UDC 621.924.093
Analysis of methods for strengthening machine parts
and cutting tools
Increasing the service life and wear resistance of machine parts and tools by hardening is an important task, the solution of which contributes to a significant increase in their durability and ensures savings in expensive and scarce materials, energy, and labor resources. Widely used in mechanical engineering various methods hardening, the choice of which depends on the properties of the cutting and processed material, operating conditions and economic efficiency of using the hardening method.
composite reinforcing coatings, laser hardening and alloying, electric spark alloying, epilation, magnetic pulse processing
INTRODUCTION
One of the most promising areas for increasing the reliability and durability of wearing parts of machines and tools is strengthening or modifying working surfaces by creating surface layers with higher mechanical and tribological characteristics.
An analysis of scientific sources has shown that surface hardening can be carried out using coatings, heat treatment or using various types of energy. At the same time, the experience of laboratory research and operating practice shows that it is difficult to choose a universal processing method, since each of them reveals its potential capabilities in a certain case, often in a very narrow range of operating parameters.
METHODS OF SURFACE HARDENING
parts FOR “HEAVY” friction modes
Units of products operating in “heavy” friction modes widely use surface hardening methods. The wear resistance of a friction pair is influenced by a complex of physical and mechanical characteristics: strength, plasticity and hardness. The surface layer has a special influence on wear resistance, since it absorbs loads and makes contact with the external environment.
On the other hand, the surface layers also have more defects (pores, microcracks, inclusions of an unusual crystalline structure, etc.) than the entire volume of the part.
During the wear process, the contacting surfaces must successfully resist plastic deformation, shearing - chipping of microvolumes of material, penetration particulate matter(abrasive particles from external environment, separated particles or growths during adhesion), as well as exposure to aggressive environments and temperatures.
The main impact is perceived by a thin surface layer, and the remaining cross-section of the material perceives only a small fraction, due to the inertia of the materials. Therefore, it is necessary to differentiate the physical and mechanical properties of the surface layers and the rest of the section, which is achieved by various methods of surface hardening.
Let's consider the most used methods, while taking hardness as the criterion for assessing the layers being strengthened - the only material characteristic obtained by non-destructive testing methods (table).
Table. Applicability of methods for surface hardening of parts depending on the hardness of the hardened surfaces
Table. The use of surface strengthening methods depending on hardness of strengthened surface
Hardening methods | Hardness, MPa |
|
Surface plastic deformation (SPD) | ||
Heat treatment (HT) | ||
Chemical-thermal treatment (CHT) | ||
Boriding | ||
Friction-diffusion hardening | 11000 …. 13000 |
|
Electrospark hardening (ES) | ||
Weldable coatings | ||
Sprayed coatings | ||
Laser hardening (LU) | ||
Detonation coating | 10000 …. 14500 |
|
Composite coatings |
The table shows that the hardness of the coatings of the hardened layers is more than twice as high. However, the merits of the method cannot be fully judged by surface hardness alone. It is necessary to consider the positive and negative aspects of other methods when comparing the mechanical properties required for the application.
PPD – increases hardness, reduces ductility, and has low abrasive wear resistance.
TO – the “working” threshold of the wear regime is the temperature in the friction zone, which is C.
CTO – increases wear resistance, which is determined by the temperature resistance of chemical compounds. Thus, nitriding can withstand temperatures up to 600-650, and boriding up to 9000C and higher.
Welded and sprayed coatings are characterized by high wear resistance, which depends on chemical composition applied coating, but requires sophisticated technology, including preparatory operations and operations to relieve internal stresses. For spraying, it is also necessary to introduce an operation to increase the adhesion strength of the coating to the base (thermal deformed delamination).
Laser hardening makes it possible to obtain thin layers that differ from the TO structure due to the high heating rates of the layers. The disadvantage of this hardening method is the low temperature threshold, which is about 2000 C.
Detonation treatment makes it possible to obtain a coating of higher quality than sprayed coating and does not require deformation resorption. The disadvantages of the method include the complexity of implementing the technological process and the difficulty of installing the part in the technological equipment.
Composite coatings have now received greatest application. Main advantages: the ability to obtain fairly thick layers (up to 4 mm); the use of wear-resistant powder compositions from hard alloys, relit, borides and special alloys; creation of coatings with solid lubricants, where graphite, molybdenum disulfide, sulfides, selenides, etc. are used as fillers.
Despite significant advantages, the methods have not been widely used due to a number of significant disadvantages: the complexity of the coating technology, including special preparation operations for sealing the hardening zone; use of high temperatures (up to 12000); temperature deformations and stresses are observed, since the entire strengthening system is subject to heating; the use of expensive materials both as a matrix (silver, nickel, cobalt, copper) and as fillers (borides, carbides, hard alloy); the need to include operations to relieve internal stress.
Analyzing methods of surface hardening, it should be noted that by increasing hardness, we reduce ductility, which leads to a decrease in the danger of seizure of mating surfaces, on the one hand. On the other hand, a decrease in plasticity increases sensitivity to local high pressures, which can even lead to local destruction of the surface.
So, in the hardened surface layer it is necessary to ensure sufficient ductility, high hardness and strength. These requirements can only be realized in a composite coating, organizing a strengthened layer consisting of a plastic base (matrix) with solid inclusions.
METHODS FOR SURFACE HARDENING OF METAL-CUTTING
tool
The performance of a metal-cutting tool can be ensured only if its working part is made of a material with sufficient hardness, strength, wear resistance, temperature resistance and thermal conductivity. The coating applied to the working surfaces of the tool is a fairly universal and reliable means with which you can take a new approach to the problems of improving the properties of the tool material, increasing its performance and controlling the cutting process.
Tool material with a wear-resistant coating is a new composite type material that optimally combines the properties of the surface layer (high values of hardness, heat resistance, passivity in relation to the material being processed, etc.) and the properties manifested in the volume of the tool body (strength, impact strength , crack resistance, etc.). A hard alloy tool with a composite coating has a high resistance to adhesive-fatigue and diffusion wear at a temperature of C. The coating increases the resistance of a high-speed steel tool to abrasive and adhesive-fatigue wear, and significantly increases resistance to corrosion-oxidative wear.
Obtaining a coated tool using chemical and physical metal deposition has disadvantages: the complexity of the coating technology, including special preparation operations to seal the hardening zone and the use of high temperatures; temperature deformations and stresses as a result of heating; use of expensive materials.
To level out these negative aspects of the process, a scheme for applying a composite multilayer coating for carbide tools is proposed. The coating contains several intermediate layers, each of which has its own functional purpose: ensuring a strong connection of the multilayer coating with the working surfaces of the tool; implementation of adhesive bond between functional layers; performing barrier functions, for example, increasing the thermodynamic stability of the coating when higher speeds cutting, etc. All compounds widely used as coatings are characterized by an increase in microhardness up to 2.5 GPa, but are quite brittle, which significantly narrows the scope of their application. Therefore, coatings made of nanomaterials are of particular interest. Surface coatings in the form of a thin film have characteristics that differ significantly from bulk (monolithic) material, and the thinner the film, the stronger it is.
Improvement of coated carbide is always aimed at combating the fragility of its surface layer. Recently, coatings called “Low stress coating” have been used; the technological process consists of applying a multilayer coating to a carbide substrate using standard technology. After this, the front surface of the plates is polished along the front surface, as a result of which the layer of titanium nitrides and the top layer of aluminum oxide with a thickness of only 2..3 microns from the total thickness of the coating are completely removed, which makes it possible to reduce the level of internal tensile stresses by 2 times and remove most of the crack nuclei.
Hardening of cutting tools made of high-speed and alloy steel by carbonitration in gaseous products, carbonitridation in hydrogen-free glow discharge plasma (GDT) increases the hardness, wear resistance and heat resistance of the tool. After nitriding, the tool can withstand temperatures up to 600-650, and when boriding - up to 9000C and higher. The diffusion layers obtained after carbonitration with a thickness ranging from several microns (for small-sized tools) to 0.01-0.02 mm ensure an increase in tool life by 1.5-2 times. Tests of cutting cutters, drills, taps, and reamers hardened by carbon nitriding have shown that their durability is 2-2.5 times greater than that of unhardened tools.
The use of a concentrated plasma jet with a power of 30 kW for surface hardening, generated by an indirect plasma torch with a sectioned interelectrode insert, ensures hardening to a significant depth (3.0-3.5 mm). In this regard, it is of practical interest to harden small-sized tools (cutters, drills, dies, etc.) made of low-alloy tool steel 9ХФ and high-speed steel R6M5 during hardening with a powerful plasma jet. But the hardening process requires sophisticated technology, including preparatory operations and operations to relieve internal stresses.
Laser hardening (HL) of cutting tools made of high-speed and alloy steels is carried out by pulsed irradiation of the working edges of the tool on a laser technological installation. In this case, the tool life can be increased by 1.5-3 times. Under the influence of laser radiation, rapid heating of the metal occurs in the region of the austenitic state and subsequent cooling of the metal. The strengthened layer has a particularly dispersed austenitic-martensitic structure. As a result, a layer 60-80 microns thick with microhardness N/mm2 is formed on the surface. But the LT process does not help maintain the level of plasticity required for the tool. Another disadvantage is the violation of the tool geometry due to melting without increasing the penetration depth.
Laser surface alloying is a promising technological method for ensuring and increasing the reliability of various tools (stamps, molds, cutting tools), the working surfaces of which have been subjected to pulsed laser alloying using paste-like compositions. After experiments, a process was introduced into production using a paste containing fine powders of boron and hard alloy with the addition of fluoride activators, mixed in glycerin until thick. The oxide film leads to a reduction in the time to achieve a fixed wear value in the running-in area along the tool’s rake surface by 2-3 times compared to an unhardened tool, to a significant expansion of the range of cutting conditions, and at the same time, the expected reduction in cutting temperature is 100-120°C.
A promising way to increase the durability of tools made of carbon and alloy steels is magnetic-pulse treatment of them with a low-intensity field at room temperature. But this method also has a number of disadvantages: limitations in application in terms of thickness and dimensions; dependence of the quality of processing on the magnetic permeability of the material. This is explained by the fact that magnetic pulse modes do not provide the required structure.
The electroacoustic spraying process, based on the combined use of electric spark discharge and ultrasound energy, increases the service life of high-speed tools. Mechanical longitudinal-torsional ultrasonic vibrations imparted to the electrode form an interelectrode gap upon contact with the surface being treated.
Electrospark alloying (ES) of drills, cutters and other cutting tools made of high-speed steel increases their durability by 1.5-2 times. At the same time, EL has a number of disadvantages, the main of which is the formation in many cases of an unacceptable level of residual tensile stresses and unacceptable roughness of the treated surfaces.
Increasing the durability of a cutting tool can be obtained by applying epilam to the surface of the tool - hardening the cutting tool by epilaming. Epilams are compositions consisting of a solvent or mixture of solvents that contain a surfactant (fluorine). From solution, the surfactant is adsorbed onto the solid surface in the form of a monomolecular layer, removing microhardness and, consequently, surface energy. When epilam is applied to the surface of a cutting tool, its wear resistance increases by 2-5 times. The extremely high chemical activity of fluorine is a significant disadvantage of epilation.
Currently, high-energy methods of surface hardening of the working profile of tool cutting edges, such as laser and plasma hardening, magnetic pulse processing, are used to a limited extent (mainly due to the high cost of equipment and the complexity of technological processes). The main methods of hardening blade tools still remain volumetric hardening and hardening with high-frequency heating, which are associated with warping of the working profile of the tool, the occurrence of thermal stresses and other disadvantages.
Numerous research results show the convincing benefits of tools and machine parts with new coatings and confirm the promise of new developments. Promising technologies for the near future include the combination of implantation with coating, as well as work in the field of combining heat treatment of products with coating.
The above allows us to conclude: the hardening methods under consideration have certain disadvantages; their use for parts and tools requires improvement of specific technological processes and further research; At the same time, it is necessary to search for new methods.
List of references used
1. Vereshchak cutting tools with wear-resistant coatings. – M.: Mashinostroenie, 1993. – 336 p.
2. Maslov high technologies: reference book // Engineering magazine. –2008. – No. 1. – P.10-24.
3. On the classification of coating application methods (terminological aspect) // Bulletin of mechanical engineering. – 1988. – No. 9. P.54-57.
4. , Matyushenko, aspects of technical hydrogenation of metals and its influence on wear resistance // Durability of rubbing machine parts. – 1986. – No. 1. – P. 191-195.
5. , Kravets of tool reliability by laser alloying // Bulletin of mechanical engineering. – 1987. – No. 1. – P. 44-46.
6. , Sidorenko, electric spark hardening treatment for wear of separation dies // Bulletin of mechanical engineering. – 1987. – No. 2. – P.53-55.
THE ANALYSIS of METHODS for strengthening MACHINE PARTS And CUTTING TOOLS
I. T. Syechjov, I. A. Sokolova
Threading the service life and wear resistance machine parts and tools by their strengthening is an important task the solution of which secures the economy of expensive and deficit materials, energy, labor resource. Mechanical engineering uses various strengthening methods, choosing of which depends on the properties of cutting material and material to be cut, performance and economical effectiveness and strengthening method.
Withomposite strengthening plating, laser strengthening and alloying, electro-spark alloying, apyloming, magnet –impulse treatment
Candidate of Technical
Sciences, Associate Professor, Department of Materials Processing Technology
Candidate of Pedagogical Sciences, Associate Professor of the Department of Materials Processing Technology
Federal State Educational Institution of Higher Professional Education "Kaliningrad State technical university", Russia, Kaliningrad, Sovetsky Ave., 1,
e-mail: *****@***ru
Dr. I. T. Syechjov, PLD, ass. prof. Mechanical Engineering department The Kaliningrad State Technical University
Russia, Kaliningrad, Soviet pr., 1, tel.: + 7
Dr. I. A. Sokolova, PLD, ass. prof. Mechanical Engineering department The Kaliningrad State Technical University
Russia, Kaliningrad, Soviet pr.,
Thermo-mechanical processing of steel
Surface hardening of steel parts
Quenching with currents high frequency.
Gas flame hardening.
Aging
Cold processing of steel
Hardening by plastic deformation
Thermo-mechanical processing of steel
One of the technological processes of hardening treatment is thermomechanical treatment (TMT).
Thermo-mechanical treatment refers to combined methods of changing the structure and properties of materials.
Thermo-mechanical processing combines plastic deformation and heat treatment (hardening of pre-deformed steel in the austenitic state).
The advantage of thermomechanical processing is that with a significant increase in strength, the ductility characteristics decrease slightly, and the impact strength is 1.5...2 times higher compared to the impact strength for the same steel after quenching with low tempering.
Depending on the temperature at which deformation is carried out, a distinction is made between high-temperature thermomechanical treatment (HTMT) and low-temperature thermomechanical treatment (LTMT).
The essence of high-temperature thermomechanical treatment is to heat steel to the temperature of the austenitic state (above A 3 ). At this temperature, the steel is deformed, which leads to hardening of austenite. Steel with this state of austenite is subjected to hardening (Fig. 16.1 a).
High-temperature thermomechanical processing virtually eliminates the development of temper embrittlement in the dangerous temperature range, weakens irreversible temper embrittlement and dramatically increases toughness at room temperature. The temperature threshold for cold brittleness decreases. High-temperature thermomechanical treatment increases resistance to brittle fracture and reduces sensitivity to cracking during heat treatment.
Rice. 16.1. Scheme of thermomechanical treatment modes of steel: a – high-temperature thermomechanical treatment (HTMT); b – low-temperature thermomechanical treatment (LTMT).
High-temperature thermomechanical processing can be 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 higher strengthening, 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 processing is applied to martensite-hardened medium-carbon alloy steels that have the secondary stability of austenite.
The increase in strength during thermomechanical treatment is explained by the fact that as a result of deformation of austenite, its grains (blocks) are crushed. The dimensions of the blocks are reduced by two to four times compared to conventional hardening. The dislocation density also increases. With subsequent quenching of such austenite, smaller martensite plates are formed and stresses are reduced.
Mechanical properties after different types TMO for engineering steels on average have the following characteristics(see table 16.1):
Table 16.1. Mechanical properties of steels after TMT
|
(steel 40 after normal hardening) |
Thermo-mechanical processing is also used for other alloys.
In addition to applying wear-resistant coatings to tool surfaces, there are four more groups of technologies for surface hardening of cutting tools:
1. Methods of mechanical hardening: vibration, shot blasting, explosion, etc. Most often used for hardening tools made of high-speed steel and hard alloys. Surface plastic deformation (SPD) – hardening of the surface layer to a depth of 0.2-0.8 mm in order to create residual compressive stress in it. During hardening, the surface layer is flattened. The elongation of the surface layer is prevented by the force of adhesion to the underlying layers of metal. As a result, biaxial compressive stresses arise in the work-hardened layer, and insignificant reactive tensile stresses arise in the thickness of the base metal. Added to the working tensile stresses, the residual compressive stresses are reduced, and at sufficiently large values the first ones are compensated. The multiple distortions of the structure that occur during hardening (grain deformation, local plastic shears) effectively inhibit the development of fatigue damage and expand the area of existence of non-propagating cracks, the increase of which causes the existence of destructive stresses. Stress hardening, which is a combination of overload hardening with hardening, is effective. With this method, the part is loaded with a load of the same stress as the working one, causing elastic or elastoplastic deformations in the material. After the load is removed, residual compressive stresses arise in the surface layer. The hardened layer is sensitive to heat. At temperatures of 400-500 o C, the effect of hardening completely disappears, due to the recrystallization process that occurs at these temperatures, eliminating the crystal-structural changes introduced by hardening. The main types of surface hardening by plastic deformation: shot blasting, rolling, embossing, diamond smoothing.
Shot blasting involves hardening the surface layer with a stream of hardened balls (diameter 0.5-1.5 mm) created by centrifugal shot blasters. The quality of the surface during this process is slightly reduced. Flat surfaces are hardened by rolling balls mounted in a rotating chuck. The workpiece is given the movement of longitudinal and transverse feed; with the correctly selected rolling mode, the residual compressive stresses in the surface layer are 600-1000 MPa. The compaction depth of the layer is 0.2-0.5 mm. This process improves the surface quality of the part. The rotation surface is hardened by rolling in hardened steel rollers. The roller pressing force is selected in such a way as to create stresses in the surface layer that exceed the yield strength of the material under conditions of uniform compression (for steel 5000-6000 MPa). Coining is carried out with strikers with a spherical working surface, driven into vibration by pneumatic devices. The oscillation frequency and the rotation speed of the workpiece must be matched so that the work-hardened areas overlap each other.
Diamond smoothing consists of processing a pre-ground and polished surface with rounded diamond cutters (radius 2-3 mm). The surface layer is compacted to a depth of 0.3-0.5 mm.
2. Methods of chemical-thermal treatment (CHT) of tool steels: nitriding, carburization, carbonitration, oxidation, boriding in gas and liquid media, glow gas electric discharge (ion nitriding). High surface strength is ensured by isothermal hardening, as well as thermomechanical treatment of the surface of the part. During surface hardening (gas-flame hardening) and chemical-thermal treatment (cementation), hardening is mainly due to the occurrence of residual compressive stresses in the surface layer due to the formation of structures with a larger specific volume (nitrides and carbonitrides during nitrocarburization and nitriding) than the structure of the base metal. The expansion of the surface layer is inhibited by the core, which retains the original pearlite structure, as a result of which two-layer compressive stresses arise in the surface layer. In the lower layers, reactive tensile stresses develop, which have a small value, due to the insignificance of the cross-section of the thermally treated layer compared to the cross-section of the core. The creation of compressive prestresses reduces the average stress in the compression area, thereby increasing the endurance limit. Gas hardening increases the endurance limit compared to the original untreated steel structure by 1.85 times. The most effective treatment method is nitriding, which almost completely eliminates external stress raisers. Nitriding does not change the shape and size of the part. The nitrided layer has increased corrosion and heat resistance. Hardness and strengthening effect are maintained up to temperatures of 500-600 o C. The optimal thickness of the compaction layer for carburizing is 0.4-0.8 mm, carburizing and nitriding is 0.3-0.5 mm, hardening with heating and gas hardening is 2-4 mm. The surface quality is significantly improved.
Electric spark, magnetic, ultrasonic hardening. These methods are rarely used for processing cutting tools.
Physical hardening: laser processing, ion implantation. Ion implantation technology is one of the most promising today in terms of creating composite materials with an optimal set of surface and volumetric properties.
Ion implantation is a process in which almost any element can be introduced into the near-surface region of any solid body - a target (substrate) placed in a vacuum chamber, through a type of high-speed ions having an energy of several megaelectronvolts.
Ions penetrate into the target (substrate) material to a depth of 0.01 µm to 1 µm, losing energy in the process of collisions with base atoms.
The profile (distribution) of impurity concentration over depth for most combinations - implanted atom - target (substrate) can be calculated. For a low ion dose (small number of ions per unit area), the depth distribution profile of the impurity concentration is usually well described by a Gaussian distribution centered in the middle of the propagation region. As a result of ion implantation, a surface layer of an alloy with a variable composition is formed, which does not have a pronounced interface characteristic of the deposited coating.
The advantages of ion implantation as a surface modification method compared to other surface hardening methods are:
Increased solubility in the solid state;
Independence of alloy formation from diffusion constants;
Possibility of quickly changing the alloy composition;
Independence from the processes occurring in the volume of the material;
Possibility of process at low temperatures;
Very slight change in the dimensions of the workpiece;
There is no problem of augesia, since there is no pronounced interface;
Controlled depth of concentration distribution;
Vacuum cleanliness;
High controllability and reproducibility.
The main disadvantage of ion implantation is the treatment of only that part of the instrument surface that is located directly in the area of action of the ion beam.
Tool coating technologies have high performance, versatility, efficiency. In addition, it becomes possible to control the formation conditions and properties of coatings, as well as the properties of the coating-tool material composition. Tool material with a wear-resistant coating is a new composite type material that optimally combines the properties of the surface layer (high values of hardness, heat resistance, passivity in relation to the material being processed, etc.) and the properties manifested in the volume of the tool body (strength, impact strength , crack resistance, etc.).
Currently, the systematization of grades of tool materials must be supplemented by systematization of the characteristics of surface layers with modified properties (SIS), otherwise it is impossible to objectively determine the possibility of using both the hardening technology in general and the many options for the composition and designs of hardened layers for specific processing conditions. This systematization is presented in Fig. 34.3
The methods used to harden the cutting part of tools are grouped in Fig. 34.3 not just by their physical characteristics, but also by the final result - the range of characteristics and design options of the resulting layers, which must be known first of all to make a decision about their use. There are four features in total, ranked in a strictly defined order.
Based on the fact that the areas of application of traditional grades of materials are clearly defined, first a sign becomes the presence of some variant of a hardened layer on the material of the cutting part of the tool.
Rice. 7.3 Systematization of material options for the cutting part of the tool
This condition delimits the ranges of properties of these layers, i.e. These are, in fact, new classes of materials with qualitatively different performance characteristics. It is obvious that strengthening the base, i.e. changing the properties of an existing “base” tool material will not significantly increase their hardness and wear resistance, in contrast to the application of coatings, the properties of which are practically not very dependent on the properties of the base.
The second sign of systematization is the possible technology for obtaining one or another version of the hardened layer of the cutting part of the tool. It determines the possibilities of using the hardened layer in production.
The third feature is the general, integral characteristic of the strengthened layer - its total thickness. The influence of the thickness of the wear-resistant coating on the performance of the tool has been studied in sufficient detail and will be discussed below. It should be noted that different hardening technologies can provide strictly defined thickness ranges and each layer option has its own clearly defined optimum.
The fourth sign of grouping is the differentiated characteristics of the strengthened layer - a specific combination of the thickness of the layer as a whole, as well as the chemical composition and structure of its constituent layers. It is known that even a slight change in just one element (the thickness or chemical composition of one of the constituent layers) can significantly increase the performance potential of tools. For hardened base layers, the optimal gradient of properties from the core to the surface of the tool is important.
Technological features of obtaining layers with changed properties are not independent signs of grouping. They only provide service characteristics of the layer structure.
As a result of the analysis of the features of industrial operation of coated cutting tools, the following can be noted:
1. Coated tools are noticeably more expensive than uncoated tools, which requires a higher production standard, the use of unworn machine tools, and a thorough economic analysis of the feasibility of using coated tools.
2. It is most advisable to operate a coated tool at speeds exceeding the cutting speed of a conventional tool by 30-60%. Such speeds correspond to the optimal economic cutting speed, which minimizes wear rates and cutting costs.
3. Currently, industry uses a variety of coated cutting tools produced by various technological methods, which requires factory technologists to have knowledge of the areas of the most rational use of such tools. The performance of coated tools under various machining conditions is highly dependent on the method used to obtain the coating, even of the same chemical composition.
7.3. Security questions:
1. What is the need for surface hardening of cutting tools?
2. What are the modern methods of surface hardening of tools? Their advantages and disadvantages
3. What is the basic principle of systematizing the materials of the cutting part of tools?
Good day, dear reader! Last time we talked about Methods and methods for 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, hardening and tempering. This method provides general strengthening of parts.
Annealing— the annealing temperature of the part is 770-900 C. The part is heated in a furnace for 1 to 4 hours, and then cooled along with the furnace. The more carbon there is in the steel, the lower the annealing temperature should be. When a part is annealed, the coarse-grained structure of the metal becomes fine-grained. Annealing is carried out to relieve internal stresses usually formed after casting, forging, stamping, rolling, surfacing and straightening.
Normalization- the part is heated to the annealing temperature and maintained at this temperature for 1-2 hours, and then cooled in air to a temperature environment. Normalization is used to improve the structure of the metal in order to increase mechanical properties.
Hardening— the hardening temperature is 750-900 C. Hardening is used for steel with a carbon content of at least 0.5%, since with a lower content the hardness during hardening increases slightly. Hardening gives the metal high hardness and strength.
Vacation- the hardened part is heated to a temperature of 150-600 C and maintained at this temperature from 5-10 minutes to 1-15 hours, and then cooled. Tempering reduces quenching stresses and changes the structure of the steel, increasing toughness.
Surface hardening methods include hardening of parts with high frequency currents (HFC), hardening in electrolytes and cold treatment.
HDTV 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. An inductor, when high frequency alternating current (2500-5000 Hz) is passed through it, creates an alternating magnetic field. The heating time for the surface of the part is 2-10 s. When the hardening 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.
Hardening in electrolytes (in salt solutions)- carried out by transmission DC voltage 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— parts are cooled to a temperature of -80 C and below, followed by heating to ambient temperature. With such cooling, additional transformations of retained austenite into martensite occur in the metal, and therefore the hardness and wear resistance of the parts increases. To reduce internal stresses, after cold treatment, parts are tempered. Parts are treated with cold immediately after hardening. Liquid nitrogen is used as refrigerant.
Thermomechanical - this method combines two operations: pressure processing of parts with heat treatment.
Thermochemical — this method includes: cementation (carburization); cyanidation (saturation with carbon and nitrogen); nitriding (saturation with nitrogen); aluminizing (saturation with aluminum); siliconization (silicon saturation); boriding (saturation with boron); oxidation (bluing), etc.
Cementation- artificial increase in carbon content in the surface layer of a low-carbon steel part with a carbon content of 0.1-0.3%. During carburization, the carbon content on the surface of the metal with a depth of 1-3 mm increases, but the middle of the part remains low-carbon. The carburized part to 0.7-1.1% is subjected to hardening.
Cyanidation— the method consists of saturating the surface layer with both carbon and nitrogen at a temperature of 820-870 C. This is achieved by soaking the part in hot molten salts containing cyanide compounds. The saturation depth is about 0.25 mm. The hardness of the cyanidated layer reaches 640-780 Nb (Brinell units).
Nitriding— saturation of steel with nitrogen at a temperature of 480-650 C.
Aluminizing— saturation of steel with aluminum.
Siliconization— saturation of steel with silicon at a temperature of 1100–1200°C to increase its anti-corrosion properties.
Boriding— 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 caustic soda, sodium nitrate and sodium nitrite at a temperature of 130-145 C for 1-2 hours. A layer of black Fe304 oxides 1-2.5 microns thick is formed on the surface.
Thermal diffusion — with this method of hardening, energy-releasing 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 to cool. 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 a metal as a result of mechanical impact on it.
The physical essence of mechanical hardening is that under the pressure of a solid metal tool, protruding micro-irregularities of the processed surface are plastically deformed, the surface roughness decreases, and the surface layer of the metal is strengthened. Mechanical hardening methods include:
Rolling with a ball or roller;
Broach;
Shot blasting;
Diamond hardening.
Rolling with a ball or roller Cylindrical surfaces are produced on lathes, and flat surfaces are produced on planers. Rollers and balls are made of 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%.
Broaching (burning) used to strengthen 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- used to harden parts using shot. The use of steel shot gives better results than cast iron shot. With shot peening, a hardened layer up to 1.5 mm deep is obtained. Hardness increases by 20-60%, and fatigue strength by 40-90%.
Diamond hardening— the instrument is a diamond crystal with a spherical working part. The part is processed with a diamond in a mandrel, pressed by a calibrated spring to the surface of the part, which is hardened.
Electric spark method- based on the impact of a directed spark electric discharge. A spark discharge occurs between an electrode made of a hard alloy (for example, stellite) and the surface being hardened under the influence of a pulsating electric current, as a result of which metal is transferred from the electrode (anode) to the workpiece (cathode) and the workpiece surface 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 increases by 2-6 times. The point is this. A current of 350-1300 A and a voltage of 2-6 V is supplied to the contact area between the part and the tool. The tool is isolated from the machine. Due to the fact that the contact area between the tool and the workpiece is small, a large resistance occurs, which leads to an increase in thermal energy, which instantly heats the contact area to a high temperature (quenching temperature). The surface layer is quickly cooled due to heat removal 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 an output power of electromagnetic waves of 0.8-5 kW are used. When such radiation is focused on the surface being treated, it concentrates high level energy.
The laser beam, when exposed to the workpiece surface, 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 allows almost instantly reaching high temperatures on the machined surface, and this leads to local hardening of a thin near-surface layer, which ensures high hardness of the treated areas.
Materials science: lecture notes Alekseev Viktor Sergeevich
3. Methods of strengthening metals and alloys
Surface hardening of metals and alloys is widely used in many industries, in particular in modern mechanical engineering. It makes it possible to obtain high hardness and wear resistance of the surface layer while maintaining a sufficiently viscous core, and helps to increase durability and fatigue strength. Some surface hardening methods are highly productive. In some cases, they are used with great efficiency instead of conventional heat treatment methods. There are a large number of parts for which the properties of the surface metal layer have different requirements than the properties of the internal layers. For example, gear teeth experience strong friction during operation, so they must have great hardness, but have low hardness and good toughness so that the teeth are not destroyed by shocks and impacts. Therefore, gear teeth must be hard on the surface and tough at the core.
The most common method of strengthening the surface layer of metals and alloys is surface hardening, in which only part of the surface layer of parts acquires high hardness. The rest is not hardened and retains the structure and properties that were before hardening. Currently, surface hardening with induction heating using high-frequency currents is most widespread. This high-performance progressive heat treatment method improves the mechanical properties of steel, including yield strength, fatigue and hardness, eliminates the possibility of decarburization, and reduces the risk of oxidation of the surface of products and their deformation.
Parts of complex shape, band saws, cutting tools (mills, drills), levers, axles are subjected to pulse surface hardening. To do this, the hardened part of the part is heated to a temperature higher than the temperature of normal heating of this material for hardening, and then cooled at a high speed due to heat removal to the rest of the part without the use of cooling media. As a result of pulse hardening, a hardened “white” layer is obtained, which is stable when tempered up to a temperature of 450 °C, has a fine-grained structure, high hardness and wear resistance.
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