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Stainless steel is one of the most important ferrous materials. It is durable, easy to clean, and can withstand significant loads, making it perfect for the production of tools, machine parts or components.
There are several different steel types, each with their pros and cons and own unique compositions. The exact chemical composition determines, for example, whether a product is rustproof, magnetic or food-safe, among other qualities.
In this article, you will find out everything you need to know about the different types and grades of stainless steel and their properties.
What is stainless steel?
Stainless steel is a high-purity metal that consists mainly of iron and carbon. Usually, steel contains other elements such as phosphorus, aluminium, sulphur or silicon. These are extracted by special processes during the production of different stainless steel grades, in order to achieve a particularly high degree of purity.
The term “stainless steel” means that substances that would otherwise impair the purity of the metal are dissolved during the production process. The chemical composition of stainless steel is subject to particularly tight tolerances. For example,the sulphur and phosphorus content may not exceed 0.025%. A higher impurity content would lead to a reduction in stainless steel quality.
Alloy and non-alloy stainless steel are divided into different grades based on their properties and composition. Alloy steels combine with other materials (e.g. chromium) specifically to further improve certain properties and qualities of the metal. Non-alloy stainless steels are typically used where strength and hardness are more important than additional properties such as corrosion or wear resistance.
In general, stainless steel grades are characterized by the following properties:
- Hardness and durability
- High temperature thresholds
- Good welding properties
- Bacteria resistance
- Rust and corrosion resistance
Stainless steel offers many advantages; however, it also comes with a far higher manufacturing cost and overall weight, compared to steel. Different stainless steel types are therefore mainly used to produce professional tools and for the furnishing of spaces where hygiene and cleanliness are critical (e.g. in laboratories or the food industry). Steel, on the other hand, is typically the preferred material for building and construction.und Konstruktion ist.
Stainless steel grades explained
The most important distinguishing property between stainless steel grades is the composition of alloys. Depending on which substances are added and in what quantities, there are more than a hundred different steel types that can be manufactured. The American Iron and Steel Institute (AISI) has developed a classification that assigns an individual material number to each grade.
General classification becomes easier if you first consider the broader categories, as all stainless steel grades can be assigned to one of five groups.
See the table below for a stainless steel grades list, their classifications, and typical properties:
| Stainless steel groups | Alloy | Properties | Stainless steel grades |
|---|---|---|---|
| Austenitic stainless steels | Chromium-Nickel Steel Chromium-Nickel-Molybdenum Steel | • Corrosion resistant (less resistant to pitting corrosion) • Sufficiently acid-resistant • Easy to work with • Sweat-resistant • Cannot be thermally hardened, but work hardening (cold forming) is possible • Very good value for money • Most commonly used stainless steel • Easy to weld • Polishable • Highly acid-resistant • Highly corrosion-resistant • Rust-free even with scratches • Chlorine resistant • Resistant to aggressive cleaning agents | AISI 303 AISI 304 AISI 304L* AISI 305 AISI 316 AISI 316L* |
| Martensitic stainless steels | Chromium (10.5-13% content) and various other substances Carbon content 0.2-1% | • Medium corrosion resistance • Can be thermally cured • Weldable • Not chlorine and chemical-resistant • Highly durable in chemical-free environments | AISI 410 AISI 416 AISI 420 AISI 431 |
| Ferritic stainless steels | Chromium (12-19% content) Nickel content (less than 1%) Carbon content (less than 0.1%) | • Corrosion-resistant • Easily deformable • Cannot be thermally hardened but can be work-hardened • Chlorine resistant | AISI 430 AISI 434 AISI 444 |
| Duplex Stainless Steel | Chromium (19.5-33% content) Nickel (1-9% content) Nitrogen (up to 0.6%) Optional addition of molybdenum, copper, tungsten, and manganese | • Resistant to pitting corrosion, stress cracking, and vibration corrosion cracking • Particularly high weldability • Low thermal expansion | AISI 2205 AISI 2507 AISI 32769 |
| Precipitation Hardening Steel | Chromium (15-17% content) Nickel (3-5% content) Carbon (0.15-0.45%) | • High strength and toughness achieved through specific heat treatment process • High corrosion resistance • Excellent machinability and fabrication | AISI 635 AISI 630 |
Not all types of stainless steel are rustproof
Stainless steel has become synonymous with ‘rustproof’ steel. However, this is not entirely accurate. As you can see in the table, the corrosion resistance of different stainless steel types vary greatly. The decisive factor in determining whether different grades of stainless steel are considered ‘rustproof’ is the chromium content – which should be at least 12% by the standardised definition. This is because chromium reacts with oxygen to form a protective layer on the surface of the metal.
Other alloy components such as nickel and molybdenum further increase corrosion resistance and ensure the metal can continue to perform, even under particularly demanding conditions (salty water, chemical exposure, etc.). The stainless steel grades AISI 316 and AISI 316L, in particular, are characterised by excellent corrosion and chemical resistance. Due to their high tolerance of chlorides, both materials are often used in shipbuilding or the marine sector, in swimming pools and in slightly alkaline environments.
Stainless steel can still rust – especially if it comes into contact with standard steel. If you intend to drill stainless steel, it is important to only use drills and screws from a suitable stainless steel group on other stainless steel components.
Is stainless steel magnetic?
Stainless steel can be magnetic, but it does not necessarily have to be. As with corrosion resistance, this depends on the composition of the alloy. Ferromagnetic stainless steel grades typically have a higher iron content, making them more magnetic. This has nothing to do with the quality of the material itself. The widespread assumption that high-quality stainless steel cannot be magnetised is simply untrue.
Stainless steel types with a low iron content can also become magnetic. For example, austenitic steels (containing primarily chromium and nickel) are not typically magnetic. However, in certain circumstances, such as when the composition and structure of the stainless steel are altered by cold working, they can become magnetised. This does not alter the quality in and of itself. Depending on the intended use, it can actually offer advantages.
Use of different grades of stainless steel in various industries
Due to its composition, stainless steel is a very versatile material that is used in numerous different applications such as low-wear tools, resilient construction parts, or hygienic surfaces.
Stainless steel plays a particularly important role in industries with high hygienic requirements, e.g. in the food and pharmaceutical industries. The material is suitable for furnishing entire hygiene-sensitive rooms because:
- It does not provide a breeding ground for germs
- Most stainless steel types are easy to clean
- It is acid and corrosion-resistant
Logistics operations also frequently make use of stainless steel. For example, pallet trucks are often made from stainless steel. These are suitable for permanent use in damp or wet rooms or in other challenging environments. Stainless steel pallet trucks are easy to clean, and the use of food-grade hydraulic oils and lubricants allows for safe use in the food and pharmaceutical industries.
For construction purposes, small components such as screws and nuts, fittings, angles or perforated plates are often made from stainless steel. They are used to provide highly stable construction and weatherproof designs in outdoor areas.
Some stainless steel grades are also used in shipbuilding, as well as in plants where components are likely to come into contact with salt or chlorinated water. Here, stainless steel offers the best protection against corrosion and enables reliable, durable designs with low maintenance requirements.
Stainless steel is also suitable for use in harsher environments and specialised areas. Ferritic stainless steel grades are particularly resistant to stress corrosion cracking, making them suitable for use in exhaust systems or water tanks.n Hydraulikölen und Schmiermitteln lässt einen sicheren Einsatz in der Lebensmittel- und Pharmaindustrie zu.
FAQ about Stainless Steel Grades
Stainless steel is a high-purity metal that consists mainly of iron and carbon. Usually, steel contains other elements such as phosphorus, aluminium, sulphur or silicon. These are extracted during the production process so that a particularly high degree of purity can be achieved.
Stainless steel is a high-purity metal that consists mainly of iron and carbon. Usually, steel contains other elements such as phosphorus, aluminium, sulphur or silicon. These are extracted during the production process so that a particularly high degree of purity can be achieved.
What types of stainless steel are there?
Depending on which substances are present in a stainless steel alloy, a distinction is made between five groups:
• Austenitic stainless steels (iron, carbon, chromium, nickel and possibly molybdenum)
• Martensitic stainless steels (iron, carbon, chromium, and large selection of other materials possible)
• Ferritic stainless steels (iron, carbon, chromium, low nickel content)
• Duplex stainless steel (primarily chromium, nickel, nitrogen, and other optional elements)
• Precipitation-hardened steel (chromium, nickel, carbon, and other optional elements)
If the exact alloy percentages and the respective properties are also taken into account, there are more than one hundred stainless steel grades. The American Iron and Steel Institute (AISI) has developed a classification that assigns an individual material number to each grade.
In general, stainless steel has the following properties:
• Considerable hardness and durability
• Resistant to large temperature fluctuations
• Good for welding
• Germ and bacteria resistance
The selection of alloy additives can influence rust behaviour and stability under different loads or high tensile forces.
Stainless steel has become synonymous with ‘rustproof’. However, this is not correct. Depending on the alloy, the corrosion resistance of the individual groups varies in strength. The decisive factor for this is the chromium content, which should be at least 12% to ensure high rust resistance. Chromium reacts with oxygen to form a protective layer on the surface of the stainless steel.
Other alloy components such as nickel and molybdenum further increase corrosion resistance and ensure the metal is protected even under demanding conditions (salty water, chemicals, etc.).
Stainless steel can be magnetic, but it does not necessarily have to fulfil this property in order to be considered stainless steel. As with corrosion resistance, this depends on the composition of the alloy. The widespread assumption that high-quality stainless steel must not be magnetisable is not true. So-called ferromagnetic stainless steels, for example, have a high iron content and, therefore, behave magnetically. This says nothing about the quality of the stainless steel itself.
In certain circumstances, such as when the composition and structure of the stainless steel is altered by cold working, austenitic steels can also become magnetisable. This does not change the quality of the stainless steel.
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