If you know how to harden metal correctly, then even at home you can increase the hardness of metal products by two to three times. The reasons why this is necessary can be very different. Such a technological operation, in particular, is required if the metal must be hardened enough to be able to cut glass.
Most often, it is necessary to harden the cutting tool, and heat treatment is performed not only if it is necessary to increase its hardness, but also when this characteristic needs to be reduced. When the hardness of the tool is too low, its cutting part will jam during operation, but if it is high, the metal will crumble under the influence of mechanical loads.
Few people know that there is an easy way to check how well a steel tool is hardened, not only in production or at home, but also in a store when buying. In order to perform such a check, you need a regular file. They are carried out along the cutting part of the purchased tool. If it is hardened badly, then the file will seem to stick to its working part, and in the opposite case, it will easily move away from the tested tool, while the hand in which the file is located will not feel any irregularities on the surface of the product.
If, nevertheless, it turned out that you had a tool at your disposal, the quality of hardening of which does not suit you, you should not worry about this. This problem is solved quite easily: it is possible to harden metal even at home, without using sophisticated equipment and special devices for this. However, you should be aware that low-carbon steels cannot be hardened. At the same time, the hardness of the carbon and easy enough to increase even at home.
Technological nuances of hardening
Tempering, which is one of the types of heat treatment of metals, is carried out 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, so their heat treatment modes do not match.
Heat treatment of metal (hardening, tempering, etc.) may be required for:
- its hardening and increase in hardness;
- improving its plasticity, 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 it 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 properly 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 fact that the heating is happening correctly is evidenced by the bright red color of the metal. This process is well demonstrated by a video that will help you get an idea of how much to heat the metal being heat treated.
As a heat source for heating to the required temperature of a metal product that needs to be hardened, 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 the country.
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 what results are to be achieved. If, for example, it is not necessary to harden the entire product, but only its separate section, then cooling is also carried out pointwise, for which a jet of cold water can be used.
The technological scheme, according to which the metal is hardened, may provide for instantaneous, gradual or multi-stage cooling.
Fast cooling, using one type of cooler, is optimal for hardening steels in the carbon or alloy category. To perform such cooling, one container is needed, which can be a bucket, a barrel, or even an ordinary bath (it all depends on the dimensions of the object being processed).
In the event that other categories or if, in addition to hardening, tempering is required, a two-stage cooling scheme is used. With this scheme, the product heated to the required temperature is first cooled with water, and then placed in mineral or synthetic oil, in which further cooling takes place. Under no circumstances should an oil coolant be used immediately, as the oil may ignite.
In order to correctly select the hardening modes for various steel grades, one should be guided by special tables.
How to harden steel over an open fire
As mentioned above, it is possible to harden steel at home, using an open fire for heating. Naturally, such a process should begin with a fire, in which a lot of hot coals should form. You will also need two containers. Mineral or synthetic oil must be poured into one of them, and ordinary cold water into the other.
In order to extract red-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 amount of hot coals has formed in the fire, objects that need to be hardened can be placed on them.
By the color of the coals formed, one can judge the temperature of their heating. So, coals are hotter, the surface of which has a bright white color. It is also important to monitor the color of the fire flame, which indicates the temperature regime in its inner part. It is best if the fire flame is painted crimson, not white. In the latter case, indicating a too high temperature of the flame, there is a risk not only of overheating, but even of burning the metal to be hardened.
The color of the heated metal must also be carefully monitored. In particular, black spots should not be allowed to appear on the cutting edges of the machined tool. The blue color of the metal indicates that it has softened a lot and become too ductile. It cannot be brought to such a state.
After the product is 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 (with a frequency of 3 seconds) and as sharply as possible. Gradually, the intervals between these dives increase. As soon as the red-hot steel loses the brightness of its color, you can start cooling it in water.
When cooling metal with water, on the surface of which droplets of hot oil remain, care should be taken, as they can flare up. After each dive, the water must be shaken to keep it cool at all times. To get a better idea of the rules for performing such an operation, a training video will help.
There are certain subtleties in the cooling of hardened drills. So, they cannot be lowered flat into a container with coolant. If you do this, then the bottom of the drill or any other metal object that has an elongated shape will cool first sharply, which will lead to its compression. That is why it is necessary to immerse such products in the coolant from the side of the wider end.
For heat treatment of special grades of steel and smelting of non-ferrous metals, the possibilities of an open fire will not be enough, since it will not be able to provide heating of 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.
Self-made 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 will be required for the manufacture of this heating device is refractory clay. The layer of such clay, which will cover the inside of the furnace, should be no more than 1 cm.
Scheme of a chamber for hardening metal: 1 - nichrome wire; 2 - the 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 the desired dimensions, it is best to make a mold from cardboard impregnated with paraffin, on which refractory 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 itself will lag behind 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.
The chamber and the door of the device after drying in the open air are additionally dried at a temperature of 100 °. After that, they are fired in a furnace, the temperature in the chamber of which is gradually brought up to 900 °. When they have cooled after firing, they must be carefully connected to each other using locksmith tools and sandpaper.
On the surface of a fully formed chamber, a nichrome wire is wound, the diameter of which should be 0.75 mm. The first and last layer of such winding must be twisted together. When winding the wire around the chamber, a certain distance should be left between its turns, which must also be filled with refractory clay in order to exclude the possibility of a short circuit. After the layer of clay applied to provide insulation between the turns of nichrome wire dries, another layer of clay is applied to the surface of the chamber, the thickness of which should be approximately 12 cm.
The finished chamber, after complete drying, 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 finished with ceramic tiles are hung on the metal body of the furnace. All existing gaps between structural elements are sealed with refractory clay and asbestos chips.
The ends of the nichrome winding of the camera, to which it is necessary to supply electrical power, are output 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 to 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 of the frame, such openings will be closed with special steel curtains. A home-made design, the manufacture of which is described above, allows you to harden locksmith and cutting tools, working elements of stamping equipment, etc. at home.
Basic methods and ways of converting electrical energy into heat are classified as follows. There are direct and indirect electrical heating.
At direct electric heating the conversion of electrical energy into thermal energy occurs as a result of the passage of an electric current directly through a heated body or medium (metal, water, milk, soil, etc.). At indirect electric heating electric current passes through a special heating device (heating element), from which heat is transferred to the heated body or medium through heat conduction, convection or radiation.
There are several types of conversion of electrical energy into thermal energy, which determine electrical heating methods.
The flow of electric current through electrically conductive solids or liquid media is accompanied by the release of heat. According to the Joule-Lenz law, the amount of heat Q \u003d I 2 Rt, where Q is the amount of heat, J; I - silatoka, A; R is the resistance of the body or medium, Ohm; t - current flow time, s.
Resistance heating can be carried out by contact and electrode methods.
contact way It is used for heating metals both according to the principle of direct electric heating, for example, in electric resistance welding machines, and according to the principle of indirect electric heating - in heating elements.
Electrode method it is used for heating non-metallic conductive materials and media: water, milk, succulent feed, soil, etc. The material or medium to be heated is placed between the electrodes, to which an alternating voltage is applied.
Electric current, flowing through the material between the electrodes, heats it. Ordinary (non-distilled) water conducts electric current, since it always contains a certain amount of salts, alkalis or acids, which dissociate into ions that are carriers of electric charges, that is, electric current. The nature of the electrical conductivity of milk and other liquids, soil, succulent feed, etc. is similar.
Direct electrode heating is carried out only on alternating current, since direct current causes electrolysis of the heated material and its deterioration.
Electrical resistance heating has found wide application in production due to its simplicity, reliability, versatility and low cost of heating devices.
Electric arc heating
In an electric arc that occurs between two electrodes in a gaseous medium, electrical energy is converted into thermal energy.
To start the arc, the electrodes connected to the power source are touched for a moment and then slowly moved apart. The resistance of the contact at the moment of dilution of the electrodes is strongly heated by the current passing through it. Free electrons, constantly moving in the metal, with an increase in temperature at the point of contact of the electrodes, accelerate their movement.
As the temperature rises, the speed of free electrons increases so much that they break away from the metal of the electrodes and fly out into the air. As they move, they collide with air molecules and split them into positively and negatively charged ions. There is an ionization of the air space between the electrodes, which becomes electrically conductive.
Under the action of the source voltage, positive ions rush to the negative pole (cathode), and negative ions - to the positive pole (anode), thereby forming a long discharge - an electric arc, accompanied by heat generation. The temperature of the arc is not the same in its various parts and is with metal electrodes: at the cathode - about 2400 ° C, at the anode - about 2600 ° C, in the center of the arc - about 6000 - 7000 ° C.
There are direct and indirect electric arc heating. The main practical application is found by direct electric arc heating in electric arc welding installations. In indirect heating installations, the arc is used as a powerful source of infrared rays.
If a piece of metal is placed in an alternating magnetic field, then a variable e will be induced in it. d.s., under the action of which eddy currents will arise in the metal. The passage of these currents in the metal will cause it to heat up. This method of heating a metal is called induction. The device of some induction heaters is based on the use of the phenomenon of the surface effect and the proximity effect.
For induction heating, industrial (50 Hz) and high frequency (8-10 kHz, 70-500 kHz) currents are used. The most widespread is induction heating of metal bodies (parts, blanks) in mechanical engineering and in the repair of equipment, as well as for hardening metal parts. The induction method can also be used to heat water, soil, concrete and pasteurize milk.
Dielectric heating
The physical essence of dielectric heating is as follows. In solids and liquid media with poor electrical conductivity (dielectrics) placed in a rapidly changing electric field, electrical energy is converted into thermal energy.
In any dielectric there are electric charges bound by intermolecular forces. These charges are called bound as opposed to free charges in conductive materials. Under the action of an electric field, bound charges are oriented or displaced in the direction of the field. The displacement of bound charges under the action of an external electric field is called polarization.
In an alternating electric field, there is a continuous movement of charges, and, consequently, the molecules associated with them by intermolecular forces. The energy expended by the source on the polarization of the molecules of non-conducting materials is released in the form of heat. In some non-conductive materials, there are a small amount of free charges that, under the action of an electric field, create a small amount of conduction current, which contributes to the release of additional heat in the material.
During dielectric heating, the material to be heated is placed between metal electrodes - capacitor plates, to which a high-frequency voltage (0.5 - 20 MHz and higher) is supplied from a special high-frequency generator. The dielectric heating installation consists of a high-frequency lamp generator, a power transformer and a drying device with electrodes.
High-frequency dielectric heating is a promising heating method and is mainly used for drying and heat treatment of wood, paper, food and feed (drying grain, vegetables and fruits), pasteurization and sterilization of milk, etc.
Electron beam (electronic) heating
When a stream of electrons (electron beam) accelerated in an electric field meets a heated body, electrical energy is converted into thermal energy. A feature of electronic heating is the high density of energy concentration, which is 5x10 8 kW/cm2, which is several thousand times higher than with electric arc heating. Electronic heating is used in industry for the welding of very small parts and for the smelting of ultra-pure metals.
In addition to the considered methods of electric heating, it is used in production and everyday life. infrared heating (irradiation).
Heating of metal by welding current. Joule-Lenz law. Electrical resistance of metal.
All current-carrying elements are heated by electric current, and the amount of heat generated in any section of the electrical circuit with active resistance R=R(t), which is a function of t and τ at a current I=I(t) depending on time t, is determined by Joule's law -Lenza:
This is a general formula that does not show or determine specific temperatures in the joint area when it is heated by welding current.
However, it must be remembered that the value of R and I largely depends on the duration of the flow of this current.
Contact machines are structurally manufactured in such a way that the greatest amount of heat is released between the electrodes.
Seam spot welding has the largest number of electrode-electrode sections, the total amount of resistance is the sum of the resistance of the electrode-piece + piece-piece + piece + electrode-piece
Ree \u003d 2Red + Rdd + 2Rd
All components of the total resistance Ree continuously change during the thermal cycle of welding.
Contact resistance - Rdd is the largest in value, because. contacting is carried out along microprotrusions and the area of physical contact is small.
In addition, oxide films and various contaminants are present on the surface of the part.
Because We mainly weld steels and alloys with significant strength, then the complete collapse of microroughnesses occurs only when they are heated by welding current to temperatures of about 600 degrees C
The resistance in the electrode-part contact is much less than Rdd, because softer and more highly thermally conductive material of the electrodes is actively introduced between the protrusions of the microroughness of the parts.
The increased resistance in the contacts is also due to the fact that there is a sharp curvature of the current line in the contact areas, which determines a higher resistance due to an increase in the current path.
Contact resistance Rdd and Red largely depends on surface cleaning for welding.
By measuring 2 plates, 3 mm thick, very strongly compressed 200N according to the ammeter-voltmeter scheme, we obtained the following values:
Cleaning surfaces with a circle and grinding: 100 µOhm
Conclusion: grind
In practice, etching is used (when welding large surfaces), surface treatment with metal brushes, sandblasting and shot blasting.
In contact welding, they try to use cold-rolled steel on the surface of which there may be oil residues.
If there is no rust on the surface, then it is enough to degrease the surfaces to be welded.
The contact resistance of clean but oxide-coated parts decreases with increasing compression forces. This is due to the greater deformation of the microprotrusions.
We turn on the current, the highest density of the streamline is concentrated on juvenile surfaces. Current through the contacts, formed during the deformation of microprotrusions.
At the initial moment of time, the current density in the material of the part is less, because The current lines are distributed relatively evenly, and in the part-to-part contact, the current flows only through the conduction zones, therefore, the current density is higher than in the bulk of the part and heat generation and heating in this area are more significant.
The metal in contact will become ductile. It is deformed under the influence of welding force, the area of conductive contacts will increase and when t=600 degC is reached (in hundredths of a second), the microprotrusions are completely deformed, oxide films are partially destroyed, partially diffuse into the mass of the part, and the role of contact resistance Rdd will cease to be paramount in the heating process. .
However, by this moment the temperature in the part-part contact area will be the highest, the resistivity of the material ρ will be the highest, and the heat release will be more intense anyway in this zone.
With sufficient current densities for the duration of its flow, it is there that the melting of the metal begins.
The appearance of a melting isotherm precisely in the part-part contact will be facilitated by the smallest heat removal from this area, the part's own resistance.
Intrinsic resistance of the part
S-section of the conductor
Coefficient A increases the spreading of the streamline into the mass of the part, while there is an increase in the real spreading area
dk - spreading diameter
A \u003d 0.8-0.95, depends on the hardness of the material, and to a greater extent on the resistivity.
From the ratio dk / δ \u003d 3-5 A \u003d 0.8
Obviously, the resistance of the part depends on the thickness, this is taken into account by the coefficient A and on the specific electrical resistance of the material of the part ρ, it depends on the chemical composition.
In addition, resistivity depends on temperature.
ρ(t)=ρ0*(1+αp*T)
In the process of welding with the flow of current, t is measured from contact to tmelt and above
Tmelt=1530 degC
When tm is reached, the resistivity increases abruptly.
αρ - temperature coefficient
αρ=0.004 1/degC - for pure metals
αρ=0.001-0.003 1/degC- for alloys
The value of αρ decreases with increasing degree of ligation.
With an increase in temperature, the metal, both in contact and in the bulk under the electrodes, is deformed, the contact area increases, and if the working surface of the electrodes is spherical, then the contact area can increase by 1.5-2 times.
Graph of the change in resistance during the welding process.
At the initial moment of time, the resistance of the part increases due to an increase in temperature and an increase in electrical resistivity, then the metal becomes plastic and the contact area begins to increase due to the indentation of the electrodes into the surface of the part, as well as an increase in the size of the contact area part-part.
The total resistance will decrease as the welding current is switched off. However, this is true for welding carbon and low alloy steels.
For welding high temperature Ni and Cr alloys, the resistance may even increase.
Electric and temperature field.
The Joule-Lenz law Q \u003d IRt shows heat generation in current-carrying elements, and heat removal processes are still taking place.
Thanks to the active cooling of the electrodes and the increase in heat removal in them, we obtain a lenticular shape of the cast core.
But such a shape is not always possible to obtain, especially when welding dissimilar, different thickness materials and thin parts.
Knowing the nature of the temperature field in the welding zone, it is possible to analyze:
1) Dimensions of the cast core.
2) Size of the HAZ (structure)
3) The magnitude of the residual stresses, i.e. connection properties.
Temperature field - a set of temperatures at various points of the part at a certain point in time.
Points with the same temperature connected by a line are called an isotherm.
The size of the pure core on the microsection indicates the melting isotherm along the boundaries of the cast core.
Ultimately, the temperature and the size of the melting isotherm, i.e. cast core, mainly affects the resistance of the part.
The founder - Gelman, took two parts 2 + 2mm, polished, etched and got a cast core; I took the parts and got a cast core too.
However, the difficulties that arise when welding heterogeneous thicknesses force us to investigate the distribution of thermal fields in the welding zone.
The current density is the number of charges passing for 1 second through a small area perpendicular to the direction of movement of the charges, divided by the length of its surface.
Heating of metals and alloys is carried out either to reduce their resistance to plastic deformation (i.e., before forging or rolling), or to change the crystal structure that occurs under the influence of high temperatures (heat treatment). In each of these cases, the conditions of the heating process have a significant impact on the quality of the final product.
The tasks to be solved predetermine the main characteristics of the heating process: temperature, uniformity and duration.
The heating temperature is usually called the final temperature of the metal surface, at which, in accordance with the requirements of the technology, it can be issued from the furnace. The value of the heating temperature depends on the chemical composition (grade) of the alloy and on the purpose of heating.
When heated before pressure treatment, the temperature of billet output from the furnace should be sufficiently high, as this helps to reduce the resistance to plastic deformation and leads to a reduction in power consumption for processing, an increase in the productivity of rolling and forging equipment, and an increase in its service life.
However, there is an upper limit to the heating temperature, since it is limited by grain growth, overheating and overburning, and the acceleration of metal oxidation. During the heating of most alloys, upon reaching a point lying 30-100 ° C below the solidus line on their phase diagram, due to segregation and non-metallic inclusions, a liquid phase appears at the grain boundaries; this leads to a weakening of the mechanical bond between the grains, intense oxidation at their boundaries; such metal loses its strength and collapses during pressure treatment. This phenomenon, called overburning, limits the maximum heating temperature. Burnt metal cannot be repaired by any subsequent heat treatment and is only suitable for remelting.
Overheating of the metal leads to excessive grain growth, resulting in deterioration of mechanical properties. Therefore, rolling must be completed at a temperature lower than the superheat temperature. Overheated metal can be corrected by annealing or normalizing.
The lower heating temperature limit is set based on the allowable temperature at the end of pressure treatment, taking into account all heat losses from the workpiece to the environment and heat release in it due to plastic deformation. Therefore, for each alloy and for each type of forming there is a certain temperature range, above and below which the workpiece should not be heated. This information is given in the relevant reference books.
The issue of heating temperature is especially important for such complex alloys as, for example, high-alloy steels, which, during pressure treatment, have a high resistance to plastic deformation, and at the same time, are prone to overheating and burnout. These factors cause a narrower range of heating temperatures for high-alloy steels compared to carbon steels.
In table. 21-1, as an illustration, data for some steels are given on the maximum allowable temperature of their heating before pressure treatment and on the burnout temperature.
During heat treatment, the heating temperature depends only on technological requirements, i.e., on the type of heat treatment and its mode, due to the structure and structure of the alloy.
Heating uniformity is determined by the temperature difference between the surface and the center (since this is usually the largest difference) of the workpiece when it is issued from the furnace:
∆T con \u003d T con pov - T con cent. This indicator is also very important, since too large a temperature difference across the cross section of the workpiece when heated before pressure treatment can cause uneven deformation, and when heated for heat treatment, it can lead to incompleteness of the required transformations throughout the entire thickness of the metal, i.e. in both cases - marriage end products. At the same time, the process of leveling the temperature over the metal section requires a long exposure to a high surface temperature.
However, complete uniformity of heating of the metal before pressure treatment is not required, since in the process of transporting it from the furnace to the mill or press and rolling (forging), the temperature is inevitably equalized over the cross-section of ingots and billets due to heat transfer to the environment from their surface and thermal conductivity inside the metal. Based on this, the allowable temperature difference over the cross section is usually taken according to practical data during heating before pressure treatment within the following limits: for high-alloy steels ∆ T con= 100δ; for all other steel grades ∆ T con= 200δ at δ<0,1 м и ∆T con= 300δ at δ > 0.2 m. Here δ is the heated thickness of the metal.
In all cases, the temperature difference across the thickness of the billet at the end of its heating before rolling or forging should not exceed 50 °C, and when heated for heat treatment, 20 °C, regardless of the thickness of the product. When heating large ingots, it is allowed to dispense them from the furnace at ∆ T con <100 °С.
Another important task of metal heating technology is to ensure uniform temperature distribution over the entire surface of blanks or products by the time they are unloaded from the furnace. The practical necessity of this requirement is obvious, since with a significant non-uniformity of heating over the metal surface (even when the required temperature difference across the thickness is reached), such defects as the uneven profile of the finished rolled product or various mechanical properties of the product subjected to heat treatment are inevitable.
Ensuring temperature uniformity over the surface of the heated metal is achieved through the correct choice of a furnace for heating a certain type of workpieces or products and the appropriate placement of heat generating devices in it, which create the necessary temperature field in the working space of the furnace, the mutual arrangement of workpieces, etc.
Heating time to the final temperature is also the most important indicator, since the productivity of the furnace and its dimensions depend on it. At the same time, the duration of heating to a given temperature determines the heating rate, i.e., the change in temperature at some point of the heated body per unit time. Usually, the heating rate changes during the course of the process, and therefore a distinction is made between the heating rate at a certain point in time and the average heating rate over the considered time interval.
The faster the heating is carried out (i.e., the higher the heating rate), the obviously higher the productivity of the furnace, all other things being equal. However, in a number of cases, the heating rate cannot be chosen to be arbitrarily large, even if the conditions of external heat transfer allow it to be carried out. This is due to certain restrictions imposed by the conditions of the processes that accompany the heating of the metal in furnaces and are considered below.
Processes occurring during heating of the metal. When the metal is heated, its enthalpy changes, and since in most cases the heat is supplied to the surface of ingots and billets, their outer temperature is higher than the temperature of the inner layers. As a result of thermal expansion of different parts of a solid by different amounts, stresses arise, which are called thermal.
Another group of phenomena is associated with chemical processes on the metal surface during heating. The surface of the metal, which is at a high temperature, interacts with the environment (i.e., with combustion products or air), as a result of which a layer of oxides forms on it. If any elements of the alloy interact with the environment surrounding the metal with the formation of a gas phase, then the surface becomes depleted of these elements. For example, the oxidation of steel carbon when it is heated in furnaces causes surface decarburization.
Thermal stresses
As noted above, in the section of ingots and blanks, when they are heated, an uneven distribution of temperatures occurs and, consequently, different parts of the body tend to change their size to different degrees. Since in a solid there are bonds between all its individual parts, they cannot independently deform in accordance with the temperatures to which they are heated. As a result, thermal stresses arise due to the temperature difference. The outer, more heated layers tend to expand and are therefore in a compressed state. The inner, colder layers are subject to tensile forces. If these stresses do not exceed the elastic limit of the heated metal, then with the equalization of the temperature over the cross section, the thermal stresses disappear.
All metals and alloys have elastic properties up to a certain temperature (for example, most steel grades up to 450-500 ° C). Above this certain temperature, metals pass into a plastic state and the thermal stresses that have arisen in them cause plastic deformation and disappear. Therefore, thermal stresses should be taken into account during heating and cooling of steel only in the temperature range from room temperature to the transition point of a given metal or alloy from an elastic state to a plastic one. Such stresses are called vanishing, or temporary.
In addition to temporary, there are residual thermal stresses that increase the risk of destruction during heating. These stresses arise if the ingot or billet has previously been subjected to heating and cooling. When cooled, the outer layers of the metal (colder) reach the transition temperature from the plastic to the elastic state earlier. With further cooling, the inner layers are subjected to tensile forces, which do not disappear due to the low plasticity of the cold metal. If this ingot or blank is heated again, then the temporary stresses arising in them will be superimposed with the same sign on the residual ones, which will aggravate the risk of cracks and ruptures.
In addition to temporary and residual thermal stresses, during heating and cooling of alloys, stresses also arise due to structural changes in volume. But since these phenomena usually take place at temperatures exceeding the boundary of the transition from the elastic state to the plastic state, the structural stresses dissipate due to the plastic state of the metal.
The relationship between strains and stresses establishes Hooke's law
σ= ( T cf -T)
where β is the coefficient of linear expansion; T cf- average body temperature; T- temperature in a given section of the body; E- modulus of elasticity (for many grades of steel, the value E decreases from (18÷22) . 10 4 MPa up to (14÷17) . 10 4 MPa with an increase in temperature from room temperature to 500 °C; σ is stress; v - Poisson's ratio (for steel v ≈ 0.3).
Of great practical interest is finding the maximum allowable temperature difference across the body section ∆T add = T sur - T price. The most dangerous in this case are tensile stresses, so they should be taken into account when calculating the allowable temperature difference. As a strength characteristic, one should take the value of the tensile strength of the alloy σ in.
Then, using the solutions of heat conduction problems (see Chap. 16) and imposing expression (21-1) on them, for the case of a regular regime of the second kind, one can, in particular, obtain:
for uniformly and symmetrically heated endless plate
∆T add \u003d 1.5 (1 - v) σ in / ();
for a uniformly and symmetrically heated infinite cylinder
∆T add \u003d 2 (1 - v) σ in / ().
The allowable temperature difference found by formulas (21-2) and (21-3) does not depend on the size of the body and its thermophysical characteristics. Body dimensions have an indirect effect on the value of ∆ T additional, since the residual stresses in larger bodies are greater.
Oxidation and decarburization of the surface during heating. Oxidation of ingots and blanks during heating in furnaces is an extremely undesirable phenomenon, since it results in irreversible loss of metal. This leads to very large economic damage, which becomes especially obvious if we compare the cost of metal losses during oxidation with other processing costs. So, for example, when steel ingots are heated in heating wells, the cost of metal lost with scale is usually higher than the cost of fuel consumed to heat this metal and the cost of electricity consumed to roll it. When billets are heated in the furnaces of section rolling shops, the losses with scale are somewhat lower, but they are still quite large and commensurate in cost with fuel costs. Since, on the way from the ingot to the finished product, the metal is usually heated several times in different furnaces, the losses due to oxidation are very significant. In addition, the higher hardness of oxides compared to metal leads to increased tool wear and increases the scrap rate in forging and rolling.
The lower thermal conductivity of the oxide layer formed on the surface compared to the metal increases the duration of heating in furnaces, which entails a decrease in their productivity, all other things being equal, and crumbling oxides form slag build-ups on the furnace hearth, making it difficult to operate and causing an increased consumption of refractory materials.
The appearance of scale also makes it impossible to accurately measure the temperature of the metal surface, which is set by technologists, which complicates the control of the thermal regime of the furnace.
The above-mentioned interaction with the gaseous medium in the furnace of any alloy element is of practical importance for steel. A decrease in the carbon content in it causes a decrease in hardness and tensile strength. To obtain the desired mechanical properties of the product, it is necessary to remove the decarburized layer (up to 2 mm), which increases the complexity of processing as a whole. Particularly unacceptable is the decarburization of those products that are subsequently subject to surface heat treatment.
The processes of oxidation of the alloy as a whole and its individual impurities during heating in furnaces should be considered jointly, since they are closely related to each other. For example, according to experimental data, when steel is heated to a temperature of 1100°C and higher in a conventional furnace atmosphere, oxidation proceeds faster than surface decarburization, and the resulting scale plays the role of a protective layer that prevents decarburization. At lower temperatures, the oxidation of many steels (even in a pronounced oxidizing environment) is slower than decarburization. Therefore, steel heated to a temperature of 700–1000 °C may have a decarburized surface. This is especially dangerous, since the temperature range of 700-1000 °C is typical for heat treatment.
metal oxidation. The oxidation of alloys is a process of interaction of oxidizing gases with their base and alloying elements. This process is determined not only by the rate of chemical reactions, but also by the formation of an oxide film, which, as it grows, insulates the metal surface from the effects of oxidizing gases. Therefore, the growth rate of the oxide layer depends not only on the course of the chemical process of steel oxidation, but also on the conditions for the movement of metal ions (from the metal and inner layers of oxides to the outer ones) and oxygen atoms (from the surface to the inner layers), i.e., on the conditions for the flow physical process of bilateral diffusion.
The diffusion mechanism for the formation of iron oxides, studied in detail by V. I. Arkharov, determines the three-layer structure of the scale layer formed when steel is heated in an oxidizing environment. The inner layer (adjacent to the metal) has the highest iron content and consists mainly of FeO (wustite): Fe B V 2 0 2 C| FeCX The melting point of wustite is 1317 °C. The middle layer - magnetite Fe 3 0 4 , having a melting point of 1565 ° C, is formed during the subsequent oxidation of wustite: 3FeO C 1 / 2 0 2 ift Fe s 0 4 . This layer contains less iron and is enriched with oxygen compared to the inner layer, although not to the same extent as the most oxygen-rich hematite Fe 2 0 8 (melting point 1538 ° C): 2Fe 3 0 4 -f V 2 0 2 - Ts 3Fe2Os. The composition of each of the layers is not constant over the cross section, but gradually changes due to impurities of more (closer to the surface) or less (closer to the metal) oxides rich in oxygen.
The oxidizing gas during heating in furnaces is not only free oxygen, but also bound oxygen, which is part of the products of complete combustion of fuel: CO 2 H 2 0 and S0 2. These gases, as well as O 2, are called oxidizing, in contrast to reducing: CO, H 2 and CH 4, which are formed as a result of incomplete combustion of fuel. The atmosphere in most fuel stoves is a mixture of N 2 , CO 2 , H 2 0 and S0 2 with a small amount of free oxygen. The presence of a large amount of reducing gases in the furnace indicates incomplete combustion and is unacceptable from the point of view of fuel use. Therefore, the atmosphere of conventional fuel furnaces always has an oxidizing character.
The oxidizing and reducing ability of all these gases with respect to metal depends on their concentration in the furnace atmosphere and on the temperature of the metal surface. O 2 is the strongest oxidizing agent, followed by H 2 O, and CO 2 has the weakest oxidizing effect. Increasing the proportion of neutral gas in the furnace atmosphere reduces the rate of oxidation, which largely depends on the content of H 2 O and SO 2 in the furnace atmosphere. The presence of even very small amounts of SO 2 in furnace gases sharply increases the oxidation rate, since low-melting compounds of oxides and sulfides are formed on the surface of the alloy. As for H 2 S, this compound can be present in a reducing atmosphere and its effect on the metal (along with SO 2) leads to an increase in the sulfur content in the surface layer. In this case, the quality of the metal deteriorates greatly, and sulfur has a particularly harmful effect on alloy steels, since they absorb it to a greater extent than simple carbon steels, and nickel forms a fusible eutectic with sulfur.
The thickness of the oxide layer formed on the metal surface depends not only on the atmosphere in which the metal is heated, but on a number of other factors, which primarily include the temperature and duration of heating. The higher the surface temperature of the metal, the higher the rate of its oxidation. However, it has been found that the growth rate of the oxide layer increases faster after reaching a certain temperature. Thus, the oxidation of steel at temperatures up to 600°C occurs at a relatively low rate, and at temperatures above 800-900°C, the growth rate of the oxide layer increases sharply. If we take the oxidation rate at 900 ° C as a unit, then at 950 ° C it will be 1.25, at 1000 ° C - 2, and at 1300 - 7.
The residence time of the metal in the furnace has a very strong influence on the amount of oxides formed. An increase in the duration of heating to a given temperature leads to an increase in the oxide layer, although the oxidation rate decreases with time due to the thickening of the formed film and, consequently, a decrease in the diffusion flux density of iron ions and oxygen atoms through it. It has been established that if the thickness of the oxidized layer is δ 1 at the heating time t1 then at the heating time t2 up to the same temperature, the thickness of the oxidized layer will be equal to:
δ2 = δ1/( t1/t2) 1/2 .
The duration of heating the metal to a given temperature can be reduced, in particular, as a result of an increase in the temperature in the working chamber of the furnace, which leads to more intense external heat transfer and, thus, contributes to a decrease in the thickness of the oxidized layer.
It has been established that the factors affecting the intensity of oxygen diffusion to the surface of the heated metal from the furnace atmosphere do not significantly affect the growth of the oxide layer. This is due to the fact that diffusion processes in the hardest surface proceed slowly and they are the determining ones. Therefore, the velocity of gas movement has practically no effect on the oxidation of the surface. However, the picture of the movement of combustion products as a whole can have a noticeable effect, since local overheating of the metal due to an uneven gas temperature field in the furnace (which can be caused by an excessively large angle of inclination of the burners, their incorrect placement along the height and length of the furnace, etc.) , inevitably lead to local intense oxidation of the metal.
The conditions for the movement of heated workpieces inside the furnaces and the composition of the heated alloy also have a significant effect on the rate of its oxidation. Thus, when the metal is moved in the furnace, mechanical exfoliation and separation of the resulting oxide layer can occur, which contributes to a more rapid subsequent oxidation of unprotected areas.
The presence of some alloying elements in the alloy (for example, for steel Cr, Ni, Al, Si, etc.) can ensure the formation of a thin and dense, well-adhering oxide film, which reliably prevents subsequent oxidation. Such steels are called heat-resistant and well resist oxidation when heated. In addition, steel with a higher carbon content is less prone to oxidation than low carbon steel. This is explained by the fact that in steel part of the iron is in a carbon-bound state, in the form of iron carbide Fe 3 C. The carbon contained in the steel, oxidized, turns into carbon monoxide, which diffuses to the surface and prevents the oxidation of iron.
Decarburization of the surface layer of steel. Decarburization of steel during heating occurs as a result of the interaction of gases with carbon, which is either in the form of a solid solution or in the form of iron carbide Fe 8 C. Decarburization reactions as a result of the interaction of various gases with iron carbide can proceed as follows:
Fe 3 C + H 2 O \u003d 3Fe + CO + H 2 ; 2Fe 3 C + O 2 \u003d 6Fe + 2CO;
Fe 3 C + CO 2 \u003d 3Fe + 2CO; Fe 3 C + 2H 2 \u003d 3Fe + CH 4.
Similar reactions take place during the interaction of these gases with carbon in solid solution.
The rate of decarburization is determined mainly by the process of two-way diffusion, which occurs under the action of the difference in the concentrations of both media. On the one hand, decarburizing gases diffuse to the surface layer of steel, and on the other hand, the resulting gaseous products move in the opposite direction. In addition, carbon from the inner layers of the metal moves to the surface decarburized layer. Both the rate constants of chemical reactions and the diffusion coefficients increase with increasing temperature. Therefore, the depth of the decarburized layer increases with increasing heating temperature. And since the density of the diffusion flux is proportional to the difference in concentrations of the diffusing components, the depth of the decarburized layer is greater in the case of heating high-carbon steel than in the case of heating low-carbon steel. The alloying elements contained in the steel also play a role in the decarburization process. Thus, chromium and manganese lower the diffusion coefficient of carbon, while cobalt, aluminum and tungsten increase it, respectively preventing or promoting the decarburization of steel. Silicon, nickel and vanadium do not have a significant effect on decarburization.
The gases that make up the furnace atmosphere and cause decarburization include H 2 0, CO 2 , O 2 and H 2 . The strongest decarburizing effect on steel is distinguished by H 2 0, and the weakest H 2 . In this case, the decarburizing ability of CO 2 increases with increasing temperature, and the decarburizing ability of dry H 2 decreases. Hydrogen in the presence of water vapor has a very strong decarburizing effect on the surface layer of steel.
Protection of steel against oxidation and decarburization. The harmful effect of the oxidation and decarburization of the metal during heating on its quality requires the adoption of measures to prevent these phenomena. The most complete protection of the surface of ingots, blanks and parts is achieved in furnaces, where the effect of oxidizing and decarburizing gases on it is excluded. These furnaces include salt and metal baths, as well as furnaces where heating is carried out in a controlled atmosphere. In furnaces of this type, either the heated metal is isolated from gases, usually covered with a special hermetic muffle, or the flame itself is placed inside the so-called radiant pipes, the heat from which is transferred to the heated metal without its contact with oxidizing and decarburizing gases. The working space of such furnaces is filled with special atmospheres, the composition of which is selected depending on the heating technology and alloy grade. Protective atmospheres are prepared separately in special installations.
There is also known a method of creating a weakly oxidizing atmosphere directly in the working space of furnaces, without metal or flame muffling. This is achieved due to incomplete combustion of fuel (with an air consumption coefficient of 0.5-0.55). In this case, the composition of the combustion products includes CO and H, and along with the products of complete combustion of CO 2 and H 2 O. If the ratios of CO / CO2 and H 2 / H 2 O are not less than 1.3, then the heating of the metal in such an environment occurs almost without surface oxidation.
A decrease in the oxidation of the metal surface during its heating in fuel-fired furnaces with an open flame (constituting a large part of the fleet of furnaces of metallurgical and machine-building plants) can also be achieved by reducing the duration of its stay at a high surface temperature. This is achieved by choosing the most rational mode of heating the metal in the furnace.
Calculations of metal heating in furnaces are performed to determine the temperature field of an ingot, billet or finished product, based on the conditions dictated by the technological purpose of heating. This takes into account the restrictions imposed by the processes occurring during heating, as well as the patterns of the selected heating mode. The problem of determining the heating time to a given temperature is often considered, provided that the required uniformity is ensured by the end of its stay in the furnace (the latter in the case of massive bodies). In this case, they are usually set by the law of change in the temperature of the heating medium, choosing the heating mode depending on the degree of thermal massiveness of the metal. To determine the degree of thermal massiveness and for the subsequent calculation of heating, the question of the heated thickness of the ingot or billet is very important.