Effect of copper addition on the toughness of new Z-phase strengthened 12% chromium steels

Two Z-phase strengthened test steels with similar chemical composition were studied. The main difference in composition is the addition of 1 wt% Cu into one of the steels (referred to as "Z-Cu"). Mechanical testing was performed. The impact strength is very different: 3 J vs. 46.3 J, for the original and the Z-Cu steel, respectively. In the original steel that contains no Cu, much more Laves-phase (Fe2(W,Mo)) precipitates had formed along the prior austenite grain boundaries than in the steel with Cu addition. This is believed to be the reason for the difference in impact strength. Furthermore, the Cu addition also influenced the morphology of Laves-phase precipitates; fine rod-shaped instead of coarse equiaxed Laves-phase particles were observed in Z-Cu steel in comparison to the original steel. No partitioning of Cu into the Laves-phase particles was detected by using atom probe tomography (APT). The main function of Cu seems to be the formation of Cu precipitates that act as nucleation site for Laves-phase. Copyright © 2014 Electric Power Research Institute, Inc. Distributed by ASM International®. All rights reserved