness at the edge of the blade. One thing to remember about the manufacturer's instructions is that they generate a lot of their data from testing thicker pieces of steel than a knife blade. Hopefully someone who uses A2 will post but I would try tempering the blade at about 450 degrees and then test the edge and adjust from there.

Doug

Knifemaker.ca
07-22-2011, 06:52 PM
What I want to know is, can I trust that if I set a temperature and time program in the kiln that corresponds to the steel manufacturer's instructions, will the steel be properly hardened?

In short, not necessarily. Even though the alloy may be close, there are so many other things that can affect the outcome - quenchant - quenchant temperature - cryo - plate quenching - time between austenizing and quench.... Also some alloys - like W2 can have a huge range of chemistry and still be W2. .6%C W2 will not come out the same as 1%C W2. You will get consistent results with your kiln, but only to the extent your alloy and all other aspects of the heat treat are the same.


I also want to know why it's so important to temper immediately.

I do try to get most carbon and tool steels into temper quickly, and if I don't have a kiln available right away, I'll put them in the kitchen oven - real low - like 250F so they don't get too hot in the imprecise oven. It won't do much to temper, but should reduce stresses a bit. It's not so much of a problem now that we're running three kilns. As for stainless, or other air quench steels (like your A2) They go straight from room temperature to liquid nitrogen for maximum cryo effect - and we haven't had one crack yet. They get tempered the next day, so I don't think "immediate" is so important with air quench steels.


Also, I read a really old post by a guy who has an evenheat and was measuring a different temperature at the top and bottom of his oven opening. The only way I have to measure temperature in my kiln is by trusting the thermocouple and controller readout.


The old guy is correct. At high temperature, it is common to have about a 25F - 40F difference from top to bottom. Evenheat has their name for a reason. They minimize this, but it does exist. It can be measured with a pyrometer. At tempering temperatures, the difference will be minimal. We have to be careful not to imply more precision that we actually have.


But my reading has shaken my confidence that my equipment is accurate enough to be trusted.


It's important to understand the difference between accuracy and precision. Accuracy is three shot close to the bullseye. Precision is three shots in the same hole - regardless of how close to the bulllseye. Your kiln among the most precise out there. Accuracy depends on so much more. You'll get the same results over and over from your kiln - though those should be tested to determine whether they are what you want. The final outcome depends on all those other variables, so be consistent with them and keep notes.

Note my tagline. :3:

PetrifiedWood
07-23-2011, 02:57 AM
Thanks for the replies. :)

I will have to do a little research and find out what a pyrometer is.

I ran a 1/8
" thick O1 blade this evening, following the manufacturer's recommended process, with a twist.

I placed the blade in cold (no wrap). Ramped up to 1325 and held for 10 minutes, then ramped up to 1500 (the top end of the recommended range, to account for a possibly cooler lower part of the oven. Let it soak there for 8 minutes, then quenched in mineral oil heated to 130 degrees.

I tried a file on the spine and it bit. I was seriously disappointed so I tried another swipe and it skated. I think it was just a decarb layer and scale that the file bit into. The file teeth have streaks on them now from the blade steel. I had it in a 450 degree oven within 10 minutes or so of removing it from the quench. 2 hours, 2 times.

The bright area where I tested it with a file has a very light blue sheen to it now after the tempering. I'm going to test it tomorrow carving some hard Chinese Elm and if it doesn't chip or roll and stays sharp I'll go ahead and put some scales on it and finish it.

Doug Lester
07-23-2011, 09:43 PM
You already have a pyrometer on that that oven. It's what gives you a temperature readout. It's hooked up to a themocouple (a heat sensor probe inside a ceramic sleave of some type) Basically it's just an eletronic thermometer. Installing a second pyrometer would just be a check on the one that's part of the programable controler.

Doug

PetrifiedWood
07-23-2011, 09:55 PM
You already have a pyrometer on that that oven. It's what gives you a temperature readout. It's hooked up to a themocouple (a heat sensor probe inside a ceramic sleave of some type) Basically it's just an eletronic thermometer. Installing a second pyrometer would just be a check on the one that's part of the programable controler.

Doug

Thank you for explaining this! I suppose adding a second one wouldn't hurt.

Knifemaker.ca
07-24-2011, 12:20 AM
You can make a portable pyrometer - and I think evenheat might sell them. The guage has long leads to the thermocouple and you can set the tip pretty much where you want to know the heat - like in the blade rack? The leads will probably have ceramic beads to keep the wires apart. It's not something you'd use all the time. Just to check your variables and if you ever doubt your main thermocouple.

Darrin Sanders
07-24-2011, 12:45 AM
When working with steels that have the potential to crack between the quench and the temper you can do a 350-375 temper for about an hour to prevent cracking. Then when all your blades are finished you can temper them at the actual temp. you want to use and it wont hurt a thing. Remember you can always re-temper at a higher temp but you cant untemper. So when testing a new steel temper on the low side and test then re-temper at a higher temp. if necessary. Hope this makes sense.

PetrifiedWood
07-24-2011, 10:25 AM
When working with steels that have the potential to crack between the quench and the temper you can do a 350-375 temper for about an hour to prevent cracking. Then when all your blades are finished you can temper them at the actual temp. you want to use and it wont hurt a thing. Remember you can always re-temper at a higher temp but you cant untemper. So when testing a new steel temper on the low side and test then re-temper at a higher temp. if necessary. Hope this makes sense.

That does make sense, thanks. So essentially you're just doing a stress relief at 375 just to stop it from cracking?

Justin King
07-24-2011, 11:41 AM
I don't know for sure but I suspect that a lot of the issues with cracking between quench and temper are related to water quenches, differential hardening (edge only quenching or clay coated spine), eyeballed austenizing temps, and other techniques that bring unqualified variables into the mix.

Fred Rowe
07-24-2011, 07:02 PM
Properly hardened high carbon steels need to be at least partially tempered, soon after they have been quenched. It is inherent in the hardening process itself that the steel will be in a state of distortion and stress. Its the result of the process of hardening.

If an error has a cured in the manufacture of the blade it will usually crack when quenched.
Normalizing after forging a blade is done for several reasons. Grain refinement is one, the other is stress relief.

Working to keep the stresses at bay is a big part of blade making.

Fred