Metallurgical question on scale

D

deker

Joined
Nov 14, 2005
Messages
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So, as I was out in the shop today forging some damascus, I got to thinking while I was waiting for my billet to re-heat, and I have a question.

So, (I think) I understand that scale is carbon being lost due to oxidation. Ok, fine. Now, why is it that more scale forms at points of deformation as you work the steel? I was twisting a billet when I thought of this. If scale is due to oxidation, why does twisting a piece of steel produce so much more scale than letting the same piece of steel sit in still air?

-d
 
Some common misconceptions ! The facts are scale is iron oxide .Decarburization is due to lower carbon levels in the air [as CO2 and Co] than the steel .Carbon diffuses to the surface and is lost as it oxidizes to CO2 and CO .Nature wants to make everything equal .If the carbon levels in the steel are higher than the atmosphere there is decarburization.If the atmosphere has more carbon than the steel ,the steel is carburized....When we forge we are putting energy into the steel .Areas of higher energy will be more chemically active .Therefore forged areas will oxidize faster.
 
scale forms, as you twist the scale craks letting air onto points the scale previously stopped air from reaching, causing more scale, which cracks again as you twist further, causing more air and more scale...

and so on until you stop twisting.
 
Thanks Mete, that makes a little more sense. I just couldn't figure out why if I took a piece of steel out of a reducing atmosphere (so it wasn't already scaled) and bent it, why there was more scale at the point of the bend. I hadn't thought of the energy being imparted to the material. I sometimes forget how much "extra" energy we add to the equation by forging.

You ever watch the steel as you're hammering and see a bright spot where your hammer blow just landed? I use this, plus the shape of what I'm working, to determine where the next hammer blow should fall. I always got that the "hot spot" was due to my extra energy being transferred into the steel, I just never though of that down to the molecular level.

Thanks again for making it all make sense to us mere mortals Mete!

-d
 
mete said:
Some common misconceptions ! The facts are scale is iron oxide ..
Click to expand...
Mete help me understand this first part of your post.

When you say that the scale that I notice popping off my blade as I bang on it, is "iron oxide' my question is, Was it good steel a moment ago?

I place the cold blade in the hot forge. There is no iron oxide yet, correct?

Now I take out the hot blade.
I look at it.
I dont see any scale yet (as it's red hot and hard to even look at)
BUT, right at this moment is the good outer skin steel (that was just there a moment ago), turning into iron oxide?

And, is the reason it pops off the blade because it's iron oxide and it's not welded to the good steel underneath?

And, here is another question I have always had but didnt know how to ask-
I was forging the tang of a knife last.
I slipped the blade in the forge tang-first so that the tang was right in the fire blast zone.
then the phone rang...

later i saw the blade on the floor, there was no longer any tang!
Did I "burn" the tang off?

or did I "scale it to death" off in layers as like when I see scale layer flakes when I forge a blade?
 
The scale is forming inside the furnace. The higher the temperature and the longer the time and the more oxidizing the atmosphere the more scale forms !! At heat you don't notice it but when you start whacking it the brittle oxide breaks off. As for the tang it just oxidized and feel off !
 
Mete,
Ok, now does the size of scale tell me about something important?

I have heard talk that the size and shape of scale can tell a person if the steel is getting too hot.
But to forge 52100 Ball Bearings I heat the crap out of the steel for the first few heats...I do see lots of scale, should I be concerned?
 
Not to hijack this thread.

If i am performing a HT and place a piece of charcoal in the foil with my blade, would the blade absorb extra carbon? I figure this cause the atmosphere around the knife should be carbon rich.
 
Charcoal in the foil pouch ? It won't do anything . [[ BTW recently I saw a History channel program dealing with sword making .The said "charcoal contains 'trace' amounts of carbon " LOL ]] .....There are some differences in how different steels oxidize but if you think there's excessive scale - use a more reducing [cabon rich ] atmoshere, reduce temperature , reduce time.
 
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