Hardening called the operation of heat treatment, consisting of heating to temperatures above the upper critical point A C 3 for hypoeutectoid steel and above the lower critical point A C1

for hypereutectoid steel and holding at a given temperature, followed by rapid cooling (in water, oil, aqueous solutions of salts, etc.).

As a result of hardening, the steel acquires a martensite structure and thereby becomes hard.

Hardening increases the strength of structural steels, gives hardness and wear resistance to tool steels.

Quenching modes are determined by the rate and temperature of heating, the duration of exposure at this temperature and especially the rate of cooling.

Selection of hardening temperature.

The heating temperature of steel for hardening depends mainly on the chemical composition of the steel. When hardening hypoeutectoid steels, heating should be carried out to a temperature of 30-50 ° above the point A C3. In this case, the steel has a structure of homogeneous austenite, which, upon subsequent cooling at a rate exceeding the critical quenching rate, turns into martensite. This hardening is called complete ... When the hypoeutectoid steel is heated to temperatures A C 1 - A C 3, a certain amount of ferrite remaining after quenching is retained in the martensite structure, which reduces the hardness of the hardened steel. This hardening is called incomplete.

For hypereutectoid steel, the best hardening temperature is 20-30 ° above A C1, that is, incomplete hardening. In this case, the preservation of cementite during heating and cooling will increase the hardness, since the hardness of cementite is greater than the hardness of martensite. It is not necessary to heat hypereutectoid steel to a temperature higher than A st, since the hardness is lower than when quenching from a temperature above A C1, due to the dissolution of cementite and an increase in the amount of retained austenite. In addition, when cooling from higher temperatures, higher internal stresses can arise.

Cooling rate.

To obtain the structure of martensite, it is required to supercool austenite by rapid cooling of steel, which is at the temperature of the lowest stability of austenite, i.e., at 650-550 ° C.

In the temperature zone of martensitic transformation, i.e., below 240 ° C, on the contrary, it is more advantageous to use delayed cooling, since the resulting structural stresses have time to equalize, and the hardness of the formed martensite practically does not decrease.

The correct selection of quench media is essential for a successful heat treatment.

The most common quenching media are water, 5-10% aqueous solution of caustic soda or sodium chloride, and mineral oil. For hardening carbon steels, water with a temperature of 18 ° C can be recommended; and for hardening most alloy steels - oil.

The hardenability and hardenability of steel.

When hardening steel, it is important to know its hardenability and hardenability. These characteristics should not be mixed.

Hardenability shows the ability of steel to increase the hardness during quenching. Some steels have poor hardenability, that is, they have insufficient hardness after hardening. Such steels are said to "do not accept" quenching.

The hardenability of steel depends mainly on its carbon content. This is due to the fact that the hardness of martensite depends on the degree of distortion of its crystal lattice. The less carbon is in martensite, the less will be distorted its crystal lattice and, consequently, the lower will be the hardness of steel.

Hardenability steel is characterized by its ability to be hardened to a certain depth. During hardening, the surface of the part cools faster, since it is directly - nasal contact with coolant that removes heat. The core of the part is cooled much more slowly, heat from the central part of the part is transferred through the mass of metal to the surface, and only on the surface is absorbed by the coolant.

The hardenability of steel depends on the critical quenching speed: the lower the critical speed, the deeper the steel parts are hardened. For example, steel with a coarse natural austenite grain (coarse-grained), which has a low critical quenching rate, is hardened to a greater depth than steel with a fine natural austenite grain (fine-grained), which has a high critical quenching rate. Therefore, coarse-grained steel is used for the manufacture of parts that must have deep or through hardenability, and fine-grained steel is used for parts with a hard surface hardened crust and a tough unhardened core.

The initial structure of the hardened steel, the heating temperature for hardening and the quenching medium also affect the depth of hardenability.

The hardenability of steel can be determined by fracture, microstructure and hardness.

Kinds hardening steel.

There are several hardening methods used depending on the composition of the steel, the nature of the workpiece being processed, the hardness that needs to be obtained, and the cooling conditions.

Hardening in one environment is shown schematically in Fig. 1 as a curve 1 ... Such hardening is easier to perform, but it can be applied not for every steel and not for any parts, since the rapid cooling of parts of variable cross-section in a wide temperature range contributes to the occurrence of temperature irregularities and high internal stresses, which can cause warpage of the part, and sometimes cracking (if the value of internal stresses exceeds the ultimate strength).

The more carbon in steel, the greater the volumetric changes and structural stresses, the greater the risk of cracking.

Figure:1

Hypereutectoid steels are hardened in the same medium if the parts have a simple shape (balls, rollers, etc.). If the parts are of complex shape, either quenching in two environments or step quenching is used.

Quenching in two environments (curve 2) is used for tools made of high-carbon steel (taps, dies, cutters). The essence of the method lies in the fact that the part is first soaked in water, rapidly cooling it to 300-400 ° C, and then transferred to oil, where it is left until it is completely cooled.

Step hardening (curve 3) is performed by rapidly cooling the parts in a salt bath, the temperature of which is much higher than the temperature of the onset of martensitic transformation (240-250 ° C). Holding at this temperature should ensure equalization of temperatures over the entire section of the part. The parts are then cooled to room temperature in oil or in still air, thereby eliminating thermal internal stresses.

Step hardening reduces internal stresses, crunches and the possibility of cracking.

The disadvantage of this type of hardening is that hot traces cannot provide a high cooling rate at a temperature of 400-600 ° C. In this regard, step hardening can be used for parts made of carbon steel with a small section (up to 8-10 mm). For alloy steels with a low critical quenching rate, step quenching is applicable to parts with a large section (up to 30 mm).

Isothermal hardening (curve 4) is carried out in the same way as stepwise, but with a longer exposure at the temperature of the hot bath (250-300 ° C) in order to ensure complete decomposition of austenite. The shutter speed required for complete decomposition of austenite is determined from points a and b and from the S-shaped curve (see Fig. 1). As a result of such hardening, the steel acquires the structure of acicular troostite with a hardness of HRC45 55 and retaining the necessary plasticity. After isothermal hardening, the steel can be cooled at any speed. As a cooling medium, molten salts are used: 55% KNO 3 + 45% NaNO 2 (melting temperature 137 ° C) and 55% KNO 3 + 45% NaNO 3 (melting temperature 218 ° C), allowing overheating to the required tempera-tours.

Isothermal hardening has the following advantages over conventional hardening:

minimal distortion of steel and no cracks; high toughness of steel.

At present, stepwise and isothermal light hardening is widely used.

Bright hardening steel parts are carried out in specially equipped furnaces with a protective environment. At some instrumental factories, to obtain a clean and light surface of hardened tools, step quenching with cooling in molten caustic alkali is used. Before quenching, the tool is heated in a salt bath of sodium chloride at a temperature of 30-50 ° C above the A C1 point and cooled at 180-200 ° C in a bath consisting of a mixture of 75% caustic potassium and 25% caustic soda with the addition of 6-8% water (based on the weight of all salt). The mixture has a melting point of about 145 ° C and, due to the fact that it contains water, has a very high quenching ability.

When step hardening steels with overcooling of austenite in molten caustic alkali, followed by final cooling in air, the parts acquire a clean, light, silvery-white surface; in this case, there is no need for sandblasting the parts and rinsing them in hot water is sufficient.

Hardening self-leave widely used in tool making. Its essence lies in the fact that the parts are not kept in a cooling medium until they are completely cooled, but at a certain moment they are extracted from it in order to keep a certain amount of heat in the core of the product, due to which the subsequent tempering is carried out. After reaching the required tempering temperature due to internal heat, the part is finally cooled in a quenching liquid.

The tempering can be controlled by the tarnishing colors (see Fig. 2) that appear on the cleaned steel surface at 220-330 ° C.

Figure: 2.Cveta temper on vacation

Self-tempering hardening is used for chisels, sledgehammers, bench hammers, center punch and other tools that require high surface hardness and preservation of a viscous core.

Cooling methods during quenching.

Rapid cooling of steel parts during hardening causes high internal stresses in them. These stresses sometimes lead to buckling of parts and, in the most severe cases, to cracks. Especially large and dangerous internal stresses arise when cooling in water. Therefore, where possible, you should cool the parts in oil. However, in most cases for parts made of carbon steel, this is not possible, since the cooling rate in oil is much lower than the critical rate required for the transformation of austenite to martensite. Therefore, many parts made of carbon steels are recommended to be quenched with cooling in water, but at the same time to reduce the inevitable internal stresses. For this, some of the described hardening methods are used, in particular, quenching in two environments, self-tempering hardening, etc.

Internal stresses also depend on how the parts are immersed in the hardening medium. You must adhere to the following basic rules:

parts with thick and thin parts should be immersed in the quenching medium first with the thick part;

parts with a long elongated shape (taps, reamer drills) must be immersed in a strictly vertical position, otherwise they will warp (Fig. 3).

Figure: 3. Correct immersion of parts and tools in the heating medium

Sometimes, according to the working conditions, not the whole part should be hardened, but only part of it. In this case, local quenching is used: the part is not completely heated, but the whole is immersed in the quenching medium. In this case, only the heated part of the part is hardened.

Local heating of small parts is carried out in a salt bath, immersing only that part of the part that needs to be hardened into it; this is how, for example, the centers of lathes are hardened. You can also step like this: heat the part completely, and cool only that part that needs to be hardened in a quenching medium.

Defects arising from steel hardening.

Insufficient hardness hardened part - a consequence of low heating temperature, short exposure at operating temperature or insufficient cooling rate.

Defect fix : normalization or annealing followed by quenching; the use of a more vigorous quenching medium.

Overheat associated with heating the product to a temperature significantly higher than the required heating temperature for hardening. Overheating is accompanied by the formation of a coarse-grained structure, as a result of which the brittleness of the steel increases.

And defect coping: annealing (normalization) and subsequent hardening from the required temperature.

Overburn arises when steel is heated to very high temperatures, close to the melting point (1200-1300 ° C) in an oxidizing atmosphere. Oxygen penetrates into the steel and oxides form along the grain boundaries. Such steel is fragile and cannot be fixed.

Oxidation and decarburization steels are characterized by the formation of scale (oxides) on the surface of parts and burnout of carbon in the surface layers. This type of defect by heat treatment is incorrigible. If the machining allowance allows, the oxidized and decarburized layer must be removed by grinding. To prevent this type of rejection, it is recommended to heat the parts in ovens with a protective atmosphere.

Warping and cracks - consequences of internal stresses. During heating and cooling of steel, volumetric changes are observed, depending on temperature and structural transformations (the transition of austenite to martensite is accompanied by an increase in volume up to 3%). The difference in time of transformation in the volume of the part to be hardened due to its different sizes and cooling rates across the section leads to the development of strong internal stresses, which cause cracks and warpage of parts during hardening.

Cracking is usually observed at temperatures below 75-100 ° C, when the martensitic transformation covers a significant part of the steel volume. To prevent the formation of cracks, when designing parts, it is necessary to avoid sharp protrusions, pointed corners, sharp transitions from thin sections to thick ones; it is also necessary to slowly cool the steel in the zone of martensite formation (quenching in oil, in two environments, step quenching). Cracks are an irreparable marriage, while warpage can be eliminated by subsequent straightening or straightening.

6. Determination of hardness by the Brinell method (see Lr # 1).

7. Determination of hardness by the Rockwell method (see Lr No. 2).

8. Concept of alloy, component, phase, system.

9. Diagram of the state of the double alloy "lead-antimony".

10. Diagram of the state of iron-carbon alloys of the "iron-cementite" system

11. Structural constituents of iron-carbon alloys.

12. Raw materials and products of blast-furnace smelting.

13. Blast furnace, its structure and work.

14. Getting steel in oxygen converters.

15. White cast irons, their scope.

16. Gray cast irons, their marking and scope.

17. Ductile cast irons, their marking and scope.

18. Malleable cast irons, their marking and scope.

19. Carbon structural quality steels, marking and scope.

20. Carbon tool steels, marking and scope.

21. Alloyed steels, their classification and marking.

22. Brass and bronze, their marking and scope.

23. Aluminum alloys, their marking and scope.

24. Corrosion of metals, its types and methods of dealing with it.

25. Antifriction alloys, their marking and scope.

26. Cemented carbide alloys, their marking and scope.

27. Annealing and normalization. Annealing types.

28. Tempering. Types of hardening.

29. Vacation. Types of vacation.

30. Chemical heat treatment, its types.

31. Model kit, its purpose and composition.

32. Casting into multiple (permanent) metal molds (chill molds)

33. Centrifugal casting

34. Casting into shell molds.

35. Precision investment casting

36. The essence of pressure treatment. Plastic deformation of metals.

37. The phenomenon of return and recrystallization.

38. The concept of rolling production. Rolling, its types.

39. Pressing, types of pressing.

40. Drawing, equipment used, products obtained.

41. Forging, types of forging operations, equipment used.

43. Metallurgical processes in welding. Welding stresses and deformations, causes of their occurrence and methods of prevention.

44. Electric arc welding, the essence of the process, the equipment used.

45. Types of electrodes, their coating.

46. \u200b\u200bSubmerged arc and gas-shielded arc welding. Electroslag welding.

47. Initial materials for gas welding.

48. Equipment and accessories for gas welding and cutting.

49. Technology of gas welding and cutting

50. Soldering, the essence of the process. Solders, fluxes, their purpose and composition.

51. The main parts and elements of the cutter.

52. Angles of the incisor.

53. Elements of the cutting mode when turning.

54. The device of the screw-cutting lathe.

55. The device of a horizontal milling machine.

56. The process of drilling and its features.

57. Electric spark processing of metals.

58. Thermosetting plastics, their types, composition and application.

59. Composition and classification of paints and varnishes.

60. Composition and classification of adhesive materials.

61. General information about rubber. Rubber compounds, their composition.

62. General information about wood, its physical and mechanical properties.

63. Varieties of wood materials

64. Lining materials.

28. Tempering. Types of hardening.

Hardening - heating the steel above the temperature of phase transformations, followed by cooling according to a certain mode to obtain the desired structure and increase the hardness and strength.

The process of hardening steel consists in heating it to a certain temperature (30 ... 50 ° above the GSK line according to the Fe -Fe 3 C diagram), holding and subsequent rapid cooling in water, oil, molten salts or other media.

Hypoeutectoid steels must be heated approximately 30 ... 50 ° above the critical point A c3 (line GS): tzak \u003d А с3 + 30 ... 50 ° С

Hypereutectoid steels should be heated for hardening above A c1 (SK line) by 30 ... 50 °.

Oils have a cooling rate in the range of martensitic transformation 10 times lower than water, which reduces the possibility of defects during quenching.

There are the following types of hardening:

Quenching in one cooler- the most common - a product heated to a quenching temperature is immersed in a cooling medium until it is completely cooled. (carbon steels in water and alloy steels in oil). This method is simple, but it can cause significant internal stress.

Intermittent quenching (quenching in two environments)used to prevent the appearance of internal stresses in the product. This method is used for hardening large items made of structural carbon and low-alloy steel. The product heated to the required temperature is first sharply cooled in water to 300 ... 200 ° C, then transferred into oil or air, where it is slowly cooled. The disadvantage is the difficulty of adjusting the holding time.

Step hardening- the heated product is cooled by immersion in a salt bath, the temperature of which exceeds the temperature of the beginning of the martensitic transformation of this steel. Then the product is kept in a bath to equalize the temperature throughout its volume and is cooled in air to normal temperature, which reduces internal stresses. It is used for thin carbon steel products.

Self-tempering (tempering tempering)consists in the fact that the product is cooled from the quenching temperature in a cooling medium only for the time that is necessary for its calcination to a certain depth. Further cooling takes place in air. In this case, tempering is carried out due to heat transfer from the inner layers of the product. This method is used for hardening percussion tools (chisels, forging tools, etc.).

Surface hardeningit is used to increase the wear resistance, hardness and strength of parts that receive shock loads (gear wheels, shafts, etc.). It includes heating the surface layer of the product to the hardening temperature and cooling to obtain a martensite structure in the surface layer while maintaining a viscous core.

There are the following types of heating during surface hardening: heating by a gas burner flame and heating by high frequency currents.

29. Vacation. Types of vacation.

Vacation - this is the heating of hardened steel to a temperature below the critical A c1, holding at this temperature and subsequent cooling (usually in air).

There are the following types of leave: low, medium, high.

Low vacation- heating of hardened steel to 250 ° C to reduce internal stresses while maintaining high hardness. It is used for tools and products that must have high hardness and wear resistance. The resulting structure is tempered martensite.

Average vacation- heating of hardened steel to 350 ... 450 ° С, which leads to a decrease in hardness and an increase in steel toughness in comparison with low tempering. The resulting microstructure is troostite. It is used for springs, stamps, springs, impact tools, etc.

High vacation- heating of hardened steel to 450 ... 650 ° C, which contributes to obtaining the highest toughness while maintaining a sufficiently high strength. The hardness of the hardened steel is greatly reduced and a sorbitol structure is formed. The hardening of machine parts for martensite followed by high tempering for sorbitol is called improvement. Tempering sorbitol with a granular form of cementite has higher strength and viscosity indices than hardened sorbitol with a plate form of cementite.

Cold treatment- consists in the treatment of hardened products with cold at temperatures of the order of - 80 ° C and below. Cold treatment is based on the fact that retained austenite in the structure of hardened steel at low temperatures decomposes as a result of internal stress. This method increases the hardness of the cutting tool, stabilizes the dimensions of measuring instruments, etc. In industry, special installations are used in which liquid oxygen (-183 ° C), liquid nitrogen (-195 ° C), a mixture of solid carbon dioxide (dry ice) serve as coolers with denatured alcohol (-78.5 ° C).

To give steel certain performance properties, heat treatment has been carried out for many decades. Today, as well as several centuries ago, steel hardening involves heating the metal and then cooling it in a certain environment. The temperature of heating steel for hardening should be selected in accordance with the composition of the metal and the mechanical properties to be obtained. Mistakes in the selection of hardening modes will lead to increased fragility of the structure or softness of the surface layer. That is why we will consider the methods of steel hardening, the features of the technologies used, as well as many other points.

What is the hardening of metal?

The ancient blacksmiths knew what hardening was for. Correctly selected steel quenching temperature allows changing the basic performance characteristics of the material, as the structure is transformed.

Hardening - heat treatment of steel, which is carried out today to improve the mechanical properties of the metal. The process is based on the rearrangement of the atomic lattice by exposure to high temperatures followed by cooling.

Steel hardening technology makes it possible to impart better performance to inexpensive metal grades. Due to this, the cost of manufactured products is reduced, and the profitability of established production increases.

The main goals that are pursued during hardening:

1. Increasing the hardness of the surface layer.
2. Increase in strength index.
3. Reducing the ductility to the required value, which significantly increases the flexural strength.
4. Reducing the weight of products while maintaining strength and hardness

There are various methods of steel hardening followed by tempering, which differ significantly from each other. The most important heating modes are:

1. Heating temperature.
2. The time it takes to heat up.
3. The holding time of the metal at a given temperature.
4. Cooling rate.

The change in the properties of steel during quenching can take place depending on all of the above indicators, but the most significant is the heating temperature. How the atomic lattice rearrangement will take place depends on it. For example, the holding time during hardening of steel is selected in accordance with the strength and hardness of the gear wheel to ensure long-term operation under conditions of increased wear.

When considering which steels are hardened, it should be borne in mind that the heating temperature depends on the level of carbon content and various impurities. The units for hardening steel are represented by the maximum temperature as well as the holding time.

When considering this process of changing the basic performance properties, the following points should be considered:

1. Hardening is aimed at increasing the hardness. However, as the hardness increases, the metal becomes more brittle.
2. A layer of scale can form on the surface, since the loss of carbon and other impurities at the surface layers is greater than in the middle. The thickness of this layer is taken into account when calculating the allowance, the maximum dimensions of future parts.

Quenching of carbon steel is carried out taking into account the speed at which cooling will take place. If the developed technologies are not followed, a situation may arise when the rearranged atomic lattice passes into an intermediate state. This will significantly degrade the basic qualities of the material. For example, cooling at too high a speed causes cracks and various defects that prevent the workpiece from being used in the future.

The process of hardening steels involves the use of chamber furnaces that can heat the medium to a temperature of 800 degrees Celsius and maintain it for a long period. This allows you to extend the steel hardening time and improve the quality of the resulting blanks. Some steels are suitable for hardening only if the medium is heated to a temperature of 1300 degrees Celsius, for which other furnaces are installed.

A separate technology is being developed for the case when the workpiece has thin walls and edges. It is presented by stage-by-stage heating.

Full hardening is commonly used for steels and parts that are not prone to cracking or warping.

Often, the step-by-step heating technology provides for reaching a temperature of 500 degrees Celsius at the first stage, after which a certain period of time is maintained to ensure uniform heating and the temperature is raised to a critical value. Cold hardening of steel does not lead to a rearrangement of the entire atomic grid, which only determines an insignificant increase in operational characteristics.

As previously noted, there are various types of hardening of steel, but it is always necessary to ensure uniform heating. Otherwise, the rearrangement of the atomic lattice will proceed in such a way that serious defects may appear.

Methods for preventing scale formation and critical carbon reduction

The purpose of steel hardening is carried out taking into account what qualities the part should have. The process of rebuilding an atomic mesh is associated with high risks of various defects, which is taken into account at the stage of developing a technological process.

Even the most common methods, for example, water hardening of steel, are characterized by the appearance of scale or a significant increase in the fragility of the structure with a decrease in the carbon concentration. In some cases, steel hardening is carried out after finishing, which does not allow eliminating even minor defects. This is why technologies have been developed that reduce the likelihood of dross or cracking. An example is the technology when steel quenching takes place in a protective gas. However, complex methods of steel hardening significantly increase the cost of the procedure, since the gaseous environment is achieved when installing furnaces with a high degree of tightness.

A simpler technology, in which hardening of carbon steel is carried out, involves the use of cast iron chips or a spent carburizer. In this case, steel for hardening is placed in a container filled with the materials in question, after which heating is only carried out. The hardening temperature is not significantly adjusted taking into account the created chip shell. The technology provides for coating the outside of the container with clay in order to avoid the ingress of oxygen, which starts the oxidation process.

As noted earlier, heat treatment also provides for the cooling of steels, for which not only a water bath, but, for example, a salt bath can also be used. When using acids as a coolant, one of the requirements is periodic deoxidation of steels. This process eliminates the likelihood of a decrease in the carbon concentration in the surface layer. Boric acid or charcoal is used to carry out the deoxidation process. Also, do not forget that the process of deoxidation of steels leads to the appearance of a flame on the workpiece during its lowering into the bathroom. Therefore, when hardening, hardening steels using salt baths, the developed safety precautions should be observed.

Considering these methods of heat treatment with subsequent cooling, it should be noted that they significantly increase the cost of the workpiece. However, today, cooling in water or quenching while filling the chamber with oxygen does not allow increasing the properties of steel without the appearance of defects.

Steel hardening - technological process

Cooling procedure

Considering all types of steel hardening, it should be borne in mind that not only the heating temperature has a strong effect on the structure, but also the holding time, as well as the cooling procedure. For many years, ordinary water has been used to cool steels, which does not contain a large amount of impurities. It should be borne in mind that impurities in the water do not allow complete quenching while maintaining the cooling rate. The optimum temperature of the water used to cool the hardened part is considered to be 30 degrees Celsius. However, it should be borne in mind that the liquid is heated when lowering the hot workpieces. Cold running water cannot be used for cooling.

Usually, water is used when cooling to obtain non-critical parts. This is due to the fact that a change in the atomic grid in this case usually leads to warping and cracking. Quenching followed by cooling in water is carried out in the following cases:

1. When cementing metal.
2. With surface hardening.
3. With a simple blank shape.

Finished parts are not cooled in this way.

To impart the required hardness to workpieces of complex shape, a cooling liquid is used, consisting of caustic soda heated to a temperature of 60 degrees Celsius. It should be borne in mind that the hardened iron becomes lighter when using this coolant. Experts pay attention to the importance of observing safety measures, since toxic substances can be released when the substances in question are heated.

Thin-walled parts are also heat treated. Quenching followed by improper cooling will cause the carbon concentration to drop to critical values. The way out of this situation is the use of mineral oils as a cooling medium. They are used due to the fact that the oil contributes to uniform cooling. However, the ingress of water into the oil will cause cracks. Therefore, the workpieces must be cooled safely using oil.

Considering the purpose of mineral oils as a coolant, some disadvantages of this method should be taken into account:

1. Observing the heating modes, it is possible to create a situation when a red-hot workpiece comes into contact with oil, which leads to the release of harmful substances.
2. The oil may catch fire during a certain range of exposure to high temperatures.
3. This cooling method allows you to withstand the required hardness, measured in certain units, and also to avoid the appearance of cracks in the structure, but plaque remains on the surface, the removal of which also creates a very large number of problems.
4. The oil itself loses its properties over time, and its cost is quite high.

What kind of liquids are used to cool steel?

The above information determines that the liquid and the cooling mode are selected depending on the shape, size of the workpiece, as well as how good the surface should be after quenching. Combined cooling is the process of using multiple coolants. An example is the hardening of a part of a complex shape, when first the cooling takes place in water and then in an oil bath. In this case, the temperature to which the metal is cooled at which stage is taken into account.

Heat treatment is one of the necessary and important operations in steel processing. It is widely used by metallurgy and mechanical engineering. Heat treatment technology of 45 steel ensures high strength characteristics. This circumstance makes it possible to significantly expand the field of application of parts processed in this way. When using the technology of hardening steel 45, the hardness of the products becomes significantly higher.

Features of heat treatment

Hardening of 45 steel is a method widely used in metallurgy and mechanical engineering. But how to temper 45 steel to get the expected result? To change the characteristics, it is necessary to carry out heat treatment. In this case, certain modes of exposure must be observed. This process can be schematically represented by the following processes:

• Annealing.
• Normalization.
• Aging.
• Quenching and tempering.

The quality of 45 steel during heat treatment depends on a number of factors.

• Temperature conditions.
• How fast the temperature rises.
• The time span during which the metal is exposed to high temperatures.
• How fast the cooling process takes place.

Heat treatment consists in heating a part to a predetermined temperature. Cool it at the same or slightly different rate... Iron-carbon alloys are characterized by transformations when heated to certain temperatures. They are called critical points. These transformations are associated with a crystallization nature. When hardening steel 45, the hardness of the products increases significantly.

Chemical composition

What does the number 45 mean to steel? This suggests that this alloy contains 0.45% carbon. The rest of the impurities are present in insignificant amounts. Among its main substitutes are steel 40 and 50. They are also characterized by high strength. If we consider the chemical compounds that make up the steel in percentage terms, then the largest share falls on iron. For him, this figure reaches 97%. Other chemical elements are included in various amounts. The lowest indicator is for phosphorus. It contains only 0.035% of it.

Structural changes in metal

In the initial state, the structure consists of two phases that are mixed with each other - ferrite and cementite. If slowly heated to low temperatures, then no changes will occur in it. If you continue heating, the ferrite will dissolve in the austenite. When heated above the critical temperature, their structure will take on a uniform character.

The atomic lattice of iron has a body-centered character. With strong heating, it becomes face-centered in type. Before heating, carbon atoms enter pearlite (cementite crystals), after which it will take on a different state and become a solid solution. In this case, its atoms will be in the iron lattice. It can be hardened if it is chilled quickly, for example with water.

In this state, it will acquire values \u200b\u200bcharacteristic of room temperature. It would seem that everything will be rearranged in the reverse order. But such temperature parameters will not give the carbon atoms pronounced mobility. The velocity in this case is so insignificant that the atoms simply do not have time to leave the solution when rapid cooling takes place. They remain in the lattice structure. This creates a strong internal stress on the metal. The use of hardened steel significantly increases the possibility of using parts, the material for the manufacture of which is just such steel.

Hardening

Heat treatment of 45 steel involves heating above the critical temperature. In the future, accelerated cooling is carried out, in other words, hardening is carried out. After that, the hardened material acquires increased strength and hardness. The temperature regime during quenching of steel 45 is determined by how much carbon and alloying additives are contained in the steel.

The technology must be carried out in accordance with the established regulations, since after hardening is carried out, a layer of scale forms on the workpiece. In this case, there is a partial loss of carbon. The metal must cool quickly... This will prevent austenite from transforming to form sorbitol or troostite. The part is cooled according to a precise schedule. If it is broken, small cracks will form. Having cooled the part to a temperature of 200-300 degrees, the process is artificially slowed down. At the same time, coolants are used.

Special ovens are used for heating. Before this, the individual parts are heated. In this case, the use is carried out:

• ovens where the temperature is 500 degrees;
• special salt baths.

The part is immersed for a few seconds 2-3 times. An indispensable condition: the heating of the entire part must be carried out evenly. All workpieces are immersed at the same time, then exposure is required. You can see more about this in the video.

HFC hardening

With the use of HDTV, the heating temperature is higher in terms of its performance.

This circumstance becomes possible due to the presence of two factors:

Heating causes accelerated change and transition of pearlite to austenite.
The process takes place within a tight time frame. At the same time, the temperature is very high about its magnitude.
However, the workpiece does not overheat. With such operations, the characteristics of the metal, which determine its hardness, increase by 3 units according to Rockwell. Using this method, the part can be hardened very thoroughly.

The hardness test, and therefore the hardenability of the parts, is determined by the Brinell method.

Vacation

This process is determined by the temperature that is required. For this purpose, the following are used:

• ovens with forced air circulation;
• saltpeter solution in special baths;
• oil baths;
• bath filled with lye.

The tempering temperature is determined by the steel grade. The process allows you to change the structure and reduce the tension in the metal... At the same time, there is no significant decrease in hardness. Then the workpiece gets into the field of view of technical control, and then it is sent to the customer.

Precautionary measures

Such operations pose a certain danger to human life and health. Electrical heating installations involve hazardous electrical current. Working with quenching baths is associated with the release of harmful vapors and gases into the surrounding space. In this regard, equipment and good serviceability of local exhaust ventilation systems are of great importance. In addition, such places are equipped with general ventilation.

If the process is carried out using oil or kerosene, it is possible that their vapors will ignite. It is necessary to carry out protection against chemical burns. The storage of nitrate is carried out in accordance with the required rules. A solution of saltpeter in a molten state should not have a temperature higher than 60 degrees. Cyanide salts are packaged only with local exhaust ventilation. All work is carried out only with the use of personal protective equipment. To prevent the formation of poisonous hydrocyanic acid, it is impossible to allow the joint storage of cyanide salts with acid solutions.

One of the most common methods of heat treatment of metals is steel hardening. It is with the help of hardening that the required characteristics of the finished product are formed, and its incorrect implementation can lead to excessive softness of the metal (non-calcination) or to its excessive fragility (overheating). This article will discuss what the correct hardening is and what needs to be done to complete it.

What is metal hardening

Even ancient blacksmiths knew that the effect of high temperature on a metal can change its structure and properties and actively used it in practice. Later, it was scientifically established that hardening of products made of steel, which involves heating and subsequent cooling of the metal, can significantly improve the mechanical characteristics of finished products, significantly increase their service life and even ultimately reduce their weight by increasing the strength of the part. Remarkably, the hardening of parts made of inexpensive grades of steel allows them to give them the required characteristics and successfully use them instead of more expensive alloys.

The meaning of the process, which is called hardening of products from steel alloys, is to heat the metal to a critical temperature and then cool it down. The main goal pursued by this heat treatment technology is to increase the hardness and strength of the metal while reducing its ductility.

There are various types of hardening and subsequent tempering, differing in the modes of carrying out, which determine the final result. The hardening modes include the heating temperature, the time and speed of its execution, the holding time of the part in the state heated to a given temperature, the speed at which cooling is carried out.

The most important parameter at is the heating temperature, upon reaching which the atomic lattice is rearranged. Naturally, for steels of different grades, the value of the critical temperature is different, which depends, first of all, on the level of carbon and various impurities in their composition.

After quenching, both the hardness and the brittleness of the steel increase, and a layer of scale appears on its surface, which has lost a significant amount of carbon. The thickness of this layer must be taken into account when calculating the allowance for further processing of the part.

When quenching products made of steel alloys, it is very important to ensure the specified cooling rate of the part, otherwise, the already rearranged atomic structure of the metal can go into an intermediate state. Meanwhile, too fast cooling is also undesirable, since it can lead to cracks on the part or to its deformation. In order to avoid the formation of such defects, the cooling rate after the temperature of the heated metal drops to 200 degrees Celsius is somewhat slowed down.

For heating parts made of carbon steels, chamber furnaces are used, which can heat up to 800 degrees Celsius. For hardening certain grades of steel, the critical temperature can be 1250-1300 degrees Celsius, so parts from them are heated in furnaces of a different type. The convenience of hardening steels of such grades is that products made of them are not subject to cracking during cooling, which eliminates the need for their preheating.

You should take a very responsible approach to the hardening of parts of complex configuration with fine edges and sharp transitions. To eliminate cracking and warping of such parts during heating, it should be carried out in two stages. At the first stage, such a part is preheated to 500 degrees Celsius and only then the temperature is brought to a critical value.

For high-quality hardening of steels, it is important to ensure not only the heating level, but also its uniformity. If the part is massive or complex configuration, it is possible to ensure uniform heating only in a few passes. In such cases, heating is performed with two exposures, which are necessary in order for the temperature reached to be evenly distributed throughout the entire volume of the part. The total heating time also increases if several parts are simultaneously placed in the oven.

How to avoid scale and decarburization during quenching

Many steel parts are hardened after they have been finished. In such cases, it is unacceptable for the surface of the parts to be decarburized or to form scale on it. There are methods for hardening steel products to avoid such problems. Quenching, carried out in a protective gas atmosphere, which is injected into the cavity of the heating furnace, can be classified as the most advanced of such methods. It should be borne in mind that this method is used only if the oven for heating is completely sealed.

The photo shows the moment of water descaling at the hot rolling mill - descaling

A simpler way to avoid decarburization of the metal surface during quenching is the use of cast iron chips and a spent carburizer. In order to protect the surface of the part when heated, it is placed in a special container, into which these components are pre-filled. To prevent the ingress of ambient air into such a container, which can cause oxidation processes, the outside is thoroughly coated with clay.

If, after quenching the metal, it is cooled not in oil, but in a salt bath, it should be regularly deoxidized (at least twice per shift) in order to avoid decarburization of the surface of the part and the appearance of oxide on it. Boric acid, brown salt or charcoal can be used to deoxidize the salt baths. The latter is usually placed in a special glass with a lid, in the walls of which there are many holes. You should be very careful to lower such a glass into a salt bath, since at this moment a flame flares up on its surface, which dies out after a while.

There is a simple way to check the quality of the salt bath deoxidation. For this, a conventional stainless steel blade is heated in such a bath for several minutes (3-5). After the salt bath, the blade is placed in water to cool. If after such a procedure the blade does not bend, but breaks, then the deoxidation of the bath was successful.

Cooling of steel during quenching

Most of the coolants used in hardening steel products are based on water. At the same time, it is important that such water does not contain impurities of salts and detergents, which can significantly affect the cooling rate. A container that contains water for hardening metal products is not recommended for other purposes. It is also important to take into account that you cannot use running water to cool the metal during the hardening process. The optimum temperature for the coolant is 30 degrees Celsius.

Tempering steel products using ordinary water to cool them has a number of significant disadvantages. The most important of these is cracking and warping of parts after they have cooled. As a rule, this method of cooling is used when metal cementation, surface hardening of steel or heat treatment of parts of simple configuration, which will be further subjected to finishing, are performed.

For products of complex shape made of structural steels, a different type of coolant is used - a 50% caustic soda solution heated to a temperature of 60 degrees Celsius. After cooling in such a solution, the hardened steel acquires a light shade.

It is very important to observe safety precautions when working with caustic soda, be sure to use a hood placed above the bathroom. When a red-hot part is dipped into the solution, vapors are formed that are very harmful to human health.

The best coolant for thin-walled carbon steel and alloy alloy parts is mineral oils, which provide a constant (isothermal) cooling temperature regardless of ambient conditions. The main thing that should be avoided when using such a technical liquid is the ingress of water into it, which can lead to cracking of parts during their cooling. However, if water does get into such a coolant, it can be easily removed from it by heating the oil to a temperature above the boiling point of water.

Quenching steel using oil as a coolant has a number of significant disadvantages that you should definitely be aware of. When the oil comes into contact with a hot part, vapors are released that are harmful to human health, in addition, the oil at this moment may catch fire. An oil bath also has such a property: after its use, a deposit remains on the parts, and the coolant itself loses its effectiveness over time.

All these factors should be taken into account when quenching metals in an oil environment and the following safety measures should be taken:

• immerse parts in an oil bath using long-handled pliers;
• all work should be carried out in a special mask made of tempered glass and gloves made of thick fabric with fire-resistant properties or of rough leather;
• reliably protect shoulders, neck, chest with work clothes made of thick fireproof fabric.

For hardening steels of certain grades, cooling is carried out using an air flow generated by a special compressor. It is very important that the cooling air is completely dry, as the moisture contained in it can cause cracking of the metal surface.

There are methods for hardening steel, which use combined cooling. They are used to cool carbon steel parts that have a complex chemical composition. The essence of such hardening methods is that first the heated part is placed in water, where in a short time (a few seconds) its temperature drops to 200 degrees, the part is further cooled in an oil bath, where it should be moved very quickly.

Performing hardening and tempering of steel parts at home

Heat treatment of metal products, including surface hardening of steel, not only increases the hardness and strength of the alloy, but also significantly increases the internal stresses in its structure. To remove these stresses, which can lead to breakage of the part during operation, it is necessary to release the steel product.

It should be borne in mind that such a technological operation leads to some decrease in the hardness of steel, but increases its ductility. To carry out tempering, the essence of which is a gradual decrease in the temperature of the heated part and keeping it at a certain temperature regime, furnaces, salt and oil baths are used.

Tempering temperatures differ for different steel grades. So, tempering of high-speed alloys is carried out at a temperature of 540 degrees Celsius, and for steels with a hardness of HRC 59-60, 150 degrees is enough. What is characteristic, when tempering high-speed alloys, their hardness even increases, and in the second case, its level decreases, but the plasticity index increases significantly.

Hardening and tempering of steel products, including stainless grades, is quite acceptable (and, moreover, it is often practiced) at home, if the need arises. In such cases, electric stoves, ovens and even hot sand can be used to heat steel products. The temperatures to which steel products should be heated in such cases can be selected according to special tables. Before hardening or tempering steel products, they must be thoroughly cleaned, their surface should be free of dirt, traces of oil and rust.

After cleaning, the steel product should be heated so that it is evenly red-hot. In order to heat it up to such a state, it is necessary to perform heating in several approaches. After the required state has been reached, the product to be heated should be cooled in oil, and then immediately placed in an oven preheated to 200 degrees Celsius. Then it is necessary to gradually reduce the temperature in the oven, bringing it to 80 degrees Celsius.

This process usually takes an hour. Further cooling should be carried out in the open air, the only exceptions are products made of chromium-nickel steels, for which oil baths are used to reduce the temperature. This is due to the fact that steels of such grades can acquire the so-called temper brittleness upon slow cooling.

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