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Material basics, types of structure
Austenitic stainless steels
This type of stainless steel grade currently rules the market and encompasses the most common qualities, such as Mat No. 1.4301 or 1.4404 and higher-alloyed Mat. Nos. 1.4539 and 1.4529. These steels characteristically have a high content of austenite-forming substances, particularly nickel. Other alloying elements include chromium, molybdenum and, occasionally, copper, titanium, niobium and nitrogen. The addition of nitrogen increases the yield strength of the steel. Austenitic steels have an extremely wide range of uses, e.g. the chemical industry and the foodstuff industry.
Molybdenum-free steels also have excellent properties at high temperatures, which is why they are frequently used in the field of furnace construction and for building heat exchangers. Their good impact resistance at low temperatures is often utilised for products such as containers for cyrogenic liquids.
- Microstructure of
austenitic steel
Low-alloy austenitic steels (such as 1.4301 or 1.4404) are relatively prone to stress crack corrosion, particularly chloride-induced and hydrogen-induced. It is not possible to harden these steels by means of heat treatment. They are generally supplied tempered (quenched and annealed), and are soft and highly malleable. Their strength can be improved by cold-working. For this reason, some grades are supplied cold-drawn (cold-stretched) or hard-rolled.
Due to the strong tendency towards strain hardening, the modulus of elasticity of the austenitic steel types is subject to large fluctuations. For this reason, the DIBt (German Institute for Building Technology, Berlin) has reduced its value from 200 to 170 kN/mm² in the field of fastening technology in the construction industry. Tests have shown that high levels of strain hardening reduces the modulus of elasticity! High strength values achieved by strain hardening must not be applied in structural analysis of welded structures. As regards the Rp0.2 value, the basis in such cases must be the strength of the steel in its annealed state (basic strength). Please refer to the latest version of the National Technical Approval (DIBt) Z-30.3-6, "Products, fasteners and components made of stainless steels".
Austenitic steel grades are not generally magnetic in their annealed state. However, this may change as a result of cold-working and the attendant formation of martensite. In such cases, a magnet would adhere to the sheet metal or profile in the area of strain hardening. One such example is punched washers, which are often slightly magnetic as a result of this process. Due to their relatively expensive alloying constituents, Ni=nickel and Mo=molybdenum, use of austenitic steel grades is currently in strong global decline. Other types of microstructure, such as ferritic steels and duplex stainless steels, have caught up in terms of technical development and are often considerably cheaper. Please contact us and we will be happy to advise you.
Ferritic stainless steels
These steels are ferritic at all temperatures. This is achieved through a low content of austenite-forming elements (primarily nickel), and a high content of ferrite forming elements (primarily chromium).
- Microstructure of
ferritic steel
Older grades, such as Mat. No. 1.4016, were primarily used for household items, and applications where the demands placed on corrosion resistance were not too high. The current ferritic stainless steel grades, such as Mat. No. 1.4521 with extremely low carbon and nitrogen content are commonly used where there is a risk of stress crack corrosion. However, the surface finish of ferritic steels is particularly sensitive in terms of corrosion resistance! The corrosion resistance is dramatically reduced when the surface is roughened by blasting or grinding.
While ferritic steels have a slightly higher yield point Rp 0.2 than austenitic steels, they also have lower flexural strength, which restricts their cold workability. Deep-drawing and hydroforming are generally not possible, for example. Another property that distinguishes ferritic from austenitic steels is the low tendency towards strain hardening. Ferritic steels are therefore of particular interest due to their high modulus of elasticity (220 kN/ mm²), see the Comparison Table on page 16. Ferritic steels have greater fatigue strength and considerably lower thermal expansion than austenitic steel grades.
Martensitic stainless steels
- Microstructure of
martensitic steel
Martensitic steels have the highest strength, but also the lowest rust resistance of all the stainless steel grades. Martensitic steels with high carbon content are used as tool steels and have good hardening characteristics. With their high strength and relatively low corrosion resistance, they are ideally utilised where the material is subject to corrosion attacks and wear and tear in equal measure. One such example would be knife blades.
Austenitic-ferritic stainless steels (duplex stainless structure)
- Microstructure of
duplex stainless steel
In terms of microstructure and content of alloying elements, this group of steels is a mixture between ferritic and austenitic steels. The key difference is the considerably higher basic strength (see yield point Rp0.2, page 16). For this reason, these steel grades are frequently used for the manufacture of machinery parts subject to dynamic loads, such as extraction rollers for paper presses.
The oil, gas and petrochemical sector represents an area of new potential for these steels as they are ideally suited for use in sea water-resistant plants and the offshore industry. These steels are generally called duplex steels due to their two-phase microstructure, although the combined term duplex stainless steel is more suitable as there are coated sheet metals and sandwich metals with the same designation. The widely used material types in the duplex stainless steel group, such as 1.4362 or 1.4462, offer the combined advantages of both basic types of microstructure (austenitic – ferritic).