Influence of initial thermomechanical treatment on high temperature properties of laves phase strengthened ferritic steels

The aim of this work was to design 17 wt%Cr Laves phase strengthened HiperFer (Highperfomrance Ferrite) steels and evaluate their properties. This class of steel is supposed to be used in Advanced Ultra Super Critical power plants. Such cycles exhibit higher eciencyand are environmentally friendly, but improved materials with high resistance to reside/steam oxidation and sucient creep strength are required. The work focused on the characterization of creep properties of 17Cr2.5W0.5Nb0.25Si heat resistant steel. Small batches of steels with nominal compositions of 17Cr3W0.5Nb0.25Si and 17Cr3W0.9Nb0.25Si were used to analyze the inuence of chemical composition on the precipitation behaviour in comparison to17Cr2.5W0.5Nb0.25Si steel. Creep strength of HiperFer steels is ensured by ne dispersion of thermodynamically stable Laves phase particles, while maintaining high corrosion resistance by a relatively high chromium content. Design of HiperFer steels was accomplished by thermodynamic modeling (Thermocalc) with the main tasks of elimination of the unwelcome brittle (Fe,Cr)- phase and maximization of the content of the strengthening C14 Fe2Nb type Laves phase particles. Long term annealing experiments of all HiperFer steels were performed at 650 °C in order to evaluate the role of chemical composition and initial thermo-mechanical treatment state on precipitation behaviour. Laves phase particles formed quickly after few hours and the size of precipitates did not change signicantly within 1,000 hours. The observed development of Laves phase particles was compared with thermodynamical calculations (TC-Prisma). The creep properties of 17Cr2.5W0.5Nb0.25Si steel in dierent initial thermo-mechanical treatment states were tested at 650 °C. The inuence of dierent cold rolling procedures, and heat treatments was investigated. Increased cold rolling deformation had a positive efect resulting not only from work hardening, but from the acceleration of Laves phase particle precipitation.The recrystallization annealed material exhibited signicantly shorter creep life in comparison to the rolled material. Two promising heat treatments restoring the creep strength up to the level of rolled material were identied. Tempering at 540 °C accelerates the formation and reduces the size of strengthening Laves phase precipitates. Short term tempering at 800 °C causesdense precipitation of Laves phase particles at grain boundaries without detrimental inuence on the morphology of Laves phase particles in the grain interiors. Dense precipitation of coarse Laves phase particles causes a retardation of grain boundary sliding in the later periods of creep exposure