Key Properties of Stainless Steel
The demand for stainless steel has been increasing at an incredible 5% each year. In 2019, its global production reached over 52 million tonnes. China produced the most stainless steel in 2019, over 29 million metric tonnes, over half of the stainless steel produced world wide.
The versatility of stainless steel sees it being used for anything from precise and delicate components in medical equipment, to heavy structural elements in massive marine structures and everything in between.
Stainless steel costs more than mild steel initially, but its properties lead to an increased service life, making the larger upfront cost pay off in the long run.
What is Stainless Steel?
Steel is an alloy of iron and carbon with a maximum carbon content of 2.1%. Stainless steels differ in that they contain a minimum of 10.5% chromium, improving corrosion resistance and strength.
The term stainless steel is used to describe a family of about 200 alloys of steel boasting excellent heat and corrosion resistance. The corrosion resistance is provided by the chromium content of stainless steel which oxidises in air or water to create a passive layer. This layer repairs light scratching and surface damage, acting as a shield against further corrosion, making stainless steel highly resistant to corrosion under normal working conditions.
The Invention of Stainless Steel
Several scientists in the early 19th century had noted that iron-chromium alloys were more resistant to attack by certain acids, but they had not realised the key importance of carbon content.
There are several claims to being the first to discover stainless steel, but the man usually given the credit is Harry Brearley. Harry Brearley was a lead researcher at Brown Firth Laboratories in Sheffield, UK in 1912. A small arms manufacturer was seeking a way to prolong the life of their gun barrels which were eroding away too quickly, so Brearley’s aim was to create an erosion resistant steel, not a corrosion resistant one. During his experiments Brearley made several alloy variations, ranging from 6% to 15% chromium, combined with differing measures of carbon. On the 13th August 1913, Brearley created a steel with 12.8% chromium and 0.24% carbon, argued to be the first known stainless steel. One of the most significant properties of stainless steel turned out to be its resistance to nitric acid, lemon juice and vinegar. Brearley’s employees were not interested in this discovery, but the potential for this new material was immediately significant to the cutlery industry.
One of the key properties of stainless steel is resistance to chemicals and acids, meaning it is widely used in food and drink production
Categories of Stainless Steel
Stainless steels may be divided into into four main types defined by their crystalline structure: austenitic, ferritic, martensitic and duplex. 70% of stainless steel in use in the world is austenitic.
| Austenitic | Ferritic | Martensitic | Duplex |
|---|---|---|---|
| enhanced surface quality, formability, increased corrosion and wear resistance | excellent corrosion resistance and elevated temperature oxidation resistance | high strength and hardness in the heat treated condition | high strength and high resistance to stress corrosion cracking, improved weldability |
| 200 and 300 series | 400 series | ||
| 16% to 30% chromium 2% to 20% nickel | usually only has chromium as an alloying element. Chromium content ranges from 10% to 30% | 11 – 17% chromium with 0.15 – 0.63% carbon | a mixture of austenite and ferrite in structure, providing higher strength and ductility |
| non-hardenable by heat treating | non-hardenable by heat treating | ||
| non-magnetic in the annealed condition (some austenitics do become slightly magnetic when cold worked) | Always non-magnetic | magnetic in both annealed and hardened (heat treated) condition | |
| We supply and laser cut 304 and 316 | We supply and laser cut 430 | – | – |
Key properties of Stainless Steel
- Corrosion resistant
- Easy formability and fabrication with high tensile strength
- Very durable and low maintenance
- Hygienic
- Temperature resistant
- Attractive appearance
- 100% recyclable. In fact, half of all stainless steel in production is recycled from scrap metal.
The key property of stainless steel, setting it apart from mild steel, is its corrosion resistance, because this gives it several other related properties. The stability of the surface means that an attractive, durable, easy to clean and consequently hygienic surface can be maintained.
Find out more about the hygienic properties of stainless steel.
TENSILE STRENGTH
Tensile strength is a measure of the stress needed to break a material by stretching. This is useful to many products in their useful life, but is particularly important to consider for several fabrication processes such as bending and rolling. The tensile strength of stainless steel is superior to aluminium, brass, and mild steel.
DUCTILITY
The ductility of stainless steel grades vary significantly. Ductility is a measure of a material’s ability to undergo significant plastic deformation. A material with high ductility can be hammered out thin or drawn out into wire or threads, undergoing change of form without breaking.
STRENGTH AT HIGH TEMPERATURES
Stainless steel shows better fire resistance than carbon steel due to its high strength retention factor at elevated temperatures (above 500°C) alongside better stiffness retention factor than carbon steel above 300°C.
HIGHER WORK HARDENING RATE
Another useful property of stainless steel, is that it can be annealed (heated and cooled slowly) and cold worked to manipulate its strength to the desired level. This means that the same grade can be varied in strength for use in different applications.
CRYOGENIC RESISTANCE
Some stainless steel grades remain stable at a wide range of temperatures. Austenitic steels display exceptional toughness and increased tensile strength at sub-zero temperatures. Ferrite and martensitic grades, on the other hand, are not as good at cryogenic temperatures, their toughness plunges with falling temperatures.
These properties of stainless steel make it particularly useful to these industries:
- Food production and catering
- Home appliances
- Medical equipment
- Water and waste water processing
- Pharmaceuticals and chemicals
- Architecture and construction
- Offshore and shipbuilding
- Automotive manufacturing
CHEMICAL PROPERTIES OF STAINLESS STEEL
The chemical properties of stainless steel are what sets it apart from other metals.
High oxidisation resistance
Corrosion resistance is a result of the chromium content of stainless steel, this goes up to 26% in some grades. The chromium forms a thin, transparent layer on the stainless steel surface. When activated by oxygen in either air or water, this layer ‘heals over’ light surface damage.
Biologically inert
The other main important property of stainless steel is that it is biologically inert. This is why one of its first uses was for cutlery, expanding into food preparation and manufacture and medical equipment from hospital furniture to surgical pins and plates.
Resistance to acids, bases and organic materials
Stainless steel is resistant to a wide range of compounds including acids, organic compounds, salt, sulphur and chloride compounds making it useful in chemical industries for storage, handling and processing.
The biologically inert properties of stainless steel make it useful for medical applications.
ALLOYING ELEMENTS
Different proportions of alloying elements give different grades of stainless steel their unique properties.
Carbon
Carbon is present in both mild and stainless steels. The carbon percentage of stainless steel can range from 0.03% to 1.2%. A steel is considered to be a high carbon steel if it contains between 0.60 to 1.00% carbon, with manganese content ranging from 0.30 to 0.90%. This makes the steel very hard, but also quite brittle and much less ductile than mild steel.
Chromium
Chromium is the key alloying element in stainless steel. A minimum of 10.5% is necessary for any stainless steel, but more chromium is common for an increased corrosion resistance. The passive surface layer of chromium oxide prevents oxygen diffusing into the metal, protecting the internal structure from corrosion.
Nickel
Nickel improves corrosion resistance, 75% of stainless steel has some nickel content. 8-9% nickel content obtains a fully austenitic structure providing excellent welding properties. Further increasing the nickel percentage improves workability.
Copper
Copper is normally present in stainless steel as a residual element. However, it is added to some alloys to enhance heat hardening properties and/or corrosion resistance particularly for use in sea water environments.
Silicon
Silicon is used as a deoxidising agent in the melting of steel and consequently most steels contain a small amounts of silicon. Silicon improves resistance to highly concentrated nitric and sulphuric acids. It also prompts the formation of ferrite.
Nitrogen
Nitrogen is a strong austenite stabilizer, improves strength and pitting corrosion resistance.
Three other additions to stainless in lesser amounts are Molybdenum, Tungsten and Manganese.
OTHER PROPERTIES OF STAINLESS STEEL
The useful properties of stainless steel are not limited to chemical and physical.
RECYCLABILITY
Stainless is 100% recyclable. On average, new steel products contain around 30% recycled steel, any stainless object contains approximately 60% recycled steel. This has a two-fold environmental advantage; a reduction in need for raw materials and a reduction in waste creation.
HYGIENE
Stainless steel surfaces are easy to clean with non-toxic products. Stainless contains no pores, cracks or open spaces for bacteria to embed itself in, plus it is highly resistant to odour and discolouration.
APPEARANCE
Stainless steel products have an attractive, shiny finish, making it a popular choice for exposed surfaces. It comes in a range of finishes, we supply and laser cut various thicknesses of mirror polished, dull polished, brushed and checker plate.
CONCLUSION
The properties of stainless steel provide all the advantages of mild steel along with a few of its own. Primarily corrosion resistance, enduring tough environments and offering a longer service life than many comparable metals. Due to the choice of grades of stainless with varying properties, there will always be options suited for any application.
We laser cut stainless steel from as thin as 0.5mm up to as thick as 20mm, find out more on our stainless steel page. If you would like to place an order, or have any queries, please contact our sales team at sales@lasered.co.uk.
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