Classification of stainless steel grades according to standard EN10088-1:2005

01/18/2022 Author: VT-METALL

From this material you will learn

:

• Characteristics of corrosion-resistant steel
• 4 types of corrosion-resistant steel
• Marking and application of corrosion-resistant steel
• Foreign grades of corrosion-resistant steels
• Nuances of welding corrosion-resistant steels

Corrosion-resistant steel (aka stainless steel)

has firmly entered our lives and is used in various fields: from the chemical and aviation industries to the manufacture of everyday goods. The thing is that this type of steel shows much better characteristics than regular steel, and the variety of grades allows you to select the material that best suits your needs.

The history of this material goes back more than a century, and the number of brands exceeds two hundred, so it is important to understand their characteristics when choosing stainless steel as a material for your needs. In our article we will tell you what characteristics corrosion-resistant steel has, what types it is divided into, and also talk about the nuances of Western and domestic markings.

Decoding stamps

The marking of alloy steels consists of letters and numbers. At the beginning there is a two-digit number that characterizes the amount of carbon in hundredths of a percent. The following are letters of the Russian alphabet, denoting a specific element:

• X – chromium;
• N – nickel,
• T – titanium;
• B – tungsten;
• G – manganese;
• M – molybdenum;
• D – copper.

After the letter designation of the alloying element in the decoding there is a number indicating its content in stainless steel, rounded to the nearest whole percent. If there is no such figure, then the additive in the alloy is in the range of 1-1.5%.

Classification by microstructure type: austenitic grade stainless steel

The resistance of this class to corrosion is increased due to the alloying elements of nickel (from 5 to 15%) and chromium (from 15 to 20%). Austenitic alloys are insensitive to intergranular corrosion, provided that the carbon content in them is less than its solubility limit in austenite (0.02-0.03% or less). Non-magnetic, well subject to welding, cold and hot deformation. They have excellent technology. This is the best steel for the manufacture of fasteners, welded structures and use in various industries.

Grades of heat-resistant and heat-resistant stainless steels

Heat resistance, otherwise called “scale resistance,” is the property of a metal to resist gas corrosion at high temperatures in an unloaded or lightly loaded state.

Definition! To improve this characteristic, chromium, silicon and aluminum are introduced into the composition of stainless steels. These elements, combining with oxygen, form dense structures that increase the resistance of steel to temperatures above +550°C. Nickel by itself does not affect heat resistance, but in combination with Cr, Al and Si it increases their efficiency.

Heat-resistant steels are steels that function at high temperatures and loads without a tendency to short-term and long-term creep.

Table of areas of application of scale-resistant and heat-resistant steels

Characteristics of corrosion-resistant steel

Corrosion-resistant steel, an invention of metallurgist Harry Brearley, was patented in 1913 in England. Thanks to this material, steel and other industries have reached a completely new level.

Conventional steel alloys acquired unique properties and were able to resist the formation of rust due to the addition of chromium to their composition. For corrosion-resistant steels and alloys, the content of this element must be at least 10.5%. In this way the following characteristics are achieved:

• very high resistance to corrosion;
• excellent strength;
• good weldability;
• ease of processing using cold deformation;
• long service life, during which the material retains its original qualities;
• attractive appearance of products.

The essential components of corrosion-resistant steels are chromium and iron. Due to the fact that these additives complement each other, the material acquires unique characteristics. Chromium combines with oxygen and forms an oxide film on the surface of the alloy - it is this film that prevents the formation of rust.

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However, the described properties of stainless steel can be further improved with the help of alloying additives, such as nickel, titanium, molybdenum, niobium, cobalt, etc. Thanks to alloying, many types of stainless steel alloys are created in production, which have different characteristics and purposes.

The carbon contained in corrosion-resistant steel provides the metal with high hardness and strength. In addition, this element is included in all steel alloys, since many significant properties depend on it.

Stainless steel has a number of unique qualities, so it is actively used in areas where the product or equipment must constantly operate under high humidity and exposure to aggressive environments. Corrosion-resistant steels are used to make objects for use in industry and even in everyday life - this metal is the material of cutlery, knives, communication elements, enclosing structures, equipment parts, etc.

Stainless steel grades for the manufacture of chimneys

When purchasing modular chimney systems, you need to find out what kind of steel they are made of. On sale you can find chimneys that are about one and a half times cheaper than other products in this category. In their production, AISI 201 steel (12X15G9ND) is used. According to international standards, it is necessary to use steel grade AISI 321 (08Х18Н12Т), the cost of which is approximately 2 times higher than the cost of AISI 201. It is impossible to visually distinguish AISI 201 from AISI 321, moreover, both alloys are non-magnetic. They can only be distinguished by chemical analysis.

Differences in chemical composition

AISI 201 steel has low anti-corrosion characteristics, instability of the structure, and the risk of cracks during drawing. Its use will lead to rapid failure of the chimney due to rapidly developing corrosion. This steel is mainly distributed in China and India.

Well-known foreign and conscientious Russian manufacturers, in addition to AISI 321 steel, use high-alloy alloys stabilized by Ti. They are acid and heat resistant. The use of cheaper steels (AISI 409, AISI 430) for gas exhaust pipes that do not meet acid resistance requirements leads to their failure soon after the start of the heating season.

Foreign grades of corrosion-resistant steels

Steel grades AISI 201 and AISI 202 belong to the austenitic group of alloys.

It is worth clarifying that the abbreviation stands for American Iron and Steel Institute or “American Institute of Steel and Alloys.” These metals contain chromium, nickel, manganese, copper, nitrogen, due to which high strength of the products is achieved. The material also lends itself well to deformation and easily changes shape.

The balanced composition of these corrosion-resistant steels allows them to stand out from the crowd with their high resistance to rust.

AISI 201 and AISI 202 are used for the production of household appliances, pipelines, and building structures.

The difference between these brands is the content of additional components. AISI 201 contains more carbon, sulfur, manganese and copper, which provides high strength and ductility. While AISI 202 contains more nickel. It should be noted that AISI 201 is an improved version of AISI 202, but both brands retain their physical characteristics even when products made from them are used in a moderately aggressive environment.

There are Russian analogues of these corrosion-resistant steels:

• AISI 201 can be replaced with 12X15G9ND;
• AISI 202 is close to 12X17G9AN4.

Steel grades of the 300 series are included in the chemical composition of the austenitic or duplex group of alloys. The type of stainless steel depends on the proportion of basic additives, such as carbon, nickel, chromium, titanium. It is important that this series is considered universal and popular in the market, as it has high strength, resistance to wear and rust.

Stainless steels for the food industry

Corrosion-resistant steels are indispensable for industries producing equipment, tools and utensils intended for contact with food products. Their advantages:

• Resistance to various types of corrosion - chemical and electrochemical. In each specific case, it is necessary to select brands that are resistant to the environments with which they will come into contact during operation. These are normal atmospheric conditions, water, salt water, acidic, alkaline, chloride solutions.
• Good machinability. Modern tools make it possible to weld, cut, form and process corrosion-resistant alloys on lathes, milling and drilling machines in the same way as “ferrous” steels.
• Compliance with sanitary and hygienic standards. Thanks to various processing methods - grinding, polishing to a mirror finish - a surface is obtained that is practically free of pores and cracks into which dirt and pathogenic microorganisms can penetrate.
• Good mechanical characteristics. Thanks to them, it is possible to produce products and structures of smaller thickness and weight without compromising technical properties. Austenitic steels are more resistant to low temperatures compared to general purpose metals.
• Aesthetics. Electropolishing, satin finishing and other surface treatment methods provide a stylish look to stainless steel products.

Read also: Hardness of copper and aluminum
Table of properties and areas of application of food grade stainless steels

Table of stainless steel grades used in the food industry

It is impossible to imagine modern life without anti-corrosion steel. The development of such an alloy has made it possible to make a qualitative breakthrough not only in metallurgy, but also in many other areas. Stainless steels differ from classical ones in that in addition to iron and carbon, they also contain chromium. It is the addition of chromium that gives the alloy anti-corrosion properties.

Stainless steel products are very diverse. You can find a wide selection of products from any manufacturer. For example, high-quality products, as confirmed by numerous reviews, can be ordered in the BSM - Metal online store.

Marking and application of corrosion-resistant steel

Today there are more than 50 grades of corrosion-resistant chromium-nickel steels. They are used as a material for pipe and flat products, fittings, channels, beams, angles, and profiles. In addition, stainless steel is actively used in the automotive, aircraft, and energy industries.

Austenites are used to produce products by welding and cold stamping, such as:

• construction tanks;
• pipes;
• installations for oil rigs, cleaning systems;
• turbines and other mechanisms that must function in water;
• power units for the energy sector;
• aircraft and car parts;
• equipment for working with food products;
• pharmacological and medical technology;
• welded metal structures;
• hardware.

In accordance with GOST, such alloys are marked

:

• 12Х18Н10Т
  . Includes nickel, titanium, and is a material for equipment for the chemical and oil refining industries.
• 12Х18Н10Т
  . Used in the production of pipelines.
• 12X15G9ND
  . It contains nickel, manganese and copper and is used for the manufacture of containers and pipelines for solutions with moderate aggressiveness.

Martensites are used in the production of products for work in aggressive environments under conditions of low or medium intensity. Elasticity makes it possible to make springs, flanges, and shafts from such corrosion-resistant steel. In addition, metal is a material for cutting surfaces in the food and chemical industries.

Grades of martensitic steels

:

• 20Х13, 30Х13
  . Used in the manufacture of household appliances.
• 14Х17Н2
  . Contains nickel and can be used for the production of compressors and other equipment that is planned to be operated at low temperatures and in aggressive environments.

Ferrites are found in the following areas:

• chemical and petrochemical industry;
• energy;
• heavy engineering and machine tools;
• instrument making;
• medical equipment;
• production of household appliances;
• food industry.

We are talking about the following types of corrosion-resistant steels

:

• 08X13
  . Suitable for making kitchen appliances.
• 12X13
  . Used to create containers intended for storing and transporting alcohol-containing liquids.
• 12X17
  . This is a corrosion-resistant and heat-resistant steel, in tanks from which food is processed at high temperatures.

Physical properties

Stainless steel has gained high popularity not only due to its anti-corrosion properties, but also due to its variety of physical properties. Modern corrosion-resistant steels are produced by adding various impurities to the steel.

The physical properties of the finished steel depend on the amount and type of impurity. It should be noted that some grades of stainless steel are susceptible to corrosion after a long period of use. This is due to the composition, that is, the addition of this or that metal. Such an alloy has other advantages that eliminate susceptibility to oxidation.

It is necessary to highlight the main physical properties of stainless steel, which qualitatively distinguish it from a number of other metals. These properties include:

1. High strength. Products made from stainless steel are characterized by increased strength in comparison with analogues. Due to its resistance to physical stress, the products are not damaged and do not lose their original shape. High-quality steel remains reliable for more than ten years.
2. Resistance to aggressive external environment. Such steel is practically not subject to changes due to environmental conditions. This allows you to maintain the performance properties of the product for a long time.
3. Heat resistance. Stainless steel products are resistant to high temperatures, even when exposed to open fire. Also without changing shape, size and properties under significant temperature changes.
4. Environmental friendliness. Anti-corrosion properties prevent the oxidation process. In addition, the material does not contain harmful components, therefore it is widely used in the food industry.
5. Anti-corrosion properties. The main property that such steel has is that it prevents rust. Moreover, the alloy does not corrode even after exposure to acids or alkalis.
6. Appearance. The appearance of stainless steel products is qualitatively different from items made of other materials. Steel has a clean, shiny appearance that does not change after a long period of use.
7. Compliance. Such an alloy is easy to process, and making an object of the desired shape from it is not difficult.

The choice of stainless steel with certain physical properties depends on the purpose of its use. Today, a variety of components for the production of stainless steel allows you to create a material with the necessary characteristics.

Why does metallic materials deteriorate?

Before moving on to the question of what corrosion-resistant steel is, let's understand the concept of corrosion and the essence of this process.

Translated from the Latin corroder - corroding. The slow spontaneous destruction of metals and alloys based on them, which occurs under the chemical influence of the environment, is called corrosion. The reason for this destruction is the chemical interaction (redox reactions) of metal materials with the gaseous or liquid medium in which they are located.

Chemical composition

The chemical composition of stainless steel depends on the type and grade of the alloy. The main features that characterize stainless steel are the presence of at least 10.5% chromium and low carbon content. Carbon is very important in steel making as it gives the required strength. The percentage component of which in the anti-corrosion alloy should not exceed 1.2%.

Stainless steel may also contain Titanium, Phosphorus, Molybdenum, Sulfur, Nickel and Niobium. Depending on the chemical composition, stainless steel is divided into several types.

The most widely used is stainless steel of group A2. Group A2 contains 10% nickel, 18% chromium and 0.05% carbon. Most of it is occupied by the base, namely iron with accompanying components.

The composition of steels in this group includes 0.05% carbon, 2% molybdenum, 12% nickel and 17% chromium. Due to the presence of molybdenum in the composition, the alloy is resistant to acid, so the name “acid-resistant” is often applied to it.

Anti-corrosion steels of group A, due to their chemical composition, are easy to weld. That is why this type is widely used in industry. From such steel it is possible to produce parts of almost any shape, with a strong connection of the component parts.

Particular attention in production is paid to steel for the food industry. In this case, corrosion-resistant steel should not contain foreign components that can negatively affect the taste of products, as well as impurities hazardous to human health.

The resistance of steel to corrosion depends on the amount of chromium. The larger its component, the more stable the alloy. Classic stainless steel used under normal conditions contains no more than 13% chromium. To withstand an aggressive environment, the proportion of chromium must exceed 17%. This corrosion-resistant alloy is suitable for use in acidic environments.

Highly resistant alloys retain their properties even in nitric acid of 50% saturation. For resistance against stronger acids, the percentage of nickel in the composition is increased and other components are added in small quantities.

Read also: Sectional design of a propane gas cylinder

Austenitic-ferritic class

Corrosion-resistant stainless steels of this class are characterized by a reduced nickel content and a high chromium content (from 21 to 28%). Additional alloying elements include niobium, titanium, and copper. After heat treatment, the ratio of ferrite and austenite is approximately one to one.

The strength of austenitic-ferritic steels is twice that of austenitic steels. At the same time, they are ductile, resist shock loads well, have a low level of stress-corrosion cracking and high resistance to intergranular corrosion. Recommended for use in construction, manufacturing industry, and for the manufacture of products that will come into contact with sea water.

Classification of stainless steels

The classification of stainless steels varies among countries, but has common principles. Stainless steel marking is carried out depending on the chemical composition, properties and internal structure of the finished material. Based on this, steel is divided into the following types:

1. Ferritic. This group of steels is characterized by a high chromium content, usually more than 20%. Therefore, this type is sometimes called chromium. This chemical composition contributes to high resistance to aggressive external environments. Alloys of this group have magnetic properties. Ferritic steels are relatively cheap and are widely used in industry, second only to austenitic steels.
2. Austenitic. A group of anti-corrosion alloys that are characterized by a high content of chromium and nickel. Due to this, they are distinguished by increased strength and flexibility in comparison with analogues. Also easy to weld and resistant to corrosion. Most widely used in industry. They belong to non-magnetic metals.
3. Martensitic. A special type of stainless alloy. It is characterized by increased strength and wear resistance. They are not exposed to high temperatures, and at the same time contain a minimal part of harmful components that do not emit vapors during intense heating. This group includes heat-resistant, corrosion-resistant steel.
4. Combined. A special type of steel that combines the properties of the above groups. Such innovative steels are developed individually depending on the properties required by the customer. Today, austenitic-ferritic and austenitic-martensitic steels are distinguished.

Stainless steel parts

In turn, grades of stainless steel of the austenitic group are divided into 4 types:

1. A1 is steel that contains a significant amount of sulfur, which is why it is more susceptible to corrosion than others.
2. A2 is the most widely used grade. Easily weldable without loss of physical properties. Frost-resistant, but susceptible to corrosion in an aggressive acidic environment.
3. A3 is a derivative of A2, but with the addition of stabilizing components. It is characterized by increased resistance to high temperatures and acidic environments.
4. A4 – alloy with the addition of molybdenum (up to 3%). Characterized by resistance to acidic environments. Widely used in shipbuilding.
5. A5 – similar to the A4 brand. It differs only in the ratio of stabilizing components. Manufactured for increased resistance to high temperatures.

Types of stainless steel are not limited to the above types. Since even the slightest changes in the percentage of components can significantly affect the properties of steel.

The most popular brands and areas of their application

To choose the right stainless steel for the manufacture of products for a specific purpose, you can use special reference books that list both all grades of such material and their main characteristics. Meanwhile, in each of these groups there are the most popular brands, which are most often chosen by the consumer. Let's list them.

• 10Х17Н13М2Т and 10Х17Н13М3Т are steels that are characterized by good weldability and excellent corrosion resistance. Thanks to these properties, stainless steel alloys of these grades are successfully used for the production of products that, during their operation, are constantly exposed to high temperatures and aggressive environments. The properties of steels of these grades are formed due to the presence in their chemical composition of the following elements: chromium (16–18%), molybdenum (2–3%), nickel (12–14%), carbon (0.1%), silicon (0 .8%), copper (0.3%), sulfur (0.02%), phosphorus (0.035%), manganese (2%), titanium (0.7%). If there is a need to select stainless steels of these grades, then you should keep in mind that their foreign analogues can also be purchased on the domestic market, namely: SUS316Ti (Japan), 316Ti (USA), OCr18Ni12Mo2Ti (China), Z6CNDN17-12 (France) ).
• 08Х18Н9 and 08Х18Н10 are stainless steel alloys from which pipes of both round and any other cross-section are made. These materials are used for the production of various structures used in the mechanical engineering and chemical industries, as well as for the production of pipeline elements and furnace devices. The chemical composition of these grades of steel contains the following elements: chromium (17–19%), carbon (0.8%), titanium (0.5%), nickel (8–10%).
• 10Х23Н18 - steel of this grade is characterized by a high content of nickel (17–20%) and chromium (22–25%), as well as manganese (2%) and silicon (1%) in its composition. This combination of elements gives the alloy the required characteristics and creates an increased tendency to temper brittleness. It should be noted that the alloy of this grade belongs to stainless steels of the heat-resistant category.
• 08Х18Н10Т - a stainless alloy of this brand is characterized by high resistance to oxidation processes, as well as good weldability, and to obtain a high-quality connection using this technology, the products do not need to be preheated, and they also do not require heat treatment after welding. To improve the strength characteristics of products made from such steel, they must be hardened, as specified in the relevant regulatory document.
• 06ХН28МДТ - an alloy of this grade is optimally suited for creating welded structures that will subsequently be used in aggressive environments. The chemical composition of this steel contains the following elements: chromium (22–25%), nickel (26–29%), copper (2.5–3.5%).
• 12Х18Н10Т – products made from steel of this grade are mainly used to equip enterprises in the chemical, pulp and paper, construction, food and fuel industries. This metal is characterized by thermal resistance, good impact strength and practicality of use.
• 12Х13, 20Х13, 30Х13 and 40Х13 – stainless steel alloys of these grades are practically impossible to weld, but they also have positive properties. The latter lies in the fact that these steels do not have a tendency to temper brittleness, and their internal structure is not affected by defects, which in professional language are called flakes. Cutting and measuring tools, as well as springs and springs for various purposes are made from these grades of stainless steel.
• 08Х13, 08Х17, 08Х18Т1 are stainless steel alloys of the ferritic group, from which products are produced that do not experience shock loads during operation, as well as exposure to low temperatures.

Types of stainless steel surfaces

Scope of application of stainless steels

Since their development, corrosion-resistant steels have been used only in high-tech production in such areas as aircraft manufacturing, nuclear energy, petrochemical production and mechanical engineering. Today, stainless steels are widely used in various areas of our lives.

Stainless steel car detail

Let us highlight the main areas of use of stainless alloys:

1. Mechanical engineering. Stainless steel is widely used for the production of cars, industrial machines and various units. Ferritic and austenitic types are commonly used.
2. Chemical industry. The chemical industry is accompanied by the use of aggressive substances, the maintenance of which requires special equipment. Austenitic alloys are used for its production. Production tanks, pipes and vessels are not exposed to chemicals and do not lose their performance properties.
3. Energy. In the electrical power industry, only high-strength materials are used, since the strength and reliability of working units are of particular importance.
4. Pulp and paper industry. Almost all equipment in this area is made of high-quality stainless steel.
5. Food industry. There are increased requirements for the production, storage and transportation of food products. Therefore, in the manufacture of equipment, you can only use glass, several types of plastic and stainless steel. This ensures an increased level of hygiene.

In the food industry, an alloy containing a small number of components is usually used, since the equipment is not exposed to ultra-high temperatures and aggressive substances. Frost-resistant materials are used for refrigeration units.

1. Aerospace sector. Special types of stainless steel began to be used to build airplanes, rockets and spaceships.
2. Construction. Stainless steel is widely used in construction and design. Such sheets are scratch-resistant and do not leave hand marks.

Corrosion-resistant steels are also used in many fields, due to the variety of types and properties.

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Corrosion-resistant steels are metal alloys that have increased resistance to corrosion in various climatic and atmospheric conditions, as well as in salt and fresh water, in some gas areas, acids and alkalis. Next, we will describe in detail the properties and characteristics of the main grades of anti-corrosion steel.

Mechanical properties

Grades of corrosion-resistant steels must have mechanical properties that meet the requirements of established manufacturing standards. These include:

• maximum hardness on the Brinell scale (HB);
• relative extension (%);
• yield strength (N/mm 2 );
• tensile strength (N/mm 2 ).

After production, each batch (melting) of commercial products is checked for compliance of the mechanical properties and microstructure of the steel grade with GOST. The results of laboratory testing of samples are indicated in the manufacturing certificate.

3 Chromium-nickel alloys - the basis of construction and production

This type of anti-corrosion steel is the most common in modern industry and production. Today, more than 50 grades of chromium-nickel alloys are known, from which hot-rolled pipes, long and sheet metal, profiles, fittings, angles, and channels are made. In addition, such steel is widely used in the chemical, energy, aircraft and automotive industries. Grades of chromium-nickel steels can be divided into several classes:

• austenitic with low carbon content and the addition of stabilizing elements;
• acid-resistant with various additives;
• heat-resistant with a high content of nickel and chromium (more than 20%);
• austenitic-martensitic and austenitic-ferritic with an average content of nickel and chromium;

The main alloys of this type are grades ОХ18Н9, ОХ18Н10, 2Х1Н9, ОХ18Н11 and other types 18-8 (that is, 18% Ni in the alloy composition), stabilized with titanium and other alloying elements. Chromium-nickel austenitic steels are widely used for furnaces, heat turbines, and outlet manifolds. They can be worked in conditions of high external aggressiveness of the environment and subjected to short-term heating to a temperature of 600-650 degrees without additional heat treatment of the surfaces.

Chromium-nickel anti-corrosion and heat-resistant steels with the addition of silicon or boron (Ox23N18, X23N18, X25N16) are used for the production of heat-resistant sheets, tapes, tubes, wires, which are used for various equipment that operates at temperatures above 850 degrees. In the structure of chromium-nickel and nickel steels there is a certain amount of ferrite, which decreases with increasing content of elements such as manganese, silicon, molybdenum, and increases when nitrogen, boron, copper or nickel are introduced into the composition in a large percentage. All these properties reflect steel grades of a similar type and are determined using special content tables.

Corrosion-resistant (stainless) steels and alloys

BASICS OF THE THEORY OF CORROSION

The destruction of metals and alloys as a result of chemical or electrochemical action on their surface of an external aggressive environment is called corrosion .

Corrosion, as a rule, is accompanied by the formation of corrosion destruction products on the metal surface. For example, on the surface of iron alloys, as a result of corrosion, rust is formed, which has a brown color. In some individual cases, metal corrosion is not accompanied by the formation of such noticeable destruction products, and then its appearance is quite difficult to detect.

Corrosive destruction is the result of the interaction of a metal with the external environment and the intensity of its development depends on the properties of the metal itself, as well as on the nature of the environment. Most metals, being resistant in some environments, are quite easily destroyed when interacting with other environments. For example, copper alloys are stable in humid atmospheres, but are highly susceptible to corrosion if even small amounts of ammonia are present in the atmosphere; Tantalum and titanium at room temperatures are very stable in many aggressive environments, but they acquire high chemical activity when heated above 600 ° C.

There are several types of corrosion: continuous or uniform, when the entire surface of the product is exposed to corrosion; point or local, if corrosion develops in individual small areas; intergranular corrosion (ICC), when corrosion spreads deep into the product along the grain boundaries; Stress corrosion is the occurrence of corrosion cracks due to the simultaneous impact of tensile stresses and an aggressive environment on the metal.

Corrosion can occur as a result of purely chemical reactions with the environment, as well as due to electrochemical processes occurring at the interface between the metal and the external environment. The largest amount of metal is destroyed as a result of electrochemical corrosion.

Electrochemical corrosion is the destruction of metals and alloys when exposed to electrolytes. This type of corrosion is characterized by the flow of electric current, the transition of atoms to an ionized state and other electrochemical processes.

The most common electrolytes in practice are aqueous solutions of salts, acids and alkalis. Thus, electrochemical corrosion includes corrosion of metal containers, pipelines, machine parts and parts of stationary structures under the influence of acids, sea, river, ground and other waters. The most common is atmospheric corrosion.

If there are two metals with different electrode potentials in the electrolyte, then the metal with a more negative electrode potential (anode) continuously releases ions into the solution (dissolves), and the resulting excess electrons continuously flow into the metal with a less negative electrode potential (cathode). The cathode in the contact pair is not destroyed; electrons from it are continuously removed into the external environment.

All metals can be arranged in a row in descending order of their electrochemical potential:

Metal…………………. Au Ag С u Η Ni Fe Ζ n Α l

Electrode potential, V +1,42 +0,80 +0,34 0 -0,23 -0,44 -0,76 -1,66

In technical metals and alloys, which are polycrystalline bodies, the microstructure consists of grains of one or more phases, non-metallic inclusions, etc. These different structural components, which have different physical and chemical properties, upon contact with the electrolyte acquire electrode potentials of unequal magnitude and sign and some of them will become anodes, and others - cathodes. Thus, industrial metals and alloys, when exposed to electrolytes, can be considered as multielectrode elements consisting of a huge number of microscopically small corrosive galvanic pairs - microgalvanic pairs. The more the electrode potentials of the phases in the alloy differ, the faster its corrosion destruction occurs (in particular, dendritic segregation is precisely why the resistance to electrochemical corrosion decreases). It follows that either very pure metals or alloys that have a uniform (homogeneous) solid solution structure can have high corrosion resistance.

The passive state is the state of a metal (alloy) when it exhibits increased corrosion resistance (even practically stops corroding) in an aggressive environment. The opposite state, when the same metal corrodes, is called the active state.

Experimental data show that the transition of a metal from an active to a passive state is associated with an increase in its potential. For example, iron in its normal state has an electrode potential of -0.4 V; in a passive state, its potential can rise to +1.0 V.

Effect of doping. There are two groups of corrosion-resistant metals. Some metals resist corrosion well due to their low chemical reactivity. Others, being by their nature active elements, acquire high chemical stability due to the phenomenon of passivity. The first group includes platinum, palladium, gold, the second group includes chromium, titanium, aluminum, etc. To increase the corrosion resistance of a reactive metal, alloying elements are introduced into it.

When a metal is alloyed with another, more noble metal, the potential of the alloy initially remains virtually unchanged. But when a certain concentration is reached, a jump in potential occurs and the corrosion resistance of the alloy in a given environment also increases abruptly, and boundaries (thresholds) of stability appear.

It was experimentally established that such sharp changes in stability occur when the ratio of atoms of the alloying element to the alloyed element is a multiple of 8, i.e. n/8, where n is an integer 1, 2, 3... This corresponds to 12.5; 25; 37.5...% (at.) alloying element.

The appearance of stability boundaries is explained by the fact that when the alloy interacts with an aggressive environment, some of the atoms of the base metal go into solution, and the remaining atoms of a more noble or easily passivating metal form a kind of barrier on the metal surface. This barrier consists either of the noble metal atoms themselves, or of protective shielding films.

In more active environments, a higher concentration of a stable element is required, i.e. in this case, the limits of stability arise at a higher value of the number n.

The stability limit is also observed in alloys in which one of the components has the ability to self-passivate. This boundary is also observed in systems when one of the components in a given aggressive environment forms protective shielding films from insoluble compounds.

The corrosion resistance of steel can be increased if, firstly, the carbon content is reduced to the minimum possible amount and, secondly, an alloying element that forms solid solutions with iron is introduced in such an amount that the electrode potential of the alloy increases abruptly.

Steel that is resistant to atmospheric corrosion is called stainless steel. A steel or alloy that has high resistance to the corrosive effects of acids, salts, alkalis and other aggressive environments is called acid-resistant.

CHROME STAINLESS STEEL

Chromium is the main alloying element that makes steel corrosion resistant in oxidizing environments. The corrosion resistance of chromium stainless steels is explained by the formation of a protective dense passive film of Cr2O3 oxide on the surface. Such a film is formed only when the chromium content is more than 12.5% ​​(at.). It is at this chromium content (n=1) that the potential changes abruptly from -0.6 to +0.2 V.

Iron and chromium form a continuous series of solid solutions (See iron - chromium diagram). Thanks to this, it is possible to obtain steel with a high chromium content in solid solution. Chromium is not a scarce metal, its cost is relatively low, so chromium steels are the cheapest stainless steels. These steels have a fairly good set of technological properties. Carbon in stainless steels, including chromium steels, is an undesirable element, since, by binding chromium into carbides, it thereby depletes the solid solution of chromium, reducing the corrosion properties of the steel. In addition, carbon expands the region of the γ-solid solution, facilitating the formation of a two-phase state (Fig. 1).

Rice. 1. The influence of carbon on the position of the γ region - solid solution on the iron - chromium diagram

With a higher chromium content, the σ phase will be present in the steel.

The higher the chromium content, the higher the corrosion resistance of chromium steels. Currently, three types of chromium steel are smelted: 1) containing 13% Cr; 2) 17% Cr- 3) 25-28% Cr.

Steels 08X13 and 12X13 have increased ductility and are used for the manufacture of parts subject to shock loads (turbine blades, cracking unit fittings, household items, etc.).

From steels 30X13 and 40X13, which acquire a martensite structure after heat treatment, measuring and medical instruments, springs and other corrosion-resistant parts that require high hardness or strength are made.

Steels containing 17 and 25-28% Cr are classified as ferritic steels. They have higher corrosion resistance compared to X13 steels. When heated above 850° C, ferritic steels tend to grow grains and their ductility decreases. To obtain a single-phase structure and reduce the tendency to grain growth and MCC, titanium and niobium (08X17T, 15X25T) are added to these steels. Strength increases, ductility remains sufficient, and the properties of welds improve. These steels are used for the manufacture of equipment operating in such aggressive environments as fuming nitric acid, phosphoric acid, and make equipment in the chemical and food industries corrosion-resistant. Heat exchangers for hot nitrous gases, pipelines and tanks for acids, etc. are made from 12X17 steel.

The introduction of molybdenum (12Х17М2Т) makes the steel resistant even to organic acids (acetic, formic). Ferritic grade steels are not susceptible to stress corrosion.

For the manufacture of ball bearings operating in aggressive environments, steel 95X18 (0.9-1.0% C, 17-19% Cr) is used.

All chromium steels are subjected to hardening at 1000-1100°C followed by tempering (for ferritic steels - at 700-750°C, martensitic class 200-250°C).

Ferritic steels do not undergo transformations when heated, so heat treatment is carried out to obtain a more homogeneous solid solution structure, which increases corrosion resistance.

CHROME-NICKEL STAINLESS STEEL

Nickel is one of the metals that easily acquire passivity, although its passivation ability is less than chromium and molybdenum. Adding nickel to iron in an amount of 1/8 mole dramatically improves the corrosion resistance of the alloy in sulfuric acid. At a nickel concentration of 2/8 mole, the corrosion resistance increases even more.

Iron-nickel phase diagram. Nickel is an austenite-forming element that greatly reduces the critical points of the γ-να transformation. Nickel also has this effect when it is introduced into chromium steels. Therefore, steel containing 18% Cr and 9% Ni has an austenite structure at room temperatures (see Fig. 2).

Rice. 2. Structural diagram of stainless steels

Stainless steels with an austenitic structure have higher corrosion resistance,

better technological properties compared to chromium stainless steels, in particular, better weldability. They retain strength to higher temperatures, are less prone to grain growth when heated, and at the same time, austenitic steels do not lose ductility at low temperatures. Like chromium steels, chromium-nickel steels are corrosion-resistant in oxidizing environments. The main element that increases the potential of iron is also chromium, so its content should be >13%. Nickel only further increases the corrosion resistance of steels.

The composition and properties of chromium-nickel stainless steels are given in GOST 5632-72. In Fig. Figure 2 shows a structural diagram that allows you to determine the structure of steel depending on its composition.

Chromium-nickel steels, depending on composition and structure, are divided into steels of austenitic, austenitic-martensitic and austenitic-ferritic classes.

The lower the carbon content, the higher the corrosion properties of stainless steels. The carbon contained in chromium-nickel steels can be in solid solution, as well as in carbides or carbonitrides of varying degrees of dispersion. Cr23C6 carbides are predominantly formed, and they are formed already at a carbon content of slightly more than 0.04% (0.04% C is the solubility limit of carbon in austenite alloyed with nickel). If steels contain nitrogen (for example, X17AG14 steel), then carbonitrides of the Me23(C,N)6 and Me(C,N) type can be formed.

Most chromium-nickel stainless steels belong to the austenitic class: 04Х18Н10, 12Х18Н9Т, 09Х14Н16Б, 08Х10Н20Т2, etc. These steels are ductile, easy to weld, have increased heat resistance, and are corrosion-resistant in many environments with moderate activity. Steel 12Х18Н10Т is the cheapest and therefore most often used.

For greater homogeneity, chromium-nickel steels are quenched at 1050-1100 ° C in water. In this case, σΒ = 50-60 kgf/mm2 and δ = 35-45% are obtained. These steels are strengthened by cold plastic deformation.

Additional alloying of chromium-nickel steels with molybdenum and copper increases their corrosion resistance and acid resistance (03Х16Н15МЗ, 03Х17Н14М2). Sometimes titanium and aluminum are introduced into these steels in small quantities, which, forming dispersed intermetallic compounds of the Ni3(Ti,Al) type, strengthen the austenite (08Х17Н13М2Т, 08Х17Н15МЗТ).

Steel 06ХН28МДТ (0.06% C; 22-25% Cr; 26-29% Ni; 2.5-3% Mo; 2.5-3.5% Cu and 0.5-0.3% Ti) has high corrosion resistance, it is used in highly aggressive environments (diluted sulfuric acid, etc.). This steel, after quenching at 1100″C in water, has an austenite structure with a small amount of carbonitrides. After short-term heating to 500-900° C, it does not show a tendency to MCC.

Nickel is a fairly expensive and scarce metal, so stainless steels are created with a lower nickel content. To do this, other austenite-forming elements are introduced into the composition of stainless steels, for example, manganese and even nitrogen (steels 10Х14Г14Н4Т, 15Х17AG14, 10Х14AG15, etc.).

Austenitic-martensitic steels (transitional steels) have lower corrosion resistance compared to austenitic steels, but exceed them in strength (σΒ = 120-130 kgf/mm2). Transitional class steels include steels 09X15N8Yu, 09X17N7Yu, 08X17N5MZ, 20X13N4G9, etc.

The heat treatment regime of these steels is characterized by great complexity: hardening, cold treatment, tempering - aging. In Fig. Figure 3 shows the effect of various types of heat treatment on the strength of stainless steels of various classes. Transitional class steels receive the greatest strengthening. Such steels are used to create lightweight structures with high resistance to corrosion damage.

Rice. 3. The influence of heat treatment on the strength of stainless steels:

1 - hardening; 2 - hardening and cold treatment; 3- hardening, cold treatment, tempering (aging)

Austenitic-ferritic steels are proposed as substitutes for chromium-nickel steels of the X18N8 type in order to save nickel. This class includes steels 12Х21Н5Т and 08Х22Н6Т. Austenitic-ferritic steels at room temperatures have higher strength and hardness than steel type 18-8, but their ductility and toughness are lower. These steels do not have stable properties: their properties depend on the ratio of ferrite and austenite phases, which in turn depends on the total influence of ferrite-forming (Cr, Ti, Mo, Si) and austenite-forming (Ni, N2, C) elements. With an increase in the amount of ferrite, the heat resistance of steels decreases, the strength increases, and the ductility decreases, but not below 30%. Good technological properties are obtained with the ratio Φ:A=1:1.

Steel 15Kh28AN, which has good mechanical properties, also belongs to this class of steels.

(σΒ = 65–70 kgf/mm2, δ = 11–23%), including in the weld.

Typical heat treatment of austenitic-ferritic steels: hardening from 1000-1150° C and tempering - aging at 500-750° C.

Austenitic-ferritic steels are not subject to stress corrosion cracking: cracks can only occur in the austenitic areas, but the ferritic areas retard their development.

Stainless steels exhibit a special type of corrosion called intergranular corrosion (sometimes also called intergranular corrosion). Such corrosion occurs mainly along grain boundaries and is very dangerous because it does not have any external signs - the metal even retains a metallic luster. At the same time, the strength drops catastrophically, the metallic sound disappears, the metal is so easily destroyed that it can be turned into powder. Intercrystalline corrosion (ICC) develops if a stainless steel product, after hardening, is heated to 500–700° C or if slow cooling is carried out in this temperature range. At the same time, a network of chromium carbides is clearly visible in electron micrographs.

The causes of ICC have been studied for many years and there are several theories explaining the causes of this dangerous phenomenon.

The most accepted is the so-called “impoverishment theory.” It is known that the grain boundary is a transition zone between them.

If the penetration of a dissolved impurity into the intergranular zone reduces the excess energy of the boundaries, the concentration of this impurity in the zone increases. It has been established that carbon reduces the excess energy of boundaries, therefore intercrystalline internal adsorption of carbon occurs along the grain boundaries of stainless steel. Thus, already during quenching, carbon atoms are nonuniformly distributed in the solid solution, their concentration along the boundaries is greater than in the grain. Although chromium carbides are not formed in this case, such an increased concentration of carbon is, as it were, a preparation for their rapid formation. When heated to 500-700° C, chromium carbides Cr23C6 are formed along the grain boundaries. At these temperatures, the diffusion of carbon in solid solution to grain boundaries proceeds faster than that of chromium. Therefore, the formation of carbides consumes not only the carbon reserve present there, but also the carbon diffusing from inside the grains. At the same time, chromium, necessary for the formation of carbides, comes, at the first stages of the process, from the boundaries or from the boundary zones of austenite. As a result, the chromium content in the border zones of grains becomes less than 13% (even up to 6.5%) and they lose their corrosion resistance.

Due to the great danger of the MCC phenomenon, all smelted stainless steels must be checked for susceptibility to this type of corrosion. In this case, samples made of hardened steel are subjected to provocative tempering for an hour at 650° C. After this, the samples are boiled in an aggressive environment and the presence of MCC is determined.

The tendency to MCC in stainless steels can be eliminated: 1) by reducing the carbon content (in steels containing 0.02% C, MCC is not observed); 2) the introduction of elements - titanium or niobium stabilizers, which have a greater affinity for carbon than chromium; 3) using stabilizing annealing (heating the product to 850°C).

Rice. 4. Microstructure of chromium-nickel stainless steel 08Х18Н9 without MCC (a) and with MCC. (b)

When welding in the heat-affected zone, the metal can heat up to dangerous temperatures (500-700 ° C). Therefore, if the steel is prone to MCC, then welded products should not be made from it, or after welding it is necessary to carry out heat treatment, at least annealing to 650°C. In Fig. Figure 4, α shows the microstructure of stainless steel 08Х18Н9 after heat treatment (quenching at 1100° C in water), heating at 650° for an hour and boiling in sulfuric acid for 48 hours (Fig. 4, b).

CORROSION-RESISTANT ALLOYS AND CAST IRONS

In addition to stainless steels, other corrosion-resistant alloys are also used in industry.

For particularly aggressive environments, nickel-based alloys such as Hastelloy (NIMO alloys) are used. The nickel content in these alloys reaches 80%. The second element present in these alloys in large quantities is molybdenum (15-30%). The composition of some alloys is given in table. 1.

Table 1.

Chemical composition (%) of acid-resistant nickel-based alloys such as Hastelloy

These alloys have very high corrosion resistance in environments where, apart from them, only a few metals are stable (for example, in boiling phosphoric acid up to a concentration of 50%, in boiling hydrochloric acid up to 20%, etc.).

Hastelloy alloys have high mechanical properties, which can be improved by heat treatment - hardening + aging at 800° C. In this case, σв = 120 kgf/mm2 and hardness ΗВ = 360.

The disadvantage of alloys is their tendency to MCC, so the carbon content in them should be minimal.

Corrosion-resistant cast irons are resistant to many aggressive environments (and not only oxidizing ones). They are usually heat resistant. Alloy cast irons are cheaper than stainless steels and have good casting properties, so products made from them are produced by casting methods. The chemical composition and properties of acid-resistant cast iron are given in GOST 2176 and GOST 2233 (Table 2).

Chromium cast irons contain 26-36% Cr. The structure of chromium cast iron is a solid solution of chromium ferrite and eutectic carbides. Carbides can also be in a free state, and Cr7C3 carbides are predominantly formed. Chromium cast irons (X34) have high hardness (HB 325-400), resist wear well, but are difficult to machine. Alloys 25X18L and 30X20L are related to steel in terms of carbon content, and cast iron in terms of properties. Their casting and mechanical properties are better than those of X28 and X34, they are less prone to the formation of hot cracks

Table 2.

Chemical composition (%) of corrosion-resistant cast irons and cast steels

* Contains 3.5-4.5 Mo.

** Alloy SChShch-1 contains 0.8-1% Ni, alloy SChShch-2 0.4-0.5% Ni.

Chromium cast irons are resistant to oxidizing environments: in nitric acid of any concentration at 20°C and 40% boiling; in concentrated sulfuric acid and other media. Scale resistance is maintained up to 1000-1100° C.

Chromium cast iron is used to make parts and equipment for the nitrogen industry, artificial fertilizers, dies, etc. They are also used as heat-resistant materials - for the manufacture of furnace equipment, grates, combs and blades in furnaces intended for firing.

Silicon cast irons are acid-resistant alloys. Silicon, like chromium, expands the range of existence of ferrite and alloys containing up to 14.5% Si have the structure of a homogeneous solid solution. The carbon content in silicon cast iron is only 03-0.8%; with a higher content, carbon can be released in the form of graphite. Cast irons are smelted with a silicon content of up to 18%, since at a higher content these alloys become brittle and cannot be used. With a sharp change in temperature, cracking is possible. In oxidizing environments, a strong SiO2 film is formed on the surface of products, which is restored in case of mechanical damage.

Products from silicon cast iron are made only by casting, without subsequent mechanical processing (only grinding is possible).

The Φ15 alloy, also called "antichlor", contains 3.5-4.5% Mo. As a result of the addition of molybdenum, the alloy is stable in 10-30% solutions of hydrochloric acid (up to 90 ° C).

Silicon cast iron is used to make centrifugal pumps, acid sprayers, taps, boilers, vats, etc. All silicon cast iron has high oxidation resistance.

Nickel cast irons contain ~1% Ni (see Table 2). These cast irons are resistant to molten salts and concentrated alkali solutions. With increasing nickel content, the corrosion resistance of cast iron increases. The composition of nickel cast iron can be more complex: nickel-silicon austenitic cast iron contains,%: 1.7-2 C; 1.8-3 Cr; 5–7 Si and 16–20 Ni; nickel copper 2-2.8 C; 3-4 Cr; 5-8 Cu; 1.5-1 Si and 12-5 Ni.

Properties of stainless steel

How are corrosion-resistant properties achieved? — Thanks to the addition of additional chemical elements at the metal production stage, an oxide film is formed on the surface, which does not dissolve, but, on the contrary, protects the alloy itself from the effects of corrosion.

The main properties of stainless steel also include:

• high strength
• high quality welded joints
• plastic
• long service life with preservation of its properties

Nickel (nickel-based alloys) and iron-nickel (iron-nickel-based alloys) can be used as the base metal.

The introduction of various alloying elements adds certain properties to the original metal:

• chromium increases the corrosion resistance, hardness and strength of the alloy; reducing the linear expansion coefficient simplifies the welding process
• Nickel additionally increases viscosity, ductility, hardenability and reduces the coefficient of thermal expansion, which allows the use of a product made from such an alloy with sulfuric, hydrochloric and phosphoric acids
• manganese in a percentage of more than 1% helps to increase durability, hardenability, hardness and wear resistance (can be partially replaced with nickel)
• titanium increases the strength of steel and its density, which provides high corrosion-resistant properties
• molybdenum increases elasticity, anti-corrosion properties, increases tensile strength and resistance to high temperatures
• Niobium provides low corrosion in welded products
• vanadium increases the strength, density and hardness of the alloy
• tungsten increases hardness and reduces brittleness during heat treatment (tempering) due to the formation of hard carbide compounds with other elements
• silicon in a percentage of more than 1% increases heat resistance, elasticity, scale resistance and acidity, and also increases electrical resistance and strength with the same viscosity parameters
• cobalt helps to increase impact resistance and improve heat-resistant properties
• copper gives the alloy high resistance to atmospheric corrosion
• aluminum helps reduce metal aging and also increases impact strength and fluidity

Range of products made from stainless steel

Corrosion-resistant steel is used to produce the following products:

• heat-treated etched and polished sheets;
• heat-treated unetched sheets;
• thermally untreated and unetched sheets;
• warm, cold and hot-deformed seamless pipes;
• hot-rolled steel strips for general purposes;
• calibrated hexagons;
• stainless steel circles;
• stainless steel wire (heat-treated and cold-drawn);
• castings with special properties;
• forgings;
• other types for which GOSTs and technical instructions (TU) have been developed.