Question : heat treat

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ss caustic

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Ok, so I've been researching heat treating info and process logic and ran across this site , seems to be a great resource .

-http://www.navaching.com/forge/heattreat.html

I did come away with a question on heat treat though and it has to do with once you get past the "nose" of the s curve from 1450f down to 900f in the example ( form web site), what effect does cooling all the way down quickly to 125F have , alternately what effect does following the curve more closely to the 125F.

is it only about managing the thermal shock or will one :

1) have a higher HRC #.?
2)finer grain structure ?
3)any thing else different ?

I'll re-read the pc. in case it is staring me in the face , but in the mean time anyone willing to clarify would be much appreciated

hope this makes scene , thanks
 
You didn't tell us what steel you are heat treating, but a basic description is ( there is a more detailed one in the stickys):

When the steel heats above 1414F, it becomes austenite. That is a different arrangement of the atoms than it was. Above this transformation, the steel is in the austenitic climbing range (Ac1,Acm, etc.)

Upon slow cooling ( annealing), the drop in temperature is slow and falls into the pearlite range . The point of that graph is called "the nose" because it resembles one. If the cooling rate enters to the right of the nose, the atoms will convert to a pearlite structure by around 1000F.

Upon rapid cooling (quench), the steel stays to the left of the nose, and once it gets to 900F it is still austenite. below the nose, it is called super-cooled austenite, because it would have normally converted at a higher temperature. This is called the austenitic cooling (reducing temperature) range ( Ar1, Ar3, etc.)

When the steel reaches about 400F, it starts converting into martensite ( a different atomic arrangement), called the martensitic start point (Ms). This conversion ends at different points depending on the steel type. For simple carbon steels, it ends around 200F. This is the martensitic finish point (Mf).

The cooling rate between 900F and 400F is where you can do straightening, and allow the blade to relax a bit before the sudden and violent conversion to martensite. It is at this conversion that the blade can crack and warp...not at the initial quench point. Generally, a slow and even cooling between 900F and 400F is desired. The cooling between 400 and room temperature should be allowed to happen at its own rate with no shocks or other stress added. during and immediately after this transformation the blade can snap in half with finger pressure.

The blade needs to be tempered to remove the brittleness of the newly formed martensite.

All that is important is that the blade stays to the left of the pearlite range until it reaches 400F. Following the pearlite curve will allow a slower transformation time. but normal air cooling is all you need. There are specialty quenching methods called ausquenching and marquenching where the drop is stopped at the Ms. These do not generally apply to knife making.

Now, in annealing, there is a difference in the rate from 900f to room temp ( after it enters the pearlite range and becomes pearlite). Air ( slow) cooling below the 900F point will make the pearlite structure fine pearlite, rapid cooling ( water cooling) will make it coarse pearlite. The latter is preferred for machining and grinding.
 
great reply, thank you. As for the type of steel to be Heat treated, This question is not "how to" HT a particular steel so much as understanding the logic and what happens if...

but the example from the site was using O1.

"The cooling rate between 900F and 400F is where you can do straightening..." now this info from the paragraph I did not read anywhere else yet.

Question: does cooling slower but still staying to the left of the curve effect grain size or anything else, or is it just to reduce stresses?

"All that is important is that the blade stays to the left of the pearlite range until it reaches 400F. Following the pearlite curve will allow a slower transformation time. but normal air cooling is all you need. "

Question re worded: if that is all that is needed , then the question is, if you drop the temp quicker what is the effect on grain, hardness, stresses or martensite development?

I hope this makes sense , it is somewhat more difficult to ask a question via. text then in person.

much appreciated.
 
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My nearly uneducated response would be that for most plain carbon steels that knifemakers use, the rate of cooling from 900F to room temp is probably not as important as long as you stay to the left of the curve. I do think that the more even you cool from 400F to finish, the better. I'm not sure if it minimalizes retained austenite or something. I do know that engineered commercial/industrial quench oils are designed to cool at specific(and varying) rates upon the initial quench through to finish.
 
IF you drop the temp quicker from 900 to 400 you are not gaining anything as far as grain size, hardness, or martensite development.
 
thanks for the replies,

so nothing gained form the quicker temp drop from 900 to 400 but is there any risks by doing so?

also,

during the martensite phase 400- down , is there any gain to a slightly quicker formation , will it form a slightly harder blade or will it only induce more stress an obtain the same hardness as a controlled slow cool down?

if there is a hardness gain then I guess their will be a sweet spot between stress and hardness that one can strive for?

or

do you only risk having retained austenite or other issues, if you cool too quickly?

if other issue what are they?




PS. I have read the HT stickies, tooooonnes of info there, more then I think I can absorb in one reading. will likely re read tonight, but it helps to discuss.

thanks
 
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another question :

I'm reading Kevin's stickies again and came across this:

"So when you have a steel well above .8% carbon be certain to normalize well without overly slow cooling, use sub critical anneals by cycling above 1100F but below non-magnetic (not only will the steel love you, your mills and drills will as well), and be careful to keep the temperatures below Accm in hardening. Unfortunately the best way to accomplish that last one is to have a well calibrated heat source. Lower temperatures and longer soaks are much better for hypereutectoid steels."

I have Iron - iron carbide phase diagram in front of me but I cannot find the Accm line , I've looked for other charts but none have it marked as such. is Acm the same as accm? or is it something other?
 
Many new makers read the metallurgical charts and info and worry about their knives being this or that. Unless you have high tech equipment, it won't really matter. Learn to use the equipment you have to its best advantage and don't sweat the metallurgy too much.

As far as "will doing it this or that way be better", if it made a difference, it would be mentioned in the HT info. The metallurgists and other knifemakers have done thousands of tests and HTs to determine the info you read. If it does not recommend a different rate that the normal one, it is either a bad idea or doesn't matter. When it says., " air cool to ambient"...that is what it means. It does not mean dunk in ice water or oven cool slowly. If it says,, "do not slow cool", then a medium to rapid rate is desired. If it says "cool quickly once below 900F", then don't waste time, ......etc.
 
hey Stacy thanks for the reply, I'm just trying to get a good understanding of effects of temp / time in order to understand what is happening and when.

I believe I mentioned this before but,
I've been working with steels for 20yrs and have always been inserted in metallurgy and heat treating but have never had the opportunity to learn anything to do with it through out the industries I've worked.

now that I'm interested in it for my own pleasure and the internet's massive wealth of info , I'm taking some time to fill in some blanks .



I've reread Kevin's highly detailed stickies and it does seem to make a difference how you cool through out the martnesite phase eg, martempering, so can one assume that trying to follow the s-curve closely but staying to the left , would lessen hardness slightly but really increase toughness similar to a martemper?

I understand one form of martemper as equalizing the temp of the part for a moment, before it enter the Ms stage. The idea being to obtain a more stress free hardening due to having the outside and inside of the part being the same temp once Ms stages starts.

and the other is equalizing the temp just after Ms, but I recall reading somewhere that if you stop the cooling during the martensite phase you stop the it from forming so I'm not sure how this one works , but I'll dig deeper in to this later.

Also

Still , with regards to the Accm is this the same as Acm on the chart? as I haven't found a chart with Accm on it yet.

thanks again to any who reply.
 
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The answer is a little tricky, but basically, Acm is the same as Accm if the heating rate is relatively slow.
 
The gain in a slow cooling from 900F to 400F is that it is still rubbery austenite. You can bend and twist as much as you wish with gloved hands. Once it starts stiffening up as it approaches 400F, quit straightening. I have a wooden anvil face and wooden mallet I use to hammer sword and knife blades straight when in this range.

The gain in a slow and even cooling below 400F is that the blade stays in one piece.

In the cooling stage of HT once the Ms is reached, a slow and undisturbed cooling from 400 to below 200f is desired. This is to avoid cracks forming during the violent transformation to martensite. If it is very slow, the steel seems less hard, but it is just that it has auto-tempered during the slow cooling.

I'll try and explain it is a simplified description for a basic carbon steel:
The steel starts to convert to martensite at 400F. It is becoming a very hard and brittle martensite at this point. As it cools, the austenite continues to convert until it is all ( or mostly) changed to martensite by 200F. By ambient temp, the steel should be hard, brittle martensite.
This cooling is usually done in the oil tank, or in ambient air. If the cooling rate is slowed more than that ( but not crazy slow), it will temper itself to a small degree as it cools through the 350-300F range. While not really tempered martensite, this slightly auto-tempered martensite is a tad softer than the brittle martensite. Once the full temper cycles are run, the steel hardness and the structure should be the same regardless of the cooling rate.


Ways to slow the cooling are to set the blade on a warm brick or block of iron that is at 400F or a little below. Let them cool to ambient together. Some smiths set or hang the blade near the forge where it is pretty warm ( 120-150F). Using an oven to slow cool from 400 to 80F would be slower than is reasonable needed, and would gain nothing.

What causes the dreaded "PING" or "TINK" in a water or brine quench on 1095 is that the edge cools faster than the thick spine. It can drop below 400F very quickly, sometime within 5-8 seconds. Then, when the spine starts contracting as it cools, and then suddenly expands as it converts to martensite it tears a crack in the edge. Once started, the tear will rocket at the speed of sound ( literally and figuratively) across the blade and break in half. The crack progressing from grain boundary to grain boundary is what we hear. By the time the sound reaches our ears, it is all over. In the case of clay coated spines during yaki-ire, it usually stops at the pearlitic spine and there is a nice crack ( or many of them) going up to the hamon.
 
Thanks again, Stacy. very informative.

good Ideas on slowing the Ms cool down with warm bricks etc.

also

I found a copy of - "Steel Metallurgy for the Non-Metallurgist", lost for reading to do now...

I'm sure I'll be back with more questions on whether or not I understand things.
 
just came across this in my reading of Verhoeven.

Acm = is in reality supposed to be "Aecm" and is determined in equilibrium in other words determined through very , very slow temp changes.
Accm = is when heating ( is actually higher in temp when compared to equilibrium)
Arcm = is when cooling (is actually lower in temp when compared to equilibrium)
 
Stacy E. Apelt - Bladesmith said:
The gain in a slow cooling from 900F to 400F is that it is still rubbery austenite. You can bend and twist as much as you wish with gloved hands. Once it starts stiffening up as it approaches 400F, quit straightening. I have a wooden anvil face and wooden mallet I use to hammer sword and knife blades straight when in this range.

The gain in a slow and even cooling below 400F is that the blade stays in one piece.

In the cooling stage of HT once the Ms is reached, a slow and undisturbed cooling from 400 to below 200f is desired. This is to avoid cracks forming during the violent transformation to martensite. If it is very slow, the steel seems less hard, but it is just that it has auto-tempered during the slow cooling.

I'll try and explain it is a simplified description for a basic carbon steel:
The steel starts to convert to martensite at 400F. It is becoming a very hard and brittle martensite at this point. As it cools, the austenite continues to convert until it is all ( or mostly) changed to martensite by 200F. By ambient temp, the steel should be hard, brittle martensite.
This cooling is usually done in the oil tank, or in ambient air. If the cooling rate is slowed more than that ( but not crazy slow), it will temper itself to a small degree as it cools through the 350-300F range. While not really tempered martensite, this slightly auto-tempered martensite is a tad softer than the brittle martensite. Once the full temper cycles are run, the steel hardness and the structure should be the same regardless of the cooling rate.


Ways to slow the cooling are to set the blade on a warm brick or block of iron that is at 400F or a little below. Let them cool to ambient together. Some smiths set or hang the blade near the forge where it is pretty warm ( 120-150F). Using an oven to slow cool from 400 to 80F would be slower than is reasonable needed, and would gain nothing.

What causes the dreaded "PING" or "TINK" in a water or brine quench on 1095 is that the edge cools faster than the thick spine. It can drop below 400F very quickly, sometime within 5-8 seconds. Then, when the spine starts contracting as it cools, and then suddenly expands as it converts to martensite it tears a crack in the edge. Once started, the tear will rocket at the speed of sound ( literally and figuratively) across the blade and break in half. The crack progressing from grain boundary to grain boundary is what we hear. By the time the sound reaches our ears, it is all over. In the case of clay coated spines during yaki-ire, it usually stops at the pearlitic spine and there is a nice crack ( or many of them) going up to the hamon.
Click to expand...
I had this happen with two knives (W2) using clay on the spine and a brine quench. One or two seconds after the clay fell off I heard the dreaded tink.
 
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