Decalescence; as seen through a lens.

Fred.Rowe

Fred.Rowe

Joined
May 2, 2004
Messages
6,848
If you have not had the opportunity to see this interesting phenomenon.

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This visual show occurs when steel is being heated through the critical range. There is a sudden slowing in the rate of temperature increase or decrease, depending on whether you are heating it up or cooling it down. The slowing is caused by a change in the internal crystal structure of the steel.

This blade was cooling from 1550fh.

You see the change beginning to take place along the edge of the blade, in the first picture.

The pictures span a four second time frame.

In the last pic, the blade edge is darkening and is down in the 8oo degree range. The decalescence is long gone at this stage of the blades cool down cycle.

Smiths throughout history have been intrigued by this phenomena. Most, until recent history, did not know, what was taking place.
Today we don't refer to it as, smoke passing across a cloud, we understand, it is phase change, taking place.

Sorry about the poor pics. My website is down for a bit, being upgraded.



Fred
 
I think the pics are quite good. They show what you are trying to very clearly.

The only suggestion I could make, if you happen to be good with Photoshop or equivalent, would be to combine them all side by side in one image.
 
dlpierson said:
I think the pics are quite good. They show what you are trying to very clearly.

The only suggestion I could make, if you happen to be good with Photoshop or equivalent, would be to combine them all side by side in one image.
Click to expand...

My computer is running on one cylinder at the moment or that would have happened. :grumpy:

Fred
 
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How's that?
 
Thanks for posting this pic. I was thinking about trying to heat treat my first knife.
So, when you are heat treating the knife does the shadow start at the edge or thicker spine?

What do you watch for, the shadow to dissapear, or make the journey from tip to handle? at what point do you quench?
 
before the shadow starts. You use decoalescence to figure out what the steel's critical temperature color is, as decoalescence is the phase change where the carbon atom does it's gymnastics routine

-Page
 
No problem. Very interesting to see it, I always thought non magnetic was much higher temperature, that looks very dark.
 
Sam Salvati said:
No problem. Very interesting to see it, I always thought non magnetic was much higher temperature, that looks very dark.
Click to expand...
The room was almost dark and the pics were taken without a flash. Since the steel used is W2 I figure the steel was at around 1400 to 1450 when the first photo was taken.
Good of you to paste the pics for me.

Fred
 
Why does W2 have to be at such a lower temperature and not up around 1500 or 1550 for hardening, does it harden better at a lower temperature, or was it just to show the decalescence?
 
Bigfattyt said:
Thanks for posting this pic. I was thinking about trying to heat treat my first knife.
So, when you are heat treating the knife does the shadow start at the edge or thicker spine?

What do you watch for, the shadow to dissapear, or make the journey from tip to handle? at what point do you quench?
Click to expand...

The shadow starts where the phase change happens. If the spine heats (or cools if going down) faster then the spine shows it first. Most of the time the edge, being thinner, responds faster to temperature changes and thus shows it first.

If you can see the shadow while the blade is heating that is best as you want to be just a bit above critical. If you see it going up, then give the steel a bit more time (give the higher alloy steels longer than simple steels) to make sure the entire knife is completely heated and go straight to the quench. You want to hit the quench before the blade cools enough to change back. You also want to be carefull not to let any part of the blade over heat (most common at the tip) and it is not critical to heat treat the tang.

Is that confusing enough for ya'?

ron
 
Sam Salvati said:
Why does W2 have to be at such a lower temperature and not up around 1500 or 1550 for hardening, does it harden better at a lower temperature, or was it just to show the decalescence?
Click to expand...
1550 is just about rite. By the time I took the first shot the temp was around the 1400 mark or so, indicated by the decalescence.

Fred
 
what was really fascinating at (I think it was 96) Ashokan was watching Tim Zowada or Kevin Cashen showing the bright line moving the other direction which indicated that the phase chang to HOT enough was happening rather than this direction which really shows that you just missed it

-Page
 
son_of_bluegrass said:
The shadow starts where the phase change happens. If the spine heats (or cools if going down) faster then the spine shows it first. Most of the time the edge, being thinner, responds faster to temperature changes and thus shows it first.

If you can see the shadow while the blade is heating that is best as you want to be just a bit above critical. If you see it going up, then give the steel a bit more time (give the higher alloy steels longer than simple steels) to make sure the entire knife is completely heated and go straight to the quench. You want to hit the quench before the blade cools enough to change back. You also want to be carefull not to let any part of the blade over heat (most common at the tip) and it is not critical to heat treat the tang.

Is that confusing enough for ya'?

ron
Click to expand...

Thanks
I think I get what you are saying. So I would be looking for the shadow to start at the thinner edge, and move toward the thicker spine. When the shadow reaches the spine, quench before it has a chance to cool back down.

Should the shadow appear after non magnetic, or around the same time? I read that non magnetic usually still have about 10+ degrees to critical temp.
 
sunshadow said:
what was really fascinating at (I think it was 96) Ashokan was watching Tim Zowada or Kevin Cashen showing the bright line moving the other direction which indicated that the phase chang to HOT enough was happening rather than this direction which really shows that you just missed it

-Page
Click to expand...

By looking at the pictures, how can you tell, outside of looking at the darkening edge, which is happening? Heat going up or heat coming down.
Does the shadow move from spine to edge when the heat is coming up?

I am trying to picture this happening and can't see it clearly.

Fred
 
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