What is the purpose of normalization of steel structures

Steel normalization - description of the process and its essence

Most of the operations associated with heat treatment involve the same algorithm of actions:

heating the product to certain temperatures;
holding under the influence of the set temperature for a specified time;
cooling, which can be carried out in different environments and at different rates.

Heat treatment of parts can act both as an intermediate technological process and as a finishing one. In the first case, those parts that will still be processed, for example, drills or aircraft turbine blades, pass through it. The second case implies that after heat treatment, the finished part will receive new properties.

Steel normalization is a type of heat treatment of metal followed by cooling in air. The result of this operation is the formation of a normalized steel structure. By the way, this is where the name comes from. The operation is used in relation to forgings, castings, etc. Normalization is used to minimize grains in the steel structure formed by the weld.

The essence of the process

The normalization procedure is as follows. The part is heated to temperatures that exceed the maximum permissible parameters (Ac1, Ac3) by 30 - 50 degrees Celsius, then it is kept at this temperature for some time, after which it is cooled.

The temperature is selected based on the steel grade. Thus, alloys containing 0.8% carbon, so-called hypereutectoid, are processed at temperatures lying between the critical points Ac1 and Ac3.

What are critical points? This is the name given to the temperatures at which phase changes and structure of the alloy occur when it is heated or cooled.

The result of this is that a certain volume of carbon enters the solid solution and austenite is fixed. That is, a structure consisting of martensite and cementite appears. It is cementite that leads to an increase in wear resistance and hardness. Heating high-carbon steel above ac3 leads to an increase in internal stresses. This is due to the fact that the amount of austenite increases due to an increase in carbon concentration.

When heated above the critical point Ac3, steel with a carbon content of less than 0.8% acquires increased viscosity. This happens because in steel of this type austenite (fine-grained) appears, turning into martensite (fine-grained).

Hypoeutectoid steel is not processed at temperatures in the range Ac1 - Ac3. Since in this case ferrite appears, which reduces the hardness parameters.

Time required to complete the operation

It takes some time to obtain a homogeneous structure of the alloy at a certain temperature. This time will be determined as the holding time of steel during normalization. It was experimentally determined that a layer of metal 25 mm thick becomes homogeneous after an hour. Thus. and determine the normalization time.

The final stage is cooling

The cooling rate plays a significant role in the formation of perlite volume and the size of its plates. Numerous studies have shown that high cooling rates increase the amount of perlite and the steel gains increased hardness and strength. Low cooling intensity leads to steel losing hardness and strength.

When processing parts with significant differences in size, for example. shafts, it is advisable to remove stresses arising under the influence of temperature fluctuations. To do this, they are preheated in a container filled with different salts. When the temperature drops, it is possible to speed up this process by placing hot parts in water or specially selected oil.

In other words, steel normalization eliminates stress inside the part and minimizes its structure. That is, it has a direct effect on changes in the microstructure of steel alloys.

Using Normalization

This form of heat treatment is used to achieve different purposes. Thus, the use of normalization can increase or decrease the hardness of a steel alloy, toughness and strength characteristics. This method of heat treatment is used when it is necessary to improve the machinability of steel using different methods - cutting, stamping, etc.

Parts produced by casting undergo normalization in order to obtain a homogenized structure and eliminate internal stresses. The same can be said about parts obtained after forging.

That is, normalization serves to obtain a homogeneous metal structure and eliminate internal stresses. In addition, this process can be used as a replacement for hardening products with complex profiles.

In addition to the mentioned results of the normalization process, you can add such results as minimizing grains in the alloy structure, removing secondary cementite, and increasing the machinability of steel.

Essentially similar heat treatment processes

In addition to normalization, the following operations can be added to the list of heat treatment of steels:

annealing;
vacation;
hardening;
cryogenic treatment and several others.

The annealing operation provides a high-quality, finer structure of pearlite; this occurs because furnaces are used to cool the parts. The purpose of this operation is to reduce the heterogeneity of the structure, remove stress, and increase machinability.

The principles underlying the hardening operation are identical to those of normalization, but there are some differences. For example, when hardening, much higher temperatures and high cooling rates are used. Hardening leads to improved strength characteristics, hardness, etc. But often workpieces that have undergone hardening are characterized by reduced viscosity and high fragility.

Tempering of parts is used after the hardening operation. Tempering reduces brittleness and internal stress. In this case, the temperature range is lower than that used in normalization. The parts are cooled in air. As the temperature increases, the tensile strength and hardness decrease and at the same time the impact strength increases.

Cryogenic treatment of steel results in a uniform metal structure and increased hardness. This processing technology is used for hardened carbon steel.

Normalization and its application in practice

When assigning a heat treatment method, the technologist must take into account the carbon concentration. Steels in which the carbon content does not exceed 0.4% can be processed by both normalization and annealing. Normalization minimizes the grain size in the structure and increases strength characteristics.

Comparing the time spent between normalization and other methods, we can conclude that processing by other methods takes longer.

Due to the speed of the operation, the coverage of a large number of steels, the quality of the resulting parameters (hardness, strength, etc.), this is why normalization is widely used in mechanical engineering.

Source: https://prompriem.ru/stati/normalizaciya-stali.html

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

The principle of steel normalization

Steel normalization refers to the hardening process through a cycle of heating to a certain temperature and cooling. Heat treatment has different modes for each type of metal. As a result of the use of technology, the material becomes stronger due to the elimination of defects. The latter inevitably appear as a result of previous stages in the manufacture of steel products.

Purpose of technology

Steel normalization can be carried out in a garage if the appropriate equipment is available. The advantage of the technology is the production of a thin eutectoid. The structure of this layer directly affects the strength and rigidity of the metal.

Since steel normalization is carried out to improve the quality of the product, the cost of its production increases accordingly. Technology is used only when necessary. For lightly loaded parts it is not required. It is often applicable for the production of long metal.

The technology can be a replacement for such procedures as hardening with high tempering and classical annealing. Normalization of medium carbon steel does not provide high strength comparable to the structure after quenching. But it does not lead to severe deformation and helps get rid of internal cracks.

Steel normalization refers to a thermal treatment method. There are several technologies for heating metal, differing in conditions:

The heating temperature of metals and alloys is different.
Heated holding time.
This type of cooling often occurs over a long period of time due to heat exchange with the environment.

It is slow cooling that makes it possible to obtain a homogeneous steel composition. The purpose of annealing is a uniform structure of the metal, the desire to remove cavities and voids, and small cracks.

The following types of annealing are used, common to reduce local thickening after hot and cold rolling:

Diffusion - changes the chemical composition.
Full - affects the entire structure, helps to achieve uniformity.
Recrystallization - removes the hardening of steels.
Incomplete - makes the steel more pliable for metalworking.
Isothermal is the most optimal way to reduce the strength of steel.
Spheroidizing - converts flat pearlite grains into spherical ones.

The steel normalization temperature was selected experimentally for each type of alloy. After casting or cold rolling, no workpiece has an ideal structure. Additional heat treatment—annealing—helps correct the situation.

Normalization and hardening of steel is necessary to correct internal inhomogeneities after casting. Shaped castings and ingots are subjected to heat treatment. This is most often required for products made of alloy steels.

To correct defects in steel, it must be heated to a very high temperature. In this state, the atoms of alloying elements begin to move. Their uniform redistribution occurs throughout the internal volume.

At 1100 degrees, optimal heat treatment of steel takes place. Diffusion normalization lasts about 10-20 hours in a heated state, followed by very slow cooling.

Full annealing

Normalization and hardening of hypoeutectoid steel is necessary to correct the structure damaged by heating during the manufacture of castings and forgings processed by pressure. The processing temperature must exceed a critical point when pearlite begins to transform into austenite.

The temperature increase should occur strictly 30-50 degrees above the critical point Ac3. This value for alloy steels is taken from tables, and for carbon steels it is determined from the state diagram. Normalization process:

The initial stage is heating 30-50 degrees above the critical temperature Ac3. Austenitic grains form.
Exposure at high temperatures is accompanied by the growth of austenite grains.
Long-term uniform cooling - small austenite crystals disintegrate into several pearlite grains. The ferrite structure is uniformly filled with a pearlite layer.

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Partial annealing is required to reduce the hardness of metals. More often this is necessary due to the conditions of metal cutting. As a result of normalization, excess tension in the steel is eliminated. Unlike full annealing, the entire process occurs at lower temperatures. Accordingly, less time is spent.

Processing of complex alloy steels

In the process of isothermal normalization, hard metals become more amenable to cutting. Heating occurs at the following temperatures:

Structural steels - no higher than 30-50 degrees of the critical point Ac3.
Tool steels are 5-100 degrees higher than point Ac1.

Unlike the methods discussed, isothermal annealing involves cooling steel immersed in molten salt. Natural cooling occurs after the temperature drops to 700 degrees. At this moment, austenite completely transforms into pearlite grains.

Correction of damaged structure of metals and alloys

Two-stage cooling of steels allows the pearlite plates to be converted into grains. Heating occurs to a temperature above point Ac1. Then it is reduced to 700 and maintained at 500 degrees. Then the metal cools down in air for a long time. This normalization is called spheroidizing. As a result, the product can be easily cut. This is how metals containing 0.65% carbon are processed.

Hardening is the formation of stronger areas of metal after cold stamping or drawing. Recrystallization annealing removes this defect - the brittleness of steels is eliminated by heating to 700 degrees (below Ac1). At this moment, the crystallization lattice of metals is restored. The structure becomes fine-grained and homogeneous. Bright annealing can also be carried out to restore the properties of steels after sheet rolling in order to maintain a shiny surface.

Source: https://FB.ru/article/345726/printsip-normalizatsii-stali

What is the purpose of normalizing steel structures?

The quality of steel is determined by the structure of its crystal lattice. During heat treatment, in some cases, the uniformity of the metal grain may be disrupted, and defects and internal stresses may arise. The more such negative aspects, the lower the grade of the material will be. To improve grade characteristics (to make the metal stronger and harder), a process called steel normalization is used. This type of processing also refers to thermal.

Most often, this type of processing occupies an intermediate position in the technological chain, but sometimes it is used at the final stage to obtain long products. High-carbon, medium-carbon and low-carbon steels, as well as tool materials and low-alloy metal products can be normalized. In each specific case, normalization is achieved by one or another change associated with the improvement of parameters.

Goals and purpose of normalization

Normalization has several purposes - it should not be considered only as a way to increase the hardness of steel. In some cases, this process achieves the opposite effect on hardness, and can also reduce the strength and toughness of the metal. It is important to understand here that any steel has a mechanical and thermal history.

The main goal of normalization is to achieve the effect of leveling stresses that have arisen in the structure of the material for one reason or another. As a result, the steel is easier to process in a variety of ways, and it gains additional characteristics as a result of processing.

If we take, for example, steel castings, then processing by the normalization method will allow us to obtain homogenization of the crystal structure, reduce residual stresses and increase the ability to thermal hardening.

Steel objects that were obtained by pressure are subjected to normalization after rolling and forging in order to reduce banding and heterogeneity of the structure, respectively.

When the normalization process is carried out simultaneously with the tempering of the metal, this is an alternative to processing by hardening for those products that have a complex shape or have sharp differences in cross-section. In this case, it is possible to avoid possible deformation of the part.

Another property of normalization: it allows you to transform the coarse-grained structure of the metal into a finer state. This treatment improves the ability to harden, process by cutting, and makes it possible to remove the network of so-called secondary cement in hypereutectoid steel. All this helps prepare the product for heat treatment at the last stage of the technological process.

Normalization process and basic principles

From the point of view of the physics of the process, steel normalization is a thermal treatment of the metal, in which it is heated above the upper critical threshold of Acm and Ac3 by 30–50 degrees Celsius. At this level, the metal is held and then cooled under normal ambient temperature conditions.

After reaching the Ac3 point, the completion of the phase is observed, when ferrite is transformed into austenite with simultaneous normalization of the structure of the resulting substance. After overcoming the Acm threshold, a process follows where secondary cementite begins to precipitate from austenite (if the temperature decreases) and its dissolution in austenite stops (with an increase in temperature relative to this point).

If the steel was too overheated and because of this the lattice grain became coarser, to reduce this size the product is subjected to such processing where the normalization temperature of the steel is increased by 100–150 degrees Celsius relative to the ACj point.

Do not confuse normalization with annealing: each process has its own characteristics. When steel is normalized, cooling occurs twice as fast. From an economic point of view, this process is more cost-effective, since it does not require the use of a furnace for gradual cooling.

The steel normalization method cannot always be applied to some grades of steel, because after such treatment they remain at increased hardness, which is not necessary in all cases. This applies to those metals where the carbon content exceeds 0.4%. In low-carbon steels this effect is usually not observed. A way out of the situation may be to use high tempering after normalization at a temperature of 650–700 degrees Celsius.

Equipment and materials

Furnaces for hardening and annealing steel are used as equipment for normalization. Gas heating can be used in furnace equipment. Such systems contain:

Camera. This is a special, hermetically sealed box where the workpieces are placed.
Heating elements in the form of burners. Designed to increase the temperature in the oven chamber. Burners can be of the flat-torch type and operate on the principle of indirect or direct heating.
Devices that perform shut-off and control functions.
Power control modules. They can be of a combined type, proportional or pulse.
Thermal insulation material.

Source: https://master-kleit.ru/origami/s-kakoj-celju-proizvoditsja-normalizacija-stalnyh/

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.

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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

Why is steel normalized?

To improve the characteristics of the finished metal product, it is subjected to heat treatment. This is a set of processes that are performed using certain technologies. Steel normalization is a heat treatment of a product, as a result of which the mechanical and physical properties of the metal are improved.

The essence of processing

Normalization is heating a metal workpiece to a temperature 50 degrees above the critical temperature. After heating, cooling occurs. However, between these processes, exposure is carried out at a normalization temperature.

The degree of heating depends on the material of the part. To calculate the time of thermal exposure, it is necessary to pay attention to the homogenization of the metal structure. The optimal indicator is exposure for 1 hour at a thickness of 25 mm.

When cooling, certain points must be taken into account.
When the temperature drops below critical, you need to speed up the cooling process. To do this, the part is dipped into a container with oil or water. The number of advantages and disadvantages of the finished product depends on correctly carried out heat treatment and subsequent cooling. Heat treatment, tempering, annealing, normalization, tarnishing

Purpose

This technology is used to achieve the following goals:

changes in the structure of an alloy or homogeneous metal;
achieving greater strength and hardness;
changes in the mechanical properties and characteristics of the part;
reducing metal stresses that appear during other processing processes.

With the help of such thermal effects, you can achieve various results, for example, change the hardness and strength indicators.

Normalization is mandatory after processing steel by pressure, since increasing and decreasing the temperature allows you to correct problems with the structure of the material.

Features of the work

Normalization is uniform heating of the workpiece to a temperature above the critical one. After heating, the parts are maintained at the same temperature. Then the workpiece is cooled. Initially, it slowly cools down to the lower critical temperature, then the master immerses it in coolant to speed up the process.

Principles of conducting

Heat treatment of the material is necessary if its structure and, consequently, technical characteristics change.

There are two types of metals that can be heat treated:

hypereutectoid;
hypoeutectoid.

The choice of temperature depends on the type of metal. For example, for hypereutectoid billets, the heating process is carried out at temperatures located between the marks AC1–AC3. As for hypoevectoid parts, they are processed at temperatures above the AC3 point. Materials belonging to the first group achieve the same hardness index.

Duration

The cooling rate depends on the amount of perlite contained in the workpiece and the size of the plates being processed. If the cooling rate is increased and the procedure time is reduced, the amount of perlite formed during the heat treatment process will increase. Strength and hardness indicators will also increase.

Other heat treatment methods

In addition to the normalization process, there are other methods of heat treatment of metals and alloys:

Tempering is a technology used to reduce brittleness and internal stresses in a material.
Annealing is a method in which the grain size in the structure of a material is reduced and internal stress is relieved.
Hardening is a technique similar to normalization. The differences are a higher heating temperature and a high cooling rate.
Cryogenic processing is a technology associated with the use of low temperatures.
Dispersion hardening is the final stage of heat treatment. The processed part is given a high strength index.

The main methods for processing metal workpieces are presented above, but the order is incorrect. You can find it out in any source on metalworking.

Steel normalization is considered one of several stages of heat treatment. With its help, the structure and characteristics of the material change. If desired, you can worsen or improve the properties of the workpiece.

Why is steel normalized? Link to main publication

Source: https://metalloy.ru/obrabotka/termo/normalizatsiya-stali

General provisions

The principle of most heat treatment technologies involves heating and holding steels and cooling, which changes their structure. Despite the same principle and similar goals, each of them has certain temperature and time regimes.

Heat treatment can serve both as an intermediate step and as a final technological process.

In the first case, such methods are used to prepare the material for subsequent processing, and in the second, new properties are imparted using this method.

Steel normalization is the process of heating, holding the material, and then cooling it in air.

As a result, a normalized structure is formed. This explains the name of this processing method.

Normalization is used for different steels, as well as castings. In addition, welded seams are subjected to this operation to refine the structure of the material.

Principles

The essence of normalization is heating the steel to a temperature exceeding the upper critical temperature values by 30 - 50 ° C, holding and cooling.

The temperature is selected based on the type of material. Thus, hypereutectoid options should be normalized in the temperature range between points Ac1 and Ac3, while for hypoeutectoid steel temperatures above Ac3 are used. As a result, all materials of the first type acquire the same hardness due to the fact that the same amount of carbon passes into the solution and the same amount of austenite is fixed. The result is a structure consisting of martensite and cement.

The second component helps to increase the wear resistance and hardness of the material. Heating high-carbon steel above Ac3 leads to an increase in internal stresses due to the growth of austenite grains and an increase in its quantity due to an increase in the concentration of carbon in it, leading to a decrease in the martensitic transformation temperature. Because of this, hardness and strength are reduced.

As for hypoeutectoid steel, when heated above Ac3 it gains increased viscosity. This is due to the fact that in low-carbon steel, fine-grained austenite is formed, which, after cooling, turns into fine-crystalline martensite. Temperatures between Ac1 and Ac3 are not used for processing such materials, since the structure of hypoeutectoid steel in this case receives ferrite, which reduces its hardness after normalization and mechanical properties after tempering.

Optimal heating temperatures for various types of heat treatment

The holding time determines the degree of homogenization of the structure. The standard indicator is considered to be an hour of exposure per 25 mm of thickness.

The intensity of cooling largely determines the amount of perlite and the size of the plates.

So, there is a direct relationship between these quantities. That is, with increasing cooling intensity, more pearlite is formed, the distance between the plates and their thickness are reduced. This increases the hardness and strength of normalized steel. Consequently, low cooling intensity contributes to the formation of a material of lower strength and hardness.

In addition, when processing objects with large differences in cross-section, they strive to reduce thermal stresses in order to avoid warping, both during heating and cooling. So, before starting work, they are heated in a salt bath.

When the temperature of the workpiece decreases to the lower critical point, it is permissible to accelerate the cooling by placing it in oil or water.

Thus, normalization reduces internal stresses and refines the coarse-grained structure of forgings, castings, and welds through recrystallization. That is, the microstructure of steel changes.

Related processes

Heat treatment of steel, in addition to normalization, includes annealing, tempering, hardening, cryogenic treatment, and dispersion hardening. The purpose of normalization, as well as the principle of implementation, coincides with the named technologies. Therefore, a comparison of these processes is carried out below.

Annealing gives a finer pearlite structure, as it involves cooling in the oven. It is used to reduce structural heterogeneity, stress after processing by casting or pressure, imparting a fine-grained structure, and improving cutting processing.

The principle of hardening is similar, except for higher temperatures than during normalization, and an increased cooling rate, due to the fact that it is produced in liquids. Tempering increases strength and hardness, as does normalizing. However, the parts obtained in this way are characterized by fragility and reduced impact strength.

Tempering is used after hardening to reduce brittleness and stress. To do this, the material is heated to a lower temperature and cooled in air. As the temperature increases, the tensile strength and hardness decrease, and the impact strength increases.

Dispersion hardening, also related to final processing, implies the release of dispersed particles in a solid solution after hardening with less heat for the purpose of strengthening.

Thanks to cryogenic processing, the material obtains a uniform structure and hardness. This technology is especially relevant for hardened carbon steel.

Application

The choice of any of the considered processing methods is determined by the carbon concentration in the steel. For materials with a value of this indicator up to 0.2%, it is preferable to use normalization. Steels with a carbon amount of 0.3 - 0.4% are processed by both normalization and annealing. In such cases, the choice of method is based on the required properties of the material.

Thus, normalization of steel gives it a fine-grained structure, greater strength and hardness compared to annealing. In addition, this technology is a more productive process. Therefore, other things being equal, it is more preferable.

It is preferred to hardening due to the fragility of the products obtained in this way and when processing objects with differences in cross-section in order to avoid defects.

Thus, normalization can be considered an intermediate technology in relation to them: it produces a material that is harder than annealing, but less brittle than quenching, improving the structure and reducing stress. In view of this, normalization has become more widespread in mechanical engineering.

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

What is the purpose of normalization of steel structures? — machines, welding, metalworking

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.

Steel grade 45 and its features

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.

drilling;
turning;
milling.

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

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

Selecting a heat treatment method for steel

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Source: https://stanki-info.com/s-kakoy-tselyu-proizvoditsya-normalizatsiya-stalnyh-konstruktsiy/

Conventional metal normalization

Metal normalization is one of the types of heat treatment of alloys (steel). The product is heated above (30-50 degrees) critical temperatures Ast (Asz) until complete recrystallization. The structure of the steel changes, becoming homogeneous and fine-grained. Slow cooling of the metal in air is twice as fast as annealing and much cheaper.

Cooling during annealing is carried out in a furnace, so normalizing steel is a more advantageous heat treatment option. Since there is no difference in the characteristics of the metal after normalization and annealing, many enterprises when processing low-carbon steel give preference to normalization of the product.

In medium-carbon (0.3-0.6%) and high-carbon products, after normalization, the steel will receive increased hardness. Its structure will consist of sorbitol-like pearlite and free ferrite (its amount depends on the carbon content).

It is recommended to anneal such types of steel.

But this type of heat treatment of the product is also possible: first, the high-carbon steel is subjected to normalization, and then (temperature 650-700 degrees) a high tempering is carried out, which reduces its hardness.

Thus, for the first type of steel, normalization can replace annealing, for the second - quenching with high tempering.

Purpose of metal normalization

Product normalization is used for:

Preparations for hardening the metal structure;
Elimination of hardening and internal stresses;
Obtaining a fine-grained structure in forgings (castings).

Such heat treatment allows you to correct the structure of metal that has been overheated during hot processing or during the annealing process. An example is hypereutectoid steel. In its structure, cementite (looks like a mesh) is placed along the grain boundaries, and this worsens its mechanical properties.

The essence of the normalization process

Internal stresses can be thermal or structural. The first ones occur after uneven heating and different cooling rates of parts. The latter appear as a result of structural transformations inside the part. Reaching a large value and adding up to the stresses arising during operation, internal stresses can destroy the metal (the tensile strength is exceeded).

They can be eliminated by special annealing. It is carried out at a temperature that is lower than the recrystallization temperature. Elevated temperature allows dislocations to be redistributed. They move from areas where there is an increased level of internal stress to places with a reduced level. The operation, which lasts several hours, relieves internal stress.

Calm air promotes a cooling rate of 150-250 degrees/hour. If it is necessary to normalize massive products, the speed is selected based on the size of the steel and its composition. An increased heating rate, minimum temperatures and holding time will make it possible to obtain finer austenite grains and a dispersed mixture of pearlite (sorbitol with ferrite).

The advantages of steel normalization are simplicity (no furnace is needed for cooling) and efficiency (less time and energy is spent on the operation). For some types of steel, normalization will be the final heat treatment (production of channels, angles, rails), for others it will be a preliminary operation.

To order metal normalization services, you can contact us right now by filling out the feedback form or calling us at the numbers listed on the website.

Source: http://pzto.pro/services/obychnaya_normalizaciya.html

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.

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