To distinguish different types of stainless steel, we measure the metal’s microstructure at room temperature to learn more about the overall composition. In total, there are five primary types of stainless steel:
With a similar structure to low alloy steels, ferritic stainless steel boasts strong resistance to corrosion cracking. Using little to no nickel — one of the priciest alloying elements — it’s less expensive than austenitic steel, and typically has a significant percentage of chromium (11.2% – 19%). However, it is also less formable than austenitic steel. Finally, ferritic steel is magnetic.
This is the most common variety of stainless steel, making up more than 70% of overall production. It’s a well-rounded structure, with adequate weldability, formability, and creep resistance. Unlike ferritic steel, austenitic varieties are for all intents and purposes non-magnetic. Austenitic steel gets its name from the crystalline microstructure, which is made of isometric crystals. When you add a large proportion of molybdenum (over 6%), the steel becomes superaustenitic, which gives it better protection against crevice corrosion and cracking.
This stainless steel type contains chromium, nickel, molybdenum, and carbon, which makes for a more brittle microstructure. However, it is generally tougher than the austenitic and ferritic varieties. Though they have relatively low formability, these alloys can be tempered as you would with carbon steels. Adding a small amount of nickel improves the martensitic steel’s lack of weldability.
These hybrid alloys are called duplex because the composition is about 50% austenitic and 50% ferritic. By combining the two microstructures, you end up with a new form that has more strength than both. The strong composition also leads to improved corrosion resistance and stress cracking resistance. Since duplex steel has such a large proportion of ferritic alloy, this type is also magnetic.
Finally, this martensitic type has been made even stronger by adding aluminum, copper, and niobium, in conjunction with a precipitation hardening process. It involves heat-treating the metal to create particles in the crystal lattice, which help to stop irregularities in the microstructure and boost the alloy’s overall strength.