What is Stainless Steel & What is it Used For?
Stainless steel is one of the most popular types of alloy, it’s multi-functional, strong and highly resistant to corrosion, which makes it ideal for commercial use.
In this blog, we’re going to explain what stainless steel is, its properties, how to judge its quality and how it’s made.
What is Stainless Steel?
Stainless steel is a form of steel containing chromium, which is resistant to tarnish and rust.
Who Invented Stainless Steel?
Stainless steel is a new phenomenon in the world of metallurgy, unlike iron which has been around for over a thousand years. Stainless steel was founded in 1913 by Harry Brearley of Sheffield, which is why Sheffield became synonymous with steel production.
There had been many attempts to create ‘rustless steel’ without any luck. Then Brierley discovered how to make stainless steel while trying to solve the problem of erosion in gun barrels in the First World War. After the initial discovery, improvements have happened at a rapid pace and stainless steel is now one of the most commonly used commercial alloys.
Commercial History of Stainless Steel
1919-1923: stainless steel is used to manufacture surgical scalpels, tools, and cutlery in Sheffield.
1925: a stainless steel tank is used to store nitric acid, cementing its position as corrosion-resistant.
1926: the first stainless steel surgical implants are used.
1928: stainless steel shows its hygiene benefits by fermenting beer in a specialised vessel, it’s now widely used by the food and hygiene industry.
1930s: the first stainless steel trains are made.
1931: the first stainless steel aircraft was built.
1935: stainless steel is commonly used in kitchens.
1954: the first stainless steel underwater camera was manufactured.
1966: the first tidal power station with stainless steel turbine blades was completed in France.
1980s: the biggest movable flood barrier in the world is built using stainless steel.
2010: global production of steel reaches 31 million tonnes.
What is the Difference Between Steel and Stainless Steel?
Steel is prone to rust and corrosion, whereas stainless steel doesn’t stain or rust easily. Both types of steel contain their base metal which is iron, plus carbon and some other trace elements. The difference in the makeup of stainless steel is the addition of chromium, nickel, nitrogen and molybdenum, which makes it less prone to corrosion and rust.
Steel is made by removing impurities from iron such as manganese, sulfur and silicon.
Steel is magnetic while most forms of stainless steel aren’t.
Is Stainless Steel an Alloy?
Stainless steel is an alloy that contains around 10-30% chromium, which makes it corrosion-heat resistant. Stainless steel does contain other elements such as nickel, molybdenum, titanium, aluminium, niobium, sulfur, copper, phosphorus, nitrogen and selenium.
What Are Different Kinds of Stainless Steel?
There are four types of stainless steel, austenitic, ferritic, martensitic and duplex. They’re identified by their microstructure which is based on the elements that are added to the steel.
Austenitic Stainless Steel
Austenitic steel is the most weldable of the groups and is divided into three ‘loose’ groups: common chromium-nickel, manganese-chromium-nickel-nitrogen and speciality alloys.
Austenitic steel is the most popular of the stainless steel groups and is used for many industrial and consumer applications, such as chemical and power plants, as well as food processing and dairy equipment.
Chromium content: 16-26%
Nickel content: up to 35%
Here are some of our austenitic steel alloys:
Ferritic Stainless Steel
Ferritic steel is an iron-chromium-based alloy and is the most ductile and formable of the three types of stainless steel, but doesn’t perform well in high-temperature structures.
Typical uses of ferritic steel include car exhausts, kitchen sinks and industrial equipment. Ferritic steel is cheaper than austenitic steel.
Chromium content: 10.5-27%
Nickel content: 0%
Martensitic Stainless Steel
Martensitic steel is a composition of steel that contains chromium, iron and carbon. Tempered martensite is resistant to corrosion and is relatively strong and tough. Untempered martensite lacks toughness and is brittle.
Martensitic steels are used for medical equipment, cutlery, and aerospace applications like driving shafts and landing gear.
Chromium content: 11.5%-18%
Carbon content: 1.2% (nickel sometimes added)
Here are some of our martensitic steel alloys:
Duplex Stainless Steel
Duplex stainless steels are usually a 50/50 mix ferritic and austenitic steel. They are used to provide higher corrosion resistance and are stronger than standard austenitic steel.
They are used in the petrochemical and oil and gas industries in the form of pipework, manifolds, pipelines and pressure vessels.
Chromium content: 21-27%
Nickel content: 1.35-8%
Other: 0.005-3% copper 0.005 5% molybdenum
Here are some of our duplex steel alloys:
Stainless Steel Advantages and Disadvantages
Steel is easy to clean, which is why it’s popular in the medical and food industries who adhere to strict food hygiene standards. It’s impact resistance means little crevices or dents can’t appear in the steel, meaning dirt and germs have nowhere to hide.
Stainless steel is a durable alloy; its strength and corrosion-resistant characteristics make it a popular choice for businesses from various industries. It also has high-temperature resistance and takes impact to its structure well. If you maintain stainless steel (which isn’t too difficult), you can count on it lasting a long time.
The main reason stainless steel was developed was to combat corrosion in regular steel, be it rust-based or other types of corrosion. However, certain environments can be damaging, so it’s important to consult with a specialist beforehand.
Because it doesn’t damage or corrode quickly, stainless steel can hold its value over a long period. If you’re using steel as an interim, you can resell it on and recoup some of the money you paid for it.
The initial cost of stainless steel can be higher than other metals like aluminium. However, if the resale value stays high, you can recoup some of the cost if you do sell.
While steel is easy to clean, it is a magnet for dirt and dust, so it’ll need to be cleaned regularly.
Stainless Steel Grades
There are numerous ways to grade steel which vary from country to country; we’re going to use the SAE steel grading series as it’s what we use for our steel and is recognised worldwide.
- Series 100 Steel
- Series 200 Steel – austenitic chromium-nickel-manganese alloys.
- Series 300 Steel – austenitic chromium-nickel alloys.
- Series 400 Steel – ferritic and martensitic chromium alloys.
- Series 500 Steel – heat-resisting chromium alloys.
- Series 600 Steel – created for proprietary alloys (which are no longer graded by the SAE).
- Series 900 Steel – austenitic chromium-molybdenum alloys.
What Grade of Stainless Steel Doesn’t Rust?
Ferritic and martensitic steel is more likely to rust because they contain less chromium than austenitic steel. Austenitic grades such as 304 or 316 have high amounts of chromium, so they’re less susceptible to rust.
What is Stainless Steel Used For?
Stainless steel is part of everyday life. Whether it’s the knife and fork you use or the oil you put in your car, steel has probably had a direct or indirect impact on what you do.
Most industries use steel in some form, but some industries have a more active interest in the product, such as manufacturers that need parts for machinery or production.
- Automotive industry – catalytic converters, exhaust systems, and structural purposes. Steel helps to reduce long-term maintenance costs, lifetime value and environmental impact.
- Construction industry – facades, cladding and roofing. Stainless steel is known for its anti-vandalism properties and is popular for public equipment like ticket machines.
- Mining & materials – stainless steel has played a part in many large-scale projects, with improved tools and better facilities for workers to live in.
- Railway industry – stainless steel is corrosion-resistant and offers excellent mechanical properties, which is ideal for the railway industry. Austenitic stainless steel has been used for carriages, and ferritic stainless steel is used for freight wagons.
- Oil and gas industries – stainless tanks, pipes, pumps and valves. The oil and gas industry relies on steel due to its corrosion-resistant properties.
Stainless steel is used in various industries, such as medical equipment, food processing, pulp and paper, power plants, civil engineering, bridge building and shipbuilding.
Stainless Steel Properties
Stainless Steel Corrosion
As we’ve already mentioned, stainless steel is known for its corrosion-resistant properties; it’s what separates it from regular steel. The chromium in the alloy gives the steel an oxide layer, which is what gives it its corrosion-resistant property. The oxide layer is self-healing, and even if the surface is damaged, it will still be resistant to corrosion.
In contrast, carbon steels are protected via coatings like galvanising. Unfortunately, any damage to the surface will expose the steel and leave it open to corrosion, so it can cause issues with its full lifetime value.
Different grades of steel have varying resistance, and chloride accelerates corrosion in some grades. Grades with nickel, chromium and molybdenum are the most resistant to corrosion.
Stainless Steel Magnetism
Magnetism is the attraction of steel to a magnet. Austenitic grades are not usually magnetic, although magnetism can be achieved with cold working. Grades with high nickel content will also be non-magnetic, and ferritic steels are magnetic.
Stainless Steel Tensile Strength
When compared with mild steel, stainless steel has more tensile strength, and duplex stainless steel has better tensile strength than austenitic stainless steel.
However, the highest tensile steel is seen in martensitic steel and can have strengths higher than that of the steel above.
Stainless Steel Cryogenic Resistance
At sub-zero temperatures, the toughness and ductility of austenitic stainless steel are increased, whereas ferritic and martensitic stainless steel should not be used at these temperatures.
How is Stainless Steel Made?
1. Melting and Casting
The raw materials are melted together in a furnace, which usually requires about 8-10 hours of intense heat. When the steel is liquidised, it’s cast into semi-finished forms such as blooms, billets and slabs, rods and tube rounds.
The steel is heated and passed through huge rolls. Blooms and billets are formed into bar and wire ,and slabs are formed into plate, strip and sheet which are all available in different sizes.
3. Heat Treatment
After the stainless steel is formed, it needs to be annealed. Annealing is a process where the metal is heated and cooled to relieve internal stressors and soften the metal. The process is delicate as the steel is sensitive to different cooling rates and temperatures.
For example, a rapid cooling process can increase the strength of steel without affecting toughness. Different types of steel receive different treatments.
Annealing causes a scale or build-up on the steel, which can be removed with several processes. Pickling uses a nitric-hydrofluoric acid bath to clean the steel. Electrocleaning applies an electric current to the surface of the steel using a cathode and phosphorous acid. The descaling process will depend on the type of steel.
Cutting is usually required to get the desired shape or size before the steel is finished. Mechanical cutting is done with a variety of methods, such as straight shearing with guillotine knives, circular shearing with circular knives, sawing using high-speed blades, blanking and nibbling. Stainless steel can also be cut using flame cutting and plasma jet cutting.
Surface finish is important in steel manufacturing, especially if the material is going to be customer-facing. In addition, steel is also easier to clean with a surface finish. There are several different types of finish, which include a dull finish, a bright finish, a reflective finish, and a mirror finish.
Once the steel has been through this process, it’s ready to be sold.
NeoNickel Steel Manufacturing
All of our steels go through this manufacturing process, so we can give you the type and grade of steel that suits your project. If you’d like to learn more about our stainless steel, contact us, and we’ll answer any questions you have.