PMPA Craftsman Cribsheet #135:
ISO Material Groups: Stainless Steel

The letter “M” is for all stainless steels in the ISO material group categories. Most manufacturers break it down into three subgroups. I have made it into five subgroups because of the tendency to put martensitic, ferritic, and precipitation hardening stainless steel into the P ( steel) categories.
Stainless steels require sharp tools and low to no dwell times. Stainless steel especially in the higher nickel alloys work hardens readily. Spending too long in the cut or pushing too much with tools with low to no rake angle can cause work hardening.
The material listings are not exhaustive but examples of materials found in that sub-group to provide context of what materials fit into that group. 

*Machinability is a percentage of 1,212 (1,212 = 100%).

Find a grade-specific reference in a reference manual to get surface feet per minute (SFM) with high-speed steel (HSS). Multiply the HSS number by 2-2.5 and it will provide a reference point for what SFM you should use with the carbide. The SFM guidelines given based on material groups above can vary greatly, which can be seen in the machinability range.