Tempering and metalurgy

Bob Ohlemann

Bob Ohlemann

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Dec 5, 2013
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Ok, so I made a comment in another thread and found myself corrected by a few people who I know have far more knowledge in metallurgy than I. Not wanting to hijack the thread and anticipating a healthy back and forth on the subject, I figured I should start a new thread.

It is my understanding that tempering increases the toughness of hardened steels by allowing some of the carbon atoms trapped between the iron atoms in the martensite to be released thereby reducing stress between the iron atoms within said martensite. I also understand that how much of the carbon that moves out is dependent on time and temperature. I believe, but would like verification, that the carbon that is released combines to form iron carbides.

First big question is about the water quench after tempering. I've never read anything in any heat treating instructions that says one way or the other. I've been told cool in still air by all whom I've trained with over the whopping (almost) two years I've been at this. So, obviously a quench after tempering is going to abruptly stop most transformations; how, specifically, is this beneficial?

I have more questions but, I want to understand this first.

Bob
 
In a simplified explanation:

After the quench the steel cools quite happily until it reaches the Ms, where the sudden conversion from austenite to martensite leaves the new structures in a very "tense" condition. This makes the metal extremely brittle once it reaches the Mf temperature ( room temp for this discussion). There also is usually some retained austenite that didn't make the conversion.

With re-heating back to around the Ms temperature , the heat allows some of these "tense" structures to "slip" and find a more comfortable position. At the same time, the austenite goes back to the range where it can turn into martensite when it cools back down to Mf.

At this point you need to cool the steel to let these changes happen and become permanent. Upon the cooling things are still in progress. Slow ari cooling to ambient may allow some of the changes to "slip" back to less desirable states. Thus, a sudden cooling in water will get to room temp as fast as is needed. This puts no stress on the blades and absolutely can not cause any warp or cracking.

You now have a less brittle blade, but there is a little new martensite formed by the retained austenite that is in its "tense" state.

Upon the second temper, the structures slide a little more comfortably into place and the new martensite slips into its comfortable position. Again, quick cooling in water helps lock in these good changes.

Upon reaching room temperature after the second temper the blade has smoothly tempered martensite and the minimal amount of retained austenite.
 
Thank you Stacy. I'm following everything you said here. My next question is why/how can the transformations caused during tempering "slip" back when they are being refined into their "correct" matrix at the given temperature? If austenite cannot be formed below critical temperature and you convert retained austenite during tempering, what does it "slip" back to? You can be technical with me; I'll just look up the big words.

Bob
 
It is complicated, and frankly, while I know the basics of what is happening, I am not confident enough to try and give a technical explanation. IIRC, the places the changes occur that are time dependent are both in the grain boundaries with the carbides taking different carbon structures as well as changes in the type of martensite formed being different from the one we most desire.


I hope stezan, marthinus, or shqxk in Bangkok will chime in with a technical explanation.
 
This is my memory of my understanding (which matches what Stacy said):

Tempering changes the composition of martinsite from the as quenched structure by diffusing some of the carbon back into the matrix (this causes carbide changes as well). The RA will convert to to as quenched martinsite.

Tetragonal martinsite (the as quenched martinsite) is not fully stable at room temperature, which could be the cause of the slippage that Stacy mentioned. You have two forms of martinsite with different volume properties so you still have stress and carbon laying around wanting to do something from the previously tempered martinsites carbon diffusion.
 
Verhoeven hurt my brain. I just finished reading it. Didn't really grasp allot of it... But I did get allot out of it!
 
FunkCoaster said:
To date, this is the most complete single information source I've found: http://www.hybridburners.com/documents/verhoeven.pdf
Warning - it's over 180 pages, and pretty dense with technical info.
It details many of the chemical processes that are being discussed and at great depth.
Specifically, page 95 starts talking about tempering.
Click to expand...

What a fantastic reference! Thank you for posting this.

Bob
 
Great reference...a must for those who heat treat steels.
Actually from tempering temperatures in the order of 400 °F you won't get any strange slip or unwanted precipitates dictating a water quench.
But you have a slight chance to scrape some speckle of sleeping RA, and if it was the case a water quench could lead it to martensite and anycase no harm would be done.
 
RangerBobTX said:
What a fantastic reference! Thank you for posting this.

Bob
Click to expand...

I purchased the book. It does break things down in a nice scholastic format. It is also right that is it technically intense. I have knowledge of phase diagrams, but Metal phase diagrams are a whole other kind of complicated with the many regions it shows.

I had to put it away for a little while after the third chapter.
 
I think I made the comment in the other thread Bob referenced. Stacy covered it far better than I could have. As I see it, the benefits are negligible. My comment was to emphasize that nothing "bad" could happen from water quenching in between tempering cycles ..... and that manatee fat could out perform Parks AAA when it comes to triple quenching a properly edge-packed blade. Or, was that another thread?... bah
 
Rick Marchand said:
I think I made the comment in the other thread Bob referenced. Stacy covered it far better than I could have. As I see it, the benefits are negligible. My comment was to emphasize that nothing "bad" could happen from water quenching in between tempering cycles ..... and that manatee fat could out perform Parks AAA when it comes to triple quenching a properly edge-packed blade. Or, was that another thread?... bah
Click to expand...

It's all good Rick. I know I have a lot to learn. Love the quote in your signature; somehow seems apropos.

Bob
 
Several makers have described it this way; if I may paraphrase:

It's not that annealing, normalizing, hardening, tempering, or even cryo-treatment are "separate" things. They are all part of the complete and continuous transformation of the steel from:

(A) an unstressed, malleable state to a...

(B) highly-stressed (possibly over-heated, forged or heavily-ground) state, to a...

(C) hard, but tough and resilient martensitic state.

(proper HT also means not completely ruining the steel at any point along the way...)

I realize that's not very helpful from a technical stand-point, but wrapping my head around heat-treatment as a full process has helped me get a grip on why "this" needs to happen "when", and in what order.
 
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I'll hurt your head ! One of the more recent things in the happenings of steel in HT is the formation of ' eta 'carbides as we do in some of the high tech steels.We go to about - 300 F to tweek the matrix so that eta carbides can form on tempering !! So there are still things to be discovered !
 
I think that I may know the thread you were referring to RangerBobTX! I'm glad I posted my heat treat blunder and it led to this. This thread is gold for a beginner with an interest in metallurgy like me.
 
Verhoeven's written work can hurt your head but as a teacher he was much more down to earth. He brought in Beck's beer to class to show us how grain growth works by watching the bubbles grow together. Brilliant stuff. On the tempering side, you are simply moving carbon atoms into a happier place in the steel. The trapped carbon atoms are what cause hardness to begin with so the tempering energy just allows them the ability to move to this less stressed state. Yes, you have conditioning of the martensite too which causes the need for more than one temper but the mechanism is the same. If you temper with enough energy to form carbides you are into another mechanism all together outside of tempering and shouldn't be confused with it except for the fact the precipitated carbides may stress the structure enough to cause a hardness bump (secondary hardening). With heat treatable alloys it's all about the carbon. It's the reason all the fun things happen.
 
NYMet said:
Verhoeven's written work can hurt your head but as a teacher he was much more down to earth. He brought in Beck's beer to class to show us how grain growth works by watching the bubbles grow together. Brilliant stuff. On the tempering side, you are simply moving carbon atoms into a happier place in the steel. The trapped carbon atoms are what cause hardness to begin with so the tempering energy just allows them the ability to move to this less stressed state. Yes, you have conditioning of the martensite too which causes the need for more than one temper but the mechanism is the same. If you temper with enough energy to form carbides you are into another mechanism all together outside of tempering and shouldn't be confused with it except for the fact the precipitated carbides may stress the structure enough to cause a hardness bump (secondary hardening). With heat treatable alloys it's all about the carbon. It's the reason all the fun things happen.
Click to expand...

Is it necessary to quench in water after the temper? If so, what does it do?

Hoss
 
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