Concurrent Precipitation of Nb(C,N) and Metastable M23C6 in Alloy 347H at 700°C and 750°C: Computer Simulations and Comparison to Experiment.

We present our results for the concurrent precipitation of metastable M₂₃C₆, Nb(C,N) secondary precipitates, and the Nb(C,N) primary crystals in 347H austenitic stainless steel. For precipitation modeling, we have accounted for the elastic contribution to interfacial energy, and for the Fe-spin-polarization for NbC/Fe and M₂₃C₆/Fe interfacial energy values: for NbC/Fe ~ 0.63 J/m². For M₂₃C₆ precipitates, an error function was used to describe the interfacial energy growth with particle size. In precipitation simulations, the average size of the primary Nb(C,N) particles remained ~ 1 μm at 700°C and ~ 0.3 μm at 750°C. The M₂₃C₆ precipitates at 750°C dissolved after 120 h (our simulations) compared to 300 h (experiments). The Nb(C,N)/Fe interfacial energy was not affected by the nitrogen additions. With these modifications, reasonable agreement with the available experimental data was obtained, which allows using them in the development of the 2nd-phase particle-informed creep theory. [ABSTRACT FROM AUTHOR]