Stabilization of martensitic microstructure in advanced 9Cr steel during creep at high temperature

In order to improve the long-term creep strength of 9%Cr steel, the stabilization of martensitic microstructure in the vicinity of prior austenite grain boundaries during creep has been investigated by the addition of boron and by a dispersion of nano-size MX nitrides. Creep tests were carried out at 923 K for up to about 3×104 h. Boron is enriched in the M23C6 carbides during aging and creep, especially in the vicinity of prior austenite grain boundaries. This reduces the coarsening rate of M23C6 carbides, which effectively stabilizes the martensitic microstructure in the vicinity of prior austenite grain boundaries. A dispersion of nano-sized MX nitrides but no M23C6 along boundaries also gives rise to excellent pinning force for migrating boundaries during creep, as shown by approximately two orders of magnitude longer time to rupture than ASME-P92. The stabilization of martensitic microstructure retards the onset of tertiary or acceleration creep, which results in lower minimum creep rate and longer time to rupture.