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

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.

Cooling modes during hardening

The most studied issues in materials science are the relationship between the chemical composition and the structure of the metal at certain temperatures. The most poorly studied area in hardening technology is methods, conditions and cooling modes. Meanwhile, it is in cooling that large reserves for controlling the structure and properties of the metal in finished products lie.

The main question of hardening is at what rate to cool? It would seem that if you cool it as quickly as possible, you will get maximum hardness, but at the same time, increased internal stresses will lead to the formation of cracks in the parts. The so-called hardening cracks, which are well known to all thermal experts. By cooling slowly, you will not achieve the required hardness and the part will need to be annealed and then undergo repeated heat treatment. Each grade of steel has its own “critical” cooling rate, which ensures maximum hardness and will not lead to cracking. For example, 45X steel, depending on the type of coolant, can be hardened to HRC 45 or HRC 60. In order to “squeeze” the maximum hardness out of steel, it is necessary to cool it as close as possible to the critical speed, for a specific grade of steel and type of workpiece. From this we can draw a simple conclusion that the cooling rate must be adjustable. There are only two widely used cooling rates: the cooling rate in water and in oil. Even taking into account that the cooling rate can be controlled, within a small range, by the temperature and circulation of the quenching liquid, the critical quenching rate may still not be obtained.

Water and oil media may only provide "critical" hardening rates in some applications. In addition, while working with water is relatively simple, oil hardening has specific features and disadvantages:

• — insufficient cooling rate for some steel grades;
• — the ability to ignite, emit harmful vapors, smoke, coke on the walls of air ducts, etc.,
• — good wetting of surfaces and, as a result, large removal of oil from workpieces, evaporation;
• — change in chemical composition under the influence of high temperatures;
• — the need to wash the workpieces in washing solutions with further regeneration of oil films.

The disadvantages of traditional hardening options have contributed to the search for more optimal hardening media and hardening techniques, at least for some variants of workpieces and alloys. As a result, there are several options for hardening technologies and media that are better suited for certain types of products. The most widely used are liquid polymer concentrates combined with water. This technology first appeared in the Soviet Union in 1980.

Characteristics of water-polymer compounds

Water-polymer compositions are a mixture of water and polymers in certain proportions. Polymers are chemical compounds formed by long chains of macroparticles obtained by combining microparticles - monomers. This reaction is called polymerization. Mixing water and polymers produces a stable coolant with adjustable heat capacity and therefore cooling capacity.

The basis of the coolant composition is water, even with modified properties. Therefore, there are restrictions on the use of water-polymer liquids. These environments are not recommended for hardening high-alloy tool and die steel grades, as well as parts of complex shapes or with variable cross-sections.

Polyacrylic iron salt of the PK-M brand is used as the initial polymer concentrate. This polymer turned out to be cheap and had advantages over other polymers of similar composition. Polymer coolants were originally designed to replace oil to eliminate flammability. Soon they developed materials that were superior to oil in efficiency for some products. Other advantages of water-polymer quenching media have also been discovered.

  Average cooling results in various environments

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.

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.

Martensite and martensitic transformation in steels

Martensite is a supersaturated solid solution of carbon in α-iron (α-Fe). Read what austenite, cementite, ferrite and pearlite are here. When eutectoid steel (0.8% carbon) is heated above point A1, the original pearlite structure will transform into austenite. In this case, all the carbon present in the steel will dissolve in austenite, i.e. 0.8%. Rapid cooling at a supercritical rate (see figure below), for example in water (600 °C/sec), prevents the diffusion of carbon from austenite, but the fcc crystal lattice of austenite will rearrange into the tetragonal lattice of martensite. This process is called martensitic transformation. It is characterized by the shear nature of the restructuring of the crystal lattice at a cooling rate at which diffusion processes become impossible. The product of martensitic transformation is martensite with a distorted tetragonal lattice. The degree of tetragonality depends on the carbon content in the steel: the more it is, the greater the degree of tetragonality. Martensite is a hard and brittle structure of steel. Found in the form of plates, under a microscope they look like needles.

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

Microstructure of steel after hardening

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

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

Partial hardening of steel

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

The microstructure of partially hardened steel looks something like this:

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

Incomplete hardening of steels

Quenching at temperatures lying between A1 and A3 (incomplete quenching) retains in the structure of hypoeutectoid steels, along with martensite, part of the ferrite, which reduces the hardness in the quenched state and worsens the mechanical properties after tempering. This is understandable, since the hardness of ferrite is 80HRC, and the hardness of martensite depends on the carbon content and can be more than 60HRC. Therefore, these steels are usually heated to temperatures 30–50 °C above A3 (full hardening). In theory, incomplete hardening of steels is not permissible and is considered a defect. In practice, in some cases, incomplete quenching can be used to avoid quenching cracks. Very often this concerns hardening with high frequency currents. With such hardening, it is necessary to take into account its feasibility: type of production, annual program, type of product responsibility, economic justification. For hypereutectoid steels, quenching at temperatures above A1 but below Acm produces excess cementite in the structure, which increases the hardness and wear resistance of the steel. Heating above the temperature Acm leads to a decrease in hardness due to the dissolution of excess cementite and an increase in retained austenite. In this case, the austenite grain grows, which also negatively affects the mechanical characteristics of the steel.

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

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

This is interesting: High-speed tool steel - characteristics, properties, analogues

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.

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.

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:

Technological nuances of hardening

Hardening, which is a type of heat treatment of metals, is performed in two stages. First, the metal is heated to a high temperature and then cooled. Different metals and even steels belonging to different categories differ from each other in their structure, therefore their heat treatment modes do not coincide.

Heat treatment modes for some non-ferrous alloys

Heat treatment of metal (hardening, tempering, etc.) may be required for:

• its strengthening and increasing hardness;
• improving its ductility, which is necessary when processing by plastic deformation.

Many specialized companies harden steel, but the cost of these services is quite high and depends on the weight of the part that needs to be heat treated. That is why it is advisable to do this yourself, especially since you can do it even at home.

If you decide to harden the metal on your own, it is very important to correctly carry out such a procedure as heating. This process should not be accompanied by the appearance of black or blue spots on the surface of the product. The bright red color of the metal indicates that heating is occurring correctly. This video demonstrates this process well, which will help you get an idea of ​​the degree to which to heat the metal subjected to heat treatment.

As a heat source for heating the metal product that needs to be hardened to the required temperature, you can use:

• a special oven powered by electricity;
• blowtorch;
• an open fire that you can make in the yard of your house or in your country house.

Hardening a knife on open coals

The choice of heat source depends on the temperature to which the metal to be heat treated must be heated.

The choice of cooling method depends not only on the material, but also on the results to be achieved. If, for example, it is not necessary to harden the entire product, but only a separate section of it, then cooling is also carried out pointwise, for which a stream of cold water can be used.

The technological scheme by which metal is hardened may include instant, gradual or multi-stage cooling.

Rapid cooling, which uses one type of coolant, is optimal for hardening steels classified as carbon or alloy. To perform such cooling, you need one container, which can be a bucket, barrel, or even an ordinary bathtub (it all depends on the size of the item being processed).

Cooling the knife blank in oil

In the event that steels of other categories need to be hardened or if, in addition to hardening, tempering is required, a two-stage cooling scheme is used. With this scheme, a product heated to the required temperature is first cooled with water and then placed in mineral or synthetic oil, in which further cooling occurs. Under no circumstances should an oil-based cooling medium be used directly, as the oil may ignite.

In order to correctly select the hardening modes of various steel grades, you should focus on special tables.

Heat treatment modes for high-speed steels

Heat treatment modes for alloy tool steels

Heat treatment modes for carbon tool 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.

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.

Hardening metal in oil - Metalworker's Handbook

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

One type 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.

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: Thermal conductivity coefficient of copper: what is thermal conductivity. What is the thermal conductivity of copper?

Self-production of a chamber for hardening metal

A muffle furnace, which is quite possible to make yourself at home, allows you to harden various grades of steel. The main component that is required to make this heating device is refractory clay. The layer of such clay that will cover the inside of the oven should be no more than 1 cm.

Diagram of a chamber for hardening metal: 1 - nichrome wire; 2 - inner part of the chamber; 3 — outer part of the chamber; 4 - rear wall with spiral leads

In order to give the future furnace the required configuration and desired dimensions, it is best to make a mold from cardboard impregnated with paraffin, onto which fireproof clay will be applied. Clay, mixed with water to a thick, homogeneous mass, is applied to the wrong side of the cardboard form, from which it will peel off after complete drying. Metal products heated in such a device are placed into it through a special door, which is also made of refractory clay.

After drying in the open air, the chamber and door of the device are additionally dried at a temperature of 100°. After this, they are fired in a furnace, the temperature in the chamber of which is gradually raised to 900°. When they have cooled after firing, they must be carefully connected to each other using metalworking tools and emery cloth.

Clay heater with a walled nichrome spiral

Nichrome wire is wound onto the surface of the fully formed chamber, the diameter of which should be 0.75 mm. The first and last layers of such winding must be twisted together. When winding the wire around the camera, you should leave a certain distance between its turns, which also needs to be filled with fireproof clay to eliminate the possibility of a short circuit. After the layer of clay applied to provide insulation between the turns of nichrome wire has dried, another layer of clay is applied to the surface of the chamber, the thickness of which should be approximately 12 cm.

After complete drying, the finished camera is placed in a metal case, and the gaps between them are filled with asbestos chips. In order to provide access to the inner chamber, doors, lined with ceramic tiles on the inside, are hung on the metal body of the oven. All existing gaps between structural elements are sealed using refractory clay and asbestos chips.

Ready homemade camera

The ends of the nichrome winding of the camera, to which electrical power must be supplied, are brought out from the rear side of its metal frame. In order to control the processes occurring in the inside of the muffle furnace, as well as measure the temperature in it using a thermocouple, two holes must be made in its front part, the diameters of which should be 1 and 2 cm, respectively. From the front part of the frame, such holes will be closed with special steel curtains. The homemade design, the manufacture of which is described above, allows you to harden metalworking and cutting tools, working elements of stamping equipment, etc. at home.

Self-manufacturing of such a furnace (as well as other types of hardening equipment) allows you not only to get at your disposal a device that fully meets your needs, but also to save a lot of money, since serial models are quite expensive.

Oils are often used as a quenching medium for some low-carbon steels and for a wider range of medium- and high-carbon steels of various alloys. The hardening ability of an oil is influenced by many factors, the main of which are physicochemical characteristics: viscosity and density at different temperatures, thermal conductivity, resistance to slag formation (resistance to aging).

Read also: DIY rail cleaver

In order to understand how and to what extent these factors influence hardenability, it is necessary to consider the steel cooling process in more detail. Hardening is not a perfectly straight line between the y-axis and the region of minimum stability of austenite.

This line has bends corresponding to different rates at different stages of cooling. Such changes in speeds are a consequence of the processes occurring in the part-cooling medium system during hardening.

When a product is immersed in a quenching bath, a steam jacket is formed on its surface, which has a low thermal conductivity coefficient. Cooling at this stage is very slow and uneven. This phase lasts a few seconds and is the most important stage of cooling, because at the end of the vapor phase, the nucleate boiling phase begins with structure-forming, critical speeds. In fact, the vapor phase shifts the diagram of the isothermal transformation of austenite to the left, exactly as long as it lasts and reduces the temperature at which intensive cooling begins. This cooling stage can be accelerated by actively stirring the oil. Here, the main indicator of the effectiveness of this measure is the kinematic viscosity of the oil. This property depends on the process temperature and the nature of the oil production. Kinematic viscosity determines the speed at which the oil will move in the quenching bath when stirred. However, it should be taken into account that high speeds of movement of the medium can cause strong foaming.

The nucleate boiling stage begins when the integrity of the vapor film is broken and the surface of the part comes into contact with the cooling medium. In this case, the surface temperature of the cooled product quickly drops to the boiling temperature of the oil and remains constant until the end of boiling. The intensity of cooling depends on the heat of vaporization of the oil used. The higher the heat value, the higher the cooling rate.

Then boiling stops, and cooling occurs as a result of convective heat exchange. The cooling rate at this stage depends on the viscosity and thermal conductivity of the oil, as well as on the temperature difference between the product and the coolant.

In addition to the described properties, other characteristics can be used to assess the quality of the oil. Flash point is a very important property in terms of fire safety. As a rule, oils with a flash point 50-60 degrees higher than the process temperature are used in production. The density of the oil can indicate the nature of its origin and the method of processing. However, additives can change this value, so the density characteristic cannot serve as an adequate indicator of quality. Resistance against aging is an indicator of the economic efficiency of using a particular oil. This is the time of normal operation of the cooling medium before the formation of combustion products and slag on the bottom and walls of the bath. The time for changing the oil is often determined practically by changes in the color of the hardened products or the appearance of soft spots on the surface. Manufacturers of quenching oils prefer not to indicate this characteristic in the documentation.

Another economic characteristic of oil quality is the rate at which the substance is carried away from the surfaces of the parts being processed. It cannot be unambiguously determined, because largely depends on the specific conditions of use (single quenching tank, tank as part of an automatic line, taking into account the drainage time or not). However, this characteristic has some correlation with the viscosity of the oil and more often does not exceed 1% of the area of ​​the processed products. When comparing the characteristics of oils, you need to pay attention to the permissible amount of water and foreign impurities. Water in oil may cause uneven hardness and fire in the quench tank. The more water in the oil, the greater the likelihood of these phenomena.

An ideal quenching oil should cool products as quickly as possible in the region of minimum austenite stability and as slowly as possible in the Mn - Mk region. From the above it follows that when choosing such an ideal and safe quenching oil, first of all you should take into account its viscosity, heat of vaporization, thermal conductivity and flash point.

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.

Heat treatment of metals and quenching oils

There are the following methods of heat treatment of metals:

• Annealing
• Normalization
• Hardening
• Vacation

Note : * Temperature depends on the type of metal being processed.

The following types of hardening are distinguished:

• Cold hardening (+30…+80 °C): for thermal improvement of open-die forging and die-forging parts, hardening of hand tools, leaf and screw springs, high-strength bolts, nuts, washers, etc.
• Hot hardening (+165…+220 °C): for hardening high-precision parts (for example, parts of the drive mechanism of cars), where it is necessary to eliminate the risk of surface curvature
• Vacuum hardening: for tool, bearing, heat-resistant, high-speed steel

When heat treating metals and alloys using the hardening method, it is very important to take into account the temperature and duration of heating, as well as the cooling rate.

Water or special oil is used as a working medium in the metal hardening process.

When quenching using oil, significantly fewer thermal cracks form on the product than when quenching in water.

The production of domestic hardening oils began to actively develop in the late 90s. Currently, there is a whole range of these products on the Russian market: from simpler and proven ones to expensive imported ones, which can affect the cooling rate.

Quenching oils make it possible to obtain steel products with specified values ​​of hardness, required structure and surface cleanliness.

When choosing an oil, it is necessary to take into account its flash point in an open crucible. It is determined by the quality of the base oil and should be 30 below the overall process temperature. Additives are added to the quenching oil in order to increase its efficiency or speed up the heat removal process.

Quenching oils must have the following properties:

• High thermal and chemical stability (preservation of properties throughout the entire service life)
• Good cleaning properties (precipitates and scale from the surface of parts accumulate in the oil)
• High resistance to evaporation (use in open quench tanks)
• Good anti-foam properties (strong swirl of hot oil in quench tanks)
• A certain level of viscosity (depends on the quenching temperature and affects oil loss when removing parts from tanks)
• Lack of water (affects oil foaming)

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.

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.

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: Metal products

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.

Cooling stages

The sequence of heating and then rapidly cooling parts through quenching is a way of achieving additional hardness in a part that would not otherwise be possible.
Heating causes changes in the crystal structure of the surface of a metal part; rapid cooling “freezes” these changes in place and makes the surface harder. The first stage
of hardening is known as the steam stage.
Since the submerged part is much hotter than the damper, a vapor barrier forms around the part. At this stage, the part cools, but this is prevented by steam, which acts as an insulator. The second stage
is the boiling stage, which is characterized by intense boiling.
Parts cool fastest in this stage because the temperature of the part has dropped enough in the previous stage for the vapor coating to dissipate. Due to the fact that the damping element can easily contact the part, it can remove the greatest amount of heat during boiling. The third stage
is the convective stage, during which convection and conduction remove heat from the part. Convection refers to the movement of fluid due to the tendency of hotter, less dense fluids to rise while cooler, denser fluids sink. Conductivity refers to the tendency of heat to dissipate throughout a substance when there are temperature differences within a liquid. The oils are vigorously agitated during quenching, causing them to flow upward through the work load. For this reason, natural convection does not occur.

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.

How to harden steel over an open fire

As mentioned above, you can harden steel at home, using an open fire for heating. Naturally, such a process should begin by starting a fire, in which a lot of hot coals should form. You will also need two containers. You need to pour mineral or synthetic oil into one of them, and ordinary cold water into the other.

In order to remove hot iron from a fire, you will need blacksmith tongs, which can be replaced with any other tool of a similar purpose. After all the preparatory work has been completed, and a sufficient number of hot coals have formed in the fire, objects that need to be hardened can be placed on them.

Read also: Wood engraving work

The color of the formed coals can be used to judge the temperature of their heating. Thus, coals whose surface has a bright white color are hotter. It is also important to monitor the color of the fire’s flame, which indicates the temperature regime in its interior. It is best if the fire flame is colored crimson rather than white. In the latter case, indicating that the flame temperature is too high, there is a risk of not only overheating, but even burning the metal that needs to be hardened.

Heated steel colors

The color of the heated metal must also be carefully monitored. In particular, black spots must not be allowed to appear on the cutting edges of the tool being processed. Blue discoloration of the metal indicates that it has softened greatly and become too flexible. It cannot be brought to such a state.

After the product has been calcined to the required degree, you can proceed to the next stage - cooling. First of all, it is lowered into a container with oil, and this is done often (every 3 seconds) and as sharply as possible. Gradually the intervals between these dives increase. As soon as the hot steel loses its brightness, you can begin to cool it in water.

Steel tarnish colors

When cooling metal with water on the surface of which droplets of hot oil remain, care should be taken, as they may burst into flames. After each dive, the water must be agitated to ensure it remains cool at all times. A training video will help you get a clearer idea of ​​the rules for performing such an operation.

There are certain subtleties when cooling hardenable drills. So, they cannot be lowered flat into a container with coolant. If you do this, then the lower part of the drill or any other metal object that has an elongated shape will cool sharply first, which will lead to its compression. That is why it is necessary to immerse such products in coolant from the wider end.

For heat treatment of special grades of steel and smelting non-ferrous metals, the capabilities of an open fire are not enough, since it will not be able to heat the metal to a temperature of 700–9000. For such purposes, it is necessary to use special furnaces, which can be muffle or electric. If it is quite difficult and expensive to make an electric furnace at home, then with muffle-type heating equipment this is quite feasible.

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.

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.

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 .

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/