It get worse, martinsitic stainless converts to austenic with work hardening. This takes a tough hard alloy and makes it very hard. This is common with 301 types which have less nickel than 304 types but there are many types after that i.e.318L,200 and muffler stock type 400. We used 301 to cut costs and to take advantage of the hardening effect so we could use a smaller blank. As the stuff stretched it hardened as it thinned and continue to pull metal from areas that had not stretched. The product was thin but real tough because of the hardening effect. I strongly recommend sticking with brass-bronze brushes. By the way if it is magnetic it is either 400 series or work hardened 300 series. That's way more than anyone wants to know about SS.
bill
