Gray Cast Iron and temperature

[Flesh]

I have a sample of pearlitic GCI (grade used in a brake application). The material was exposed to an unknown elevated temperature in service. When looking at the microstructure, some of the pearlite is starting to break up (disassociate to form secondary ferrite). At what temperature or temperature range would I expect this to occur? Assumption: the elevated temperature exposure occurs for 1 to 5 hours. Essentially, I am trying to determine how hot this material could have reached.

 

[metengr]

Excerpt from ASM Handbook, Volume 4,

Ferritizing Annealing. For an unalloyed or low-alloy cast iron of normal composition, when the only result desired is the conversion of pearlitic carbide to ferrite and graphite for improved machinability, it is generally unnecessary to heat the casting to a temperature above the transformation range. Up to approximately 595 °C (1100 °F), the effect of short times at temperature on the structure of gray iron is insignificant. As the temperature increases above 595 °C (1100 °F), the rate at which iron carbide decomposes to ferrite plus graphite increases markedly, reaching a maximum at the lower transformation temperature (about 760 °C, or 1400 °F, for unalloyed or low-alloy iron).