Recent work by the U.S. Bureau of Mines and others has detailed how the microstructure on ni-hard 4 (ASTM A532-87 Class I, Type D--white cast iron) is influenced by solidification and solid-state transformation kinetics. Studies using differential thermal analysis, electron microprobe analysis, x-ray diffraction, and electron and optical microscopies have clarified factors that influence carbide structure and morphology. The effects of silicon and nickel in promoting m7c3 as opposed to m3c carbides in these irons is explained by their effects on hindering the growth of m3c carbide during solidification. Transformations that occur on destabilisation and subsequent air cooling are also described, and it is shown that the hardenability of these irons may be calculated. It is further shown how such air hardening, subzero treatments, or combined or special heat treatments may be used to develop high- hardness Ni-hard 4. Lastly, the improvements in abrasive wear life obtainable from using castings of high hardness are briefly discussed.