What is steel improvement?

Annealing of steels

According to the book definition, annealing is heating steel to a temperature above the critical temperature, holding it at this temperature and slowly cooling it along with the furnace. In fact, this is a general definition that does not cover all types of annealing. Annealing modes depend primarily on the final requirements for the steel or product, primarily the requirements for the mechanical or technological properties of the metal.

Annealing of the first kind (I-th kind)

Annealing of the first kind is a thermal operation consisting of heating a metal in an unstable state, obtained by previous treatments, to bring the metal into a more stable state.

This type of annealing may include processes of homogenization, recrystallization, hardness reduction and residual stress relief. The peculiarity of this type of annealing is that these processes occur regardless of whether phase transformations occur during heat treatment or not.

There are homogenization (diffusion), recrystallization annealing and annealing, which reduces stress and reduces hardness.

Homogenization annealing

Homogenization annealing is a heat treatment in which the main process is the elimination of the effects of dendritic and intracrystalline segregation in steel ingots. Liquation increases the susceptibility of steel processed by pressure to brittleness, anisotropy of properties and defects such as slate (layered fracture) and flakes.

Elimination of segregation is achieved through diffusion processes. To ensure a high diffusion rate, the steel is heated to high (1000–1200 °C) temperatures in the austenitic region. At these temperatures, long (10–20 hours) holding and slow cooling with a furnace are done. Diffusion processes are most active at the beginning of aging.

Therefore, in order to avoid a large amount of scale, cooling with a furnace is usually carried out to a temperature of 800 - 820 ° C, and then in air. During homogenization annealing, large austenite grains grow. You can get rid of this undesirable phenomenon by subsequent pressure treatment or heat treatment with complete recrystallization of the alloy.

Leveling the composition of steel during homogenization annealing has a positive effect on mechanical properties, especially ductility.

Recrystallization annealing of steel

Recrystallization annealing, used for cold worked steels, is a heat treatment of a deformed metal or alloy. Can be used as a final or intermediate operation between cold forming operations.

The main process of this type of annealing is recovery and recrystallization, respectively. Return refers to all changes in the fine structure that are not accompanied by changes in the microstructure of the deformed metal (the size and shape of the grains do not change). The return of steels occurs at relatively low (300–400°C) temperatures.

During this process, restoration of crystal lattice distortions is observed.

Recrystallization is the nucleation and growth of new grains with fewer defects in the crystal structure. As a result of recrystallization, completely new, most often equiaxed crystals are formed. There is a simple relationship between the temperature threshold of recrystallization and the melting temperature: TR ≈ (0.3–0.4) TPL., which is 670–700°C for carbon steels.

Stress Relief Annealing

Stress relief annealing is a heat treatment in which the main process is complete or partial relaxation of residual stresses.

Such stresses arise during pressure or cutting processing, casting, welding, grinding and other technological processes. Internal stresses remain in parts after the end of the technological process and are called residual.

You can get rid of unwanted stresses by heating steels from 150 to 650°C, depending on the grade of steel and the method of previous processing.

High annealing steel

This operation is often called high tempering. After hot plastic deformation, the steel has a fine grain and a satisfactory microstructure. Steel obtains this state during accelerated cooling after plastic deformation.

However, the structure may contain components: martensite, bainite, troostite, etc. The hardness of the metal can be quite high. To increase ductility and, accordingly, reduce hardness, high annealing is done.

Its temperature is below the critical Ac1 and depends on the requirements for the metal for the next processing operation.

Annealing of the second kind (ΙΙ-th kind)

Annealing of the ΙΙ kind is based on the use of phase transformations of alloys and consists of heating above the transformation temperature, followed by slow cooling to obtain a stable structural state of the alloys.

Full annealing

Full annealing is carried out for hypoeutectoid steels. To do this, the steel part is heated above the critical point A3 by 30–50°C and after heating, slow cooling is carried out. As a rule, parts are cooled together with the furnace at a rate of 30–100°C/hour. The structure of hypoeutectoid steel after annealing consists of excess ferrite and pearlite.

The main goals of full annealing are:

— elimination of structural defects that arose during previous processing (casting, hot deformation, welding, heat treatment), such as coarse grain and Widmanstätten structure;

- softening of steel before cutting - obtaining coarse grain to improve surface quality and greater fragility of low-carbon steel chips;

- reduction of stress.

Partial annealing

Incomplete annealing differs from complete annealing in that heating is carried out 30–50 °C above the critical point A1 (line РСК on the “Iron - cementite” diagram). Incomplete annealing of hypoeutectoid steels is carried out to improve cutting machinability.

With incomplete annealing, partial recrystallization of the steel occurs due to the transition of pearlite to austenite. Excess ferrite is only partially converted to austenite.

Such annealing is carried out at a temperature of 770 - 750 ° C, followed by cooling at a rate of 30 - 60 ° C / s to 600 ° C, then in air.

Partial annealing is widely used for hypereutectoid carbon and alloy steels. Heating these steels by 10 - 30°C above Ac1 causes almost complete recrystallization of the alloy and makes it possible to obtain a granular (spherical) form of pearlite instead of a lamellar one. This annealing is called spheroidization.

Particles of cementite that did not dissolve during heating, or areas of austenite with an increased carbon concentration due to its incomplete homogenization after cementite dissolution, serve as crystallization centers for cementite, which is released upon subsequent cooling to a temperature below A1 and in this case takes on a granular form.

As a result of heating to a temperature well above A1 and the dissolution of most of the cementite and more complete homogenization of austenite, the subsequent precipitation of cementite below A1 occurs in lamellar form.

If excess cementite was in the form of a network, then before this annealing it is necessary to carry out normalization with heating above Acm (preferably with cooling in a directed air flow).

Steels close to the eutectoid composition have a narrow heating temperature range (750 - 760°C) for annealing to granular cementite; for hypereutectoid steels, the range expands to 770 - 790°C. Alloyed hypereutectoid steels can be heated to higher temperatures of 770 - 820°C. Cooling and spheroidization of cementite occurs slowly. Cooling should ensure the decomposition of austenite into a ferrite-carbide structure, spheroidization and coagulation of the resulting carbides to 620 - 680°C.

Annealing on granular pearlite (pendulum annealing)

To obtain granular pearlite, annealing is used with various variations of thermal cycling in the supercritical and intercritical temperature range, pendulum types of annealing with different shutter speeds and number of cycles.

Steel with granular pearlite has lower hardness, tensile strength and, accordingly, higher values ​​of plasticity characteristics. For example, eutectoid steel with lamellar pearlite has a hardness of 228HB, and with granular pearlite it has a hardness of 163HB and, accordingly, a tensile strength of 820 and 630 MPa, a relative elongation of 15 and 20%.

The microstructure of steel after annealing to granular pearlite (GP) looks like this:

After annealing to granular pearlite, steels have the best machinability and a higher surface finish is achieved. In some cases, annealing for granular pearlite is a mandatory preliminary operation. For example, to avoid crack formation when setting bolts and rivets.

Isothermal annealing

Isothermal annealing consists of heating the steel to a temperature Ac3 + (30–50°C), subsequent accelerated cooling to an isothermal holding temperature below point A1 and further cooling in still air. Isothermal annealing has two advantages over conventional annealing:

- greater time gain, since the total time of accelerated cooling, holding and subsequent cooling may be less than the slow cooling of the product together with the furnace;

— obtaining a more homogeneous structure over the cross-section of the product, since during isothermal holding the temperature across the cross-section of the product is equalized and the transformation throughout the entire volume of steel occurs at the same degree of supercooling.

Patenting

Patenting is an annealing operation, usually assigned to spring wire, with a carbon content of 0.65 - 0.9%, before drawing.

The process consists of austenitizing the metal and then passing it through molten salts at a temperature of 450 - 550 ° C (in DIPA these are isothermal holding temperatures in the region of minimum stability of austenite).

This leads to the formation of thin-plate troostite or sorbitol, which makes it possible to obtain reduction rates of more than 75% for drawing and a final tensile strength of 2000 - 2250 MPa after CPD.

Normalization annealing (steel normalization)

Normalization annealing or normalization of steel is used as an intermediate operation to soften the steel before cutting and to generally improve its structure before hardening. During normalization, hypoeutectoid steel is heated to temperatures Ac3 + (30–50°C), hypereutectoid steel to Acm + (30–50°C) and after holding it is cooled in still air.

Accelerated cooling compared to annealing causes a slightly greater undercooling of austenite, therefore, upon normalization, a finer eutectoid structure (fine pearlite or sorbitol) and a smaller eutectoid grain are obtained.

The strength of steel after normalization is slightly higher than after annealing. In hypereutectoid steel, normalization eliminates the coarse network of secondary cementite. When heated above the Acm point, secondary cementite dissolves, and with subsequent accelerated cooling in air it does not have time to form a coarse network, which reduces the properties of steel. In hypoeutectoid steel, as mentioned above, normalization makes it possible to eliminate large grains after overheating and Widmanstätt after a violation of the GPD cycle.

Source: https://HeatTreatment.ru/otzhig-stalej

Products – Tekhmashholding – group of companies, official website

There are more than two dozen alloying elements used in steels. Here we look at the effects on steel of the most common (often unavoidable) alloying elements - carbon, manganese and silicon.

The influence of carbon on the properties of steels

Carbon is the main strengthening element in all steels except austenitic stainless steels and some other high-alloy steels. The strengthening effect of carbon consists of solid solution strengthening and strengthening due to dispersed precipitation of carbides. With increasing carbon content in steel, its strength increases, but ductility and weldability decrease. Carbon has a moderate tendency to macrosegregate during crystallization. Macrosegregation of carbon is usually more significant than that of all other alloying elements. Carbon has a strong tendency to segregate at defects in steels such as grain boundaries and dislocations. Carbide-forming elements can react with carbon to form “alloyed” carbides.  

The influence of manganese on the properties of steels

Manganese is present in almost all steels in amounts of 0.30% or more. Manganese is used to remove oxygen and sulfur from steel. It has less tendency to segregate than any other alloying element. Manganese has a beneficial effect on surface quality over the entire carbon content range, with the exception of very low carbon steels, and also reduces the risk of red brittleness. Manganese has a beneficial effect on the ductility and weldability of steels. Manganese does not form its own carbide, but only dissolves in cementite and forms alloyed cementite in steels. Manganese promotes the formation of austenite and therefore expands the austenite region of the phase diagram. High manganese content (more than 2%) leads to an increased tendency to cracking and warping during hardening. The presence of manganese in steels encourages impurities such as phosphorus, tin, antimony and arsenic to segregate to the grain boundaries, causing temper brittleness.

The influence of silicon on the properties of steels 

Silicon is one of the main deoxidizing agents used in steel smelting. Therefore, the silicon content determines the type of steel produced. Mild carbon steels can contain silicon up to a maximum of 0.60%. Semi-quiet steels may contain moderate amounts of silicon, for example 0.10%. Silicon is completely dissolved in ferrite at silicon contents up to 0.30%. It increases the strength of ferrite without almost reducing its ductility. When the silicon content is above 0.40% in general purpose carbon steel, there is a significant decrease in ductility. In combination with manganese or molybdenum, silicon provides higher hardenability of the steel. The addition of silicon to chromium-nickel austenitic steels increases their resistance to stress corrosion. In thermally hardenable steels, silicon is an important alloying element; it increases the ability of steels to be thermally hardened and their wear resistance, increases the elastic limit and yield strength. Silicon does not form carbides and does not contain cementite or other carbides. It dissolves in martensite and slows down the decomposition of alloyed martensite up to 300 °C.

Source: https://pellete.ru/stal/na-chto-vliyaet-kremnij-v-stali.html

What kind of processing of steel products is called improvement?

Heat treatment (heat treatment) of steel, non-ferrous metals is the process of changing the structure of steel, non-ferrous metals, alloys during heating and subsequent cooling at a certain speed.
Heat treatment (heat treatment) leads to significant changes in the properties of steel, non-ferrous metals, and alloys. The chemical composition of the metal does not change.

Annealing is a thermal treatment (heat treatment) of a metal that involves heating the metal and then slowly cooling it. This heat treatment (i.e. annealing) comes in different types (the type of annealing depends on the heating temperature and the cooling rate of the metal).

Hardening

Hardening is a heat treatment (heat treatment) of steel and alloys, based on the recrystallization of steel (alloys) when heated to a temperature above critical; After sufficient exposure to the critical temperature to complete the heat treatment, rapid cooling follows. Hardened steel (alloy) has a nonequilibrium structure, so another type of heat treatment is applicable - tempering.

Vacation

Tempering is a heat treatment (heat treatment) of steel and alloys, carried out after hardening to reduce or relieve residual stresses in steel and alloys, increasing toughness, reducing the hardness and brittleness of the metal.

Normalization

Normalization is a heat treatment (heat treatment) similar to annealing. The differences between these heat treatments (normalization and annealing) are that during normalization the steel is cooled in air (when annealing, it is cooled in a furnace).

Heating the workpiece is a critical operation. The quality of the product and labor productivity depend on the correctness of its implementation.

You need to know that during the heating process the metal changes its structure, properties and characteristics of the surface layer and as a result of the interaction of the metal with atmospheric air, scale is formed on the surface; the thickness of the scale layer depends on the temperature and duration of heating, the chemical composition of the metal. Steels oxidize most intensively when heated above 900°C; when heated to 1000°C, oxidation increases 2 times, and at 1200°C - 5 times.

Chrome-nickel steels are called heat-resistant because they practically do not oxidize.

Alloy steels form a dense, but not thick layer of scale, which protects the metal from further oxidation and does not crack during forging.

When heated, carbon steels lose carbon from a surface layer of 2-4 mm. This threatens the metal with a decrease in the strength and hardness of the steel and hardening deteriorates. Decarburization is especially harmful for small-sized forgings followed by hardening.

Carbon steel blanks with a cross-section of up to 100 mm can be quickly heated and therefore they are placed cold, without preheating, in a furnace where the temperature is 1300°C. To avoid cracks, high-alloy and high-carbon steels must be heated slowly.

When overheated, the metal acquires a coarse-grained structure and its ductility decreases. Therefore, it is necessary to refer to the iron-carbon diagram, which defines the temperatures for the start and end of forging.

However, overheating of the workpiece can, if necessary, be corrected by heat treatment, but this requires additional time and energy.

Heating the metal to an even higher temperature leads to burnout, which disrupts the bonds between grains and such metal is completely destroyed during forging.

Burnout is an irreparable marriage. When forging products from low-carbon steels, less heating is required than when forging a similar product from high-carbon or alloy steel.

When heating metal, it is necessary to monitor the heating temperature, heating time and temperature at the end of heating. As the heating time increases, the scale layer grows, and with intense, rapid heating, cracks may appear. It is known from experience that on charcoal a workpiece 10-20 mm in diameter is heated to forging temperature in 3-4 minutes, and workpieces with a diameter of 40-50 mm are heated for 15-25 minutes, monitoring the color of the heat.

Chemical thermal treatment (CHT) of steel is a set of heat treatment operations involving saturation of the surface of the product with various elements (carbon, nitrogen, aluminum, silicon, chromium, etc.) at high temperatures.

Surface saturation of steel with metals (chromium, aluminum, silicon, etc.), which form substitutional solid solutions with iron, is more energy-intensive and longer lasting than saturation with nitrogen and carbon, which form interstitial solid solutions with iron. In this case, the diffusion of elements proceeds more easily in the alpha-iron lattice than in the more densely packed gamma-iron lattice.

Chemical-thermal treatment increases hardness, wear resistance, cavitation, and corrosion resistance. Chemical-thermal treatment, creating favorable residual compressive stresses on the surface of products, increases reliability and durability.

Steel cementation is a chemical-thermal treatment of low-carbon steel (C

Source: https://steelfactoryrus.com/kakaya-obrabotka-stalnyh-izdeliy-nazyvaetsya-uluchsheniem/

What is steel normalization and a description of this process

Often, for production purposes, it becomes necessary to change the parameters of steel; one way to do this is heat treatment . By their principle, most heat treatment technologies involve changing the structure of steels through heating, holding and cooling.

Despite the fact that all these technologies have the same goals and operating principles, they all differ in temperature and time conditions. Heat treatment can be either an intermediate or a final process during production. In the first case, the material is prepared for subsequent processing, and in the second, new properties are given to it.

One such technology is steel normalization. This is the name for heat treatment, in which the material is heated to a temperature 30-50 degrees above Ast or Ac3, and then it is cooled in still air.

Principles of normalization

Like other heat treatment technologies, normalization can be either an intermediate or a final operation to improve the structure of steel. Most often it is used in the first case; as a final procedure, normalization is mainly used in the production of long products such as rails, channels and more.

The key feature of normalization is that the steel is heated to a temperature that is 30-50 degrees higher than the upper critical values, and the material is also held and cooled.

This or that temperature is selected depending on the type of material. Hypereutectoid materials are normalized at a temperature between points Ac 1 and Ac 3, but hypoeutectoid materials are normalized at temperatures above Ac 3. As a result, materials of the first type obtain the same hardness, since carbon passes into the solution in the same amount, and austenite is also fixed in the same amount. The structure includes cement and martensite.

Thanks to this composition, the wear resistance and hardness of the hypereutectoid material increases. If high-carbon steel heats up above Ac 3, the growth of austenite grains will increase and, accordingly, internal stresses will increase. The carbon concentration will also increase, and as a result, the martensitic transformation temperature will decrease. As a result, the material becomes less durable and hard and can be changed.

And hypoeutectoid steel, when heated above a critical value, becomes very viscous. This is explained by the fact that fine-grained austenite is formed in low-carbon steel.

This component, after cooling, transforms into fine-crystalline martensite.

Temperature values ​​in the interval between Ac 1 and Ac 3 cannot be used for processing, since in this case the structure of hypoeutectoid steel receives ferrite, which reduces its hardness after normalization, and after tempering, its mechanical properties.

The holding time depends on the degree of homogenization of the material structure. The standard indicator is an hour of exposure per 25 mm of thickness. The intensity of cooling to one degree or another determines the size of the plates and the amount of perlite.

These quantities are interdependent. Even more perlite will form with increasing cooling intensity, reducing the distance between the plates and their thickness. All this increases the hardness and strength of the normalized material. Due to the low cooling intensity, a material with less hardness and strength is formed.

If objects with large differences in cross-section are processed, then the thermal stress must be reduced to prevent warping during heating or cooling. Also, before starting work, they should be heated in a salt bath.

When the temperature of the product decreases to the lower critical point, cooling can be accelerated by placing it in water or oil.

Purpose of the process

Normalization is designed to change the microstructure of steel; it does the following:

• reduces internal stress;
• through recrystallization, it refines the coarse-grained structure of welds, castings or forgings.

The goals of normalization can be completely different. Using this process, the hardness of steel can be increased or decreased, the same applies to the strength of the material and its toughness. It all depends on the mechanical and thermal characteristics of the steel. Using this technology, it is possible to both reduce residual stresses and improve the degree of machinability of steel using one or another method.

Steel castings are subjected to this treatment for the following purposes:

• to homogenize their structure;
• to increase susceptibility to heat hardening;
• to reduce residual stresses.

Products obtained by forming are subjected to normalization after forging and rolling in order to reduce the heterogeneity of the structure and its banding.

Normalization together with tempering is needed to replace the hardening of products with complex shapes or with sharp changes in cross-section. This will prevent defects.

This technology is also used to improve the structure of the product before hardening, increase its machinability through cutting, eliminate the secondary cement network in hypereutectoid steel, and also prepare the steel for final heat treatment.

This steel is an alloy of iron and carbon. Steel grade 45 due to its hardness is traditionally in high demand in various industrial sectors. In this alloy, the share of iron is about 45 percent.

The properties of a material are directly related to its alloying elements and the amount of carbon, which is very important in the production of rolled metal products. This or that temperature treatment allows you to obtain a durable product.

After normalization, the hardness of grade 45 is directly related to the temperature during operation.

This steel is carbon structural steel. Normalization should be carried out outdoors, and not in a special oven, unlike other stages of processing. Grade 45 is easily and quickly amenable to mechanical processing, in particular:

• drilling;
• turning;
• milling.

The following products are produced on the basis of this steel:

• bandages;
• cams;
• cylinders;
• gears;
• crankshafts and camshafts;
• gear shaft;
• spindles.

Other heat treatment methods

In addition to normalization, heat treatment of steel includes the following processes:

• annealing;
• hardening;
• vacation;
• cryogenic treatment;
• dispersion hardening.

The principle of implementation and goals of each technology are the same, however, each has its own distinctive features:

• annealing - thanks to it, the structure of pearlite will be as thin as possible, since cooling occurs in the furnace. Annealing can reduce structural heterogeneity, as well as stress after processing by casting or injection, give the structure a fine grain or improve cutting;
• hardening - the technology principle is the same, but the temperatures are higher compared to normalization and the cooling rate is also higher. The process occurs in liquids. Thanks to hardening, the strength and hardness of the material increases, and the parts will eventually have low impact strength and fragility;
• Tempering - Tempering done after hardening reduces stress and brittleness. For this purpose, the material is heated to a low temperature and cooled outside. As the temperature rises, tensile strength and hardness fall, and impact strength increases;
• cryogenic treatment - thanks to it the material will have a uniform structure and hardness; this technology is most suitable for hardened carbon steel;
• dispersion hardening is a final treatment during which dispersed particles are released in a solid solution after hardening at low heat to impart strength to the material.

To perform heat treatment you will need the following:

• tanks with water and oil;
• sanding paper;
• metallographic microscope;
• furnace with thermoelectric pyrometer;
• Rockwell hardness testers;
• sets of microsections (sorbitol, martensite, ferrite-martensite, etc.).

Normalization or another method of heat treatment of steel is chosen depending on the concentration of carbon in it. If the material contains it in amounts up to 0.2%, then the most acceptable method is normalization. If 0.3−0.4% carbon is present, then both normalization and annealing will do.

The choice of one or another processing method should also depend on the required properties. For example, normalization will give the product a fine-grained structure, and, compared to annealing, greater hardness and strength.

In many cases, normalization is the most preferred method of processing materials, since it has many advantages over others. In many industries, in particular mechanical engineering, it is most often used for heat treatment .

Source: https://tokar.guru/metally/stal/normalizaciya-stali-opisanie-i-harakteristiki.html

What kind of processing of steel products is called improvement - Metalworker's Handbook

Heat treatment of metal is an important part of the production process in non-ferrous and ferrous metallurgy. After this procedure, the materials acquire the necessary characteristics. Heat treatment has been used for quite some time, but it was imperfect. Modern methods allow you to achieve better results with less effort and reduce costs.

To impart the desired properties to a metal part, it is subjected to heat treatment. During this process, a structural change occurs in the material .

Metal products used in the household must be resistant to external influences. To achieve this, the metal must be strengthened by exposure to high temperature. This treatment changes the shape of the crystal lattice, minimizes internal stress and improves its properties.

Types of heat treatment of steel

Heat treatment of steel comes down to three stages: heating, holding and rapid cooling. There are several types of this process, but the main stages remain the same.

The following types of heat treatment are distinguished:

• Technical (tempering, hardening, cryogenic treatment, aging).
• Thermo-mechanical, which uses not only high temperature, but also physical impact on the metal.
• Chemical-thermal involves heat treatment of the metal followed by exposure of the surface to nitrogen, chromium or carbon.

Annealing

This is a manufacturing process of heating metal to a predetermined temperature, and then slowly cooling, which occurs naturally. As a result of this procedure, the heterogeneity of the metal is eliminated, internal stress is reduced, and the hardness of the alloy is reduced, which greatly facilitates its processing. There are two types of annealing: first and second kind.

During first-order annealing, the phase state of the alloy changes slightly. It has varieties:

• Homogenized - the temperature is 1100−1200 °C, the metal is kept for 7−14 hours in such conditions.
• Recrystallization - annealing temperature 100−200 °C, this procedure is used for riveted steel.

During second-order annealing, a phase change in the metal occurs. The process has several types:

• Full annealing - the metal is heated 25−40 °C above the critical value for this material and cooled at a special speed.
• Incomplete - the alloy heats up to a critical point and takes a long time to cool down.
• Diffusion - annealing is carried out at a temperature of 1100−1200 °C.
• Isothermal - heating of the metal occurs as during complete annealing, but cooling below the critical temperature, cooling in the open air.
• Normalized - the metal is completely annealed and cooled in air.

Cryogenic treatment

Changes in the structure of the metal can be achieved not only by high temperature, but also by low temperature. Alloy processing at temperatures below 0 °C is widely used in various industries. The process occurs at a temperature of 195 °C.

Advantages of cryogenic processing:

• Reduces the amount of austenite, which gives stability to the dimensions of parts.
• Does not require subsequent tempering, which shortens the production cycle.
• After this treatment, the parts are better suited for grinding and polishing.

Chemical-thermal treatment

Chemical-thermal treatment includes not only high temperature exposure, but also chemical exposure. The result of this procedure is increased strength and wear resistance of the metal, as well as making it fire and acid resistant.

There are the following types of processing:

• Cementation.
• Nitriding.
• Nitrocarburization.
• Borating.

Steel carburization is a process of additional processing of metal with carbon before hardening and tempering. After the procedure, the product’s endurance during torsion and bending increases.

Before cementation begins, the surface is thoroughly cleaned, after which it is coated with special compounds. The procedure is carried out after the surface has completely dried.

  Steel 30hgsa characteristics application

There are several types of cementation: liquid, solid, gas. In the first type, a special bath oven is used, into which 75% soda, 10% silicon carbide, 15% sodium chloride are poured. After which the product is immersed in a container. The process takes place for 2 hours at a temperature of 850 °C.

Hard cementation can be conveniently performed in a home workshop. For it, a special paste is used based on soda ash, soot, sodium oxalate and water. The resulting composition is applied to the surface and waits to dry. After this, the product is placed in an oven for 2 hours at a temperature of 900 °C.

Gas cementation uses mixtures of gases containing methane. The procedure takes place in a special chamber at a temperature of 900 °C.

Nitriding of steel is the process of saturating the metal surface with nitrogen by heating to 650 °C in an ammonia atmosphere. After processing, the alloy increases its hardness and also becomes resistant to corrosion.

Nitriding, unlike carburization, allows you to maintain high strength at high temperatures. And also the products do not warp when cooled.

Metal nitriding is widely used in industry to impart wear resistance to the product, increase hardness and protect against corrosion.

Source: https://ssk2121.com/kakaya-obrabotka-stalnyh-izdeliy-nazyvaetsya-uluchsheniem/

Improved steels

Improved steels are medium-carbon structural steels containing (0.30.5)% C, subjected to hardening at a temperature of 820880 0C and subsequent high-temperature tempering at 550680 0C. After such heat treatment, the steel acquires a sorbitol structure that can withstand shock loads well.

Chrome steels

For moderately loaded small-sized parts, chromium steel grades 30Х, 38Х, 40Х, 50Х are used. With increasing carbon content, strength increases, but ductility and toughness decrease.

The hardenability of steels is low and to increase it it is alloyed with boron (0.0020.005%). The critical diameter of 35ХР steel when quenched in water is 3045 mm, and in oil 2030 mm.

The introduction of 0.10.2% vanadium (40CFA) increases the mechanical properties of chromium steels, mainly toughness, due to better deoxidation and grain refinement without increasing hardenability. These steels are used for products operating under increased dynamic loads. The mechanical properties of some steels to be improved after heat treatment are given in Table 10.

Chromium-manganese steels

Joint alloying of steels with chromium (0.91.2%) and manganese (0.91.2%) makes it possible to obtain sufficiently high strength and hardenability (for example, 40KhG), however, they have a reduced viscosity, a lower threshold of cold brittleness (from 20 0C to minus 60 0С). The introduction of titanium reduces the tendency to overheat, and the addition of boron increases hardenability.

Table 10 - Mechanical properties of some alloyed steels that can be improved

Steel grade	Calcined diameter, mm	sigmaв, MPa	sigma0.2,MPa	d, %	y, %	KCU,MJ/m2
30X40X40XFA40ХGTR30ХГС40ХН30ХН3А40ХН2МА36Х2Н2МФА38ХН3МФА	25-3525-3525-3550-7550-7550-7575-10075-100more than 100more100	90010009001000110010001000110012001200	70080075080085080080095011001100	12101011101110121212	45455045454550505050	0,70,60,90,80,40,70,80,80,80,8

Chrome-silicon-manganese steels

They have high hardenability and mechanical properties. These include steel grades 20KhGS, 25KhGS, 30KhGS. Chromansil steels are used in the form of sheets and pipes for critical welded structures. With the introduction of additional nickel 1.41.8% (30HGNA), the strength of steel increases: sigmaв = 1650 MPa, sigma0.2 = 1400 MPa.

Chrome-nickel steels

They have high hardenability, strength, and good toughness. They are used for the manufacture of large products of complex configurations operating under vibration and dynamic loads. Nickel, especially in combination with molybdenum, greatly reduces the cold brittleness threshold.

The higher the nickel content, the lower the permissible temperature for using steel and the higher its resistance to brittle fracture. It is recommended to introduce up to 3% Ni. With a higher content, a lot of retained austenite is obtained.

For heavily loaded parts with a cross-sectional diameter of up to 70 mm, steel grades 40ХН, 45ХН, 50ХН are used.

Chromium-nickel-molybdenum-vanadium steels

In addition to molybdenum, vanadium is added, which helps to obtain a fine-grained structure. Steel grades 38KhN3MF and 36Kh2N2MFA are used for parts with large sections (1000-1500 mm or more). After quenching, bainite is formed in the core, and sorbitol is formed after tempering. Steels have high strength, ductility and toughness, and a low cold brittleness threshold.

Molybdenum present in steel increases its heat resistance.

These steels can be used at temperatures of 400-450 0C in the manufacture of the most critical parts of turbines and compressors, which require material of special strength in large sections (forgings of shafts and solid forged turbine rotors, shafts of high-stress turbo-blowing machines, gearbox parts, etc.).

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Source: https://dprm.ru/materialovedenie/uluchshaemye-stali

Tensile and tensile strength of a material - what is it, how is the fluidity of the metal and the tensile strength of steel measured?

14Nov

articles

When constructing objects, it is imperative to use calculations that include detailed characteristics of building materials. Otherwise, too much, unbearable load may be placed on the support, which will cause destruction. Today we’ll talk about the tensile strength of a material at break and tension, we’ll tell you what it is and how to work with this indication.

Tensile strength

PP - we will use this abbreviation, and we can also talk about the official combination “temporary resistance” - this is the maximum mechanical force that can be applied to an object before its destruction begins. In this case, we are not talking about chemical effects, but we mean that heating, unfavorable climatic conditions, and a certain environment can either improve the properties of the metal (as well as wood, plastic) or worsen it.

No engineer uses extreme values ​​when designing, because it is necessary to leave a permissible error - for environmental factors, for the duration of operation. We told you what is called tensile strength, now let’s move on to the specifics of the definition.

How is the strength test performed?

Initially there were no special events. People took an item, used it, and as soon as it broke, they analyzed the breakdown and reduced the load on a similar product. Now the procedure is much more complicated, however, until now the most objective way to find out PP is the empirical way, that is, experiments and experiments.

All tests are carried out under special conditions with a large amount of precise equipment that records the condition and characteristics of the experimental material. Usually it is fixed and experiences various influences - tension, compression.

They are performed by instruments with high precision - every thousandth of a newton of the applied force is noted. At the same time, each deformation is recorded as it occurs. Another method is not laboratory, but computational.

But usually mathematical analysis is used in conjunction with testing.

Definition of the term

The sample is stretched on a testing machine. In this case, first it lengthens in size, and the cross-section becomes narrower, and then a neck is formed - the place where the thinnest diameter is, this is where the workpiece will rupture. This is true for ductile alloys, while brittle alloys, such as cast iron and hard steel, stretch very slightly without necking. Let's take a closer look at the video:

Types of PP

Tensile strength is determined by various influences, according to this it is classified according to:

• compression – mechanical pressure forces act on the sample;
• bending - the part is bent in different directions;
• torsion – suitability for use as a rotating shaft is checked;
• stretching - we gave a detailed example of testing above.

Tensile strength of steel

Steel structures have long replaced other materials, as they have excellent performance characteristics - durability, reliability and safety. Depending on the technology used, it is divided into brands. From the most common with a PP of 300 MPa, to the hardest with a high carbon content - 900 MPa. This depends on two indicators:

• What heat treatment methods were used - annealing, hardening, cryotreatment.
• What impurities are contained in the composition. Some are considered harmful, they are discarded for the purity of the alloy, and others are added to strengthen them.

Yield strength and tensile strength

The new term is designated in the technical literature by the letter T. The indicator is relevant exclusively for plastic materials and indicates how long a sample can be deformed without increasing the external load on it.

Usually, after overcoming this threshold, the crystal lattice changes greatly and is rearranged. The result is plastic deformation. They are not undesirable; on the contrary, self-strengthening of the metal occurs.

Fatigue of steel

The second name is endurance limit. It is denoted by the letter R. This is a similar indicator, that is, it determines what force can act on an element, but not in a single case, but in a cycle. That is, certain pressures are applied to the experimental standard cyclically, over and over again. The average number of repetitions is 10 to the seventh power. This is exactly how many times the metal must withstand the impact without deformation or loss of its characteristics.

If you carry out empirical tests, it will take a lot of time - you need to check all the force values, applying it over many cycles. Therefore, the coefficient is usually calculated mathematically.

Proportionality limit

This is an indicator that determines the duration of the loads applied to the deformation of the body. In this case, both values ​​should change to different degrees according to Hooke’s law. In simple words: the greater the compression (tension), the more the sample is deformed.

The value of each material lies between absolute and classical elasticity. That is, if the changes are reversible after the force ceases to act (the shape becomes the same - for example, compression of a spring), then such parameters cannot be called proportional.

How are the properties of metals determined?

They check not only what is called tensile strength, but also other characteristics of steel, for example, hardness. The tests are carried out as follows: a ball or cone made of diamond, the most durable rock, is pressed into the sample.

The stronger the material, the smaller the mark left. Deeper, wider-diameter prints are left on soft alloys. Another experience - for a blow. The impact occurs only after a pre-made cut on the workpiece.

That is, the destruction is checked for the most vulnerable area.

Mechanical properties

There are 5 characteristics:

• The tensile and tensile strength of steel is temporary resistance to external forces, stress arising internally.
• Plasticity is the ability to deform, change shape, but maintain the internal structure.
• Hardness – willingness to meet harder material without causing significant damage.
• Impact strength is the ability to resist impacts.
• Fatigue is the duration of preservation of qualities under the influence of cyclic loads.

Strength classes and their designations

All categories are written down in regulatory documents - GOSTs, according to which all Russian entrepreneurs produce any rolled metal and other metal products. Here is the correspondence between the designation and parameter in the table:

Class	Tensile strength, N/mm2
265	430
295	430
315	450
325	450
345	490
355	490
375	510
390	510
440	590

We see that for some classes the PP indicators remain the same, this is explained by the fact that, with equal values, their fluidity or relative elongation may differ. Depending on this, different maximum thickness of rolled metal is possible.

Specific strength formula

R with the index “y” is the designation of this parameter in physics. It is calculated as PP (in writing – R) divided by density – d. That is, this calculation has practical value and takes into account theoretical knowledge about the properties of steel for use in life. Engineers can tell how the temporary resistance changes depending on the mass and volume of the product. It is logical that the thinner the sheet, the easier it is to deform.

The formula looks like this:

Ry = R/d

Here it would be logical to explain how the specific tensile strength is measured. In N/mm2 - this follows from the proposed calculation algorithm.

Using the properties of metals

Two important indicators - plasticity and PP - are interrelated. Materials with a large first parameter degrade much more slowly. They change their shape well and are subjected to various types of metal processing, including die stamping - that’s why car body elements are made from sheets. With low ductility, alloys are called brittle. They can be very hard, but at the same time have poor stretching, bending and deformation, for example, titanium.

Resistance

There are two types:

• Regulatory - prescribed for each type of steel in GOSTs.
• Calculated – obtained after calculations in a specific project.

The first option is rather theoretical; the second is used for practical tasks.

Ways to increase strength characteristics

There are several ways to do this, two main ones:

• addition of impurities;
• heat treatment, for example, hardening.

Sometimes they are used together.

General information about steels

All of them have chemical and mechanical properties. Below we’ll talk in more detail about ways to increase strength, but first, let’s present a diagram showing all the varieties:

Also watch a more detailed video:

All of them have chemical and mechanical properties. Below we’ll talk in more detail about ways to increase strength, but first, let’s present a diagram showing all the varieties:

Carbon

The higher the carbon content of a substance, the higher the hardness and the lower the ductility. But the composition should not contain more than 1% of the chemical component, since a larger amount leads to the opposite effect.

Manganese

A very useful additive, but with a mass fraction of no more than two percent. Mn is usually added to improve machinability. The material becomes more susceptible to forging and welding. This is due to the displacement of oxygen and sulfur.

Silicon

Effectively increases strength characteristics without affecting ductility. The maximum content is 0.6%, sometimes 0.1% is enough. Combines well with other impurities; together, they can increase corrosion resistance.

Nitrogen and oxygen

If they get into the alloy, but worsen its characteristics, they try to get rid of them during manufacturing.

Alloying Additives

You can also find the following impurities:

• Chrome – increases hardness.
• Molybdenum – protects against rust.
• Vanadium – for elasticity.
• Nickel – has a good effect on hardenability, but can lead to brittleness.

These and other chemicals must be used in strict proportions according to the formulas. In the article we talked about tensile strength (short-term resistance) - what it is and how to work with it. They also gave several tables that you can use while working. To finish, let's watch the video:

To clarify the information you are interested in, contact our managers by phone 8 (908) 135-59-82; (473) 239-65-79; 8 (800) 707-53-38. They will answer all your questions.

Source: http://rocta.ru/info/predel-prochnosti-materialov-razryv-metallov-pri-rastyazhenii-i-szhatii-chto-ehto-takoe-vidy-foto/

Hardness after steel improvement – ​​Steel improvement: process, technology, steels being improved

alexxlab | 09/24/2019 | 0 | Questions and answers

Steel Improvement | KVADRO LLC

Steel improvement is a complex heat treatment of steel, consisting of hardening the part followed by high tempering of the steel , providing good strength and ductility.

The essence of the steel improvement process

After hardening of steel, martensite structures predominate in it. High tempering of steel consists of heating at least 20-40°C below the Ac1 point (see Iron-carbon diagram), but not lower than 500°C, holding and controlled cooling of the part.

Improvement of steels on the iron-carbon diagram

At the second stage of steel improvement - the process of high tempering of steel - diffusion decomposition of martensite occurs until the formation of tempered sorbitol (see Elements of the theory of heat treatment). Dispensed sorbitol has a homogeneous and dispersed structure.

Application of improving steels

It is the sorbitol tempering structure that provides the excellent combination of toughness, ductility and strength while reducing hardness in parts that have undergone the steel improvement .

The steel improvement process involves parts made of carbon and alloy steels with a carbon content of 0.30-0.55%. For example, steel 45, 40Х, 30ХГСА, 38Х2МУА.

If higher surface hardness is required, these parts are subjected to other processing methods after the steel improvement procedure: high-frequency hardening or nitriding.

In the absence of high requirements for ductility and toughness, instead of improving steels , steel normalization can be used as a more economical process.

Improvement of steels at KVADRO LLC

Our company has been performing heat treatment of metals to order in St. Petersburg for almost a quarter of a century, including the improvement of steels .

We carry out heat treatment of steels (including stainless steel, tool steel, etc.), as well as other metals and alloys (aluminum and titanium, brass and bronze, etc.) according to the Customer’s drawings or heat treatment modes.

In addition to steel improvement, we also produce other types of heat treatment of metals to order, for example:

Source: https://stankotec.ru/raznoe/tverdost-posle-uluchsheniya-stali-uluchshenie-stali-process-texnologiya-uluchshaemye-stali.html

Improvement of steel

Steel improvement is a set of heat treatment operations, which includes hardening and high tempering. For processed parts the following increases:

• strength;
• plastic;
• impact toughness;
• fatigue strength;
• the cold brittleness threshold decreases.

Improvement of steel

The essence of the improvement process

Structural upgradeable steels of three categories are subjected to the improvement process:

1. Carbonaceous. The average content, which ranges from 0.25% to 0.6%.
2. Low alloyed. The average total content of alloying elements is no more than 3%.
3. Medium alloyed. The number of introduced elements ranges from 3% to 10%.

During hardening, the part is heated to a temperature 30°C lower than at point Ac1. At this stage, it is necessary to ensure through hardenability. The internal structure of the part is dominated by martensite.

Improved steel structure

High tempering is carried out at temperatures from 550°C to 650°C. Due to this, the metal structure transforms into sorbitol and becomes homogeneous and fine-grained.

The maximum effect can be achieved if ferrite and bainite are not formed during hardening.

Thermal improvement of metals allows you to change such indicators as:

1. Strength characteristics:
   1. ϬВ – tensile strength;
   2. Ϭ0.2 – yield strength;
   3. KCU – impact strength;

1. Plasticity characteristics:
   1. δ%—relative elongation;
   2. ψ% - transverse narrowing;
2. Fatigue characteristics:
   1. Ϭ-1 – fatigue strength;
   2. Ψ-1 – torsional fatigue limit;
3. Hardness (HB, HRC).

Improvement technology

When hardening or hardening, the heating temperature is selected based on the composition of the metal. If for structural medium-carbon steels it can be selected according to the iron-carbon diagram, then to obtain austenite in a metal containing alloying elements (chromium, molybdenum, vanadium, nickel and others), it is necessary to increase the heating temperature.

Fe-C diagram

Intensive cooling is carried out in two media: water and oil. Carbon metals should be cooled in water, and alloyed metals should be cooled in oil, since the aqueous environment can provoke the formation of internal cracks and deformations.

The internal structure of martensite can be transformed by medium or high tempering. The tempering temperature largely depends on the percentage of alloying elements.

Applying an improvement

After improvement, carbon steels are used to produce parts that require increased strength. These are parts such as shaft, bushing, gear, gear, bushing. The use of carbon steels is due to low cost of production and manufacturability.

Steel improvement is used in the manufacture of worm shaft

Materials with a high carbon content (60, 65) after improvement are used for the manufacture of spring and spring products.

The introduced alloying elements make it possible to manufacture from these steels critical parts with larger diameters that experience greater loads. After heat treatment, they retain their viscosity and plasticity with increased strength and hardness, and the cold brittleness threshold is reduced.

The mechanical properties of structural elements depend on the homogeneity of the metal structure, which directly depends on through hardenability and the minimum diameter. This parameter characterizes the formation of more than half of martensite. So the table shows some indicators at which the critical diameter is maintained.

steel grade	Carrying out hardening at temperature, °C	Critical diameter, mm
Intensive cooling environment
water	oil
45	840850	until 9	up to 25
45G2	840850	before 18	up to 34
40ХН2МА	840850	up to 110	up to 142
38Х2МФА	930	up to 72	up to 86

As practice shows, alloying elements have a great influence on hardenability. This is especially noticeable in the presence of nickel. Its presence makes it possible to harden parts of large diameter. Thus, a critical part with a diameter of over 100 mm can be turned from steel 40ХН2МА and subjected to heat treatment while maintaining the given properties throughout the entire volume.

Cold brittleness

Negative temperatures contribute to the transition to a brittle state, which affects ductility and toughness. When exposed to dynamic loads at low temperatures, parts are destroyed. When selecting the material from which parts will be made that operate under extreme conditions, first of all they use such a parameter as cold brittleness.

Cold brittleness threshold depending on nickel content

The graph characterizes that the increased presence of nickel increases the cold brittleness threshold. This value is also influenced by molybdenum.

The fine-grained structure obtained during high tempering helps to increase the cold brittleness index.

Dependence of the cold brittleness threshold on grain size

The graph shows the dependence on grain size:

1 – grain size 0.002-0.01 mm;

2 – grain size 0.05-0.1 mm.

The presence of sulfur and phosphorus negatively affects the formation of a fine-grained structure.

The wrong choice of material for the manufacture of products operating in the far north and polar regions has more than once led to catastrophic consequences. For example, a shaft made of st. 40 and improved in a temperate climate, it works for more than one year. And in Chukotka, when the frost is more than 50°C, it will break down in the first months of operation.

Mechanical properties after improvement

Improved carbon steels have low hardenability. Therefore, steels from 30 to 50 are used for the manufacture of parts with a diameter of no more than 10 mm. After improvement, they are characterized by the following parameters:

• ϬB (ultimate strength) - 600700 MPa;
• KCU (impact strength) – 0.40.5 MJ/m2;
• HRC (hardness) – 4050.

If an element requires greater surface strength due to operating conditions, it is subjected to hardening with high frequency currents (HFC).

For products with a diameter of more than 30 mm, alloy metals are used to impart improved qualities. With a high hardening rate, a larger critical diameter, along with fine grains, they exhibit low residual stresses after HT and high resistance to tempering.

Thus, an iron alloy containing chromium and nickel, after improvement, has the following parameters:

• ϬB (ultimate strength) - 1020 MPa;
• Ϭ-1 (fatigue limit) – 14 MPa;
• ψ% (transverse narrowing) – 41%;
• HB (hardness) – 241.

In addition to widely used alloying elements, titanium, niobium and zirconium are used for grain refinement. To increase hardenability, boron is used.

Improvement of steel in the manufacture of parts

As an example, we can consider the route for manufacturing a gear part from 40ХН steel. This type of part requires high hardness values ​​of the working surface, as well as good ductility and toughness.

The technological process looks like this:

1. Obtaining a workpiece by volumetric stamping.
2. Annealing. Hardness HB = 172175.
3. Improvement. Heat in oil at t = 820-840°C. Vacation at t = 600-620°C. Hardness HB = 241244.
4. Mechanical restoration.
5. Heat treatment. Heat no deeper than 3 mm. Then low tempering at t = 220°C. Hardness HRC 5662.
6. Grinding teeth.

When choosing heat treatment modes for improvement, the following factors should be taken into account:

• degree of alloying;
• diameter and size of the workpiece;
• transitions that are sources of stress;
• applied dynamic loads;
• working conditions;
• required hardness.

Improved steels

Improved steels are structural materials:

1. carbon;
2. low alloyed;
3. medium alloyed.

I	II	III
Carbon	low alloy	medium alloyed
GOST 1050-82	GOST 4543-71	GOST 4543-71
30-60	Morganzovye 30G-65G, chromium 30Х-40Х	38Х2МУА and others, but with a carbon content of no more than 0.4%
Chromium-molybdenum 30ХМ-40ХМ, 50Г2
Multicomponent 30-40KhGSA, 30-40KhMFA	45ХН2МФА

Alloy steels can be divided into several categories:

• chromium;
• chromomanganese (chromansil);
• nickel-containing;
• with the addition of tungsten and molybdenum.

It is especially worth noting the poor weldability of the metals being improved. It is produced subject to certain measures that maintain the required characteristics.

Source: https://stankiexpert.ru/spravochnik/materialovedenie/uluchshenie-stali.html

Steel improvement: process, technology, improved steels - Website about

Steel improvement is a complex heat treatment of steel, which consists of hardening the part followed by high tempering of the steel, providing good strength and ductility.

Steel normalization

Normalization of steel is often considered from two points of view - thermal and microstructural.

In the thermal sense and classical sense, steel normalization is the heating of steel to an austenitic state, followed by cooling in still air. Sometimes normalization also includes operations with accelerated air cooling.

The location of the normalization temperature on the iron-carbon phase diagram is shown in Figure 1.

Figure 1 – Simplified iron-carbon phase diagram.
The shaded strip is the normalization temperature of steels

From the point of view of microstructure, the normalized structure is pearlite for steel with a carbon content of 0.8%, and for steels with a lower carbon content - hypoeutectoid steels - a mixture of pearlite and ferrite.

The normalization operation is used for most steels, including steel castings. Very often, welded steel seams are normalized to refine the steel structure in the zone of influence of welding.

The purpose of steel normalization

The purposes of steel normalization can be different: for example, to both increase and decrease strength and hardness, depending on the thermal and mechanical history of the product.

The purposes of normalization often overlap or even get confused with annealing, heat hardening, and stress-relieving tempering. Normalization is used, for example, to improve the machinability of a part by cutting, refine the grain, homogenize the grain structure, or reduce residual stresses. A comparison of temperature-time cycles for normalization and annealing is shown in Figure 2.

Figure 2 ─ Comparison of temperature-time cycles of normalization and full annealing. Slower cooling during annealing results in a higher ferrite-pearlite transformation temperature and a coarser microstructure than normalization.

For steel castings, normalization is used to homogenize their dendritic structure, reduce residual stresses and make them more susceptible to subsequent thermal hardening.

Products obtained by pressure treatment can be normalized to reduce banding of the structure after rolling or different grain sizes after forging.

Normalization followed by tempering is used instead of conventional hardening when products have a complex shape or sudden changes in cross-section. This is done to avoid cracking, warping and excessive thermal stress.

Steel cooling rate during normalization

The cooling rate during normalization is usually not a critical value. However, when the product has large differences in cross-sectional dimensions, measures are taken to reduce thermal stresses to avoid warping.

Holding at normalization temperature

The role of the duration of exposure at the normalization temperature is only to ensure homogenization of the austenitic structure before cooling begins. One hour of exposure for every 25 mm of section thickness is the norm.

The cooling rate during normalization significantly affects the amount of perlite, its size and the thickness of the pearlite plates. The higher the cooling rate, the more perlite is formed, and its plates become thinner and closer to each other. Increasing the proportion of pearlite in the structure and its grinding increases the strength and hardness of steel. Lower cooling rates mean less strong and harder steel.

After the products have cooled uniformly across their cross-section below the lower critical point Ar1, they can be cooled in water or oil to reduce the overall cooling time.

Source: https://steel-guide.ru/termicheskaya-obrabotka-stali/normalizaciya-stali.html