Heat treatment of HSS tools

For high durability and performance of tools needed to work part of them had the suitable hardness and wear resistance. This is possible through the creation of a core of fine-grained martensitic structure, which is achieved by pre-forging and isothermal annealing before machining and after machining — quenching and tempering. For these thermal processes are characterized is the following:

1) the heating of the tool under forging, annealing and hardening should be rapid so that the whole material had evenly warm; at the same time, too slow heating causes the appearance of scale;

2) exposure tool in a furnace should be sufficient for structural transformations, but to avoid burnout of carbon from the surface of the tool and the growth of grains of austenite not to overdo it at a high temperature;

3) cooling the tool you need to make depending on the chemical composition of the steel with a certain intensity

.heat treatment

These operations of heat treatment are applied to all classes of tool steel. However, the temperature regimes of these processes largely depend on the steel grade and the size of the tool. Given the relatively wide application of high-speed steel for making cutting tools, consider the specific characteristics of the operations of heat treatment.

high Speed steel usually supplied in the annealed condition with the structure of fine-grained (sarbatoarea) perlite with excess carbides. The hardness of the steel in the usual performance of R9 and R18, in accordance with GOST, should be no more than NV = 207-255, and of steels of high performance — up to NV = 269-293. The thickness bezoperatsionnogo layer on the side for rods with a diameter of 5-100 mm and should not exceed 0,45—1,3 mm.

polished steel (silverfish) obesplozhennym layer is generally not allowed. Ductility supply of high speed steel is determined by mechanical properties, if required, shall be checked by the technological testing trial cutting, punching, and also the samples for the bend.

as carbide heterogeneity in theaters with a diameter of over 50 mm, usually more than 3 cylinder, billet from bars, rods of large diameter to reduce the carbide heterogeneity should be subjected to additional forging. Forging are also bars of high speed steel. Thus, heat treatment of high speed steels include, in addition to the main operations are quenching and tempering, the operations of forging and subsequent annealing. The selection of species and modes of heat treatment should take into account obtaining the necessary grit, toughness (at least HRC = 62 to 65), strength and heat resistance of steel. You must also avenge that changing modes of heating, soaking and cooling different impact on obtaining the optimal values of the above characteristics. Thus, the hardness of hardened steel with the increase of quenching temperature first increases and then decreases. The heat resistance also depends on alloy™ solid solution and increases with the increase of quenching temperature, while the strength — structural factors and Prel<de only of grain size and terms of distribution of carbides. Therefore, the increase of quenching temperature, an even growth of grain, but to ensure the dissolution of fine carbides, including those present in the annealed steel at the grain boundaries to increase the strength. In addition, the quenching temperature, which ensures the highest red hardness, does not coincide with temperatures conducive to maximum hardness.

Heating for forging because of the poor thermal conductivity of high speed steel should be delayed. Billets from steel R18 with a diameter of 50-60 mm is first placed in the oven with a temperature of 400-600° C and slowly heated at the rate of 7— 8 minutes for every 10 mm thickness and then heated to 780— 820° C and kept at this temperature, i.e. in the field of the transformation of pearlite into austenite, also the rate of 7-8 min for every 10 mm of diameter. Smaller workpiece should be placed in an oven with a temperature of 780-820° C. Further heating to the temperature of the beginning of forging /= 1140-1180° C, and ingots up to /=1150-1200° C produced relatively quickly. To prevent excessive hardening and cracking forging (rolling) stops at a temperature of 875-900° C and the ingots at a temperature 975-1000° C. As high-speed steel quenched by cooling in air, to prevent cracking of large blanks are cooled slowly in hot sand, and then performed isothermal annealing.heat treatment process

Isothermal annealing high-speed steel required for the stress relief obtained from the treatment pressure, and for lowering hardness and also create the structure of granular perlite or sarbatoarea reporting relatively good machinability.

Isothermal annealing high-speed steel R18 is produced by heating to a temperature of 850-870° C with endurance up to 12 hours at this temperature. After the end of exposure produce cooling the steel to a temperature of 720-740° C with a relatively small speed (40-50° C per hour) and then at 720-740° C, the steel is again aged for at least 4 hours. It should be particularly noted that high-speed steel containing molybdenum or cobalt and are particularly sensitive to decarburization annealing, it is advisable to anneal in a furnace with a protective atmosphere or in a cast iron shavings.

When cutting with great speed is called the uneven heating of the surface layer of the treated tool, which is the reason for the creation of surface stresses and formation of sites with the martensitic-austenitic structure. In this regard, for large tools with complex shape and a large length, such as broaches, after machining to relieve stresses and the preparation of steel to quenching, it is recommended to produce a high holiday. This vacation is made by heating the tool to a temperature of 650-680° C With exposure to 2— 3 hours with subsequent cooling in air or in oil.

High red hardness and cutting ability of high speed steel depend not only on chemical composition, initial grain structure with uniform distribution of carbides, but also from the particular conditions of quenching and tempering. Therefore, heating in the hardening of such steels to high temperatures spaced near the melting temperature, is a specific feature of heat treatment of these steels. Given a reduced (to 2-3 times) the thermal conductivity of the high-speed steels compared with carbon, to prevent the occurrence of high stresses and cracks produced stage heating for hardening. The first heating, which is done only for large (diameter of over 30 mm) and complex instruments, is performed at a temperature of 400-500° C in a furnace of any design. The second heating up to 840° C required for the transformation of pearlite into austenite, avoiding bezuglaja, na and oxidation is performed in furnaces with protective atmosphere or baths. The duration of exposure in this heating depends on the shape and size of the tool. When heated, the salts the exposure is about 20 seconds, stoves, and 25-30 seconds on a 1 mm section of the tool. Such a large exposure is necessary not only to warm the tool over the entire section up to the specified temperature, but for a complete transformation of pearlite into austenite. Final heating to the quenching temperature necessary to dissolve carbides of alloying elements in aumente, is made relatively quickly, as prolonged exposure possible grain growth of austenite. In addition, the grain boundaries of austenite can be formed of a carbide mesh, and also occur decarburization of the surface layers tool. In addition, prolonged exposure close to the temperature of formation of the liquid phase changes the shape of the carbides. They receive the angular shape and greatly increase in size due to dissolution of fine carbides.

the Duration of heating up to high temperatures, as established practice, to some extent proportional to the cross section of the tool and is 8-9 seconds for 1 mm of diameter or thickness by heating in salt and 10-12 seconds to heat in a furnace.

it Should be noted that the quality of tempering depends on the accuracy of the setting of the quenching temperatures, as minor changes in the chemical composition of the steel quenching temperature is changed. Thus the range of changes in the hardening temperature for the particular steel should be reduced to 10-5° C, which increases cutting ability of the tool to 1,5 times.

Final heating of the tools of ordinary high speed steels is most appropriate to perform in a molten salt ВаС12, and steels containing cobalt or more than 1.5—2% molybdenum for the best protection against decarburization — in controlled atmospheres or in a molten salt, but with a reduced shutter speed, using heated to 1100-1150° C.

cooling heated to the hardening temperature of the tool should be set depending on its size and shape. Accelerated cooling to 500-400° C delays the release of carbides from austenite and contributes to the best heat resistance.

Continuous quenching with cooling in oil (30-100° C) can be used for tools of simple shapes (cutters, drills) having a diameter (thickness) up to 30-40 mm. the Tools of small diameter, or thickness, 3-5 mm is possible to cool with compressed air or directly on the air.

Step annealing with advanced podstugivaniya air exposure time of 2-5 minutes in the hot cooling medium (potassium nitrate) at g = 450-500° C, or 250-350° C, i.e., p region of greatest stability of austenite, it is advisable to for a contoured medium-sized and very long and thin instruments. After the end of the exposure tool is cooled directly by air. In the absence of the bath with molten mixture of tools can also be cooled in oil up to 300-450°, and then on the air.

Isothermal hardening recommended for large and long tools, is cooled in the molten salt with a temperature of 200-300° C with an exposure of 30-60 minutes. In addition, for very large tools with sharp transitions can be applied intermittent annealing, which is carried out first in oil heated to 90-100° C, is obtained when the partial martensitic transformation, and then the tools are transferred to the oven for the holidays. Also of note is the hardening under pressure. It is used to reduce deformation of the tools of simple form and of very small size. In this case, the instruments are first cooled in a salt to 500-600° C, and then the oil under pressure.

Despite the high temperature, correctly hardened high-speed steel must have a fine-grained structure, consisting of alloyed martensite (about 50%), high alloyed retained austenite (about 30%) and complex carbides (about 20%). For further conversion of residual austenite into the secondary, more alloyed martensite immediately after quenching produced multiple vacation. Latest is for tools of high speed steels of normal performance at the temperature of 560-570° C aging for 60-75 minutes during heating in the salt bath and aged for 60 minutes while heating in a furnace. This is followed by a cooling of the tool in air to room temperature. During exposure of the residual austenite stand out the fine carbides, with the result that the depleted austenite with carbon and alloying impurities and becomes less stable. While the transformation of residual austenite into the secondary, more alloyed martensite increases the hardness of steel to HRC = 63-65. Such leave for steel R18 is recommended 2-3 times, and for steel P9—3—4 times. In addition, tools are subjected to isothermal incomplete or intermittent sakamkam, as well as all the major tools with a diameter greater than 80 mm are subjected to sin, a four-time vacation.

Number of holidays can be reduced to one, if just hardened high-speed steel subjected to a single release, and then placed in the environment with sub-zero temperatures (-75—120° C). Thus there is a more intensive process of further transformation of the residual austenite into the secondary martensite.

the Increase in the tempering temperature of high speed steels conventional performance up to 580-600° C is known to reduce the hardness and wear resistance, but little increases the strength and the vacation at a temperature below 550е difficult enough With a full transformation of austenite. On the contrary, became enhanced performance to obtain a hardness of HRC = 65-66 should be released at a higher temperature (575-585° C). This provides better cutting properties of the instruments used mainly in finishing operations. For instruments as complex forms on these steels used in the removal of chips large cross-section, the release is performed at even higher temperatures (580-590′ S).

a Significant increase in the resistance of high speed tool can also provide additional release of the tool after grinding, if the tool is not subjected to such chemical and thermal methods of refining, as cyanidation and processing of hot steam.

Editing tools from high speed steels in contrast to tools from other steels is best done immediately after hardening or when heated, and hardened steel up to 300-350° C, i.e. in conditions where the steel has low resistance to plastic deformation. In this regard, after the holiday is done when necessary, just an edit.

you Should refer to some feature of the hardening of the weld tool during heating in the hardening should take in the bath to the upper zone of the working part of the tool was above the level of salt in the bath, because the tool due to the heat warms up even on some part of its length, projecting above the surface of the bath. In this regard, in order to avoid a strong heating of the weld is usually located above the working part by an amount approximately equal to the diameter of the working part.

Heating for hardening by high frequency currents (HFC) is a more powerful process than with heating in a furnace and salts. The speed of induction heating is 20 to 1000 deg/sec (sometimes longer), while the heating rate in the molten salts does not exceed 10, and the furnace is 0.8 deg/s. However, the quality of heat treatment of tools made of high speed steel, heated by high frequency currents depends not only on temperature but also on the speed. It is known that increasing the heating rate shifts the start and finish of phase transformations to higher temperatures. In addition, with the increase of heating rate temperature range hardening, providing the highest hardness of the tool also shifts to higher temperatures because of a change of time of completion of processes of formation of austenite and dissolution of carbides. Although induction deformation of the entire product is less than the volumetric hardening with heating in a furnace or salt, the projecting parts of the tool can get very large distortions. Besides the increase of the heating rate HDTV also reduces the thickness of the hardened layer, creates a more abrupt change in hardness over the cross section and increases stress and deformation. Therefore, to obtain higher performance properties of the cu-moorebank tool you need to very accurately set the basic parameters of high-frequency quenching: temperature and heating rate. When you select the speed of heating is necessary to consider the required thickness of the hardened layer depending 01 size, shape and operation instrument.

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