Stainless steel is a class of iron-based alloys with a minimum chromium content of 10.5 wt.%. It is characterised by its superior resistance to corrosion in comparison to other steels.
The corrosion resistance of stainless steel is mostly a result of the chromium content. A stable layer of chromium oxide is formed on the surface of the steel, which prevents chemical reactions with the bulk of the material. This oxide layer is extremely thin, 2 nm – 3 nm in thickness, is passive (highly resistant to corrosion), tenacious (adheres well to the bulk) and self-healing (reforms when cracked or damaged).
Nevertheless, stainless steel can corrode under certain circumstances. Uniform corrosion can occur when exposed to acidic solutions such as strong sulfuric acid and hydrochloric acid, and to basic solutions such as sodium hydroxide. Localised corrosion can occur in the form of pitting corrosion, crevice corrosion and stress corrosion cracking, for example when exposed to chloride ions.
As the term stainless steel encompasses a wide range of materials, the mechanical properties, of course, are quite diverse. In general, the values tested for include yield strength, tensile strength, ductility, hardness, toughness, creep resistance and fatigue resistance. Specific values can be found on Matmatch for thousands of different stainless steels.
Stainless steel is a relatively poor conductor of electricity. The electrical conductivity of 18 wt.% Cr. 8 wt.% Ni stainless steel at 20 °C is 1.45 ✕ 106 S/m compared to 5.96 ✕ 107 S/m for pure copper.
Ferritic, martensitic and duplex stainless steels are classified as magnetic, whereas austenitic is classified as non-magnetic. An important value often used to quantify this is the relative magnetic permeability 𝜇r, which is related to the magnetic susceptibility 𝜒m via the equation m=r-1. Magnetic stainless steels typically have permeability values of around 14, whereas non-magnetic stainless steels are close to the minimum value of 1.
All stainless steels are completely recyclable. As they contain significant amounts of valuable elements such as chromium and nickel, recycling of scrap stainless steel is in fact extremely cost-effective. Modern stainless steels are usually manufactured with 60% scrap, including reclaimed scrap, such as from consumer products and industrial equipment, and industrial scrap, such as offcuts from the manufacturing process.
Stainless steels are classified into four main categories according to their crystal structure: ferritic, austenitic, martensitic and duplex.
Ferritic stainless steels possess a body-centred cubic crystal structure, similar to that of pure iron. They generally have a high chromium content of between 10.5 wt.% and 18 wt.%. They have a low carbon content, which gives them a relatively low strength and contain little to no nickel. Ferritic stainless steels are not hardenable by heat treatment. After annealing, they have yield strengths in the range of 275 MPa – 350 MPa.
Ferritic stainless steels are known to have low ductility, resulting in reduced formability, are magnetic, have limited toughness and often lower corrosion resistance compared to other stainless steels. Their absence of nickel, however, gives them a lower cost.
One of the main advantages of ferritic stainless steels is their high resistance to stress corrosion cracking.
409, used in the automotive industry for parts such as exhaust systems and tailpipes.
410S, used in the oil and gas industry, ore processing and thermal processing.
430, used in dishwasher linings, refrigerator cabinet panels, element supports and fasteners, stove trim rings and chimney liners and automotive trim and lashing wires.
Austenitic stainless steels are the most common category of stainless steel. They possess a face-centred cubic crystal structure. In addition to the main alloying elements of iron and chromium, these steels also contain nickel, manganese and nitrogen.
They are not hardenable via heat treatment but only by cold working (also known as work hardening). These steels have relatively low strength compared to the other steels and lower resistance to corrosion
Austenitic stainless steels are split into two subcategories: the 300 series and 200 series. For the 300 series, the austenitic structure is achieved via the addition of nickel, whereas for the 200 series, this is achieved primarily via the addition of manganese and nitrogen.
Austenitic stainless steels are non-magnetic, however, the 300 series can become magnetic after cold working. The nickel content in the 300 series makes it suitable for low-temperature cryogenic applications.
304, the most common stainless steel contains 18 wt.% chromium and 8 wt.% nickel and therefore is often referred to as 18/8. It is used for many applications including cutlery, kitchen equipment, food processing equipment, automotive and aerospace structural components and marine fasteners.
316, the second most common stainless steel, is used for example in food preparation equipment, chemical processing equipment, laboratory benches and equipment, boat fittings, heat exchangers and pharmaceutical and textile equipment and surgical equipment.
317, a low-carbon stainless steel with increased chromium, nickel and molybdenum for greater corrosion resistance. This is used for paper handling equipment, chemical and petrochemical processing equipment, condensers in power generation stations, food processing equipment and textile equipment.
Martensitic stainless steels posses the body-centred tetragonal crystal system. They can range from having a low to high carbon content, up to 1.2 wt.%, contain 12 wt.% – 15 wt.% chromium and 0.2 wt.% to 1.0 wt.% molybdenum. They do not contain nickel. Due to the presence of carbon, they are hardenable via heat treatment, similarly to carbon steels.
Martensitic stainless steels are magnetic and possess relatively high ductility and toughness, making them easier to form. They can be moderately hardened via cold working. After annealing, they typically have yield strengths of around 275 MPa.
This is dependant on carbon content: higher carbon content results in increased strength and hardness but decreased ductility and toughness. They exhibit moderate corrosion resistance and poor weldability.
403, used in compressor blades and turbine parts.
410, used for highly stressed parts, for examples in turbine blades, kitchen utensils, bolts, nuts and screws, pump and valve parts, dental and surgical instruments, nozzles and parts for oil well pumps.
416, which has the highest machinability of any stainless steel, is used for electrical motors, nuts and bolts, pumps, valves, automatic screw machine parts and gears.
420, used for cutlery, knife blades, surgical instruments, needle valves, shear blades, scissors and hand tools.
Duplex stainless steels possess a mixed microstructure of ferrite and austenite. They contain a high chromium content of 22 wt.% – 25 wt.%, a molybdenum content of up to 5 wt.% and a low nickel content.
Duplex stainless steels exhibit roughly twice the strength of austenitic stainless steels and possess greater resistance to chloride stress corrosion cracking compared to austenitic stainless steels, albeit less so than ferritic. They exhibit toughness between that of ferritic and austenitic.
Due to these properties, duplex stainless steels are ideal for demanding environments where good mechanical properties are required in addition to good corrosion resistance. Common examples of duplex stainless steels include:
2205, used in chemical processing, oil and gas processing equipment, marine and other high chloride environments, effluent scrubbing systems, pulp and paper industry, cargo tanks for ships and trucks, food processing equipment, biofuels plant.
2304, used in chloride containing environments, welded pipe systems, transportation, heat exchanger tubes, construction, pressure vessels, caustic solutions and the food industry.
2507, used in oil and gas industry equipment, offshore platforms, chemical process industries, desalination plants, mechanical and structural components and power industry FGD systems.
There are many grading systems for stainless steel, originating from different standards organisations in various countries. These group stainless steels according to their composition and physical properties. Equivalent standards can be determined from comparison tables, or through materials databases such as Matmatch. The most common are:
SAE (also AISI, USA)
British Steel (UK)
International Standards Organisation
 American Welding Society, 'Classifications of Stainless Steel'. [Online]. Available: app.aws.org/wj/1998/11/kotecki/. [Accessed: 26 Sept. 2018].
 Aalco Metals Ltd., 'Stainless Steel St St Introduction'. [Online]. Available: www.aalco.co.uk/datasheets/Aalco-Metals-Ltd_Stainless-Steel-St-St-Introduction_61.pdf.ashx. [Accessed: 26 Sept. 2018].
 D. J. Schaeffler, 'An overview of austenitic and ferritic stainless steels', thefabricator.com, 24 Jul. 2017. [Online]. Available: www.thefabricator.com/article/metalsmaterials/an-overview-of-austenitic-and-ferritic-stainless-steels. [Accessed: 26 Sept. 2018].
 British Stainless Steel Association, 'Category: Classification of Stainless Steels'. [Online]. Available: www.bssa.org.uk/topics.php?sub_category=26. [Accessed: 26 Sept. 2018].
 Penn Stainless Products Inc., 'Stainless Grades'. [Online]. Available: www.pennstainless.com/stainless-grades/. [Accessed: 26 Sept. 2018].
Composition: iron-based, minimum 10.5 wt.% chromium, other alloying elements include nickel, molybdenum, manganese, nitrogen and carbon.
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