Need an explanatin - steel in freezer

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jawilder

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I am not trying to start any arguments here or bring up old any old bitterness. I am just trying to look for an explanation. Now that we got that out of the way, here is the scnario:

I am a big fan of "test it yourself before you form an opinion". With that in mind I profiled some 3/32 blades from O1 and heat treated them as usual - digital oven and peanut oil. I pulled them out and straightened them by hand with no problems. To make sure they stayed straight I used the Marchand system of clamping them between 1/4" pieces of steel during a 2 hour 400 temper.

Here is where my testing came in. I have been lead to understand that O1 does benefit from some sort of subzero temper, so I put the blades in the deep freeze at -23f for 8 hours. they went in straight and came out straight. But I set a blade on the counter when it was still cold and it warped like crazy. I have been able to take the warp out with another 2 hr temper/press.

Why did the blade warp after the freezer? In my very limited experience, I would have to assume that SOMETHING was done to the steel while in the freezer, I just don't know what that SOMETHING is.

Any suggestions or ideas?
 
Most likely uneven cooling/warming.The counter allowed the upper part to warm at a different rate than the bottom. No internal structure change happened in the freezer.
 
It is obvious that the uneven cooling caused the steel to warp, but doesn't it mean that since it was cold enough to warp mean that some sort of internal structural change took place?
 
This is a great question. I had a hard time understanding what was happening during the straightening/jig tempering cycles.... Was it inducing stress? Was it relieving stress? Was plastic deformation occuring?.... I still try to wrap my head around it. I can't bring myself to believe that -23F has ANY effect on common carbon steel but how much do I actually know? O1 has more alloying. You say it warped like crazy on the table and I am willing to believe that without proof(why would you make it up, right?). Did it RETURN to the original shape that came out of the quench? THAT would be interesting to note. I work with blades in the field in temperatures that drop below -40F and have never noticed warp... but I have experienced cold embrittlement.

I think the mystery here lies in the fact that the warped blade took a SET. Like Stacy, I can't see anything changing at those temps... but there it is!!! There is either a simple explanation or metallurgy gets turned upside down. I think the answer is simple... but obviously, I haven't reached the level of SIMPLE metallurgical understanding..... which is humbling.

Could you repeat the question?:p

Rick
 
I'm with you Rick. I know enough to know that I don't know that much.

The blade did go back to shape during tempering. At first I just clamped it flat between the steel for an hour. That made it better but not perfect. Then I put some washers on it and forced it to bend the other way for another hour and that seemed to work better. I don't know if it matters but there are two blades in this batch. One is a 4" pearing knife and the other is a large 8" Chef knife. The pearing knife did not bend. Only the much larger Chef knife.
 
Thermal shock
From Wikipedia, the free encyclopedia
Thermal shock is the name given to extreme temperature difference (gradient) across an object, which can result in cracking and/or breaking. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness and low thermal conductivity. However, they are used in many high temperature applications due to their high melting point.

Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail.
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Thermal shock can also result in plastic deformation before actual cracking. An example would be cylinder head warp resulting in gasket failure. I saw some partly BS explanation out there involving metals expanding at "the molecular level" with rising temp, I believe expansion at the crystalline level is more correct. Basically, uneven or too rapid heating or cooling can cause metals to take an uneven set. I have warpage problems sometimes when grinding wide thin blades, the uneven heating of one side of the blade will cause it to expand more, resulting in warpage even before cooling with water.

It's different than thermal expansion involved with phase change. I read that stainless steels have a more uniform rate of thermal expansion overall, as they do not undergo phase change when heating past the AC1 point for carbon steels. I guess they weren't talking about martensitic stainless alloys? Also that stainless alloys have lower thermal conductivity, which would tend to make them more vulnerable to thermal shock.
 
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I know this won't be helpful... but I'll add it anyway.

You said you wanted to test the theory, but your effort to straighten out the warp effectively invalidated the test. The correct procedure for testing the theory would have been to check hardness levels at various points before and after the tempering, before and after the freezing on both blades.

Testing a theory means more than just seeing if anything happens. It means gathering data to explain why something did (or didn't) happen. As it stands, all we have now is another anecdotal example with no real data. It's going to be hard for anyone to offer a factual explanation with no real data.
 
That's weird. I would love to see some pictures if you do this again. Sorry I can't contribute any knowledge on this subject.
 
For my own curiousity....what does "warped like crazy" constitute? Not trying to be a smartass...it's an honest question. I know you probably didn't measure it, but would you please take a guess?
 
I don't know if a RC test would be beneficial here as I am not necessairly trying to make it harder, but to make it tougher by converting retained austenite into martensite.

The only reason I thought to put this in the freezer is because before I ordered this O1 from Aldo, he mentioned that "you get the benefits of being able to Cryo it".

I could be completely off track here, and like I said, I'm not trying to start any arguments. Just trying my best to understand.

Salem, isn't the whole HT process not much more than different levels of "thermal shock"?
 
watercrawl said:
For my own curiousity....what does "warped like crazy" constitute? Not trying to be a smartass...it's an honest question. I know you probably didn't measure it, but would you please take a guess?
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I didn't take any measurments, but if I were to guess, I'd say that each end was raised about 3/16" if laying flat.
 
My guess is that it would be the uneven cooling.

I make it a rule to never set a blade down flat during heat treat. (of course I would have set that one down flat! I never would have thought about it out of the freezer)

This is kind of off the subject but once I was normalizing a 1095 blade I accidentally dropped it and in a rush set it flat on my KMG arm. The blade actually formed a large bubble maybe 1" high then returned back to the normal shape. I couldn't believe it happened before my eyes... it was super distorted and returned right back to normal. It looked fine after this happened but I kept it for scrap.
 
jawilder said:
I don't know if a RC test would be beneficial here as I am not necessairly trying to make it harder, but to make it tougher by converting retained austenite into martensite.
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As you'll find in the manufacturer's datasheet, you'll get virtually no retained austenite conversion until -90°F

It's easy to prove to yourself: if you do a sub-zero quench in dry ice, you'll get an additional ~2 Rockwell points of hardness. Freezer, none.
 
Thermal shock is a separate concept from phase transformation. It can occur at temperatures far lower than those necessary for steel to form austenite. Quenching can be seen as "thermal shock" but is an entirely different proces and result from a metallurgical POV.

Speaking of austenite, what you're looking for with sub zero treatment is an extension of the quench because of the following:

(From Eastern Tool Steel)
In certain cases, a combination of variables including high alloy content, long austenitizing time or high temperature, discontinuing the quench process too soon, or other factors in the process may cause the MS temperature to become depressed to below room temperature. In this case some of the high temperature structure, austenite, will be retained at room temperature (martensite is not completely formed). This retained austenite condition is usually accompanied by an unexpected shrinkage in size, and sometimes by less ability to hold a magnet. This condition can often be corrected simply by exposing tools to a low temperature (cryogenic or refrigeration treatments), to encourage continuation of the transformation to martensite by cooling the steel to below its MS.


Tool steels transform to martensite during quenching from about 600øF down to about 200øF. In some cases, as described above, the transformation to martensite may not be complete at the end of the quench (125øF). In such cases, some austenite may still be retained after the normal heat treatment. This retained austenite can sometimes lead to unexpected growth in service, causing loss of accuracy. A2 and D2 are two grades commonly prone to retained austenite after heat treating. By cooling the steel to sub-zero temperatures, this retained austenite may be transformed to martensite. The newly formed martensite is similar to the as-quenched martensite, and must be tempered. Cryogenic or refrigeration treatment should include a temper after freezing. The cold treatment is often performed between normally scheduled tempers. By minimizing retained austenite, certain kinds of dimensional stability problems can be avoided.
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With O1 I believe from my reading that barring faulty heat treatment, cryo should not be necessary as retained austenite should be negligible- Mf for O1 is above room temp.

In steels which exhibit greater proclivity for retained austenite, such as D2, cryo can benefit. But, (again, just regurgitating info I've read) the steel would then need to be frozen down to -100F or colder to see much benefit.

How about this- an experiment. Take a piece of mild steel, grind it into a similar blade, freeze it, and set it on the countertop. If it warps as it warms up, you know that the warpage has nothing to do with martensite or carbides and more with crystalline size.

Also, I too believe in trying things to test them- and I admit I've not put any of my blades in my home freezer, then tempered and test cut/bent.
 
jawilder said:
So if -90 = 2 points, then would -23 = 1/4 point?
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Good question. Short answer: the conversion is continuous, but not linear, with very little retained austenite conversion until -90°F.

The datasheets will show an Ms (Martensite start, the base of the pearlite nose) and Mf (Martensite finish) temperatures on the retained austenite graphs. Depending on the amount and type of alloy, Mf is around -110° F, and the retained austenite conversion is negligible until -90° F, where there's a sharp elbow in the curve, which tails off at -110° F.
 
jawilder said:
So if -90 = 2 points, then would -23 = 1/4 point?
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Nope... it doesn't work like that. If it did, you could just as easily assume that dropping your austenizing temperature controls as quenched hardness.

If 1500F = 66HRC as quenched, then 1350F = 59HRC as quenched?

If conversion doesn't begin until -90F, what would you expect at -24F? You can't bake a cake at 185F no matter how long you keep it in the oven.
 
I can maybe see maybe a little warpage from laying it on the counter, uneven warming (?), but if the only things you did different are the Marchand straightening and the freezer thing (or just one or the other) and it "warped like crazy",... I'd say that's pretty weird.
 
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To correct myself, upon going back into Verhoeven it seems that retained austenite can be present in hypo-eutectoid carbon steels when normally quenched. What Rick and lazlo have stated makes that superfluous in the case of your freezer, though.
 
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