This paper reviews the physical metallurgy of grain refinement produced by standard heat treatment practices. Grain refinement by normalization is shown to be dependent upon the starting microstructure. High alloy steels with as-cast microstructures that are bainitic are shown to be problematic with respect to obtaining grain refinement by standard normalization practices. These as-cast bainitic microstructures contain remnant austenite that is stable during heating, which subsequently grows during austenitization, and upon impingement reproduces the original parent austenite grain. The addition of a tempering heat treatment for these high alloy steels is shown to eliminate the austenite, increase grain multiplication upon heating, and produce grain refinement.