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.
Stainless steel properties
Stainless steel properties can be divided into three categories:
Mechanical properties
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.
Physical properties
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.
Corrosion and the corrosion resistance properties
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.
There are two main types of corrosion that can occur on stainless steel: wet corrosion and high-temperature corrosion.
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 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.
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 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 (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, 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.
