Unlike water, the cooling ability of oil depends little on temperature, and the cooling rate in oil is many times lower than in water. Therefore, in order to reduce stresses and avoid quenching cracks, mineral oil is used to quench alloy steels with a lower thermal conductivity than carbon steels. In the absence of oil, hot water (80 ° C) is used.

Table 2.3 Cooling rate of steel.

The main methods of steel hardening are hardening in one cooler, in two environments, jet, self-tempering, step and isothermal.

Quenching in one cooler. The part, heated to the quenching temperature, is immersed in the quenching liquid, where it is kept until it is completely cooled. This method is used for hardening simple parts made of carbon and alloy steels.

Parts made of carbon steels are cooled in water, and parts made of alloy steels are cooled in oil, this method is also used in mechanized hardening, when parts are automatically transferred from the unit to the quenching liquid.

High-carbon steels are hardened with cooling, i.e. the heated part is kept in air for some time before cooling. This reduces internal stresses in the parts and eliminates the formation of cracks.

Quenching in two environments (or intermittent quenching).

The part is first cooled in a fast-cooling medium - water, and then transferred to a slow-cooling medium - oil; used for hardening tools made of high carbon steel.

Jet hardening. Parts heated to the hardening temperature are cooled with a water jet. This method is used for hardening internal surfaces, upsetting dies, dies and other tools in which the working surface must have a martensite structure. Jet quenching does not form a steam jacket, which provides deeper hardenability than simple water quenching.

Self-tempering hardening. Parts are kept in a cooling environment not until complete cooling, but up to a certain stage in order to keep the heat necessary for self-release in the core of the part.

Step hardening. In this quenching method, heated parts are first cooled to a temperature slightly above the M n point (in hot oil or molten salt), then after a short exposure at this temperature (before the onset of intermediate transformations) they are cooled in air. In the second stage of cooling, the steel is hardened.

Isothermal hardening. Parts are heated to a predetermined temperature and cooled in an isothermal environment to 220 ° -350 ° C, which is slightly higher than the temperature of the onset of martensitic transformation. Holding the parts in a quenching medium should be sufficient for the complete transformation of austenite into acicular troostite. This is followed by air cooling. With isothermal hardening, the holding time is significantly longer than with step hardening.

Isothermal hardening eliminates the large difference in cooling rates of the surface and core of parts, which is the main cause of thermal stresses and hardening cracks. After isothermal hardening, the parts acquire high or medium hardness, high toughness and good resistance to impact loads. Isothermal hardening in some cases excludes the tempering operation, which reduces the heat treatment cycle by 35-40%.

Parts and tools made of alloy steels of grades 6ХС, 9ХС, 65Г, ХВГ, etc. are subjected to isothermal hardening.

Light hardening. In this method of hardening, the parts are heated in a neutral, non-oxidizing atmosphere or in molten neutral salts. With light hardening, parts or tools are heated in liquid salts that do not cause metal oxidation, followed by their cooling in molten caustic alkalis; in heating furnaces with the use of a controlled protective gas amtosphere, which makes it possible to regulate the interaction of furnace gases with steel during heating; in vacuum (10 -1 -10 -4 mm Hg) hardening furnaces. Any of these processes can produce parts with a clean light gray surface.

/ 11.08.2019

Cooling rate of steel in air deg s. Determination of the cooling rate during hardening of steel

The cooling capacity of media is largely determined by whether their state of aggregation changes or does not change in the process of cooling products.
Cooling media are divided into two groups depending on the boiling point. The first group is the environment, the state of aggregation of which changes during the cooling period. It includes water, aqueous solutions of salts, alkalis, liquid nitrogen, as well as oils, aqueous solutions of emulsins, etc., the boiling point of which is lower than the temperature of the refrigerated product. The second group includes media, the state of aggregation of which does not change during the cooling of products. Their boiling point is higher than the temperature of refrigerated items. These are molten metals, salts, alkalis and their mixtures. The same group should include air and helium, which also do not change their state of aggregation.
Depending on the cooling capacity of the media, they are divided into three groups:
1. Strong coolers: solutions in water 5% NaOH, 2% K4Fe (CN) 6 * 3H2O1 10-15% NaOH and water at 15-30 ° C, especially with heavy traffic, liquid nitrogen, helium.
2. Intermediate coolants: molten salts, alkalis and metals, vegetable and mineral oils, fuel oil, aqueous solutions of sodium silicates, hot aqueous solutions of salts, etc.
3. Weak coolers: strings of dry air and steam-air mixtures, water at 80-90 ° C, soapy water, etc.
The cooling capacity of a cooler depends on its latent heat of vaporization, heat capacity, thermal conductivity and viscosity. The cooling rate in the considered cooler is influenced by the amount of the cooler and the hydrodynamic mode of its movement.
When a product with a high temperature is immersed in a cooling medium with a low boiling point, the following processes are observed. At the initial moment, due to increased vaporization, the entire product is covered with a vapor film, which stably adheres to the metal surface. It is a poor conductor of heat, and therefore cooling at this stage, called the film boiling stage, is slowed down.
As the temperature of the product decreases, the amount of heat released becomes insufficient to maintain a continuous vapor film on the surface. The destruction of the vapor film leads to the fact that the surface of the product begins to come into contact with the cooling liquid. In this case, steam bubbles are intensively formed, and the cooling intensity increases sharply, since a large amount of heat is spent on the formation of steam bubbles, which quickly detach from the surface, and the metal temperature quickly drops to the boiling point of the cooling medium. The corresponding cooling period is called nucleate boiling.
With subsequent cooling, vaporization is practically not observed, and heat is transferred by convection from the surface of the product to the cooling medium. The intensity of heat transfer in the third period is low, and, accordingly, the cooling rates are low.
The requirements for an ideal cooling medium during quenching are that it must provide cooling at rates higher than critical in a certain temperature range. At lower temperatures, the cooling rate should not be high, as this leads to high residual stresses and warpage of products. So, for example, when quenching aluminum alloys, high cooling rates are required at temperatures of 500-300 ° C to fix a supersaturated solid solution. Ox cooling in the temperature range of 200-20 ° C is desirable to be carried out with a much lower intensity to reduce residual stresses.
Cooling in water. To assess the cooling capacity of the media from experimental data, the dependences of the heat transfer coefficients α on the surface temperature of the product are plotted.As noted earlier, the cooling capacity also depends on the condition of fluid movement relative to the surface of the product.
In fig. 1 shows the values ​​of the heat transfer coefficients for calm and circulating water. It can be seen that, depending on the temperature, the heat transfer coefficients change with a pi curve with a maximum. At high temperatures, the α coefficients are small, which corresponds to the film boiling regime. With the development of nucleate boiling, the coefficients α increase sharply and then decrease at such surface temperatures during the transition to convective heat transfer.

The curves of the dependence of the heat transfer coefficients α on the surface temperature do not give an unambiguous idea of ​​the cooling rates at different temperatures, since the amount of heat removed from the product is proportional to α and the temperature difference tp-twater. The cooling rate for thermally “thin” bodies is determined by the formula

where c and ρ are the specific heat and density of the metal, respectively; V - the volume of the cooled body
From equation (I) it follows that to estimate the cooling capacity of the medium, you can use the parameter k = α (tp-twater), which for given physical properties ah metal and dimensions of the product uniquely determines the cooling rate. In fig. 2 shows the dependences of the parameter k on the surface temperature plotted according to the data in Fig. 1.
As can be seen from the data in Fig. 2. The rates of cooling products in water, depending on the temperature of the cooled surface, also change along a curve with a maximum. At a water temperature of 20 ° C, this maximum is in the surface temperature range of 200-300 ° C. With an increase in the water temperature, the cooling rates decrease, and the maximum shifts to the region of lower surface temperatures. It should be noted that in the surface temperature range of 100-200 ° C, the cooling rate when using water with an elevated temperature is greater than for water with a temperature of 20 ° C. An increase in cooling rates at low temperatures leads to an increase in residual stresses, and therefore overheating of water in quenching tanks is limited to temperatures of 25-45 ° C. Water circulation increases cooling rates at high temperatures.

Cooling in aqueous solutions and emulsions. Studies have shown that even a small amount of impurities significantly alters the cooling capacity of water. It was found that this is due to the effect of impurities on the stability of the vapor film on the surface of the cooled product. In this case, soluble impurities decrease, while insoluble impurities, on the contrary, increase the stability of the vapor film. This property of water has been used to develop quench media with high and low cooling capacity.
An increase in the cooling capacity of water is achieved by dissolving acids, alkalis and salts in it. Such highly soluble compounds as KCl, NaCl, CaCl2, Na2CO3, K2CO3, Na2SO4, H2SO4, NaOH, KOH, reduce the stability of the vapor film and increase the temperature of the transition from the film boiling mode to the bubble mode.
On the contrary, the addition of insoluble and poorly soluble substances that form an emulsion in water (oils, fats, oil products, soaps, etc.) increases the stability of the vapor film and lowers the temperature of changing boiling modes. The cooling capacity of water containing these substances is significantly reduced in the high temperature range.
In the practice of heat treatment, aqueous solutions of 5-15% NaCl have found wide application.They have a greater cooling capacity compared to water at elevated temperatures, and at temperatures below 200-250 ° C they cool at about the same rate as water.
Solutions of alkalis, which are similar in cooling capacity to NaCl solutions, have found less application. Solutions of acids are not used as quenching media because of their aggressiveness and release of harmful vapors during quenching. Suspensions and emulsions are characterized by a reduced cooling rate at high and medium temperatures, and at low temperatures they are cooled in the same way as pure water.
In fig. 3 shows the values ​​of the heat transfer coefficients depending on the surface temperature for various solutions and emulsions, which make it possible to obtain quantitative characteristics of their cooling capacity.

V recent times As quenching media, they begin to use aqueous solutions of polymers, which provide a reduced cooling rate during quenching and, accordingly, less warpage. In foreign practice, aqueous solutions of polyalkylene glycol are used, and in our country, aqueous solutions of polyethylene glycol. At temperatures above 70-77 ° C, these substances are insoluble in water, and therefore, during quenching cooling, the polymer molecules cover the surface of the product with a thin film, which slows down the cooling. At sufficiently low temperatures, the polymer film transforms into an aqueous solution and the retarding effect of the film is eliminated.Therefore, when quenching in aqueous solutions of polymers, there is no colossal difference in the cooling rates in the range of high, medium, and low temperatures, which is characteristic of water.
The viscosity of a solution, which largely determines its cooling capacity, depends on the concentration of the polymer. Thus, a change in the concentration of polyethylene glycol from 30 to 70% increases the kinematic viscosity of the solution by almost 30 times, which allows for sharp quenching in solutions of low concentration and quenching with a moderate cooling rate in solutions of high concentration (Fig. 4).

When quenching aluminum alloys in solutions of polyethylene glycol with a concentration above 50%, warpage can be reduced by 85–95%. At the same time, the mechanical properties and corrosion resistance of alloys are not significantly reduced. It is most expedient to use these media in the conditions of machine-building plants, where it is often necessary to carry out repeated quenching.
Cooling in oils. Currently, only distillate mineral oils are used for quenching. The boiling points of oils are 150-300 ° C higher than those of water. The maximum permissible temperature of overheating of oils is chosen 25-30 ° C below the flash point. Compared to water, oils are characterized by a significantly lower cooling capacity, especially at the stage of convective heat transfer. Cooling rates in oil at elevated temperatures are 5-8 times less than when cooling in water.
The cooling capacity of oils is weakly dependent on their circulation and temperature, within the limits that are encountered in practice (25-65 ° C).
For quenching operations in the practice of heat treatment, oils with low, normal, high and high viscosity are used. Most often, oils of grades 20 and 20B are used, which have a normal viscosity.
Low viscosity oils have a higher cooling capacity than high and high viscosity oils and have a low flash point. Oils of high and high viscosity have a high flash point, which allows them to be heated to 160-200 ° C to reduce the viscosity. They are used for isothermal and step hardening.
The disadvantage of mineral oils is their aging, which leads to thickening and loss of cooling capacity. Aged oil is subjected to regeneration, consisting of sedimentation, filtration from impurities and the addition of a certain amount of fresh oil.
Quenching in molten lead, salts and alkalis. For stepwise and isothermal quenching, cooling media are used, which, during heat treatment, do not change their state of aggregation. These include molten lead, nitric acid salts and alkalis.
Molten lead satisfies the requirements from a thermal engineering point of view, but is scarce, and its vapors are harmful to service personnel. Therefore, the lead is replaced by other media.
Molten salts and alkalis and their mixtures have several advantages over lead. Firstly, they are easily washed off the surface of the products. The surface after quenching in alkalis is not oxidized, which allows firing without cleaning the parts after heat treatment. Secondly, the use of salt and alkaline baths allows for internal electrical heating, it is easy to automate temperature control, and it is easy to solve the problems of mixing the melt. Research shows that the cooling capacity of saline and alkaline media is somewhat nicer than that of lead. However, when handling alkaline media, precautions should be taken against splash burns.
The compositions of the mixtures recommended as quenching media are given in table. 3.

The cooling capacity of molten media depends on the viscosity: the lower the viscosity, the higher the cooling capacity. With an increase in the melt temperature and a corresponding decrease in viscosity, the heat transfer coefficients increase, reach a maximum, and then fall (Fig. 5).
Water additions strongly affect the cooling capacity of salts and alkalis. Water increases the fluidity and the cooling capacity of the molten medium. For this purpose, 2-10% of water is introduced into salt melts, and 8-15% of water is added to alkali melts.

After quenching in alkali melts, the products must be thoroughly washed and passivated to prevent corrosion.
Air cooling. When cooled in air, heat is transferred by radiation and convection, and the heat transfer coefficient α is the sum of the convective αc and radiant αl components.
When cooling in a calm air environment, the following formulas are used to determine αk:

where Δr = tp-tair is the difference between the average surface temperature of the product and the air temperature; l - characteristic size, m, is taken for a ball and a horizontal cylinder (pipe) equal to their diameter; for a vertical cylinder and a vertical plate - the height of the cooled section, for a horizontal plate - its smallest length.
For a plate facing upward heat exchange surface, the values ​​of αc increase by 30%, and for a plate facing downward heat exchange surface, decrease by 30% in comparison with those calculated by formulas (2) and (3).
The values ​​of the coefficients B, A1, and A2 are determined depending on the average temperature t = 1/2 (tp + tair) and are given in table. 4.

The intensity of convective heat exchange during forced movement of air or gas depends on the speed of gas movement, its physical properties and the geometry of the heated or cooled products.
For the forced motion of the surface, αc is determined by the Yurges formulas:
a) at a speed of air movement w0≤4.65 m / s:
for polished surfaces

with rolled surface

for rough surfaces

b) at a gas velocity w0≥4.65 m / s:
for polished surfaces

for rolled surface

for rough surfaces

Here W0 is the reduced (0 ° C; 0.1 mPa) air velocity. If the speed of movement w at temperature t is known, then

Heat treatment often requires cooling at a controlled rate. Therefore, it becomes necessary to calculate the cooling processes, which can be carried out in any of the environments described above.
The calculation of cooling processes under various conditions is, in principle, no different from the calculation of heating processes. It's just that in the formulas given in the literature on heat transfer, instead of the furnace temperature t1, it is necessary to substitute the value of the cooling temperature among.

Heat treatment of steels is one of the most important operations in mechanical engineering, on the correct implementation of which the quality of products depends. Quenching and tempering of steels is one of the various types of heat treatment of metals.

The thermal effect on the metal changes its properties and structure. This makes it possible to increase the mechanical properties of the material, the durability and reliability of products, as well as to reduce the size and weight of mechanisms and machines. In addition, due to heat treatment, cheaper alloys can be used for the manufacture of various parts.

As the Steel Was Tempered

Heat treatment of steel consists in the thermal effect on the metal according to certain modes to change its structure and properties.

Heat treatment operations include:

• annealing;
• normalization;
• aging;
• hardening of steel and tempering of steel (etc.).

Heat treatment of steel: hardening tempering - depends on the following factors:

• heating temperature;
• heating time (speed);
• the duration of exposure at a given temperature;
• cooling rate.

Hardening

Steel hardening is a heat treatment process, the essence of which is heating the steel to a temperature above the critical one, followed by rapid cooling. As a result of this operation, the hardness and strength of the steel increases, and the ductility decreases.

When steels are heated and cooled, the atomic lattice is rearranged. The critical temperatures for different steel grades are not the same: they depend on the content of carbon and alloying impurities, as well as on the heating and cooling rate.

After hardening, the steel becomes brittle and hard. The surface layer of products when heated in thermal furnaces becomes scaled and decarburized the more, the higher the heating temperature and the holding time in the furnace. If the parts have a small allowance for further processing, then this marriage is irreparable. Tempering modes of hardening steel depend on its composition and technical requirements for the product.

Parts should be cooled quickly during quenching so that austenite does not have time to transform into intermediate structures (sorbitol or troostite). The required cooling rate is ensured by the choice of the cooling medium. At the same time, excessively rapid cooling leads to the appearance of cracks or warping of the product. To avoid this, in the temperature range from 300 to 200 degrees, the cooling rate must be slowed down by using combined hardening methods. Great importance to reduce the warpage of the product, it has a method of immersing the part in a cooling medium.

Metal heating

All methods of steel hardening consist of:

• heating steel;
• subsequent exposure to achieve end-to-end heating of the product and completion of structural transformations;
• cooling at a certain rate.

Carbon steel products are heated in chamber furnaces. Preheating is not required in this case, since these steels are not subject to cracking or warping.

Complex products (for example, a tool with protruding fine edges or sharp transitions) are preheated:

• in salt baths by two or three immersions for 2 - 4 seconds;
• in separate ovens up to a temperature of 400 - 500 degrees Celsius.

All parts of the product must be heated evenly. If this cannot be achieved in one step (large forgings), then two exposures are made for through heating.

If only one piece is placed in the oven, the heating time is reduced. For example, one 24 mm disc milling cutter heats up in 13 minutes, and ten such products in 18 minutes.

Product protection against scale and decarburization

For products whose surfaces are not polished after heat treatment, carbon burnout and scale formation are unacceptable. Protect surfaces from such defects by applying supplied to the cavity of an electric furnace. Of course, this technique is only possible in special sealed ovens. Shielding gas generators serve as a source of gas supplied to the heating zone. They can operate on methane, ammonia and other hydrocarbon gases.

If there is no protective atmosphere, then the products are packed in containers before heating and covered with spent carburizing agent, shavings (a thermist should know that charcoal does not protect tool steels from decarburization). To prevent air from entering the container, it is coated with clay.

When heated, salt baths do not allow the metal to oxidize, but they do not protect against decarburization. Therefore, in production, they are deoxidized at least twice per shift with brown salt, blood salt or boric acid. Salt baths operating at temperatures of 760 - 1000 degrees Celsius deoxidize very effectively charcoal... To do this, a glass, which has many holes over the entire surface, is filled with dried charcoal, closed with a lid (so that the coal does not float) and, after heating, is lowered to the bottom of the salt bath. First, a significant number of flames appear, then it decreases. If during the shift in this way the bath is deoxidized three times, then the heated products will be completely protected from decarburization.

The degree of deoxidation of salt baths is very simple to check: an ordinary blade, heated in a bath for 5 - 7 minutes in a high-quality deoxidized bath and hardened in water, will break, not bend.

Coolants

The main coolant for steel is water. If you add a small amount of salt or soap to the water, the cooling rate will change. Therefore, in no case should the hardening tank be used for any other purpose (for example, for washing hands). To achieve the same hardness on a hardened surface, it is necessary to maintain a coolant temperature of 20 - 30 degrees. The water in the tank should not be changed frequently. It is absolutely unacceptable to cool the product in running water.

The disadvantage of water quenching is cracking and warping. Therefore, this method only hardens products of uncomplicated shape or cemented.

• When hardening products of complex configuration from structural steel, a fifty percent solution of caustic soda is used (cold or heated to 50 - 60 degrees). Parts heated in a salt bath and hardened in this solution are light. The temperature of the solution must not be allowed to exceed 60 degrees.

Modes

The vapors generated during quenching in a caustic solution are harmful to humans; therefore, the quenching bath must be equipped with exhaust ventilation.

• Alloy steel is hardened in mineral oils. By the way, thin carbon steel products are also carried out in oil. The main advantage of oil baths is that the cooling rate does not depend on the oil temperature: at a temperature of 20 degrees and 150 degrees, the product will cool at the same rate.

Avoid getting water into the oil bath, as this can lead to cracking of the product. What's interesting: in oil heated to temperatures above 100 degrees, water ingress does not lead to cracks in the metal.

The disadvantage of an oil bath is:

1. emission of harmful gases during quenching;
2. plaque formation on the product;
3. the tendency of the oil to be flammable;
4. gradual deterioration of the hardening ability.

• Steels with stable austenite (for example, X12M) can be cooled with air supplied by a compressor or fan. In this case, it is important not to allow water to enter the air duct: this can lead to the formation of cracks on the product.
• Step hardening is carried out in hot oil, molten alkalis, low-melting salts.
• Intermittent quenching of steels in two cooling media is used for machining complex parts made of carbon steels. First, they are cooled in water to a temperature of 250-200 degrees, and then in oil. The product is kept in water for no more than 1 - 2 seconds for every 5 - 6 mm of thickness. If the exposure time in water is increased, then cracks will inevitably appear on the product. Transferring a part from water to oil should be done very quickly.

Depending on the required temperature, the vacation is carried out:

• in oil baths;
• in saltpeter baths;
• in furnaces with forced air circulation;
• in baths with molten alkali.

The tempering temperature depends on the steel grade and the required hardness of the product, for example, tools that require a hardness of HRC 59 - 60 should be released at a temperature of 150 - 200 degrees. In this case, the internal stresses decrease, and the hardness decreases slightly.

High speed steel is tempered at a temperature of 540 - 580 degrees. This tempering is called secondary hardening, as it increases the hardness of the product.

Products can be tempered by heating them on electric stoves, ovens, even in hot sand. The oxide film, which appears as a result of heating, acquires different tarnishing colors, depending on the temperature. Before starting to leave for one of the tarnish colors, it is necessary to clean the surface of the product from scale, oil carbon deposits, etc.

Usually, after tempering, the metal is cooled in air. But chromium-nickel steels should be cooled in water or oil, since slow cooling of these grades leads to temper brittleness.

As a result of hardening, the steel receives a martensitic structure, very hard (over 6000 HB) and brittle. Martensite is a supersaturated solid solution of carbon in α-Fe. The transformation of austenite into martensite is a diffusion-free process: upon rapid cooling (at a rate of more than 150 0 C / s), the face-centered crystal lattice of austenite transforms into an α-Fe lattice. In this case, the diffusion of carbon atoms does not have time to occur, and they retain their previous positions. As a result, a stressed state of the crystal lattice is created, which leads to high hardness and brittleness of the hardened steel.

To reduce brittleness after quenching, tempering is always carried out, as a result of which internal stresses are reduced and the steel acquires the necessary physical and mechanical properties.

Tempering of hardened steel is carried out by heating to a temperature below the critical points Ac 1, holding at this temperature and then slow or rapid cooling. Rapid cooling in water is recommended when tempering alloy steels to avoid temper brittleness. Carbon steels are air cooled.

Conventionally distinguish between low, medium and high leave. Low tempering is performed when heating up to 200 __ 300 0 С. The resulting structure is tempered martensite, hardness over 5000 HB. Cutting tools, gauges, etc. are subject to low tempering.

Medium tempering is carried out with heating from 300 to 500 0 С. As a result of medium tempering, the steel acquires the tempering troostite structure, which is characterized by a hardness of about 4000 HB. Leave for troostitis is used when processing springs, springs, stamps, percussion tools, etc. Intermediate heating produces troosto-martensite or troosto-sorbitol structures.

High tempering is performed when heating 550-650 0 С. The resulting structure is tempering sorbitol, hardness is about 3000 HB. Crankshafts, axle shafts, connecting rods, connecting rod bolts and many other machine parts are subjected to high tempering.

Thus, as the tempering temperature rises, the strength characteristics decrease, the plasticity characteristics and the toughness increase. For different brands steel, the values ​​of these characteristics will be different, but the general tendency of their change remains the same. Steel after hardening and high tempering (sorbitol structure) has the best combination of strength and ductility properties.

Work order

The work is carried out by a group of 10-12 people. Every two students perform normalization, hardening, low and high tempering of the steel sample.

Determine the hardening temperature of the steel using the lower part of the iron-cementite diagram. For medium-carbon, hypo-eutectoid steels (grades 40, 45, 50), the normal hardening temperature is 30-50 0 С above the GS line, i.e. Ac 3 + (30-50) 0 C.

Determine the heating and holding time of the samples, using the data given in table. 5.2.

Determine the cooling rate in different environments. To do this, take the most common quenching media that cool at different rates: water (cooling rate 600 0 С / s) and oil (cooling rate 150 0 С / s).

The samples are placed in a furnace heated to the hardening temperature for steel of this grade, and kept in the furnace for the required time. When heated to the hardening temperature of samples made of steel 40, the original ferrite-pearlite structure will turn into austenite structure.

Quench the sample in water. To do this, it is necessary: ​​a) to quickly transfer the sample with forceps into a quenching bath with water in order to avoid cooling the sample below the temperature Ac 3 and obtaining incomplete quenching; b) vigorously move the sample in the bath in order to eliminate the formed steam jacket, which slows down the cooling process.

Wipe the samples cooled in oil with a rag, grind both ends on sanding paper. Determine the hardness of hardened samples according to HRC.

Determine the tempering temperature of the steel. Since during tempering there is a change in the structure and properties of steel and the more so, the higher the tempering temperature, you should apply different temperatures vacation from low (200 0 С) to high (600 0 С).

Determine the holding time at the tempering temperature at the rate of 2-3 minutes per 1 mm of sample thickness and write it down in the appropriate column of the protocol.

Determine the cooling conditions. Usually, cooling after tempering is carried out in air, but it is possible to cool in both water and oil, since the cooling rate does not affect the hardness and structure of the steel. To speed up the work, the samples after tempering should be cooled in water.

Measure the hardness of the samples after each type of tempering, recording the measurement results in the workbook and setting the approximate value of the tensile strength according to the dependence

12. In the report, give the schedule and all the necessary data for the heat treatment mode, give the name of the obtained microstructure and explain the effect of heat treatment on the mechanical properties of steel.

LABORATORY WORK No. 5

STEEL HARDENING IN VARIOUS ENVIRONMENTS

Purpose of work: to evaluate the effect of various cooling media on the transformation of austenite during quenching by controlling the hardness.

Devices, materials, tools:

1) electric muffle furnace MP-2U;

2) samples from steel 50;

3) Rockwell hardness tester;

4) cooling tanks with quenching media.

Cooling during quenching should ensure the formation of a martensite structure within a given section of the product and should not cause quenching defects - cracks, warpage, residual stresses in the surface layers, etc. During quenching, for subcooling of austenite to the temperature of martensitic transformation, rapid cooling is required, but not over the entire temperature range in which austenite is least stable. Above 650 ° C, the rate of transformation of austenite is low, and therefore the steel during quenching can be cooled slowly in this interval, but not so much that the formation of P + P begins. The interval 650 ºС-400 ºС must be passed very quickly.

At the moment the product is immersed in the quenching medium, a film of superheated steam forms around it; cooling takes place through the layer of this steam jacket, i.e. slowly (film boiling). At a certain temperature, the steam jacket bursts, the liquid begins to boil on the surface of the part, and cooling occurs rapidly (bubble boiling). The third stage (convective heat exchange) begins when the liquid can no longer boil. The wider the interval of the second stage, the more effective the quenching medium.

If the intensity of water cooling in the middle of the second stage is taken as a unit, then for mineral oil it will be equal to 0.3; for a 10% NaCl solution in water - 3; for 10% - NaOH solution in water - 2.5.

When quenching carbon and some low-alloy steels, water and aqueous solutions (8-12%) NaCl and NaOH are used as a cooling medium. Water, as a cooling medium, has a drawback. A high cooling rate in the temperature range of martensitic transformation leads to the formation of quenching defects. NaCl and NaOH solutions have the most uniform cooling capacity; in addition, the alkaline environment does not cause subsequent corrosion of the remaining parts. Oil, as a quenching medium, has the advantage of a low cooling rate in the range of martensitic transformation, which reduces the occurrence of quenching defects. The disadvantage is increased flammability.

The structure of hardened steel - martensite - is obtained by the sharp cooling of austenite during hardening. Considering the diagrams of isothermal transformation of carbon and alloy steel (Fig. 21), it is easy to make sure that the line of the beginning of transformation in alloy steel is shifted to the right from the ordinate axis as compared to carbon steel. Consequently, the stability of austenite alloy steel, characterized by the distance from the y-axis to the point TO the inflection of the line of the beginning of pearlite and intermediate transformation is much higher than that of carbon steel.

If one depicts the cooling rate during quenching in various media on the isothermal transformation diagram, they will have the form of curves , . The higher the cooling rate, the steeper the curve. According to the diagram with the curves of cooling rates, one can judge about the structural transformations that take place in the parts made of a given steel when quenched in a certain cooling medium.

Let - the rate of cooling in water, - in oil, - in calm air.

Consider the transformation to steel by quenching. Carbon steel has low austenite resistance. When it is cooled in water, the cooling rate curve does not cross the line of the onset of martensitic transformation. The austenitic structure is completely preserved until the beginning of the martensitic transformation, and the structure after the end of cooling is quenched martensite. When it is cooled in oil, it turns out that the curve of the cooling rate crosses the line of the beginning of the transformation in the region of troostite transformation, but does not go beyond the line of the end of the transformation, and then crosses the lines of the beginning and end of the martensitic transformation. Consequently, part of the supercooled austenite transforms into hardening troostite, and part is retained up to the region of martensitic transformation, and the structure of the steel after the end of cooling consists of hardening troostite and hardening martensite. This leads to a decrease in hardness, and the part goes to waste.

If we cool a part made of this steel in air, it turns out that the cooling rate curve intersects the lines of the beginning and end of the transformation in the region of pearlite and sorbitol transformation; the structure of the steel after cooling consists of hardened pearlite and sorbitol.

Thus, to obtain a quenched martensite structure, we must select the cooling medium so that the cooling rate curve does not cross the pearlite transformation lines.

Rice. 21. Isothermal transformation diagram of austenite
for steel with a carbon content of 0.8%.

Extremely low cooling rate, the curve of which does not intersect the pearlite transformation line, but touches it at the point TO is called the critical quenching rate. For each steel, the critical hardening rate is a constant value, but different from the critical hardening rate of another steel. It depends on the least stability, i.e. from the distance from the ordinate to the point TO at the bend of the curve of the beginning of the transformation. The critical quenching rate is the lowest cooling rate sufficient to supercool austenite before the onset of martensitic transformation, and, therefore, to obtain a quenched martensite structure. When choosing a cooling medium for quenching a certain steel grade, an environment is selected that gives a cooling rate slightly higher than the critical one when quenching the steel to the full depth determined by the hardenability of the steel. An unnecessarily high cooling rate is undesirable, since it is accompanied by the formation of high residual stresses and leads to distortion of the part and even to the formation of cracks.

If the selected speed is below the critical one, then this causes a decrease in hardness due to the formation of a troost-martensite structure, which is undesirable.

Work order:

1. Quench the samples from a temperature of 820 ° C in water, oil, 10% solution in NaCl water and in air.

2. Determine the hardness of the samples after each type of processing.

3. Explain the results obtained, fill in the table. 5.

4. Draw up a report.

Table 5

Table of the dependence of the properties of steel on the cooling medium

Questions for self-test:

1. What media are used for hardening carbon and alloy steels?

2. How does the cooling medium affect the hardness of steel?

3. What structure is obtained as a result of quenching carbon steel in water, oil, molten salts, in air?

Technologies for hardening metals and alloys have been improved over the centuries. Modern equipment allows for heat treatment in such a way as to significantly improve the properties of products, even from inexpensive materials.

Quenching (martensitic transformation)- the main method of imparting greater hardness to steels. In this process, the product is heated to such a temperature that the iron changes its crystal lattice and can be additionally saturated with carbon. After holding for a certain time, the steel is cooled. This must be done at high speed to prevent the formation of intermediate forms of iron.
The rapid transformation results in a carbon-supersaturated solid solution with a distorted crystal structure. Both of these factors are responsible for its high hardness (up to HRC 65) and fragility.
Most carbon and tool steels are heated to temperatures from 800 to 900C during hardening, but high-speed steels P9 and P18 are calcined at 1200-1300C.

Microstructure of high-speed steel R6M5: a) cast state; b) after forging and annealing;
c) after hardening; d) after vacation. × 500.

Tempering modes

• Quenching in one medium

The heated product is immersed in a cooling medium, where it remains until it has completely cooled down.This is the easiest hardening method to perform, but it can only be used for steels with a low (up to 0.8%) carbon content or for parts of a simple shape. These limitations are associated with thermal stresses that arise during rapid cooling - parts of complex shapes can warp or even get cracked.

• Step hardening

With this method of hardening, the product is cooled to 250-300C in a saline solution with an exposure of 2-3 minutes to relieve thermal stresses, and then cooling in air is completed. This prevents cracks or warpage of parts. The disadvantage of this method is a relatively low cooling rate, so it is used for small (up to 10 mm in diameter) parts made of carbon or larger alloyed steels, for which the quenching rate is not so critical.

• Hardening in two environments

It starts with a quick cooling in water and ends with a slow cooling in oil. Typically, this hardening is used for tool steel products. The main difficulty lies in calculating the cooling time in the first environment.

• Surface hardening (laser, high frequency currents)

It is used for parts that must be hard on the surface, but at the same time have a viscous core, for example, gear teeth. During surface hardening, the outer layer of the metal is heated to supercritical values, and then cooled either in the process of heat removal (during laser hardening), or by a liquid circulating in a special circuit of the inductor (when hardening with a high frequency current)

Vacation

The hardened steel becomes excessively brittle, which is the main disadvantage of this hardening method. To normalize the structural properties, tempering is performed - heating to a temperature below the phase transformation, holding and slow cooling. When tempering, there is a partial "cancellation" of hardening, the steel becomes slightly less hard, but more ductile. Distinguish between low (150-200C, for tools and parts with increased wear resistance), medium (300-400C, for springs) and high (550-650, for highly loaded parts) tempering.

Quenching and Tempering Temperature Table of Steels

P / p No.	steel grade	Hardness (HRCэ)	Temperature. hardening, degrees C	Temperature. vacation, degrees C	Temperature. order HDTV, degrees C	Temperature. cement., degrees C	Temperature. annealing, degrees C	Temper. Wednesday	Approx.
1	2	3	4	5	6	7	8	9	10
1	Steel 20	57…63	790…820	160…200		920…950		Water
2	Steel 35	30…34	830…840	490…510				Water
		33…35		450…500
		42…48		180…200	860…880
3	Steel 45	20…25	820…840	550…600				Water
		20…28		550…580
		24…28		500…550
		30…34		490…520
		42…51		180…220					Sech. up to 40 mm
		49…57		200…220	840…880
							780…820		With oven
4	Steel 65G	28…33	790…810	550…580				Butter	Sech. up to 60 mm
		43…49		340…380					Sech. up to 10 mm (springs)
		55…61		160…220					Sech. up to 30 mm
5	Steel 20X	57…63	800…820	160…200		900…950		Butter
		59…63		180…220	850…870	900…950		Water solution	0.2 ... 0.7% polyacrylanide
		«-					840…860
6	Steel 40X	24…28	840…860	500…550				Butter
		30…34		490…520
		47…51		180…200					Sech. up to 30 mm
		47…57			860…900			Water solution	0.2 ... 0.7% polyacrylanide
		48…54							Nitriding
							840…860
7	Steel 50X	25…32	830…850	550…620				Butter	Sech. up to 100 mm
		49…55		180…200					Sech. up to 45 mm
		53…59		180…200	880…900			Water solution	0.2 ... 0.7% polyacrylanide
							860…880
8	Steel 12ХН3А	57…63	780…800	180…200		900…920		Butter
		50…63		180…200	850…870			Water solution	0.2 ... 0.7% polyacrylanide
							840…870		With oven up to 550 ... 650
9	Steel 38Х2МЮА	23…29	930…950	650…670				Butter	Sech. up to 100 mm
				650…670					Normalization 930 ... 970
		HV> 670							Nitriding
10	Steel 7ХГ2ВМ						770…790		With oven up to 550
		28…30	860…875	560…580				Air	Sech. up to 200 mm
		58…61		210…230					Sech. up to 120 mm
11	Steel 60S2A						840…860		With oven
		44…51	850…870	420…480				Butter	Sech. up to 20 mm
12	Steel 35ХГС						880…900		With oven up to 500 ... 650
		50…53	870…890	180…200				Butter
13	Steel 50KHFA	25…33	850…880	580…600				Butter
		51…56	850…870	180…200					Sech. up to 30 mm
		53…59		180…220	880…940			Water solution	0.2 ... 0.7% polyacrylanide
14	Steel ШХ15						790…810		With oven up to 600
		59…63	840…850	160…180				Butter	Sech. up to 20 mm
		51…57		300…400
		42…51		400…500
15	Steel U7, U7A	HB					740…760		With oven up to 600
		44…51	800…830	300…400				Water up to 250, oil	Sech. up to 18 mm
		55…61		200…300
		61…64		160…200
		61…64		160…200				Butter	Sech. up to 5 mm
16	Steel U8, U8A	HB					740…760		With oven up to 600
		37…46	790…820	400…500				Water up to 250, oil	Sech. up to 60 mm
		61…65		160…200
		61…65		160…200				Butter	Sech. up to 8 mm
		61…65		160…180	880…900			Water solution	0.2 ... 0.7% polyacrylanide
17	Steel U10, U10A	HB					750…770
		40…48	770…800	400…500				Water up to 250, oil	Sech. up to 60 mm
		50…63		160…200
		61…65		160…200				Butter	Sech. up to 8 mm
		59…65		160…180	880…900			Water solution	0.2 ... 0.7% polyacrylanide
18	Steel 9ХС						790…810		With oven up to 600
		45…55	860…880	450…500				Butter	Sech. up to 30 mm
		40…48		500…600
		59…63		180…240					Sech. up to 40 mm
19	Steel KhVG						780…800		With oven up to 650
		59…63	820…850	180…220				Butter	Sech. up to 60 mm
		36…47		500…600
		55…57		280…340					Sech. up to 70 mm
20	Steel X12M	61…63	1000…1030	190…210				Butter	Sech. up to 140 mm
		57…58		320…350
21	Steel R6M5	18…23					800…830		With oven up to 600
		64…66	1210…1230	560 ... 570 3-fold				Oil, air	In oil up to 300 ... 450 degrees, air up to 20
		26…29		780…800					Exposure 2 ... 3 hours, air
22	Steel R18	18…26					860…880		With oven up to 600
		62…65	1260…1280	560 ... 570 3-fold				Oil, air	In oil up to 150 ... 200 degrees, air up to 20
23	Springs. steel Cl. II			250…320					After cold winding of springs 30 minutes
24	Steel 5ХНМ, 5ХНВ	>= 57	840…860	460…520				Butter	Sech. up to 100 mm
		42…46							Sech. 100..200 mm
		39…43							Sech. 200..300 mm
		37…42							Sech. 300..500 mm
		НV> = 450							Nitriding. Sech. St. 70 mm
25	Steel 30HGSA	19…27	890…910	660…680				Butter
		27…34		580…600
		34…39		500…540
		«-					770…790		With oven up to 650
26	Steel 12Х18Н9Т		1100…1150					Water
27	Steel 40ХН2МА, 40ХН2ВА	30…36	840…860	600…650				Butter
		34…39		550…600
28	Steel ЭИ961Ш	27…33	1000…1010	660…690				Butter	13X11N2V2NF
		34…39		560…590					At t> 6 mm water
29	Steel 20Х13	27…35	1050	550…600				Air
		43,5…50,5		200
30	Steel 40Х13	49,5…56	1000…1050	200…300				Butter

Heat treatment of non-ferrous metals

Alloys based on other metals do not respond to hardening as brightly as steels, but their hardness can also be increased by heat treatment. Typically a combination of quenching and pre-annealing (heating above the transformation point with slow cooling) is used.

• Bronzes (copper alloys) are annealed at a temperature slightly below the melting point, and then quenched with water cooling. The hardening temperature is from 750 to 950C, depending on the composition of the alloy. Vacation at 200-400C is carried out within 2-4 hours. The highest hardness values, up to HV300 (about HRC 34), can be obtained for products from beryllium bronzes.
• The hardness of silver can be increased by annealing to a temperature close to its melting point (dull red) followed by quenching.
• Various nickel alloys are annealed at 700-1185C, such a wide range is determined by the variety of their compositions. For cooling, salt solutions are used, the particles of which are then removed with water or protective gases that prevent oxidation (dry nitrogen, dry hydrogen).

Equipment and materials

To heat metal during heat treatment, 4 main types of furnaces are used:
- salt electrode bath
- chamber furnace
- continuous combustion furnace
- vacuum furnace

Liquids (water, mineral oil, special water polymers (Termat), salt solutions), air and gases (nitrogen, argon) and even low-melting metals are used as quenching media in which cooling takes place. The unit itself, where the cooling takes place, is called a quenching bath and is a container in which laminar mixing of the liquid takes place. An important characteristic of the quench bath is the removal quality of the steam jacket.

Aging and other methods of increasing hardness

Aging- another type of heat treatment, which allows increasing the hardness of alloys of aluminum, magnesium, titanium, nickel and some stainless steels, which are subjected to preliminary hardening without polymorphic transformation. With aging, hardness and strength increase, and ductility decreases.

• Aluminum alloys, for example, duralumin (4-5% copper) and alloys with the addition of nickel and iron, are kept within an hour at a temperature of 100-180C
• Nickel alloys are aged in 2-3 stages, which in total takes from 6 to 30 hours at temperatures from 595 to 845C. Some alloys are subjected to preliminary hardening at 790-1220C. Details from nickel alloys placed in additional containers to prevent contact with air. Electric furnaces are used for heating; salt electrode baths can be used for small parts.
• Maraging steels (high-alloy non-carbon iron alloys) age for about 3 hours at 480-500C after preliminary annealing at 820C

Chemical heat treatment- saturation of the surface layer with alloying elements,

• non-metallic: carbon (carburizing) and nitrogen (nitriding) are used to increase the wear resistance of knees, shafts, gears from low-carbon steels
• metallic: for example, silicon (siliconizing) and chromium helps to increase the wear and corrosion resistance of parts

Cementing and nitriding are performed in mine electric furnaces. There are also universal units that allow you to carry out the entire range of work on the thermochemical treatment of steel products.

Pressure treatment (work hardening) - an increase in hardness as a result of plastic deformation at relatively low temperatures. In this way, low-carbon steels are hardened by cold forging, as well as pure copper and aluminum.

In the process of heat treatment, steel products can undergo amazing transformations, acquiring wear resistance and hardness, many times greater than that of the original material. The range of changes in the hardness of non-ferrous metal alloys during heat treatment is much smaller, but their unique properties often do not require large-scale improvement.

Cooling of workpieces in different heat treatment operations is performed at different rates. When annealing, the cooling should be slow, and when quenching some steels, on the contrary, very fast. The cooling rate is regulated by the use of different cooling media.

Cooling of workpieces with a furnace, those. very slow, use during annealing. For all other heat treatment operations, cooling is performed with more speed. Air cooling used for normalization, and also when hardening steels with very high hardenability (air-hardened steels).

The minimum permissible cooling rate during hardening of steels (the lower the rate, the lower the value of quenching stresses, see 11.6 and Fig. 11.16) is determined by their hardenability. The higher the hardenability of the steel, the slower the quenching cooling can be performed (see Fig. 5.22), therefore, for different steels, quenching fluids are used that provide different cooling rates.

Cooling (quenching) medium must provide a high cooling rate at temperatures of lowest stability of supercooled austenite (650 ... ... 550 ° С, see Fig. 5.7) in order to prevent its decomposition. On the contrary, in the temperature range of martensitic transformation (Mn ... Mk), slow cooling is advisable to reduce quenching stresses. The characteristics of the hardening media most used in the practice of heat treatment are given in table. 15.2.

Table 15.2

Cooling rate in various quenching media

				Cooling rate, ° С / s, at temperature, ° С
			Emulsion
			Machine oil
			Transformer oil
			Copper plates
Solution (10%) in water			Iron plates
			The air is calm
			Air under pressure

Water and aqueous solutions Are cheap and widespread coolers. Their advantage is a high cooling rate in the area of ​​minimum stability of supercooled austenite; the disadvantage is also a high cooling rate in the region of martensitic transformation (see Table 15.2). The use of these media increases hardenability, but increases the likelihood of deformations and cracks. Water is used in the hardening of carbon steels.

When quenched in water, a patchy hardness may appear (see 5.2.2). To prevent this rejection, aqueous solutions of salts and alkalis, which have a higher vaporization temperature, are used as quenching liquids. But at the same time, the cooling rate sharply increases (see Table 15.2), which determines a larger value of quenching stresses.

Oils in the range of Mn ... Mk provide, compared with water, a significant decrease in the cooling rate, this leads to a decrease in quenching stresses and deformations. However, cooling in the range of minimum stability of supercooled austenite slows down (see Table 15.2), so oils are used for hardening alloy steels with a higher hardenability.

Oil in water emulsion(emulsions consist of the smallest suspended droplets of oil in water) and water with temperature 30 ... 40 ° С reduce the cooling rate in the range of 650-550 ° С (see table 15.2) and thereby the probability of deformations, while reducing hardenability. These media are used for HFC hardening, when it is necessary to harden only the surface of the part.

For steels with deep hardenability, use as a quenching medium air - quiet, which provides a very low cooling rate, or under pressure when it is necessary to cool faster (see table 15.2). In both cases, the quenching stresses are low.

Cooling under metal plates also happens with low speeds(see table 15.2). This technology combines hardening with straightening (shape correction) and virtually eliminates deformations.

When hardening large-sized parts, use water-air mixtures. They are fed to the part through special nozzles. The cooling capacity of the mixtures can be adjusted by changing the amount of water in it and the air pressure.

Use as coolants aqueous solutions of polymers allows you to change the cooling rate within a wide range - between the rates of cooling in water and in oil. They are used for volumetric and surface hardening.

For many structural steels, Mn temperatures are in the range of 170-330 ° C. For their isothermal hardening(performed by holding at a temperature slightly above the Mn point) use molten salts. In particular, the mixture of NaNO3 (45%) and KNO3 (55%) already considered above, which is efficient in the range of 160 ... 650 ° C, is used.