Nucleation of W-rich carbides and Laves phase in a Re-containing 10% Cr steel during creep at 650 °C

The nucleation of the Laves phase particles in a Re-containing 10% wt. Cr-3% Co-3% W steel with a low nitrogen and a high boron contents during creep is characterized by distinctive features. The precipitation process can be written as M23C6 carbide -> M6C carbide -> Laves phase. The nucleation of all phases in this precipitation sequence is heterogeneous. The M23C6 carbides precipitate on the boundaries of martensitic structure during tempering at temperature of 770 degrees C. The M6C (Fe3W3C) carbides precipitate during both tempering at 770 degrees C and creep at 650 degrees C on the M23C6/ferrite surfaces and on the martensitic lath boundaries. The M6C carbides have the mutual relationship of crystal orientations with both the M23C6 carbides and ferrite. The chemical composition of the M6C carbides depends on their nucleation sites. After 83 h of creep, the M6C carbides starts to dissolve, whereas the Laves phase particles are nucleated on the M6C/ferrite surface and separately on the martensitic lath boundaries. The Laves phase particles nucleate on the M6C/ferrite surface are smaller than those nucleated on the martensitic lath boundaries. The Laves phase particles exhibit the unique orientation relationships with a high misfit. The transformation of M23C6 -> M6C -> Laves phase is an in-situ transformation accompanied by chemical compositional changes.