An integral turbine includes a forward hub section and an aft hub section. The forward hub section and the aft hub section are metallurgically coupled to one another along an annular interface that resides within a plane generally orthogonal to a rotational axis of the axially-split turbine. The turbine further includes an airfoil blade ring metallurgically coupled to a radial outer surface of the coupled forward and aft hub sections and an impingement cavity formed within an interior portion of the coupled forward and aft hub sections. The impingement cavity includes an interior surface that is positioned proximate to the radial outer surface of the coupled forward and aft hub sections. Further, an impingement cooling air flow impinges against the interior surface of the impingement cavity to provide convective and conductive cooling to the radial outer surface of the coupled forward and aft hub sections.