Depends on what they are made of.
The Union Stongbolt genuine blanks are really hard and a lot of the universal Rim and Safe blanks seem to be really hard again.as well.
I use a lot of SKS copies for most mortice ER-1,4,5G and UN-1G and these are really good quality and as they are brass allow my cutters
to last a lot longer, i also use the coated cutters on my 2 Blade Vectis machines as well as on my Lancer.