Color change in gray cast iron

[Beggar]

I have a large gray cast iron roller that's subject to friction heating (it's internally cooled). In certain areas of the roller, the color of the material has changed from bright silver to a yellowish color to a dark blue.

Obviously it's been heated above some sort of transition temperature but I'm wondering if anybody could give me an estimate of how high. The highest measured temperature is about 125C

I don't know the specific composition of the material.

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[unclesyd]

Your temperature isn't high enough for any transition to take place.
You have some type stain or a light oxidation due to the enviroment.

What is the enviroment the roller sees?

 

[Carburize]

If these are really produced by heat and not by some other corrosion process the dark blue color would suggest that the affected area had been close to 600F.

 

[Beggar]

The roller is operating inside of a size-reduction mill. The discoloration does not appear to be corrosion because it's in discrete bands that appear due to friction and because of the color transitions from silver to blue.

Another data point: An earlier roller was apparently statically cast and has not seen the same discoloration while (presumably) seeing the same environment.

Would there be some difference in a statically cast roller vs. a centrifugally cast roller that would account for a different response to the heat?

Also, in order to maintain full strength & toughness, what would be the maximum permissible operating temperature of gray cast iron?

--------------------
Bring back the HP-15

http://www.hp15c.org

--------------------

 

[Carburize]

I would not expect any difference in the color change due to heat from a static cast compared to a centrifugal cast part.
At temperatures up to 600 - 700F all types of unalloyed cast irons show little if any decrease in strength. At temperatures above 700F the tensile strength decreases rapidly.