The purpose and classification of steel
These are the parameters characterizing the usefulness of steel in the economy. Their size depends on the alloy composition and its treatment. The following values are characteristic of steels used in construction.
Tensile strength is determined by the amount of stress caused in the cross-section of the sample by the force causing it to break. Other parameters determining the stress in steel samples, such as compressive strength, bending, shear, and torsion, are also tested. When testing steel samples at the break, the following are also determined: bursting stress, i.e. the actual value of stress at the point of narrowing of the stretched sample immediately before its breaking (this is the value of the force causing the breaking concerning the cross-section of the broken sample at its narrowest point); relative elongation, i.e. the percentage increase in the length of the broken sample concerning its initial length, relative narrowing, i.e. the percentage reduction in cross-sectional area of the broken specimen at the point of rupture to its original cross-section.
Resilience is understood as the ability of a material to recover its original form after cessation of deformation forces. Hooke's law applies to elastic stresses. The elasticity of the material is determined by:
- compression modulus (Young's modulus) E, which, for steel, has a value in the range from 205 to 210 GPa (Gigapascals)
- shear modulus G (Kirchhoff's modulus), which for steel is 80GPa.
Plasticity is the ability of the material to retain its form deformed as a result of stress from loads after cessation of their operation. These are permanent deformations that arise after exceeding the value of the so-called yield point, beyond which a significant increase in the elongation of the tensile sample occurs, even without an increase and often with a decrease in the value of the tensile force. The yield strength is conventionally assumed for the value of stress at which the permanent elongation of the sample is 0.2%.
Tenacity - the ability of a material to maintain its properties during processing consisting of its pressing, bending or straightening, etc. This property is used in the production of products (e.g. trapezoidal sheets, door frames, etc.).
Impact resistance is a resistance to dynamic loads.
Hardness is the ability to resist material when trying to push harder objects. The hardness of steel is related to the content of carbon, manganese, chromium, etc.
Weldability is a feature of steel that allows permanent joints to be made by welding.
Construction steels are divided into:
- general-purpose construction steel (structural steel for constructing structures and parts of general-purpose machinery and equipment, wherever its characteristics are sufficient to perform its function. General-purpose construction steels are the lowest grade steels and contain significant amounts of impurities, mainly sulfur, silicon and phosphorus in its production. A low technological regime is used, which affects the wide dispersion of carbon content. General-purpose steels are not heat treated. These steels are usually calmed, unless the lack of calming is indicated separately).
- higher quality construction steel (steel characterized by narrow limits of carbon and manganese content and low content of impurities, mainly silicon (below 0.7%) and phosphorus. Usually, they are supplied as solids and suitable for heat treatment).
- low-alloy construction steel (steel with low carbon content up to 0.22% having alloy additions in limited quantities. Low-alloy steels are used for the construction of structures exposed to weather conditions such as bridges, masts, railway wagons, etc. - wherever the application is economically justified. They are characterized by greater strength than higher quality structural steels and higher corrosion resistance).
Steel for carburizing - steel used for chemical carburizing. High quality, low-alloy, and alloy steels with low or medium (0.08 to 0.25%) carbon content are used for carburizing. Products made of such steel after carburizing retain high tenacity and resistance to core spreading and surface hardness.
Steel for nitriding - steel used for chemical treatment of nitriding. Low-alloy structural steels and alloy steels with aluminum content above 1.0% are used for nitriding. Products made of such steel after nitriding obtain high abrasion resistance.
Steel for toughening - steel used for heat treatment, thermal-improvement. Higher quality structural steels, low alloy, and alloy steels with carbon content 0.25 to 0.6% are used for thermal-improvement.
Spring steel - steel used for the production of springs, torsion bars, etc. Spring steels are carbon steels containing 0.6% - 0.85% carbon low alloy steels containing the addition of silicon, manganese, chromium, and vanadium. Most spring steels have increased silicon content. In these applications, it is a required additive to reduce steel tenacity. Free-machining steel - steel used for the production of small parts, e.g. bolts, nuts, washers, etc. Used for parts that are not subject to heavy loads. Such steel, supplied in the form of bars, is used in machines that, with minimal human supervision, require forming steel short and brittle shavings. This is ensured by the increased addition of sulfur to 0.35% and phosphorus to 0.15%. Sulfur, forming brittle sulfides with metals, most affects shavings breaking.
Steel for rolling bearings - steel for the production of rolling bearings. Elements of rolling bearings operating under extreme stress conditions require high-quality steel, produced in a particularly strict technological regime. Bearing steels require a narrow and strictly maintained tolerance of alloying elements and impurities, and an appropriate structure.
Transformer steel - steel with special magnetic properties, it is used for transformer sheets. There is no, or there is a little effect of eddy current and magnetostriction. Both factors are responsible for energy losses in the transformer, so-called power losses in iron. Transformer steel is low-alloy steel with the increased silicon content.
Tool steel - steel for the production of tools, measuring instruments and their holders. Tool steels are characterized by high hardness, wear resistance, low deformability and insensitivity to overheating. These features are achieved by the high carbon content and appropriate heat treatment for low-responsible tools, and the use of appropriate alloying additives combined with appropriate heat treatment for responsible tools.
Tool steel is divided into:
Carbon tool steels (tool steel that does not have a larger amount of alloying additions except from carbon, which content is in the range of 0.5% - 1.3%. Other features that distinguish tool carbon steels from structural steel are reduced manganese content and fineness. From most carbon tool steels shallow hardening is required. Cheaper deep hardening steel is used for less responsible tools. Shallow hardening is desirable because it ensures the hardness of the tool surface, while the core of the tool is impact resistant.)
Alloy tool steel:
- steel for cold work (tool alloy steel used for machining and plastic tools that can only heat up slightly during operation. This type of steel is also used for the production of measuring instruments. Cold work steel is required to keep its properties up to +200 ° C)
- steel for hot work (tool steel used for hot forming tools and for the construction of casting molds exposed to very high temperatures during operation. These steels are required to retain their properties up to + 600 ° C. This is achieved by using tungsten and molybdenum as alloying additives up to 8-10%, as is the case with WWV steel)
- high-speed steel (tool alloy steel used to produce tools for machining at high cutting speeds. they retain hardness and shape up to a temperature of + 600 ° C. This feature is achieved by using alloying elements - carbon 0.75-1.3%, chromium 3.5-5.0%, tungsten 6-19%, vanadium 1, 0-4.8%, molybdenum 3.0 to 10%, and in some grades also cobalt 4.5-10.0%, and appropriate heat treatment. During this time annealing is carried out, so alloy additions have formed compounds with carbon, so-called carbides, which must significantly dissolve in ferrite. This requires very careful and long-term processing).
Today, tool steels, especially those of high quality, are displaced by stellite and cemented carbides.
Special steel - steel intended for special applications. Special steels contain a large amount of alloying additives, require very complicated heat treatment and a high treatment and assembly regime. Due to the high price they are not widely used.
Stainless steel (weather resistant steel, dilute acids, alkaline solutions and the like. Stainless steel is obtained by increasing the chromium content. The higher the chromium content, the greater the corrosion resistance of steel. Usually, 12% to 25% chromium is used. Increased carbon content also increases stainless steel, but too high content makes the steel brittle. Stainless steels are heat treated, hardened and tempered. Stainless steels are used for tanks for petroleum products, swimming pool basins, rectification columns, installations in the coke industry, steam turbine blades, industrial and home fittings, medical tools, cutlery, installations in the food industry, rigging, and nautical fittings, etc.).
Acid-resistant steel (steel resistant to acids with lower power than sulfuric acid. Acid resistance is obtained thanks to the stabilization of austenite under normal conditions, which can be achieved due to the high content of chromium (17-20%) and nickel (8-14%), and other additives alloys such as manganese, titanium, molybdenum, and copper.
Acid-resistant steels (they are used after polishing. As alloying materials tend to combine with carbon to form hard carbides at high temperatures, they must be heat treated after welding of stainless steel. Acid-resistant steels are used for the construction of acid tanks and industrial installations, for their production and distribution, and other installations containing acids, e.g. in the dyeing industry, in the production of fertilizers, etc.).
Creep-resistant steel (a variety of heat-resistant steel, which is also required to maintain strength over a wide temperature range).
Heat resistant steel (steel resistant to high temperatures and working periodically or constantly under their influence. Heat resistance is obtained by a high content of chromium 5-30%, nickel 4-30% and significant amounts of molybdenum 0.5 to 1.0% and tungsten to 2% as alloying elements. High content of alloying elements allows obtaining an austenitic structure under normal conditions. The upper limit of heat resistance is 800 ° C to 1200 ° C depending on the composition of the steel. Elements of furnaces, steam boilers, fans for hot gases, carburizing boxes, gas turbine combustion chambers, and internal combustion piston valves).
Magnetic steel (steel with special magnetic properties. They are constantly possessing ferromagnetic properties used for permanent magnets. Very low carbon steel, so-called ferrite, is used for permanent magnets).
Wear-resistant steel (cast steel with very slow surface wear, so-called abrasion. It is high carbon steel containing 11 to 14% manganese. It is so hard that it cannot be machined. Finished parts are cast in molds, therefore is called steel, technically it is cast steel).
Hadfield's steel - (designation: X120Mn13) - steel with high resistance to abrasion. Contains 1.1-1.3% C and 12-13% Mn. Above 950 ° C Hadfield's steel has an austenitic structure. After cooling to room temperature, it is a mixture of ferrite and manganese cementite. This steel is characterized by a very high propensity to strengthen because, under the influence of crumple, it creates micro twins. The hardness of such steel is approx. 500 HB. Hadfield's steel strengthens during work. It is mainly used for elements particularly exposed to abrasion at high pressures: excavator baskets, tractor tracks, railway turnouts.
DUE TO THE TYPE AND SHARE OF ALLOY COMPONENTS:
Carbon steel - steel in which carbon is an element that shapes its characteristics. Carbon steel is widely used in the manufacture of structures and parts of mechanical equipment, wherever its characteristics are sufficient.
Carbon steels are divided into:
- ordinary quality carbon steels - used without further treatment
- higher quality carbon steels - often subjected to additional heat or chemical treatment.
Depending on the carbon content, we divide them into:
- low carbon steels - with carbon content up to about 0.3%
- medium-carbon steels - with 0.3 to 0.6% content
- high-carbon steels - with content over 0.6%.
Alloy steel - steel in which, apart from carbon, there are other alloying additives with the content of several to even tens of percent, significantly changing the characteristics of steel. Alloy additions are added to increase steel hardenability, achieve higher steel strength, change certain physical and chemical properties of steel. Alloy steels, usually very expensive, are used in special applications and it is economically justified.
Steel has been used in various fields of technology. In construction, it is one of the few basic construction materials.
Steels most commonly used in this field of the economy are low-alloy and general-purpose steels (also called unalloyed steels).
In the first group, the most popular are (markings according to PN-88 / H-84020) groups with the symbols St0S, St3S, and St4S.
In the second group there are constantly:
- increased strength (marked by PN-86 / H-84018) with symbols 18G2, 18G2A and 18G2AV;
- hard rust (marked according to PN-82 / H-84017) with symbols 10HA, 10H, 12HIJA, 12PJA;
- steels for pipe production (marked by PN-89 / H-84023.7) with symbols R, R35, R45, 12X. 18G2A;
- St3S steels are also used for pipe production.
THE FREQUENTLY USED ADDITIVES IN STEEL INCLUDE:
- nickel (reduces the austenitic transformation temperature and hardening speed. In practice, this facilitates the hardening process and increases the hardening depth. Nickel dissolved in the ferrite strengthens it, significantly increasing impact strength. The addition of nickel in the amount of 0.5% to 4% is added to the steel to improve warm, and in quantities of 8% to 10% for acid-resistant steel. In steel symbols, its addition is marked with the letter N).
- chromium (causes grain fragmentation. Increases steel hardenability. Increases its strength. Used in tools and special steels. In the latter even in amounts up to 30%. In steel symbols, its addition is marked with the letter H).
- manganese (reduces the temperature of austenitic transformation, and at content above 15% stabilizes and allows obtaining an austenitic structure at normal temperatures. Already at contents of 0.8% to 1.4% significantly increases tensile strength, impact, and abrasion. In steel symbols, its addition is marked with the letter G).
- tungsten (increases steel graininess, increases strength, abrasion resistance. A large addition of tungsten 8% to 20% increases steel tempering resistance. In steel symbols, its addition is marked with the letter W).
- molybdenum (increases the hardenability of steel. It increases strength and reduces brittleness and increases creep resistance. In steel symbols, its addition is marked with the letter M). * vanadium (increases the fine graininess of steel and significantly increases its hardness. In steel symbols, its addition is marked with the letter V (F).
- cobalt (increases the graininess of steel and significantly increases its hardness. In steel symbols, its addition is marked with the letter K)
- silicon (normally treated as an undesirable admixture, increases the fragility of steel. It becomes a desirable component in spring steels. Because it reduces energy losses current in steel, it is added in amounts up to 4% to transformer steel. In steel symbols, its addition is marked with the letter S)
- titanium (in steel symbols, its addition is marked with the letter T)
- niobium (in steel symbols, its addition is denoted by the letters Nb)
- aluminum (in steel symbols its addition is marked with the letter A)
- copper (it has similar physical properties as pure iron, but it is much more resistant to corrosion. Copper is a desirable additive and its content increases systematically with the use of scrap steel when smelting new steel. In steel symbols, its addition is marked with the letters Cu).
PARAMETERS DETERMINING THE CHARACTERISTICS OF STEEL AS A MATERIAL include PHYSICAL, MECHANICAL AND TECHNOLOGICAL CHARACTERISTICS.
Steel physical properties:
- density ρ = 7850 kg / m3
- coefficient of linear expansion αT = 0.000012 0C-1
- thermal conductivity coefficient λ = 58 W / mK
- Poisson's ratio ν = 0.30 resistivity (20 oC, 0.37-0.42% carbon) = 171 • 10-9 [Ω • m]