What is steel hardening called?

Types of metal hardening

Based on the cooling method, the following types of hardening are distinguished.

Quenching in one environment

This type of hardening is easier to perform, but it cannot be used for any steel and not for all products.

Rapid cooling in a wide temperature range of products of variable cross-section contributes to the occurrence of temperature unevenness and large internal stresses, called thermal.

In addition to thermal stresses, during the transformation of austenite into martensite, additional so-called structural stresses due to the fact that the transformation of austenite into martensite occurs with an increase in volume.

If the part has a complex shape or variable cross-section, then the increase in volume occurs unevenly and causes the occurrence of internal stresses.

The presence of large stresses can cause warping of the product, lead, and sometimes cracking if the magnitude of internal stresses exceeds the tensile strength.

The more carbon, the greater the volumetric changes and structural stresses, the greater the risk of cracks.

Steel with a carbon content of more than 0.8% is hardened in one environment if the products are of simple shape (balls, rollers, etc.). Otherwise, they prefer hardening either in two environments, or using the step hardening method.

Hardening in two environments

This method has found wide application for hardening high-carbon steel .

It consists of the following:

1. the part is first soaked in water and cooled to temperatures of 500-550°,

2. then quickly transferred to oil, where it is left until completely cooled.

Step hardening

With this method, the part is quickly cooled by immersion in a salt bath with a temperature of 300-250°. Hold at this temperature for 1.5-2 minutes. should ensure equalization of temperatures throughout the entire cross-section of the product, thereby eliminating thermal internal stresses. Subsequent cooling is carried out in air.

Molten salts, nitrate, and fusible metals are used as a cooling medium

Step hardening reduces internal stresses, warping and the possibility of cracking of parts.

Disadvantages of step hardening

The disadvantage of this type of hardening is that cooling in hot environments cannot provide a high cooling rate in the range of 400-600°.

In this regard, step hardening for carbon steel can be used for small cross-section products (diameter up to 10 mm, for example, drills).

For alloy steels with small values ​​of the critical hardening rate, step hardening is applicable to products of larger cross-sections.

Hardening with cooling

With this method, the part is removed from the oven and kept in air for some time before being immersed in the coolant. The exposure time in air should be such that

decomposition into the structure of perlite or sorbitol. This time is determined by the practice of hardening.

Cooling reduces internal stress and warping and is used for thin and long parts.

Surface hardening of steel

Some parts in operation require high surface hardness while maintaining a sufficiently viscous core, for example, a gear tooth, a crankshaft journal, etc.

In this case, the steel is deliberately hardened to a shallow depth. There are several methods for surface hardening steel.

Surface hardening when heated with an acetylene-oxygen flame

The product is heated with an acetylene-oxygen flame. A flame burner (Fig. 67), moving along the product at a certain speed, heats its surface.

Following the burner, a tube moves at the same speed, supplying water, with the help of which the product is cooled.

The heating depth and heating temperature are regulated by the speed of movement of the burner and the distance of the burner from the product.

Surface hardening with high frequency currents

Heating of products by high frequency currents causes heating of the surface layer of the product.

This is explained by the fact that high-frequency currents spread with uneven density across the cross section. The higher the frequency of the current, the shallower the depth of the product the currents penetrate.

Due to this, a high current density occurs at the surface of the product, causing very rapid heating of the surface layers of the metal.

This method has a number of advantages: high productivity, sufficient ease of adjusting the depth of the hardened layer, obtaining greater hardness than with conventional hardening methods, and the absence of scale and warping.

The electric current used for this purpose is obtained from special generators that produce alternating current with a frequency of up to 10 million Hz (i.e., changes in current direction per second). The city network current has a frequency of 50 Hz.

The product is heated by an inductor through which high-frequency and high-power currents pass.

The inductor induces (induces) currents in the product placed inside it (Fig. 68).

The inductor is made of hollow copper tubes, inside which cooling water circulates, so it itself does not heat up in the short period of time during which the part manages to heat up to the required temperature.

The shape of the inductor must exactly repeat the shape of the product, only then the product will be hardened to the same depth throughout the entire cross-section. Difficulties arise when the part has a complex shape, which limits the use of this method.

Cooling of the heated part is most often carried out either by an additional rain device or by water circulating inside the inductor.

Due to the fact that a new type of part requires the manufacture of a new inductor, this method is advisable to use when there are parts of the same type in mass or large-scale production.

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Source: http://www.Conatem.ru/tehnologiya_metallov/vidy-zakalki-metalla.html

Hardening of steels

Hardening is a heat treatment process that involves heating steel to a temperature above the critical temperature and then rapidly cooling it at a rate that suppresses the decomposition of austenite into a ferrite-cementite mixture and provides the martensite structure.

Martensite and martensitic transformation in steels

Martensite is a supersaturated solid solution of carbon in α-iron (α-Fe). Read what austenite, cementite, ferrite and pearlite are here. When eutectoid steel (0.8% carbon) is heated above point A1, the original pearlite structure will transform into austenite. In this case, all the carbon present in the steel will dissolve in austenite, i.e. 0.8%.

Rapid cooling at a supercritical rate (see figure below), for example in water (600 °C/sec), prevents the diffusion of carbon from austenite, but the fcc crystal lattice of austenite will rearrange into the tetragonal lattice of martensite. This process is called martensitic transformation.

It is characterized by the shear nature of the restructuring of the crystal lattice at a cooling rate at which diffusion processes become impossible. The product of martensitic transformation is martensite with a distorted tetragonal lattice.

The degree of tetragonality depends on the carbon content in the steel: the more it is, the greater the degree of tetragonality. Martensite is a hard and brittle structure of steel. Found in the form of plates, under a microscope they look like needles.

The hardening temperature for most steels is determined by the position of the critical points A1 and A3. In practice, the hardening temperature of steels is determined using steel graders. How to choose the hardening temperature of steel, taking into account points Ac1 and Ac3, read the link.

Microstructure of steel after hardening

Most steels after hardening are characterized by the structure of martensite and retained austenite, the amount of the latter depending on the carbon content and the qualitative and quantitative content of alloying elements. For structural steels of medium alloying, the amount of retained austenite can be in the range of 3-5%. In tool steels this amount can reach 20-30%.

In general, the structure of steel after hardening is determined by the final requirements for the mechanical properties of the product. Along with martensite, after quenching, ferrite or cementite may be present in the structure (in case of incomplete quenching). When steel is isothermally hardened, its structure may consist of bainite. The structure, final properties and hardening methods of steel are discussed below.

Partial hardening of steel

Partial quenching is called quenching, in which the cooling rate is not sufficient for the formation of martensite and it turns out to be below critical. This cooling rate is indicated by the blue line in the figure. During partial hardening, the “nose” of the C-curve steel seems to be touched. In this case, in the structure of the steel, along with martensite, troostite will be present in the form of black island inclusions.

The microstructure of partially hardened steel looks something like this:

Partial hardening is a defect that is eliminated by complete recrystallization of the steel, for example, during normalization or during reheating for hardening.

Incomplete hardening of steels

Quenching at temperatures lying between A1 and A3 (incomplete quenching) retains in the structure of hypoeutectoid steels, along with martensite, part of the ferrite, which reduces the hardness in the quenched state and worsens the mechanical properties after tempering. This is understandable, since the hardness of ferrite is 80HRC, and the hardness of martensite depends on the carbon content and can be more than 60HRC.

Therefore, these steels are usually heated to temperatures 30–50 °C above A3 (full hardening). In theory, incomplete hardening of steels is not permissible and is considered a defect. In practice, in some cases, incomplete quenching can be used to avoid quenching cracks. Very often this concerns hardening with high frequency currents.

With such hardening, it is necessary to take into account its feasibility: type of production, annual program, type of product responsibility, economic justification. For hypereutectoid steels, quenching at temperatures above A1 but below Acm produces excess cementite in the structure, which increases the hardness and wear resistance of the steel.

Heating above the temperature Acm leads to a decrease in hardness due to the dissolution of excess cementite and an increase in retained austenite. In this case, the austenite grain grows, which also negatively affects the mechanical characteristics of the steel.

Thus, the optimal quenching for hypoeutectoid steels is quenching from a temperature 30–50 °C above A3, and for hypereutectoid steels – at 30–50 °C above A1.

The cooling rate also affects the hardening result. The optimal cooling medium is one that quickly cools the part in the temperature range of minimum stability of supercooled austenite (in the range of the nose of the c-curve) and slowly in the temperature range of martensitic transformation.

Cooling stages during hardening

The most common quenching media are water of various temperatures, polymer solutions, alcohol solutions, oil, molten salts. When hardening in these environments, several cooling stages are distinguished:

— film cooling, when a “steam jacket” is formed on the surface of the steel;

- nucleate boiling, which occurs with the complete destruction of this steam jacket;

— convective heat transfer.

More details about the cooling stages during quenching can be found in the article “Characteristics of quenching oils” 

In addition to liquid quenching media, cooling in a gas flow of different pressures is used. It can be nitrogen (N2), helium (He) and even air. Such quenching media are often used in vacuum heat treatment. Here it is necessary to take into account the fact of the possibility of obtaining a martensitic structure - the hardenability of steel in a certain environment, i.e. the chemical composition of the steel on which the position of the c-curve depends.

Factors influencing the position of c-curves:

- Carbon. Increasing the carbon content to 0.8% increases the stability of supercooled austenite, and accordingly the c-curve shifts to the right. When the carbon content increases above 0.8%, the c-curve shifts to the left;

— Alloying elements. All alloying elements increase the stability of austenite to varying degrees. This does not apply to cobalt; it reduces the stability of supercooled austenite;

— Grain size and homogeneity. The larger the grain and the more uniform its structure, the higher the stability of austenite;

— An increase in the degree of distortion of the crystal lattice reduces the stability of supercooled austenite.

Temperature affects the position of c-curves through all of the above factors.

Methods of hardening steels

In practice, various cooling methods are used depending on the size of the parts, their chemical composition and the required structure (diagram below).

Diagram: Cooling rates for different methods of hardening steels

Continuous hardening of steel

Continuous hardening (1) is a method of cooling parts in one environment. After heating, the part is placed in a quenching medium and left there until completely cooled. This technology is the most common and is widely used in mass production. Suitable for almost all types of structural steels.

Isothermal hardening of steels

Isothermal hardening (speed 4) is done to obtain the bainitic structure of the steel. This structure is characterized by an excellent combination of strength and plastic properties. During isothermal hardening, parts are cooled in a bath of molten salts, which have a temperature 50–150 °C above the martensite point Mn, maintained at this temperature until the end of the transformation of austenite into bainite, and then cooled in air.

When hardening onto bainite, it is possible to obtain two different structures: upper and lower bainite. Upper bainite has a feathery structure. It is formed in the range of 500-350°C and consists of ferrite particles in the form of laths thick

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

What is steel hardening, types of metal hardening and the temperature used

To give metals certain qualities, for example, strength, they are subjected to a special heat treatment called hardening. During this process, the metal is heated at very high temperatures, bringing the steel to its critical point and then rapidly cooling it. For rapid cooling of steel, compressed air, water mist, and liquid polymer quenching medium can be used as a coolant.

This is a complex type of metal processing, since in this case the metal becomes not only strong, but also not as viscous and elastic as before processing. In order for a metal product to obtain the necessary qualities after hardening, various types of hardening are used. Whatever the method of hardening, it is necessary to observe certain safety measures.

1. If the part needs to be lowered into an oil bath, do this only with long-handled tongs.
2. Face masks should only be used with tempered glass.
3. Gloves for work must have fire-resistant properties.
4. Fire-resistant fabric must be used to make clothing.

Steel hardening methods

There are several types of hardening, the choice of which depends on what composition the metal has, what the nature of the workpiece is, how much it is necessary to increase the strength of the material and under what conditions cooling will occur. The methods by which metal is processed can also be divided into several subtypes.

Using one environment

The method is quite simple, but it is not suitable for every grade of steel and not for all parts. In this case, rapid cooling is used over a wide temperature range. During processing, temperature unevenness and high internal stress occur in the material, which can lead to deformation of the product and even its destruction. Material that has a high carbon content in its composition is not suitable for such processing.

Hardening of metal in several stages

In this method, after heating the steel to the desired temperature, it is immersed in a salt bath. This helps even out her temperature. The part is then cooled to normal temperature using oil or air. This method relieves internal stress and improves the mechanical qualities of the product. This method is suitable for processing small parts.

Isothermal

This type of processing is carried out in almost the same way as step hardening, but the product is kept in a salt bath for a longer time. When isothermal hardening is used, the cooling rate does not affect the quality of the part. The advantages of this type of hardening are that the steel practically does not warp and there are completely no cracks. The metal becomes more viscous.

Light

For this procedure, special ovens are used that have a protective environment. Before placing the instrument in such a furnace, it is heated in a salt bath, which contains sodium chloride, and then cooled in a bath containing a mixture of caustic potassium and caustic sodium with a small addition of water.

Hardening with self-tempering

This method is suitable for tool production. The essence of this method is that the heated parts are removed from the cooling environment before they have completely cooled down. This way it is possible to retain some heat in the core of the part.

It is this heat that makes it possible to temper the product. Only when it is produced is the product completely cooled using a special liquid.

This heat treatment is used for steel, which is used to make tools that require high strength during operation.

Methods used for cooling

When rapidly cooled, hardened steel acquires internal stress, which over time causes parts made from it to begin to warp and cracks may appear in them. Steel can acquire these negative qualities if it is cooled in water.

It is better to use oil for cooling. However, for some parts made from carbon steel, the use of oil is not suitable because the cooling process is not fast enough.

In this case, it is better to use hardening in two environments, with self-tempering or another method.

The internal stress in the metal depends on how the part is immersed in the hardening medium . Basic rules that must be followed when cooling are as follows:

• if, according to its configuration, the part has thin and thick parts, then the thick part is cooled first;
• To prevent long and elongated parts from warping, they must be lowered vertically into the quenching medium;
• if it is necessary to harden only part of the product, local hardening is used, but the entire part is immersed in the cooling medium.

If technological standards were violated during heat treatment of steel, the product may have insufficient hardness. This occurs when the temperature during heating is not high enough and the holding time is short, and also if the cooling rate was insufficient. This can be corrected by annealing and re-quenching or by using a more vigorous quenching medium.

Sometimes hardened steel has a coarse-grained structure, which entails increased brittleness. This is a consequence of overheating of the product. It is necessary to anneal and re-harden at the required temperature. If fragility appears after a burnout, it is impossible to correct such a defect.

If after hardening the part warps and cracks appear, it means that the metal has high internal stress. Such defects appear due to an uneven change in the volume of the hardened part if it has unequal dimensions and the cooling mode is disrupted.

Cracks cannot be fixed , but warping can be eliminated by straightening or straightening. Sometimes scale appears on the workpiece after hardening. It is impossible to fix such a marriage.

This can be avoided by heating parts in ovens that have a protective atmosphere.

Source: https://tokar.guru/metally/stal/zakalka-stali-temperatura-zakalki-i-ee-vidy.html

Hardening of steel: description of the heat treatment process, temperatures and types of hardening, cooling methods and defects

It is impossible to do without heat treatment when working with metals. The quality characteristics of the metal product depend on how correctly the heat treatment was carried out. Its strength and durability in service. In this article you can learn how to properly heat treat (harden) steel products.

Steel hardening

Hardening is a heat treatment operation of metal. It consists of heating the metal to a critical temperature at which the crystal lattice of the material changes , or to a temperature at which the phase dissolves in the matrix, which exists at a low temperature.

It is important to understand:

• After reaching a critical temperature, the metal undergoes rapid cooling.
• After hardening, the steel acquires the structure of martensite (named after Adolph Martens) and therefore becomes hard.
• Hardening increases the strength of steel. The metal becomes even harder and more wear-resistant.
• A distinction should be made between conventional quenching of the material and quenching to obtain excess vacancies.

Hardening modes differ in the speed of the process and heating temperature. There are also differences in the duration of exposure at a given temperature and cooling rate.

Temperature selection for hardening

The decision at what temperature to harden the metal is determined by the chemical composition of the steel.

There are two types of hardening:

Based on the diagram of critical points, one can see that hypoeutectoid steel during the process of complete hardening should be heated above the Ac3 point by 30–50 degrees C. As a result, the steel will have a homogeneous austenite structure. Subsequently, under the influence of the cooling process, it will turn into martensite.

Figure No. 1. Critical points .

Incomplete hardening is more often used for tool steel. The purpose of incomplete hardening is to reach a temperature at which the process of formation of excess phases takes place. Heating of steel occurs in the temperature range from Ac1 - Ac2 . At the same time, some amount of ferrite remaining after hardening of the steel will remain in the martensite structure.

To harden hypereutectoid steel, it is better to maintain a temperature 20–30 degrees higher than Ac1 - incomplete hardening. Because of this, cementite will be retained during heating and cooling, which increases the hardness of martensite. When hardening, hypereutectoid steel should not be heated above the specified temperature. This may affect hardness.

Cooling rate

The martensite structure is obtained by rapid cooling of austenite at the moment when the temperature of the steel contributes to the least stability of austenite (about 650-550 degrees).

When moving to the temperature zone in which martensitic transformation occurs (below 240 degrees), slow cooling is applied. As a result, the resulting structural stresses have time to level out while the hardness of the resulting martensite does not decrease.

To carry out successful heat treatment, it is very important to choose the right hardening medium. Often the following can be used as a quenching medium:

• water;
• sodium hydroxide solution (5–10%) or table salt;
• mineral oil.

To harden carbon steel, it is better to use water at a temperature of 18 degrees. Oil is suitable for hardening alloy steel.

Characteristics of steel: hardenability and hardenability

The important characteristics of steel - hardenability and hardenability - should not be confused.

Hardenability

This characteristic indicates the ability of steel to gain hardness after hardening. There are types of steel that are difficult to harden and after the heat treatment process the steel becomes insufficiently hard. They say about such material that it “did not accept hardening.”

The hardness of martensite is related to the degree of distortion of its crystal lattice. A lower carbon content in martensite contributes to less distortion in the crystal lattice, which means the hardness of the steel will be lower. If the steel contains less than 0.3% carbon, then the hardenability of such an alloy is low, and usually such alloys are not hardened.

Hardenability

This characteristic can indicate how deeply the steel has been hardened. When hardening, the surface of a steel part cools faster than the core . This occurs because the surface is in direct contact with the cooling fluid, which removes heat. And the central part of the steel part gives off its heat through the thickness of the metal and the surface, where it is absorbed by the coolant.

Hardenability is affected by the critical hardening rate - the lower it (speed), the deeper the steel is hardened. For example, coarse-grained steel, which has a low critical hardening rate, is annealed deeper than fine-grained steel, which has a high critical hardening rate.

The depth of hardenability depends on the initial structure of the alloy being hardened, the heating temperature and the quenching medium. The hardenability of steel is determined by fracture, microstructure and hardness.

Types of steel hardening

There are many ways to harden metal. Their choice is determined by the composition of the steel, the nature of the product, the required hardness and cooling conditions. Stepwise, isothermal and light hardening are often used.

Hardening in one environment

Referring to the graph of cooling curves for various hardening methods, you can see that hardening in one environment corresponds to curve 1. It is easy to perform such hardening. However, it is not suitable for every steel part. Due to the rapid decrease in temperature in steel of variable cross-section, temperature unevenness and high internal stress occur in the temperature range. This can cause the steel part to warp and crack.

Figure No. 2. Cooling curves .

A high carbon content in steel parts can cause volumetric changes in structural stresses, and this, in turn, threatens the appearance of cracks.

Hypereutectoid steels, which have a simple shape, are best quenched in one environment. To harden more complex shapes, hardening in two environments or step hardening is used.

Hardening in two environments (in Figure No. 2 this is curve 2) is used for tools made of high-carbon steel. The method itself consists in the fact that the steel is first cooled in water to 300-400 degrees , after which it is transferred to an oil environment, where it remains until it has completely cooled.

Isothermal hardening

Isothermal hardening on the graph corresponds to curve 4. Hardening is carried out similarly to step hardening. However, steel is aged longer in a hot bath. This is done in such a way as to cause complete disintegration of the austenite . In the diagram, the shutter speed is shown on the S-shaped line by points a and b. Steel that has undergone isothermal hardening can be cooled at any speed. The cooling medium can be molten salts.

Advantages of isothermal hardening:

• steel is almost impossible to warp;
• no cracks appear;
• viscosity.

Light hardening

To carry out such hardening, a specially equipped furnace equipped with a protective environment is required. In production, to obtain a clean and bright surface of hardened steel, step hardening should be used. After this, the alloy is cooled in molten caustic alkali.

Before the hardening process, the steel part is heated in a salt bath of sodium chloride at a temperature 30–50 degrees above point Ac1 (see “Critical point diagram”). The part is cooled in a bath at 180–200 degrees.

The cooling medium is a mixture consisting of 75% potassium hydroxide, 25% sodium hydroxide, to which 6–8% water (based on the weight of salt) is added.

Hardening with self-tempering

Used in the production of tool steel. The main idea of ​​hardening is to remove the steel part from the cooling medium until it is completely cooled. The withdrawal occurs at a certain point.

A certain amount of heat is retained in the core of the steel part. Subsequent vacations are made at his expense .

After the steel product reaches the required temperature for tempering due to internal heat, the steel is placed in a quenching liquid for final cooling.

Figure No. 3 - Temperature table.

Tempering is controlled by the colors of tarnish (see Figure No. 3), which is formed on a smooth metal surface at 220–330 degrees.

Using self-tempering hardening, sledgehammers, chisels, plumbing hammers and other tools are made, which require high surface hardness while maintaining internal viscosity.

Cooling methods during hardening

When steel products are rapidly cooled during hardening, there is a risk of large internal stresses occurring, which leads to warping of the material and sometimes cracks. To avoid this, where possible, it is better to cool steel parts in oil. Carbon steel, for which such cooling is not possible, is better cooled in water.

In addition to the cooling environment, the internal stress of steel products is affected by how they are immersed in the cooling environment. Namely:

• products with a thick and thin part in the quenching liquid with the bulky part first;
• If the product has an elongated shape (drills, taps), it must be immersed strictly vertically, otherwise they may warp.

Sometimes it is not necessary to harden the entire part, but only part of it. Then local hardening is applied. The product is not completely heated, but the entire part is immersed in the quenching liquid.

Defects during hardening of steel

1. Insufficient hardness . Occurs if there was a low heating temperature, short holding time at operating temperature, or insufficient cooling rate. The fix is ​​to apply a more energetic environment; anneal and then harden.
2. Overheat. Occurs if a steel part is heated to a temperature exceeding the permissible one.

   When overheated, a coarse-grained structure is formed, which leads to brittleness of the part. Can be corrected: by annealing and hardening at the right temperature.

3. Burnout. When a steel part is heated to a high temperature close to the melting point (1200–1300 degrees) in an oxidizing atmosphere. Oxygen penetrates into steel products and oxides form along the grain boundaries. This kind of steel cannot be repaired.

4. Oxidation and decarbonization . In this case, scale (oxides) form on the surface of steel parts, and carbon burns out in the surface layers of the steel. This marriage cannot be fixed. To prevent defects, ovens with a protective atmosphere should be used.
5. Warping and cracking. They arise due to internal stress. Cracks are an irreparable defect.

   Warping can be removed by straightening or straightening.

Conclusion

The most important thing when hardening metal is strict adherence to technology. Any deviation to the side leads to undesirable consequences. If you do everything correctly, then even at home you can carry out the process of hardening steel.

Source: https://stanok.guru/metalloobrabotka/termoobrabotka-metalla/zakalka-stali-process-termoobrabotki.html

What is hardening, tempering of steel and tarnish color

You may have heard these terms more than once when talking about forged knives, and steels in general. It's time to figure out what they mean.

Hardening, in its essence, is heating the finished product to a certain temperature, followed by cooling at a certain speed, and tempering is additional heating following hardening to lower temperatures with a different cooling mode; exactly which one depends on the grade of steel. The speed is regulated by the so-called. “quenching medium” - a liquid in which the blade is cooled at a certain speed: machine oil, saline solutions, air flow, etc. For example, oil cools at a rate approximately 6 times slower than circulating water.

To get to specific numbers, you need to understand why these two processes are needed at all.

What does proper hardening of steel improve?

If you ask the average person who has nothing to do with knife forging, the question “What does hardening give?” he will first talk about strength. In general, he will be right, although of the several qualities that hardening improves, hardness will still be the leader. But first things first.

• The hardness of blade steels is typically measured using the Rockwell Hardness Scale (HRC); European knives barely reach 60 HRC, Asian knives slightly exceed this mark. If we scratch two identical alloys of different hardness against each other, marks will remain on the softer one; Thus, hardness gives us an idea of ​​how well an alloy resists mechanical damage.
• Strength usually means steel’s resistance to destruction (bending, impact, etc.) - for a knife this is important when, for example, we test it “for bending”. If the steel is damp, the blade will remain partially deformed after bending. True, if the steel is overheated, it will be even worse - the blade will break; Therefore, when hardening, it is important to maintain a golden mean.
• Elasticity. This is exactly what we talked about a little higher - the ability to return to its original shape after removing the load. If the hardening is done according to all the rules, everything will be fine with this indicator: when bent by about 10 degrees (and for thin kitchen knives up to 30), the blade will return to its original shape.
• Wear resistance. The correct hardening regime improves all the indicators that are included in this concept: the ability to resist mechanical and abrasive wear, the ability to hold an edge and resistance to shock loads.

The main thing in the pursuit of all these qualities is to achieve by hardening such a compromise of all the above properties so that the knife cuts well and is durable.

How to do hardening and tempering

After the blade blank has been given the required shape, it is hardened. Of course, everything is very individual for different grades of steel, for specific products, but on average, craftsmen call the heating temperature for hardening about 700–800 degrees Celsius. The optimal color of the product in this case will be scarlet or cherry.

If the redness goes away, giving way to orange and yellow hues, the temperature has most likely exceeded 1,100 degrees - this is already too much for most steels.

The white color indicates that the temperature has reached at least 1,300 degrees, and it is not suitable for hardening - it will cause overheating; in this case, it will be impossible to restore the strength of the steel.

It is these colors that are called the colors of incandescence. We will meet with them again when we consider a vacation.

The heat colors show us the temperature the workpiece has reached. They should not be confused with tarnished colors - shades of oxides

When a blade is hardened, it gains high hardness, but at the same time loses strength. Now the strength must be restored: vacation serves this purpose. Vacation, as we remember, is reheating to lower temperatures followed by cooling; Let's add to this that between repeated heatings the blade must cool completely - naturally or by cooling it in a saline solution or oil. We select the heating temperature for tempering as follows.

• Most likely, we do not need high-temperature tempering - it is done for parts that are subjected not so much to deformation as to shock loads, and this clearly does not apply to knives. However, let’s say about it that its temperature limits are 500–680 degrees.
• Medium-temperature tempering is heating to 350–500 degrees; This is also a lot, only suitable for throwing knives.
• Low temperature holiday is what you need. Warming up here goes up to 250 degrees. Of course, the knife will not be so resistant to lateral impact loads, but we don’t need this: we have already achieved the required hardness during hardening, and now we are interested in strength. At this temperature it will turn out just right.

The desired temperature will again be shown by the heat colors: the optimal color in this case (for the knife) will be light yellow.

After each stage at which oxide products (tarnish) appear, the product should be cooled in salt water or oil. In clean water, the workpiece should not be cooled either after hardening or during tempering - due to too high a cooling rate, the product may crack.

Neither water nor oil fully meets the necessary requirements for hardening carbon steel: rapid cooling to 550 °C and slower cooling from 300 °C to 200 °C. Therefore, water is used in combination with oil: first into water, and then into oil. This method is used on tool steels and is called “into oil through water.”

But alloy steels can only be hardened in oil.

The colors of tarnish on the blade of the “Zombie” collection knife are oxides that were not removed after tempering

Selection of steel for hardening

To begin with, let’s conditionally divide all steels into high-carbon and alloyed.

All steels are alloys of iron with carbon and various alloying elements; The name of the steel will depend on whether one carbon predominates in it or whether alloying elements are present in significant quantities.

It cannot be said that this or that group is worse or better at hardening; They initially have very different characteristics and different tasks, so we will simply talk about the hardening of both steels.

Hardening of carbon steels

We have accumulated vast experience working with this steel, as well as with products made from it. By itself, it requires lower quenching temperatures than those alloyed with various elements - it already has fairly high hardness and strength indicators, which are so valued on the market.

• Low-carbon steels are hardened at temperatures from 727 to 950 °C.
• Medium and high carbon steels are hardened at temperatures from 680 to 850 °C.

It must be remembered that steels with very low carbon content cannot be hardened at all.

If we want to make and harden a blade from carbon steel at home, the following brands are suitable for us.

Russian:

American:

These grades, when properly heat treated, are characterized by great strength and hardness, although low resistance to corrosion.

Hardening of alloy steels

In addition to iron and carbon, such steels contain a significant amount of various alloying elements, which give the alloy special properties needed in a particular area.

• Chromium makes steel corrosion-resistant if its content exceeds 12–16%.
• Molybdenum and nickel increase the strength of steel and its ability to withstand high loads.
• Vanadium improves the wear resistance of the alloy and gives its blades the ability to hold an unusually sharp edge.

Due to the presence of these elements in the alloy, steel has worse thermal conductivity than pure carbon steel, therefore: 1) it will take more time to heat and cool - if the process is artificially accelerated, then cracks may appear in the alloy; 2) for hardening it needs a high temperature - from 850 to 1,100 ° C.

Unfortunately, correct heat treatment of complex alloy steels is quite difficult, since to give the blade high performance properties, both precise temperature and special equipment for deep cooling are needed. Therefore, it will not be possible to harden them qualitatively “by eye”.

The most common brands include the following:

• 420;
• 440A;
• D2;
• ATS34;
• CPM S320V.

We can say about the last sample that it is extremely wear-resistant.

Hardening knife steel at home

For simple carbon steels, even in artisanal conditions, satisfactory hardening can be done, the main thing is to arm yourself with the right knowledge.

Used tools, springs and files can be used as sources; Make sure there is no rust on them. A workpiece made from brand new melted metal is, of course, better, since parts that have served for a long time have a quality called fatigue, which reduces their strength.

Although for high-quality materials it is enough to carry out annealing, which consists of heating the steel, holding it at a certain temperature and then slowly cooling it with a furnace or in sand at a speed of two to three degrees per minute.

As a result of annealing, a stable structure is formed, free from residual stresses.

For both annealing and heating the part for hardening, you can use a homemade forge made from a pit lined with bricks, a blowtorch and a pipe. Ideally, of course, use a muffle furnace.

It’s easy to check at home whether the hardening has reached the required degree: you can run a file over the hardened product - if the hardening is not complete, the file will simply stick to the knife. Overheating can be checked in artisanal conditions by a strong blow of the workpiece against a hard object - a stone or a rail: the overheated blade shatters into pieces with such a blow.

Source: https://www.tojiro.ru/clients/blog/kukhonnye-nozhi/zakalka-i-otpusk-stali-tsveta-kaleniya-i-pobezhalosti/

What steels can be hardened?

One of the most common methods of heat treatment of metals is steel hardening.

It is with the help of hardening that the required characteristics of the finished product are formed, and its incorrect implementation can lead to excessive softness of the metal (non-hardening) or to its excessive fragility (overheating). Our article will talk about what proper hardening is and what needs to be done to accomplish it.

Steel hardening

What is the hardening of metal?

The ancient blacksmiths knew that the effect of high temperature on metal can change its structure and properties and actively used this in practice.

Subsequently, it was scientifically established that hardening of products made of steel, which involves heating and subsequent cooling of the metal, can significantly improve the mechanical characteristics of finished products, significantly increase their service life and even ultimately reduce their weight by increasing the strength of the part. What’s noteworthy is that hardening parts made from inexpensive steel makes it possible to give them the required characteristics and successfully use them instead of more expensive alloys.

The meaning of the process, which is called hardening of steel alloy products, is to heat the metal to a critical temperature and then cool it.

The main goal pursued by this heat treatment technology is to increase the hardness and strength of the metal while simultaneously reducing its ductility.

There are various types of hardening and subsequent tempering, differing in modes of implementation, which determine the final result.

Hardening modes include the heating temperature, the time and speed of its implementation, the time the part is kept in a state heated to a given temperature, and the speed at which cooling is carried out.

The most important parameter when hardening metals is the heating temperature, upon reaching which the atomic lattice is rearranged.

Naturally, for different grades of steel, the critical temperature value is different, which depends, first of all, on the level of carbon content and various impurities in their composition.

After hardening, both the hardness and brittleness of the steel increases, and a layer of scale appears on its surface, which has lost a significant amount of carbon. The thickness of this layer must be taken into account when calculating the allowance for further processing of the part.

Iron-carbon phase diagram

When hardening products made of steel alloys, it is very important to ensure a given cooling rate of the part, otherwise the already rearranged atomic structure of the metal may go into an intermediate state.

Meanwhile, too rapid cooling is also undesirable, as it can lead to the appearance of cracks on the part or to its deformation.

In order to avoid the formation of such defects, the cooling rate after the temperature of the heated metal drops to 200 degrees Celsius is somewhat slowed down.

To heat parts made of carbon steel, chamber furnaces are used, which can heat up to 800 degrees Celsius.

For hardening certain grades of steel, the critical temperature can be 1250–1300 degrees Celsius, so parts made from them are heated in a different type of furnace.

The convenience of hardening steel of these grades lies in the fact that products made from them are not subject to cracking when cooled, which eliminates the need for preheating.

You should take a very responsible approach to hardening parts of complex configurations that have thin edges and sharp transitions. To prevent cracking and warping of such parts during the heating process, it should be carried out in two stages. At the first stage, such a part is preheated to 500 degrees Celsius and only then the temperature is brought to a critical value.

Heating of steel during hardening with high frequency currents

For high-quality hardening of steels, it is important to ensure not only the level of heating, but also its uniformity.

If the part is massive or has a complex configuration, it is possible to ensure uniform heating only in several approaches.

The total heating time also increases if several parts are placed in the oven at the same time.

How to avoid scale formation and decarburization during quenching

Many steel parts are hardened after they have been finished.

In such cases, it is unacceptable for the surface of the parts to be decarburized or for scale to form on it.

There are methods for hardening steel products that avoid such problems.

Hardening, carried out in a protective gas environment, which is injected into the cavity of the heating furnace, can be classified as the most advanced of these methods. It should be borne in mind that this method is used only if the heating oven is completely sealed.

The photo shows the moment of hydrobeating at the hot rolling mill - descaling

A simpler way to avoid decarburization of the metal surface during hardening is to use cast iron shavings and used carburizer.

In order to protect the surface of the part when heated, it is placed in a special container into which these components are previously poured.

To prevent ambient air from entering such a container, which can cause oxidation processes, the outside of it is thoroughly coated with clay.

If, after hardening the metal, it is cooled not in oil, but in a salt bath, it should be deoxidized regularly (at least twice per shift) to avoid decarburization of the surface of the part and the appearance of oxide on it.

Boric acid, brown salt or charcoal can be used to deoxidize salt baths. The latter is usually placed in a special glass with a lid, the walls of which have many holes.

Such a glass should be lowered into the salt bath very carefully, since at this moment a flame flares up on its surface, which dies out after a while.

There is a simple way to check the quality of deoxidation of a salt bath. To do this, a regular stainless steel blade is heated in such a bath for several minutes (3–5). After the salt bath, the blade is placed in water to cool. If after such a procedure the blade does not bend but breaks, then the deoxidation of the bath was successful.

Volumetric hardening of thick-walled workpieces

Cooling steel during hardening

The basis of most coolants used in hardening steel products is water.

It is important that such water does not contain impurities of salts and detergents, which can significantly affect the cooling rate.

A container containing water for hardening metal products is not recommended for other purposes.

It is also important to take into account that running water cannot be used to cool the metal during the hardening process. The optimal temperature for coolant is 30 degrees Celsius.

Hardening steel products using ordinary water to cool them has a number of significant disadvantages. The most important of them is cracking and warping of parts after they have cooled.

As a rule, this cooling method is used when cementing metal, surface hardening of steel, or heat treatment of parts of a simple configuration that will later be subjected to finishing.

For products of complex shapes made from structural steels, another type of coolant is used - a 50% caustic soda solution heated to a temperature of 60 degrees Celsius. After cooling in such a solution, the hardened steel acquires a light shade.

It is very important to follow safety precautions when working with caustic soda; be sure to use a hood placed above the bathtub. When a hot part is lowered into a solution, vapors are formed that are very harmful to human health.

Hardening steel in a muffle furnace

The best coolant for thin-walled parts made of carbon steels and products made of alloys are mineral oils, which provide a constant (isothermal) cooling temperature, regardless of environmental conditions.

The main thing to avoid when using such a technical fluid is getting water into it, which can lead to cracking of parts during their cooling.

However, if water does get into such a coolant, it can be easily removed from it by heating the oil to a temperature above the boiling point of water.

Tempering steel using oil as a coolant has a number of significant disadvantages that you should definitely be aware of.

When oil comes into contact with a hot part, vapors are released that are harmful to human health; in addition, the oil may catch fire at this moment.

An oil bath also has the following property: after its use, a residue remains on the parts, and the coolant itself loses its effectiveness over time.

All these factors should be taken into account when hardening metals in an oil environment and the following safety measures should be taken:

• immerse parts in an oil bath using tongs with long handles;
• carry out all work wearing a special mask made of tempered glass and gloves made of thick fabric with fire-resistant properties or rough leather;
• reliably protect your shoulders, neck, chest with work clothes made of thick fire-resistant fabric.

Oil bath cooling

To harden certain grades of steel, cooling is carried out using an air flow created by a special compressor.

It is very important that the cooling air is completely dry, as the moisture it contains can cause the metal surface to crack.

The essence of such hardening methods is that the heated part is first placed in water, where in a short time (a few seconds) its temperature drops to 200 degrees, further cooling of the part is carried out in an oil bath, where it should be moved very quickly.

Performing hardening and tempering of steel parts at home

Heat treatment of metal products, including surface hardening of steel, not only increases the hardness and strength of the alloy, but also significantly increases internal stresses in its structure. To relieve these stresses, which can lead to breakage of the part during operation, it is necessary to release the steel product.

It should be borne in mind that such a technological operation leads to a slight decrease in the hardness of steel, but increases its ductility. To perform tempering, the essence of which is to gradually reduce the temperature of the heated part and maintain it at a certain temperature, furnaces, salt and oil baths are used.

Quenching and tempering steel at home

Temperatures at which tempering is carried out differ for different grades of steel.

What is typical is that when high-speed alloys are tempered, their hardness even increases, and in the second case its level decreases, but the ductility index increases significantly.

Hardening and tempering of steel products, including stainless steel varieties, is quite acceptable (and, moreover, often practiced) at home, if the need arises.

In such cases, electric stoves, ovens, and even hot sand can be used to heat steel products.

The temperatures to which steel products should be heated in such cases can be selected using special tables.

Before hardening or tempering steel products, they must be thoroughly cleaned; their surface should be free of dirt, traces of oil and rust.

After cleaning, the steel product should be heated so that it becomes evenly red-hot.

In order to heat it to such a state, it is necessary to perform heating in several approaches.

After the required state has been achieved, the heated product should be cooled in oil and then immediately placed in an oven preheated to 200 degrees Celsius. Then you need to gradually reduce the temperature in the oven, bringing it to 80 degrees Celsius.

This process usually takes an hour. Further cooling should be carried out in the open air, with the only exception being products made of chromium-nickel steels, for which oil baths are used to reduce the temperature. This is due to the fact that steel of such grades, when cooled slowly, can acquire so-called temper brittleness.

Source: https://steelfactoryrus.com/kakie-stali-podvergayutsya-zakalke/

Features of steel hardening

Heat treating a metal changes its characteristics. Tempering steel makes it harder and stronger. In some cases, heat treatment is carried out to refine the grain and level the structure. A simple heating and rapid cooling technology for small parts can be done at home. It is necessary to know the grade of steel and its heating temperature for hardening.

What is metal hardening?

One type of heat treatment is metal hardening. It consists of several stages performed in a certain sequence:

1. Heating metal to a certain temperature. Dwell time for leveling over the entire depth of the part.
2. Fast cooling.
3. Tempering to relieve stress and correct hardness to a specified value.

During the manufacturing process, complex parts can undergo several different types of hardening.

Based on the depth of treatment, hardening is divided into two types:

Basically, in mechanical engineering, volumetric heat treatment is used, when the part is heated to its entire depth. As a result of sudden cooling, after the completion of heat treatment, the hardness inside and outside differs by only a few units.

Surface hardening is used for parts that must be hard on top and ductile on the inside. The inductor heats the steel to a depth of 3–20 mm and immediately behind it there is a sprayer that pours water on the hot metal.

The steel is heated to austenite state. Each brand has its own temperature, determined from the table of the state of iron-carbon alloys. During sudden cooling, carbon remains inside the grain and does not enter the intercrystalline space. The transformation of the structure does not have time to occur, and the internal structure contains pearlite and ferrite. The grain becomes finer, the metal itself becomes harder.

What steels can be hardened?

When heated and rapidly cooled, internal changes in structure occur in all steels. Hardness increases only with carbon content greater than 0.4%. St. 35 according to GOST has it 0.32 - 0.4%, which means it can “get hot” - slightly change the hardness if the carbon is located at the upper limit.

Steels starting from CT45 and higher in carbon content are considered hardenable. At the same time, hardening of stainless steel with low carbon content, type 3X13, is possible. Chromium and some other alloying elements replace it in the crystal lattice and increase the hardenability of the metal.

High-alloy carbon steels contain substances that accelerate the cooling process and increase the steel's ability to harden. They require a complex step cooling system and high temperature tempering.

Temperature and heating rate

The heating temperature for hardening increases with the content of carbon and alloying substances in the steel. For St45 it is, for example, 630–650⁰, St 90HF - more than 800⁰.

High-carbon and high-alloy steels, when heated quickly, can “crack” - form small cracks on the surface and inside. They are heated in several stages. At temperatures of 300⁰ and 600⁰, exposure is done. In addition to equalizing the temperature throughout the depth, there is a structural change in the crystal lattice and a transition to other types of internal structure.

Properties of steel after hardening

After hardening of parts, structural changes occur that affect the technical characteristics of the metal:

• increases hardness and strength;
• grain decreases;
• flexibility and ductility decreases;
• fragility increases;
• abrasion resistance increases;
• fracture resistance decreases.

It is easy to obtain a high class of cleanliness on the surface of a hardened part. Raw steel is not polished, it drags on and on.

How to harden steel at home

The decision on how to heat metal is made based on several parameters:

• steel grades;
• required hardness;
• operating mode of the part;
• dimensions

Not all heat treatment methods are available to amateurs. You should choose the simplest ones. Most often, at home, you have to harden stainless steel when making knives and other home cutting tools.

The hardening temperature of chromium-containing steels is 900–1100⁰C. Heating should be checked visually. The metal should have a light orange - dark yellow color, uniform over the entire surface.

You can dip a thin stainless steel into hot water, lifting it into the air and lowering it again. The higher the carbon content, the more time the steel spends in air. One cycle lasts approximately 5 seconds.

Plain weldable steels are heated to a cherry color and cooled in water. Medium alloy materials should have a red color before immersion in water. After 10–30 seconds, they are transferred to oil, then placed in the oven.

When hardening, the maximum hardness that steel gives with this technology is obtained. Then it is reduced to the required value by high-temperature tempering.

Hardening at home

Equipment

Metal is heated in various ways. You just need to remember that the combustion temperature of wood cannot provide heating to the metal.

If you need to improve the quality of 1 part, just light a fire. It must be lined with bricks around the perimeter and, after laying the workpiece, partially closed on top, leaving gaps for air access. It's better to burn coal.

A separate area and a small part are heated with a gas and kerosene burner, constantly running the flame and heating it from all sides.

Making a muffle furnace requires a lot of time and resources. It is advisable to build it for constant use.

The coolant can be in a bucket or any other container that will ensure complete immersion of the part with an oil thickness of the 5 largest sections of the part:

• one part under the hardened product;
• two on top.

The part must be moved slowly in the coolant. Otherwise, a steam jacket will form.

Self-production of a chamber for hardening metal

The simplest semblance of a muffle furnace is made from refractory brick, fireclay clay and asbestos:

1. Wind copper wire onto the mandrel. For home voltage, a cross section of 0.8 mm is suitable. Leave long ends.
2. Place the spiral inside the bricks and fix it with clay, coating the entire inner surface.
3. Make a pallet inside - a platform for placing workpieces. To do this, you need to mix clay with asbestos.
4. Thermal insulating material can also be placed outside, reducing the heat transfer of the walls.
5. Connect the ends of the wire to the wires with a plug.
6. At the back, seal the hole between the bricks hermetically.
7. Build a lid in front that will open.

All materials should dry at room temperature.
This will take several days. Then you can lay the part on the insulating material and heat it. Hardening an ax at home.

Steel hardening

To impart certain performance qualities to steel, heat treatment has been carried out over many decades. Today, like several centuries ago, steel hardening involves heating the metal and its subsequent cooling in a certain environment.

The heating temperature of steel for hardening should be selected in accordance with the composition of the metal and the mechanical properties to be obtained. Mistakes made when choosing hardening modes will lead to an increase in the fragility of the structure or the softness of the surface layer.

That is why we will consider methods of steel hardening, features of the technologies used, as well as many other points.

Steel hardening

What is the hardening of metal?

The ancient blacksmiths knew why steel was hardened for. The correctly selected steel hardening temperature allows you to change the basic operational characteristics of the material, as the structure is transformed.

Hardening is the heat treatment of steel, which today is carried out to improve the mechanical properties of the metal. The process is based on the rearrangement of the atomic lattice due to exposure to high temperature followed by cooling.

Steel hardening technology makes it possible to give inexpensive grades of metal higher performance qualities. Due to this, the cost of manufactured products is reduced and the profitability of established production is increased.

The main goals pursued during hardening:

1. Increasing the hardness of the surface layer.
2. Increase in strength index.
3. Reducing ductility to the required value, which significantly increases bending resistance.
4. Reducing the weight of products while maintaining strength and hardness

There are a variety of methods for hardening steel followed by tempering, which differ significantly from each other. The most important heating modes are:

1. Heating temperature.
2. Time required for heating.
3. The holding time of the metal at a given temperature.
4. Cooling rate.

Changes in the properties of steel during hardening can take place depending on all of the above indicators, but the most significant is the heating temperature. How the restructuring of the atomic lattice will occur depends on it. For example, the holding time when hardening steel is selected in accordance with the strength and hardness the gear must have to ensure long-term operation under conditions of increased wear.

Steel hardening colors

When considering which steels are subject to hardening, it is worth considering that the heating temperature depends on the level of carbon content and various impurities. The units of steel hardening are represented by the maximum temperature as well as the holding time.

When considering this process of changing basic performance properties, the following points should be taken into account:

1. Hardening is aimed at increasing hardness. However, as hardness increases, the metal becomes more brittle.
2. A layer of scale may form on the surface, since the loss of carbon and other impurities in the surface layers is greater than in the middle. The thickness of this layer is taken into account when calculating the allowance and the maximum dimensions of future parts.

Carbon steel is hardened taking into account the rate at which cooling will occur. If the developed technologies are not followed, a situation may arise when the rearranged atomic lattice goes into an intermediate state. This will significantly deteriorate the basic qualities of the material. For example, cooling at too high a rate causes the formation of cracks and various defects that prevent the workpiece from being used in the future.

The steel hardening process involves the use of chamber furnaces, which can heat the environment to a temperature of 800 degrees Celsius and maintain it for a long period. This allows you to extend the steel hardening time and improve the quality of the resulting workpieces. Some steels are suitable for hardening only if the environment is heated to a temperature of 1300 degrees Celsius, for which other furnaces are installed.

A separate technology is being developed for the case where the workpiece has thin walls and edges. It is represented by gradual heating.

Full hardening is usually used for steels and parts that are not subject to cracking or warping.

Often, staged heating technology involves reaching a temperature of 500 degrees Celsius in the first stage, after which a certain period of time is maintained to ensure uniform heating and the temperature is raised to a critical value. Cold hardening of steel does not lead to the restructuring of the entire atomic network, which determines only an insignificant increase in operational characteristics.

As previously noted, there are different types of hardening of steel, but it is always necessary to ensure uniform heating. Otherwise, the restructuring of the atomic lattice will proceed in such a way that serious defects may appear.

Methods for preventing scale formation and critically reducing carbon concentrations

The purpose of steel hardening is carried out taking into account what qualities the part should have. The process of rebuilding the atomic grid is associated with high risks of the appearance of various defects, which is taken into account at the stage of development of the technological process.

Even the most common methods, for example, quenching steel in water, are characterized by the appearance of scale or a significant increase in the fragility of the structure with a decrease in carbon concentration. In some cases, steel hardening is carried out after finishing, which does not allow even minor defects to be eliminated.

This is why technologies have been developed that reduce the likelihood of scale or cracks. An example is a technology where steel is hardened in a protective gas environment.

However, complex methods of steel hardening significantly increase the cost of the procedure, since the gas environment is achieved when installing furnaces with a high degree of tightness.

A simpler technology, in which carbon steel is hardened, involves the use of cast iron shavings or used carburizer.

In this case, the steel for hardening is placed in a container filled with the materials in question, after which only heating is carried out. The hardening temperature is slightly adjusted taking into account the created shell of chips.

The technology involves coating the outside of the container with clay in order to prevent the ingress of oxygen, which begins the oxidation process.

Heating temperature of steel during heat treatment

As previously noted, heat treatment also involves cooling steels, for which not only a water bath, but, for example, a salt bath can be used. When using acids as a coolant, one of the requirements is periodic deoxidation of steels.

This process eliminates the possibility of a decrease in the carbon concentration in the surface layer. To carry out the deoxidation process, boric acid or charcoal is used. Also, do not forget that the process of deoxidation of steels leads to the appearance of flames on the workpiece while it is lowered into the bath.

Therefore, when hardening or hardening steels using salt baths, the developed safety precautions should be followed.

Considering these methods of heat treatment with subsequent cooling, it should be noted that they significantly increase the cost of the workpiece. However, today cooling in water or quenching while filling the chamber with oxygen does not allow increasing the properties of steel without the appearance of defects.

Steel hardening - technological process

Cooling procedure

When considering all types of steel hardening, it is worth considering that not only the heating temperature has a strong effect on the structure, but also the holding time, as well as the cooling procedure. For many years, ordinary water, which does not contain many impurities, was used to cool steel.

It is worth considering that impurities in water do not allow complete hardening to be carried out while maintaining the cooling rate. The optimal temperature of water used to cool a hardened part is considered to be 30 degrees Celsius. However, it should be borne in mind that the liquid is heated when the hot workpieces are lowered.

Cold running water cannot be used for cooling.

Typically, water is used for cooling to produce non-critical parts. This is due to the fact that changes in the atomic network in this case usually lead to warping and the appearance of cracks. Hardening followed by cooling in water is carried out in the following cases:

1. When cementing metal.
2. With surface hardening.
3. With a simple workpiece shape.

The parts are not cooled in this way after finishing.

To impart the required hardness to workpieces of complex shape, a coolant consisting of caustic soda heated to a temperature of 60 degrees Celsius is used. It is worth considering that when using this coolant, hardened iron acquires a lighter shade. Experts pay attention to the importance of following safety precautions, since toxic substances can be released when the substances in question are heated.

Steel hardening process

Thin-walled parts are also subject to heat treatment. The quenching effect followed by improper cooling will cause the carbon concentration to decrease to critical values.

The way out of this situation is to use mineral oils as a cooling medium. They are used because the oil promotes uniform cooling. However, the ingress of water into the oil composition causes cracks to appear.

Therefore, workpieces must be cooled when using oil in compliance with safety precautions.

When considering the purpose of mineral oils as a coolant, some disadvantages of this method should be taken into account:

1. By observing heating modes, you can create a situation where a hot workpiece comes into contact with oil, which leads to the release of harmful substances.
2. Within a certain range of exposure to high temperatures, the oil may ignite.
3. This cooling method makes it possible to maintain the required hardness, measured in certain units, and also to avoid the appearance of cracks in the structure, but a coating remains on the surface, the removal of which also creates a very large number of problems.
4. The oil itself loses its properties over time, and its cost is quite high.

The above information determines that the liquid and cooling mode are selected depending on the shape, size of the workpiece, as well as how high quality the surface should be after hardening.

  The combined cooling method is the process of using several coolants. An example is the hardening of a part of complex shape, when cooling first takes place in water and then in an oil bath.

In this case, it is taken into account to what temperature the metal is cooled at what stage.

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