Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of tool steel letter grades (a, d, h, m, o, s, w). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
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).
Common misuse
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.