Microstructure evolution of HP40-Nb alloys during aging under air at 1000 °C

Two as-cast HP 40 alloys provided by different manufacturers were aged at 1000 °C under laboratory air. They had the same as-cast microstructure consisting of austenite dendrites delineated by a network of eutectic Nb-rich MC and Cr-rich M7C3 carbides. After aging for several months, they showed similar microstructures in the bulk materials, though M7C3 carbides have been replaced by M23C6 carbides. As expected, a sub-surface zone depleted in chromium has appeared where a tetragonal CrNbC could be identified in both materials. However, the composition of the transition zones between the surface and the bulk materials differed, mainly because one of the materials underwent significant nitrogen pick-up with associated precipitation of M6(C,N) and M2(C,N) phases. On the contrary, the other alloy did show only one intermediate zone with a mix of CrNbC, M23C6 and MC carbides. A full account of the microstructures observed in the aged materials is given.