Exploring the Fe-C Phase Diagram: 0.2% C Content

Click For Summary

Discussion Overview

The discussion revolves around the Fe-C phase diagram, specifically focusing on the behavior of iron-carbon alloys with 0.2% carbon content during cooling from melt to room temperature. Participants express confusion regarding the implications of the phase diagram, particularly concerning temperatures outside the provided range and the relationships between different phases at specific compositions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks clarification on the phase behavior of a 0.2% C alloy as it cools, noting the diagram only provides temperature data from 500 to 1536 degrees Celsius.
  • Another participant provides a link to a resource, but the original poster expresses that it did not clarify their confusion.
  • A participant discusses the relationship between ferrite and austenite at 0.3% C and 800 degrees Celsius, proposing to use the Lever Arm rule to determine the percentages of each phase.
  • Calculations are presented for the percentages of ferrite and austenite based on the Lever Rule, with some values being approximated from a diagram.
  • Questions arise regarding the source of specific values used in calculations, particularly concerning the intersection points of the lever line with phase boundaries.
  • Concerns are raised about the reliability of the phase diagram used by one participant, with a request for verification of the carbon content in the austenite phase.
  • A participant introduces the concept of the cementite iron-carbon phase diagram being metastable, suggesting that it may not represent true equilibrium conditions and discussing the implications for phase formation at room temperature.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confidence regarding the phase diagram and its implications. There is no consensus on the specific values derived from the diagram, and multiple interpretations of the phase behavior exist.

Contextual Notes

Participants note that the phase diagram does not cover room temperature explicitly, leading to uncertainty about phase behavior at lower temperatures. The discussion also highlights the potential limitations of using a metastable phase diagram for certain analyses.

TSN79
Messages
422
Reaction score
0
Fe-C phase diagram

Can someone explain to me what happens in the Fe-C phase diagram during cooling from melt to room temperature when the content of C is 0,2%? All those lines in the diagram confuse me a bit... What I don't get is how I can say anything about what happens in room temperature, the diagram only has temps from 500 to 1536 degrees celsius. :confused:
 
Last edited:
Engineering news on Phys.org
Maybe this will help you out...

http://engr.bd.psu.edu/rcv/470/phase_diag.pdf
 
Last edited by a moderator:
Didn't really get that much wiser, but thanks anyway. I'm wondering about one more thing. I'm supposed to find the relationship between the structures that excist at 0,3% C, at 800 degrees celsius. Looking at the diagram I see that ferrite and austenite excist at this point, but how do I find this relationship? I'm supposed to use the Lever Arm rule. In other words, what percentage is there of each at this point?

Thanks
 
As for your second quetion, let me see if I can remember how to do this...

At 800°C and .3%C, that puts you in a mix of ferrite and austenite. I am going to say that my three points at the temp A=.05% (where the lever line intersects the ferrite curve), B=.3% (your stated point) and C=.35% (where the lever line intersects the austenite curve). These are eyeball on a small chart, so you may think the values are a bit different.

% Ferrite = (C-B)/(C-A) = (.35-.3)/(.35-.05) = .05/.3 = 17% Ferrite @ 800°C

% Austenite = (B-A)/(C-A) = (.3-.05)/(.35-.3) = .25/.3 = 83% Austenite @ 800°C

EDIT: I should have taken out those decimals on the percentages...whoops.
 
Last edited:
Hmm, where does the values 0,05% and 0,35% really come from? Can you read those from the diagram? You said "where the lever line intersects the ferrite curve". Could you explain this a bit? I'm really trying here...
 
TSN79 said:
Hmm, where does the values 0,05% and 0,35% really come from? Can you read those from the diagram? You said "where the lever line intersects the ferrite curve". Could you explain this a bit? I'm really trying here...
Draw a vertical line from the required composition (in this case 0.3% C), up to the required temperature (800C). This lands you in the phase composed of ferrite and austenite. Draw the horizontal "lever" line through this point (isothermal line) such that it spans the width of the specific phase region - in this case, up to the phase boundaries with ferrite and austenite respectively. The points of intersection of the lever line with each of these phase boundaries tells you the compositions of these phases (ferrite and austenite). I get about 0.02%C (the max C content in ferrite is about 0.03%) in ferrite and about 0.6%C in the austenite phase.

The lengths of the two segments of the lever arms are proportional to the amounts of the phases. From my numbers (also rough - don't have a text nearby), the ratio is about 0.30 : 0.28 ferrite to autenite, or about 48% austenite and 52% ferrite (each of whose compositions is given above).
 
Last edited:
It's been a while, but just to check...

Gokul, on your C percentages, you said .6% C in the austenite phase. On the phase diagram I have in my book, .6% at 800°C puts me smack out in the middle of the austenite phase, not on the phase boundary. Are you sure about that value? Like I said, it's been a while since i have done one of these...
 
FredGarvin said:
It's been a while, but just to check...

Gokul, on your C percentages, you said .6% C in the austenite phase. On the phase diagram I have in my book, .6% at 800°C puts me smack out in the middle of the austenite phase, not on the phase boundary. Are you sure about that value? Like I said, it's been a while since i have done one of these...
I thought I was, but I was merely using a chart I googled on the spur (don't know how reliable it is). I'll have to go home to double-check.
 
Last edited:
Groovy. Thanks.
 
  • #10
What I don't get is how I can say anything about what happens in room temperature, the diagram only has temps from 500 to 1536 degrees celsius.

The cementite iron-carbon phase diagram is a metastable phase diagram meaning that it isn't a "true" equilibrium diagram. Most undergraduates and professionals do not know this probably because that is what they use mostly. Cementite is a intermediate phase that the precipitate has to go through before equilibrium graphite is formed. The formation of GP-zones is a similar pheonomena. It all has to do with the activation energy of kinetics. However, for most purposes the process kinetics do not allow the formation of graphite for steels so the cementite phase diagram is mostly used. The Lever Rule is can be used for both metasable and stable phase diagrams.

Modey3
 

Similar threads

Reading a Binary Phase Diagram (lever rule etc.)
  • · Replies 1 ·
Replies
1
Views
3K
Phase Diagrams at Very Low Concentration of one Component
  • · Replies 3 ·
Replies
3
Views
2K
Finding Values Using Fe-C Diagram for Materials Engineering Assignment
  • · Replies 3 ·
Replies
3
Views
2K
Gibb's phase rule degrees of freedom
  • · Replies 2 ·
Replies
2
Views
3K
Phase change material steady melt temp, like ice water?
  • · Replies 2 ·
Replies
2
Views
2K
Fe-C Phase Diagram Gibbs-Comp Slopes
  • · Replies 1 ·
Replies
1
Views
7K
Undergrad Melting point of graphite and diamond?
  • · Replies 3 ·
Replies
3
Views
6K
What exactly happens during a phase change?
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Four Questions on vacuum phase transitions in the Universe...
  • · Replies 29 ·
Replies
29
Views
4K
Potential Energy vs x phase diagram
  • · Replies 6 ·
Replies
6
Views
2K