Development of High Temperature Resistant Ferritic Steels

  DFG-Logo Copyright: © DFG   SEM-BSD picture Copyright: © IEHK Figure 1: SEM-BSD picture of the Laves phase (bright) in a ferritic matrix (dark) after 30 min of heat treatment at 700° C.

Single phase ferritic steels with a chromium content of 17 wt-% show potentially sufficient resistance to steam oxidation at high temperatures. The required creep strength cannot be achieved on the basis of precipitation of carbides, nitrides and carbonitrides, because of the low solubility of carbon and nitrogen in ferritic steels. Alloying with tungsten, niobium and silicon enables reaching the desired combination of high oxidation resistance and creep strength by solid solution and precipitation strengthening effects. The alloying of these elements results in the precipitation of intermetallic (Fe,Si,Cr) 2 (Nb,W) - Laves phase particles, finely dispersed in the matrix (Figure 1).

The research deals with the development of a production process of high temperature resistant steels. The effect of a thermomechanical treatment on the Laves phase precipitation and the way to control targeted the precipitation are investigated. The focus is to create a fundamental understanding of the interaction of the alloy composition, material and deformation structure and precipitation microstructure with the mechanical properties.

The oxidation and creep resistant ferritic steels open up numerous fields of high temperature application, such as steam transfer components in steam power plants, ceramic fuel cells or thermal energy conversion and storage systems.