How to harden metal at home: on fire, in oil or oven

• Hardening
• Heating the metal
• Product protection from scale and decarburization
• Coolants
• Vacation process

Heat treatment of steels is one of the most important operations in mechanical engineering, the correct implementation of which determines the quality of the products. Quenching and tempering of steels are one of the various types of heat treatment of metals.

Thermal effects on metal change its properties and structure. This makes it possible to increase the mechanical properties of the material, the durability and reliability of products, as well as reduce the size and weight of mechanisms and machines. In addition, thanks to heat treatment, cheaper alloys can be used for the manufacture of various parts.

It also won’t hurt you to know how to cook with a semi-automatic machine.

As the Steel Was Tempered

Heat treatment of steel involves applying heat to the metal under certain conditions to change its structure and properties.

Heat treatment operations include:

• annealing;
• normalization;
• aging;
• steel hardening and steel tempering (etc.).

Heat treatment of steel: hardening, tempering - depends on the following factors:

• heating temperatures;
• heating time (speed);
• duration of exposure at a given temperature;
• cooling rate.

Introduction

There is a characteristic of steel - hereditary and acquired grain size. The grain size can be smaller or larger, and it also changes under the influence of high temperatures. How quickly depends on the amount of impurities. It is impossible to say unambiguously which crystal lattice and which compounds are better. In some cases, strength depends on this, in others ductility. This indicator must be changed depending on what kind of processing is to be done. If sheet steel or a profile is planned to be cut, then a procedure should be carried out that leads to grain coarsening. And if the work is to be done with high-carbon steel, then workpieces with a fine-grained structure are better processed.

Changing the grain size is quite difficult. In this case, hereditary propensity must be taken into account. This does not mean that the alloy will in any case have large grains, but with the same heating of two bars with different heredity, one will produce growth of compounds faster than the other. Therefore, the factor is very important when selecting heating. So not everyone can only selectively harden metal at home; you should know the chemical composition.

The alloy has many impurities. Among them:

• Ferrite. This is the fundamental element that is most abundant. It carries the basic properties, other substances only increase or decrease them.
• Perlite. Increases hardness and tensile and compressive strength.
• Cementite. Chemical formula – iron with carbon. And although element “C” increases the strength characteristics, if you use pure FeC, you may be surprised at its fragility.
• Graphite. High-carbon Damascus steels are obtained by saturation with this impurity at the time of processing by forging.
• Austenite. Formed at a moment of very high heat. At the same time, plasticity increases, and magnetic properties disappear.

If the carbon content is from 0% to 2.18%, then we are dealing with steel - low carbon (up to 0.8%) or carbon. And if it is more than 2.18%, then we have durable cast iron. We conclude: the characteristics depend on two reasons:

• amount of impurities;
• degree of thermal treatment.

And if you can’t change the first one yourself, then the second one certainly can.

Types of steel tempering

The main technical parameter of the OS is the heating temperature. There are 3 types of OS - high, medium and low. Of course, high-temperature tempering is the optimal means of processing, since the higher the heating temperature, the more actively recrystallization of the metal will occur. However, low- and medium-temperature processing methods also have practical benefits that should not be underestimated. Below we will look at each type of OS separately.

High tempering of steel is a variant of tempering treatment at temperatures from 500 to 700 degrees. This method is the most effective, since with such heating, polygonization and recrystallization of the material occurs, which eliminates all stress inside the metal. Usually lasts from 2 to 3 hours. In the case of processing complex structures, the recommended time can increase to 6 hours.

During high annealing, a process of recrystallization occurs (bringing the substance to a state of greater thermodynamic stability) in combination with spheroidization of cementite. Cementite particles acquire a round shape with a size of 0.5 to 2 microns, and the structure of tempered sorbitol with a granular shape is acquired. Tempering sorbitol gives steel increased toughness. Alloy steels acquire a granular pearlite structure. Structural stability is ensured and internal stress is relieved.

Technological processes at our plant are carried out in modern computerized equipment under the control of qualified personnel. This helps to achieve the highest performance in the field of chemical-thermal processing of metals. We practice an individual approach to each client and each order.

The main disadvantage of high-temperature tempering is a slight decrease in the strength of the material. Therefore, the technique is not suitable for processing parts that will experience extremely high loads during operation. The high temperature technique applies to all types of steel, but note that in the case of some alloy alloys, so-called reversible high temperature embrittlement may occur during processing.

The main feature of medium tempering is the active diffusion of carbon without polygonization and recrystallization of the alloy. In the case of medium-temperature treatment, the elasticity of the material improves and its relaxation resistance increases. The steel tempering temperature in this case ranges from 350 to 500 degrees. The average processing time is 2-4 hours. The optimal environment is oily or alkaline. Medium processing is well suited for durable parts of complex shapes - springs, springs, impact structures. However, in practice this technology is rarely used due to a number of limitations:

• In the temperature range from 250 to 300 degrees there is a so-called island of brittleness of the first kind, which should be avoided. At the same time, at temperatures above 500 degrees, there is another island of fragility of the second type (it is also recommended to avoid it). We will discuss the features of these islands below. A slight deviation in temperature up or down during the holidays can lead to fatal consequences.
• The technique has no advantages in comparison with alternative technologies (low and high). At the same time, weak processing furnaces usually cannot heat the working environment to such temperatures, and stronger furnaces can reach higher temperatures, which is inconvenient from a practical point of view.

Low tempering of steel is a method of processing a steel alloy or product in which heating is carried out to a temperature of 100 to 250 degrees. The processing time is usually 1-3 hours depending on the type of part and its dimensions. During low-temperature processing, diffusion of particles of carbon components occurs without polygonization and recrystallization of the atomic lattice. This allows you to increase some of the physical characteristics of the material - strength, ductility, hardness, chemical inertness.

Low tempering is a universal technology, but in fact it is used mainly for tempering products made from low-alloy and high-carbon steels (knives, utensils, simple parts). You also need to avoid heating the material above a temperature of 250 degrees (otherwise it will fall into a type I island of fragility, which can lead to irreversible damage to the metal).

Temperature table for quenching and tempering steels

Differences between hardenability and hardenability

Each grade of steel has a certain hardenability, which is characterized by its ability to acquire the required hardness during hardening. The main factors affecting the hardenability of steel are the percentages of carbon and alloying additives. The lower limit of carbon content, after which steel does not accept hardening, is 0.2%. Hardenability is characterized by the depth of penetration into the volume of metal of a hardened structure (fully martensitic or consisting of troostite and martensite). Alloying additives in the form of molybdenum, chromium, nickel, etc. increase both hardenability and hardenability, and the addition of cobalt reduces them.

Types of steel hardening

The main parameters for hardening steel: heating temperature and cooling rate. They completely depend on the grade of steel - carbon content and alloying substances.

Hardening in one environment

When hardening steel, the environment determines the cooling rate. The greatest hardness is obtained when the part is dipped in water. This way you can heat medium-carbon low-alloy steels and some stainless steels.

If the metal contains more than 0.5% carbon and alloying elements, then when cooled in water, the part will crack - become covered with cracks or completely collapse.

High-alloy steels increase their hardness even when cooled in air.

When quenching in water, alloy steel is heated to 40–60⁰. The cold liquid will bounce off the hot surface, forming a steam jacket. The cooling rate will be significantly reduced.

Step hardening

Hardening of steels with complex composition can be carried out in several stages. To speed up the cooling of large parts made of high-alloy steels, they are first dipped in water. The residence time of the part is determined by several minutes. After this, quenching continues in oil.

Water quickly cools the metal on the surface. After this, the part is dipped in oil and cools to the critical temperature of structural transformations of 300–320⁰. Further cooling is carried out in air.

If you heat massive parts only in oil, the temperature from the inside will slow down the cooling and significantly reduce the hardness.

Isothermal hardening

It is difficult to harden metal with a high carbon content, especially tools made of tool steel - axes, springs, chisels. When rapidly cooled, strong stresses are formed in it. High temperature tempering removes some of the hardness. Hardening is carried out in stages:

1. Normalization to improve structure.
2. Heating to hardening temperature.
3. Dipping into a bath of saltpeter, heated to 300–350⁰, and soaking in it.

After hardening in a saltpeter bath, tempering is not necessary. Stresses are released during slow cooling.

Isothermal hardening

Light hardening

There is no technical term for “light hardening”. When alloy steels are hardened, including heating, in a vacuum or inert gases, the metal does not darken. Hardening in a protective gas environment is expensive and requires special equipment separately for each type of part. It is used only for mass production of the same type of product.

In a vertical furnace, the part is heated, passing through an inductor, and immediately lowered below - into a salt or nitrate bath. The equipment must be sealed. After each cycle, the air is pumped out of it.

Hardening with self-tempering

During rapid cooling during the hardening process of steel, heat remains inside the part, which gradually leaves and releases the material - relieving stress. Self-tempering can only be done by specialists who know how much the time a part remains in the coolant can be reduced.

Self-tempering can be done at home if you need to slightly increase the hardness of fasteners or small parts. It is necessary to lay them on heat-insulating material and cover with asbestos on top.

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.

Tempering and aging of metal

Often, by hardening, not only the hardness of the metal increases, but also its fragility, so it is necessary to perform one more stage - tempering, during which the strength and hardness are somewhat reduced, but the material becomes more ductile. Tempering is done at a temperature lower than in the previous process, and the metal is cooled gradually.

Hardening can be carried out without changing the structure of the metal (polymorphic transformation). In this case, there will be no problems with brittleness, but the required hardness will not be achieved. And it can be increased through another heat treatment process called aging. During aging, the supersaturated solid solution decomposes, as a result of which the strength and hardness of the material increases.

Steel tempering is a type of heat treatment used for parts hardened to a critical point at which a polymorphic change in the crystal lattice occurs. It involves keeping the metal in a heated state for a certain period of time and slowly cooling it in the open air. Tempering is done to reduce internal stress, as well as eliminate the fragility of the metal and increase its ductility.

Through aging, the required hardness of hardened steel is achieved. Aging can be:

• natural, in which the strength of the hardened metal spontaneously increases and its ductility decreases. This process occurs when kept in a natural environment;
• thermal. Aging is the process of increasing the hardness of a metal through exposure to high temperatures. Compared to the first type, in this case overaging may occur - this is when hardness, strength and fluidity limits, having reached their maximum value, begin to decrease;
• deformation. This aging is achieved through plastic deformation of a hardened alloy having the structure of a supersaturated solid solution.

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.

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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” forms on the surface of the steel;
• nucleate boiling, which occurs with the complete destruction of this steam jacket;
• convective heat transfer.

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.

Types of hardening - with and without polymorphic transformation

Hardening of steels proceeds with a polymorphic transformation, non-ferrous metals and alloys - without them.

Hardening of steels with polymorphic transformation

In carbon steels, when temperatures rise above a certain level, a series of phase transformations occur, causing changes in the crystal lattice. At critical temperatures, the value of which depends on the percentage of carbon, the decomposition of iron carbide occurs and the formation of a solution of carbon in iron, called austenite. With slow cooling, austenite gradually disintegrates, and the crystal lattice acquires its original state. If carbon steels are cooled at a high speed, then, depending on the quenching mode, various phase states are formed in them, the strongest of which is martensite.

To obtain a martensitic structure, hypoeutectoid steels (up to 0.8% C) are heated to temperatures above the Ac3 point by 30-50°C, for hypereutectoid steels - 30-50° above Ac1. Using this technology, metal-cutting tools are hardened and products are strengthened, which are subject to friction during operation: gears, shafts, races, bushings. When heated to lower temperatures, the structure of hypoeutectoid steels, along with martensite, retains softer ferrite, which reduces the hardness of the metal and worsens its mechanical characteristics after tempering. Such hardening of steel is called incomplete and in most cases is a defect. But it can be used in some cases to avoid cracks.

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 homogeneous 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.

What is temper brittleness

The release temperature affects the quality of processing - the higher the temperature, the higher the quality of processing. However, metallurgical scientists have found that this rule has 2 exceptions, when an increase in temperature does not lead to an improvement, but to a deterioration in the quality of the material. These two exceptions are often called temper brittleness islands in practice. Fortunately, several effective, safe ways to bypass these islands have been developed, so the issue of tempering ability is not significant in modern metallurgy. Let's look at each of the islands separately + learn how to get around them.

Irreversible low temperature embrittlement

Another name is fragility of the first kind. It occurs during prolonged processing of the material at temperatures from 250 to 300 degrees, and this fragility extends to all types of steel alloys. Explanation of the phenomenon: when heated in a given temperature range, carbon begins to actively distribute over the surface of the crystal lattice. However, the distribution of carbon is extremely uneven - this leads to disruption of the crystalline structure of the metal, which leads to a serious increase in fragility. As the name implies, this fragility is irreversible (that is, the islands remain stable for an unlimited time, and damaged material is only suitable for remelting). The method of dealing with this fragility is trivial - you need to use either low or medium heat treatment - but not “intermediate” between them.

Reversible high temperature embrittlement

Another name is fragility of the second kind. It occurs only when three factors are combined simultaneously. The first factor is that the metal is heated above a temperature of 500 degrees (that is, this fragility is characteristic of high tempering processing). The second factor is that steel is an alloy with a high content of chromium, manganese or nickel. The third factor is a very low cooling rate. Explanation of the phenomenon: with a combination of three factors, an uneven distribution of carbon, chromium, manganese and nickel atoms also occurs, which leads to disruption of the crystal lattice of the alloy. There are many ways to combat this fragility - let's look at two of them:

• Method No. 1: after the formation of brittleness, the material is reheated to a given temperature - only heating is carried out in an oil environment, and the metal is cooled after tempering very quickly.
• Method No. 2: during tempering, tungsten (about 1% of the total mass) or molybdenum (0.3-0.4%) is additionally added to the alloy - after this, high tempering is performed using standard technology.

Technological nuances: how to properly harden metal

The procedure itself includes three steps - heating, holding and cooling. Depending on what result you want to get and what material you are working on, you choose different parameters: limit, duration, and cooling methods. Here is a table with several steel grades:

There are two main purposes of heat treatment:

• increasing strength - this is necessary for knives, axes, drills and other tools used to process hard surfaces;
• increasing the plasticity of the product. For example, before forging or bending - it is used not in everyday life, but in a small private business.

When carrying out the heating technology, you should monitor the color of the workpiece. It should be deep red with an orange or yellowish tint depending on the type. There should be no black or other colored spots on the surface.

When carrying out the heating technology, you should monitor the color of the workpiece. It should be deep red with an orange or yellowish tint depending on the type. There should be no black or other colored spots on the surface.

How to properly harden metal and iron if there is no special kiln for firing? Use a blowtorch or make a regular fire - its temperature and burning time are high enough to do work that does not exceed domestic needs.

Cooling can be carried out in various ways. If you urgently need to reduce the heat in one area of ​​the product, you can use a directed stream of cold water. Water, and therefore rapid, cooling is necessary for alloy and carbon steels. After heating, you should take the element with tongs (if it is a small knife, an ax) and place it in a previously prepared container with liquid. When leaving, cool gradually - first with water and then with oil.

And the third option is gradual cooling in the fresh air. This is also an effective method when you need to leave a slight plasticity effect. Let's watch a video on this topic:

What kind of steel is hardened?

Only metal that contains at least 0.45% carbon is thermally treated, as well as tool and alloy steel, the hardness of which becomes several times higher after hardening. The metal in which the carbon content does not exceed 0.45% is not thermally treated. Below is a table of heat treatment modes for some types of steels.

Heat treatment: how best to harden iron at home

This is a heating process followed by further cooling to change properties. We place a regular alloy in the furnace, and take out a hardened one, which is less susceptible to external deformations. What is it for? During primary processing, for example during stamping, cutting or casting, internal stresses appear inside the alloy, which have a very negative effect on the strength characteristics and increase brittleness. There are four types of heat treatment:

• Annealing. Necessary for the formation of ferrite and pearlite. It consists of heating in a furnace to 680-740 degrees, when the recrystallization threshold has already passed. As a result, old molecular bonds break down and new ones form. Then follows some exposure at a temperature of 400-500, at the end - cooling, slow, together with the heating element and simply open doors.
• Normalization is similar to the procedure for relieving internal stress, but heating is higher and cooling is much faster.
• Hardening. The main process that occurs is a change in grain size, which leads to the desired results. Cooling is very rapid, often in water or oil.
• Vacation. It comes in several modes. Let's talk about it separately.

Steel hardening and tempering technology

Heat treatment of steels is one of the most important operations in mechanical engineering, the correct implementation of which determines the quality of the products. Quenching and tempering of steels are one of the various types of heat treatment of metals.

Thermal effects on metal change its properties and structure. This makes it possible to increase the mechanical properties of the material, the durability and reliability of products, as well as reduce the size and weight of mechanisms and machines. In addition, thanks to heat treatment, cheaper alloys can be used for the manufacture of various parts.

Heat treatment of steel involves applying heat to the metal under certain conditions to change its structure and properties.

Heat treatment operations include:

• annealing;
• normalization;
• aging;
• steel hardening and steel tempering (etc.).

Heat treatment of steel: hardening, tempering - depends on the following factors:

• heating temperatures;
• heating time (speed);
• duration of exposure at a given temperature;
• cooling rate.

Checking hardness after hardening metal at home

The word familiar to everyone in everyday life is a precise term and is applied mainly to solid products. To check, a ball or cone made of tool steel is pressed into the surface, and then a calculation is made using formulas depending on how deep the mark is left and what force was applied. There is another option - a Rockwell device, but using it at home or in an apartment is almost impossible.

The unit of hardness measurement is HRC. To compare values:

• kitchen knife, strong, expensive - from 55 to 63;
• small gears in cars - from 52 to 58;
• tips, drill tools, drills - from 60 and above.

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.

Common media for self-heating

For hardening steel at home, the following cooling media are usually used: air, water and aqueous solutions, mineral oil. 10-15% sodium chloride (table salt) is usually used as aqueous solutions, and mineral oil in home workshops is most often used as a regular engine oil. To harden individual parts of a product with different hardness, hardening with sequential cooling in two environments is used. Each of these quenching media is characterized by its own cooling rate, which directly affects the structure of the metal being processed. For example, air cools steel at a rate of 5÷10 °C per second, oil - 140÷150 °C, and water (depending on temperature) - 700÷1400 °C.

In order to properly and without problems harden your product, you need to know the brand of metal from which it is made, since both the heating temperature and the cooling method depend on this. For their products, craftsmen most often use used products made from high-speed and tool steels, which can be hardened in a home workshop, as starting materials. The table below shows the recommended temperature conditions and cooling environments for various steels.

Hardening metal in oil

Oil conducts heat rather poorly, which contributes to the slower formation of structural elements of steel. Therefore, if it is hardened in an oil environment, it will acquire strength and elasticity along with hardness. In production, industrial oil I-20 or modern quenching oils such as “Thermoyl”, “Thermo” or “Voltex” are usually used for hardening. In home workshops, craftsmen use what is available. Most often this is new or used motor oil. To safely quench a part in such oil at home, you need to remember that it has a much lower flash point compared to industrial quenching liquids, and when hot metal is immersed in it, it ignites for a short time, releasing acrid smoke. Therefore, a hardening container used in a home workshop should have a minimal exposed surface and be used only outdoors or in a ventilated area. In addition to ordinary buckets and cans, one of the most common designs of such containers, which are used by home craftsmen, is an elongated section of pipe of a suitable diameter with a welded bottom.

This is interesting: Specific density of copper and its specific gravity. Applications of copper

Hardening a knife with graphite

Heat treatment of metal with graphite is good when you need to harden not the entire object, but only part of it. For a knife, this is the edge. The sequence of the knife heat treatment process at home:

• We check the tip of the knife for hardness using a needle file. If the metal grinds off easily, and the needle file makes a dull sound, then the knife is not heat-treated;
• for this process you will need graphite, which can be obtained from round batteries, take the leads of a simple pencil, or use the graphite brushes of a generator;
• we turn the mined graphite into powder;
• We use a DC welding machine as a power source. Set to minimum;
• We make the substrate from galvanized sheet. Pour graphite powder onto it;
• We connect the “plus” of the welding device to the substrate, and the “minus” to the knife handle;
• then carefully move the knife blade along the graphite so that it does not touch the substrate. We also make sure that the graphite does not ignite, otherwise our knife will be damaged;
• when the blade moves across the graphite, the latter will produce sparks. As soon as we see that the tip of the knife has heated up, we stop the process. Approximate hardening time: no more than 5 minutes;
• let the knife cool naturally, then take a file and check the hardness. If the sound made by the file upon contact with the knife is loud, and the tip cannot be sharpened, then the hardness of the blade is high.

The hardening process in production is much easier than at home. If necessary, you can try to harden the desired object or tool using “clumsy” methods using improvised means.

Literature and sources used:

• Surface phenomena in metals and alloys / V.K. Semenchenko. - M.: Gostekhizdat
• Ultra-fast hardening of liquid alloys. — Moscow: Mechanical Engineering
• Wikipedia article

Source

Equipment and features of the process

To carry out the technological process of processing the material, it is necessary to use certain equipment. Special ovens are used for heating. They can run on electricity, gas or solid fuel. In addition to the heating structure, you need to prepare a container filled with water or oil. It is needed for rapid cooling of the workpiece.

Manufacturing a chamber for hardening metal

The main materials for the manufacture of home furnace bodies for steel hardening are solid refractories in the form of blocks of various sizes and fireclay clay. In such a furnace, a temperature of over 1200 °C is reached, so it is possible to harden products not only from carbon or tool steel, but also from high-alloy steel. When making home stoves from fireclay clay, a cardboard frame is first made according to the shape and size of the working chamber, which is then covered with a layer of fireclay. A heating coil is wound over it, and then the main heat-insulating layer is applied. With this design, the heating area is isolated from the heating element, which is important when it is necessary to harden steel that is sensitive to oxides and carbon burnout.

The most common design of home hardening furnaces are installations whose thermal bodies are made of fireclay bricks or similar refractories. The operating temperature of such materials is more than 1400 °C, so in such furnaces it is possible to harden almost any type of steel and many refractory alloys. Structurally, such a home oven is similar to a conventional wood-burning oven, only it is much smaller in size. The metal in it is heated using an electric spiral placed in grooves along the perimeter of the internal space. If it is necessary to qualitatively harden steel, it must be heated to a precisely specified temperature, so most of these homemade products are equipped with thermostats (they can be freely purchased on Aliexpress).

The video below shows the design of such a home furnace with end loading and a thermostat, which allows you to harden steel with precise temperature conditions. Its thermal body is made of mullite-silica refractory plates ShPT-450.

A detailed description of the design and recommendations for creating a top-loading furnace, in which you can harden products up to 54 cm in length, can be seen in the following video. Here the thermal body of the furnace is made of fireclay bricks (ShB type) and a thermostat is also used. In addition to the top loading, a special feature of this device is a kanthal spiral, which lasts many times longer than traditional nichrome and fechral.

Features of aluminum hardening

The need to harden any aluminum product at home rarely arises, since all finished products made from casting and wrought alloys usually undergo the required heat treatment and practically do not lose their hardness and rigidity during operation.
A home craftsman may have such a need after welding parts made of aluminum alloys together, since in this case they very often lose their rigidity in the area adjacent to the weld. But it is very difficult to harden aluminum at home, because to do this you need to know exactly the type of alloy and maintain thermal parameters with an accuracy of at least ±5 °C.

Cooling also requires certain skills, because if the technology is not followed accurately, the product may malfunction. If you still want to master this type of heat treatment for use at home, then first of all you need to acquire a furnace with an accurate thermostat, and also be prepared for the fact that each time you will have to harden several samples in turn to select the necessary parameters of the thermal process.

How to take a holiday on your own

Tempering of steel is carried out to reduce its brittleness and increase its ductility, which occurs during its heating to a low (compared to quenching) temperature, followed by slow cooling. Most steels (carbon and low alloy) that can be hardened in a home workshop are tempered at temperatures ranging from 150 to 250 °C (see table above). Unlike hardening, such heating does not require special equipment, so many home craftsmen use household stove ovens with thermostats for this purpose. The heating temperature during tempering can be determined by the color of the tarnish - a multi-colored oxide film that appears on the surface of the steel when heated (see figure below). If you harden steel “to martensite,” that is, with rapid cooling in water, you will get a very hard but brittle structure. Therefore, tempering is a mandatory procedure when heat treating a cutting tool.

Checking the quality of hardening

In order to determine whether a steel product has been hardened to the required hardness, the home craftsman does not have many ways.
The traditional way is to try to scratch the metal with a file (not a diamond one), which usually has a hardness of 55÷60 HRC. If grooves remain on the surface, this means that it was not possible to harden the steel to the required value and its hardness is below this value. If the file slides over the surface of the hardened metal, then its hardness is normal.

Another way to test the quality of home tempering is to scratch the surface of a bottle glass with tempered steel (see photo below). In addition to hardness, at home, if you have certain skills, you can also check the structure of the metal. To do this, it is necessary to harden several samples of the same steel in different modes, and then compare the structure and grain size by eye.

Other hardening methods

The essence of any hardening is the transformation of austenite into martensite (iron-carbon diagram). Depending on the temperature regime, hardening can be complete or incomplete. The first method is to harden tool steel, and the second is to harden non-ferrous steel.

During hardening, one or more coolants can be used. The method of heat treatment also depends on this. Depending on the cooling medium, heat treatment of the metal can be:

• using one cooler;
• with cooling;
• intermittent;
• stepped;
• isothermal.

Quenching in one cooler

This method is used for heat treatment of simple parts made of alloy and carbon steel. The part is heated to the required temperature and then cooled in liquid. Carbon steel with a diameter of 2 to 5 mm is cooled in water, parts of smaller diameter and all alloy steel are cooled in oil.

Hardening with cooling

When heat treating with a single coolant, thermal and structural internal stress conditions often occur. They develop when the temperature difference reaches a minimum. Tensile stress is formed on the surface of the metal, and compressive stress is formed in the center. To reduce these stresses, before lowering the heated part into the liquid, it is kept in the open air for a short time. The temperature of the part in this case should not be below the 0.8 K line on the iron-carbon diagram.

Intermittent

This hardening is carried out in two environments - water and oil or water and air. A part heated to a critical point is first quickly cooled in water, and then slowly in oil or in the open air. This heat treatment method is used for high-carbon steel. This method is complex, since the cooling time in the first environment is very short and only a highly qualified specialist can determine it.

Stepped

With intermittent heat treatment, the part cools unevenly—thinner surfaces cool faster than others. In addition, it is very difficult to adjust the time the part is in the first medium (water). Therefore, it is better to use step hardening. This method allows the part to be cooled in an environment at a temperature above the martensitic point. The first stage is cooling and holding the part in a given environment until all sections of the part reach the same temperature. The second stage is the final slow cooling (transformation of austenite into martensite).

Isothermal

In isothermal heat treatment, the part is heated to a critical point and then lowered into an oil or salt bath at a temperature of 250 degrees. Leave for half an hour and then cool in the open air. This hardening provides high structural strength and is used for alloy and structural steels in which the decomposition of austenite in the intermediate region does not completely occur. Subsequently, it turns not into martensite, but into bainite + 20% retained austenite, enriched in carbon. With this hardening, high strength and good toughness can be achieved.

Methods for household hardening of metal

To harden a metal product at home, you first need to decide on a method for heating it to the required temperature, and also select containers for coolants.

In addition, you need to choose a home or a place in the yard where you can do hardening in compliance with all safety requirements. Open flame sources can be used for heating. But in this way it will be possible to heat and harden only small parts.

In addition, an open flame causes oxidation and decarbonization, which negatively affect the surface layer of the metal. Home craftsmen, as a rule, determine the heating temperature by the color of the heated workpiece.

The figure below shows a color chart, without which it is impossible to properly harden a carbon steel product. For alloy steels, the temperature range is usually shifted upward by 20÷50 °C.

In order to harden a steel product with complete and uniform heating, it is best to use heat sources such as forges and closed furnaces. This equipment is easy to make yourself in a home workshop, and it can be used both indoors and outdoors.

An industrial hair dryer is usually used to pressurize a forge, and charcoal, which is sold in any supermarket, is suitable as fuel. A small closed oven can be easily made from a couple of dozen fireclay bricks. Moreover, depending on the method of hardening the metal, it is possible not only to harden it, but also to temper it with heating of the entire volume of the product.

The easiest way is with cooling containers and a clamping tool. Any non-flammable vessel of sufficient size will be suitable for the quenching liquid, and the part can be held and moved with tongs or hooks with handles of a suitable length. The video below shows how you can harden an ax at home using a homemade forge and two containers with different cooling media.

Hardening on an open fire

The easiest way to harden a small part at home is to heat it over an open flame to the desired temperature, guided by color tables.
In such cases, you can use a gas burner, a blowtorch, or even a burner on a home gas stove as a heating source. The main disadvantage of such hardening is the difficulty of uniformly heating the product throughout the entire volume, since the flame creates a high temperature in a narrow, limited area.

This method is suitable when it is necessary to harden the end of an elongated product, for example, the cutting part of a drill or a chisel blade, or a small part several centimeters in size.

Another problem that a home craftsman may encounter if he decides to harden carbon steel with an open flame is severe oxidation and burning of carbon in the surface layer of iron, which leads to degradation of its structure.

Temperature

The correct temperature regime for hardening stainless steel products is an important condition for their quality. To achieve good characteristics, they are uniformly heated to 750-850°C, and then quickly cooled to a temperature of 400-450°C.

Important: Heating the metal above the recrystallization point leads to a coarse-grained structure, which worsens its properties: excessive brittleness, leading to cracking!

To relieve stress after heating the metal to the desired hardening temperature, step-by-step cooling of products is sometimes used, gradually lowering the temperature at each heating stage. This technology allows you to completely remove internal stress and obtain a durable product with the required hardness.

Gas flame hardening

The temperature regimes associated with heating and cooling can be continuous or cyclical. Surface hardening is performed in four ways.

1. Heating and cooling of a part area: hardening of wheel teeth, rail ends, valves, etc.
2. Hardening of small rotating bodies with a small width of the treated area: axle and shaft journals.
3. Continuous-sequential method: moving a flame along the surface, followed by a coolant. Sequential heating and cooling of narrow areas with water jets is carried out. The surfaces of large-diameter parts are similarly hardened with their slow rotation relative to stationary burners and nozzles. Temper zones remain at the edges of the strips during secondary heating from neighboring areas.
4. Combined method: moving along the generatrix of the flame jets, and behind them - the cooling medium while rotating the cylindrical part. The technology is used for hardening long products. The method provides a uniform solid layer on the surface of the part.

Cooling medium

Achieving the required properties of stainless materials largely depends on the choice of cooling method.

Different grades of stainless steel undergo cooling differently. If low-alloy steels are cooled in water or its solutions, then for stainless alloys oil solutions are used for these purposes.

Important: When choosing a medium in which to cool the metal after heating, it should be taken into account that cooling occurs faster in water than in oil! For example, water at a temperature of 18°C ​​can cool an alloy by 600°C in a second, but oil by only 150°C.

In order to obtain high metal hardness, cooling is carried out in running cold water. Also, to increase the hardening effect, a brine solution is prepared for cooling by adding about 10% table salt to the water, or an acidic medium containing at least 10% acid (usually sulfuric) is used.

This is interesting: Countersinking and countersinking - how to process metal parts

In addition to the choice of cooling medium, the cooling mode and speed are also important. The temperature decrease rate must be at least 150°C per second. Thus, in 3 seconds the temperature of the alloy should drop to 300°C. A further decrease in temperature can be carried out at any speed, since the structure fixed as a result of rapid cooling will no longer be destroyed at low temperatures.

Important: Cooling the metal too quickly leads to its excessive fragility! This should be taken into account when hardening yourself.

The following cooling methods are distinguished:

• Using one medium, when the product is placed in a liquid and kept there until completely cooled.
• Cooling in two liquid media: oil and water (or saline solution) for stainless steels. Products made of carbon steel are first cooled in water, since it is a fast cooling medium, and then in oil.
• Using the jet method, when the part is cooled with a stream of water. This is very convenient when you need to harden a specific area of ​​the product.
• Using the method of step cooling in compliance with temperature conditions.

Features of copper hardening

Heat treatment technologies for steel and copper have fundamental differences. Heating copper to red heat (over 600 °C) and rapidly cooling it in water causes it to release (i.e., it becomes soft).

Tempering copper at home is more difficult than tempering it, because to do this it only needs to be heated to 400 °C, at which point it does not glow. After heating to the specified temperature, the copper product is slowly cooled in air, after which it acquires hardness, as after hardening.

If there is still an urgent need to harden a certain amount of copper parts in a home workshop, you will have to acquire a pyrometer to control the heating temperature.

We have described two ways to check the quality of hardening at home. Which ones do you know? Please share information in the comments to this article.

Hardening using household appliances

For hardening, some craftsmen try to use a regular gas stove. The diameter of the 2.5 kW burner is 130 mm. When burning, a circle with an internal diameter of 85...90 and an external diameter of 130...170 mm is heated. Only the ring gets hot. The metal can be heated to a temperature of 800 ⁰C.

Heating on a gas burner:

To heat the part evenly, you need to set restrictions. A metal square contour is made, inside which the temperature can be equalized. It is advisable to thermally insulate the circuit to limit heat exchange with the environment.

For hardening, containers are used in which waste mineral oil is used.

Using a blowtorch you can get a temperature of 850...1000 ⁰C. At this temperature, it is easier to heat a suitable part to the desired temperature. To limit heat loss, place it in a thick-walled pipe. The flow of fuel combustion products is also directed there.

Heating with a blowtorch:

Attention! High-quality hardening is carried out by heating in a muffle furnace or in a forge, where the entire product is located in the heating zone.

Heating the workpiece in a charcoal forge:

Video: hardening steel at home.

Hardening steel at home or cottage

Sometimes it happens that heat treatment of metal at home or in the country is necessary. This happens if the purchased tool turns out to be under-hardened or not hardened at all. Often there is a need to harden a knife, ax or drill. Of course, good hardening can only be done under production conditions, but male craftsmen are excellent at doing this over an ordinary fire. Sequence of home hardening:

• prepare two containers. Pour mineral oil into one, water into the other;
• We also need to prepare a tool with which we will put the metal to be hardened into the fire and remove it from it. Pincers are suitable for this procedure;
• Next, we make a fire and wait for the coals to form. We place a metal object on them that needs to be hardened;
• We monitor the color of the coals and the color of the flame. Hot coals are white. And the flame should not be white. The crimson color of the flame is optimal for the hardening process at home. A white flame indicates that the temperature inside the fire is too high, and our part may simply burn out;
• It is also necessary to ensure that black or blue spots do not appear on the metal product, which indicate deformation of the metal as a result of excessive softening. And if the metal has turned white, then such a part can be safely thrown away.
• As soon as the metal object heats up to the temperature we need, we take it out and first lower it into the oil. We do this three times, the first time for three seconds. Each time we increase the time by the same amount. We lower and take out sharply;
• Next, lower the metal tool into a container of water and leave it there until it cools completely.

Parts or objects that have an elongated shape are placed vertically in the water. To estimate the hardening temperature in a fire, we use a color table. Instead of a fire, you can use any stove.

How to drill through hardened metal

First of all, we list the main features of drilling workpieces and products made of hardened metal. For successful processing you need:

1. choose the right drill;
2. prepare a workpiece or product;
3. use cutting fluid.

Which tool to choose for drilling hardened metal

For drilling hardened metal, tools made from the following grades of steel are best suited.

1. P18. Tools made from steel of this brand are the best choice. These drills for hardened metal appeared back in Soviet times. The material contains up to 18% tungsten. This gives the steel high strength. The surfaces do not overheat and wear out slowly.
2. R6M5K5. This grade of steel contains 6% tungsten and 5% each of molybdenum and cobalt. These hardened metal drills can withstand maximum heat loads when machining hardened parts and products.
3. HSS-Co. This is a foreign analogue of the previous steel.

Hardened metal drill made from HSS-Co steel

Craftsmen choose drills made from these steel grades because of the optimal combination of price and efficiency in processing high-strength hardened metals.

Note! Before drilling, it is necessary to thoroughly clean the workpiece or product from oils, grease and other contaminants.

Tips for using coolant when processing hardened metal

1. Add coolant to tool cutting edges. During processing, the liquid scatters and evaporates. The lubricant must be updated promptly.
2. Before processing a part or product, it is also necessary to apply coolant to the target surface.
3. When drilling hot metal, take short breaks to allow the workpiece and tool to cool.

Heat treatment of tool alloys

The following statement is true for almost all metals: with increasing tempering temperature, strength decreases and ductility increases. The only exceptions are high-speed steels used in the production of tools. To ensure better heat resistance and wear resistance characteristics, they are alloyed with carbide-forming elements: molybdenum, cobalt, tungsten and vanadium. And for hardening, heating to temperatures above 1200 °C is used, which allows the formed carbides to be most completely dissolved.

The thermal conductivity of iron itself and its alloying elements varies significantly, therefore, to prevent deformation and cracking during heating, temperature pauses should be performed. This occurs when temperatures reach 800 °C and 1050 °C, and for large objects the first interval is assigned at a temperature of 600 °C. The duration of the stop ranges from 5 to 20 minutes, which allows for the best conditions for dissolving carbides. Cooling is most often carried out in oil.

Deformation can be significantly reduced by stepwise heat treatment of steel in molten salts, where hardening is performed at a temperature of about 500 °C. To increase the hardness of the products, this is followed by double tempering at 570 °C. The duration of the process is 1 hour, and its mode is influenced by the chemical properties of alloying elements and temperature, which determines the rate of carbide release.

How to bend hardened metal

The following types of presses are usually used for bending metal blanks and products in production.

1. Pneumatic and hydraulic. This is standard metal bending equipment. The blanks are placed between punches and dies. This allows you to bend even thick parts and products. Hydraulic presses are used more often. Their advantages are low cost and ease of operation.
2. Rotary. Metal bending occurs between special beams and plates. The technology is excellent for processing simple hardened metal products with small dimensions.
3. Rotary. On these machines, special rollers bend the hardened metal. Rotary machines are most often used for small-scale production of large-sized products.

Bending metal on a machine

Note! Good productivity is achieved when using rotary and rotary presses. Processing occurs automatically. There is no need to calculate the effort in advance.

Classification of hot steel

Types of steel hardening are classified according to the type of heating source and method of cooling the metal. The main equipment for heating parts before hardening is still muffle furnaces, in which metal products of any size can be evenly heated. A high heating rate during continuous processing of products is ensured by hardening using high-frequency currents (induction hardening of steels) (see photo below). To harden the top layers of steel products, fairly inexpensive and effective gas-flame hardening is used, the main disadvantage of which is the inability to accurately set the heating depth. Laser hardening does not have these disadvantages, but its capabilities are limited by the low power of the radiation source. Methods for cooling a hardened part are usually classified according to the type of cooling medium, as well as sets and cycles of work operations. Some of them include tempering procedures, while others, such as various types of isothermal hardening, do not need it.

Hardening in one environment

With this method of hardening, a steel product heated to a given temperature is placed in a liquid, where it remains until it cools completely. Water is used as a quenching medium for carbon steels, and mineral oil is used for alloy steels. The disadvantage of this method is that after such hardening, significant stresses remain in the metal, so in some cases additional heat treatment (tempering) may be required.

How to cut threads in hardened metal

Tools made from high-speed steels and carbide alloys are also best suited for this operation. Taps are used to cut internal threads, and dies are used for external threads.

Internal threading technology

To cut internal threads of a certain size, three taps are usually used: rough (No. 1), semi-finish (No. 2) and finishing (No. 3).

Proceed according to the following scheme.

1. Make the markings.
2. Punch the hole.
3. Lubricate the future hole and drill.
4. Secure the part.
5. Install the drill.
6. Set the cutting mode. Start processing at low speeds. After the drill is immersed in the metal, the speed can be gradually increased.
7. Drill a hole for the thread and countersink. Remove the shavings. Lubricate the #1 tap and the workpiece.
8. Install the tool. The axes (its and the holes) must match.
9. Make the first pass. After each full turn of the tap, make a half turn in the opposite direction. If necessary, remove chips.
10. Make passes using semi-finishing and finishing taps.

External thread cutting technology

For this, dies are used. Process workpieces using this technology.

1. Place the tool in a holder of the appropriate size. Secure the die with screws.
2. Make a chamfer at the end of the workpiece.
3. Apply coolant to the surfaces.
4. Place the die on the workpiece. Its plane must be perpendicular to the axis of the workpiece.
5. Cut the thread. After one or two turns, return by half a turn.
6. Make sure the threads are cut accurately.

Defects during hardening of steel

The cause of defects during steel hardening is a number of physical and chemical factors that arise when there is a deviation from the specified parameters of the thermal process or due to the heterogeneity of the workpiece being hardened. Uneven heating or cooling of the product can lead to its deformation and internal cracks. The same reason can cause uneven phase transformations in different parts of the product, as a result of which the metal will have a structure that is heterogeneous in composition and hardness. Burnout of steel occurs due to the penetration of oxygen into the surface layer of the metal, which leads to the formation of oxides that separate its structural elements and change the physical properties of the surface layer. The reason for decarburization during steel hardening is the burnout of carbon when excess oxygen enters the furnace. These types of defects are irreparable, and the only way to deal with them is to check the tightness of the furnace or hardening in a vacuum and inert gases.

Scales and a critical decrease in carbon concentration during heating

Even a small concentration of oxygen in a hardening furnace leads to the appearance of surface scale, which is a consequence of the oxidation of the metal during its heat treatment. The same reason can cause a decrease in the amount of carbon in the surface layer of the workpiece. It is possible to completely get rid of such phenomena only by using vacuum furnaces, which provide so-called light hardening, as well as by heating the product in a nitrogen or argon environment. To minimize oxidation and decarburization, the hardening furnace must be as sealed as possible, which to some extent limits the flow of oxygen into its working space.

For hardening metals, it is recommended to use transformer or industrial oil I-20. It is not easy for a private owner to get it, so I would like to hear in the comments to this article your opinion on the possibility of using used car oil or other automobile oil for hardening steel.

Sources

• https://www.rocta.ru/info/kak-pravilno-samomu-zakalit-metall-i-stal-v-domashnih-usloviyah/
• https://WikiMetall.ru/metalloobrabotka/zakalka-stali.html
• https://HeatTreatment.ru/zakalka-stalej
• https://WikiMetall.ru/metalloobrabotka/kak-zakalit-metall.html
• https://metalloy.ru/obrabotka/termo/zakalka-metalla-v-domashnih-usloviyah
• https://martensit.ru/termoobrabotka/zakalka-metalla/
• https://plavitmetall.ru/obrabotka/zakalka-stali-v-domashnix-usloviyax.html
• https://metmastanki.ru/kak-zakalit-stal-v-domashnih-usloviyah
• https://www.rinscom.com/articles/kalenyy-metall-kharakteristiki-i-osobennosti-materiala-instrumenty-dlya-obrabotki-primenyaemye-tekhn/

Areas of application of HDTV hardening

High-frequency hardening is used in a number of technological processes for the manufacture of the following parts:

• shafts, axles and pins;
• gears, cogwheels and rims;
• teeth or grooves;
• cracks and internal parts of parts;
• crane wheels and pulleys.

Most often, high-frequency hardening is used for parts that consist of carbon steel containing half a percent carbon. Such products acquire high hardness after hardening. If the presence of carbon is less than the above, such hardness is no longer achievable, and with a higher percentage, cracks are likely to occur when cooled with a water shower.

In most situations, hardening with high-frequency currents makes it possible to replace alloyed steels with more inexpensive ones - carbon ones. This can be explained by the fact that such advantages of steels with alloying additives, such as deep hardenability and less distortion of the surface layer, lose their significance for some products. With high-frequency hardening, the metal becomes stronger and its wear resistance increases. Just like carbon steels, chromium, chromium-nickel, chromium-silicon and many other types of steels with a low percentage of alloying additives are used.