Grain refinement strategies in high strength cast steel

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 standard normalization practices. These as-cast bainitic microstructures contain remnant austenite that is stable during heating, which subsequently grows during austenitization and upon grain impingement reproduces the original parent austenite grain.  Subcritical annealing heat treatments for high alloy steels used in ground engaging tools are shown to eliminate the austenite and increase grain multiplication upon heating.  Lower alloy steels such as 8620, 8630 and 4140, can also benefit from subcritical annealing although the effects are not as dramatic. The addition of a subcritical anneal for the lower alloy steels may affect grain size uniformity.