Stainless Steel Properties, Grades and Applications
In this blog we look at stainless steel in detail – its composition, benefits and applications, types and grades.
What is stainless steel?
Stainless steel is a corrosion and heat resistant iron-based alloy. Stainless steel properties include durability, a high level of hygiene, easy maintenance and aesthetic qualities. Consequently, it’s an important metal for many industries, particularly construction, medical, food and manufacturing.
In comparison with aluminium, stainless steel is approximately three times heavier. It’s completely (and infinitely) recyclable making it the green material of choice, particularly in meeting the sustainability requirements of the construction industry.
Composition of stainless steel
Stainless steel composition varies according to its intended use. Like steel itself, it’s an alloy which always consists of different materials. For example, a minimum of 11% chromium is what makes steel corrosion-resistant and, therefore, ‘stainless’. There are hundreds of different grades of stainless steel, each covered by national and international standards. These grades refer to the various mechanical and physical properties of stainless steel which are dependent upon the presence of elements such as chromium, nickel, molybdenum, titanium and niobium. On contact with oxygen, a chromium oxide layer is formed on the surface of the steel. This passive layer protects the steel, giving it the unique ability to repair itself.
Benefits of stainless steel
There are six main benefits of stainless steel. Each one is important but together, they really do make the case for using stainless steel wherever possible.
Corrosion resistance: one of the most significant characteristics of stainless steel is that it is corrosion resistant. So stainless steel is ideal in environments where industrial acids, alkaline solutions and high quantities of saline (eg salt water) are present.
Fire and heat resistance: stainless steel is resistant to oxidation even at high temperatures. This means it retains its strength under harsh and extreme temperature conditions, both high and low.
Hygienic: stainless steel is easy to sanitise due to its smooth, non-porous surface which means dirt and bacteria can be wiped away easily.
Impact resistance and strength: stainless steel is an extremely tough and highly durable material with resistance to high impact. This is in part due to stainless steel resisting brittleness at both high and low temperatures (such as cryogenic applications). The metal can be welded, cut, and fabricated while retaining is shape.
Aesthetic appearance: right from the word go, stainless steel has been regarded as an elegant, attractive and modern material. It’s always in fashion for both functional and ornamental purposes in domestic and commercial properties. It also complements other materials such as wood and glass, and creates the perfect backdrop for colour.
Sustainability: stainless steel is 100% recyclable in its original form. It’s typically created from about 70% scrap metal and won’t leach toxic chemicals during the recycling process, unlike some other materials.
Applications and uses of stainless steel
There are so many industries that use stainless steel so we’ve concentrated on five key ones: construction, aerospace & MoD, medical technology, food & catering and architectural design.
Construction
Strength, resistance and flexibility, together with ‘green credentials’ make stainless steel a vital material in the building trade and wider construction industry. It features in both residential and commercial construction due to its weldability, easy maintenance and attractive finish. Stainless steel is used internally on items as diverse as sinks and handrails. Externally, it’s very popular from a structural point of view, including as cladding on high impact buildings.
Aerospace & MoD
The aerospace and defence industries value stainless steel for its strength, durability and ability to withstand extreme temperatures. It’s the ideal material for diverse applications, from parts to actual structure and frame-work.
Medical technology
Stainless steel is the preferred material in sterile environments because it is easy to clean and resistant to corrosion. It’s used in the production of a wide range of medical equipment, including surgical and dental instruments. It’s also a key component in theatre furniture, MRI scanners, cannulas, and steam sterilisers. Most surgical implants (eg replacement joints and artificial hips) are made from stainless steel, as are pins and plates to repair broken bones.
Food & catering
Because it doesn’t affect the flavour of food, stainless steel is ideal for food production and kitchen accessories, such as cookware and cutlery. Its resistance to corrosion makes it idea for use with acidic food stuffs while its sterile properties keep bacteria at bay.
Architectural design
Stainless steel’s aesthetic qualities combined with its weldability, strength and durability make it a highly valuable material in modern architecture. A variety of finishes can be specified, such as brushed, polished and matt. It can be engraved and tinted, and its easy maintenance means it continues to look good.
Types of stainless steel
There are many different types of stainless steel, each graded according to characteristics and intended use.
Martensitic stainless steel
Martensitic stainless steels are in the 400 grade series. They have a low to high carbon content, contain 12% to 15% chromium and up to 1% molybdenum. Martensitic is used whenever corrosion and / or oxidation resistance are required. It also offers high strength at low temperatures and creep resistance at elevated temperatures. Martensitic steels are also magnetic and possess relatively high ductility and toughness, which makes them easier to form. They’re used for many different parts and components: from compressor blades and turbine parts, nuts and bolts to dental and surgical instruments, kitchen utensils and hand cutting tools.
Ferritic stainless steel
Ferritic stainless steels (400 grade) have a high chromium content ranging from 10.5% to 27%. They too have magnetic properties, offer good ductility and tensile stability. Resistance to corrosion, thermal fatigue and stress-corrosion cracking are also key characteristics. Typical applications for ferritic stainless steels include automotive components, heat exchangers, furnaces, appliances and food equipment.
Austenitic stainless steel
Austenitic steels are divided into the 300 and 200 series subcategories as determined by the alloys involved. Austenitic is the most common category of stainless steel (grade 304 being the most popular stainless steel) – high in chromium, with varying amounts of nickel, manganese, nitrogen, and some carbon. Sometimes referred to as ’18/8′ because of its 18% chromium and 8% nickel content, austenitic stainless steel is commonly used in kitchen equipment, cutlery, food processing equipment, and structural components in the automotive and aerospace industries. Grade 316 is another common stainless steel found in food preparation equipment, laboratory furniture, medical and surgical equipment, boat fittings and processing equipment.
Duplex
Duplex stainless steels are in the 2000 grade series, and share a mixed microstructure of ferrite and austenite. The chromium and molybdenum content is high (22% to 25%, and up to 5%, respectively), with very low nickel content. The duplex structure gives the stainless steel many desirable properties: double the strength of ordinary austenitic or ferritic stainless steels, with excellent corrosion resistance and toughness. Duplex stainless steel is ideal for applications in demanding environments such as found in the chemical, oil, and gas industries. Also marine, high chloride environments and desalination plants.
Grades of stainless steel
Grade compositions, mechanical properties and production specifications are governed by a range of international and national standards for stainless steel. Here’s a quick history lesson, courtesy of Wikipedia:
The SAE steel grades system is a standard alloy numbering system maintained by SAE International. In the 1930s and 1940s, the American Iron and Steel Institute (AISI) and SAE were both involved in efforts to standardise the numbering system for steels. These efforts were similar and overlapped significantly. For several decades the systems were united into a joint system designated the AISI/SAE steel grades. In 1995 the AISI turned over future maintenance of the system to SAE because the AISI never documented any of the specifications. Today steel quotes and certifications commonly make reference to both SAE and AISI, not always with precise differentiation. It’s worth noting that the alloy number is simply a general classifier, whereas the specification narrows down the steel to a very specific standard. (European countries are adopting unified Euronorm standards.)
For more on metal grades and specifications, see our blog ‘What is a sheet metal gauge’.
The value and versatility of stainless steel
Stainless steel is an extremely versatile metal with an impressive list of qualities: strength, durability (at extreme ends of the temperature range), sustainability, resistance to corrosion, hygienic, easy to maintain and aesthetically pleasing. Understandably, these qualities are highly valued across a range of industries, for example construction, aerospace and architecture / design where our own experience and knowledge lies.
We work with our clients to ensure everything is in order to meet their stainless steel project objectives – design, specification and process. Read more about our sheet metal services or, if you’d like to discuss a specific stainless steel project with us, please get in touch.