Impeller internal thermal cooling holes

An impeller may include a hub section, a plurality of blades, and a shroud. The hub section may be mounted on a rotatable shaft. The hub section may define a central opening for the rotatable shaft to extend therethrough and may define a plurality of holes disposed in a circular manner about the central opening. The plurality of blades may be connected to or integral with the hub section. The shroud may be connected to or integral with the hub section and the plurality of blades. The plurality of holes may be either through holes or partially drilled holes. A bottom of some or all of the partially drilled holes may be flat, conical, or rounded. Some or all of the partially drilled holes may have one or more bleed holes that may permit quenching material to flow therethrough and prevent the quenching material from stagnating therein.