Stainless steel types for every application

Stainless steel types

Stainless steels have traditionally been divided into categories since the microstructure has a decisive effect on the properties. The categories are assigned depending on the chemical composition and the manufacturing steps and parameters that determine the microstructure at room temperature.

What is stainless steel?

Stainless steel is an iron-based alloy known for its resistance to rusting, corrosion, and heat. Stainless steels are typically iron-chromium alloys, with different types of stainless steel containing other elements like nickel, carbon, and molybdenum. The chromium in stainless steel gives it the superior corrosion resistance that sets it apart from other steel types.

Stainless steel is a very versatile material that can be produced in many different forms – including rolls, sheets, bars, and tubes. Stainless steel applications include everything from cooking utensils and surgical instruments to construction materials and industrial equipment.

Fully recyclable, stainless steel is a key building block for the sustainable development of our global infrastructure.

Where and when was stainless steel invented?

Stainless steel was invented as a result of work by scientists and industrial developers from France, Germany, and the United Kingdom. As early as 1821, a French scientist noted that iron-chromium alloys were resistant to certain acids.

British scientists first showed that chromium-iron alloys were resistant to oxidizing agents in the early 1800s, and German scientists Robert Bunsen discovered that chromium was resistant to strong acids.

Steelmakers in the UK and Germany began manufacturing chromium steel in the 1840s.

Stainless steel properties

Stainless steel properties can be divided into three categories:
Mechanical properties, Physical properties, and Corrosion properties.

Mechanical properties of stainless steel

The mechanical properties of different stainless steel types are usually compared using stress strain curves. These are produced from the data gathered during tensile tests. The different stainless steel types – austenitic, ferritic, duplex, and martensitic – have similar mechanical properties at room temperature within each group.

The mechanical properties of stainless steel are strongly affected by the amount of cold work that the material is subject to. Cold work, also known as strain hardening or work hardening, is a process used to strengthen the stainless steel by plastic deformation.

Mechanical properties also vary depending on the form of the product. Typically, stainless steel strength increases as its thickness decreases. For example, thick hot rolled plate has a lower strength than thin cold rolled coil stainless steel.

The toughness of stainless steel describes its ability to absorb energy in the plastic range. Toughness is dependent on temperature, and generally increases as temperature increases. However, austenitic stainless steels also have excellent toughness at low temperatures, making them ideal for very low-temperature (cryogenic) applications.

The mechanical strength data provided in national and international standards and design codes is used when designing load-bearing structures like bridges, pressure vessels, and structural components for vehicles uses.

Learn more about stainless steel mechanical properties

Physical properties of stainless steel

Different types of stainless steel can have very different physical properties.

Density – Stainless steel density usually increases with an increasing level of alloying elements, particularly molybdenum. Austenitic stainless steel grades typically have a higher density than other types of stainless steel.

Elastic modulus – Also known as Young’s modulus, the elastic modulus of stainless steel is a measure of its stiffness, which is particularly important in construction applications. The elastic modulus decreases as temperature increases.

Thermal properties – Thermal expansion and thermal conductivity are the two physical properties that vary the most between types of stainless steel. Austenitic grades have higher thermal expansion than ferritic grades. Thermal conductivity decreases as the alloying level increases.

Electrical resistivity and magnetic properties – The higher the alloying element content of stainless steel grades, the higher their electrical resistivity. All ferritic, martensitic and duplex stainless steel grades are ferromagnetic, while stable austenitic grades are not magnetizable.

Compare physical properties of different stainless steel grades in Stainless Steel Finder

Corrosion and the corrosion resistance properties of stainless steel


What makes stainless steel corrosion resistant?

The corrosion resistance of stainless steel comes from the thin passive film that forms on its surface in oxidizing environments if the chromium content is a minimum of approximately 10.5%. All types of corrosion that can occur on stainless steel relate to permanent damage to this passive film.

Corrosion negatively affects stainless steel properties like mechanical strength, appearance, and impermeability to liquids and gases. Although stainless steel is often chosen because of its excellent corrosion resistance, it is not immune to corrosion. Whether a stainless steel is corrosion resistant in a specific environment depends on the chemical composition of the stainless steel and the aggressiveness of the environment. 

Learn more about corrosion resistance.

There are two main types of corrosion that can occur on stainless steel: wet corrosion and high-temperature corrosion.

Wet corrosion

Wet (or aqueous) corrosion is an electrochemical process involving an anode and a cathode. The metal oxidizes (corrodes) at the anode and forms rust or some other form of corrosion product. There are many different types of wet corrosion, including pitting and crevice corrosion, atmospheric corrosion, and intergranular corrosion.

Watch a webinar about corrosion in water

High-temperature corrosion

High-temperature corrosion and oxidation happens when stainless steel is exposed to a hot atmosphere containing oxygen, sulfur, halogens, or other compounds that react with the material. When stainless steel is exposed to an oxidizing environment at elevated temperatures, an oxide layer forms on the surface.

Watch a webinar about high-temperature corrosion

Corrosion data

Corrosion data for the environment that the stainless steel will be exposed to is used to determine which grade is best suited to the application in question. Important criteria include temperature, concentration of main species and impurities, and pH.

Learn more about corrosion tables and charts

Stainless steel grades

Stainless steels have traditionally been divided into categories depending on their microstructure at room temperature. This is because the microstructure of stainless steel has a decisive effect on its properties.

The stainless steel products that Outokumpu produces can be divided into four main groups:
Ferritic, martensitic and precipitation hardening, duplex, and austenitic.

Austenitic grades

Austenitic grades are the largest group of stainless steels, including Cr-Ni, Cr-Mn, Cr-Ni-Mo, high-performance, and high-temperature grades. They have good to excellent corrosion resistance plus good formability and weldability. The strength of austenitic stainless steel can be increased with cold working.

Learn more about austenitic stainless steel grades

Ferritic grades

Ferritic grades, also known as Cr-steels, are alloyed with chromium and have little or no nickel added. As the price of nickel is highly volatile, ferritic grades are more price stable.

Learn more about ferritic stainless steel grades

Duplex grades

Duplex grades have a ferritic-austenitic microstructure, with a phase balance of approximately 50% ferrite and 50% austenite. They combine many of the beneficial properties of ferritic and austenitic stainless steels, and their duplex microstructure also contributes to their high strength and high resistance to stress corrosion cracking.

Learn more about duplex stainless steel grades

Martensitic grades

Martensitic grades are the smallest group of stainless steels. Their higher carbon content improves the strength and hardenability of these grades, and they sometimes contain nitrogen to further improve strength. Precipitation-hardening stainless steel grades are hardened by a special mechanism involving the formation of precipitates within the microstructure.

Learn more about martensitic stainless steel and precipitation hardening (PH) grades

The stainless steel manufacturing process

The main raw material used in the stainless steel manufacturing process is recycled stainless steel scrap. The rest is virgin material in the form of iron and different alloying elements like nickel, carbon, and molybdenum.

Steel scrap is tested and analyzed so that it can be sorted according to its alloying content. This helps to ensure that as little virgin material as possible is used to get the right chemical composition of the stainless steel end product.

 

The stainless steel manufacturing process can be divided into these steps:

The meltshop

The meltshop is the first step in the stainless steel manufacturing process. Raw materials are melted in an electric arc furnace before carbon, sulfur, and any nitrogen are removed in a steel converter. The steel composition is then fine-tuned in ladle treatment before the molten steel is cast into slabs or ingots.

hot rolled coil

Hot rolling

In hot rolling, the microstructure of semi-finished casting products such as slabs, blooms, or billets is broken down and deformed. The deformed grains recrystallize, which maintains an equiaxed microstructure (a structure in which the grains have approximately the same dimensions in all directions) and prevents the steel from work hardening.

Cold roll icon

Cold rolling

Cold rolling involves rolling wide sheets of stainless steel between small-diameter working rolls backed by two or three layers of supporting rolls. The steel strip profile and flatness are carefully controlled during the process.

Annealing and pickling

Annealing and pickling (AP) is a process used to restore the properties of stainless steel after cold rolling by recovering and recrystallizing the deformed microstructure. A typical cold rolled AP line has a pre-treatment phase, which is often an electrolytic pickling step in a neutral solution based on sodium sulfate or an acidic step based on nitric acid. A bright annealing (BA) line can also be used to restore the material properties, where the steel strip is treated in a protective atmosphere of hydrogen, or a mixture of hydrogen and nitrogen.

Final processing

Final processing, also called finishing, is where stainless steel coils and plates are tailored according to the requirements of the end customer. This can involve processes like levelling of a plate or coil, slitting or cutting coils into rows or sheets, edge preparation for welding, and surface preparation like grinding, pattern rolling, or brushing.

Stainless steel applications

Stainless steel is one of the most versatile and widely used materials in the world. It can be found in almost every aspect of life – from construction, transportation, and manufacturing to energy, agriculture, and household appliances. Its strength, durability, and malleability make it an essential material in modern society.

In terms of stainless steel applications, usage is dominated by a handful of major areas:

  • consumer products
  • equipment for the oil and gas industry
  • the chemical process industry, and
  • the food and beverage industry.

Stainless steel is widely used because it is a hygienic, aesthetically pleasing, and low-maintenance material. Stainless steel’s strength and corrosion resistance help other materials, such as concrete, to perform better and last longer, and at the end of its life stainless steel can be fully recycled without losing any of quality.

The most commonly used stainless steel type is cold rolled sheet. The most widely used stainless grades are the austenitic Cr-Ni 18-8 type steels (EN 1.4301/1.4307) which account for more than 50% of the global production of stainless steel. The next most widely used grades are the ferritic Cr-steels such as 1.4512 and 1.4016, followed by the molybdenum-alloyed Cr-Ni-Mo austenitic steels 1.4401/1.4404.

Together these grades make up over 80% of the total tonnage of stainless steels. The remainder includes other austenitic grades and duplex and martensitic grades.

Learn more about stainless steel applications

Stainless steel price

Business meetingThe total price of stainless steel is made up of three parts:

  1. The base price – this is negotiated individually with each customer and is separated from volatile alloy price movements so that it does not fluctuate on a daily basis. Base prices are usually negotiated with each customer on a monthly basis.
  2. Extras – these are material-specific charges for non-standardized stainless steel product dimensions. These extras are added or subtracted from the base price, and typically include charges for things like special thicknesses, surface treatments, and packaging.
  3. The alloy surcharge – an instrument used to pass changes in alloy costs along the value chain in the stainless steel industry. Stainless steel alloy surcharges are product-specific as alloy compositions differ between grades and calculation periods may be different. Standard practise in the industry is to use a monthly average calculation – meaning the average alloy costs of a fixed period prior to the delivery of the product are applied to determine the alloy surcharge. Alloy surcharges are usually published monthly.

You can subscribe to the latest stainless steel alloy surcharge reports from Outokumpu for Europe and the Americas on our website.

Stainless steel sustainability

As a material, stainless steel has low carbon footprint is due to the fact that the primary raw material used to produce it is recycled scrap.

Stainless steel is the most recycled material in the world, with 82% of used stainless steel estimated to be recycled into new steel. When stainless steel is recycled it does not negatively impact the properties of the material. In theory, stainless steel can be recycled an infinite number of times.

 

Interchange of roads and green areas in between. 

How stainless steel helps protect the environment

The inert nature of stainless steel products offers many advantages. One of the biggest advantages from an environmental point of view is that the passive layer that prevents the release of metals from the surface of stainless steel. For example, this layer means that stainless steel can be used in applications like roofing and water piping for buildings without the risk of metals leaching into and contaminating groundwater.

Research by Outokumpu and its partner fka, a research partner for the automotive industry, compared stainless steel with standard carbon steel and aluminum for the manufacture of battery casings for electric vehicles. The results showed that using stainless steel can reduce the carbon footprint of this component by 112.8% compared to aluminum and by 29.9% compared to carbon steel.

Circle Green

The most sustainable stainless steel producer in the world Outokumpu_Circle_Green_Emblem_RGB_pos

Steel accounts for 7–9% of global greenhouse gas emissions. Outokumpu’s stainless steel products have a total carbon footprint that is up to 75% lower than the industry average. By 2030, Outokumpu’s aim is to further reduce emissions by 42% per ton of stainless steel with 2016 as the base year.

Outokumpu Circle Green® is the world’s first step toward zero-carbon stainless steel. It has the lowest carbon footprint in the industry – up to 93% lower compared to the industry average.

Outokumpu Circle Green
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Can stainless steel overtake aluminum for constructing low carbon vehicles?

Read our latest study and learn more.

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How to choose the right material for different stainless steel applications

Stainless steel coil from TornioThe right choice of stainless steel grade for your specific application will depend on the service conditions for your application.

When selecting a stainless steel, you should first think about material-related aspects like:

  • Corrosion resistance
  • Mechanical strength
  • Fabricability
  • Physical properties
  • Surface aspects (e.g. appearance and ease of cleaning)

You should also think about:

  • Possibilities for weight reduction
  • Availability of the stainless steel in question
  • Cost, including the full lifecycle cost
  • Recyclability of the material
  • Any legislation, standards, and approvals you need to comply with

In load-bearing structural applications like bridges, pressure vessels, and structural automotive components you can use the mechanical strength data provided in national and international standards and design codes.

If your application involves elevated temperatures (above 550 °C) you will need to examine the creep and oxidation data for the stainless steel.

Useful resources to help you choose the right material for different stainless steel applications

Steel Finder

The Outokumpu Steel Finder tool is an invaluable resource to help you find the right stainless steel for your application. You can search for stainless steel grades by name, standard, product category, product form, thickness, and width.

You can also search corrosion tables by chemical formula or alphabetically.

Steel Finder

Handbook of Stainless Steel

The Outokumpu Handbook of Stainless Steel is a comprehensive introduction to the world of stainless steel. The handbook covers stainless steel grades, the stainless steel manufacturing process, stainless steel properties, and more. There are also sections to help you choose the right stainless steel for your specific application, help you calculate the life cycle cost of the material, and learn more about stainless steel welding and fabrication.

Handbook of Stainless Steel