Stainless steel

Stainless steels, which can also be attributed to a broader class of corrosion-resistant steels-materials that have high resistance to corrosion in a humid atmosphere and slightly aggressive aqueous solutions.

Corrosion is the destruction of metals and alloys due to their chemical or electrochemical interaction with the external environment. Corrosion resistance is the ability of a material to resist the corrosive effects of the environment.

The basis of stainless steels is iron. The main alloying element that provides resistance to corrosion is chromium (Cr). Also, these materials usually include carbon (C), silicon (Si), manganese (Mn), sulfur (S) and phosphorus (P). Many of the stainless steels contain nickel (Ni) as alloying elements, which improves the corrosion resistance and heat-resistance of the steel; molybdenum (Mo), niobium (Nb), which increases the operating temperature of the steel; cobalt (Co), which increases the wear resistance of the material.

The most common is the classification of steels by their structure. There are the following types of corrosion-resistant steels:

• ferritic;
• martensitic;
• austenitic;
• ferritic-martensitic;
• austenitic-martensitic;
• austenitic-ferritic.

It is worth noting that, as a rule, corrosion-resistant alloys based on nickel, chromium and nickel, nickel and molybdenum are allocated to a special class.

The structure of the steels are different due to the different ways of cooling after high temperature processing. The structure, along with the chemical composition, has a great influence on the material's resistance to corrosion in various aggressive environments, which, in turn, determines the application areas of products made of a particular alloy or steel. The properties of stainless steels are determined by the chemical composition of the steel, as well as its structure. These features are particularly important for determining the environment in which a particular material is standing.

Martensitic and martensitic-ferritic steels have good corrosion resistance in atmospheric conditions, mildly aggressive environments (for example, in weak solutions of salts and acids), and also have high mechanical properties.

The main working medium of ferritic steels is solutions of nitric acid of ammonia, ammonium nitrate, a mixture of phosphoric, nitric, hydrofluoric acids, as well as some other oxidizing aggressive media. Steel of this class becomes brittle at a temperature of 475 °C, and also has relatively low strength and heat resistance. It is worth noting the poor weldability of ferritic steels and low corrosion resistance of welds.

Austenitic steels have good mechanical and technological properties, and are resistant to a large number of aggressive environments. Steels of this class have high ductility and strength, and are also well processed.

Austenitic-ferritic and austenitic-martensitic steels are similar in corrosion resistance to austenitic steels, but they are superior in mechanical characteristics. Thus, austenitic-ferritic steels have an increased yield strength, and austenite-martensitic steels have an increased strength.

It is necessary to say a few words about the marking of alloy steels. It is based on the alphanumeric system (GOST 4543-71). Alloying elements: manganese - G, silicon - S, chromium - H, nickel - N, tungsten - W, vanadium - F, titanium - T, molybdenum - M, cobalt - K, aluminum - YU, copper - D, boron - R, niobium - B, zirconium - C, nitrogen - A. The amount of the alloying element as a percentage is indicated by the number after the corresponding index. At the beginning, before the letter designation is written (regulated by the grade) in the form of a digital value multiplied by 10 percent of the carbon content in steel. The absence of a number after the element index indicates that its content is less than 1.5 %. High-quality steels have the letter A in the designation, and especially high-quality steels have the letter SH at the end.

For example, 12H2H4A steel contains 0.12% C, about 2% Cr, about 4% Ni, and less than 0.025% S and P.

Advantages:
• high corrosion resistance in various aggressive environments;
• lower cost compared to corrosion-resistant nickel-based alloys.

Disadvantages:
• lower heat-resistance compared to corrosion-resistant nickel-based alloys.

These materials are used in the production of items for power engineering and baking. These products include working blades, bolts, nuts, disks and rotors, and other elements of gas turbines, as well as parts of furnaces and other products that require protection from corrosion in aggressive environments. Stainless steels have lower operating temperatures compared to heat-resistant alloys and nickel-based steels, so they are used in cases where operating temperatures do not exceed 500-700 °C.

Various semi-finished products made of stainless steel are produced. It is worth noting stainless rods and round bars, wire and filament, stainless sheets and flat bars, pipes. These semi-finished products are used in industrial areas where high requirements are imposed on the corrosion resistance of products.