Laves Phase Precipitation Behavior in HiperFer (High Performance Ferritic) Steel with and without Boron Alloying

Abstract

High-chromium ferritic stainless HiperFer steels were developed for high-temperature applications in power conversion equipment. The presented research describes the precipitation behavior of the Laves phase after the thermomechanical treatment of Fe-17Cr-0.6Nb-2.4W HiperFer alloys with and without the addition of 55 ppm boron. The boron-alloyed variant was produced with the aim of enhancing grain boundary strengthening and consequently increasing creep resistance. The focus is set on the effect of boron on the thermomechanically induced precipitation of (Fe,Cr,Si)2(Nb,W) Laves phase at grain boundaries. The addition of boron modifies the diffusion conditions in the area of grain boundaries. Consequently, the formation of Laves phase is promoted and the particle growth and coarsening process are suppressed. The impact of boron addition was validated by performing creep and thermomechanical fatigue testing in the standard processing state of HiperFer steel. In the B-alloyed variant, increased creep ductility through the modification of the particle-free zone widths at high-angle grain boundaries was encountered. Nevertheless, an optimized thermomechanical treatment is necessary to fully utilize the increased ductility effect for the creep strength optimization of the B-alloyed grade.