Carbon steel U8 after quenching and tempering. Take a look at the transformations that occur under the selected heat treatment modes and the final structure

Tool steel U8A

Tool steel U8A is used for the manufacture of blades and knives.
It has high hardness and strength, which allows it to be used for the production of cutting tools, including household knives (according to GOST R 51015-97). The advantages of the metal include high strength, which is due to the heat treatment of the alloy. This ensures that the cutting edge remains sharp for several months, as well as the material’s resistance to mechanical stress. Threading the blade is quite simple; at home, abrasive wheels and bars are used for this.

The disadvantage of U8A steel is its low corrosion resistance, which leads to the need for constant maintenance of the product. After use, the surface of the knife is wiped with a dry cloth, and in some cases coated with special oil. The characteristics of a carbon alloy are determined based on its chemical composition. The knife remains usable unless the blade overheats.

U8 steel - pros and cons of using it for knives

For the production of bladed weapons, many steels are used, including U8, which, due to its characteristics, has both advantages and disadvantages for the manufacture of knives.

Decoding steel U8

U8 includes the following components:

• iron up to 97%;
• carbon up to 0.83%.

Chemical composition

According to regulatory documents, the metal contains less than 0.83% carbon and 97% iron, the rest is manganese, nickel, etc.

The material belongs to the eutectoid group, i.e. the amount of pure carbon remains constant during the transition from the pearlite cementite state to the final structure, and secondary carbides are not formed. Heat treatment of U8A tool steel is quite difficult. The hardening temperature differs from that of traditional materials, which negatively affects the strength and toughness of the finished product. Before hardening, the product is heated, which improves the structure of the metal. The latter is characterized by low hardenability, so it is necessary to carefully monitor the timing of all manipulations during heat treatment.

The austenitic state of the alloy is maintained at 740-720 C, the pearlitic state at 700 C, and the martensitic transformation occurs at 810-245 C.

Steel grade U8A has a homogeneous structure and can be forged. A unique feature is the presence of this alloy in the composition of Damascus steel.

Induction installation

The HDTV induction heat treatment unit is a high-frequency generator and inductor for HDTV hardening. The part to be hardened can be located in or near the inductor. The inductor is made in the form of a coil, with a copper tube wound on it. It can have any shape depending on the shape and size of the part. When alternating current passes through the inductor, an alternating electromagnetic field appears in it, passing through the part. This electromagnetic field causes eddy currents known as Foucault currents to occur in the workpiece. Such eddy currents, passing through layers of metal, heat it to a high temperature.

HDTV induction heater

A distinctive feature of induction heating using HDTV is the passage of eddy currents on the surface of the heated part. This way, only the outer layer of the metal is heated, and the higher the frequency of the current, the smaller the depth of heating, and, accordingly, the depth of hardening of the high-frequency frequency. This makes it possible to harden only the surface of the workpiece, leaving the inner layer soft and tough to avoid excessive brittleness. Moreover, you can adjust the depth of the hardened layer by changing the current parameters.

The increased frequency of the current allows you to concentrate a large amount of heat in a small area, which increases the heating rate to several hundred degrees per second. Such a high heating rate moves the phase transition to a higher temperature zone. In this case, the hardness increases by 2-4 units, to 58-62 HRC, which cannot be achieved with volumetric hardening.

For the correct implementation of the HDTV hardening process, it is necessary to ensure that the same clearance is maintained between the inductor and the workpiece over the entire hardening surface, and mutual touching must be avoided. This is ensured, if possible, by rotating the workpiece in the centers, which allows for uniform heating, and, as a consequence, the same structure and hardness of the surface of the hardened workpiece.

The inductor for hardening HDTV has several versions:

• single- or multi-turn annular - for heating the outer or inner surface of parts in the form of bodies of rotation - shafts, wheels or holes in them;
• loop - for heating the working plane of the product, for example, the surface of the bed or the working edge of the tool;
• shaped - for heating parts of complex or irregular shape, for example, the teeth of gear wheels.

Depending on the shape, size and depth of the hardening layer, the following HDTV hardening modes are used:

• simultaneous - the entire surface of the workpiece or a certain zone is heated at once, then also cooled simultaneously;
• continuous-sequential - one zone of a part is heated, then when the inductor or part is displaced, another zone is heated, while the previous one is cooled.

Simultaneous high-frequency heating of the entire surface requires large amounts of power, so it is more profitable to use it for hardening small parts - rolls, bushings, pins, as well as part elements - holes, necks, etc. After heating, the part is completely lowered into a tank with coolant or sprayed with a stream of water.

Continuous-sequential hardening of high-frequency particles allows you to harden large-sized parts, for example, the crowns of gear wheels, since during this process a small zone of the part is heated, which requires less power of the high-frequency generator.

Application

The metal is used to produce working elements of dies necessary for cold stamping of sheets. It is used to produce knives for cold heading machines, clamps, stops, metal-cutting equipment and tools (taps, dies, etc.). The scope of application allows it to be used for the manufacture of cross-cutters, chisels, and files.

Specifications

The main technical characteristics of U8A steel include:

The material does not become brittle after tempering, is not sensitive to flakes, and is not subject to welding.

The price of a kilogram is at least 50 rubles. depending on the quality of U8A steel.

U8 steel, description of properties and hardening mode, heat treatment

Interpretation of steel grade U8: the letter U indicates that this is a high-quality non-alloy tool steel, in which carbon is present in an average amount of 0.8%.

Tools made of U8 steel and its heat treatment: metalworking hammers are made from U7 and U8 steels. The striker and tail are hardened. Heating is best done in a salt or lead bath. When heating a hammer in a chamber furnace, the head is first hardened, and then the tail, after which they are cooled alternately until the middle part is completely darkened. For final cooling, the hammer is transferred to oil. Release at 260-340° for 30-40 minutes. Hardness Rc = 49 -56. Check the hardness using the RV device.

For the manufacture of pneumatic tools, U8 or U7 steel is used. The working tail section is subjected to hardening. Heating the pneumatic tool completely should be avoided, so it is best to heat it in salt or lead baths. The working part is quenched in water and transferred to oil, and the tail part is quenched in oil. After this, the tool is released depending on the required hardness of the working part, namely: chisels, crosscuts, punches, coins and notches are released at 240-270° with a holding time of 30 - 40 minutes. Required hardness Rc = 56-59.

Crimpers, supports, strikers and drifts are released at 270-300° for 30-40 minutes. Required hardness Rc = 53-56. Determine the hardness with a calibrated file. It is not uncommon for a pneumatic tool to break at the point of transition from a smaller diameter to a larger one during operation, and the fracture structure to a depth of 5-8 mm around the circumference is very fine-grained, and deeper, coarse-grained. The main cause of breakdowns is insufficient cleanliness of the surface at the transition points (risks, scratches, etc.).

Machine chisels are made from steels U8, U9, 65X. The transition point from the thin part of the chisel to the thick one, as well as the walls of the hole in the hollow chisel, should be hardened to a low hardness. Failure to comply with this may cause the bit to bend or break during operation. Obtaining a slight hardness of the transition part is achieved by intermittent quenching in water for solid carbon steel bits or by complete quenching followed by tempering in a salt bath to a gray tarnish for all other bits. The tail part is not hardened. Solid chisels are tempered at a temperature of 260-280°, and hollow ones at 320-360°; stand for 20-30 minutes. The required hardness for rough bits is Rc = 56-58, and for hollow bits Rc = 50-52.

Chisels and chisels for carpentry and carpentry are made from the same steels. Heating for hardening is carried out in bath furnaces to a length of 60-80 mm. When heated in chamber ovens, the tool is soaked to a length of 60-80 mm. The tail part is not hardened. Release at a temperature of 250-300° for 20-40 minutes. Required hardness Rc = 53-58.

Combined pliers and wire cutters are made from U7 and U8 steels. They are subjected to heat treatment in assembled form with the jaws open, only the working part is heated - in pliers the jaws are 8-10 mm long, and in combined pliers the jaws are heated for the length, including the slots at the hinge. Cool in oil or kerosene with vigorous stirring. Tempering is carried out at a temperature of 220-300° for 30-40 minutes. Hardness Rc = 52-60. Hardness is controlled using a PB device or a calibrated file, as well as by biting off a steel wire with a diameter of 2 mm.

The stamps are made from the above-mentioned steels, hardened and then tempered at a temperature of 220-250°. The shank is tempered by heating in a lead bath until it turns gray. Required hardness of the working part Rc = 54-58.

Annealing

The annealing features are established based on:

• heating principle;
• processing temperatures;
• part dimensions;
• the principle of placing parts in the oven.

The greatest efficiency of heat treatment of U8A steel is ensured when the workpieces are laid in one layer on thermally insulated asbestos sheets. The gap between the parts must exceed three times the size of the largest section parameter. In this case, the duration of heating to 1000-1200 C is:

• 5.5 minutes – for a section of 2 cm;
• 9 minutes – 3 cm;
• 10.5 minutes – 4 cm;
• 13.5 minutes – 5 cm;
• 16.5 minutes – 7.5 cm;
• 22 minutes – 10 cm.

When heated for a long time, a large amount of carbon is formed on the surface of the metal. Therefore, heat treatment of U8A steel is traditionally carried out in furnaces with controlled pressure or in an inert gas environment (CO2 or Ar). In other cases, the heating rate is reduced by 15-20%.

Hardening

If all the rules for hardening U8A steel are followed, its strength is ensured at 59-62 HRC and the martensitic-austenitic structure is preserved. Requirements for the procedure:

1. Temperature 800-820 C.
2. Preliminary and final heating of parts is the same in duration and is carried out at 400-500 C.
3. The duration of heating depends on the dimensions and area of ​​the outer surface of the product. It is necessary to soak the product in molten salts for 8-14 minutes, in salt water for 15-30 minutes. When hardening thin objects of considerable length, this indicator may change.
4. Cooling a product made from U8A steel grade in water at 18-25 C. When this value decreases, there is a risk of cracks; when it increases, there is an uneven distribution of hardness throughout the volume of the metal. Similar consequences can occur in the presence of mineral and organic residues in the hardening environment.
5. No processes in air.

Physical properties of U8A steel

Vacation

Tempering of U8A steel is the next stage after hardening. This process is designed to complete the martensitic transformation, reduce internal stresses and increase the toughness of the central part of the part.

Tempering is carried out at 140-200 C for 2-3 minutes, which ensures hardness. For the measuring instrument, it is planned to reduce the temperature to 90-180 C, as well as to carry out a repeat procedure after sharpening and grinding (300-350 C for 1.5-2 hours) with cooling under natural conditions.

The essence of the process

The normalization procedure is as follows. The part is heated to temperatures that exceed the maximum permissible parameters (Ac1, Ac3) by 30 - 50 degrees Celsius, then it is kept at this temperature for some time, after which it is cooled.

The temperature is selected based on the steel grade. Thus, alloys containing 0.8% carbon, so-called hypereutectoid, are processed at temperatures lying between the critical points Ac1 and Ac3.

What are critical points? This is the name given to the temperatures at which phase changes and structure of the alloy occur when it is heated or cooled.

The result of this is that a certain volume of carbon enters the solid solution and austenite is fixed. That is, a structure consisting of martensite and cementite appears. It is cementite that leads to an increase in wear resistance and hardness. Heating high-carbon steel above ac3 leads to an increase in internal stresses. This is due to the fact that the amount of austenite increases due to an increase in carbon concentration.

When heated above the critical point Ac3, steel with a carbon content of less than 0.8% acquires increased viscosity. This happens because in steel of this type austenite (fine-grained) appears, turning into martensite (fine-grained).

Hypoeutectoid steel is not processed at temperatures in the range Ac1 - Ac3. Since in this case ferrite appears, which reduces the hardness parameters.

Time required to complete the operation

It takes some time to obtain a homogeneous structure of the alloy at a certain temperature. This time will be determined as the holding time of steel during normalization. It was experimentally determined that a layer of metal 25 mm thick becomes homogeneous after an hour. Thus. and determine the normalization time.

The final stage is cooling

The cooling rate plays a significant role in the formation of perlite volume and the size of its plates. Numerous studies have shown that high cooling rates increase the amount of perlite and the steel gains increased hardness and strength. Low cooling intensity leads to steel losing hardness and strength.

When processing parts with significant differences in size, for example. shafts, it is advisable to remove stresses arising under the influence of temperature fluctuations. To do this, they are preheated in a container filled with different salts. When the temperature drops, it is possible to speed up this process by placing hot parts in water or specially selected oil.

In other words, steel normalization eliminates stress inside the part and minimizes its structure. That is, it has a direct effect on changes in the microstructure of steel alloys.

The purpose of steel normalization

The purposes of steel normalization can be different: for example, to both increase and decrease strength and hardness, depending on the thermal and mechanical history of the product.

The purposes of normalization often overlap or even get confused with annealing, heat hardening, and stress-relieving tempering. Normalization is used, for example, to improve the machinability of a part by cutting, refine the grain, homogenize the grain structure, or reduce residual stresses. A comparison of temperature-time cycles for normalization and annealing is shown in Figure 2.

Figure 2 ─ Comparison of temperature-time cycles of normalization and full annealing. Slower cooling during annealing results in a higher ferrite-pearlite transformation temperature and a coarser microstructure than normalization.

For steel castings, normalization is used to homogenize their dendritic structure, reduce residual stresses and make them more susceptible to subsequent thermal hardening.

Products obtained by pressure treatment can be normalized to reduce banding of the structure after rolling or different grain sizes after forging.

Normalization followed by tempering is used instead of conventional hardening when products have a complex shape or sudden changes in cross-section. This is done to avoid cracking, warping and excessive thermal stress.

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Hardening of U8 steel

Carbon tool steel U8 GOST 1435 is a popular material for the manufacture of working parts of dies for cold sheet stamping of highly plastic metals, stops, clamps, cutting knives of cold heading machines. In addition, some types of metal-cutting equipment are produced from this steel, in particular, taps and dies. Hand tools – files, chisels, crosscutters, etc. – can also be made of U8 steel.

Disadvantages of U10A alloy

The knife, cast from U10A steel and hardened to 52 hrc, has gained fame as the sharpest; the blade retains its sharpness for a long time, but is afraid of blows. The high hardness of the material makes it difficult to sharpen blades in the field. An excellent solution to the problem would be a combination of U10A alloy in a package with 7ХНМ tool steel. This composite manufacturing technology has been mastered by ZZOSS. The final product has qualities typical of Damascus knives. The only drawback of the U10A-7KhNM alloy is its susceptibility to corrosion. Timely care will prolong the useful properties of the knife.

Characteristics and properties

Having in its chemical composition 0.75...0.85% carbon, as well as a small amount of other elements - silicon, manganese, chromium, nickel and copper - U8 steel is ectectoid. With reduced manganese and silicon contents, the critical cooling rate always increases. Therefore, in practice, U8 steel is used only for the manufacture of metalworking tools with small overall dimensions. Hardening of steel of this type allows the use of very harsh cooling media (water or aqueous solutions of salts). Thus, this steel is not hardenable: the core remains viscous, and the hardness obtained as a result of preliminary annealing of the workpieces remains almost the same.

Decoding the alloy

When considering how steel decoding is carried out, we note that different designation standards are used in different countries. The designation of material U8 or U8a is carried out in accordance with GOST standards:

1. The letter “U” indicates that the material belongs to the group of tool steels. This symbol does not indicate any component or property.
2. The next number in decimal form indicates the concentration of the main component - carbon. In the case under consideration, the concentration is 0.8%.
3. If you decipher U8a, you should take into account that the letter “A” indicates increased quality, which is achieved by eliminating various harmful impurities from the composition or reducing their concentration.

There are no other designations of substances in the labeling, but the chemical composition contains a fairly large number of different impurities. Examples include silicon and manganese. In addition, there are harmful impurities, for example, phosphorus and sulfur, the concentration of which determines the quality of steel. With an increase in the concentration of harmful impurities, the performance qualities of the material deteriorate, strength and hardness are lost.

General characteristics of hardenability

The hardenability factor for unalloyed tool steels is considered very important. High operating speeds of tools, ranging from automatic presses that operate in a continuous cycle, to thread-cutting tools, require a combination of high surface hardness with sufficient core viscosity. Otherwise, the working edges of the tooling quickly become chipped, and the tool loses its accuracy.

The intensity of hardenability depends on the total number of impurities, which for U8 steel should not exceed the following limit values:

Therefore, when purchasing large quantities of U8 steel, they always pay attention to the presence and content of certificates of conformity, which must contain data on harmful impurities.

Temperature of the workpiece depending on the color when heated

The percentage of carbon also has a significant influence on the toughness of the core of the unalloyed steel, and therefore on the wear resistance of the surface layer. From this point of view, U8 steel is considered tough. Therefore, special requirements for the accuracy of compliance with heat treatment regimes are not required here.

The manufacture of tooling from U8 steel (even small-sized ones) is made difficult by the inherently low ductility of the material.

In its initial state, steel grade U8 can be supplied in the following types of assortment according to GOST 5210:

1. rolled wide strip. The strip width is 12...48 mm, with a thickness of 3...10 mm;
2. round rod with a diameter of 4...18 mm;
3. square profile with side sizes from 4×4 to 18×18 mm;
4. special profiles.

Annealing of metal from the specified range is not performed. For other types of supplies, as well as for hot forging of ingots, annealing is required.

Rules of care

To minimize the development of corrosion processes, it is necessary to properly care for the knife after each use. The following rules must be observed.

Wipe dry after use and washing. Treat with machine oil or a special protective agent. The use of autopropylene has a good effect: it creates a film that protects the material from moisture and dirt. If you don’t have the product, you can use vegetable oil; clove oil or even sunflower oil will do.

It will help maintain cleanliness, but will not create a protective layer. It is important to ensure that U8 steel does not react with acids. Otherwise, a gray coating will appear on the surface.

Knives made of U8 steel will be ideal helpers in the household, and they will also come in handy in field conditions. Durability, sharpness retention and strength are the main advantages of this material. The only disadvantage we can note is the need for constant cleaning of the weapon.

Annealing production technology

The annealing mode of U8 steel is determined by the following factors:

• the method of laying workpieces under a thermal furnace;
• the ratio of the height and thickness of the workpieces;
• heating temperature;
• type of heating furnace.

It has been experimentally established that the most effective annealing mode is laying the workpieces in one layer on heat-insulating asbestos supports, with a distance between adjacent workpieces of at least 3D (D should be understood as the maximum overall dimension of the section in plan). Then to heat to the required temperature (1000...1200 ° C) you will need:

• for cross sections up to 20 mm – 5…6 min;
• for cross sections up to 30 mm – 8…10 min;
• for cross sections up to 40 mm – 9…12 min;
• for cross sections up to 50 mm – 12…15 min;
• for cross sections up to 75 mm – 15…18 min;
• for cross sections up to 100 mm – 19…25 min;

Hardening

If the hardening technology is followed, then the final hardness of the products after heat treatment should be within 59...62 HRC. To fulfill this condition and maintain the required structure (martensite + austenite), you must adhere to the following recommendations:

1. Hardening processes proceed in full if they begin at 800...820 °C.
2. The ratio of the time of preliminary and final heating of the tool for hardening should be the same, and be in the temperature range of 400...500 °C.
3. The exact heating time is usually calculated depending on the surface area of ​​the tool and its volume. This is especially important when heating workpieces in molten salts: for melts it should be 8...14 minutes, for aqueous solutions - 15...30 minutes (increased standards are applied for tools with sharply different longitudinal and transverse dimensions).
4. Cooling of the tool after hardening is carried out in water, the temperature of which (regardless of the time of year and the temperature in the thermal compartment) should be within 18...25 °C. At lower temperatures, the risk of cracking of products increases, and at higher temperatures, the hardness of the tool is uneven. The same defect is possible when the quenching medium is contaminated with mineral and organic residues.
5. Hardening of U8 steel in air is impossible.

After hardening, the products are released. In this case, the martensitic transformation occurs fully, internal stresses are reduced, and the viscosity of the core increases. The tempering temperature of U8 steel after hardening is 140...200 °C: it is after these temperatures that the final product will retain sufficient hardness and will have a sufficiently viscous core. The holding time is taken within 120...200 s; for the measuring instrument, the temperature can be further reduced by 20...50 °C.

Sometimes, after sharpening and grinding a tool made of U8 steel (mainly a measuring tool), additional tempering is carried out. In this case, the temperature is 300...350 °C, and the holding time is 1.5...2 hours, followed by cooling of the part in air.

Source

Cooling parts

Cooling is the second important stage of the hardening process; the quality and hardness of the entire surface depends on its speed and uniformity. Cooling occurs in coolant tanks or by spray. For high-quality hardening, it is necessary to maintain a stable temperature of the coolant and prevent it from overheating. The holes in the sprayer must be of the same diameter and spaced evenly, this way the same metal structure on the surface is achieved.

To prevent the inductor from overheating during operation, water is constantly circulated through the copper tube. Some inductors are made combined with a workpiece cooling system. Holes are cut in the inductor tube through which cold water enters the hot part and cools it.

Hardening with high frequency currents