It should be noted that nitrocarburising and nitriding are both used in the production of certain cast-iron piston rings as well. Tool steel piston rings are generally used where the durability of other materials is insufficient.As a piston ring, they can be an expensive option, but where ring wear or breakage is the limiting factor for engine durability, they can reduce rebuild frequency.t should be noted that nitrocarburising and nitriding are both used in the production of certain cast-iron piston rings as well. Tool steel piston rings are generally used where the durability of other materials is insufficient.