The main requirements for tool materials are hardness, resistance to wear, heat, etc. Compliance with these criteria allows cutting. In order to penetrate into the surface layers of the product being processed, the blades for cutting the working part must be made of strong alloys. Hardness can be natural or acquired.
For example, factory-made tool steels cut easily. After mechanical and thermal processing, as well as grinding and sharpening, their level of strength and hardness increases.
How is hardness determined?
A characteristic can be defined in many ways. Tool steels have Rockwell hardness, hardness has a numerical designation, as well as the letter HR with a scale of A, B or C (for example, HRC). The choice of tool material depends on the type of metal being processed.
The most stable performance and low wear blades thathave been heat treated, can be achieved with an HRC of 63 or 64. At a lower value, the properties of tool materials are not so high, and at high hardness, they begin to crumble due to brittleness.
Metals with a hardness of HRC 30-35 are perfectly machined with iron tools that have been heat treated with an HRC of 63-64. Thus, the ratio of hardness indicators is 1:2.
To process metals with HRC 45-55, tools should be used, which are based on hard alloys. Their index is HRA 87-93. Synthetic-based materials can be used on hardened steels.
Strength of tool materials
During the cutting process, a force of 10 kN or more is applied to the working part. It provokes high voltage, which can lead to the destruction of the tool. To avoid this, cutting materials must have a high safety factor.
The best combination of strength characteristics have tool steels. The working part made of them perfectly withstands heavy loads and can function in compression, torsion, bending and stretching.
Effect of critical heating temperature on tool blades
When heat is released when cutting metals, their blades are subject to heating, to a greater extent - surfaces. When the temperature is below the critical mark (for each material it has its own)structure and hardness do not change. If the heating temperature becomes higher than the permissible norm, then the level of hardness drops. The critical temperature is called red hardness.
What does the term "red hardness" mean?
Red hardness is the property of a metal to glow dark red when heated to a temperature of 600 °C. The term implies that the metal retains its hardness and wear resistance. At its core, it is the ability to withstand high temperatures. For different materials there is a limit, from 220 to 1800 ° C.
How can cutting tool performance be increased?
The tool materials of the cutting tool are characterized by increased functionality while increasing temperature resistance and improving the removal of heat generated on the blade during cutting. Heat raises the temperature.
The more heat is removed from the blade deep into the device, the lower the temperature on its contact surface. The level of thermal conductivity depends on the composition and heating.
For example, the content of elements such as tungsten and vanadium in steel causes a decrease in its thermal conductivity, and an admixture of titanium, cob alt and molybdenum causes it to increase.
What determines the coefficient of sliding friction?
The coefficient of sliding friction depends on the composition and physical properties of the contacting pairs of materials, as well as on the stress value on the surfaces,subjected to friction and slip. The coefficient affects the wear resistance of the material.
The interaction of the tool with the material that has been processed proceeds with constant moving contact.
How do instrumental materials behave in this case? Kinds of them wear out equally.
They are characterized by:
- the ability to erase the metal it comes into contact with;
- ability to show resistance to wear, that is, to resist the abrasion of another material.
Blade wear happens all the time. As a result of this, the devices lose their properties, and the shape of their working surface also changes.
Wear resistance may vary depending on cutting conditions.
What groups are tool steels divided into?
Main instrumental materials can be divided into the following categories:
- cermet (hard alloys);
- cermets, or mineral ceramics;
- boron nitride based on synthetic material;
- synthetic diamonds;
- Carbon-based tool steels.
Tool iron can be carbon, alloy and high speed.
Carbon-based tool steels
Carbonaceous materials began to be used to make tools. Their cutting speed is slow.
How are tool steels marked? Materials are designated by a letter (for example, "U" means carbon), as well as a number (indicators of tenths of a percent of carbon content). The presence of the letter "A" at the end of the marking indicates the high quality of steel (the content of substances such as sulfur and phosphorus does not exceed 0.03%).
Carbon material has a hardness of 62-65 HRC and low temperature resistance.
U9 and U10A grades of tool materials are used in the manufacture of saws, and the U11, U11A and U12 series are designed for hand taps and other tools.
The temperature resistance level of steels of the U10A, U13A series is 220 °C, therefore it is recommended to use tools made of such materials at a cutting speed of 8-10 m/min.
Alloyed iron
Alloyed tool material can be chromium, chromium-silicon, tungsten and chromium-tungsten, with an admixture of manganese. Such series are indicated by numbers, and they also have letter markings. The first left figure indicates the coefficient of carbon content in tenths if the content of the element is less than 1%. The numbers on the right represent the average alloying content as a percentage.
The tool material grade X is suitable for making taps and dies. B1 steel is suitable for making small drills, taps and reamers.
The level of temperature resistance of alloyed substances is 350-400 ° C, so the cutting speed is one and a half times faster than forcarbon alloy.
What are high-alloy steels used for?
Various fast cutting tool materials are used in the manufacture of drills, countersinks and taps. They are labeled with letters as well as numbers. Important constituents of the materials are tungsten, molybdenum, chromium and vanadium.
HSS are divided into two categories: normal and high performance.
Normal performance steels
The category of iron with a normal level of productivity includes grades R18, R9, R9F5 and tungsten alloys with an admixture of molybdenum of the R6MZ, R6M5 series, which retain a hardness of at least HRC 58 at 620 ° C. Suitable for carbon and low alloy steels, gray cast iron and non-ferrous alloys.
High performance steels
This category includes grades R18F2, R14F4, R6M5K5, R9M4K8, R9K5, R9K10, R10K5F5, R18K5F2. They are able to maintain HRC 64 at temperatures from 630 to 640 °C. This category includes superhard tool materials. It is designed for iron and alloys that are difficult to machine, as well as titanium.
Hardmetals
Such materials are:
- cermet;
- mineral ceramic.
The shape of the plates depends on the properties of the mechanics. These tools operate at high cutting speed compared to high speed material.
Metal ceramics
Cermet carbides are:
- tungsten;
- tungsten titanium;
- tungsten with the inclusion of titanium and tantalum.
VK series includes tungsten and titanium. Tools based on these components have increased wear resistance, but their level of impact resistance is low. Devices on this basis are used for processing cast iron.
Tungsten-titanium-cob alt alloy is applicable to all kinds of iron.
The synthesis of tungsten, titanium, tantalum and cob alt is used in special cases when other materials are ineffective.
Carbide grades are characterized by a high level of temperature resistance. Materials made of tungsten can maintain their properties with HRC 83-90, and tungsten with titanium - with HRC 87-92 at a temperature of 800 to 950 ° C, which makes it possible to operate at high cutting speeds (from 500 m/min to 2700 m /min when machining aluminum).
For machining parts that are resistant to rust and high temperatures, tools from the OM fine-grain alloy series are used. Grade VK6-OM is suitable for finishing, while VK10-OM and VK15-OM are suitable for semi-finishing and roughing.
Even more efficient when working with "difficult" parts are superhard tool materials of the BK10-XOM and BK15-XOM series. They replace tantalum carbide with chromium carbide, making them more durable even when subjected to high temperatures.
In order to increase the strength level of the solid plate, they resort to coating it with a protective film. Titanium carbide, nitride and carbonite are used, which are applied in a very thin layer. The thickness is from 5 to 10 microns. As a result, a layer of fine-grained titanium carbide is formed. These inserts have three times the tool life of uncoated inserts, increasing cutting speed by 30%.
In some cases, cermet materials are used, which are obtained from aluminum oxide with the addition of tungsten, titanium, tantalum and cob alt.
Mineral ceramics
Mineral ceramics TsM-332 are used for cutting tools. It has high temperature resistance. The hardness index HRC is from 89 to 95 at 1200 °C. Also, the material is characterized by wear resistance, which allows the processing of steel, cast iron and non-ferrous alloys at high cutting speeds.
To make cutting tools, B-series cermet is also used. It is based on oxide and carbide. The introduction of metal carbide, as well as molybdenum and chromium into the composition of mineral ceramics, helps to optimize the physical and mechanical properties of cermet and eliminates its brittleness. The cutting speed is increased. Semi-finishing and finishing with a cermet-based tool is suitable for gray ductile iron, hard-to-machine steel and a number of non-ferrous metals. The process is carried out at a speed of 435-1000 m/min. Cutting ceramics are temperature resistant. Its hardness is HRC90-95 at 950-1100 °С.
For the processing of hardened iron, durable cast iron, as well as fiberglass, a tool is used, the cutting part of which is made from solid substances containing boron nitride and diamonds. The hardness index of elbor (boron nitride) is about the same as that of diamond. Its resistance to temperature is twice that of the latter. Elbor is distinguished by its inertness to iron materials. The strength limit of its polycrystals in compression is 4-5 GPa (400-500 kgf/mm2), and in bending - 0.7 GPa (70 kgf/mm 2). Temperature resistance is up to 1350-1450 °C.
Also noteworthy are the synthetic-based diamond ballas of the ASB series and the carbonado of the ASPK series. The chemical activity of the latter towards carbon-containing materials is higher. That is why it is used when sharpening parts made of non-ferrous metals, alloys with a high silicon content, hard materials VK10, VK30, as well as non-metallic surfaces.
The tool life of carbonade cutters is 20-50 times that of hard alloys.
Which alloys are used in industry?
Instrumental materials are released all over the world. The kinds used in Russia, the USA and in Europe, for the most part, do not contain tungsten. They belong to the KNT016 and TN020 series. These models have become a replacement for the T15K6, T14K8 and VK8 brands. They are used for processing steels for structures, stainless steel and tool materials.
New requirements for tool materials due to shortage of tungsten andcob alt. It is precisely with this factor that alternative methods for obtaining new hard alloys that do not contain tungsten are constantly being developed in the USA, European countries and Russia.
For example, the Titan 50, 60, 80, 100 series tool materials manufactured by the American company Adamas Carbide Co contain carbide, titanium and molybdenum. An increase in the number indicates the degree of strength of the material. The characteristic of tool materials of this release implies a high level of strength. For example, the Titan100 series has a strength of 1000 MPa. She is a competitor to ceramics.
