sub critical heat treatment in high chromium white cast iron

[carbidesformation]

We apply direct secondary heat treatment (from 350 OC to 750 0C) at our material (18% Cr) presumably to have secondary carbide precipitation and martensite formation during cooling. The secondary carbides that we get have high percent of chromium and a lower percent of fe contrary to the primary ones that have high percent of fe and a low percent of cr.
In literature this is widely mentioned but I can't find any references regarding why is that happening. Do you have any suggestions?
Moreover I would be very interested if you have any good etching to propose for secondary carbides?

Thank you

 

[metengr]

Plenty of information

http://www.google.com/search?hl=en&...f+high+chromium+white+cast+iron&start=10&sa=N

http://www.springerlink.com/content/m777348828873471/

For chemical etchants

http://www.kaker.com/steel/demo/list/cast_steels.html