Effects of low temperature (~300 F) exposure of ATS-34 after final temper.

[Walt Welch]

Fellow posters; I apologize in advance for the bandwidth of this post, but I believe it contains a great deal of info about the effect of low (~300 degrees F) temperature exposure of steel, particularly ATS-34, after final tempering.

This is the text of an e-mail I sent to Les de Asis, attempting to obtain enlightenment:

Mr. de Asis, you may wish to reconsider your 11-23-98 answer regarding the
effect on temper of raising ATS-34 steel to around 300 degrees (which I
understand is about average for Black Teflon coatings; here is Anthony
Lombardo's post regarding this):

Anthony Lombardo posted 28 October 1998 05:38 PM ET (US)
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Black T will not screw up your heat treat.
All of the coatings out there are put on after heat treating-BlackT, BT2,
Black Ti, hard chrome.

They all use some degree of heat.

Black T only gets about 300 degrees for a period of time. (end quote)


Here is Mike Turber's post regarding heat treating:

Mike Turber posted 03 October 1998 01:21 PM ET (US)
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Christian,
Iron will, at common temperatures, organize itself into an atomic structure
that is called "body centered cubic." This consists of overlapping cubes
with an atom at each corner, and one more in the center of the cube. But
above roughly 1400 degrees F there is a change in structure to "face
centered cubic" and the central atoms migrate to the faces of the cubes.
This latter form is not magnetic.

Steel is basically iron with some carbon mixed in, though modern alloys have
various other metals and substances as well. When steel is heated to the
critical temperature (about 1400 degrees F), the iron will change to face
centered, and the carbon atoms will migrate into the central position
formerly occupied by an iron atom. This form of red-hot steel is called
austentite. Since it is not magnetic, a magnet may be used to determine when
the critical temperature has been reached (though the magnetism may be lost
before the transition, so this is only approximate). Complete migration of
the carbon atoms may take a minute or two.

If you let this cool slowly, the iron atoms migrate back into the cube and
force the carbon back out, resulting in soft steel called pearlite. If the
sample was formerly hard, this softening process is called annealing.

If you cool (quench) the sample suddenly by immersing it in oil or water,
the carbon atoms are trapped, and the result is a very hard, brittle steel.
Too brittle for most uses. The structure is now a body centered tetragonal
form called martensite.

So, the next step is to heat it back up, to between 200 and 800 degrees F or
so, depending on the desired end hardness. This allows some of the hardness
to relieved and is called tempering. The amount of tempering that is
desirable depends on the final use. Cutting tools are very hard, knife
blades less so because they must flex under use rather than break. Tempering
is a trade-off between hardness and flexibility.

Accurately measuring the tempering temperature is important. A nice,
expensive thermostatically-controlled oven is great. Or, some special
compounds can be applied that melt or change color at the right temp, such
as Tempilstik and Tempilaq. If the steel is clean to start with, then you
may notice that it goes through certain color changes as it heats up, with
understandably vague descriptions such as "light straw" indicating about 440
degrees F, and purple=520. These colors are not incandescence colors, but
are viewed in normal room light. The colors are due to types of surface
oxidation that are temperature dependent.

When quenching, it is often very important to avoid stirring a part because
this will cool one side much more quickly than the other, and might cause
warping. For knife blades, as an example, move it strictly up and down
during the quench.

(snipped)


Most of this is from info I gathered around the web so hopefully it answered
your question and then some.

Great place huh?

Best Regards,
Mike Turber (end quote)


Here is what Jim March has to say about heat treating ATS-34:

would like other options..
Jim March posted 16 November 1998 11:00 PM ET (US)
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Ernie Mayer is just starting up. His EMail address is blackcloud@home.com -
he'll be doing up to 30" of steel for sure, due to his shortsword line (20"
blade, 10" full tang).
He's the #1 proponent of the "low temp cycle" wherein the second and third
temper cycles on ATS34 are taken to around 350 F versus 900-something as
Paul Bos does. Which is better we don't have a final 100% definitive answer
on...I suspect Ernie's right but Paul is known for excellent consistency
over decades. I made some mistakes jumping into that debate and as I said,
we just don't have proof yet...several people, myself included and according
to Paul, Ernie phrased the whole problem in such as way as to piss off Paul
rather severely and now he doesn't seem inclined to even try the lower-temp
process.

It's a bit of a mess and it doesn't help us decide how to do our steel up
right.

Jim March (end quote)

So it would seem that the 300 degrees F of the Black T process is quite
close to the secondary and tertiary tempering levels. This, it seems to me,
might well affect the temper. Certainly this temperature does not approach
the Austentizing temperature, but it is close to the tempering temperature.

Respectfully submitted. Walt Welch
********************

Mr. de Asis' reply:


Walter,

Thanks for your update. I based my comments on our actual experiences.
When we first began using ATS-34, we obtained hardness levels of 61-63+ Rc
right out of the initial quench. We then used approx 350 deg F tempering
process and still encountered high brittleness. (When we pushed thumb lugs
into some knives using a press fit, the blades often cracked around the
thumb stud hole) When Mr. Pardue began working with us, he suggested we
try the secondary higher temp 950 deg F tempering process.

We noticed that during the first temper, the blade lost hardness to about
54-55 Rc and then when put through the secondary cycle, gained secondary
hardness characteristics (up to 59-61Rc) and exhibited substantial gains in
abrasion resistance and toughness. In fact, it became almost twice as
difficult to grind.

Our experience with TiN processing led us to abandon that process due to
our supplier not delivering on time, nor were the parts consistent. Also,
it appeared that the heat treatment was affected. We shared our
experiences with Spyderco and they confirmed their early experiences as
quite similar.

I'm not claiming to be an expert but we can't seem to detect any
substantial (meaningful) differences in blade metal characteristics after
the relatively low temp BT2 process is applied. (BTW, our supplier guards
their processes very closely, so I can't really claim to know their
formulas)

It is a very interesting process though. We're enbarking on trying to
determine objective fact from subjective opinion. It is tough sometimes
because everyone is very passionate about their beliefs.

Thanks again for taking the time to write!
_ _ _ _ _ _ _ _ _ _ _

Les de Asis
President/CEO

Benchmade Knife Co., Inc.
300 Beavercreek Road
Oregon City, OR 97045

Tel: 503-655-6004
Fax: 503-655-6223

ldeasis@benchmade.com
http://www.benchmade.com
*************************

My conclusion from this is that even if an ATS-34 blade is subjected to a temperature close to that of the final temper in the heat treating cycle, no apprecable effect on the steel results. True, Mr. de Asis does not use a low temp tertiary temper cycle, as I read his note, but the raising of the blade to ~300F during the coating process would constitute, I believe, a tempering cycle itself.

I intended this to be a reply to the Kydex / temper thread below, but it seems that the thread has expanded beyond this narrow concern, so have posted it separately.

At the risk of being banished from yet another forum (for excessive bandwidth), let me say that Kydex really wouldn't be a problem, as the specific heat of plastics is so low that insufficient heat is present to warm the steel to the initial temperature of the Kydex. Jim March has already confirmed this by experimentation.

Your thoughts, comments, criticisms, and questions, as always, are welcome.

Respectfully submitted, Walt Welch

 

[m]

Thank you Dr. Welch for shedding light on a subject that I and probably many others have often wandered about. I am also curious about the effects of temperatures at the other end of the spectrum on steel and other materials we use every day. Please keep us informed of any other findings on the matter.
Thanks
m

 

[Edward R Schott]

I have a heat treat sheet from hitachi its in Japanese.
Starting a hardness of 62 and heating to 392 F. will result in a hardness of around 59/60RC heating to 572F will result in hardness of 57RC heating to 950F will result hardness of around 60.5.Heating to secondary hardness is the best for cuting,edge holding while lower temperatures give better corrosion resistance. Heating any martensistic stainless in the 425F-800F range will result in a loss of corrosion resistance.
But we want knives to cut we don't want them to sit in bearing housings in corrosive environments.
so high temp for cuting low temp for corrosion resistance.
ED

 

[RJ Martin]

Gentlemen: 300F will have virtually no effect on the hardness of ATS34, no matter how long the exposure. ATS34 exhibits some of the characteristics of a High Speed Steel, that is, it has a secondary hardening range. This was described in the post by Les-where he described an increase of hardness when tempered at 950F. The steel actually increases in hardness after the temper!! That is why these types of steels require multiple tempering-you need to repeat the temper so that the NEW martensite gets tempered, too. As I understand it, most of the martensite forms during the first temper, so, if you temper two tomes, virtually all of the martensite gets tempered either once or twice. Tempering three times is even better.
This tempering treatment is indeed the way to go for ATS34, as well as 154CM and BG42. These steels are highly alloyed, and there is a lot going on inside them during hardening and tempering.
In the case of BG42, the service temperature is well above 300F. By tempering at the higher temperature, you are providing the microstructure that is properly equipped to handle exposure to these high temperatures. A low tempering temperature will soften the blade slightly, perhaps to what is considered a good working hardness value, like 58-60. However, all is not well just because the hardness is at the right value!! When low temperature tempered, the microstructure has plenty of retained Austenite mixed in with the Martensite, which contributes to weakness and cracking problems. Better to temper 2 or even 3 times at the higher temperature. Oh, and, don't forget to Liquid Nitrogen deep freeze after the first temper. You can do this at any time, even after the last temper. If you do, you'll want to stress relieve at 300F for an hour or two. You should note that the commercial outfits offering cryogenic treatment usually stress relieve at 300F, regardless of the steel.

So, don't worry-be happy and Ti coat those blades-you're not hurting a thing!!

RJ Martin

 

[Walt Welch]

My thanks to Edward R. Schott for his reply. You HAVE to respect anyone who buys CPM steel in ton quantities!!

 

[Jim March]

A couple of points. The "Ernie Mayer recipe" seems to call for 375F on the second and third strokes.

Paul Bos said that ATS34 started out in the Jet engine turbine biz - as did Paul. Ernie says his lower-temp temper cycle results in less resistence to high heat but I understood that to mean higher temps than 300F. My dad was a machinist for United Air Lines so I know just a bit about the temps involved in a jet. This is one reason Ernie's contention that his heat treat is tuned for "human survivable temp range performance" makes sense to me.

Walt, don't worry about bandwidth. ATS34 is a good, widely available steel and figuring out how best to deal with the stuff is *important*. At this moment I own two blades heat/cryo-treated in the "Ernie recipe low-temp fashion". As soon as I finish the sword I'll do more formal testing and we'll have a bit more data, although not enough to put it to rest.

Jim March

[This message has been edited by Jim March (edited 06 December 1998).]

 

[Sid Post]

If I understand things correctly so far, A low temperature temper will produce a blade with higher corrosion resistance.

For superior cutting ability, a high temperature temper is best.

For the ultimate temper, we would cryo treat it between the initial and secondary high temperature temper cycles and would high temperature temper it a third time. This would result in the "best" grain structure for a using knife.

Looks like a good little tid bit to remember if/when I ever get my CPM M4 knife made.

Sid

[This message has been edited by Sid Post (edited 07 December 1998).]

 

[Edward R Schott]

This high temp low temp thing wont work on CPM M4 IF you temperd it at 350 it would chip like glass.it needs a min temp of 1000F.
ED

 

[Jim March]

Ed, this is purely about ATS34. Every steel needs a different recipe, there's no similarity unless we're talking two near-clones like CPM154 (I think) and ATS34.

All: Ernie Mayer claims an improvement of around 15% in BOTH edgeholding and toughness using the low-temp versus high-temp cycle.

It's that claim that's at issue here. BOTH Paul Bos and Ernie do Cryo-treat.

It's *possible* that the cryo process is the difference, there's a couple of variations on it. Lesse, there's burying it in dry ice in a large freezer for a while (16 hours or so?) and then there's liquid nitrogen, usually in farm equipment made to hold frozen bull semen which is why a lot of smaller treaters are limited to 2" wide. (Ooops, off topic! Both Ernie and Paul are past this point anyways

. You find out the DAMNDEST stuff researching this crap.)

Jim March

 

[Walt Welch]

Jim; I certainly value your insight in this matter, but I, for one, do not think we have to keep the thread so narrow.

Further, Ed was just replying to Sid Post's post about using CPM M4 if and when he got any. Ed was just giving us the benefit of his experience and cautioning us in general about not trying low temp. tempering cycles with M4. It is clear from Sid's post that he was NOT contemplating doing this.

I think cautionary information, which may well save life and or limb is never out of place.

I have opened another thread on the heat treatment of alloys in general, so perhaps further posts dealing solely with alloys other than ATS-34 could be posted there, but I think the primary job of the forum is to get the information on the forum; even if it is in a less than appropriate place. Better that than not getting the info at all.

So post away, gentlemen and ladies; I value your input.

I will again say that IMHO, you have got to respect a guy that buys his CPM in ton lots! Ed must have a wealth of heat treating information; let's pick his brains!!

Thanks again, Jim and Ed. Walt