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Ferrite, Austenite, etc.


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I've looked all over the internet, can't find one. I'm looking for a good article on defenitions, properties, etc. of Ferrite, Austenite, and such terms as those. I'm pretty sure they are stages of steel, but I need something good to study. If you could point me to a good article on it, or explain them yourselves, that would be awesome.

 

Thx,

 

-Caleb

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Take this for what it's worth, I'm probably not 100% correct. But a simple definition:

 

Ferrite is a plain iron matrix, the iron molecules are under as little stress as possible and very soft and malleable, I think some people might also call it pearlite.

 

Austenite is the stage where the iron molecules are spread open by enough heat to allow molecules of the alloying elements to intermingle, and when cooled quickly, the iron collapses tightly around these alloying elements, putting the matrix under incredible tension, which hardens it.

 

Martensite would be the stage of steel that is fully hardened by the iron molecules tightly constricted around the alloying elements, tempering at this stage eases the tension just enough to allow flexibility, which causes the tempered steel to spring to it's original shape after pressure is applied.

 

I've had a hard time finding articles as well, so I'm probably missing a HUGE amount of information here, but hopefully this will serve at least a basic understanding. I'm sure someone will chime in with more and better information for you.

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Take this for what it's worth, I'm probably not 100% correct. But a simple definition:

 

Ferrite is a plain iron matrix, the iron molecules are under as little stress as possible and very soft and malleable, I think some people might also call it pearlite.

 

Austenite is the stage where the iron molecules are spread open by enough heat to allow molecules of the alloying elements to intermingle, and when cooled quickly, the iron collapses tightly around these alloying elements, putting the matrix under incredible tension, which hardens it.

 

Martensite would be the stage of steel that is fully hardened by the iron molecules tightly constricted around the alloying elements, tempering at this stage eases the tension just enough to allow flexibility, which causes the tempered steel to spring to it's original shape after pressure is applied.

 

I've had a hard time finding articles as well, so I'm probably missing a HUGE amount of information here, but hopefully this will serve at least a basic understanding. I'm sure someone will chime in with more and better information for you.

OH, THANK YOU!! I've really been needing this!

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You could go to KnifeDog.com and go to the makers section then to the heat treating sub section and then click on the sticky on heat treating at the top of the page. That should have some of the definitions. Freya is close but ferrite and austinite deal with the basic iron crystals. Think of an iron crystal in a cube shape with the iron atoms at the corners. Ferrite, the "low" temperature form that exists at less than around 1420° or there abouts. This form has an iron atom at each of the corners and in the middle of the cube. These cubes form together to produce a matrix of interlocking structures rather that individual molecules. This is what allows a metal to act as a metal. This structure is also refered to as a body centered cube or alpha iron. Iron crystals in this configuration also don't want to dissolve much carbon; slightly less than 0.02%

 

Austinite forms when the steel or iron is heated above this temperature to where the iron matrix reshapes itself to from cubes with iron atoms on each corner and each face. This is also called a face centered cube or gamma iron. It will hold up to around 0.77% carbon in solid solution. This is important because this dissolving of carbon allows martinsite to form on rapid cooling.

 

Martinsite is formed when the steel is cooled rapidily enough to trap carbon atoms in the iron matrix. The carbon doesn't allow the iron to switch back to a body centered cube but a body centered "oblong box" structure which puts strain on the atomic bonds between the iron atoms in the matrix and makes the steel harder and more brittle. How quickly this has to be done depends on the alloy. A shallow hardening steel may need to be cooled very rapidly and something like a complex tool steel or stainless steel can harden in still air.

 

Cementite is iron carbide, Fe3C. It contributes to hardness and wear resistance. It will occur within a product of austinite conversion known as pearlite in all steels and outside the pearlite structure in steel over 0.77% carbon. Even though cementite is a carbide it is frequently referred to as a separate structure. Because the base is iron it will break down at temperatures that are close to what will convert ferrite to austinite. It is also the softest on the carbides that you will find in steel.

 

Pearlite is a structure of ferrite and cementite arranged in alternating layers. This forms in non air quenching steels at room temperatures. In steel with less than 0.77% carbon cementite will not form outside the pearletic structure but will form ferrite structures outside the pearletic structure. In steels over 0.77% cementite will form outside of the pearletic structure but ferrite won't.

 

This should be enough to get you started. If you have anymore questions on definitions just ask.

 

Doug

Edited by Doug Lester
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You could go to KnifeDog.com and go to the makers section then to the heat treating sub section and then click on the sticky on heat treating at the top of the page. That should have some of the definitions. Freya is close but ferrite and austinite deal with the basic iron crystals. Think of an iron crystal in a cube shape with the iron atoms at the corners. Ferrite, the "low" temperature form that exists at less than around 1420° or there abouts. This form has an iron atom at each of the corners and in the middle of the cube. These cubes form together to produce a matrix of interlocking structures rather that individual molecules. This is what allows a metal to act as a metal. This structure is also refered to as a body centered cube or alpha iron. Iron crystals in this configuration also don't want to dissolve much carbon; slightly less than 0.02%

 

Austinite forms when the steel or iron is heated above this temperature to where the iron matrix reshapes itself to from cubes with iron atoms on each corner. This is also called a face centered cube or gamma iron. It will hold up to around 0.77% carbon in solid solution. This is important because this dissolving of carbon allows martinsite to form on rapid cooling.

 

Martinsite is formed when the steel is cooled rapidily enough to trap carbon atoms in the iron matrix. The carbon doesn't allow the iron to switch back to a body centered cube but a body centered "oblong box" structure which puts strain on the atomic bonds between the iron atoms in the matrix and makes the steel harder and more brittle. How quickly this has to be done depends on the alloy. A shallow hardening steel may need to be cooled very rapidly and something like a complex tool steel or stainless steel can harden in still air.

 

Cementite is iron carbide, Fe3C. It contributes to hardness and wear resistance. It will occur within a product of austinite conversion known as pearlite in all steels and outside the pearlite structure in steel over 0.77% carbon. Even though cementite is a carbide it is frequently referred to as a separate structure. Because the base is iron it will break down at temperatures that are close to what will convert ferrite to austinite. It is also the softest on the carbides that you will find in steel.

 

Pearlite is a structure of ferrite and cementite arranged in alternating layers. This forms in non air quenching steels at room temperatures. In steel with less than 0.77% carbon cementite will not form outside the pearletic structure but will form ferrite structures outside the pearletic structure. In steels over 0.77% cementite will form outside of the pearletic structure but ferrite won't.

 

This should be enough to get you started. If you have anymore questions on definitions just ask.

 

Doug

THANK YOU SO MUCH!!!

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http://www.feine-klingen.de/PDFs/verhoeven.pdf

 

I shouldn't be posting this, but encouraging you to buy it from ASM. However, the mirror site is still up after all this time, and they have not pursued it, so there it is. Very accessible, very good, everything you ever needed to know, but didn't know enough to ask. :)

Edited by Howard Clark
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Just noticed that there was an error in my explanation of austine and corrected it. The austinite crystal has an iron atom at each corner of the cube and each face.

 

Doug

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  • 3 weeks later...

What Doug Lester said looks right on the money to me.

 

If you want the full meal deal, then: +1 on the public domain Verhoeven PDF that Howard Clark posted - but note that it reads like temperatures were initially written in C and F translations were added at the last minute - with several goofs. His updated and physically published book is on my wish list.

 

Wayne Goddard - in his folksy style - has a couple of chapters covering phases of steel and heat treat in his "Wonder of Knifemaking" and a chapter on it in his "$50 Knife Shop".

 

And what Alan Longmire said: ABS Master Smith Kevin Cashen *is* an expert and has been great in sharing his knowledge in postings such as this one over on Knife Dogs: http://knifedogs.com/showthread.php?22568-Your-Heat-Treating-Tool-Box

 

I'm not a metallurgist by any stretch - but I summed up my understanding of this at the end of the latest 5160 Club newsletter - pretty much what Doug said with more added in: http://www.elementalforge.com/5160Club/201306Newsletter.pdf

Edited by Michael Kemp
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