The way I would do it would be to output two different versions of the same iron rod - one completely cool (black), and one completely hot (red).
It would then be a simple matter to bring it into After Effects (or any compositing software) and, using a feathered mask, control the exact timing of the cooling. In this way, if the client wants a slightly different timing on it, for example they want it to look exactly the way it looks in reality, you can shift your mask.
While a gradient map should work, I think that shifting it along the length of the rod to show the cooling process could be much more difficult than the above process. Good luck!
Joe Bourke
Owner/Creative Director
Bourke Media
http://www.bourkemedia.com
