General Characteristics

Stainless steel is a complex alloy that is resistant to corrosion under the influence of atmospheric and soil moisture, as well as in aggressive environments. In 1913 Harry Brearley, who experimented with different kinds of alloys, discovered the ability of steel with high chromium content to resist acid corrosion.

Chemical composition

When selecting the chemical composition of a corrosion resistant alloy, the following rule applies. If a metal that is not corrosive (e.g. iron) is added to a metal that forms a solid solution with it and is resistant to corrosion (such as chromium), the protective effect is not uniform. If a second metal is added, corrosion resistance increases not in proportion to the amount of the alloying component, but in steps. The main alloying element in stainless steel is chromium Cr (12-20%); in addition to chromium, steel contains elements that accompany iron (C, Si, Mn, S, P) and elements introduced into steel to give it the necessary physical and mechanical properties and corrosion resistance (Ni, Mn, Ti, Nb, Co, Mo).

Chromium

Corrosion resistance is directly related to the alloy's chromium content. At its content of 13% and more, the metal becomes stainless under normal conditions and in mildly aggressive environments. Over 17% Cr makes it corrosion-resistant in more aggressive oxidizing and other environments. In particular, in nitric acid of strength up to 50%. The reason for corrosion resistance is that a thin film of insoluble oxides forms on the surface of the chrome-containing part in contact with aggressive media. In this case the state of the material surface, the absence of internal stresses and crystalline defects is of great importance. In strong acids (sulfuric, hydrochloric, phosphoric and their mixtures) complex alloys with high content of Ni and additives Mo, Cu, Si are used.

Classification:

According to the chemical composition, stainless chromium steels are divided into: martensitic, semi-ferritic (martensitic-ferritic), ferritic. Chrome-nickel steels are divided into austenitic, austenitic-ferritic, austenitic-martensitic, austenitic-carbide. The classification of chromium-manganese-nickel steels coincides with that of chromium-nickel stainless steels.

Austenitic stainless steels.

They are divided into those prone to intergranular corrosion, and stabilized - with Ti and Nb additions. A significant reduction in the susceptibility to intergranular corrosion is achieved by reducing the carbon content to 0.03%. Stainless steels that are prone to intergranular corrosion are usually heat-treated after welding. The alloys Fe + Ni are widespread, in which due to the nickel the austenitic structure of iron is stabilized and the alloy acquires weakly magnetic properties.

Martensitic and martensitic-ferritic steels

They have good corrosion resistance in the open air and in slightly aggressive environments (weak solutions of salts and acids) and have high mechanical properties. They are mainly used for wear goods, as cutting tools such as knives, for resilient elements and structures and in the food and chemical industry for parts in contact with mildly aggressive media. This includes steels such as 30X13, 40X13 , etc.

Ferritic steels.

Ferritic steels are used in the manufacture of parts used in oxidizing environments (e.g. nitric acid solutions), domestic appliances, food, light industry and heat exchange equipment in power machinery. Such steels have high corrosion resistance in nitric acid, aqueous ammonia solutions, in ammonium nitrate, in a mixture of nitric, phosphoric and hydrofluoric acids and in other aggressive environments. This type includes the 400 series steels.

Austenitic steels

High service characteristics (strength, ductility, corrosion resistance in most working environments) and good processability are the advantage of steels in this class. That is why austenitic corrosion-resistant steels are widely used as structural material in various branches of mechanical engineering. The 300 series steels belong to this class.

Austenitic-ferritic steels

The advantage of steels in this group is the improved yield strength in comparison with the austenitic single-phase steels, no tendency for grain growth while retaining the two-phase structure, lower content of severely deficient nickel and good weldability. Such steels are widely used in various branches of modern technology, especially in chemical engineering, shipbuilding and aviation. This type of steel includes, 08Cr22Ni6T, 08Cr21Ni6M2T, 08Cr18G8Ni2T.

Austenitic-martensitic steels.

The demands of the new branches of modern equipment in corrosion-resistant steels of increased strength and adaptability have resulted in the development of transition-type steels. These are steels of 07Cr16Ni6, 09Cr15Ni9U, 08Cr17Ni5M3 types.

Alloys on iron-nickel and nickel base

When making chemical equipment, especially for work in sulfuric and hydrochloric acids, it is necessary to use alloys with higher corrosion resistance than austenitic steels. For these purposes use alloys on iron-nickel base type 04ChN40MTDTU and alloys on nickel-molybdenum base H70MF, on chromium-nickel base XN58V and chromium-nickel-molybdenum base XN65MV, XN60MB

Production and application

According to ISSF, in 2010 the global stainless steel production was about 25 million tons. Chromium steels are mainly used in hydraulic presses, turbine blades, cracking equipment, springs, cutting edges of knives, and household appliances.

Stabilized austenitic steels are used in welding equipment operating in aggressive environments and in products operating at high temperatures of 550-800°C. Chrome-nickel and chromium-manganese-nickel steels are used in the production of household items, particularly tableware (food grade steels). In the food industry;
Stainless steel is used in both deformed and cast state.

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