What is the most effective hardening method. Modern methods of surface hardening of tools. Methods for changing structure

Thermomechanical treatment of steel

One of the technological processes of hardening treatment is thermomechanical treatment (TMT).

Thermomechanical treatment refers to the combined methods of changing the structure and properties of materials.

Thermomechanical treatment combines plastic deformation and heat treatment (hardening of pre-deformed steel in the austenitic state).

The advantage of thermomechanical treatment is that with a significant increase in strength, the plasticity characteristics decrease insignificantly, and the impact strength is 1.5 ... 2 times higher than the impact strength for the same steel after quenching with low tempering.

Depending on the temperature at which deformation is carried out, there is a distinction between high-temperature thermomechanical treatment (HTMT) and low-temperature thermomechanical treatment (HTMT).

The essence of high-temperature thermomechanical treatment consists in heating steel to the temperature of the austenitic state (above A 3). At this temperature, the steel is deformed, which leads to austenite work hardening. Steel with this state of austenite is hardened (Figure 16.1 a).

High-temperature thermomechanical treatment practically eliminates the development of temper brittleness in a dangerous temperature range, weakens irreversible temper brittleness, and sharply increases the impact strength at room temperature. The temperature threshold of cold brittleness decreases. High-temperature thermomechanical treatment increases brittle fracture resistance, decreases sensitivity to cracking during heat treatment.

Fig. 16.1. Scheme of the modes of thermomechanical treatment of steel: a - high-temperature thermomechanical treatment (HTMT); b - low-temperature thermomechanical treatment (NTMO).

High-temperature thermomechanical processing is effectively used for carbon, alloy, structural, spring and tool steels.

Subsequent tempering at a temperature of 100 ... 200 o C is carried out to maintain high strength values.

Low-temperature thermomechanical processing (ausforming).

The steel is heated to an austenitic state. Then it is kept at a high temperature, cooled to a temperature above the temperature of the onset of martensitic transformation (400 ... 600 o C), but below the recrystallization temperature, and at this temperature, pressure treatment and quenching are carried out (Fig. 16.1 b).

Low-temperature thermomechanical treatment, although it gives a higher hardening, does not reduce the tendency of steel to temper brittleness. In addition, it requires high degrees of deformation (75 ... 95%), so powerful equipment is required.

Low temperature thermomechanical treatment is applied to martensite hardened medium carbon alloy steels that have secondary austenite stability.

The increase in strength during thermomechanical treatment is explained by the fact that as a result of deformation of austenite, its grains (blocks) are fragmented. Block sizes are reduced by two to four times compared to conventional quenching. The dislocation density also increases. During the subsequent quenching of such austenite, smaller martensite plates are formed, and the stresses are reduced.

The mechanical properties after different types of TMT for machine-building steels, on average, have the following characteristics (see Table 16.1):

Table 16.1. Mechanical properties of steels after TMT

Thermomechanical treatment is also used for other alloys.

Surface hardening of steel parts

Structural strength often depends on the state of the material in the surface layers of the part. One of the methods of surface hardening of steel parts is surface hardening.

As a result of surface hardening, the hardness of the surface layers of the product increases with a simultaneous increase in abrasion resistance and endurance limit.

Common to all types of surface hardening is the heating of the surface layer of the part to the hardening temperature, followed by rapid cooling. These methods differ in the methods of heating the parts. The thickness of the hardened layer during surface hardening is determined by the depth of heating.

The most widespread are electrothermal quenching with heating of products by high-frequency currents (HFC) and flame quenching with heating with a gas-oxygen or oxygen-kerosene flame.

Hardening by currents of high frequency.

The method was developed by the Soviet scientist V.P. Vologdin.

It is based on the fact that if a metal part is placed in an alternating magnetic field created by a conductor-inductor, then eddy currents will be induced in it, causing the metal to heat up. The higher the current frequency, the thinner the hardened layer is.

Typically, machine generators with a frequency of 50 ... 15000 Hz and lamp generators with a frequency of more than 10 6 Hz are used. The depth of the hardened layer is up to 2 mm.

The inductors are made of copper tubes, inside of which water circulates so that they do not heat up. The shape of the inductor corresponds to the external shape of the product, while a constant gap between the inductor and the surface of the product is required.

A schematic diagram of the technological process of hardening HFC is shown in Fig. 16.2.

Fig. 16.2. Flow chart of HFC hardening

After heating for 3 ... 5 s of inductor 2, part 1 quickly moves into a special cooling device - sprayer 3, through the holes of which a quenching liquid is sprayed onto the heated surface.

A high heating rate shifts phase transformations to higher temperatures. The quenching temperature during heating with high frequency currents should be higher than during conventional heating.

With the correct heating conditions, after cooling, a fine-acicular martensite structure is obtained. The hardness increases by 2 ... 4 HRC in comparison with conventional hardening, wear resistance and endurance limit increase.

Before HFC quenching, the product is subjected to normalization, and after quenching, low tempering at a temperature of 150 ... 200 o С (self-tempering).

It is most advisable to use this method for steel products with a carbon content of more than 0.4%.

The advantages of the method:

· High efficiency, there is no need to heat the entire product;

· Higher mechanical properties;

· Lack of decarburization and oxidation of the surface of the part;

· Reduction of rejects due to warpage and formation of hardening cracks;

· The ability to automate the process;

· Use of HFC hardening allows replacing alloyed steels with cheaper carbon steels;

· Allows hardening of individual parts of the part.

The main disadvantage of the method - high cost of induction installations and inductors.

It is advisable to use in serial and mass production.

Flame hardening.

Heating is carried out by an acetylene-oxygen, gas-oxygen or kerosene-oxygen flame with a temperature of 3000 ... 3200 o С.

The surface layer structure after quenching consists of martensite, martensite and ferrite. The thickness of the hardened layer is 2 ... 4 mm, the hardness is 50 ... 56 HRC.

The method is used for hardening large products with a complex surface (helical gears, worms), for hardening steel and cast iron rolls. It is used in mass and individual production, as well as in repair work.

When heating large products, burners and cooling devices move along the product, or vice versa.

Disadvantages of the method:

· Low productivity;

· Difficulty of regulating the depth of the hardened layer and heating temperature (the possibility of overheating).

Aging

Tempering applies to alloys that have been quenched with polymorphic transformation.

For materials quenched without polymorphic transformation, apply aging.

Quenching without polymorphic transformation is a heat treatment that fixes at a lower temperature the state characteristic of an alloy at higher temperatures (supersaturated solid solution).

Aging - heat treatment, in which the main process is the decomposition of a supersaturated solid solution.

As a result of aging, the properties of hardened alloys change.

Unlike tempering, after aging, strength and hardness increase and ductility decreases.

Aging of alloys is associated with variable solubility of the excess phase, and strengthening during aging occurs as a result of dispersion precipitates during the decomposition of a supersaturated solid solution and the resulting internal stresses.

In aging alloys, precipitates from solid solutions occur in the following basic forms:

· Thin-lamellar (disc-shaped);

· Equiaxial (spherical or cubic);

· Needle-like.

The shape of the precipitates is determined by competing factors: surface energy and energy of elastic deformation, which tend to a minimum.

Surface energy is minimal for equiaxed precipitates. Elastic distortion energy is minimal for precipitates in the form of thin plates.

The main purpose of aging is to increase strength and stabilize properties.

Distinguish between natural, artificial and after plastic deformation.

Natural agingis called a spontaneous increase in strength and a decrease in the ductility of a hardened alloy, which occurs during its holding at normal temperature.

Heating the alloy increases the mobility of the atoms, which speeds up the process.

The increase in strength during holding at elevated temperatures is called artificial aging.

The tensile strength, yield strength and hardness of the alloy increase with the aging duration, reach a maximum and then decrease (over-aging phenomenon)

With natural aging, over-aging does not occur. As the temperature rises, the overaging stage is reached earlier.

If a hardened alloy with a structure of a supersaturated solid solution is subjected to plastic deformation, then the processes occurring during aging are also accelerated - this is strain aging.

Aging encompasses all processes occurring in a supersaturated solid solution: the processes that prepare the isolation and the processes of isolation themselves.

For practice, the incubation period is of great importance - the time during which preparatory processes are performed in the hardened alloy, when high plasticity is maintained. This allows cold deformation after quenching.

If during aging only excretion processes occur, then the phenomenon is called precipitation hardening.

After aging, the strength increases and the ductility of low-carbon steels decreases as a result of dispersed precipitation in the ferrite of tertiary cementite and nitrides.

Aging is the main method of hardening aluminum and copper alloys, as well as many high-temperature alloys.

Cold treatment of steel

High carbon steels and many alloy steels have endpoint martensitic temperatures (M to) below 0 o C. Therefore, a significant amount of retained austenite is observed in the structure of the steel after quenching, which reduces the hardness of the product and also impairs the magnetic characteristics. To eliminate the residual austenite, additional cooling of the part is carried out in the region of negative temperatures, to a temperature below t. M to (- 80 o C). Usually dry ice is used for this.

This processing is called cold steel processing.

Cold treatment must be carried out immediately after quenching to prevent stabilization of the austenite. The increase in hardness after cold treatment is usually 1 ... 4 HRC.

After cold treatment, the steel is subjected to low tempering, since cold treatment does not reduce internal stresses.

Cold treatment is applied to parts of ball bearings, precision mechanisms, measuring instruments.

Strengthening by plastic deformation

The main purpose of mechanical surface hardening methods is to increase fatigue strength.

Mechanical hardening methods - riveting of the surface layer to a depth of 0.2 ... 0.4 mm.

Varieties are shot blasting and roller blasting.

Shot blasting - shot processing of finished parts surface.

It is carried out using special shot blasting machines that throw steel or cast iron shot onto the surface of the processed parts. Shot diameter - 0.2 ... 4 mm. Shot impacts cause plastic deformation to a depth of 0.2 ... 0.4 mm.

They are used to harden parts in grooves, on protrusions. They expose products such as springs, springs, chain links, tracks, sleeves, pistons, gears.

When roller processing deformation is carried out by the pressure of a hard metal roller on the surface of the workpiece.

When forces on the roller exceed the yield point of the material being processed, work-hardening occurs to the required depth. Processing improves microgeometry. The creation of residual compressive stresses increases the fatigue limit and durability of the product.

Rolling in rollers is used when processing shaft journals, wire, when calibrating pipes and rods.

No special equipment is required, turning or planing machines can be used.

Good day, dear reader! The last time we talked about the Methods and methods of restoring parts of ship technical equipment,today we'll talk about ways to harden parts.

Thermal (thermal) - this method of processing parts includes: annealing, normalization, quenching and tempering. This method provides a general hardening of the parts.

Annealing - the annealing temperature of the part is 770-900 C. The part is heated in a furnace from 1 to 4 hours, and then cooled together with the furnace. The more carbon is in steel, the lower the annealing temperature should be. When the part is annealed, the coarse-grained structure of the metal becomes fine-grained. Annealing is performed to relieve internal stresses usually formed after casting, forging, stamping, rolling, surfacing and straightening.

Normalization - the part is heated to the annealing temperature and kept at this temperature for 1-2 hours, and then cooled in air to ambient temperature. Normalization is used to improve the structure of the metal in order to improve its mechanical properties.

Hardening - the hardening temperature is 750-900 C. Quenching is used for steel with a carbon content of at least 0.5%, since at a lower content, the hardness during hardening increases insignificantly. Hardening gives the metal high hardness and strength.

Vacation - the hardened part is heated to a temperature of 150-600 C and kept at this temperature from 5-10 minutes to 1-15 hours, and then cooled. Tempering reduces quenching stresses and changes the structure of steel, increases toughness.

Surface hardening methods include hardening of parts by high-frequency currents (HFC), quenching in electrolytes and cold treatment.

HFC hardening - the part is heated in an inductor, the shape of which is consistent with the shape of the surface of the part being hardened. The inductor, when passing through it an alternating current of high frequency (2500-5000 Hz), creates an alternating magnetic field. The heating time of the part surface is 2-10 s. When the quenching temperature reaches 750-900 C, the current is turned off and water is supplied for cooling. The depth of the hardened layer of the crankshaft journal is 4-7 mm.

Quenching in electrolytes (in salt solutions) - carried out by passing a direct current with a voltage of 220 V through a part (cathode) immersed in an electrolyte (Na2C03 solution). The part is heated to a temperature of 250-450 C.

The use of such hardening makes it possible to increase the wear resistance of parts by 2-5 times or more.

Cold treatment - the parts are cooled to a temperature of -80 C and below, followed by heating to an ambient temperature. With such cooling, additional transformations of retained austenite into martensite occur in the metal, which increases the hardness and wear resistance of the parts. To reduce internal stresses after cold treatment, the parts are tempered. Parts are cold treated immediately after hardening. Liquid nitrogen is used as freon.

Thermomechanical - this method combines two operations: the processing of parts by pressure with heat treatment.

Thermochemical - this method includes: cementation (carburization); cyanidation (saturation with carbon and nitrogen); nitriding (saturation with nitrogen); aluminizing (saturation with aluminum); siliconizing (saturation with silicon); boriding (saturation with boron); oxidation (bluing), etc.

Cementation- artificial increase in the carbon content in the surface layer of a part made of low-carbon steel with a carbon content of 0.1-0.3%. Carburizing increases the carbon content on the metal surface with a depth of 1-3 mm, while the middle of the part remains low-carbon. The carburized part is hardened up to 0.7-1.1%.

Cyanide - the method consists in saturating the surface layer with carbon and nitrogen at the same time at a temperature of 820-870 C. This is achieved by holding the part in hot molten salts containing cyanide compounds. The saturation depth is about 0.25 mm. The hardness of the cyanide layer reaches 640-780 HB (Brinell units).

Nitriding - saturation of steel with nitrogen at a temperature of 480 - 650 C.

Alimentation - saturation of steel with aluminum.

Siliconizing - saturation of steel with silicon at a temperature of 1100-1200 ° C to increase its anti-corrosion properties.

Boring - saturation of steel with boron to increase hardness and wear resistance.

Oxidation (bluing) - saturation of steel with oxygen by thermal or chemical means to protect parts from corrosion. Oxidation is carried out in baths filled with a mixture of solutions of sodium hydroxide, sodium nitrate and sodium nitrite at a temperature of 130-145 C for 1-2 hours. A layer of black Fe304 oxides with a thickness of 1-2.5 microns is formed on the surface.

Thermal diffusion - with this method of hardening, energetic pastes are used, which are spread on the part and set on fire! When the paste burns, the part heats up to a temperature of 600-800 C, and the alloying elements contained in the paste diffuse (penetrate) into the upper layers of the part. After 2-3 minutes, the burnt part is immersed in water for cooling. Mixtures of oxygen-containing substances with powders of aluminum, magnesium, calcium and other metals are used as energy-releasing components in the paste.

Mechanical hardening - This is a deliberate distortion of the crystal lattice of the metal as a result of mechanical action on it.

The physical essence of mechanical hardening is that under the pressure of a solid metal tool, the protruding microroughnesses of the treated surface are plastically deformed, the surface roughness decreases, and the surface layer of the metal is hardened. Mechanical hardening methods include:

Running in with a ball or roller;

Broach;

Shot blasting;

Diamond hardening.

Ball or roller rolling cylindrical surfaces are produced on lathes, and flat surfaces - on planers. Rollers and balls are made from tool steels.

Rolling the surface of a part with a ball or roller increases its hardness by 40-50%, and fatigue strength by 80-100%.

Broach (mandrel) used to harden and improve the accuracy and cleanliness of processing the internal surfaces of parts. The essence of the process is to pull a special mandrel (mandrel) or ball through a hole in the part.

Shot blasting - It is used to harden parts with shot. The use of steel shot gives better results than cast iron. When shot peening, a hardened layer up to 1.5 mm deep is obtained. The hardness increases by 20-60%, and the fatigue strength increases by 40-90%.

Diamond hardening - the tool is a diamond crystal with a spherical working part. The part is machined with a diamond in a mandrel, pressed by a calibrated spring to the surface of the part, which is hardened.

Electrospark method - based on the impact of a directional electric spark discharge. A spark discharge occurs between the hard alloy electrode (for example, stellite) and the surface to be hardened under the action of a pulsating electric current, as a result of which the metal from the electrode (anode) is transferred to the part (cathode) and the workpiece surface to be treated is hardened.

Electromechanical method - used for surface hardening to a depth of 0.2-0.3 mm. At the same time, wear resistance increases up to 11 times, fatigue strength 2-6 times. The bottom line is as follows. A current of 350-1300 A, with a voltage of 2-6 V is supplied to the contact zone of the part and the tool. The tool is isolated from the machine. Due to the fact that the contact area of \u200b\u200bthe tool and the part is small, a large resistance arises, which leads to an increase in thermal energy, which instantly heats the contact area to a high temperature (hardening temperature). The surface layer is quickly cooled by transferring heat into the part. The result is the effect of surface hardening to a depth of 0.2-0.3 mm with simultaneous surface hardening, which significantly increases the wear resistance and fatigue strength of the part.

Laser hardening - for laser hardening of parts, lasers (optical quantum generators) with a radiation power of electromagnetic waves at the output of 0.8-5 kW are used. When such radiation is focused, a high level of energy is concentrated on the surface to be treated.

The laser beam, when exposed to the workpiece surface to be treated, is partially reflected, and the rest of the radiation flux penetrates to a depth of 10 6-10 7 m. The high power density of laser radiation makes it possible to almost instantly reach high temperatures on the treated surface, and this leads to local hardening of a thin surface layer. which ensures high hardness of the treated areas.

Strengthening of parts

The service life of construction machine parts can be increased by hardening the surface layer by plastic deformation (pressure hardening), thermomechanical, thermal and chemical-thermal treatment.

Strengthening of the surface layers of metal by plastic deformation is carried out by shot blasting and rolling with rollers or balls, embossing, backing (hardening by calibration), cutting with special cutters.

Plastic surface deformation increases the endurance limit of the part and increases the cleanliness of its surface, which in some cases makes it possible to refuse the use of finishing operations.

Shot blasting is carried out on special installations with steel or cast iron shot with a diameter of 0.4 ... 2 mm. In these installations, shot at a high speed (50 ... 70 m / s) is directed to the surface to be treated, causing compressive stresses in it, reaching several tens of kilograms per 1 mm2. This treatment is most often used to increase the fatigue strength and hardening of heat-treated steel parts operating under variable loads. These parts include springs, coil springs, gear wheels, connecting rods, hammers, stone crusher cheeks, etc. After shot blasting, the service life of leaf springs increases 4 ... 6 times, and fine-modular gear teeth 2.5 ... 3 times.

The depth of the work-hardened layer, usually not exceeding 1 mm, depends for a given part on the processing time (ranges from 5 ... 10 s to 2 ... 3 minutes), impact force, size and angle of incidence of the shot (the greatest work-hardening is observed at an angle of 75 ... 90 ° С ).

Rolling the surface with hardened rollers is an effective means of hardening large parts in the form of bodies of revolution. Knurling improves the surface microgeometry and creates a hardened hardened layer, which leads to an increase in the fatigue limit and wear resistance of parts.

Rolling is carried out by freely rotating rollers brought into contact with a rotating part installed in a lathe. The roller on the mandrel is fixed in the machine support or a special device.

Reinforcement by embossing is used for significant local work hardening of areas of surfaces of parts with a high concentration of stresses (fillets, holes, slots, welds, etc.). Embossing is carried out with special strikers, rollers, balls by impact on the surface to be hardened.

Thermomechanical treatment gives good results in terms of surface hardening and obtaining a deposited layer without pores and cavities. In this case, surfacing and surface hardening are combined. The weld layer immediately behind the weld pool is rolled or struck by a striker.

Construction machines have many parts, the surface layers of which must have high wear resistance, and the core must have sufficient strength and toughness. Changing the properties of only the surface layer of parts is achieved by saturating the surface with carbon (carburizing), nitrogen (nitriding), carbon and nitrogen (cyanidizing) and surface hardening.

Cementation is applied to parts operating at high specific pressures and friction, as well as those experiencing shock loads during operation (gear teeth, piston pins, camshafts, etc.). Parts made of steel containing no more than 0.25% carbon are subjected to cementation - these are low-carbon steels of grades 0, 8, 10, 15, 20, alloyed steels of grades 15X, 20X, etc.

Cementation is carried out in a carburizing medium (carburizer) at temperatures of 900 ... 950 ° C without air access. The carburizers used for this purpose can be solid, liquid or gaseous. After carburizing the parts, they are subjected to normalization, quenching and tempering.
The most widespread is carburizing in a solid carburizer (fine charcoal mixed with barium carbonate).

The average rate of carburization is 0.8..0.1 mm / h, therefore, to obtain a cemented layer 0.5… 2 mm deep, 12… 15 hours are required, which is a significant disadvantage.

Liquid carburizing is used to produce a shallow carburized layer in small and thin-walled parts. This type of carburizing is produced by immersing parts in a bath containing a mixture of sodium chloride, sodium carbonate, sodium cyanide and barium chloride. The process is carried out at a temperature of 840 ... 860 ° G for 0.5 ... 2.5 hours. During this time, it is possible to obtain a cemented layer with a depth of 0.2 ... 0.6 mm, which after appropriate hardening heat treatment reaches a hardness of 40 ... 60 HRC ...

Gas carburizing is widely used in batch and mass production plants. It can significantly reduce the duration of carburizing and reduce its cost. Gas carburizing is carried out in shaft and muffle furnaces, which are fed with gas containing carbon (natural, light, etc.). For 6 ... 7 hours at a temperature of 900 ... 950 ° С this way a carburized layer up to 1 mm deep is obtained.

Nitriding makes it possible to obtain the hardness of the surface layer of steel parts, 1.5 ... 2 times higher than by cementation and hardening. Moreover, the hardness obtained without the use of heat treatment is retained when the parts are heated up to 500 ... 600 ° C. In addition, nitriding sharply increases the corrosion resistance, wear resistance and fatigue strength of steel parts.

Nitriding is carried out mainly on alloyed parts that have special requirements in terms of wear resistance and fatigue resistance, for example, cylinder liners and crankshaft journals of internal combustion engines, gear wheels, measuring instruments and other parts.

Nitriding consists in saturating the surface layers of steel parts with nitrogen by heating them for a long time at a temperature of 480, .. 650 ° C in an ammonia atmosphere. Before nitriding, the parts are subjected to heat treatment (quenching and tempering), then machining (including grinding) and then flushing with gasoline. To do this, they are placed in a special furnace, ammonia is also fed there, which decomposes at high temperatures with the release of atomic nitrogen and hydrogen. Nitrogen, diffusing into the surface layer of steel parts, forms chemical compounds with alloying elements (chromium, molybdenum) - nitrides.

The total depth of the nitrided layer usually does not exceed 0.5 mm. The diffusion rate at a temperature of 500 ° C is approximately 0.1 mm every 10 hours.
Cyanidation is used for low-carbon steels instead of carburizing and has significant advantages over it (increased wear resistance and shock resistance, higher process speed). The process consists in heating parts to 820 ° C in molten cyanide salts of sodium, potassium or calcium containing active carbon and nitrogen, holding the parts at this temperature for a certain time (from 20 minutes to 2 hours) and then slowly cooling them. At the end of cyanidation, the parts are quenched and tempered. The thickness of the cyanide layer is 0.15 ... 0.3 mm, the rest of the part thickness remains viscous.

A significant drawback that restrains the use of cyanidation is the toxicity of cyanide baths, which necessitates special precautions.

Surface hardening is used to strengthen parts made of carbon steel grades 40, 45, 50, low-alloy chromium and manganese steel. Parts made of these steels with conventional methods have reduced ductility and toughness, since they are annealed throughout the entire section.

In order to give hardness only to the surface layer and at the same time to preserve the viscous core, it is necessary to heat the part so that only the surface layer 1 ... 6 mm thick is heated to the hardening temperatures. Then, during the cooling process, the core of the part will not be hardened and retain the properties inherent in unhardened metal.

The surface of the part for hardening is heated with an oxygen-acetylene flame (flame hardening) and high-frequency currents (hardening of high frequency current). In flame hardening, standard welding torches can be used for heating, in which the nozzles are replaced with special multi-flame hardening tips. The dimensions and profile of these tips depend on the shape of the parts to be hardened.

Fig. 1. Diagrams of profiles and tips

Flat tips (Fig. 1, a) are used to harden the surface of different sizes of bodies of revolution (rollers, wheels, etc.). Ring (Fig. 1, b) and semi-ring lugs are designed for hardening the necks of the shafts and other cylindrical parts. Contoured multi-flame tips (Fig. 1, c) are used for hardening gear teeth.

The movement of the burner must be uniform. The distance between the torch tip and the surface to be hardened is kept within 10 ... 15 mm. The surfaces are heated to a light red color, and cooled with water, which is supplied to the hardening tips and, flowing out through special holes, creates a water shower located 10 ... 20 mm from the burner flame.

Depending on the design features of the parts, the following two methods of flame surface hardening are used: cyclic and continuous-sequential.

In the cyclic method, first, the surface is heated for hardening, and then it is cooled. When heated, the part can remain stationary (stationary method) or rotate at a speed of 10 ... 12 m / min (rotational-cyclic method). The surface hardening of small parts is carried out in a cyclic manner: rollers, fine-modular gears, shaft journals with a diameter of up to 100 mm, etc.

The continuous-sequential method is used for surface hardening of flat parts (bed guides of metal-cutting machines) and parts of large diameter (traveling wheels, cranes, runners, etc.).

Fig. 2. Surface hardening in a continuous-sequential way
1- welding torch;

With this method of hardening, the surface is heated and cooled continuously due to the constant movement of parts relative to the torch and the cooling source (Fig. 2). The speed of movement of the part relative to the torch is selected in the range of 60 ... 300 mm / min.

Surface hardened parts are subjected to low tempering at a temperature of 180 ... 200 ° C in oil baths with electric heating. The holding time of parts at these temperatures is determined at the rate of 1 hour per 1 cm of the radius of the part. The final processing of parts (grinding and lapping) is carried out after heat treatment, which ensures the required hardness and structure of the metal.

Surface hardening of parts when heated by currents of high frequency (up to 106 Hz) is used in serial and mass production to harden parts of automobiles and construction machines. HFC heating can be used with equal success for hardening both external and internal surfaces with a diameter of more than 11 mm.

The essence of surface heating by high-frequency currents is as follows. Eddy currents are excited in a part placed in an alternating magnetic field created by an inductor. These currents are pushed back to the surface of the product under the influence of a magnetic field. With an increase in the frequency of the current, the effect of currents displacement to the surface layers, and, consequently, the current density in them increases. As a result of the thermal action of eddy currents in 3 ... 5 s, the surface layers are heated to the hardening temperature, after which the parts are cooled in water, oil or emulsion.

Inductors are made of copper tubes with a diameter of 4 ... 20 mm with a wall thickness of 0.5 ... 2 mm, in which water circulates, preventing their heating. To increase the efficiency of the inductor, the round tubes are profiled, giving the section a square or rectangular shape.

Separate areas of large parts (crankshaft journals, gear teeth) are heated and hardened in parts, alternating heating and cooling. When quenching medium-sized parts, continuous successive heating and cooling are used. After HFC hardening, the parts are processed in the same way as after flame surface hardening.

High-frequency hardening ensures high productivity, low deformation of the hardened parts, the absence of scale on the surface of the part after hardening, and a significant improvement in the sanitary and hygienic working conditions of workers.

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