How to temper metal in oil. Metal hardening at home: hardening steel correctly. How to properly harden steel. How to harden metal at home: video, methods

Many are interested in how to properly harden metal at home and in what cases is this process required? Hardening of products from this material is carried out when it is necessary to increase its strength. For example, for hardening edges for cutting kitchen utensils (knife, pruning shears) or tools (chisel, chisel, etc.) In addition, the metal acquires a certain amount of plasticity, which makes it easier to process the material. Blacksmiths are familiar with this rule. The article will describe how to harden metal at home.

What is hardening for?

Hardening of the metal increases the hardness of the product by about 4 times. In this case, an object made of this material can easily cut the glass surface. Hardening is required due to insufficient strength of the object, or vice versa. In the first case, metal products will be jammed, and in the second, they will crumble.

Checking the level of hardening of a metal product

To check the hardening level of a metal product, take a file and run the tool along the edge of the object to be cut, for example, an ax or a knife. If you feel that the file begins to stick or stick to the metal, then this indicates that the product is not sufficiently hardened. At the same time, its edge will be soft and pliable.

If the instrument easily moves away from the object and it feels like it is stroking it, and the hand does not feel irregularities, then this means that the metal is too tempered.

It is possible to harden the metal at home. In this case, one should not turn to complex technologies. The process is done by hand.

It should be remembered that mild steels are not subject to the process. But to achieve an increase in the strength of carbon products or really.

How is hardening carried out?

The hardening technology involves two processes - heating a metal product to a high temperature and subsequent cooling.

Heat treatment of the surface is advisable if:

there is a need to give the metal additional strength;
an increase in the level of ductility is required, for example, for subsequent hot forging.

The price of hardening a metal product at a professional level is 200 rubles. for 1 kg. Fire treatment of small parts is cheaper. The price for this service is 20 rubles.

How to harden metal at home? It is necessary to familiarize yourself with some of the nuances of this case.

Heating should be uniform. There should be no black or blue stains on the metal. Do not heat the product to the extreme temperature. The fact that the process is proceeding correctly is evidenced by the appearance of a bright red color.

What equipment is used for hardening?

For example, in order to harden metal at home in the form of a drill, an electric or a lamp for soldering or a fire is used. What is suitable in a particular case depends on what temperature is required for the material being processed.

Cooling of various instruments

Instrument cooling rules vary. The process can be carried out in one or more steps. It all depends on the type of metal.

How to harden the metal not over the entire surface, but only in a certain place? In this case, jet hardening is used. It involves directing a stream of cold water to an object pointwise.

If the action is carried out with one cooler, then specialized devices are required in the form of a barrel or bucket. For this purpose, even a bath is used. This cooling method is suitable for items based on carbon steel or alloy steel.

If a two-stage cooling circuit is required to lower the temperature of the product, different media are used. This process provides metal tempering. Initially, drills or discs are cooled with water, then with machine or mineral oil. Cooling with it is the second stage of the process, since there is a risk of ignition under the influence of high temperature.

Application of water for cooling

The main cooling liquid is water. If you add a little salt or soap to it, the cooling rate will change. Therefore, the hardening tank cannot be used for hand washing. To ensure the same hardness index on the metal surface, the liquid temperature should be maintained at 20 - 30 ° C. Do not change it frequently in the tank. Do not cool the product in running water.

The disadvantage of hardening with water is the appearance of many cracks on the metal surface. In this way, objects of simple shape or cemented objects should be subjected to the process.

What is used for hardening parts of complex shapes?

How to temper complex metal? For using a fifty percent solution of caustic salt cold or heated to 50 - 60 ° C. Parts heated in a salt bath and hardened in it come out bright. The temperature of the solution must not be higher than 60 ° C.

The vapors that arise during hardening are harmful to health, so the bathtub must be equipped with a ventilation hood.

How is alloy steel hardened?

How to temper metal? At home, alloy steel is subjected to the process in a mineral oil bath. Thin carbon steel objects are hardened in the same way. The advantage of oil baths is that the cooling rate does not depend on the oil temperature. It will proceed equally quickly for any indicator.

How to quench metal in oil correctly? Water should not get into such a bath, as this can provoke the appearance of cracks on the surface of a metal object. It is noticed that if the oil is heated to a temperature of 100 ° C, then the ingress of water will not cause cracking of the product.

Cons of oil bath

Quenching produces poisonous gases.
A plaque forms on the subject.
The oil can catch fire.
The quality of the oil bath quenching is gradually decreasing.

How is metal tempering carried out?

All hardened parts are tempered. This relieves internal stress. As a result of this process, the hardness decreases and the plasticity of the material increases.

How to release hardened metal? Depending on the required temperature, the process is carried out:

in baths with oil;
in baths with saltpeter;
in ovens with air circulation;
in alkaline baths.

What determines the choice of the tempering temperature?

How to loosen hardened metal correctly, what should be considered? An important factor is the tempering temperature. It depends on the type of steel and the required hardness of the product. For example, a product that requires an HRC of 59 - 60 is tempered at a temperature of 150 - 200 ° C. In this case, the internal stress decreases, and the hardness practically does not change.

It is lowered at a temperature of 540 - 580 ° C. This process is called secondary hardening. Its result is an increase in the hardness of the product.

The metal is tempered to a tarnish color. It is heated in ovens or hot sand. The oxide film that appears when heated turns into different colors. At the same time, the surface of the metal product is cleaned of scale, carbon deposits and oil.

After tempering, the metal is usually air-cooled. Nickel-chromium products are cooled in water or oil, since the slow cooling of these grades leads to temper brittleness.

on an open fire?

How to harden metal on fire? For an easy process of metal hardening at home, you should make a fire and prepare two large containers. A lot of hot coals should be present in the fire.

Diesel or engine oil should be poured into one container, and clean water into the other. Better if it is well. Initially, a tool is prepared, with the help of which the metal, red-hot to the limit, will be held. Use blacksmith tongs. But if they are not there, then you can use something similar.

After completing the preliminary work, metal drills or other tools are placed in the center of the flame on hot coals. The white coals are much hotter than the rest. The hardening process must be monitored carefully. The flame of the fire should be crimson. If the fire turns white, then there is a threat of overheating and even combustion of the metal.

It is necessary to ensure that the crimson color is evenly distributed over all areas of the fire. Black stains should not appear on the edge of a metal product. If blue spots appear on the material, this indicates excessive softening of the material and its excessive plasticity. This should not be allowed.

After the metal product has been ignited in a fire, it should be removed from the high temperature center. The red-hot object is lowered into a container of oil many times at intervals of 3 seconds. The time span is gradually increased. It is impossible to hesitate at this stage. The operation is carried out quickly and abruptly. The product is dipped in oil until its color is no longer bright and saturated.

Then the object is immersed in a bucket of water, which must be shaken a little. At this stage, you should be extremely careful, as oil droplets on a knife or ax may dry out when immersed in an aquatic environment. The drills should be dipped into the liquid with the thick end.

You already know how to properly harden the metal. If you follow all the recommendations, then the process will bring the desired effect.

When do you use an electric oven?

At home, you have to harden products made of non-ferrous metals or steel. In this case, a very high temperature up to 900 ºС and higher will be required.

Only an electric or muffle stove can heat a metal product to such an indicator. The latter can be made by hand, but it is impossible to make an electric one.

How to make a muffle furnace?

How to harden metal at home using a handmade muffle furnace? Such a device will become very useful in the household. It will allow heat treatment of metal products without unnecessary manipulations. To make a furnace with your own hands, you need refractory clay used for coating. A chamber no more than 1 cm thick is created from this material. Its dimensions should be 210x105x75mm.

Carrying out the modeling of a muffle furnace with their own hands, they resort to using a pre-prepared form made of cardboard. So that it does not stick, it is soaked in paraffin.

Clay is smeared on the mold from the inside out. In this case, during drying, it will not shrink. When solidified, the material will itself move away from the edges of the form. Refractory clay can form the basis of the oven door.

A homemade muffle furnace must be allowed to dry in the open air. Then it is dried to the end in an oven at 100 ° C. The doors and the chamber are fired gradually as the temperature rises to 900 ºС.

The parts should be allowed to cool without removing them from the oven. Then a door joins it. Its surface is sanded with a file.

The camera is wound with 18 m of nichrome wire. Its thickness should be 0.75 mm. The first and last turns must be twisted. To avoid a short circuit, the distance between the turns is coated with clay. On a dry layer of material, you need to spread another layer 12 cm thick.

A self-made muffle furnace consists of a metal frame, the size of which is 270x200x180 mm.

For easy assembly of the case, it should be designed with two removable covers fixed by screws.

A door is hinged to the front cover. It should open horizontally. A ceramic piece must be installed on this door using bolts and gaskets.

All gaps are covered with clay, and the edges of the wire are removed to the back cover of the frame.

Then a connector and a standard cord with a plug are made. All openings between the heating parts and the frame are filled with asbestos chips.

To install the thermocouple and be able to track the metal hardening process in the chamber, two holes must be drilled. The diameter of the first one should be 1 cm, and the second 2 cm. Closing metal curtains are attached to them.

The homemade muffle furnace weighs 10 kg. It heats up to a temperature of 900 ºС within an hour. With its help, you can facilitate the process of hardening drills, files, dies and many other metal products.

Muffle equipment for metal hardening is not the only device. For this purpose, a chamber device, an electric or thermal oven, and a bath oven are used. Making a muffle furnace with your own hands is more profitable than purchasing ready-made equipment. For example, the average price of such a device on the market is 40,000 rubles.

Heat treatment of metal is one of the main methods of increasing its parameters: hardness, strength. The most common and used procedure is metal hardening. It has been in the arsenal of mankind for many centuries. Now this procedure is successfully carried out not only in industrial enterprises, but also by craftsmen in everyday life to improve metal products. If you have knowledge of how to harden metal at home, then the hardness of the object can be increased up to several times. There can be many reasons for this operation. For example, such a technological operation is used when it is necessary to give such strength, for example, to a knife, so that they can cut glass.

Cutting tools are most often hardened. It should be noted that heat treatment is carried out by him not only when an increase in hardness is required, but also when it is necessary to reduce this characteristic. If the hardness is too low, the cutting part will become difficult to use, it will become jammed. If it is very high, it will crumble under load.

When there is a need to increase the strength of the product, then there is no need to be upset - hardening steel at home can help. You do not need special equipment and special tools. But it should be understood that steels with a low percentage of carbon (low carbon) will not succumb to this procedure. Tool steels and carbon steels are easy to handle.

What is hardening?

The technology involves the implementation of heat treatment of iron. It includes heating to certain temperature values, at which changes occur in the structure of the crystal lattice, and then rapid cooling in a liquid medium (water, oil). The goal is to increase hardness.

There is a procedure in which the heating temperature is not brought up to the moment of transformation of the crystal lattice. In such a situation, a fixation of a state characteristic of a heated metal occurs. This effect is called a supersaturated solid solution.

Lattice change hardening is used for steel and its alloys. For non-ferrous metals, a procedure is provided without polymorphic changes.

Upon completion of this procedure, the steel alloy will have a higher hardness, but increased brittleness appears. Plastic properties are lost.

To reduce excessive brittleness after heating with changes in the crystal structure, another process is used - tempering. It is carried out at a lower temperature, followed by gradual cooling of the workpiece. Due to this, stress reduction occurs in the metal and brittleness decreases.

Features of the technological process

Hardening is carried out in 2 stages. In the first, the workpiece is heated to the required temperature, and in the second, it is cooled. Different types of metals and steels differ from each other in structure. Because of this, the heat treatment process is different.

Quenching is offered by many organizations, but the price of services will not be small. It will depend on the mass of the workpiece being processed. For this reason, it is worthwhile to carry out appropriate heat treatment of the metal at home.

When doing the operation yourself, it is extremely important to carry out the heating correctly. When heated, no black or blue spots should appear on the surface of the part. The correct heating process is accompanied by a bright red color of the metal. A video showing how to carry out heating will help to understand the procedure.

To heat the material to the required temperature, the following can be used:

Special electric oven;
Blowtorch;
Open fire from a bonfire.

The choice of a heating source should be based on the temperature to which the workpiece should be heated.

The cooling method should also be selected based on the characteristics of the metal, as well as on the desired end result. For example, if it is necessary to harden not the entire workpiece, but only a separate part, then it is also necessary to cool it pointwise. A jet of water is suitable for this.

The quenching technology can include instant, gradual or multi-stage cooling.

The rapid cooling process uses the same type of coolant. It is suitable for hardening carbon steel or alloy steel. To cool this way, you need one suitable container.

When hardening of another type of steel is required or tempering is required, two-stage cooling methods are used. In this case, the heated workpiece at the first stage is placed in a tank with water, and then transferred to oil - mineral or synthetic, in which the subsequent cooling process is carried out. But it is strictly forbidden to immediately place a heated part in oil, since it can ignite.

For the correct selection of hardening modes for different types of steels, it is necessary to use the corresponding tables.

For high speed steels

For alloyed tool steels

For carbon tool steels

Cooling environment

The achieved hardening result will largely depend on the cooling scheme. Different steels, as mentioned above, are cooled by different types. So, for low-alloy steels, water or solutions are used, and for stainless steels, oil and appropriate solutions.

An important point is that the choice of the cooling medium must take into account the fact that water cools the part faster than oil. For example, water at 18 ° C can cool an alloy with a temperature of 600 ° C in 1 second. But oil can only lower the temperature by 150 ° C.

To obtain high metal hardness, the cooling process is carried out under a stream of cold water. Salt solutions can also be prepared to increase the hardening effect. Approximately 10% salt is added to the water. Also used are acidic liquids containing at least 10% acid, mainly sulfuric.

In addition to coolants, cooling mode and speed will be important. The temperature must drop at least 150 ° C / sec. So, in three seconds, the temperature value should drop to 300 ° C. Subsequently, it can be cooled at any speed, because the resulting structure in the process of rapid cooling will not be further destroyed.

Please note that an overly fast cooling process will lead to increased fragility. This point must be taken into account during self-hardening.

The following cooling methods can be distinguished:

Using a single environment. The workpiece is placed in a liquid and completely cooled there;
On 2 Wednesdays. Oil and water (saline solutions) are used. Carbon steels are first cooled with water, and then in oil;
Inkjet method. The workpieces are cooled under running water. Convenient way to harden individual areas;
Step-by-step method with keeping temperature conditions.

Tempering steel over an open fire

As mentioned above, it is possible to harden the metal at home, using an open fire for this. It all starts, of course, with the creation of a fire and the preparation of a large amount of hot coals. You will also need 2 containers. Cold water is poured into the first, and oil (synthetic / mineral) into the other.

A pliers or similar tool is required to remove the hot metal. Once the tool has been prepared and a sufficient amount of coal has been generated, the workpieces can be stacked.

The color of the coals in the fire can signal their temperature. The hottest are those with a bright white hue. It is also necessary to observe the color of the fire in the fire. It also signals the degree of heating of the interior. The optimal situation is for the flame to be crimson rather than white. The last case speaks of an excessively high temperature of the fire. There is a risk of overheating.

The color of the heated iron should also be observed with care. Do not allow dark spots to form on the cutting edge. If the metal turns blue, it means that it has become excessively soft and ductile. It is not worth reaching this state.

Once calcination is completed to the required level, the subsequent cooling step can begin. To begin with, the workpiece is placed in a tank of oil. This is done in several passes with an interval of three seconds, with maximum sharpness. The interval between lowering must be gradually increased. After the steel has lost its brightness, it is possible to start the cooling process in water.

Care is required when a part is cooled in water. Oil droplets may remain on its surface and may ignite. Also, after immersing the part, the water must be mixed to maintain coolness. It is possible to visually study the process in the video.

For heat treatment of certain grades of steel and non-ferrous metals, the temperature of the open flame of the fire will not be enough, because will not be able to heat iron up to 9000 degrees. This requires the use of special furnaces - muffle or electric. Making an electric house at home is not an easy undertaking, but muffle equipment can be created.

DIY hardening chamber

If you make a muffle furnace at home, then it will allow you to temper special types of steel. The main element that you need for assembly is refractory clay. She will need to cover the inside of the stove. The thickness of the coating should be up to 1 cm.

To create the required shape and dimensions, it is recommended to prepare in advance a cardboard form that is impregnated with paraffin. Clay will be placed on it. She divorces water. Next, the seamy side of the cardboard blank is coated with a thick homogeneous mass. The cardboard itself will lag behind in drying. The metal blank will be placed inside the hole closed by the door (also made of clay).

The chamber and the door must first dry out in the open air, and then additionally at a temperature of 100 ° C. Then they are fired in an oven with a gradual rise in temperature to 900 ° C. Upon completion of firing and cooling, the elements are connected.

The finished chamber is wrapped with nichrome wire. Its diameter is 0.75 mm. The first and last layers are twisted together. When winding, it is necessary to leave gaps between the turns. They should also be filled with clay to avoid short circuits. After the clay with insulation and wire has dried, the surface is again covered with clay. The thickness is about 12cm.

When the surface layer is dry, the camera should fit into the metal case. The gap between the metal and the clay chamber is filled with asbestos chips. To provide access to the interior, the metal enclosure must also have a door lined with ceramic tiles inside. Any remaining gaps should be filled with clay and chips.

Nichrome wire is brought out from the back of the frame. An electrical voltage will be applied to it. To control the temperature and processes inside, a couple of 1-2 cm holes can be made on the front part.

Such a homemade product will allow you to temper any steel tools that require an elevated temperature (up to 950 ° C) on your own. Its weight will be approximately 10 kg. In addition, it will significantly save on production models, which are quite expensive.

They learned how to harden metal for a long time - such a procedure significantly strengthens products from it. Now industrial use is mainly used with the help of thermal furnaces, but even household hardening of steel at home can deprive a metal object of unwanted softness and ductility.

There is also a downside - excessive hardening endows the metal with excessive brittleness, but it can also be eliminated on its own by subjecting the product to tempering.

Tempering to the required degree will not allow the metal to bend easily, and at the same time will not allow it to crumble. How to properly thermally treat a steel product in order to achieve this on your own is the topic of our review of hardening methods.

What happens to the metal during quenching

Hardening is essentially red-hot or white-hot, depending on the material, the finished product, or its parts, followed by rapid cooling - single or stage-wise, in order to increase the degree of its strength.

The answer to the question - why hardening increases the strength of materials, was accurately given only after studying the crystal structure of their lattice. Prior to this, the craftsmen, without a reliable understanding of the mechanism of what constitutes metal hardening, have empirically come to the conclusion that it increases the hardness in comparison with the raw material.

When metals and alloys are heated above the critical point, their original crystal structure is destroyed.
The metal becomes soft, and the crystals become mobile and fine-grained.
After immersion in a quenching medium (rapid cooling), the grains retain their fine-grained structure, and the bond between them is strengthened.

The hardened material acquires a denser and therefore stronger structure, but at the same time fragility is added. Therefore, only the tips, edges of the cutting surface and other working parts of the products are often hardened, leaving the core itself plastic so that the wear resistance is not lost and the loads are sustained.

How to test a metal for hardness

In order to decide whether a particular material needs heat treatment, you need to find out the degree of its hardness. And only then choose a suitable method for hardening the metal at home in order to obtain the desired balance of hardness / ductility.

In essence, the hardness of a metal is the degree to which it resists the impact of a more durable object.

There are laboratory and industrial methods, reference tables, but the Rockwell technique remains the most popular and simplest method, where by pressing a diamond tip or a high-strength steel ball on the device, the degree of deepening is checked and correlated with the scale.

But if the exact numbers of the indicator on the Rockwell hardness scale are not needed, then you can estimate it by eye for metal at home. To do this, you will have to arm yourself with a file if you need to check a flat or rounded surface, or with a piece of glass if you need to test a sharp edge.

Soft metal (raw material that has not been hardened) is almost effortlessly taken with a file and does not cut the glass, only slightly scratching.
Relatively hard metal (moderate hardening) is taken with a file heavily, with significant efforts, and leaves a clear, confident groove on the glass.
Strong metal (strong hardening) the file already refuses to take, but the glass gives in to it without effort, accompanying cutting with a characteristic crunch.

Having decided on the initial degree of hardness, you can select methods for self-hardening of steel and metal objects in order to achieve the desired strength. By applying these simple tests at the end of the process, you can similarly check the hardness obtained after hardening, making sure that the result is satisfactory.

What are the types of household hardening

Depending on the task at hand and the initial steel grade, self-hardening is carried out using various methods, forcing the metal to become as strong as necessary. These types of process differ in cooling modes that are most suitable for a specific metal. If you apply an incorrect cooling mode, the result will be unsatisfactory, and the product will be damaged.

Quenching in a single cooling medium is the most popular method due to its simplicity, but it should not be used for metal with a high (from 0.8%) carbon content. Otherwise, due to the appearance of internal stresses in the structure, excessive brittleness and cracks will occur, and the product itself may deform. Therefore, this technique is only suitable for low carbon metal.
Intermittent, in 2 stages, quenching with cooling in 2 different media - water, and then in oil or in air. It is this variety that is suitable for high-carbon material or alloy steels, because it does not lead to deformations and cracks. Due to the complexity of the method, it is worth resorting to it to temper large products.
Stepwise step-by-step hardening, when, after incandescence, the product is placed in a hot salt bath for several minutes, provides even cooling over the entire section, which prevents thermal stress leading to cracks and fragility of the product. Then the metal cools in air. This method is best used for thin products with a high carbon content in the material.
Surface (partial) hardening endows metal products with surface strength and wear resistance, while maintaining the ductility of the core. This method is applicable for parts on the surface of which a significant load is applied.
Quenching followed by tempering allows you to harden the product to give it hardness only to a given depth, and leave the deeper layer plastic. This method increases the strength of the percussion tool.

For your information! It is not scary if excessive fragility is obtained - it can be eliminated using the vacation procedure.

What quenching media are suitable for self quenching

The choice of the medium where the self-hardening process will be carried out is just as important a stage as the heating itself, since crystallization and polymorphic transformations occur in different media in different ways.

In everyday life, water, oil, solutions of salts and polymers, and air are suitable for hardening cooling.

Water quickly enough is able to cool the incandescent material, which with an increased carbon content can lead to some disadvantages - deformation, brittleness, cracking. Therefore, low-carbon materials are hardened in water, or products with partial hardening.
Mineral oil cools the hot steel much more slowly, and therefore more evenly, which minimizes the appearance of uneven structure and its stress, and, accordingly, defects due to hardening. Usually, alloy steel or material with a high percentage of carbon is cooled with oil.
Aqueous solutions of sodium chloride or sodium hydroxide, with a concentration of about 10%, will cool a heated product much more evenly than just water. This will make it possible to achieve the same structural transformation over the entire section of the metal. More suitable for hardening of low alloy and high carbon steel products.
Polymer solutions (silicate, detergents) reduce the cooling rate of the material, and therefore reduce defects and product deformation.

For cooling, water is taken with a temperature of 20 ° to 80 ° C, oil - with a temperature of 20 ° to 200 ° C, salt solutions - with a temperature of 20 ° C to the maximum.

Ways to increase the hardness of the metal by hardening yourself

To make quenching or tempering, the metal should be very heated - at least to a crimson color. For these purposes, a thermal oven is optimal, and in its absence - an open flame of a fire, gas burner, blowtorch, or a high voltage current. When preparing to harden, you must first take into account many points.

The higher the initial hardness of the material, the more it needs to be heated.
The more carbon a material has in the composition, the slower it should cool down.
If the task is to harden the whole object, then it will need uniform heating over the entire surface.
It is not necessary to overheat the product, it is better to avoid the appearance of blue or black blotches on the hot surface.
Tongs and a container with a cooler (coolers, if there are several) are prepared in advance.

Full, total hardening is best done on the flame of a fire made of coals - they keep high heat for a long time, and the fire will allow you to completely place a dimensional part there and warm it up evenly.

Partial hardening, for example of the cutting edge, can be done with a blowtorch, but it can also easily harden small parts - bolts, drills, nails.

As soon as the material heats up to the required point, it is immediately taken out and transferred to a cooler (bath, container, vessel).

By applying a high current to the carbon plate, the tip strength of a metal product can be greatly improved when it is made of carbon-free or low-carbon metal.

Important! Be careful when working with oils - they can be flammable!

The hardening procedure, if the material has not yet acquired the required strength, can be repeated - but for this, each time it will have to be heated more. If the part is too fragile, then leave is applied.

How to remove the excessive hardness of the metal yourself with the help of tempering

Tempering removes the excessive hardness and brittleness of the material acquired during quenching. Vacation is essentially the same heating to the critical point and slow cooling in air, when the structural lattice changes again.

Vacation at low temperatures requires slight heating up to 250 ° C. It removes structural stress and maintains high strength. Suitable for cutting and piercing tools made of carbon material and also for low alloy steel.
Tempering at medium temperatures already requires intense heating in the range of 350 ° C to 500 ° C. It makes it possible to achieve such transformations of atoms when the structure becomes uniformly fine-grained, and therefore elastic and wear-resistant. Parts under dynamic load - springs, spirals - are subjected to such tempering.
Vacation at high temperatures requires a strong glow in the range from 500 ° C to 700 ° C. Then a structural shift occurs, returning the overly hardened part to toughness and ductility while maintaining the highest strength. Parts for shock loads require such tempering.

In summary, the picture of the vacation process should be clarified. In the first case, a weak decay will be observed in the metal, in the second, decay will occur, but the restructuring of the structure will not begin, in the third, a restructuring of the structure or crystal structure of the grains will occur.

Heat treatment of metals is one of the main ways to improve their mechanical and physicochemical characteristics: hardness, strength, and others.

One of the types of heat treatment is hardening. It has been successfully used by man in a handicraft way since ancient times. In the Middle Ages, this method of heat treatment was used to improve the strength and hardness of metal household items: axes, sickles, saws, knives, as well as military weapons in the form of spears, sabers and others.

And now they use this method of improving the characteristics of metal, not only on an industrial scale, but also at home, mainly for hardening metal household items.

What is metal hardening and its types

Quenching is understood as a type of heat treatment of a metal, consisting of heating it to a temperature at which a change in the structure of the crystal lattice (polymorphic transformation) occurs and further accelerated cooling in water or an oil medium. The purpose of this heat treatment is to increase the hardness of the metal.

Quenching is also used, at which the heating temperature of the metal does not allow the polymorphic transformation to take place. In this case, its state is fixed, which is characteristic of the metal at the heating temperature. This state is called a supersaturated solid solution.

The technology of quenching with polymorphic transformation is used mainly for products from steel alloys. Non-ferrous metals are quenched without achieving polymorphic changes.

After such processing, steel alloys become harder, but at the same time they acquire increased brittleness, losing ductility.

To reduce unwanted embrittlement after polymorphic heating, a heat treatment called tempering is applied. It is carried out at a lower temperature with a gradual further cooling of the metal. In this way, the stress of the metal after the hardening process is removed, and its fragility is reduced.

When quenching without polymorphic transformation, there is no problem with excessive brittleness, but the hardness of the alloy does not reach the required value, therefore, during repeated heat treatment, called aging, it is, on the contrary, increased due to the decomposition of a supersaturated solid solution.

Features of steel hardening

Mainly hardened stainless steel products and alloys intended for their manufacture. They have a martensitic structure and are characterized by increased hardness, leading to fragility of the products.

If such products are heat treated with heating to a certain temperature, followed by rapid tempering, then an increase in viscosity can be achieved. This will allow the use of such products in various fields.

Types of steel hardening

Depending on the purpose of stainless products, it is possible to temper the entire object or only that part of it, which should be working and have increased strength characteristics.

Therefore, the hardening of stainless products is divided into two methods: global and local.

Cooling medium

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

Different grades of stainless steels are cooled in different ways. 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 the metal is cooled after heating, it should be borne in mind that cooling occurs faster in water than in oil! For example, water with a temperature of 18 ° C can cool an alloy by 600 ° C in a second, and oil by only 150 ° C.

In order to obtain a high hardness of the metal, cooling is carried out in running cold water. Also, to increase the quenching effect for cooling, a brine solution is prepared by adding about 10% sodium chloride to the water, or an acidic environment is used, in which at least 10% acid (usually sulfuric).

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

Important: Too fast cooling of the metal leads to its excessive fragility! This should be taken into account when self-hardening.

There are the following cooling methods:

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

Temperature regime

The correct temperature regime for hardening stainless 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 that worsens its properties: excessive brittleness, leading to cracking!

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

How to temper metal at home

Using basic knowledge, you can harden steel at home. Metal heating is usually carried out using a fire, muffle electric furnaces or gas burners.

Hardening the ax at the stake and in the furnace

If you need to give additional strength to household tools, for example, to make an ax more durable, then the easiest way to harden it can be done at home.

During the manufacture of axes, a stamp is put by which you can recognize the grade of steel. We will consider the hardening process using the example of U7 tool steel.

The technology must be carried out in compliance with the following rules:

1. Annealing... Blunt the sharp edge of the blade before processing and place the ax in a burning brick oven to heat up. The heat treatment procedure must be carefully monitored to prevent overheating (permissible heating 720-780 ° C). More advanced craftsmen recognize the temperature by the color of the heat.

And for beginners, the temperature can be found out with the help of a magnet. If the magnet stops sticking to the metal, then the ax has heated up over 768 ° C (red-burgundy color) and it's time to cool down.

With a poker, move the red-hot ax to the oven door, remove the heat inward, close the door and latch, leave the heated metal in the oven for 10 hours. Let the ax cool down gradually with the stove.

2. Steel hardening... Heat the ax on a fire, stove or stove to a dark red color - temperature 800-830 ° C (the magnet has stopped magnetising, wait another 2-3 minutes).

Quenching is carried out in heated water (30 ° C) and oil. Lower the ax blade into the water 3-4 cm, vigorously moving it.

3. Letting go of the ax blade... Tempering reduces the brittleness of the steel and relieves internal stress. Grind the metal with emery to better distinguish between the colors of the shade.

Keep the ax in the oven for 1 hour at a temperature of 270-320 ° C. After aging, take out and cool in air.

Video: heat treatment of the ax at home, three stages: annealing, hardening, tempering.

Hardening the knife

It is advisable to use furnaces independently for hardening metals. For household items in the form of knives, axes, drills and others, small muffle furnaces are most suitable. In them, it is possible to achieve a hardening temperature much higher than on a fire and it is easier to achieve uniform heating of the metal.

You can make such a furnace yourself. On the Internet, you can find many simple options for its design. In such ovens, a metal product can be heated up to 700-900 ° C.

Consider how to harden a stainless steel knife at home using an electric muffle furnace. For cooling, instead of water or oil, melted sealing wax is used (you can get it in a military unit).

The sequence of the hardening process is as follows:

a knife (without a handle, if it is wooden) is placed in a cold oven;
turning on a closed oven, heat it together with a knife until a bright red color of the blade is obtained (800-900 ° C);
with a red-hot knife blade, they cut the sealing wax up to 10 times, plunging into it by 1.5 cm;
the procedure is repeated up to 5 times, heating the knife blade and cooling in sealing wax;
the remains of the sealing wax are removed with turpentine using a moistened cloth.

The procedure is best done in the fresh air, the wax smells awful when melting. Also, the knife blade can be heated over an open fire.

Video: other ways to harden the knife at home.

P.S. Knowing the behavior of the metal upon heating and its properties after heat treatment, as well as the technology of hardening, it can be successfully carried out at home to improve the characteristics of small metal products.

Metal hardening is a heat treatment in which steel workpieces are heated to a temperature exceeding the critical temperature, held at it for a certain time, and sharply cooled in water or oil.

The main purpose of hardening steel is to obtain a hard, durable, wear-resistant metal. The quality of hardening depends on the temperature and speed of heating / cooling, holding time.

The heating temperature for hardening for most metals is set according to the location of the critical points. Quenching of metals such as stainless steel is carried out at a higher temperature level than hardening of conventional steel. For example, 4X13 stainless steel is hardened at 1100 degrees. Steel R18 is hardened at 1250 degrees. This is necessary to ensure complete dissolution of excess carbide elements.

The speed of heating depends on the shape of the blanks, their hardenability, the type of heating furnaces and the heating environment. For example, a ball part heats up 3 times, and a cylinder part heats up 2 times slower than a plate. The higher the heating rate, the more efficient the heating furnace.

If the workpieces are located next to each other in a dense manner, then it will take a lot of time to heat them. To determine the heating time of products, specialists usually use flow charts. They include a list of all procedures for processing workpieces, all the necessary information is indicated (temperature level, heating duration, cooling method, devices used).

The average heating time of the workpieces is as follows (the time for heating one millimeter is indicated):

electric oven (800 degrees) - 50 seconds;
flame oven (1300 degrees) - 18 seconds;
salt bath (1300 degrees) - 9 seconds.

During heat treatment, it is necessary not only to heat the iron to the desired temperature level, but also to withstand it until the end of the structural changes. Consequently, the total residence time of products in a heating environment is the sum of the duration of heating and holding.

Cooling media

In order to cool steel products, different hardening media are usually used: water, salt solutions, fused salt, mineral oils, and so on. Quenching media vary considerably in physical characteristics.

The best medium for metal hardening is one in which cooling takes place quickly at 500-650 degrees and slowly at 200-300 degrees. There is no universal environment at the moment.

Hardening types

The different types of hardening differ in the cooling method. The more complex the shape of the workpiece, the more responsible you need to treat the selection of the cooling medium. Products should become hard, not have cracks.

Full hardening is divided into the following varieties:

1. In one cooler. The easiest and most popular method. The workpiece, heated to the hardening temperature, is immersed in a cooling medium. It sits there until it cools down completely. This method is used when hardening simple products that are made of carbon / alloy steel. Billets made of carbon steel are cooled in water (except for products with a radius of less than 2 millimeters), from alloyed steel in an oil liquid. This method can be used when carrying out mechanized hardening of metals.

2. In 2 environments (intermittent). A method in which the product is cooled in water and then immersed in another cooling medium for quenching (oil liquid). The method is used when processing tools made of metal with a high carbon content.
The disadvantage of this method is that it is not easy to determine the length of time a product has been in water. If you overexpose a part in water, it will warp, cracks will appear on it. A specialist using such hardening methods must be experienced and highly qualified.

3. Jetted. The workpieces, for which the quenching temperature has been reached, are cooled with a water jet. A similar one is used when processing internal sections, upsetting stamps, dies, and other stamping tools, in which the working part must have a martensitic structure.

When using these quenching methods, the steam jacket is not formed. Parts are hardened deeper than with ordinary water hardening. The speed of cooling depends on the temperature level, water pressure, radius and number of holes in the spray, the angle that the jet forms with the workpiece.

4. Self-leave. The method consists in keeping the products in a cooler until they have completely cooled down. At some point, the cooling is stopped to ensure that the heat required for self-tempering remains in the middle of the workpiece. This moment is determined empirically. The quality of heat treatment is directly dependent on the qualifications of the worker.

Hardening and tempering is controlled by the tarnishing colors that appear on the light part of the workpiece. The appearance of tarnishing colors at 200-300 degrees is due to the appearance of a thin oxide film on the product.

Such hardening methods are used for processing impact tools (chisels, barbs, cores). For these fixtures, the hardness should decrease in a uniform and gradual manner.

5. Stepped. The preheated workpieces are cooled in a slowly cooling medium (melted salt, hot oily liquid). During a short exposure time, the temperature level is equalized. The final cooling is then performed.

Stepped full hardening reduces stress within the part. It is often used in industry, especially in the manufacture of tools. It makes it possible to straighten and straighten hot products.

6. Isothermal. Such complete hardening consists in the fact that the product is heated to the required temperature level and cooled in an isothermal environment to 230-340 degrees. Holding the blanks in the hardening environment should be sufficient for the austenite to completely transform into troostite. After transformation, the hardened product is cooled in air.

This type of hardening is used when the purpose of hardening is to make the product as strong, ductile and viscous as possible.

Defects that occur during hardening

When the product is hardened and tempered, stresses arise inside it. Cracks are formed, the part is deformed, warped, decarburized, oxidized, soft spots appear.

Cracks. This marriage cannot be corrected, it is formed during heat treatment. In large products, such as dies and forging dies, cracks can occur even when quenched in an oil liquid. In view of this, such products need to be cooled to one hundred and fifty degrees with a sharp vacation.
Cracks occur when warming up errors and also when the cooling rate during quenching is too high. They usually appear in the corners of the workpieces, they look arched or sinuous.
Deformation, warpage. They arise due to the fact that the transformations of the structure and volume are uneven, stresses appear inside the part during cooling. In view of this, when lowering the product into the quenching medium, its shape and size must be taken into account. For example, workpieces that have thick / thin elements are dipped into the hardening medium first with the part that is thicker.

In large-scale production, special fixtures are made for each product. The cost of developing them pays off. Products such as wheels with teeth, discs, plates are hardened in pressing / stamping devices. This avoids warping.

Decarburization. This process, by definition, is that most of the carbon is removed from the metal. The part can be decarbonized when heated in electric furnaces, liquids (salt baths). This is a significant defect that greatly reduces the strength of the product. It is difficult to find it. Usually a microsection is used for this.
Soft spots. Are areas of the workpiece with reduced hardness. The defect can be caused by the presence of scale, dirt, decarburization, steam jacket. It is disposed of by jet hardening in salt water.
Insufficient hardness. Usually, a defect manifests itself during tool processing, it can be caused by slow cooling in a hardening medium, low temperature. To correct it, the product is released at a temperature of six hundred degrees, after which it is normally tempered.
Overheating. The structure of the overheated part is coarse-grained, broken. Because of this, the mechanical characteristics of the metal are low. In order to grind the grains and prepare the workpiece for a new hardening, the metal is annealed.
Underheating. With this defect, the metal structure contains martensite and ferrite grains. They have low hardness. The defect is eliminated by annealing the metal and re-hardening.

What is hardenability? This is a property of a metal that characterizes its quenching ability. For each type of metal, you need to select the optimal hardening method. When choosing it, you must also take into account the type of product. In no case should the critical quenching rate be exceeded. This can lead to a variety of defects that must be eliminated. You also need to cool the part for a sufficient amount of time.

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