Chromium (Cr) -nickel (Ni) -cobalt (Co)-based superalloys are used in structural parts of steel annealing furnaces. This application requires the alloy to be modified by silicon (Si) to enhance the service life. This investigation firstly attempts to identify the microstructural changes that occur following silicon modification of the Cr-Ni-Co superalloy in as cast and heat treated conditions and then goes on to study the structure-high temperature properties correlations.