In order for the working surface of the tool to retain the specified properties for a long time, it is necessary to use special alloys and steels for the manufacture of such elements. To date, parts of the cutting tool are made from hard alloys, tool steel grades. For milling cutters, chisels, chisels mainly high-speed steel is used.
Basic requirements for cutting tool alloys
Details of this type for a long time working in conditions of friction and elevated temperatures. However, the working surface must retain its properties, have high wear resistance and hardness. At high speeds, which the tool picks up during the cutting process, both its edge, and the part itself, and chips are heated. Therefore, the main characteristic that high-speed steel should have is heat resistance. For difficult-to-work materials, powder high-speed steels are used. They have higher cutting properties. The disadvantage of such alloys is difficult processing of the blanks. All the necessary characteristics are achieved by the introduction of certain alloying elements and special heat treatment.
Effect of alloyed elements on steel properties
Tungsten, molybdenum provide high heat resistance of high-speed steel grades. On their basis, carbides are formed, which are partially converted into solid solution. After heat treatment, the structure of martensite is provided. Tungsten, molybdenum, and vanadium slow down its decay. This is what provides the necessary redness. For a long time high-speed steel alloyed with tungsten was used. However, due to the scarcity of this metal, it began to be partially replaced by molybdenum. This element also has a positive effect on the tendency of tungsten steel grades to carbide heterogeneity. The hardest carbide forms vanadium. However, the carbon content should be sufficient for greater saturation of the solid solution. The more vanadium is introduced, the more carbon must be in the alloy. The main task of chromium is to impart high hardenability to steel. Redness also increases cobalt.
High-speed steel (the hardness when alloying with this element increases to 70 HRC) in this case will have reduced strength. It is worth noting that the introduction of chromium is not widely used due to the high cost of the element.
Heat treatment of high-speed steel
These steel grades come in the forging state (temperature is about 1200 ° C). Heated to 860 ° C, then the metal is maintained at a temperature of about 760 ° C. Heat treatment tools include quenching and tempering. It should be noted that such processing has its own characteristics. First, slow, gradual heating is needed. Since the steel is high alloyed, its thermal conductivity is rather low, rapid heating can lead to the formation of cracks. It is very important to heat the workpiece evenly. Electric furnaces, salt baths are used. The processing of high-speed steel is quite time-consuming, it requires strict adherence to all stages of the process.
Hardening steel for cutting tools
The main task of quenching is the dissolution of carbides in austenite. As a rule, carbides based on tungsten and chromium dissolve at 1200 ° C, vanadium requires higher temperatures. After this stage, the structure has excess (those that are not dissolved) carbides. They restrain grain growth. High temperatures provide fine-grained austenite. Cooling takes place in oil or molten salts. The temperature over the cross section of the parts is equalized. Such processing of high-speed steel avoids the appearance of cracks. After quenching, the steel has the following structure: martensite, residual austenite, carbides.
High-speed steel tempering
The tempering of steel contributes to the transformation of tempering martensite to tempering martensite, austenite to martensite (since the former does not have sufficient hardness), and the removal of residual stresses. As a rule, heat treatment of high-speed steel includes multiple tempering. This process begins at a temperature of 150 ° C. Further, at 550 ° C, precipitation hardening occurs (carbides are precipitated from the solid solution). As a result, the hardness of the alloy increases. Higher tempering temperatures are undesirable, since the process of decomposition of martensite, and, accordingly, a decrease in hardness, will occur. After a single tempering, tungsten steels have residual austenite. It completely turns into martensite during the second holiday. Residual stresses are removed in the process of the third vacation. Steel with cobalt content may be subject to tempering for the fourth time.
Violation of heat treatment technology
Reducing the amount of carbon on the surface of the workpiece can be a consequence of poor salt content of the salt bath, as well as overheating during austenization. Excess temperature leads to the melting of the grain boundaries. Also, the treated part may have cracks. This phenomenon occurs due to the rapid heating of the metal. Another reason is accelerated cooling. Low hardness may be due to insufficient doping of the structure of martensite, a violation of the temperature regime during tempering, in which residual austenite remains. Another possible defect in the workpiece is mothball fracture.
The most common grades of high-speed steel
High-speed steel (GOST 19265-73) is divided into alloys of normal and high heat resistance. The first group includes brands such as R18, R6M5. Their hardness reaches 63 HRC. Their main purpose - the processing of cast iron, copper, aluminum alloys. Tungsten steels have a higher heat resistance. They are used for the manufacture of drills, cutters, cutters. Steel R6M5, which contains molybdenum, is slightly inferior in cutting properties, but it is significantly cheaper. In addition, its plasticity is somewhat higher, and the tendency to the formation of cracks is not so high. More heat-resistant steels are composed of vanadium and cobalt (10Р6М5, Р9Ф5). Their hardness reaches 66 HRC. They are used to process more durable structural steels, high-temperature alloys, in the manufacture of finishing tools. It is characteristic that these brands have a higher wear resistance (due to the presence of vanadium in the composition). Recently, the method of powder metallurgy has been increasingly used. Such tools have higher cutting properties.