It is known that metals with a high degree of purity (99.99 and more percent of a pure substance) have low strength, which makes it difficult to use them. Exceptions are aluminum and copper, used in electrical engineering. Steel, in connection with their functional, must have rigidity, wear resistance, hardness, and also in some cases, plasticity and elasticity, so iron in its pure form is not suitable for their creation.
The alloy steels from ordinary are distinguished by the presence of artificially introduced additives, which predetermine certain properties of the future alloy. So, ordinary carbon steel contains in some or other proportions the "grains" of ferrite, cementite and perlite. When introducing alloying elements , the amount of carbon in the perlite is most often reduced (the strength of steel increases).
The alloyed steels, due to the introduction of additional substances, often have a distorted crystal lattice, which can provide additional viscosity (when grinding the perlite and ferrite grains), reduce internal stress, reduce the probability of cracking during quenching, or increase the calcination depth of the material,
Properties of alloy steel directly depend on additional components. For example, elements of chromium and nickel save metal parts from corrosion, manganese increases resistance to shocks, increases wear resistance and hardness. Such an element as silicon allows the products to resist the effects of acids better, and cobalt increases the heat resistance.
The alloyed steels are divided into high-, medium- and low-alloy steels (the content of additives is more than 10%, 2.5-10% and less than 2.5%, respectively). In medium order, medium-alloy steels (additions of about 5-6%) of pearlite structure are produced. The remaining structural compositions of alloys (martensitic, carbide, austenitic, ferritic) are less common.
For materials of this type, as for other industrial products, there is a GOST. Alloyed steels are classified according to state standards No. 4543 - 71, from which it is possible to find out the number of additional components in the steel of a particular brand. For example, a chromo-manganese nickel alloy with titanium and molybdenum of 25HGNMT sample contains up to 0.29% of carbon, up to 0.37% of silicon, up to 0.8% of manganese, up to 0.6% and 1.10% of chromium and nickel ), Up to half a percent of molybdenum and up to 0.09 percent of titanium. In addition to the assortment and technical requirements, GOST contains complete data on methods of product testing, acceptance, transportation, packaging, etc.
The alloyed steels are also divided into several groups according to their purpose: structural (used in engineering, construction of bridges, wagons, oil and gas pipelines, springs, springs, etc.), instrumental (they are made with cutting tools, such as drills, files, saws, mills, etc. .) And steel special purpose, having high resistance against corrosion of the electrochemical type.