HeatTreating Question

B

Bushrat

Joined
Jun 21, 2006
Messages
37
After heat treating in your kiln, does tempering have a significant effect on Rockwell hardness?
For example, if 440c comes out of the kiln at 58HRC, will there be much of a difference after tempering in the oven at 350' for 1-2 Hrs?
 
Tempering lowers the hardness to the target point. More importantly, untempered martensite is brittle,tempered martensite is not. The loss of several points of Rc is desirable as the gain in toughness makes the blade usable. Most steels come out of quench at Rc60-64 (depending on the steel). After temper most like a Rc of 58-59 on a using knife.
Stacy
 
Thanks bladsmith. I was hoping you would come out and answer this one. So my next question is, in order to achieve a higher Rc out of quench, do I have to run my kiln a little hotter or longer?
 
How long did you soak at what temp? How did you "quench"?

440C at that hardness post-quench would like a cold treatment to give you 2-3 points HRC more.
 
Soaked for 20min, @1850'.(not sure if digital pyrometer is accurate) Air quench (440C). The reason I ask is after treating and tempering, I had some low Rc tests. Trying to find out where in the process I went wrong. ie in the heat treatment or the tempering.
 
I can't help but think people expect too much out of 440C. The crucible spec sheet shows as quenched hardness as RHC59. That's not inconsistent with your results - especially considering even your test block for your rockwell tester is likely rated +/- 1 point. Cryo may give you a point or two if done right after quench.

Yes, it should still be tempered.

Reputable companies like PUMA used to use 440C a lot and they routinely aimed it for the 57 range. It's a corrosion resistant, easy to sharpen steel that takes a nice polish and processes consistently. High wear resistance is not one of it's strong points.

:rolleyes: Now lets see if I've started something. :D

Rob!
 
Sounds like it wasn't hot enough for long enough.Try higher and longer.You should have got a littkle higher RC. Also, is your Rc tester accurate? Rob is right, a better steel will make a better blade.
Stacy
 
Bushrat, you should run experiments where you soak a small tab of 440C at 1850 for 30 and another for 45 minutes. It will not hurt the steel. Then test as-quenched hardness. Then notch partway through the tab with the grinder or an abrasive wheel, put it into a vise and whack with a hammer to snap it off so you can look at the grain with a magnifying glass/jeweler's loupe to assure yourself the grain hasn't grown. Repeat the experiment at 1875 and 1900F. Then you'll have data to base your process on.

If you can find two plates of steel or aluminum to use as quench plates, that will help you a bit, too. Just lay the foil-wrapped blade on one plate, then lay the other plate on top and weight it down, stand on it, etc., for about a minute. Air cool after that.

Cold treatment with dry ice or even cycling in and out of the kitchen freezer several times over a couple days will help further. Monitor the hardness as you go if you have the tester right there.

Get the most out of your chosen steel before you consider moving on. A better steel is only a better steel if the heat treat is right. Pick one and stick to it until you learn the optimization process. 440C with a good heat treat has skinned a lot of deer over the years. "Steel of the month" with a half-assed HT is way too common nowadays.
 
Tempering should be very important because a blade of maximum martensite that is tempered back to desired hardness will be superior in almost every way to a blade that did not reach full hardness in the quench. Now I am always very careful with words like “superior” but I feel very safe in this instance. Mixed products from incomplete solution or inadequate quenching, will simply not hold up as well as complete martensite that is tempered properly.

This is not as applicable to this circumstance of stainless, but many smiths feel that slowing down the quench, until the as-quenched hardness is the same, is equivalent to tempering a fully hard blade back to the same hardness, nothing could be farther from the truth. Slowing the quench will result in patches or colonies if pearlite mixed into your martensite, fully hardening the blade and then tempering will result in a homogenous tempered martensitic structure, which will out-perform the former in both impact toughness and edge holding. This holds true even in very soft steel, lamellar pearlite will not be as tough or even as ductile as spheroidized martensite. Not surprisingly I am sure mete will be happy to chime in on the effectiveness of mixed structures over homogenous.

For the stainless it is still desirable to heat to some level of tempering in order to deal with the body centered tetragonal nature of the martensite you do have, and stress relieve.
Tempering is also capable of dealing with some of the retained austenite that could be present, not as well as a cold treatment, but well enough to make tempering well worth it.

fitzo said:
"Steel of the month" with a half-assed HT is way too common nowadays.
Click to expand...

Fitzo, it is very sad how true these words are:(
 
No disagreements with anything here. FWIW - using Admiral 440C, I have been getting ~60 Rc as quenched, using the following method. Ramp to 1200 fast and hold for 15 minutes. Ramp fast to 1450 and hold for 15 minutes. Ramp fast to 1885 and hold for 20 minutes and plate quench in the foil. Temper twice at 375 F. I have been getting a consistent 58 Rc, with NO cryo.
 
OK. thanks for all the replys. The blades in question came out a Rc of 52 - 54.
I would like to redo them. I think that I would have to anneal them, and do the hole process over again. What is the best recomended way to anneal 440c?
Thanks for all replys.
 
Yes..........
You don't have to anneal unless you are going to be grinding or shaping.
 
440C likes 40 minutes@1875F. Press quenched will be harder (and straighter )than air quenched.
 
You must log in or register to reply here.
Back
Top