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Kyle Johnson

Tempering questions

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In the case of sword blades it doesn't have to be an either/or situation... too hard, or too soft.  One way around that is pattern welded blades.  With the right combination of steels, the blades can be (all, or in part) very hard *and* very springy.  And such a blade would be my choice for a weapon.

 

And then there are fully hardened, selectively tempered monosteels...

 

As always, your mileage may vary.  :;):

 

Jimmy

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Don, Here is an interesting comparison.  The first quote is from "Physical Metallurgy Principles" by Robert E. Reed-Hill, 1964 (note the date).  " During the second stage of tempering, the retained austenite transforms to bainite.  The bainite reaction becomes measureable at 100C and continues to about 300C, WHILE FOR TEMPERATURES BELOW 100C (212F) AUSTENITE MAY BE ASSUMED TO REACT TO MARTENSITE AND NOT BAINITE".  

 

Now a quote from "Steels: Heat treatment and Processing Priciples" by Krauss, 1990. "Stage two tempering: The transformation of retained austenite to ferrite and cementite."

 

Now, bainite IS ferrite and cementite, as is pearlite.  Martensite is NOT ferrite, which is body-center cubic.  Martensite is body-center tetragonal.  In the course of about 25 years, the understanding of what is happening has changed considerably.  Anyway, this is an iteresting discussion that sent me back to the books more than once.  When I get some time, I will look up some info on "blue brittle".

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RK and Don...

 

I must have read this thread forty times and am no wiser.  It's apparent that I've reached the age where my mind is impervious to new knowledge.  :(

 

Nothing for me to do but to keep on casting the runes and following their suggestions as to what to do with a piece of steel... or ask Don or Randal to kindly draw out some diagrams in crayon so I'll be able to grok them.  Or I could just forget about the *why* and keep focused on trying to remember the *how*.

 

Old, old, OLD man Fikes

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RK, the thinking may have changed in the last 25 years, but certainly they would have recognized the difference between fresh martensite and banite in their samples and in every book I have in my library there is mention of retained austenite converting to martensite.

 

What I am hearing you say is that it would be impossible. What comes to mind is age hardening and tool failure due to retained austenite. I will continue to look for references, this is a key point for my understanding. Thanks for the dialog.

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Its very interesting....  can I also add..... that their was also the idea the wootz steel was brittle in extreme cold....such as russia.....   wootz being a high carb alloy it may make sense

( maybe with the under cooling in the cold climate... brought the steel closer to Martensite finish and the production of fresh martensite)

 

and from what I've read... there was no second tempering process written of.......  sometimes there was an extra ordinarily long tempering session.... such as placing the sword in a smoldering dung fire for a whole day

 

anyhow.... the second tempering is a good practise

 

thank you for the interesting info....  it just made my day

 

Greg

 

 

Super

 

 

ps... we could prepare some samples of alloy and test it....and look for the structures that the texts are tallking bout ???

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I was wondering if embrittlement happens with bainite.

 

For instance, would something like L6 have the same embrittlement issues with bainite formed at 550f as martensite tempered to the same temp?

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Don, the conversion of retained austenite to martensite is easily done by undercooling the steel; this is continuing the QUENCH. What I cannot find clear reference to is the conversion of austenite to martensite by HEATING during a second temper.  As I said, if retained austenite is converted to martensite by tempering, the hardness after tempering should go up.  But it goes down.  Krauss, who is known as Mr. Martensite and was the past president of ASM, said retained austenite transforms to FERRITE and CARBIDE in tempering, not martensite. I will keep looking for this in my books.  Don, I have not had such a spirited discussion with any of my metallurgist collegues in a long time.  That, sir, is a tribute to your mastery of this craft though self-education!

AS for the embrittlement thread, let me refresh my memory with some more book-learning.  Bainite does not appear to suffer from embrittlement like martensite does.  In fact, Blue Brittleness is more correctly called Tempered Martensite Embrittlement.  I will post more when I am smarter.... :D

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Guest Kevin H
RK - Back in ancient history when I was the heat treat metallurgist for Crucible Steel, we stress relieved tensile specimens machined from heat treated 4140 bar product prior to pulling them. Had to avoid a range of about 500 to 700 degrees F to avoid the embrittlement problem. We had done studies that verified for the grades we used the thermal treatment was the equivalent of very slow machining that did not affect the heat treated steel. The minimum tempering temperature we used was I believe 1100 degrees F - end use was bolting stock per ASTM specification B7 (?) hardness was I believe 262 to 321 BHN. As a  comparison, if memory serves, 302 BHN is equivalent to 32 Rc.

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Guest bainite or martensite ?

RK and Don: Can I conclude that on tempering a hardened blade (some sort of heat-treatable steel), that some or all of any retained austenite will convert to bainite or martensite depending on:

 

 

    1. Where the tempering temperature falls in relation to the Ms/Mf range.

    2.  Dwell time at tempering temperature.

    3.  Whether any still remaining retained austenite has been "conditioned" enough to tranform on the cool down from tempering temperature.

 

Grandpa Meier

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RK, as I understand it, the stress relief created during the first tempering cycle allows room for the retain austenite to convert to fresh martensite on cooling. As Daryl said what product it produces depends on how it was conditioned.

 

Secondary hardening is a known and real problem especially with high speed steels.

 

What goes on in the Ms to Mf range is not often discussed by bladesmiths, but I think it is crucial to understanding the process. I have been reviewing my shop practice in my head so I can filter the book information and this has been a good review.

 

What books do recommend by "Mr. Martensite"? I can see it is time to expand my library.

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Kevin and Grandpa Meier, jump in on this topic!  We need some other viewpoints.  Let me see if I can summarize the points I think are unclear:

Most steels will have some retained austenite after quenching; high carbon steels usually have the most.

1.The retained austenite can be transformed to martensite by:

          -rkn: cooling more

          -df: tempering

2. The books I have said that tempering retained austenite converts it to bainite or pearlite; Don's books say it can convert to martensite (fresh or tempered?).

3.  I don't think the retained austenite need be conditioned in any way to decompose to bainite or transform to martensite. This is determined by whether it is cooled more or heated after quenching.

 

Don, I am not sure where the extra room would come from in the retained austenite because it has a fairly stable lattice parameter and it has the largest spacing between iron atoms of the common crystal shapes for iron.  Secondary hardening is indeed well documented and it is attributable to the precipitation of alloy carbides when tempering in the 1100F range.

 

Kevin's website has a good discussion of basic metallurgy as does iforgeiron.com in the blueprint section.  I would encourage those of you who are interested check it out.

 

Don, The best book by Mr. Martensite is the one I mentioned earlier.  It is actually a re-write of a classic text by Bain and Grosman.  He has also re-written the book "Tool Steels" which is available from ASM.  Both books are $$$$$$$$ !

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"Steels: Heat treatment and Processing Priciples" by Krauss, 1990

 

This book is a re-write of my beloved "Principles of Heat Treament" by Bain and Grossman???

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Now I am confused. I found the information you referenced in the "Metals Handbook." Excuse me while I go walk off my paradigm shift. :P

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Don, that loud noise you heard two posts ago was my own pardigm shifting without a clutch......  

 

Do I understand your point correctly?  Did you find info in the literature that clearly states that retained austenite will transform to martensite by tempering it?  I am not saying it is impossible, I am saying I can't find a clear reference to it.  

 

Guy, yep.  And the re-write is a 4th year engineering school text.   Bang Head

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RK, that's what the problem is, I find it in mentioned in several books. In "Tool Steels" 4th edition by G. Roberts and R. Cary published by ASM it even has a chart titled Influence of tempering temperature on Ms point for transformation of retained austenite to martensite on subsequent cooling.

 

My confusion came when I found that secondary banite is the most common product in carbon and low alloy steel reheated to 350 to 400F. "The secondary banite assumes an identical hardness with that of martensite tempered at the same temperature."

 

But it also says that, "Retained austenite is unstable and may transform to fresh martensite under the influence of cold work or deformation, during tempering operations, or simply aging at room temperature, causing unexpected dimensional change, brittleness, or cracking." That experience of blades cracking before they get to the tempering oven, prompted the homework in the first place.

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Guest daryl meier

RK and Don:  Am I detecting two definitions of tempering?

 

 RK: Does your use of the term "tempering" consider only those reactions occuring during the hold at tempering temp.??

 

 Don: Do you use the term "tempering" to mean the complete cycle from room temp ---heat up-- hold-- cool back to room temp.??

 

Grandpa Meier ( currently have 7 grandchildren )

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Grandpa, I am talking about the whole cycle up and down.

 

My first grandchild is on the way any day now. :D

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Yes, congratulations Don!

 

So the gist of the last few posts seems to be that any retained austenite is converting to bainite during time at the tempering temperature but that any austenite that is not converted to bainite or other structures, then may trip to martensite during the cooling interval betwen tempering sessions when the steel is coming back down to room temperature?

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Guy, I think you are essentially correct.  I still cannot say with certainty that retained austenite will transform to martensite as it is cooling from tempering.  It will transform if it is cooled below the QUENCH temperature (as in cryo treating) after tempering.

 

Two grandkids, here.  One is 2 and the other is 12 going on 21.  Talk about precocious!  She could read, write and do basic arithmetic before she got into school. WFT

 

Daryl: In fact, residual heat is tempering the martensite as it forms during quenching.  Most of the definitions include heating time and holding time but more recent work suggests that some carbides can precipitate as the piece cools from tempering.  Quenching from tempering prevents the caribde size from coasting past the desired point of hardness. :P

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Some info which won't add much but may help.

Although I'm not a metalurgist, I give you this from my classes in metallurgy in Europe 20 years ago (so things may have changed).

It stated that (translated):

"retained austenite transforms starting at 250 celsius and until 500celsius according to complex reactions that can form either martensite, cementites or carbides. Any and all of those transformations can superpose or substitute itself to the standard transformations (Martensite alpha prime into martensite alpha second, Alpha second into ferrite and Fe3C, Carbide epsilon into carbide Eta plus ferrite into cementite)" "when the martensite reaction was incomplete the perlite, troostite, cementite and ferrite that resulted instead of martensite are not transformed by tempering, they only change shape and dimensions." All of the processes are in reality related to the diffusion of carbon atoms within the structure. Ultimately it always tends towards the cementite/ferrite compound.

In certain cases hardness increases with tempering due to carbide precipitation. "It was believed that the increase in hardness in tempering was due to retained austenite transformation, But it doesn't seem exact.[ long paragraph on physico-chemical details of transformation of different phases as well as illustrations with specific alloyed steels..] Carburigen elements create an increase in secondary hardening. [..] secondary hardening is without a doubt created by a complex structural hardening provoked by the critical dispersion of carbides created from alloying elements."

 

"If Mf is below normal temperature, cryo quenching will result in a more complete transformation into martensite."

 

I see also clear references to "tempering fragility" and "blue brittleness" but I found no specific information yet other than silicium alloying increases the temperature of that brittleness, therefore reducing its influence.

 

As a note, it seems that everybody is right :D . As for the hardness increase if residual austenite were transformed into martensite, it seems clear that there are multiple transformations taking place simultaneously and without micro homogeneity (on a macro scale it may seem homogeneous) and they have complementary and adverse effects. Therefore it seems hard to relate a simple increase or decrease in hardness (or anything else) to a characterization of one transformation or another. (just like bending up or down in water and oil).

 

I also seem to find conflicting info on retained austenite becoming martensite at room temperature. Some texts mention age hardening (retained austenite becoming martensite I presume) over time. Some other text refer to "martensite transformation is strictly a function of temperature, not time" (between Ms and Mf). I have a bit of skepticism with the latter as nothing is ever not a function of time (including C diffusion to transform austenite into martensite).

I love steel, are we ever going to understand it all?

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Michael, You may be right about nothing not being related to time but....the transformation of austenite to martensite is a diffusionless shear transformation.  Carbon is trapped in the octahedral sites when the shear takes place.  And something has  to drive the shear transformation and I think the books are saying it is lowering the temperature.  Sigh..............this subject is wearing me out.  I will be back in the office tomorrow and can do some literature searches on blue brittlenes.  I no longer trust what I think I know or remember! WFT

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