Microstructures
by
George
Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA,
1966
Copyright©2005 by George Langford
Cast Irons, High Alloy Steels,
and Superalloys - Lesson 1 - Ninth specimen
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This is a nodular cast iron with an unusual microstructure,
shown at left at 500X.
What has been done to it ?
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Explanation: It is
martensitic, as the result of a heat treatment after casting that
consisted of austenitization for 8 hours at 940C followed by water
quenching. Normally, no cast iron would be martensitic in the as
cast condition. On the other hand, welded cast irons usually
freeze as white iron and harden so as to look like this upon cooling to
room temperature. This sequence of transformations is difficult
to
avoid at the heat flow rates common for welding. Better to braze
with copper or bronze and a gas torch than to attempt electric arc
welding a cast iron.
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