1080 Steel Fine Pearlite

[MechEng1977]

I am looking for a Fine Pearlite stucture after a piece of rail is thermite welded. The rail steel is essentially a 1080 Carbon Steel. Specifically the composition is:
Weight Percentage, %

Carbon, C
0.72 - 0.84

Manganese, Mn
0.70 – 1.25

Silicon, Si
0.10 – 1.00

Phosphorus, P
0.02 Max.

Sulphur, S
0.02 Max.

Copper, Cu
0.40 Max.

Chromium, Cr
0.20 – 0.40

Nickel, Ni
0.15 Max.

Molybdenum, Mo
0.05 Max.

Vanadium, V
0.01 Max.

Aluminium, Al
0.01 Max.

Based on a CCT Diagram for 1080 steel it should be cooled from Eutectoid temp of 727°C to 25°C no quicker than 2 minutes and no longer than 166 minutes. Does this make sense. See attached.

 

[Wrenchbender]

Yes, that would be my interpretation of the diagram, but you may want to do some experimenting and lab work for verification. As compared to spec-grade 1080, the chemistry you have listed has boosted levels of Si, Mn, and Cr among others that could affect 'hardenability' and bias the resulting microstructure, probably toward the P+M zone.

 

[MechEng1977]

What I don't get is the 40°C/s and the 5°C/s it doesn't match with the time scale on the x-axis?

 

[metengr]

The time scale on the x-axis is just seconds (s) of time and your comparing a cooling rate of deg C/s to seconds.

 

[MechEng1977]

Yes I understand that, I'm not that slow.

I don't understand a) how is the cooling rate constant?

b)If we take a constant cooling rate of 40C/s, it means that from 730C to 0C, it would take 18.3 seconds. That doesn't match the x-scale which says 10^2.

 

[TVP]

The cooling rates associated with the three curves (5, 40, 140 C/s) are incorrect with respect to the x-axis. The author and editor(s) of this textbook did not catch the error. If you look at other CCT diagrams you can see that a cooling rate of 40 C/s is associated with the 10 second order of magnitude, not 10².

 

[MechEng1977]

Ok so I should cool the steel from the critical temp no quicker than 10 seconds and no longer than 166 minutes?

 

[TVP]

Yes, that is correct. Here is a link to a free CCT/TTT diagram calculator that was originally developed by Oak Ridge National Lab:

http://calculations.ewi.org/vjp/secure/TTTCCTPlots.asp