If you know how to harden the metal correctly, even at home you can increase the hardness of the products from it in two or three times. The reasons for which the need arises may be the most different. Such a technological operation, in particular, is required if the metal must be given hardness sufficient so that it can cut the glass.
Most often, it is necessary to harden the cutting tool, and thermal processing is performed not only if it is necessary to increase its hardness, but also when this characteristic is required to reduce. When the hardness of the tool is too small, its cutting part will be fused during operation, if it is high, the metal will crumble under the influence of mechanical loads.
Few know that there is a simple way to check how well the tool from steel is hardened, not only in production or home conditions, but also in the store, when buying. In order to perform such a check, you will need a regular file. They are carried out along the cutting part of the acquired tool. If he hardered badly, the file will seem to stick to its working part, and in the opposite case - it is easy to move away from the test tool, while the hand in which the file is located, will not feel any irregularities on the surface of the product.
If still it happened that at your disposal turned out to be a tool, the quality of quenching whom does not suit you, you should not worry about this. This problem is solved fairly easy: we can harden the metal even at home without using for this complex equipment and special devices. However, you should know that the ordering is not amenable to small carbon steel. At the same time, the hardness of carbon and is enough to simply increase even at home.
Technological nuances quenching
Hardening, which is one of the types of thermal processing of metals, is performed in two stages. First, the metal is heated to a high temperature, and then cooled. Various metals and even steel relating to different categories differ from each other with their structure, so the thermal processing modes do not coincide.
Thermal processing of metal (hardening, vacation, etc.) may be required for:
- his hardening and improving hardness;
- improving its plasticity, which is necessary when processing by plastic deformation.
Many specialized companies hardened steel, but the cost of these services is high enough and depends on the weight of the part that is required to be subjected to heat treatment. That is why it is advisable to do it yourself, especially since this can be done even at home.
If you decide to harden the metal on your own, it is very important to correctly implement such a procedure as heating. This process should not be accompanied by the appearance on the surface of the product of black or blue spots. The fact that the heating occurs correctly is evidenced by the bright red color of the metal. Well demonstrates this video process, which will help you get an idea of \u200b\u200bwhat extent heated the metal subjected to thermal processing.
As a heat source to heat up to the desired metal temperature, you can harden, you can use:
- special furnace working on electricity;
- soldering lamp;
- open fire, which can be divorced in the yard of your home or in the country.
The choice of heat source depends on how temperature it is necessary to heat the metal subjected to heat treatment.
The choice of cooling method depends not only on the material, but also on what results you need to achieve. If, for example, you need to harden not all the product, but only its separate section, then cooling is also carried out point, for which the cold water jet can be used.
The technological scheme for which the metal is hardened may provide for instant, gradual or multi-stage cooling.
Fast cooling for which the cooler of the same type is used is optimally suitable in order to order steel related to the category of carbon or alloyed. To perform such cooling, you need one container, which can use a bucket, barrel or even a regular bath (it all depends on the dimensions of the object being processed).
In the event that other categories or if there is no extraction, a two-stage cooling scheme is required. With such a scheme, the product is heated to the desired temperature first cooled with water, and then placed in a mineral or synthetic oil in which further cooling occurs. In no case cannot be used at once the oil coolant, as the oil can ignite.
In order to correctly select the modes of hardening different stamps, you should focus on special tables.
How to harden steel on open fire
As mentioned above, harden steel and at home, using an open fire for heating. Begin such a process, naturally, it follows from the dilution of a fire, in which many hot coal should be formed. You will also need two containers. In one of them it is necessary to pour mineral or synthetic oil, and in the other - ordinary cold water.
In order to extract hot iron from a fire, you will need blacksmith mites that can be replaced by any other tool for such a destination. After all the preparatory work was performed, and a sufficient amount of hot coal was formed in the fire, it is possible to lay items that need to harden.
The color of the resulting coal can be judged by the temperature of their heating. So, the coals are more hot, the surface of which is bright white. It is important to monitor both the color of the fire flame, which indicates the temperature mode in its inner part. It is best if the fire flame will be painted in a raspberry, not white. In the latter case, testifying to too high flame temperatures, there is a risk not only to overheat, but even burn metal to harden.
The color of the heated metal is also needed to closely. In particular, it is impossible to assume that black spots appeared on the cutting edges of the treated tool. The formation of the metal indicates that he strongly softened and became too plastic. It is impossible to bring it to such a state.
After the product is rolled to the desired degree, you can proceed to the next step - cooling. First of all, it is lowered into a container with oil, and they do it often (with a frequency of 3 seconds) and as sharply as possible. Gradually, the gaps between these dips increase. As soon as the hot steel will lose the brightness of its color, it can be cooled in water.
When cooled by water, on the surface of which droplets of hot oil remained, care should be taken, as they can flare. After each dive, water must be scolded so that it constantly remains cool. Get a more visual idea of \u200b\u200bthe rules for performing such an operation will help the learning video.
There are certain subtleties when cooling the hardening drills. So, they can not be lowered into a container with coolant plastics. If you do this, then the lower part of the drill or any other metal object with an elongated shape sharply cools the first, which will lead to its compression. That is why it is necessary to immerse such products into the cooling fluid from a wider end.
For thermal processing of special varieties of steel and smelting of non-ferrous metals of the open-end fire capabilities, as it will not be able to ensure the heating of the metal to a temperature of 700-9000. For such purposes, it is necessary to use special furnaces that can be muffle or electric. If the electrical furnace is made at home is quite difficult and expensive, then with the heating equipment of the muffle type it is quite feasible.
Independent manufacture of metal chamber
The muffle furnace, which is quite possible to do on its own at home, allows you to harden the various steel brands. The main component that will be required for the manufacture of this heating device is a refractory clay. The layer of such a clay, which will be covered with the inside of the furnace, should be not more than 1 cm.
Camera diagram for hardening metal: 1 - nichrome wire; 2 - the inner part of the chamber; 3 - outer part of the chamber; 4 - Rear Wall with Spiral Conclusions
In order to give a future furnace the required configuration and desired dimensions, it is best to make a form of cardboard impregnated with paraffin, to which a refractory clay will be applied. The clay, mixed with water to thick homogeneous mass, is applied to the wrong side of the cardboard form, from which it herself will last after complete drying. Metal products heated in such a device are placed in it through a special door, which is also manufactured from refractory clay.
The chamber and the device door after drying outdoors are additionally dried at a temperature of 100 °. After that, they are subjected to burning in the furnace, the temperature in the chamber of which is gradually adjusted to 900 °. When they are cooled after firing, they must be accurately combined with each other, using laminated tools and emery skins.
The surface of the fully formed chamber is wound with nichrome wire, the diameter of which should be 0.75 mm. The first and last layer of such a winding must be twisted. Waving a wire on the camera, you should leave a certain distance between its turns, which also need to fill the refractory clay to eliminate the possibility of a short circuit. After the clay layer, applied to ensure isolation between the turns of the nichrome wire, dried, 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 the metal housing, and the gaps between them are covered with asbestos crumb. In order to provide access to the inner chamber, the doors trimmed from the inside with ceramic tiles are placed on the metal body of the oven. All existing gaps between the structural elements are closed with refractory clay and asbestos crumbs.
The ends of the nichrome winding of the camera, to which the electric power must be tested, are removed from the back side of its metal frame. To control the processes occurring in the inside of the muffle furnace, as well as measure the temperature in it using the thermocouple, in its front part it is necessary to perform two holes whose diameters should be 1 and 2 cm accordingly. With the front part of the frame, such holes will be closed with special steel curtains. A homemade design, the manufacture of which is described above, allows at home to order plumbing and cutting tools, working elements of stamping equipment, etc.
Basic methods I. methods for transformation of electrical energy into thermal Classified as follows. Distinguish direct and indirect electrical heating.
For straight ElectronAgreage The transformation of electrical energy into thermal occurs as a result of the passage of the electric current directly along the heated body or medium (metal, water, milk, soil, etc.). For indirect electronagev The electric current passes through a special heating device (heating element), from which heat is transferred to the heated body or medium by means of thermal conductivity, convection or radiation.
There are several types of electrical energy transformation into thermal, which determine electric heating methods.
The flow of electrical current on electrically conductive solid bodies or liquid media is accompanied by heat release. According to the law of Joule - Lenz, the amount of heat Q \u003d i 2 Rt, where Q is the amount, heat, J; I - Silatoka, A; R is the resistance of the body or environment, Ohm; t - current flow, p.
Heating with resistance can be carried out by contact and electrode methods.
Contact method It is used to heat the metals both according to the principle of direct electric heating, for example, in the devices of electrocontact welding, and on the principle of indirect electric heating - in the heating elements.
Electrode method It is used to heat non-metallic conducting materials and media: water, milk, juicy feed, soil, etc. Heated material or medium is placed between the electrodes to which the variable voltage is supplied.
Electric, current, flowing through the material between the electrodes, heats it. Normal (unlawted) water conducts electric current, as it always contains some amounts of salts, alkalis or acids, which dissociate on ions that are carriers of electrical charges, that is, an electric current. Similar to the nature of the electrical conductivity of milk and other liquids, soil, juicy feed, etc.
The direct electrode heating is carried out only on alternating current, since the constant current causes the electrolysis of the heated material and its damage.
Electricheating resistance was widely used in production due to its simplicity, reliability, versatility and low cost of heating devices.
Electric arc heating
In an electrical arc arising between two electrodes in the gaseous medium, the electric energy transformation into thermal.
For the ignition of the arc, the electrodes attached to the power source are in contact, and then slowly diluted. Contact resistance at the time of dilution of the electrodes is strongly heated by a current passing. Free electrons that constantly moving in metal, with an increase in temperature in the place of contact of the electrodes accelerate their movement.
With increasing temperature, the speed of free electrons increases so much that they are torn from the metal of the electrodes and fly into the airspace. When driving, they face air molecules and break them on positively and negatively charged ions. The ionization of the airspace between the electrodes is occurring, 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-term discharge - an electric arc accompanied by heat release. The temperature of the arc is non-etinakov in different parts and is at metallic electrodes: at the cathode - about 2400 ° C, at the Anode - about 2600 ° C, in the center of the arc - about 6000 - 7000 ° C.
Distinguish direct and indirect electric arc heating. The main practical application finds direct electric arc heating in arc electrical welding installations. In the installations of indirect heating of the arc, it is used as a powerful source of infrared rays.
If you put a piece of metal into the alternating magnetic field, then the variable e is induced. D. C, under the action of which vortex currents will occur in the metal. The passage of these currents in the metal will cause its heating. This method of heat heating is called induction. The device of some induction heaters is based on the use of the phenomenon of the surface effect and the effect of proximity.
For induction heating, industrial (50 Hz) and high frequency (8-10 kHz, 70-500 kHz) are used for induction heating. The induction heating of metallic bodies (parts, blanks) in mechanical engineering and in the repair of equipment, as well as for hardening metal parts, was the greatest distribution. Induction method can also be used to heat water, soil, concrete and milk pasteurization.
Dielectric heating
The physical essence of dielectric heating is as follows. In solid bodies and liquid media with poor electrical conductivity (dielectrics) placed in a fast-hot electric field, electrical energy turns into thermal.
In any dielectric there are electrical charges associated with intermolecular forces. These charges are called associated in contrast to free charges in conductive materials. Under the action of the electric field, the associated charges are oriented or shifted in the direction of the field. The displacement of the associated charges under the action of an external electric field is called polarization.
In a variable electric field, there is a continuous movement of charges, and therefore associated with the intermolecular forces of molecules. The energy spent by the source to the polarization of the molecules of the weatherproof materials is highlighted in the form of heat. In some ruling materials, there is a small amount of free charges that are created under the influence of an electrical field of conduction current in the magnitude of conductivity current, which promotes the release of additional heat in the material.
With dielectric heating, the material to be heated is placed between metal electrodes - the capacitor plays to which the high frequency voltage (0.5 - 20 MHz and above) is supplied from a special high-frequency generator. The dielectric heating unit consists of a high-frequency lamp generator, a power transformer and a drying device with electrodes.
High-frequency dielectric heating is a perspective method of heating and is used mainly for drying and heat treatment, paper, products and feeds (drying grain, vegetables and fruits), pasteurization and sterilization of milk, etc.
Electron beam (electronic) heating
When meeting the flux of electrons (electron beam), accelerated in an electric field, with a heated body, electrical energy turns into thermal. A feature of the electron heating is the high density of the energy concentration, which is 5x10 8 kW / cm2, which is several thousand times higher than with electric arge heating. Electronic heating is used in industry for welding of very small parts and smelting of ultrapure metals.
In addition to the considered methods of electrical heating, in production and everyday life infrared heating (irradiation).
Heating metal with welding current. Law of Joule Lenza. Electrical metal resistance.
All current-key elements are heated by electric current, and the amount of heat released on any section of the electrical circuit with the active resistance R \u003d R (T), which is a function from T and τ with a current i \u003d i (t), depending on time T is determined by the Jouel law -Lenty:
This is a general formula that does not show and does not determine the specific temperatures in the compound zone when it is heated with its 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.
The contact machines are structurally made so that the greatest amount of heat is highlighted between the electrodes.
At the suture point welding, the largest number of the electrode electrode, the total number of resistance is made up of the electrode resistance - the part + part-detail + part + electrode- detail
RE \u003d 2DD + RDD + 2D
All components of the overall resistance resistance are continuously changed during the thermal cycle of welding.
Contact resistance - RDD is the largest largest, because Contacting is carried out in microwaves and the area of \u200b\u200bphysical contact is small.
In addition, oxide films and various contaminants are present on the surface of the part.
Because Welded mainly steel and alloys with significant strength, then the complete crumple of microennesses occurs only when they are heated by their welding current to heat passenger, about 600 grades
Resistance in contact electrode- Detail is significantly less than RDD, because Soften and more high-temperature electrode material of the electrodes is actively implemented between the protrusions of the micronether parts.
Increased resistance in contacts is also due to the fact that in the contact regions a sharp curvature of the current line, which determines the higher resistance due to the increase in the current path.
Contact resistance RDD and RED largely depends on the cleaning of the surface for welding.
Measuring 2 plates, 3 mm thick very strongly compressed 2007 according to the ammeter-voltmeter scheme, obtained the following values:
Skipping surfaces around and grinding: 100 mp
Conclusion: Grind
In practice, etching is used (when welding large surfaces), surface treatment with metal brushes, sandblasting and shot blasting.
During contact welding, it is trying to use cold-rolled rolling 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 coiled surfaces.
Contact resistance of clean, but covered with oxide parts decreases with increasing compression effort. This will be explained by the larger deformation of microprotes.
Turn on the current, the largest density of the current line focuses on juvenile surfaces. Current through the contacts formed during the deformation of microprotes.
At the initial moment of time, the current density in the material is less, because The current lines are cut relatively evenly, and in contact, the detail of the current flow only through the conductivity 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.
Metal in contact will become plastic. It is deformed under the action of welding effort, the area of \u200b\u200bconducting contacts will increase and when the hundredth lobes of a second) is completely deformed, the oxide films are completely deformed, the oxide films are partially collapsed, partially diffuse into a mass of the part and the role of contact resistance Redd will cease to be paramount during heating. .
However, by this time, the temperature in the area of \u200b\u200bthe contact part-detail will be the highest, the resistivity of the material ρ is the largest and heat dissipation will be more intense anyway in this zone.
With sufficient densities of the current duration of its flow, it is precipitated that metal melting begins.
The appearance of melting isotherms It is in contact that the part-detail will contribute to the smallest heat sink from this area, its own resistance of the part.
Own resistance details
System conductor
The coefficient A increases the spreading of the current line into the mass of the part, and the increase in the real spread area occurs
dK-diameter spreading
A \u003d 0.8-0.95, depends on the hardness of the material, and to a greater extent of 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, the specific resistance depends on temperature
ρ (t) \u003d ρ0 * (1 + αp * t)
In the process of welding when current flow T is measured from contact to T Pl and above
TPL \u003d 1530 Grads
When the TPL is reached, the resistivity is increased by jumping.
αρ- temperature coefficient
αρ \u003d 0.004 1 / Grads- for pure metals
αρ \u003d 0.001-0.003 1 / Grads- for alloys
The value of αρ falls with increasing degree of ligation.
With the increasing temperature of the metal both in contact and in the bulk, under the electrodes, the contact area increases and if the operating case of the electrodes is spherical, then the area of \u200b\u200bcontact may increase by 1.5-2 times.
Schedule change resistance in the process of welding.
In the initial moment of time, the parts resistance increases due to an increase in temperature and growth of the electrical resistance, then the metal becomes plastic and the contact area begins to increase due to induction of the electrodes into the surface surface, as well as an increase in the size of the contact area of \u200b\u200bthe part-item.
The overall resistance will decrease as the welding current is turned off. However, this is true for welding carbon and low alloy steels.
For welding heat-resistant NI and CR alloys, resistance may even grow.
Electric and temperature field.
The law of Joule-Lenza Q \u003d IRT shows heat dissipation in current-carrying elements, and there are still heat sink processes.
By gravating the active cooling of the electrodes and an increase in the heat sink in them we obtain a lentil shape of a cast kernel.
But this form is not always possible to get, especially when welding of heterogeneous, multi-power materials and thin parts.
Knowing the temperature of the temperature field in the welding zone can be analyzed:
1) sizes of cast kernel.
2) Size of the GVT (structure)
3) the magnitude of residual stresses, i.e. Properties of compounds.
Temperature-set temperature at different points of part at a certain point in time.
Points with the same temperature, the connected line are called isotherm.
The size of the pure core on the microclife calls the melting isotherm on the boundaries of the cast kernel.
Ultimately, the temperature and size of melting isotherm, i.e. Cast kernel affects mainly parts resistance.
The founder, Gelman, took two parts 2 + 2 mm, polished, stealing and received a cast kernel; He took the details and received the cast kernel too.
However, the difficulties arising during welding of heterogeneous thicknesses are forced 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 platform, perpendicular to the direction of movement of charges, attributed to the length of its surface.
The heating of metals and alloys is produced either to reduce their resistance of plastic deformation (i.e., before forging or rolling), or to change the crystal structure of what is happening under the influence of high temperatures (heat treatment). In each of these cases, the conditions for the flow of the heating process have a significant impact on the quality of the final product.
The solid tasks are predetermined by 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 it can be issued from the furnace in accordance with the requirements of the technology. The temperature of heating temperature depends on the chemical composition (brand) of the alloy and the goal of heating.
When heated before processing pressure, the temperature of issuing billets from the furnace should be sufficiently high, as it helps to reduce the resistance of plastic deformation and leads to a reduction in electricity consumption for processing, an increase in the performance of rolling and blacksmithing equipment, as well as an increase in its service life.
However, there is an upper limit of the heating temperature, since it is limited by the growth of grain, the phenomena of overheating and the fifth, as well as the acceleration of metal oxidation. In the process of heating the majority of alloys while reaching a point lying on 30-100 ° C below the line of solidus on their status diagram, due to the course and non-metallic inclusions, the liquid phase appears on the grain boundaries; This leads to a weakening of mechanical communication between grains, intense oxidation on their borders; Such a metal loses strength and destroyed when processing pressure. This phenomenon called the fault, limit the maximum heating temperature. The checked metal can not be corrected by any subsequent heat treatment and is only suitable for melting.
Metal overheating leads to excessive grain growth, as a result of which mechanical properties deteriorate. Therefore, rolling should end at a temperature of lower than the overheating temperature. Superheated metal can be corrected by annealing or normalization.
The lower limit of the heating temperature is determined based on the temperature allowed at the end of the pressure processing, taking into account all the heat loss from the workpiece into the environment and the heat release in it itself due to plastic deformation. Consequently, for each alloy and for each type of pressure treatment, there is a specific temperature range, above and below which should not be heated by the workpiece. This information is given in the relevant reference books.
The heating temperature is especially important for such complex alloys, such as high-alloy steel, which in the process of pressure processing have a large resistance of plastic deformation, and at the same time, prone to overheating and facing. These factors cause a narrower range of heating temperatures of high-alloy steels compared to carbon monoxide.
In tab. 21-1 As an illustration, data are given for some steels about the maximum permissible temperature of their heating before pressing pressure and the temperature of the face.
When heat treatment, the heating temperature depends only on the technological requirements, that is, on the type of heat treatment and its regime caused by the structure and structure of the alloy.
Uniform heating The size of the temperature difference between the surface and the center is determined (since it is usually the greatest difference) of the workpiece when it is issued from the furnace:
ΔТ Kon \u003d ton pov - t con Price. This indicator is also very important, since too much temperature difference in cross section of the workpiece when heated before treatment may cause uneven deformation, and when heated under heat treatment - entail the incompleteness of the required transformations over the entire thickness of the metal, i.e., in both cases - marriage final products. At the same time, the level of alignment temperature in a metal cross section requires long exposure at high surface temperature.
However, the complete uniform of the heating of the metal before processing pressure is not required, since in the process of transporting it from the furnace to the mill or press and rolling (forging), the temperature of the ingots and blanks in connection with the impact of heat into the environment from their surface is inevitable. thermal conductivity inside the metal. Based on this, the permissible temperature difference in cross section is usually taken according to practical data when heated before processing by pressure during the following limits: for high-alloyed steels Δ Ton. \u003d 100δ; For all other steel grades Δ Ton. \u003d 200δ at Δ<0,1 м и ∆Ton. \u003d 300δ at δ\u003e 0.2 m. Here δ is the heated thickness of the metal.
In all cases, the temperature difference in the thickness of the workpiece at the end of its heating before rolling or forging should not exceed 50 ° C, and when heated under heat treatment 20 ° C, regardless of the thickness of the product. When heating large ingots is allowed to be issued from the furnace at δ Ton. <100 °С.
Another important task of the metal heating technology is to ensure a uniform temperature distribution over the entire surface of the blanks or products by the time they are unloading from the furnace. The practical need for this requirement is obvious, since with a significant uneven heating along the metal surface (even when the necessary temperature difference is achieved in thickness), such defects such as the uneven profile of the finished rolling profile or various mechanical properties of the heat treatment is inevitable.
Ensuring the uniformity of the temperature over the surface of the heated metal is achieved by means of the correct selection of the furnace for heating a certain type of blanks or products and the corresponding placement of heat generating devices in it, creating the necessary field of temperatures in the workspace of the furnace, the relative position of the workpieces, etc.
Duration of heating Until the final temperature is also the most important indicator, since the performance 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 temperature change at some point of the heated body per unit of time. Typically, the heating rate varies in the course of the proceeding process, and therefore the heating rate at some point in time and the average heating rate for the time interval in question are distinguished.
The faster heating is carried out (i.e., the greater the heating rate), the one is obviously higher than the performance of the furnace with other things being equal. However, in some cases, the heating rate cannot be chosen as much as large, even if the conditions of the external heat exchange and allow it to be implemented. This is due to certain limitations imposed by the conditions for the flow of processes accompanying the heating of the metal in the furnaces and the following.
Processes flowing with metal heating.When the metal heated, its enthalpy occurs, and since in most cases the heat supply is produced to the surface of the ingots and blanks, their outer temperature is higher than the temperature of the internal layers. As a result of the thermal expansion of different parts of the solid, the voltage called the thermal name occurs in different magnitude.
Another group of phenomena is associated with chemical processes on the metal surface when heated. The surface of the metal, which is at high temperature, enters into interaction with the environment (i.e., with combustion products or with air), as a result of which the oxide layer is formed on it. If any elements of the alloy interact with the surrounding metal medium to form the gas phase, then the surface is depleted with these elements. For example, the oxidation of carbon steel when it is heated in the furnaces, it causes superficial decarbing.
Thermal stresses
As noted above, in the cross section of ingots and billets, with their heating, an uneven temperature distribution arises and, therefore, different parts of the body seek to change their size to varying degrees. Since there are connections between all individual parts in the solid, they cannot independently deform in accordance with those temperatures to which they are heated. As a result, thermal stresses arise due to the difference in temperatures. Exterior, more heated layers, strive to expand and are, therefore, in a compressed state. Internal, colder layers are subject to stretching effort. If these voltages do not exceed the limit of the elasticity of the heated metal, then with the leveling of temperature by cross section, thermal stresses disappear.
All metals and alloys have elastic properties to a certain temperature (for example, most steel brands up to 450-500 ° C). Above this certain temperature, the metals are transferred to the plastic state and the thermal stresses arising in them cause plastic deformation and disappear. Therefore, temperature stresses should be taken into account when heated and cooling steel only in the temperature range from room temperature to the transition point of this metal or alloy from the elastic state into the plastic. Such voltages are called disappearing, or temporary.
In addition to temporary, there are residual temperature stresses that increase the risk of destruction when heated. These voltages occur if the ingot or billet was previously heated and cooling. When cooled, the outer layers of metal (coolest) earlier reach the transition temperature from plastic to an elastic state. As further cooling, the inner layers are under the influence of tensile efforts that do not disappear due to the low plasticity of the cold metal. If this ingot or billet will be rearranged again, then the temporary voltages arising in them will be applied with the same residual sign, which will aggravate the risk of cracking and breaks.
In addition to time and residual temperature stresses, voltages caused by the structural changes in volume occur during the heating and cooling of alloys. But since these phenomena usually take place at temperatures exceeding the border of the transition from the elastic state into plastic, then the structural stresses are dissipated due to the plastic state of the metal.
The dependence between deformations and stresses establishes the law
σ= ( T SR -T.)
where β is the linear expansion coefficient; T cf. - average body temperature; T. - temperature in this section of the body; E. - modulus of elasticity (for many brands steel E. Reduced from (18 ÷ 22). 10 4 MPa to (14 ÷ 17). 10 4 MPa with increasing temperature from room temperature up to 500 ° C; σ - voltage; V is the ratio of Poisson (for steel V ≈ 0.3).
Large practical interest is the foundation of the maximum permissible temperature difference over the body cross section ΔТ additional \u003d t. The most dangerous in this case are tensile stresses, so they should be considered when calculating the permissible temperature difference. As a strength characteristic, the value of the time resistance of the alloy rupture σ in is to.
Then, using solutions of thermal conductivity problems (see ch. 16) and imposing expression on them (21-1), for the case of regular mode II, it is possible, in particular, to obtain:
for uniformly and symmetrically heated infinite plate
∆T. extra \u003d 1.5 (1 - v) σ in / ();
for uniformly and symmetrically heated endless cylinder
∆T. Dop \u003d 2 (1 - V) σ in / ().
The permissible temperature difference found by formulas (21-2) and (21-3) does not depend on the size of the body and its thermophysical characteristics. Body sizes have an indirect effect on the value of Δ T. Extra, as residual stresses in larger bodies more.
Oxidation and decarburization of the surface when heated.The oxidation of ingots and billets when heated in the furnaces is extremely undesirable phenomenon, since its consequence is non-refundable metal loss. This leads to very large economic damage, which becomes especially obvious if you compare the cost of metal losses during oxidation with other expenses on the redistribution. For example, when heating steel bars in heating wells, the cost of a metal lost with a scale, usually above the cost of fuel consumed to heat this metal, and the cost of electricity consumed on its rolling. When the blanks are heated in the furnaces of sorts of rolling shops loss with a scale somewhat lower, but they are still large enough and at the cost of commensurate with fuel expenditures. Since, on the way from the ingot to the finished product, the metal is usually heated several times in different ovens, then the loss due to oxidation is a very substantial value. In addition, the higher hardness of oxides compared with the metal leads to increased wear of the tools and increases the percentage of marriage when forging and rolling.
The thermal conductivity of the oxide in relation to the metal in relation to the metal increases the duration of heating in the furnaces, which entails a decrease in their performance, all other things being entrusted, and the sprinkled oxides form slag growths on the snoves, making it difficult and causing an increased consumption of refractory materials.
The appearance of scale also does not allow accurate to measure the temperature of the metal surface specified by technologists, which complicates the control of the thermal regime of the furnace.
The above interaction with the gas medium in the furnace of any element of the alloy has practical importance for steel. Reducing the carbon content in it causes a decrease in hardness and strength limit. To obtain the specified mechanical properties of the product, it is necessary to remove the decarburized layer (reaching 2 mm), which increases the complexity of treatment as a whole. Especially unacceptable shuttering of those products that are subsequently subject to superficial heat treatment.
The oxidation processes of the alloy as a whole and its individual impurities when heating in the furnaces should be considered jointly because they are closely related. For example, according to the experimental data, when heated steel to a temperature of 1100 ° C and above in a conventional stove atmosphere, oxidation flows faster than the surface decarburization, and the hydrogen-generating layer plays the role of a protective layer warning decarbing. At lower temperatures, the oxidation of many steels (even in a pronounced oxidative environment) is slower than the decarburization. Therefore, steel heated to a temperature of 700-1000 ° C may have a decarbered surface. This is especially dangerous, as the temperature range of 700-1000 ° C is characteristic of heat treatment.
Metal oxidation. Alloy oxidation is the process of interaction of oxidizing gases with their basis and alloying elements. This process is determined not only by the rate of flow of chemical reactions, but also the patterns of the formation of an oxide film, which, as it is grown, the metal surface from the effects of oxidative gases. Therefore, the growth rate of the oxide layer depends not only on the flow of the chemical process of steel oxidation, but also on the conditions of movement of metal ions (from metal and inner layers of oxides to the outer) and oxygen atoms (from the surface to the inner layers), i.e. from the conditions of 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 during the heating of steel in the oxidative medium. The inner layer (adjacent to the metal) has the highest iron content and consists mainly of FEO (Wytitis): Fe in V 2 0 2 C | FeCX Wasteit Melting Temperature 1317 ° C. The average layer - Magnetite Fe 3 0 4, having a melting point of 1565 ° C, is formed under the subsequent oxidation of the avala: 3Feo C 1/2 0 2 IFT Fe S 0 4. This layer contains less iron and compared with the inner layer is enriched with oxygen, although not to such an extent, as the richest oxygen hematite Fe 2 0 8 (melting point 1538 ° C): 2Fe 3 0 4 -F v 2 0 2 - C 3Fe 2 O S. The composition of each of the layers is not constant in cross section, but gradually changes due to impurities (closer to the surface) or less (closer to the metal) oxygen rich oxygen.
The oxidizing gas with heating in the furnaces is not only free oxygen, but also oxygen associated, which is part of the fuel combustion products: CO 2 H 2 0 and S0 2. These gases, as well as about 2, are called oxidative, in contrast to the restoration: CO, H 2 and CH 4, which are formed as a result of incomplete combustion of fuel. The atmosphere in most of the fuel furnaces is a mixture of N 2, C0 2, H 2 0 and S0 2 with a small amount of free oxygen. The presence of a large amount of reductive gases in the furnace testifies to incomplete combustion and is unacceptable in the point of view of fuel use. Therefore, the atmosphere of ordinary fuel furnaces always has an oxidative character.
The oxidative and reductive capacity of all listed gases relative to the metal depends on their concentration in the atmosphere of the furnace and on the surface temperature of the metal. The strongest oxidizer is about 2, it is necessary for it. 2 o and the weakest oxidizing effect differs from 2. The increase in the proportion of neutral gas in the stove atmosphere reduces the oxidation rate, which largely depends on the content of H 2 O and SO 2 in the stove atmosphere. The presence in the furnace gases even very small amounts of SO 2 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 may be present in the reducing atmosphere and its impact on the metal (along with SO 2) leads to an increase in the sulfur content in the surface layer. The quality of the metal is strongly deteriorating, and the sulfur has a particularly harmful effect of sulfur, as they absorb it to a greater extent than simple carbon monoxide, and nickel forms with a gray-melting eutectic.
The thickness of the formed oxide layer on the surface of the metal depends not only on the atmosphere, in which the metal is heated, but to from a number of other factors to which the temperature and duration of heating are primarily among the first. The higher the temperature of the metal surface, the higher the speed of its oxidation. However, it was established that the growth rate of the oxide layer increases faster after the achievement of some temperature. Thus, the oxidation of steel at temperatures up to 600 ° C occurs with a relatively low velocity, 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 per unit, then at 950 ° C it will be 1.25, at 1000 ° C- 2, and at 1300 - 7.
The duration of the residence of the metal in the furnace has a very strong effect on the amount of oxides formed. The increase in the duration of heating to a given temperature leads to an increase in the oxide layer, although the oxidation rate drops with time due to thickening of the resulting film and, consequently, to reduce the density of the diffusion flow through it iron ions and oxygen atoms. It has been established that if the thickness of the oxidized layer is δ 1 at heating time t 1. then at heating time t 2. Up to the same temperature, the thickness of the oxidized layer will be equal to:
δ 2 \u003d Δ1 / ( T 1./ T 2.) 1/2 .
The duration of the metal heating to a given temperature can be reduced, in particular, as a result of an increase in temperature in the furnace working chamber, which leads to more intensive external heat exchange and, thus, helps to reduce the thickness of the oxidized layer.
It has been established that factors affecting the intensity of the diffusion of oxygen to the surface of the heated metal from the atmosphere of the furnace does not have a significant effect on the growth of the oxide layer. This is due to the fact that the diffusion processes in the solid surface itself slowly and they are determining. Therefore, the speed of movement of gases is 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 the local metal overheating caused by an uneven gas temperature in the furnace (which can be caused by an excessively large angle of tilt of the burners, their improper placement in height and the length of the furnace, etc.) Inevitably lead to local intensive metal oxidation.
The conditions for the movement of heated blanks inside the furnaces and the composition of the heated alloy also have a noticeable effect on the speed of its oxidation. Thus, when moving the metal in the furnace can occur mechanical peeling and separation of the oxide layer, which contributes to a more rapid subsequent oxidation of unprotected areas.
The presence in alloying of certain alloying elements (for example, for steel CR, Ni, Al, Si, etc.) can provide the formation of a thin and dense, well-adjacent oxide film, reliably warning subsequent oxidation. Such steels are called heat-resistant and well resist oxidation when heated. In addition, steel with a higher carbon content is less susceptible to oxidation than small carbon. This is explained by the fact that in steel part of the iron is in the carbon-related state, in the form of carbide FE 3 C. Carbon contained in steel, oxidizing, turns into carbon monoxide, diffusing to the surface and preventing iron oxidation.
Steel Surface Layer. Steel blazer when heated 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. Debrewing reactions as a result of the interaction of various gases with iron carbide may proceed as follows:
Fe 3 C + H 2 O \u003d 3FE + CO + H 2; 2fe 3 C + O 2 \u003d 6FE + 2SO;
Fe 3 C + CO 2 \u003d 3FE + 2SO; Fe 3 C + 2H 2 \u003d 3FE + CH 4.
Similar reactions occur in the interaction of these gases with carbon in solid solution.
The decarburization rate is determined mainly by the process of two-way diffusion occurring under the action of the difference in the concentrations of both environments. On the one hand, the shut-off gases diffuse to the surface layer of steel, and on the other - the resulting gaseous products move in the opposite direction. In addition, carbon from the inner layers of metal moves to the surface dishthumous layer. Both the rate constants of chemical reactions and diffusion coefficients increase with increasing temperature. Therefore, the depth of the decarburized layer increases with increasing temperature of heating. And since the density of the diffusion flow is proportional to the difference in the concentrations of diffing components, the depth of the dismissed layer is larger in the case of heating of high carbon steel than in the case of heating is low-carbon. The alloying elements contained in steel also play a role in the process of decarburing. Thus, chromium and manganese lower the coefficient of carbon diffusion, and cobalt, aluminum and tungsten increase it, respectively, preventing or contributing to decarburization of steel. Silicon, Nickel and Vanadium do not have a significant effect on decarburization.
The gases that are part of the furnace atmosphere and causing decarbers include H 2 0, CO 2, O 2 and H 2. The strongest decarburizing effect on steel is characterized by H 2 0, and the weakest H 2. At the same time, the decarburizing ability of C 2 increases with increasing temperature, and the dismounting ability of dry H 2 decreases. Hydrogen in the presence of water vapor has a very strong shut-off effect on the surface layer of steel.
Protection of steel from oxidation and decarburization. The harmful effect of the oxidation and decarburization of the metal when heated on its quality requires the adoption of measures warning these phenomena. The most complete protection of the surface of the ingots, billets and parts is achieved in the furnaces, where the effects of oxidizing and decarburizing gases are eliminated. Such furnaces include saline and metal baths, as well as furnaces where heating is conducted in a controlled atmosphere. In the furnaces of this type, it is isolated from gases or heated metal, usually closed by a special hermetic muffle, or the flame itself is placed inside the so-called radiant pipes, heat from which is transmitted to the heated metal without contact with oxidizing and decarburing gases. The workspace of such furnaces is filled with special atmospheres, the composition of which is chosen depending on the heating technology and alloy brand. Protective atmospheres are prepared separately in special installations.
Also known is the method of creating a weakly oxidative atmosphere directly in the workspace of the furnaces, without moweling of a metal or flame. This is achieved by incomplete fuel combustion (with air flow rate of 0.5-0.55). The composition of combustion products are consisted of CO and n and along with products of complete combustion CO 2 and H 2 O. If the ratios of C / C02 and H 2 / H 2 O are not less than 1.3, then the heating of the metal in such a medium occurs almost without Oxidation of its surface.
Reducing the oxidation of the metal surface when it is heated in open flame fuel furnaces (components most of the park of metallurgical and machine-building plants) can also be achieved by reducing the duration of its stay at high surface temperature. This is achieved by choosing the most rational metal heating mode in the furnace.
The calculations of the heating of the metal in the furnaces are performed to determine the temperature of the ingot, blanks or finished product, based on the conditions dictated by the technological purpose of heating. At the same time, the flavories 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 provided to the end of its stay in the furnace (the latter - in the case of massive bodies). At the same time, the law of changing the temperature of the brass medium is usually asked by the law, choosing the heating mode depending on the degree of thermal massiveness of the metal. To identify 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.