Effect of thermal ageing on mechanical properties and microstructures of a standard G-X 12 CrMoVWNbN 1011 grade of cast steel for turbine casing

The mechanical properties, creep-rupture strength (CRS) and microstructural changes of G-X 12 CrMoVWNbN 1011 (G-911) in as received (AR) and thermally aged (AG) condition have been evaluated and compared. The ageing has introduced Laves phase in the matrix. This has resulted in 8.3% drop in tensile strength, 9.7% drop in hardness and 11.5% drop in CRS. Creep deformation parameters like stress exponent (n) and activation energy (Qc) have been determined. These are found to be rather high in both AR and AG material. This behavior has been attributed to the combined effect of solid solution strengthening and precipitation hardening. Microstructural examination under transmission electron microscope of AR sample revealed the presence of M23C6 at grain boundaries (GB) and lath boundaries (LB), whereas, M2X and MX type of precipitates was within the intra-lath matrix. In AG, amidst coarsened microstructure, presence of Laves phase was observed. After creep exposure, the coarsening rates of M23C6 and Laves phase in AR and AG were measured and compared. The coarsening rate of M23C6 was found to be more in AR than in AG. Laves phase was found to cause the embrittlement in the matrix.