The AISI classification of tool steels by hardening method and application.
The letter is the family: W water-hardening, O oil-hardening, A air-hardening, D high-chromium cold work, S shock-resisting, H hot work, M and T molybdenum and tungsten high-speed.
The trade-off runs from W (cheap, tough to control in hardening, poor wear resistance) through A and D (dimensionally stable, good wear) to H (hot hardness) and M/T (red hardness for cutting tools).
Choosing a tool steel by wear resistance alone. The hardening method and dimensional stability in heat treatment matter as much — D2 wears better than A2 but moves more.
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