From brittle to robust
The addition of carbon (C content > 0.1 %) gives ferritic steels a martensitic structure (hardening) during rapid cooling. As martensite is brittle and prone to stress cracking, these materials must be tempered after hardening to make the steel storable and machinable. Carbon martensites have high hardness and are corrosion-resistant. However, they only have low toughness at low temperatures.
