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 to make such elements. To date, the details of the cutting tool are made of hard alloys, instrumental steel grades. For mills, cutters, chisels, mainly high-speed steel is used.
Basic requirements for alloys for cutting tools
Details of this type work for a long time under conditions of friction and elevated temperatures. However, the working surface must retain its properties, have high wear resistance and hardness. At high speeds, which is accumulated by the tool during the cutting process, its edge, and the part itself, and chips are heated. Therefore, the main characteristic that high-speed steel must have is heat resistance. For hard-to-work materials, powder high-speed steels are used. They have higher cutting properties. A disadvantage of such alloys is the difficult processing of blanks. All necessary characteristics are achieved by introducing certain alloying elements and special heat treatment.
The effect of alloyed elements on the properties of steel
High heat resistance of high-speed steel grades is provided by tungsten, molybdenum. On their basis, carbides are formed, which partially pass into a solid solution. After heat treatment, a martensite structure is provided. Tungsten, molybdenum, and also vanadium slows down its decay. This is what provides the necessary redness. For a long time, high-speed steel, alloyed only with tungsten, was used. However, due to the shortage of this metal, it was 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 to increase the saturation of the solid solution. The more vanadium is introduced, the more carbon must be in the composition of the alloy. The main task of chromium is to give the steel a high hardenability. Red resistance also increases cobalt.
High-speed steel (hardness at doping with this element rises to 70 HRC) in this case will have a lower strength. It is worth noting that the introduction of chromium is not widely used because of the high cost of the element.
Heat treatment of high-speed steel
These steel grades are delivered in the forging state (temperature about 1200 ° C). The heating is carried out to 860 ° C, then the metal is kept at a temperature of about 760 ° C. Heat treatment of the tool includes hardening and tempering. It is worth noting that this treatment has its own characteristics. First, slow, gradual heating is necessary. Since the steel is highly 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 process of processing high-speed steel is quite laborious, it requires strict adherence to all stages of the technological process.
Hardening of steel for cutting tools
The main task of quenching is 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 the growth of grain. High temperatures are provided by fine-grained austenite. Cooling occurs in oil or salt melt. The temperature along the section of the part is leveled. This treatment of high-speed steel prevents the appearance of cracks. After hardening, steel has the following structure: martensite, residual austenite, carbides.
High-speed steel production
The release of steel promotes the transformation of martensite quenching into a martensite of tempering, austenite into martensite (since the former does not have sufficient hardness), and the removal of residual stresses. Typically, the heat treatment of high-speed steel includes multiple tempering. This process begins at a temperature of 150 ° C. Further at 550 ° C, precipitation hardening takes place (carbides are isolated from the solid solution). As a result, the hardness of the alloy increases. Higher tempering temperatures are undesirable, since the decay of martensite will occur, and, accordingly, a decrease in hardness. Tungsten steels have residual austenite after a single tempering. Completely it turns into martensite on the second vacation. Residual stresses are removed during the third tempering process. Steel with the content of cobalt can be tempered and the fourth time.
Infringement of technology of heat treatment
Reducing the amount of carbon on the surface of the workpiece can be the result of poor depletion of the salt bath, as well as overheating during austenitization. Exceeding the temperature leads to a reflow of grain boundaries. Also, the treated part may have cracks. This phenomenon arises from the rapid heating of the metal. Another reason is accelerated cooling. A low value of hardness can be the result of insufficient doping of the martensite structure, a violation of the temperature regime during tempering, in which residual austenite remains. Another possible defect in the billet is the melt fracture.
The most common high-speed steel grades
High-speed steel (GOST 19265-73) is divided into alloys of normal and increased heat resistance. The first group includes such brands as P18, P6M5. Their hardness reaches 63 HRC. Their main purpose is to handle cast iron, copper, aluminum alloys. Tungsten steels have a higher heat resistance. They are used to make drills, cutters, cutters. Steel P6M5, 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 crack formation is not so high. More heat-resistant steels have vanadium and cobalt (10P6M5, P9F5) in the composition. Their hardness reaches 66 HRC. They are used for the processing of stronger structural steels, high-temperature alloys, in the manufacture of a finishing tool. 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.