General Characteristics

All stainless steels without exception, regardless of class, have something that they have in common - alloying additives. These are elements such as chromium, molybdenum, nickel, niobium, titanium, tungsten, vanadium, manganese, silicon and a number of others which can be used to produce steels with special properties. The corrosion-resistant grades are in turn subdivided into groups based on differences in the proportions of the elements and the microstructure.

Letter grades

First of all, the carbon content in hundredths of a percent is indicated. For example, 20X23H18 steel contains 0.20% carbon. Then the alloying elements are indicated: (X) chromium, (N) nickel, (G) manganese, © silicon, (M) molybdenum, (B) tungsten, (T) titanium, (TT) tantalum, (U) aluminum, (F) vanadium, (D) copper, (P) boron, (K) cobalt, (B) niobium, (C) zirconium, (E) selenium. Capital Russian letters are used for designations, and the numbers in the steel grade designation indicate the percentage of a particular element. If there is no number after the letter, the content of the element is less than 1%.

Classification

All stainless steels, depending on their microstructure, are divided into five main groups:

• austenitic;
• ferritic;
• duplex;
• martensitic;
• Heat-resistant steels.

Austenitic steels

They are the most common and contain in addition to chromium, nickel, usually in the range of 18%. Ni increases corrosion resistance and makes the steel non-magnetic. A distinctive feature of these alloys is the ability to resist corrosion under the influence of high temperatures. This is the group of high-temperature corrosion-resistant steels.

Ferritic steels

This group differs from the previous one in the low chrome content (not more than 17% (for comparison, in austenitic steels - up to 25% Cr) and the presence of carbon. Ferritic steels are magnetic.

Duplex steels

They combine the properties of austenitic and ferritic groups. The nickel content is much lower than in the austenitic group, at no more than 8%, while the chromium content is higher, at up to 28%. Such a ratio gives the steel high hardness - above all, and sufficiently high corrosion resistance.

Martensitic steels

This group has one property that distinguishes it from all the others - it is the possibility of heat treatment (hardening and tempering) to improve mechanical properties - in this case we are talking about hardening. The steels in this group have carbon and an average chromium content of 12%. They can also be alloyed with nickel, niobium and copper.

Heat-resistant (heat-resistant) steels

In addition to other alloys they contain tungsten, which is the most refractory metal. These alloys are intended mainly to work at high temperatures. Their disadvantage is the low corrosion resistance compared to other steels.

Additional alloying elements

Exotic elements such as zirconium, cobalt, boron and some others are also used to give steels some special properties, but because of their high cost those additives are very rare, and are mainly used by the space and military industry. The most common are the first two groups - austenitic and ferritic steels.

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