Superhard materials (STM). Superhard tool materials (SHM) Superhard materials composition properties

The processes of metal processing with blade tools obey the classical laws of the theory of metal cutting.

Throughout the development of metal cutting, the emergence of qualitatively new tool materials with increased hardness, heat resistance and wear resistance was accompanied by an increase in the intensity of the processing process.

Created in our country and abroad in the late fifties and early sixties of the last century and widely used tools equipped with artificial superhard materials based on cubic boron nitride (CBN), they are characterized by great diversity.

According to the information of domestic and foreign firms - manufacturers of tools, the use of materials based on CBN is currently significantly increasing.

In industrialized countries, the consumption of blade tools made of artificial superhard materials based on CBN continues to grow by an average of 15% per year.

According to the classification proposed by VNIIinstrument, all superhard materials based on dense modifications of boron nitride are given the name composites.

In the theory and practice of materials science, a composite is a material that is not found in nature, consisting of two or more components that are different in chemical composition. The composite is characterized by the presence of distinct
boundaries separating its components. The composite consists of a filler and a matrix. The filler has the greatest influence on its properties, depending on which the composites are divided into two groups: 1) with dispersed particles; 2) reinforced with continuous fibers and reinforced with fibers in several directions.

The thermodynamic features of boron nitride polymorphism led to the emergence of a large number of materials based on its dense modifications and various technologies for its production.

Depending on the type of the main process that occurs during synthesis and determines the properties of superhard materials, three main methods can be distinguished in modern technologies for obtaining instrumental materials from boron nitride:

phase transformation of hexagonal boron nitride into cubic. Polycrystalline superhard materials obtained in this way differ from each other in the presence or absence of a catalyst, its type, structure, synthesis parameters, etc. The materials of this group include: composite 01 (elbor-R) and composite 02 (belbor). The materials of this group are not published abroad;
partial or complete transformation of wurtzite boron nitride into cubic. Individual materials of this group differ in the composition of the initial charge. In our country, one- and two-layer composite 10 (hexanit-R) and various modifications of composite 09 (PTNB, etc.) are produced from materials of this group. Abroad, the materials of this group are produced in Japan by Nippon Oil Fate under the trademark Wurtzip;
sintering particles of cubic boron nitride with additives. This group of materials is the most numerous, since various bonding options and sintering technologies are possible. According to this technology, composite 05, cyborite and niborite are produced in the domestic industry. The most famous foreign materials are boron, amborite and sumiboron.

Let us give a brief description of the most known superhard tool materials.

Composite 01(elbor-R) - created in the early 70s.

This material consists of randomly oriented crystals of cubic boron nitride obtained by catalytic synthesis. As a result of high-temperature pressing under high pressure, the original BN K crystals are crushed to sizes of 5…20 µm. The physical and mechanical properties of composite 01 depend on the composition of the initial charge and the thermodynamic parameters of synthesis (pressure, temperature, time). Approximate mass content of components of composite 01 is as follows: up to 92% BN K, up to 3% BN r , the rest is impurities of additives-catalysts.

Modification of composite 01 (elbor-RM), unlike elbor-R, is obtained by direct synthesis of BN r -> BN k, carried out at high pressures (4.0 ... 7.5 GPa) and temperatures (1300 ... 2000 ° C). The absence of a catalyst in the charge makes it possible to obtain stable operational properties.

Composite 02(belbor) - created at the Institute of Solid State Physics and Semiconductors of the Academy of Sciences of the BSSR.

Obtained by direct transition from BN r in high-pressure apparatus under static load application (pressure up to 9 GPa, temperature up to 2900 °C). The process is carried out without a catalyst, which ensures high physical and mechanical properties of composite 02. With a simplified manufacturing technology, due to the introduction of certain alloying additives, it is possible to vary the physical and mechanical properties of polycrystals.

Belbor is comparable in hardness to diamond and significantly surpasses it in heat resistance. Unlike diamond, it is chemically inert to iron, and this allows it to be effectively used for processing cast iron and steel, the main engineering materials.

Composite 03(ismit) - was first synthesized in the ISM Academy of Sciences of the Ukrainian SSR.

Three grades of material are produced: ismit-1, ismit-2, ismit-3, differing in physical, mechanical and operational properties, which is a consequence of the difference in the feedstock and synthesis parameters.

Niborite- received by IHPP of the Academy of Sciences of the USSR.

High hardness, heat resistance and significant size of these polycrystals predetermine their high performance properties.

cyborite- synthesized for the first time in the ISM Academy of Sciences of the Ukrainian SSR.

Polycrystals are obtained by hot pressing of the mixture (sintering) at high static pressures. The composition of the mixture includes cubic boron nitride powder and special activating additives. The composition and amount of additives, as well as sintering conditions, provide a structure in which intergrown BN K crystals form a continuous framework (matrix). Refractory hard ceramics are formed in the intergranular spaces of the framework.

Composite 05- the structure and production technology were developed at NPO VNIIASH.

The material basically contains crystals of cubic boron nitride (85...95%), sintered at high pressures with additions of aluminum oxide, diamonds and other elements. In terms of its physical and mechanical properties, composite 05 is inferior to many polycrystalline superhard materials.

A modification of composite 05 is composite 05IT. It is distinguished by high thermal conductivity and heat resistance, which are obtained by introducing special additives into the charge.

Composite 09(PTNB) was developed at the Institute of Chemical Physics of the USSR Academy of Sciences.

Several grades are produced (PTNB-5MK, PTNB-IK-1, etc.), which differ in the composition of the initial charge (a mixture of BN B and BN K powders). Composite 09 differs from other composite materials in that it is based on particles of cubic boron nitride 3–5 µm in size, and wurtzite boron nitride acts as a filler.

Abroad, the production of materials of this class using the transformation of wurtzite boron nitride is carried out in Japan by the Nippon Oil Fate company together with the Tokyo State University.

Composite 10(hexanit-R) was created in 1972 by the Institute of Problems of Materials Science of the Academy of Sciences of the Ukrainian SSR together with the Poltava plant of artificial diamonds and diamond tools.

This is a polycrystalline superhard material based on the wurtzite modification of boron nitride. The technological process for obtaining hexanite-R, like the previous composites, consists of two operations:

synthesis of BN B by the method of direct transition BN r -> BN B with impact on the source material and
sintering of BN B powder at high pressures and temperatures.

Composite 10 is characterized by a fine-grained structure, but the crystal sizes can vary considerably. Structural features also determine the special mechanical properties of composite 10 - it not only has high cutting properties, but can also work successfully under shock loads, which is less pronounced in other grades of composites.

On the basis of hexanit-R at the Institute of Problems of Materials Science of the Academy of Sciences of the Ukrainian SSR, an improved grade of composite 10 - hexanit-RL, reinforced with whiskers - fibers of "sapphire whiskers" was obtained.

Composite 12 obtained by sintering at high pressures a mixture of wurtzite boron nitride powder and polycrystalline particles based on Si 3 N 4 (silicon nitride). The grain size of the main phase of the composite does not exceed 0.5 µm.

The prospect of further development, creation and production of composites is associated with the use of whiskers or acicular crystals (whiskers) as a filler, which can be obtained from materials such as B 4 C, SiC, Si 2 N 4 . VeO and others.

What materials are considered superhard? What is the range of their application? Are there materials harder than diamond? Professor, PhD in Crystallography Artem Oganov talks about this.

Superhard materials are materials that have a hardness above 40 gigapascals. Hardness is a property that is traditionally measured by scratching. If one material scratches another, then it is considered that it has a higher hardness. This is relative hardness, it does not have rigid quantitative characteristics. Strict quantitative characteristics of hardness are determined by a pressure test. When you take a pyramid, usually made of diamond, apply some force and press the pyramid against the surface of your test material, measure the pressure, measure the area of the impression, a correction factor is applied, and this value will be the hardness of your material. It has the unit of pressure because it is force divided by area, hence gigapascals (GPa).

40 GPa is the hardness of cubic polycrystalline boron nitride. This is a classic superhard material that is widely used. The hardest material known to mankind so far is diamond. For a long time there were attempts, which do not stop now, to discover a material harder than diamond. So far, these attempts have not been successful.

Why are superhard materials needed? The number of superhard materials is small, on the order of ten, maybe fifteen materials known today. First, superhard materials can be used in cutting, polishing, grinding, drilling. For tasks related to machine tool building, jewelry, stone processing, mining, drilling, and so on, all this requires superhard materials.

Diamond is the hardest material, but it is not the most optimal material. The fact is that, firstly, diamond is fragile, and secondly, diamond burns in an oxygen atmosphere. Imagine a drill that is heated to a high temperature in an oxygen atmosphere. Diamond, being elemental carbon, will burn. And, besides, steel cannot be cut with a diamond. Why? Because carbon reacts with iron to form iron carbide, meaning your diamond will simply dissolve into steel at a high enough temperature, and so you need to look for some other materials. In addition, diamond is, of course, quite expensive, even synthetic diamond is not a cheap enough material.

Moreover, superhard materials can still be useful in bulletproof vests and other protective military devices. In particular, a material such as boron carbide, which is also superhard and fairly light, is widely used. Such is the range of application of superhard materials.

It is known that superhard materials are formed in substances with a strong covalent bond. The ionic bond lowers the hardness. The metallic bond also lowers the hardness. Bonds should be strong, directed, i.e. covalent, and as short as possible. The density of matter should also be as high as possible, density in the sense of the number of atoms per unit volume. And if possible, the symmetry of the substance should also be very high, so that the substance is equally strong in this direction, and in this, and in this. Otherwise, there will be the same story as in graphite, where the bonds are very strong, but only in two directions, and in the third direction, the bonds between the layers are extremely weak, as a result, the substance is also soft.

Many institutes, many laboratories around the world are engaged in the synthesis and development of superhard materials. In particular, these are the Institute for High Pressure Physics in the Moscow Region, the Institute for Superhard and New Carbon Materials in the Moscow Region, the Institute for Superhard Materials in Kyiv, and a number of laboratories in the West. Active developments in this area began, I think, in the 50s, when artificial diamond was first obtained in Sweden and America. At first, these developments were secret, but soon enough, the synthesis of artificial diamonds was also established in the Soviet Union, thanks to the work of researchers from the Institute of High Pressure Physics and the Institute of Superhard Materials.

There have been various attempts to create materials harder than diamond. The first attempt was based on fullerenes. are football-like molecules, hollow molecules, round or somewhat elongated. The bonds between these molecules are very weak. That is, it is a molecular crystal consisting of healthy molecules. But the bonds between molecules are weak, van der Waals. If such a crystal is squeezed, then bonds will begin to form between the molecules, between these balls, and the structure will turn into a three-dimensionally connected covalent very solid structure. This material was named tisnumite in honor of the Technological Institute of Superhard and New Carbon Materials. It was assumed that this material had a higher hardness than diamond, but further research showed that this was most likely not the case.

There have been suggestions and a rather active discussion that carbon nitrides can be harder than diamond, but, despite active discussion and active research, such a material has not yet been presented to the world.

There was a rather amusing work by Chinese researchers, in which they suggested, based on theoretical calculations, that another modification of carbon is similar to diamond in many respects, but slightly different from it, and is called lonsdaleite. According to this work, lonsdaleite is harder than diamond. Lonsdaleite is an interesting material, thin lamellae of this material have been found in shock-compressed diamond. This mineral was named after the famous woman Kathleen Lonsdale, the great British crystallographer, who lived in the 50s-70s of the XX century. She had an extremely interesting biography, she even had a chance to sit in prison when she refused to put out fires during World War II. She was a Quaker by religion, and Quakers were prohibited from doing anything related to war, even putting out fires. And for this she was placed in a paddy wagon. But nevertheless, everything was fine with her, she was the president of the International Union of Crystallographers, and this mineral was named after her.

Lonsdaleite, judging by all available experimental and theoretical data, is still softer than diamond. If you look at the work of these Chinese researchers, you can see that even according to their calculations, lonsdaleite is softer than diamond. But somehow the conclusion was made contrary to their own results.

Thus, it turns out that there is no real candidate for the removal of diamond from the position of the hardest substance. However, the issue is worth pursuing. Still, many laboratories are still trying to create such material. With the help of our method of predicting crystal structures, we decided to ask this question. And the problem can be formulated as follows: you are not looking for a substance that has maximum stability, but a substance that has maximum hardness. You set a range of chemical compositions, for example, from pure carbon to pure nitrogen, and everything in between, all possible carbon nitrides are included in your calculation, and evolutionarily try to find more and more solid compositions and structures.

The hardest substance in this system is the same diamond, and the addition of nitrogen to carbon does not improve anything in this system.

Thus, the hypothesis of carbon nitrides as substances harder than diamond can be buried.

We tried everything else that was suggested in the literature, different forms of carbon and so on - in all cases, diamond always won. So, it seems that the diamond cannot be removed from this pedestal. But it is possible to invent new materials that are preferable to diamond in a number of other respects, for example, in terms of crack resistance or in terms of chemical resistance.

For example, elemental boron. We have discovered a structure, a new modification of boron. We published this article in 2009, and it caused a huge response. The structure is obtained by applying slight pressure to ordinary bur and heating it to high temperatures. We called this form gamma-boron, and it turned out that it contains a partial ionic chemical bond. In fact, this is something that will somewhat reduce the hardness, but due to the high density, this modification still turns out to be the hardest of the known modifications of boron, its hardness is about 50 GPa. The pressures for synthesis are small, and therefore, in principle, one can even think about its synthesis in sufficiently large volumes.

We have predicted a number of other superhard phases, such as phases in the system "tungsten - boron", "chromium - boron" and so on. All of these phases are superhard, but their hardnesses still belong to the lower part of this range. They are closer to the 40 GPa mark than to the 90–100 GPa mark, which corresponds to the hardness of diamond.

But the search continues, we do not despair, and it is quite possible that we or our other colleagues working on this topic around the world will be able to invent a material that can be synthesized at low pressures and that will approach diamond in hardness. Something in this area has already been done by us and other colleagues. But how to apply it technologically is not yet entirely clear.

I'll tell you about a new form of carbon, which was actually produced experimentally back in 1963 by American researchers. The experiment was conceptually quite simple: they took carbon in the form of graphite and squeezed it at room temperature. The fact is that you can’t get a diamond like that, a diamond requires strong heating. Instead of diamond in their experiments, a transparent superhard non-metallic phase was formed, but nevertheless it was not a diamond. And this was in no way consistent with the characteristics of any of the known forms of carbon. What's the matter, what is this structure?

Quite by accident, while studying various structures of carbon, we came across one structure that was only slightly inferior to diamond in terms of stability. Only three years after we saw this structure, looked at it, even published it somewhere between the lines, it dawned on us that it would be nice to compare the properties of this structure with what had been published by all those researchers since 1963 and up to the most recent years. And it turned out that there is a complete match. We were happy, we quickly published an article in one of the most prestigious magazines, The Physical Review Letters, and a year later an article in the same journal was published by American and Japanese researchers who found that a completely different carbon structure also describes the same experimental data. The problem is that the experimental data were of rather poor resolution. So who is right?

Soon, Swiss and Chinese researchers proposed a number of modifications. And in the end, one Chinese researcher published about forty carbon structures, most of which also describe the same experimental data. He promised me that, if he was not too lazy, he would offer about a hundred more structures. So what is the correct structure?

To do this, we had to study the kinetics of the transformation of graphite into various carbon structures, and it turned out that we were very lucky. It turned out that our structure is the most preferable from the point of view of transformation kinetics.

A month after the publication of our article, an experimental work came out in which the experimenters did the most accurate experiment with data of much better resolution than before, and it really turned out that out of all those dozens of published structures, only one structure explains the experimental data - this is still our structure. We called this new material M-carbon, since its symmetry is monoclinic, from the first letter M.

This material is only slightly inferior in hardness to diamond, but whether there is any property in which it surpasses diamond is still not clear.

Until now, it can be said, "a thing in itself." We continue the search and hope that we will be able to invent a material that, while not much inferior to diamond in hardness, will significantly outperform it in all other characteristics.

One way to improve the mechanical characteristics of substances is their nanostructuring. In particular, the hardness of the same diamond can be increased by creating diamond nanocomposites or diamond nanopolycrystals. In such cases, the hardness can even be increased by a factor of 2. And this was done by Japanese researchers, and now you can see the products that they produce, quite large, on the order of a cubic centimeter, nanopolycrystals of diamond. The main problem with these nanopolycrystals is that they are so hard that it is almost impossible to even grind them, and the whole laboratory grinds it for weeks.

This is how you can both change chemistry, change the structure of a substance in search of improving its hardness and other characteristics, and change the dimension.

Synthetic superhard materials (STM) used for blade tools are dense modifications of carbon and boron nitride.

Diamond and dense modifications of boron nitride, which have a tetrahedral distribution of atoms in the lattice, are the hardest structures.

Synthetic diamond and cubic boron nitride are obtained by the method of catalytic synthesis and catalystless synthesis of dense modifications of boron nitride under static compression.

The use of diamond and boron nitride for the manufacture of blade tools became possible after they were obtained in the form of large polycrystalline formations.

Currently, there is a wide variety of STMs based on dense modifications of boron nitride. They differ in the technology of their production, structure and basic physical and mechanical properties.

The technology for their production is based on three physical and chemical processes:

1) phase transition of graphite-like boron nitride to cubic:

BN Gp ® BN Cub

2) phase transition of wurtzite boron nitride to cubic:

BNVtc ® BN Cub

3) sintering of BN Cub particles.

The unique physical and chemical properties (high chemical resistance, hardness, wear resistance) of these materials are explained by the purely covalent nature of the bonding of atoms in boron nitride, combined with the high localization of valence electrons in atoms.

The heat resistance of the tool material is its important characteristic. The wide range of values of the thermal stability of BN (600–1450°С) given in the literature is explained both by the complexity of the physicochemical processes that occur during heating of BN and by the uncertainty to some extent of the term “thermal stability” as applied to STM.

When considering the thermal stability of polycrystalline STMs based on diamond and dense modifications of boron nitride (they are often composite and the amount of binder in them can reach 40%), it should be taken into account that their thermal stability can be determined both by the thermal stability of BN and diamond and by the change in the properties of the binder upon heating. and impurities.

In turn, the thermal stability of diamond and BN in air is determined both by the thermal stability of high-pressure phases and their chemical stability under given conditions, mainly relative to oxidative processes. Consequently, thermal stability is associated with the simultaneous occurrence of two processes: the oxidation of diamond and dense modifications of boron nitride by atmospheric oxygen and the reverse phase transition (graphitization), since they are in a thermodynamically nonequilibrium state.

According to the production technology, STM based on diamond can be divided into two groups:

1) diamond polycrystals obtained as a result of the phase transition of graphite into diamond;

2) diamond polycrystals obtained by sintering diamond grains.

The most common grain size is approximately 2.2 µm, and there are practically no grains larger than 6 µm.

The strength of ceramics depends on the average grain size and, for example, for oxide ceramics decreases from 3.80–4.20 GPa to 2.55–3.00 GPa with an increase in grain sizes, respectively, from 2–3 to 5.8–6.5 µm.

In oxide-carbide ceramics, the granulometric composition is even finer-grained, and the average grain size of Al 2 O 3 is mainly less than 2 μm, while the grain size of titanium carbide is 1–3 μm.

A significant disadvantage of ceramics is its fragility - sensitivity to mechanical and thermal shock loads. The fragility of ceramics is estimated by the coefficient of crack resistance - K FROM.

Crack resistance coefficient K C, or critical stress intensity factor at the crack tip, is a characteristic of the fracture resistance of materials.

High hardness, strength, and modulus of elasticity, the complexity of machining, and the small size of STM samples limit the use of most currently used methods for determining the crack resistance coefficient.

To determine the crack resistance coefficient - K With STM, the method of diametrical compression of a disk with a crack and the method of determining the fracture toughness of ceramics by the introduction of an indenter are used.

To eliminate the brittleness of ceramics, various compositions of oxide-carbide ceramics have been developed.

The incorporation of monoclinic zirconia ZrO 2 into alumina-based ceramics improves the structure and thus noticeably increases its strength.

The tool, equipped with polycrystalline diamonds (PCD), is designed for finishing non-ferrous metals and alloys, non-metallic materials instead of carbide tools.

Composite 01 and composite 02 - polycrystals of cubic boron nitride (CBN) with a minimum amount of impurities - are used for fine and fine turning, mainly without impact, and face milling of hardened steels and cast irons of any hardness, hard alloys (Co > 15%) with depth of cut 0.05–0.50 mm (maximum allowable depth of cut 1.0 mm).

Composite 05 - polycrystals sintered from CBN grains with a binder - is used for preliminary and final turning without impact of hardened steels (HRC< 60) и чугунов любой твердости с глубиной резания 0,05–3,00 мм, а также для торцового фрезерования заготовок из чугуна любой твердости, в т. ч. по корке, с глубиной резания 0,05–6,00 мм.

Composite 10 and two-layer inserts made of composite 10D (composite 10 on a hard alloy substrate) - polycrystals based on wurtzite-like boron nitride (WNB) - are used for preliminary and final turning with and without impact and face milling of steels and cast irons of any hardness, hard alloys (Co > 15%) with a depth of cut of 0.05–3.00 mm, intermittent turning (presence of holes, grooves, foreign inclusions on the machined surface).

Thus, STM tools based on boron nitride and diamond have their own areas of application and practically do not compete with each other.

The wear of cutters made of composites 01, 02 and 10 is a complex process with a predominance of adhesive phenomena in continuous turning.

With an increase in contact temperatures in the cutting zone above 1000°C, the role of thermal and chemical factors increases - the following are intensified:

– diffusion;

– chemical decomposition of boron nitride;

– phase α-transition;

– abrasive-mechanical wear.

Therefore, when turning steels at speeds of 160–190 m/min, wear increases sharply, and at v > 220 m/min it becomes catastrophic almost regardless of the hardness of the steel.

In intermittent turning (with impact), abrasive-mechanical wear predominates with chipping and tearing of individual particles (grains) of the tool material; the role of mechanical impact increases with an increase in the hardness of the matrix of the processed material and the volume content of carbides, nitrides, etc.

The greatest influence on wear and tool life in continuous turning of steels is the cutting speed, in turning with impact - speed and feed, in turning cast irons - feed, and the machinability of malleable cast irons is lower than that of gray and high-strength cast irons.

Work order

1. Study the grades and chemical composition of steels and alloys, the classification of steels according to the method of manufacture and purpose, depending on the content of chromium, nickel and copper, the requirements for the macrostructure and microstructure, the normalization of hardenability. Pay attention to the order of sampling to check the hardness, microstructure, depth of the decarburized layer, surface quality, fracture.

2. Investigate the microstructure of U10 steel samples. Assess the microstructure of heat-treated steel by examining it under an MI-1 microscope. Fix the microstructure in the computer and print.

When compiling a report, it is necessary to give a brief description of the theoretical foundations of the structure, the properties of materials for cutting tools made of tool carbon, high-speed steels, hard, superhard alloys and ceramic materials. Provide photographs of the microstructure of U10 steel obtained during examination under the MI-1 microscope, indicate the heat treatment mode and structural components in the caption. The results of measurements of the main parameters of several inclusions of the considered steel are entered in table. 3.19.

Table 3.19

test questions

1. Classification of materials for cutting tools.

2. Structure and properties of tool carbon steels.

3. Structure and properties of die steels.

4. Structure and properties of high-speed steels.

5. Structure and properties of hard and superhard tool alloys.

6. Structure and properties of ceramic tool materials.

7. Structure of tool carbon steels.

8. The main properties that the material for cutting tools should have.

9. Wear resistance and heat resistance of cutting tools.

10. What determines the heating temperature of the cutting edge of tools?

11. Chemical composition and modes of heat treatment of the most used tool steels.

12. Hardenability of carbon steels, hardenability score, hardness distribution.

13. Effect of carbon content on the properties of carbon tool steels.

14. What determines the tempering temperature of tools?

15. Hot hardness and red hardness of high speed steel.

16. Reversible and irreversible hardness of high-speed steels.

17. How the red hardness of high-speed steels is structurally created.

18. How red hardness is characterized, its designation.

19. Modes of heat treatment of high speed steel tools, cold treatment, multiple tempering.

20. Steels for hot stamps, their heat resistance, heat resistance, toughness.

21. Operating temperatures for cutting hard alloy tools.

22. The hardness of cermet hard alloys, how is it determined?

23. Steels used for blade tools.

24. What explains the unique physical and chemical properties (high chemical resistance, hardness, wear resistance) of synthetic superhard materials?

25. A significant drawback of ceramics.

26. How is the brittleness of ceramics assessed?

Lab #4

Dependency research

composition - structure - properties For cast irons

Objective: study of the structure, composition and properties of conversion and machine-building cast irons; their classification and application.

Materials and equipment: collection of unetched sections of cast iron; a metallographic complex, including an MI-1 optical microscope, a Nikon Colorpix-4300 digital camera with a photo adapter; etchant (4% solution of HNO 3 in alcohol).

Theoretical part

cast iron called iron-carbon alloys containing more than 2.14% carbon and permanent impurities - silicon, manganese, sulfur and phosphorus.

Cast irons have lower mechanical properties than steels, since an increased carbon content in them leads either to the formation of a hard and brittle eutectic, or to the appearance of free carbon in the form of graphite inclusions of various configurations that disrupt the continuity of the metal structure. Therefore, cast irons are used for the manufacture of parts that do not experience significant tensile and shock loads. Cast iron is widely used in mechanical engineering as a casting material. However, the presence of graphite gives a number of advantages to cast iron over steel:

- they are easier to process by cutting (brittle chips are formed);

– have the best antifriction properties (graphite provides additional lubrication of friction surfaces);

– have higher wear resistance (low coefficient of friction);

– cast irons are not sensitive to external stress concentrators (grooves, holes, surface defects).

Cast irons have high fluidity, well fill the mold, have low shrinkage, so they are used for the manufacture of castings. Parts made from cast iron are significantly less expensive than those made by machining from hot-rolled steel sections or from forgings and stampings.

The chemical composition and, in particular, the carbon content do not characterize the properties of cast iron sufficiently reliably: the structure of cast iron and its basic properties depend not only on the chemical composition, but also on the smelting process, casting cooling conditions, and heat treatment mode.

Carbon in the structure of cast iron can be observed in the form of graphite and cementite.

Depending on the state of carbon, cast irons are divided into two groups:

1) cast irons, in which all carbon is in a bound state in the form of cementite or other carbides;

2) cast irons, in which all carbon or part of it is in a free state in the form of graphite.

The first group includes white cast irons, and the second group includes gray, malleable and high-strength cast irons.

According to their purpose, cast irons are divided into:

1) for conversion;

2) engineering.

Converting plants are mainly used to produce steel and ductile iron, while machine-building plants are used to make castings for parts in various industries: automotive and tractor building, machine tool building, agricultural engineering, etc.

White cast irons

In white cast irons, all carbon is in a chemically bound state (in the form of cementite), i.e., they crystallize, like carbon steels, according to the Fe - Fe 3 C metastable diagram. They got their name from the specific dull white color of the fracture, due to the presence of cementite in the structure.

White cast irons are very brittle and hard, difficult to machine with a cutting tool. Pure white cast irons are rarely used in mechanical engineering; they are usually processed into steel or used to produce ductile iron.

The structure of white cast irons at normal temperature depends on the carbon content and corresponds to the "iron-cementite" equilibrium state diagram. Such a structure is formed as a result of accelerated cooling during casting.

Depending on the carbon content, white cast irons are divided into:

1) hypoeutectic, containing from 2 to 4.3% carbon; consist of perlite, secondary cementite and ledeburite;

2) eutectic, containing 4.3% carbon, consist of ledeburite;

3) eutectic, containing from 4.3 to 6.67% carbon, consist of perlite, primary cementite and ledeburite.

a B C

Rice. 4.1. Microstructure of white cast irons, × 200:

a– hypoeutectic (ledeburite, perlite + secondary cementite);

b– eutectic (ledeburite);

in– hypereutectic (ledeburite + primary cementite)

Perlite in white cast iron is observed under a microscope in the form of dark grains, and ledeburite is observed in the form of separate sections of colonies. Each such area is a mixture of small rounded or elongated dark grains of perlite, evenly distributed in a white cementite base (Fig. 4.1, a). Secondary cementite is observed in the form of light grains.

With an increase in the carbon concentration in hypoeutectic cast iron, the proportion of ledeburite in the structure increases due to a decrease in the areas of the structure occupied by pearlite and secondary cementite.

Eutectic cast iron consists of one structural component - ledeburite, which is a uniform mechanical mixture of perlite with cementite (Fig. 4.1, b).

The structure of hypereutectic cast iron consists of primary cementite and ledeburite (Fig. 4.1, in). With an increase in carbon, the amount of primary cementite in the structure increases.

Similar information.

One of the directions for improving the cutting properties of tools, which makes it possible to increase labor productivity during machining, is to increase the hardness and heat resistance of tool materials. The most promising in this respect are diamond and synthetic superhard materials based on boron nitride.

Diamonds and diamond tools are widely used in the processing of parts made of various materials. Diamonds are characterized by exceptionally high hardness and wear resistance. In terms of absolute hardness, diamond is 4–5 times harder than hard alloys and tens and hundreds of times higher than the wear resistance of other tool materials in the processing of non-ferrous alloys and plastics. In addition, due to the high thermal conductivity of diamonds, heat is better removed from the cutting zone, which contributes to the guaranteed production of parts with a burn-free surface. However, diamonds are very fragile, which greatly narrows the scope of their application.

For the manufacture of cutting tools, the main application received artificial diamonds which are close to natural in their properties. At high pressures and temperatures in artificial diamonds, it is possible to obtain the same arrangement of carbon atoms as in natural ones. The mass of one artificial diamond is usually 1/8-1/10 carat (1 carat - 0.2 g). Due to the small size of artificial crystals, they are unsuitable for the manufacture of tools such as drills, cutters, and others, and therefore are used in the manufacture of powders for diamond grinding wheels and lapping pastes.

Bladed diamond tools produced on the basis of polycrystalline materials such as "carbonado" or "ballas". These tools have long tool life and high surface quality. They are used in the processing of titanium, high-silicon aluminum alloys, fiberglass and plastics, hard alloys and other materials.

Diamond as a tool material has a significant drawback - at elevated temperatures, it enters into a chemical reaction with iron and loses its efficiency.

In order to process steels, cast irons and other iron-based materials, superhard materials, chemically inert to it. Such materials are obtained using a technology close to the technology for obtaining diamonds, but not graphite, but boron nitride is used as the starting material.

Polycrystals of dense modifications of boron nitride are superior in heat resistance to all materials used for blade tools: diamond by 1.9 times, high-speed steel by 2.3 times, hard alloy by 1.7 times, mineral ceramics by 1.2 times.

These materials are isotropic (the same strength in different directions), have a microhardness that is less but close to that of diamond, increased heat resistance, high thermal conductivity, and chemical inertness with respect to carbon and iron.

The characteristics of some of the considered materials, which are currently called "composite", are shown in the table.

Comparative characteristics of STM based on boron nitride

Brand	original title	Hardness HV, GPa	Heat resistance, o С
Composite 01	Elbor-R	60...80	1100...1300
Composite 02	Belbor	60...90	900...1000
Composite 03	Ismit	60	1000
Composite 05	Composite	70	1000
Composite 09	PKNB	60...90	1500
Composite 10	Hexanite-R	50...60	750...850

The effectiveness of the use of bladed tools from various grades of composites is associated with the improvement of the design of tools and the technology of their manufacture and with the determination of a rational area for their use:

composites 01 (elbor-R) and 02 (belbor)

composite 05

composite 10 (hexanit-R)

At the same time, the tool life increases tenfold compared to other tool materials.

Materials of high hardness are mainly used in mechanisms subject to abrasive wear.

Of the simple substances, only diamonds and boron have high hardness.

The overwhelming majority of substances of high hardness are refractory chemical compounds (carbides, nitrides, borides, silicides).

Due to the high brittleness of solid joints and the difficulty of processing them, the manufacture of parts from them is in most cases impractical or economically unprofitable. Their main area of application is the solid components of composite materials and coatings applied in various ways.

Superhard materials

These include cubic modifications of carbon (diamond) and boron nitride.

Synthetic diamonds in the form of powders are used for the preparation of abrasive tools and abrasive crusts, in the form of dense polycrystalline formations (Ballas, Carbonado) for the production of abrasive tools, cutters, drawing dies.

By sintering a mixture of micropowders of synthetic and natural diamonds, dense polycrystalline diamond formations are obtained - CB and Dismit.

SV grade diamonds are used for drilling bits and drill bits, as well as for cutting non-metallic materials.

Dismit is used for the manufacture of mining tools, cutting tools (cutters, drills and others) used for processing non-ferrous metals and alloys, plastics, fiberglass.

Cubic boron nitride

Obtained only synthetically from the hexagonal modification. It is mainly used for the manufacture of abrasive tools. In terms of hardness, it is inferior to diamond, but significantly surpasses it in heat resistance.

In the USA, cubic boron nitride is produced under the name Borazon, in the CIS - Elbor and Cubonite. Their brands, respectively, are LO and KO of normal strength and LR and KR - increased.

Varieties of polycrystalline material based on Elbor and Cubonite - Elbor-R, Hexanite-R, ISMIT, PNTB, COMPOSITE and others ... are produced in the form of plates of various shapes. They are used to make metal-cutting tools used in the processing of hard-to-cut hardened steels, cast irons and alloys with a hardness of HRC> 40. The durability of such a tool is 10…20 times greater than the durability of a carbide tool, the productivity increases by 2…4 times.