Influence of Intermetallic Precipitates and Heat Treatment on the Mechanical Properties of High-Temperature Corrosion Resistant Ferritic Steels

Abstract

High-temperature corrosion resistant ferritic steels are commonly used in heat exchangers for auxiliary power units (APU), automotive exhaust systems and structural parts of solid oxide fuel cells (SOFC) due to their excellent thermal fatigue resistance. As the process temperatures in these applications are primarily limited by the materials high temperature strength, the main focus of this work is on the improvement of this property by adjustments in the material design. Generally, two mechanisms were used to increase the high temperature strength, solid solution strengthening and precipitation hardening. Due to their large atomic radii and high solubility in α-Fe, W and Mo were used for solid solution strengthening. Furthermore, the content of niobium, which is well known to form Laves phase precipitates, was raised. This led to a higher content of Laves phase precipitates compared to the reference material. The analyses concentrated on the effect of the Laves phase. Strength at elevated temperature was investigated in compression tests at 900̊C with respect to the annealing time which was varied between 1h and 1440 h.