Precipitation process of Z-phase in 9-12%CR steels

Precipitation of Z-phase, Cr(V,Nb)N, is known to negatively affect creep properties of 9-12%Cr steels for power plant applications as it dissolves finely distributed MX particles, (V,Nb)N, especially in high Cr steels. As the Z-phase precipitates slowly as large particles, this causes a net drop in precipitation strengthening. Two model alloys containing 9 and 12%Cr, but otherwise having similar composition, were produced in order to quantify the difference in Z-phase precipitation speed at different Cr levels. The nitride precipitation behavior was followed at different temperatures using TEM and XRD, allowing for a quantification of the Z-phase precipitation. The Z-phase was found to precipitate 20-50 times faster in the 12%Cr steel compared to 9%Cr steel in the temperature range 600-650°C. The transformation of MX into Z-phase was followed in a Ta containing alloy without V or Nb. In this alloy the Z-phase precipitates very quickly, and thus appears as finely distributed particles which have the same strengthening effect as MX particles. Investigations using atomic resolution microscopy showed how Cr diffuses from the matrix into the TaN MX particles and gradually transforms them both chemically and crystallographically into Z-phase CrTaN particles. Copyright © 2014 Electric Power Research Institute, Inc. Distributed by ASM International®. All rights reserved.