Surface treatement is usually done for a specific depth. You can model the surface treatement depth as a seperate body and apply the properties for that body. give bonded contact for the surface treatement body and the main body.
Usually if you want to take specific surface treatment into account, you will simply apply a coefficient that takes this into account.
This coef is called if I remeber well "Fatigue-Strength-Reduction-Factor".
You may look at this web site :