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Is grain refinement a visible phenomenon?


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I was working on a blade for a knife earlier that I put out in record time for me. It just had gotten dark, and I wanted to heat treat so I could etch it with my katana tomarrow. I had to do another normalization cycle because I had a twist in the blade after forging that I corrected with a wrench. So, I did the usual; shut off the lights, got the forge hot, got my blade to critical, and pulled it out to cool. I guess I never paid much attention to it before, but the decalescense was very cool to watch. It cooled at the edge first, (like it should've) and the shadow moved toward the spine. Suddenly, the edge got hot again and the shadow took it's place in the center of the blade. Then the edge cooled again and finally the spine. I swear I could have quenched it when the edge re-heated. It moved in a wave like patten as if some force pushed through the steel. Was this caused by the geometry of the blade, or something more scientific? I just don't understand the edge re-heating it's self like that. The heat didn't fade back into the edge from the center of the blade either. It kind of "jumped" the shadow. It was very interesting to watch. I ended up doing 2 more cycles just to watch it.

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If I am understanding what you are describing, that reheating of the edge is not reheating, that is recalescense.  That thin shadow that you describe as forced across the blade is the area of recalescense.  The phase change of the steel actually requires a little energy so it emits less photons than the surrounding steel causing that thin shadow to move across the blade.   Here is a video I recorded of it.  The geometry does have something to do with it since thinner areas will cool faster causing them to change first. 
 

 

 

  

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You know what? I bet that cold metal has better thermal conductivity than hot steel, so when the edge returns to magnetic the heat reservoir that is the thicker spine releases its heat rapidly into the more conductive edge. Or, it could be photons and such.... Thats a foreign language to me. I think I need to do some research.

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16 minutes ago, Wes Detrick said:

If I am understanding what you are describing, that reheating of the edge is not reheating, that is recalescense.  That thin shadow that you describe as forced across the blade is the area of recalescense.  The phase change of the steel actually requires a little energy so it emits less photons than the surrounding steel causing that thin shadow to move across the blade.   Here is a video I recorded of it.  The geometry does have something to do with it since thinner areas will cool faster causing them to change first. 
 

 

 

  

Oh, so what you're sayin is... The phase change (being a change in the stucture of the steel) requires energy. Since energy is released in heat, light, or force; the steel in the shadow is the same temperature, but cannot emit the light because it is using the heat energy for the phase change correct?

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Yes, that is my understanding of it.  Since it has a fixed amount of energy, and the phase change will need some of it, it has to come from somewhere, and the one visible to us is in the color and light.  It is actually still emitting photons, but less and even that less makes it look dark in comparison to the bright surrounding steel.

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Interestingly enough, I did some breif research that yeilded that the metal actually re-heats due to the change in crystalline structure. The definition of recalescence is in a paraphrased form: the temporary heating of metal due to a change in chrystaline structure. So, technically you can witness grain refinement! Because if you overheated it, the grains should have grown some and the christaline stucture should not have changed from when you rough forged it (in theory, it may not be the case). 

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I'm not aware of any way to actually see the new, smaller crystals taking the place of the larger ones as it normalizes, recalescence is just an indicator of it. If you polish the steel after every normalization, etch it with special chemicals, and look at it with a powerful microscope, you could potentially see the grain being refined without destroying the piece. I wonder if it is possible to record crystal structure transformation in real-time? If a piece of steel was placed in a vaccuum and heated to critical with a heat -resistant microscope pointed at it, I wonder if you could see the change on a red-hot but polished surface. It would have to be recorded with an extreme slow-motion camera because the change occurs at the speed of sound. Maybe the test was done already, because how else would they know that it occurs at the speed of sound without viewing it?

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It's not heat, nor is it grains.  It's photons and individual crystal structure.  When the crystal goes from face-centered cubic to body-centered cubic it takes energy to accomplish, thus the momentary darkening.  It does not cool off (much), and when it brightens again after transformation is complete it is because the photons are being emitted again rather than absorbed.

Exactly the same thing happens in reverse when you heat it up.  The swirling shadows you see are the crystals transforming from body-centered to face-centered, absorbing energy. This is the dimming via lack of photon release, it is not cooling off.

We're in the realm of subatomic phenomena here; where visible light is due to electrons jumping up or down one step in energy level, releasing or absorbing photons in the process.  Matter is energy and energy is matter, light becomes solid and vice-versa.  E=mc^2 and all that.  

Grains are just groups of crystals growing in the same alignment, not unlike quartz crystals. You can have big ones you can see or tiny ones you can't, but that make up a large mass anyway.  

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Well that's just cool! I feel a bit like a wizard :lol: So, if the change requires so much energy it can not emit it's light energy... The shadow of recalescence is sort of like watching it all happen. It's a tell-tale sign right? If I heated it to a high yellow color and soaked it for 5 minutes, then let it cool would you still see this? Or, will it only occur when normalizing temperatures are used? I was just wondering if it's something you could look for and know you're doing it right. 

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Yes, decalesence and recalesence are tell-tale signs that many of us look for when heat treating.  It doesn't matter how hot above decalesence that you heat the steel or iron.  It will still go through recalesence if allowed to cool enough to go through the phase change.  It's the phase change, up or down, that's the ticket here.

Doug

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Yes it would still change. The crystal structure change happens no matter what at about the same temperature for a given alloy.

What is useful is this temp is approximately the critical temp. This means you can use the shadows to help lock in the temperature for quenching. 

Before I had a thermocouple I'd use the shadows to help me heat treat. For the batch of 1095 I had I would shoot for 3 seconds between removing the blade from the forge and the beginning of recalcalescence as my target quench temp. It gave decent results.

You can also use it for normalization. Cycle 1 10 seconds, cycle 2 5 seconds, cycle 3 right at recalc and cycle 4 just below. For beginners it's a way to lock in some measure of consistency, albeit not perfect.

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To add on to Doug and J's replies:

You can use that phenomenon to know that you are doing it right.  That is why I heat treated at night before I had an heat treating oven (and why i recommended that guy heat treat at night in the hamon thread).  I would watch for the shadow when normalizing to know that I had reached the critical temp.  I could also know if i was too hot or too cold by watching for how long it took for the shadow to appear. It wasn't precise, but it was good enough to know if I was overshooting or undershooting my temps

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2 hours ago, Wes Detrick said:

To add on to Doug and J's replies:

You can use that phenomenon to know that you are doing it right.  That is why I heat treated at night before I had an heat treating oven (and why i recommended that guy heat treat at night in the hamon thread).  I would watch for the shadow when normalizing to know that I had reached the critical temp.  I could also know if i was too hot or too cold by watching for how long it took for the shadow to appear. It wasn't precise, but it was good enough to know if I was overshooting or undershooting my temps

I offered the same advice to him beforehand. That's how I heat treat also. I knew the shadow was a sign you were ready for quenching, but I just didn't understand why. It's just something I just came to know through trial and error. I just never paid attention to it cooling. So, that's why I was just wondering if you could use it as some kind of guage for finer grains, or something. J answered my question perfectly (Thanks J!) Just trying to learn every trick I can. It's all very fascinating. Thanks for the info. Wes, and Alan,J, Wesley and Doug. I really appreciate it :) I might try and find some kind of metallurgy literature on all this. The kind that you can put on a shelf and look at when you forget something. 

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It is indeed wizardry.  We are messing about with the fundamental structure of matter here, after all!  When you quench after the shadows are gone on a rising heat, you are locking the crystalline lattice into the face-centered cubic configuration.  We call this hardening, and it's why steel is hard and iron is not. The carbon gets stuck inside the lattice if there is enough of it, which is what holds the crystal in that configuration.  If you just let it cool the carbon gets out of the lattice and forms new grain boundaries (see, you knew it was connected!).

This is also why it is good to watch for the effect.  As J. Broddick mentioned, decalescence occurs at the transformation point of each alloy at the critical temperature for that alloy.  If said alloy is, for instance, 5160, it occurs at around 1550 F.  If you are using a magnet, all alloys go nonmagnetic at exactly 1423.5 degrees F.  If, therefore, you quench 5160 from nonmagnetic, it has not transformed and will not fully harden.  

Nifty, huh?

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You know when I saw the first post on this a thought crept into my mind. I have heard this same thing discussed before but,..........it was called by another name! So after a little searching on the net I came up with the conversation, I remembered from another forum!

http://www.knifenetwork.com/forum/showthread.php?t=50189

Interesting note no matter how you label it. The science side of knife making. I tend sometimes to say, Oh crap I don't need to know that. However I have found that when I understand the why, the whole process makes more sense. It is kind of like the little kid when you tell them something and they pop off with,...........why!

There little brains are learning. It is nice to know that no matter how old we get, our brains are still looking for more info!!! :lol:

 

 

 

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