Heat resistant alloys. Special steels and alloys. Production and use of heat-resistant alloys

Modern industry can not be imagined without such material as steel. We encounter it almost at every step. By introducing various chemical elements into its composition, mechanical and operational properties can be significantly improved.

What is steel?

Steel is an alloy that has carbon and iron in its composition. Also such an alloy (the photo is located below) can have impurities of other chemical elements.

There are several structural states. If the carbon content is in the range of 0.025-0.8%, then the data are called pre-eutectoid and have perlite and ferrite in their structure. If the steel is hypereutectoid, then perlitic and cementite phases can be observed. A feature of the ferrite structure is a large plasticity. Cementite has considerable firmness. Perlite form both the previous phases. It can have a granular form (on the grains of ferrite there are inclusions of cementite, which have a circular shape) and lamellar (both phases are plate-like). If the steel is heated above the temperature at which polymorphic modifications occur, the structure changes to austenitic. This phase has an increased plasticity. If the carbon content exceeds 2.14%, then such materials and alloys are called cast iron.

Types of steel

Depending on the composition, the steel can be carbon and alloyed. Carbon content less than 0.25% is characteristic of low-carbon steel. If its quantity reaches 0.55%, then we can speak of a medium-carbon alloy. Steel, which in its composition has more than 0.6% carbon, is called high-carbon. If, as the alloy is manufactured, the technology implies the introduction of specific chemical elements, then this steel is called alloyed. The introduction of various components significantly changes its properties. If their number does not exceed 4%, then the alloy is low-alloyed. Medium- alloy and high-alloy steel has up to 11% and more than 12% inclusions, respectively. Depending on the sphere in which steel alloys are used, their types are distinguished: instrumental, structural and special steels and alloys.

Manufacturing technology

The process of smelting steel is quite laborious. It includes several stages. First of all, raw materials are needed - iron ore. The first stage involves heating up to a certain temperature. Oxidation processes occur. At the second stage, the temperature becomes much higher. The processes of carbon oxidation are more intense. Additional alloy enrichment with oxygen is possible. Unnecessary impurities are removed into the slag. The next step is to remove oxygen from the steel, since it significantly reduces the mechanical properties. This can be done by diffusion or precipitation. If the process of deoxidation does not occur, the resulting steel is called boiling. A calm alloy of gases does not secrete, oxygen is completely removed. Intermediate position is occupied by half-stable steel. The production of iron alloys occurs in open-hearth furnaces, induction furnaces, oxygen converters.

Alloying of steel

In order to obtain those or other properties of steel, special alloying substances are introduced into its composition. The main advantages of such an alloy are increased resistance to various deformations, the reliability of parts and other structural elements is significantly increased. During quenching, the percentage of cracks and other defects decreases. Often such a method of saturation with different elements is used to impart resistance to chemical corrosion. But there are a number of shortcomings. They require additional processing, there is a high probability of the appearance of flokens. In addition, the cost of the material also increases. The most common alloying elements are chromium, nickel, tungsten, molybdenum, cobalt. Their area of application is quite large. This is engineering, and the manufacture of parts of pipelines, power plants, aviation and much more.

The concept of heat resistance and heat resistance

The concept of heat resistance means the ability of a metal or alloy to retain all its characteristics when operating at high temperatures. In such an environment, gas corrosion is often observed. Therefore, the material must have and resistance to its action, that is, be heat resistant. Thus, the characteristics of alloys that are used at a significant temperature should include both of these concepts. Only then such steel will provide the necessary resource of work for parts, tools and other structural elements.

Features of heat-resistant steel

In cases where the temperature reaches high values, the use of alloys is required, which will not be destroyed and deformable. In this case, heat-resistant alloys are used. The operating temperature for such materials is above 500 ° C. Important moments characterizing such steel are high endurance, plasticity, which persists for a long time, as well as relaxation stability. There are a number of elements that can significantly improve the resistance to high temperatures: cobalt, tungsten, molybdenum. An obligatory component is also chrome. It does not affect the strength as much as it improves the hardness of the slag. Also, chrome prevents corrosion processes. Another important characteristic of alloys of this type is slow creep.

Classification of heat-resistant steels by structure

Heat-resistant and heat-resistant alloys are of a ferritic class, martensitic, austenitic and with a ferritic-martensitic structure. The first have in their composition about 30% chromium. After special treatment, the structure becomes fine-grained. If the heating temperature exceeds 850 ° C, the grains increase, and such heat-resistant materials become brittle. The martensitic class is characterized by a chromium content: from 4% to 12%. Also in minor quantities, nickel, tungsten and other elements may be present. Of these, parts of turbines, valves in cars are manufactured. Steels that have martensite and ferrite in their structure are suitable for operation at constant high temperatures and long-term operation. The chromium content reaches 14%. Austenite is obtained by introducing nickel into heat-resistant alloys. Steel with a similar structure have many brands.

Nickel-base alloys

Nickel has a number of useful properties. It has a positive effect on the machinability of steel (both hot and cold). If a part or tool is designed to work in an aggressive environment, doping with this element significantly increases the resistance to corrosion. Heat-resistant materials based on nickel are divided into the following groups: high-temperature and actually heat-resistant. The latter should also have minimum heat-resistant characteristics. Operating temperatures reach 1200ºС. In addition, chromium or titanium is added. It is characteristic that nickel- doped steel has a small amount of such impurities as barium, magnesium, boron, so the grain boundaries are more hardened. High-temperature alloys of this type are available in the form of forgings and rolled products. It is also possible to cast parts. The main area of their application is the manufacture of gas turbine components . Heat-resistant nickel-base alloys have up to 30% chromium in the composition. They are well suited to stamping, welding. In addition, the scale resistance is at a high level. This makes it possible to use them in gas pipeline systems.

Heat resistant steel alloyed with titanium

Titanium is introduced in a small amount (up to 0.3%). In this case, it increases the strength of the alloy. If its content is much higher, then some mechanical properties deteriorate (hardness, strength). But the plasticity at the same time increases. This facilitates the processing of steel. When introducing titanium together with other components, it is possible to significantly improve the heat-resistant characteristics. If there is a need to work in an aggressive environment (especially if the design implies welding), then doping with this chemical element is justified.

Cobalt alloys

A large amount of cobalt (up to 80%) goes to the production of materials such as heat-resistant and heat-resistant alloys, since it is rarely used in pure form. Its introduction increases plasticity, as well as resistance to work with high temperatures. And the higher it is, the higher the amount of cobalt introduced into the alloy. In some brands, its content reaches 30%. Another characteristic feature of such steels is the improvement of magnetic properties. However, due to the high cost of cobalt, its use is rather limited.

The effect of molybdenum on high-temperature alloys

This chemical element significantly affects the strength of the material at high temperatures. Particularly effective is its use together with other elements. It significantly increases the hardness of steel (even at a content of 0.3%). The strength limit also increases. Another positive feature, which is found in refractory alloys doped with molybdenum, is a large degree of resistance to oxidative processes. Molybdenum promotes grain refinement. The disadvantage is the difficulty of welding.

Other special steels and alloys

To perform certain tasks, materials that have certain properties are required. Thus, we can talk about the use of special alloys, which can be both doped and carbon. In the latter, the set of required characteristics is achieved due to the fact that the production of alloys and their processing takes place behind a special technology. Still special alloys and steel are divided into structural and tool. Among the main tasks for this type of materials can be identified the following: resistance to corrosion and wear processes, the ability to work in an aggressive environment, increased mechanical characteristics. This category includes both heat-resistant steels and alloys with a high operating temperature, and cryogenic steels that can withstand up to -296 ° C.

Tool steel

To produce tools in production, special tool steel is used. In view of the fact that the working conditions are different, the materials are also selected individually. Since the requirements for the tools are high enough, the characteristics of the alloys for their production are the same: they must be free of foreign impurities, inclusions, the deoxidation process is well carried out, and the structure is homogeneous. For measuring instruments, it is very important to have stable parameters and to counteract wear. If we talk about cutting tools, they work in conditions of elevated temperatures (there is heating of the edge), constant friction and deformation. Therefore, it is very important for them to maintain the initial hardness when heated. Another type of tool steel is high-speed. Basically, it is doped with tungsten. Hardness is maintained to a temperature of about 600 ° C. There are also stamped steels. They are designed for both hot and cold deformation.

The field of application of alloys for special purposes

The industries in which alloys with specific characteristics are used are many. In view of their improved qualities, they are indispensable in engineering, construction, oil industry. Heat-resistant and heat-resistant alloys are used in the manufacture of parts of turbines, spare parts for cars. Steels that have high corrosion properties are indispensable for the production of pipes, carburetor needles, discs, all kinds of chemical industry elements. Rails for the railway, buckets, caterpillars for transport - the basis for all this is wear-resistant steel. Alloys of automata are used in the mass production of bolts, nuts and other similar parts. Springs should be sufficiently resilient and wear-resistant. Therefore, the material for them is spring steel. To improve this quality, they are additionally alloyed with chromium, molybdenum. All special alloys and steels with a set of certain characteristics can reduce the cost of parts where previously used non-ferrous metals.