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Aiden CC

What Does Decalesence Look Like?

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I have been heat treating knives in a coke fire and judging temperature by color and with a magnet and recently read about the phenomenon of decalesence as way of visually determining when the steel is at the critical temperature. However, I couldn't find much information about what to look for. I searched the forum and saw the advice to do heat treating in a dark place and look for "dancing shadows," but not much other than that. I looked for videos, and while I did find some illustrating recalesence very well, I couldn't find any for decalesence. If anyone knows of one I would be very interested in seeing it. My question is: what should I look for in the color of the steel while heating to determine if it has reached its critical point/what techniques do people use to make the transition easier to notice?

 

Is what's happening the same kind of effect as when water stays at 100˚C until all of it has boiled, except with a transition between two crystalline structures instead? In that case, would it appear as the steel reaching a certain color and then staying the same, maybe with flickers of brighter colors that quickly subside? Would it help to "practice" by heating non-blade pieces of the steel I'm using? I find the subject of metallurgy a fascinating part of bladesmithing and any information about the subject is appreciated.

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Decalesence looks just like recalesence. Both are caused by the energy from the heat going into changing the phase of the iron matrix rather than producing light. That's what creates the shadow effect. With decalesence it's the iron matrix changing from a body centered cube to a face centered cube. It is much easier to see this "shadow" in dim light. Of course the steel being down in the coke is going to make the shadow harder to see than if you were using a gas forge. It takes practice to spot it but once you've seen it you will know what to look for.

 

Doug

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Although I'm not sure the original origins of the trick, I saw Alan post something about using a pipe in your fire to see your steel.

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It is very difacult to see in a gas fore gue to the burner flair masking it.

As I understand it decolessance is a drop in temp of 100 degf or that happens as the steel uses energy to take the carbon into solution and convert it to austinite. Recollassaance is the inverse and is much easier to see.

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As I understand it decolessance is a drop in temp of 100 degf or that happens as the steel uses energy to take the carbon into solution and convert it to austinite. Recollassaance is the inverse and is much easier to see.

 

No, the temp stays constant (well, there is a little variability to it as it will be sitting in a fire, but it is pretty close to stable). Aiden CC's comparison to boiling water is pretty good. As Doug mentioned, the stack of atoms is going from BCC to FCC, so some of the energy that was being spent giving off light (black body radiation) is spent shifting atoms/bonds. Since it has less energy available to radiate as light, it only looks cooler, but it actually is not cooler. Since it is still in the fire it is in fact probably getting a bit hotter. It is able to get a little hotter because solid state phase change happens over a temperature range, rather than a specific temperature like melting and boiling. Those temps are the latent heat of fusion and latent heat of vaporization, respectively, and are pretty set for a given composition, whether that be a specific metal alloy, pure metal, or something else like H2O. The tricky part of dealing with metal alloys is that they are very rarely perfectly uniform in composition, and even the smallest chemistry differences can lead to noticeable effects, especially upon solidification.

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The guy who first taught me bladesmithing told me about recalescence but I did not take it seriously because it sounded a bit like voodoo... 15 years later I finally figured out the old guy knew what he was talking about after all... :)

 

The other guys have given you a very good description of what is happening.... I will try to describe what it looks like. The steel will go from black, to dull red, to cherry red as the heat increases. Somewhere around the cherry red stage it will loose its magnetism, and this is where you will want to start watching the colors very closely. It will stay cherry red, not glowing any brighter as the heat increases, then suddenly the thinner parts of the blade will start glowing brighter, towards red-orange, while the thicker, cooler sections of the blade will lag behind... I've heard this described as shadows... when the shadows disappear, quench.

 

It is easier to see on the way down (recalescence), the reason being the blade is no longer in the fire... as the blade cools, you will see the same thing, only in reverse.

 

This method of austenizing (heating the blade up in preparation for the quench) should work in a gas or solid fuel forge:

If you're using a gas forge, only have one burner going. Thrust the blade through the fire to the point you are only heating the tang/blade junction. Slowly at first, draw the blade back through the fire towards you. As the blade thins towards the point, pull continuously faster... you want to be pulling the areas with the most mass through the hot spot slowly, and as the mass decreases your speed should increase. Repeat... and again. Keep this up, watching as the blade starts to glow to be sure the blade is heating evenly along its length, and adjusting your 'draw' as necessary, you do not want the thinner sections of the blade heating up faster than the ricasso area. After repeating this process around 30 times (depending on the mass of the blade) the blade will begin to glow, and around 50 times it will loose its magnetism. At this point, every 5 draws, pull the blade entirely out of the fire for a second (and only a second) and look at it, watching for decalescence... then plunge it right back into the fire. Once the blade reaches decal, quench.

 

I once watched a prominent swordsmith heat-treat a 25" katana in a forge that had a 6" hot spot using this method, I've only managed a 17" blade, so far....

Edited by GEzell

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One thing you can do when you are normalizing your blades is keep a bucket handy. When you pull the blade from the forge hold the bucket up and stick the blade up into the interior where it's dark. As the blade cools you can easily see when recalescence occurs and the blade brightens. At that point pull the blade out into the ambient light of the area you are working and take note of the color of the blade. This is handy so you can make rough judgments by eye using the color of the blade to let you know are closing in on decalescense when you are heating your blade prior to quenching. Just remember the color you are judging by only only applies in the ambient light of your work space at that particular time.

 

Before I built my heat treating drum forge of Don's design and started using a thermocouple I used my propane forge for heat treating, passing the blade back and forth through the hot spot and using a magnet. When I started also using the bucket and recalescense during normalization I discovered I had been heating my blades way hotter than necessary as I often worked outside in fairly bright sunlight. That drum forge is worth it's weight in gold by the way.

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One thing you can do when you are normalizing your blades is keep a bucket handy. When you pull the blade from the forge hold the bucket up and stick the blade up into the interior where it's dark. As the blade cools you can easily see when recalescence occurs and the blade brightens. At that point pull the blade out into the ambient light of the area you are working and take note of the color of the blade. This is handy so you can make rough judgments by eye using the color of the blade to let you know are closing in on decalescense when you are heating your blade prior to quenching. Just remember the color you are judging by only only applies in the ambient light of your work space at that particular time.

 

Before I built my heat treating drum forge of Don's design and started using a thermocouple I used my propane forge for heat treating, passing the blade back and forth through the hot spot and using a magnet. When I started also using the bucket and recalescense during normalization I discovered I had been heating my blades way hotter than necessary as I often worked outside in fairly bright sunlight. That drum forge is worth it's weight in gold by the way.

I would totally agree, I still use my Don Fogg Drum furnace in preference to my other more techno HT furnaces. A great bit of kit. and I have a length of chimney as my portable dark space.

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Things suddenly got very busy, so I haven't had much time to think about knives, but I finally found a bit of time.

 

Although I'm not sure the original origins of the trick, I saw Alan post something about using a pipe in your fire to see your steel.

I can't remember where I heard about this, but I actually just started doing it. It made it a lot easier to see the steel and get an even heat, and some crushed charcoal in the pipe/some protection from the air blast helped to cut down on oxidation a little bit.

 

The other guys have given you a very good description of what is happening.... I will try to describe what it looks like. The steel will go from black, to dull red, to cherry red as the heat increases. Somewhere around the cherry red stage it will loose its magnetism, and this is where you will want to start watching the colors very closely. It will stay cherry red, not glowing any brighter as the heat increases, then suddenly the thinner parts of the blade will start glowing brighter, towards red-orange, while the thicker, cooler sections of the blade will lag behind... I've heard this described as shadows... when the shadows disappear, quench.

Thank you for the detailed description/process. This weekend I'll have some time to forge, so I'll practice looking for those stages while the steel is coming to temperature between heats.

 


 

I once watched a prominent swordsmith heat-treat a 25" katana in a forge that had a 6" hot spot using this method, I've only managed a 17" blade, so far....

And then there's me who built an extra long custom forge out of cinderblocks because I couldn't handle an 11" tanto... I'll have to try that method next time I do a longer blade. The problem I have is that all of my long blades are Japanese style, and all of that back and forth can scrape off some of the clay if you aren't careful (though I did buy some satanite powder which might be a bit tougher than the furnace cement I was using).

 

Thanks all for the helpful and interesting responses.

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