Ferritic stainless steels are essentially iron-chromium alloys. The ferritic alloys are ferromagnetic. They can have good ductility and formability, but high-temperature strengths are relatively poor compared to those of the austenitic grades. Toughness may be somewhat limited at low temperatures and in heavy sections. Unlike the martensitic stainless steels, the ferritic stainless steels cannot be strengthened by heat treatment. Corrosion resistance, that of the ferritic stainless steels can range from moderate for the low-to-medium, chromium-content alloys to outstanding for the superferritics.