The Relationship Between Reheat-Treatment and Hardness Behaviour of Cast Nickel Superalloy, GTD-111

The microstructure of cast polycrystalline nickel base superalloy, GTD-111, operated for long-term services were investigated by SEM after different re-heat-treatment conditions. It was found that the exposed specimen after long-term service could be recovered in some extent by re-solution treatment followed by two-step aging treatment. The obtained dispersed precipitation of gamma prime particles was more uniform than those of the long-term exposed microstructure. The results showed that the couple of the lower (1125 degrees C) and/or higher (1175 degrees C) solution annealing temperatures resulted in less total volume fraction of coarse gamma prime precipitates and less homogeneous distribution in microstructure than the medium (1150 degrees C) one. Furthermore, the increase of inserted primary aging temperatures (925 degrees C and 1055 degrees C) resulted in an increase of gamma prime particle size, especially of coarse gamma prime particles. The choice of re-heat treatment conditions with solutioning at 1150 degrees C for 2 hours followed by primary aging at 1055 degrees C for 1 hour and secondary aging at 845 degrees C for 24 hours was the most effective when comparing to other reheat treatment conditions in this study to optimize microstructure. Concerning the hardness test results, the hardness properties depended on the total volume fraction of gamma prime precipitated phase in case of specimens after solutioning and/or primary aging. However, in case of specimens after secondary aging, the hardness behavior trends to vary with the volume fraction of very fine gamma prime precipitated phase.