Influence of nitrogen on 475°C embrittlement of high-chromium ferritic steels

Abstract

The hardening of iron-chromium alloys containing more than 13 wt-%Cr in the temperature range 550–800°C is well known and is ascribed to the formation of σ-FeCr. Severe embrittlement at lower temperatures, commonly referred to as ‘475°C embrittlement’, is less well understood and has been variously ascribed to σ-phase, ordered Fe-Cr superlattices, and coherent precipitation of Cr-rich ferrite. The microstructure and properties of Fe-22 wt-%Cr alloys resulting from controlled changes in nitrogen concentration are described. The variation of mechanical properties with ageing time at 500°C was followed using microhardness measurements, and corresponding changes of microstructure were observed by transmission electron microscopy. A pronounced change in microstructure accompanied by severe embrittlement occurs when a critical nitrogen concentration is exceeded. The alloy then contains a high density of coherent disc-shaped particles with high associated strain and is consistent with the proposed model in which embrittlement is a result of substitutional-interstitial solute-atom clustering. The tendency of high-Cr alloys to cluster as a result of the miscibility gap in the Fe-Cr system is enhanced by the presence of nitrogen, and the lattice strains associated with interstitial clusters result in embrittlement. A chemical-strengthening model for the shearing of Cr-N clusters accounts satisfactorily for the observed increase in hardness. The morphology of the clusters depends on the ageing temperature of the alloy relative to the coherent spinodal in the Fe-Cr system.