Stainless steels have traditionally been divided into types depending on their microstructure at room temperature, which gives a rough indication of their composition and properties.
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.
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 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.
The austenitic grades are the largest type of stainless steels, and can be divided into five sub-groups:
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.
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