A: Air quench
D: Die Steel
M: Molybdenum (very long production runs, better machinability than T
series, although not as robust)
O: Oil quench
P: Plastic mold
S: Shock-resistant
H: Hot work
T: Tungsten (very long production runs, too brittle for shock applications)
W: Water quench

HTH

Regards,

Robin

"Karl Townsend" wrote in message
news
I just bought the entire tool steel inventory at a mold making shop
auction. 1000 lbs. of 4140, A2, D2, H13, S7, 420 stainless, O1, W1
and P20. Also about 2000 lbs. of 1018 and Cold Rolled and 1000 lbs.
of Aluminum, several grades.