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

[TSN79]

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.

 

[FredGarvin]

Maybe this will help you out...

http://engr.bd.psu.edu/rcv/470/phase_diag.pdf

 

[TSN79]

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

 

[FredGarvin]

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.

 

[TSN79]

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...

 

[Gokul43201]

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).

 

[FredGarvin]

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...

 

[Gokul43201]

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.

 

[FredGarvin]

Groovy. Thanks.

 

[Modey3]

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