There are different types of Stainless Steel and it depends on their microstructure at room temperature, which gives a rough indication of their composition and properties.
Stainless steels can be divided into four main types: ferritic, martensitic and precipitation hardening, duplex, and austenitic – which steel to select depends on the application.
Ferritic Stainless Steel
- Low carbon and nickel content
- Good corrosion resistance
- Good weldability
The standard ferritic grades are alloyed with chromium (11.2–19%), but with no, or a very small, addition of nickel. As nickel is one of the most expensive alloying elements, and experiences high price volatility, the low nickel content of the ferritic grades makes them more price stable compared to grades with high nickel content.
Molybdenum is added to some grades to improve corrosion resistance, while alloying with niobium and/or titanium improves weldability. The ferritic grades are magnetic due to their ferritic microstructure.
There are also ferritic grades with increased resistance to high temperatures (800–1,150 °C). These grades are typically alloyed with more carbon than standard ferritic grades to increase creep strength, and with silicon and aluminum to improve resistance to oxidation.
Martensitic and precipitation hardening stainless steel
- High strength
- High wear resistance
- Limited corrosion resistance
The martensitic grades are the smallest group of stainless steels. For improved strength and hardenability they have a higher carbon content compared to other grades, and nitrogen is sometimes added to further improve strength.
These grades contain no, or small, amounts of nickel, and molybdenum is seldom added. Adding some nickel and reducing the carbon content improves the poor weldability of martensitic grades. Sometimes sulfur is added to improve the machinability.
The precipitation hardening grades are hardened by a special mechanism involving the formation of precipitates within the microstructure. Both martensitic and precipitation hardening stainless steels are magnetic.
Duplex stainless steel
- High strength and toughness
- Very good corrosion resistance
- Good weldability
Duplex grades have a ferritic-austenitic microstructure that combines many of the beneficial properties of ferritic and austenitic stainless steels. The duplex microstructure also contributes to high strength and high resistance to stress corrosion cracking.
Duplex stainless steels are characterized by high chromium content (20.1–25.4%) and low nickel content (1.4–7%) compared to austenitic grades. The low nickel content makes duplex grades more price stable.
Molybdenum (0.3–4%) and nitrogen are added to improve corrosion resistance, while nitrogen also increases strength. The duplex grades LDX 2101 and 2304 are sometimes referred to as lean duplex grades, while the duplex grades 2507 and 4501 are also called 25Cr Superduplex grades. Due to their ferrite content the duplex grades are magnetic.
Austenitic stainless steel
- Good to excellent corrosion resistance
- Good weldability and formability
- Good creep resistance
The austenitic grades are the largest type of stainless steels, and can be divided into five sub-groups:
- Cr-Mn grades – also referred to as 200-series grades have some of the nickel replaced with manganese and nitrogen
- Cr-Ni grades – general-purpose grades mainly alloyed with chromium and nickel, but with no molybdenum
- Cr-Ni-Mo grades – also general-purpose grades, but with increased corrosion resistance due to alloying with molybdenum
- High-performance grades – high alloying content for use in very demanding environments
- High-temperature grades – high chromium and nickel content, but no molybdenum, for use at temperatures exceeding 550 °C
The austenitic grades have good to excellent corrosion resistance, as well as good formability and weldability. Their high impact strength at low temperatures is often exploited in cryogenic applications. The austenitic grades are non-magnetic in the solution- annealed condition. Cold working increases their strength and certain grades are therefore supplied in the temper rolled condition and may then be magnetic due to the presence of some martensite.