Formation of cementite in tempered Fe-Co-C alloys

In Fe-Co-C alloys, undesirable grain coarsening results from the specific a ustenite orientation variants that form after the gamma --> alpha --> gamma transformations. Tempering of martensite before reheating prevents austeni te returning to its original orientation and also limits grain coarsening. However, the reasons for this are unclear. It may be assumed that some diff erences between cementite formed in tempered and rapidly heated alloys may cause the variation in the final austenite structure. In the present work t he orientation relationships between cementite and martensite in two temper ed Fe-Co-C alloys have been studied using microbeam electron diffraction in a transmission electron microscope. In both alloys after short-term (rapid heating at 100 degreesC s(-1) followed by quench) and long-term (I and 3 h ) tempering treatments the orientation relationships were shown to obey the Isaichev orientation relationships: ((1) over bar 03)(c)//(110)(alpha)[010](c)//[1 (1) over bar(1) over bar](al pha),[311](c) 0.91 degrees from [1 (1) over bar1](alpha). However, after rapid tempering, only one carbide variant was found in each alpha crystal, while after long-term tempering, up to three variants were p resent. This might account for the observed crystallographic reversibility in rapidly heated alloys, contrary to the multiplication of gamma variants formed from the long-term tempered martensite. (C) 2001 Elsevier Science Lt d. All rights reserved.