Mark Aspery

Metallurgy Question Iron/Carbon phase diagram

Recommended Posts

Why does the Upper transformation temp (AC3) slope down to meet the AC1 line?

I see that AC1 (pearlite) remains at a constant temperature across the diagram.

Why is the free ferrite line not the same- set at a constant until it reaches the Eutectoid?

I want to say that the pearlite when it changes to Austenite is acting as a catalyst upon the free Ferrite - but I know that is not correct. What is happening?

Any thoughts?

Share this post


Link to post
Share on other sites

Coincidentally this was posted to theforge list yesterday. I don't know if it'll help any, I don't know enough to judge.

Transformation diagrams (CCT & TTT)

It has a couple of approx. real-time animations of the grains forming in
steel as it cools. (at 3 choices of constant temp)

Frosty

Share this post


Link to post
Share on other sites
Why does the Upper transformation temp (AC3) slope down to meet the AC1 line?


Pretty tough question. I guess it comes down to the electronic structure of the iron-carbon system, but I guess we don't want to go to that level of detail. :D

One reason for that could be that the temperature at which pure ferrite changes to austenite is higher than that of iron-carbon alloy. The closer to the pure ferrite your system is (i.e. low C concentration) the more there will be ferrite in the system and thus the transition point to pure austenite phase will occur at higher temperatures than in the case of systems with higher C concentration.

Pearlite will change directly to austenite only at eutectoid composition (0.76 wt% C). At lower C concentrations pearlite will first change to ferrite+austenite phase before going completely to austenite phase. This ferrite+austenite phase lies between A1 and A3 lines.

Share this post


Link to post
Share on other sites
Pretty tough question. I guess it comes down to the electronic structure of the iron-carbon system, but I guess we don't want to go to that level of detail. :D

One reason for that could be that the temperature at which pure ferrite changes to austenite is higher than that of iron-carbon alloy. The closer to the pure ferrite your system is (i.e. low C concentration) the more there will be ferrite in the system and thus the transition point to pure austenite phase will occur at higher temperatures than in the case of systems with higher C concentration.


I accept this. This describes the phase diagram with %C along the bottom. But why does AC3 slope, why is it not a constant temperature as the pearlite? Something must be happening to the free ferrite to change its phase change temperature.


Pearlite will change directly to austenite only at eutectoid composition (0.76 wt% C). At lower C concentrations pearlite will first change to ferrite+austenite phase before going completely to austenite phase. This ferrite+austenite phase lies between A1 and A3 lines.


I'm not sure about the above statement.

I see that Pearlite changes to Austenite in the range of AC1 regardless of the %. At Hyper-eutectiod, then the excess Carbon (cementite) now takes longer to go into solution. At the Eutectiod, there is no more free ferrite, so no 'Austenite / Free Ferrite' range or phase.

Share this post


Link to post
Share on other sites

I see that Pearlite changes to Austenite in the range of AC1 regardless of the %.


You are right. I should have written that the pearlite+ferrite phase changes to austenite+ferrite phase. It really seems that the pearlite part changes directly to austenite.

Share this post


Link to post
Share on other sites

I was happier when I thought that the Pearlite phase change to Austenite acted as a kind of catalyst to the remaining free ferrite in hypo-eutectoid steel.


I was told that that was not correct - but I cannot find anything that tells me what is happening.


Ignorance was bliss!!

Share this post


Link to post
Share on other sites

The key point here is the difference in structure between austenite and ferrite. Austenite has its iron atoms arranged in a face centered cubic atomic structure (fcc) while in ferrite, the iron atoms are arranged in a structure called body centered cubic (bcc). In pure iron, austenite changes to ferrite below 1666 F (i.e. goes from fcc to bcc).

The addition of carbon, however, makes things more interesting since carbon atoms fit more easily in the fcc structure of austenite than then can in bcc ferrite. As such, much more carbon can be dissolved in austenite than in ferrite. Because of this difference, the addition of carbon to iron allows austenite to remain stable at lower and lower temperatures. It's like adding salt to water: since salt can more easily dissolve in liquid water than in ice, it lowers the melting point. This is why the A3 line slopes downward - higher carbon contents allow austenite to remain stable at lower temperatures.

The limit to this is the eutectoid point. Austenite of this composition (0.8% C) will transform to pearlite below 1333 F. Note that pearlite is not a single substance, but rather a fine mixture of ferrite (essentially pure iron) and cementite/iron carbide (a chemical compound of iron and carbon). Under the microscope these two components or phases are arranged in alternating plates. When polished and etched with dilute acid, pearlite appears iridescent like a pearl - hence its name.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.