One of the key advantages of stainless steel lies in the materials sheer versatility. The fact that the precise composition of stainless steel can be altered to create a metal which is perfectly suited to a particular task means that it’s basic properties – strength, malleability and resistance to staining and corrosion – can be brought to bear in a wide range of situations.
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.
Yaang's stainless steels can be divided into four main types: ferritic stainless steel, martensitic stainless steel and precipitation hardening stainless steel, duplex stainless steel, and austenitic stainless steel – the best steel for you depends on the application.
The different stainless steel types are created by adding different levels of various alloys such as chromium or nickel during the manufacturing process. Put simply, there are five basic stainless steel types, each with its’ own individual composition and therefore with particular properties which are called for in specific working environments. The types of stainless steel in common use, their properties and composition, are as follows:
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 stainless steel grades have a ferritic-austenitic microstructure that combines many of the beneficial properties of ferritic and austenitic stainless steels. The duplex stainless steel 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 stainless steel grades more price stable.
Molybdenum (0.3–4%) and nitrogen are added to improve corrosion resistance, while nitrogen also increases strength. The duplexstainless steel grades LDX 2101 and 2304 are sometimes referred to as lean duplex stainless steel grades, while the duplexstainless steel grades 2507 and 4501 are also called 25Cr super duplex stainless steel grades. Due to their ferrite content the duplex stainless steel grades are magnetic.
The austenitic stainless steel grades are the largest stainless steel types, and can be divided into five sub-groups:
The austenitic stainless steel 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.
Hardening by a special mechanism involving the formation of precipitates within the microstructure.
The ability to absorb energy in the plastic range.
The time-dependent slow plastic deformation of metals under a constant stress.
(1) Loss of electrons in a chemical reaction.
(2) Corrosion of a metal that is exposed to an oxidizing gas at elevated temperatures. The stainless steel reacts with O2, H2O, CO2 and forms an oxide on the stainless steel surface.
Lean duplex stainless steel
The duplex stainless steel grades LDX 2101 and 2304 are sometimes referred to as lean duplex grades due to their “lean” chemical composition.
25Cr super duplex stainless steel
The duplex stainless steel grades 2507 and 4501 are sometimes referred to as super duplex stainless steel grades as the chromium content is close to 25% in these grades.
Heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility.
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