444 High-Chromium White Irons for Wear Resistance Applications

Friday, November 6, 2009: 3:40 PM
Pancho Villa (Camino Real Hotel)
Arnoldo Bedolla-Jacuinde , Instituto de Investigaciones Metalurgicas, Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico
High-chromium white irons can be described as in situ composites with large, hard eutectic and/or proeutectic M7C3 carbides in a softer iron matrix (commonly austenitic in the as-cast conditions and mostly martensitic after a destabilization heat treatment). These microstructure combinations make high-chormium white irons very suitable for applications where abrasion resistance is the main requirement. Their exceptional abrasive and erosive wear resistance results primarily from their high volume fraction of hard carbides, although the toughness of the matrix also contributes to the wear resistance. 
         Attempts to improve wear resistance in high-chromium irons have led researchers to try different alloying elements such as vanadium, tungsten, titanium and niobium. Even the addition of carbides such as TiC has been undertaken to obtain composites in these irons. The aim of these additions is usually to achieve some modification of the eutectic carbide structure by obtaining harder carbides, though they may improve the hardenability of the matrix since these carbide forming elements may partition also to the matrix.
It has been recognised that a possible strategy for improving the toughness of white iron alloys as well as the wear resistance under sliding conditions involves the refinement of the eutectic carbide structure by producing finer, more globular carbides. Rapid cooling or lower superheat have been used as a means of increasing nucleation and hindering carbide growth, producing finer carbides. Conversely, slow cooling of castings results in larger dendrite arm spacing, reducing the number of sites for nucleation of the eutectic and hence, larger, coarser, eutectic forms. Alloying additions of some elements that segregate to the matrix/carbide interphase, such as rare earths, silicon, boron, etc., have also been used to modify the eutectic carbide structure.