The composition of eta carbide phase in 2 1/4 Cr-1 Mo Steel

The metal-to-carbon ratio in the eta phase in 2 1/4 Cr-1 Mo steel exposed to sodium at 566°C for 26,500 h, and probably under a variety of service conditions, approaches four, rather than six, a fact which may be influenced by the presence of Si, O, and perhaps Cr. Silicon is present in the eta phase, probably on a metal sublattice, while oxygen is probably present on the metalloid sublattice. The observation is supported by both structural and compositional data from the literature.