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.
Stainless steel is a form of steel containing chromium, which is resistant to tarnish and rust.
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.
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.
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.
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.
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 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 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 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 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:
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.
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.
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.
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.
Stainless steel is used in various industries, such as medical equipment, food processing, pulp and paper, power plants, civil engineering, bridge building and shipbuilding.
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.
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.
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.
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.
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.
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.
The exact process for a grade of stainless steel will differ in the later stages. How a grade of steel is shaped, worked and finished plays a significant role in determining how it looks and performs.
Before you can create a deliverable steel product, you must first create the molten alloy.
Because of this most steel grades share common starting steps.
Manufacturing stainless steel starts with melting scrap metals and additives in an electric arc furnace (EAF). Using high-power electrodes, the EAF heats the metals over the course of many hours to create a molten, fluid mixture.
As stainless steel is 100% recyclable, many stainless orders contain as much as 60% recycled steel. This helps to not only control costs but reduce environmental impact.
Exact temperatures will vary based on the grade of steel created.
Carbon helps to increase the hardness and strength of iron. However, too much carbon can create problems—such as carbide precipitation during welding.
Before casting molten stainless steel, calibration and reduction of carbon content to the proper level is essential.
There are two ways foundries control carbon content.
The first is through Argon Oxygen Decarburization (AOD). Injecting an argon gas mixture into the molten steel reduces carbon content with minimal loss of other essential elements.
The other method used is Vacuum Oxygen Decarburization (VOD). In this method, molten steel is transferred to another chamber where oxygen is injected into the steel while heat is applied. A vacuum then removes vented gases from the chamber, further reducing carbon content.
Both methods offer precise control of carbon content to ensure a proper mixture and exact characteristics in the final stainless steel product.
After reducing carbon, a final balancing and homogenization of temperature and chemistry occurs. This ensures that the metal meets requirements for its intended grade and that the steel’s composition is consistent throughout the batch.
Samples are tested and analyzed. Adjustments are then made until the mixture meets the required standard.
With the molten steel created, the foundry must now create the primitive shape used to cool and work the steel. The exact shape and dimensions will depend on the final product.
Common shapes include:
Forms are then marked with an identifier to track the batch through the various processes to follow.
From here steps will differ depending on the intended grade and final product or function. Slabs become plates, strips and sheets. Blooms and billets become bars and wires.
Depending on the grade or format ordered, a steel might go through some of these steps multiple times to create the desired appearance or characteristics.
The following steps are the most common.
Performed at temperatures higher than the recrystallization temperature of the steel, this step helps to set the rough physical dimensions of the steel. Precise temperature control throughout the process keeps the steel soft enough to work without altering the structure.
The process uses repeated passes to adjust the dimensions of the steel slowly. In most cases, this will involve rolling through multiple mills over time to achieve the desired thickness.
Often used when precision is required, cold rolling occurs below the recrystallization temperature of the steel. Multiple supported rollers are used to shape the steel. This process creates a more attractive, uniform finish.
However, it can also deform the steel’s structure and often requires heat treatment to recrystallize the steel to its original microstructure.
After rolling, most steel undergoes an annealing process. This involves controlled heating and cooling cycles. These cycles help to soften steel and relieve internal stress.
The exact temperatures and times involved will depend on the grade of steel, with both heating and cooling rates impacting the final product.
As steel is worked through the various steps, it often accumulates scale on the surface.
This accumulation isn’t simply unattractive. It can also impact the stain-resistance, durability and weldability of the steel. Removing this scale is essential to creating the oxide barrier that gives stainless it’s characteristic corrosion and stain resistance.
Descaling or pickling removes this scale using either acid baths (known as acid pickling) or through controlled heating and cooling in an oxygen free environment.
Depending on the final product, the metal might return to rolling or extruding for further processing. This is followed by repeated annealing phases until achieving the desired properties.
Once the steel is worked and ready, the batch is cut to fit order requirements.
The most common methods are mechanical methods, such as cutting with guillotine knives, circular knives, high-speed blades or punching with dies.
However, for complex shapes, flame cutting or plasma jet cutting may be used as well.
The best option will depend on both the grade of steel requested and the desired shape of the delivered product.
Stainless steel is available in a variety of finishes from matte to mirror. Finishing is one of the last steps involved in the manufacturing process. Common techniques include acid or sand etching, sand blasting, belt grinding, belt buffing and belt polishing.
At this point, the steel is gathered in its final form and readied for shipping to the customer. Rolls and coils are common ways to both store and ship large quantities of stainless for use in other manufacturing processes. However, the final form will depend on the type of steel required and other factors specific to the order.
Understanding the proper stainless steel grades and types for specific uses and environments is an essential part of ensuring long-lasting results and optimizing costs. Whether you’re looking for something strong and corrosion-resistant for marine environments or something stunning and easy to clean for restaurant use, there’s a stainless steel alloy available to suit your needs.
If you’re curious how stainless steel might work for your next project, consult with Unified Alloys. As a leading source of stainless steel throughout Canada for more than 40 years, we have the knowledge and resources to help you find the ideal product for your requirements.