Hardening of steels in water-polymer media

Machine parts and mechanisms are mainly made of steel. Particularly critical or loaded parts are made of carbon steels with mandatory hardening relative to the original state. The strength characteristics of materials are determined not only by their chemical composition, but also by the structure of the crystal lattice. Metals have different strength and hardness depending on the structure of the crystal lattice.

By heating and cooling metals, you can change the structure, and therefore influence their hardness and strength. The product at the workpiece level must be soft to facilitate machining. It becomes soft after annealing, when it has a pearlite crystal structure.

When steels are heated above the recrystallization temperature (GSE points on the iron-carbon diagram), the metal changes from α (alpha iron) to γ ​​(gamma) iron, this crystal lattice structure is called austenite. If γ iron is quickly cooled, then the majority of the atoms will not have time to rearrange into their usual α lattice. This produces a solid product that has predominantly a martensite structure – i.e. solid solution of carbon in γ iron. The martensite lattice is significantly deformed and changes from cubic to tetragonal. A structure consisting of martensite will have the highest possible hardness.

In practice, finished parts have a martensite and pearlite structure in various proportions. The required ratio between structures, and therefore hardness and viscosity, is obtained using a subsequent heating operation called tempering. When tempered, some of the atoms from the γ lattice are rearranged into their usual α lattice, and internal stresses and, accordingly, hardness are reduced. Moreover, the higher the tempering temperature, the more atoms will rearrange, and the product will be less hard and more viscous.

Martensite and martensitic transformation in steels

Martensite is a supersaturated solid solution of carbon in α-iron (α-Fe). Read what austenite, cementite, ferrite and pearlite are here. When eutectoid steel (0.8% carbon) is heated above point A1, the original pearlite structure will transform into austenite. In this case, all the carbon present in the steel will dissolve in austenite, i.e. 0.8%. Rapid cooling at a supercritical rate (see figure below), for example in water (600 °C/sec), prevents the diffusion of carbon from austenite, but the fcc crystal lattice of austenite will rearrange into the tetragonal lattice of martensite. This process is called martensitic transformation. It is characterized by the shear nature of the restructuring of the crystal lattice at a cooling rate at which diffusion processes become impossible. The product of martensitic transformation is martensite with a distorted tetragonal lattice. The degree of tetragonality depends on the carbon content in the steel: the more it is, the greater the degree of tetragonality. Martensite is a hard and brittle structure of steel. Found in the form of plates, under a microscope they look like needles.

The hardening temperature for most steels is determined by the position of the critical points A1 and A3. In practice, the hardening temperature of steels is determined using steel graders. How to choose the hardening temperature of steel, taking into account points Ac1 and Ac3, read the link.

Microstructure of steel after hardening

Most steels after hardening are characterized by the structure of martensite and retained austenite, the amount of the latter depending on the carbon content and the qualitative and quantitative content of alloying elements. For structural steels of medium alloying, the amount of retained austenite can be in the range of 3-5%. In tool steels this amount can reach 20-30%.

In general, the structure of steel after hardening is determined by the final requirements for the mechanical properties of the product. Along with martensite, after quenching, ferrite or cementite may be present in the structure (in case of incomplete quenching). When steel is isothermally hardened, its structure may consist of bainite. The structure, final properties and hardening methods of steel are discussed below.

Partial hardening of steel

Partial quenching is called quenching, in which the cooling rate is not sufficient for the formation of martensite and it turns out to be below critical. This cooling rate is indicated by the blue line in the figure. During partial hardening, the “nose” of the C-curve steel seems to be touched. In this case, in the structure of the steel, along with martensite, troostite will be present in the form of black island inclusions.

The microstructure of partially hardened steel looks something like this:

Partial hardening is a defect that is eliminated by complete recrystallization of the steel, for example, during normalization or during reheating for hardening.

What is the hardening of metal?

The ancient blacksmiths knew that the effect of high temperature on metal can change its structure and properties and actively used this in practice. Subsequently, 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. What’s noteworthy is that hardening parts made from inexpensive steel makes it possible 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 steel alloy products, is to heat the metal to a critical temperature and then cool it. The main goal pursued by this heat treatment technology is to increase the hardness and strength of the metal while simultaneously reducing its ductility.

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

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

After hardening, both the hardness and brittleness of the steel increases, 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.

Iron-carbon phase diagram

When hardening products made of steel alloys, it is very important to ensure a given cooling rate of the part, otherwise the already rearranged atomic structure of the metal may go into an intermediate state. Meanwhile, too rapid cooling is also undesirable, as it can lead to the appearance of 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.

To heat parts made of carbon steel, 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 made from them are heated in a different type of furnace. The convenience of hardening steel of these grades lies in the fact that products made from them are not subject to cracking when cooled, which eliminates the need for preheating.

You should take a very responsible approach to hardening parts of complex configurations that have thin edges and sharp transitions. To prevent cracking and warping of such parts during the heating process, 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.

Heating of steel during hardening with high frequency currents

For high-quality hardening of steels, it is important to ensure not only the level of heating, but also its uniformity. If the part is massive or has a complex configuration, it is possible to ensure uniform heating only in several approaches. In such cases, heating is carried out with two delays, which are necessary so that the achieved temperature is evenly distributed throughout the entire volume of the part. The total heating time also increases if several parts are placed in the oven at the same time.

Incomplete hardening of steels

Quenching at temperatures lying between A1 and A3 (incomplete quenching) retains in the structure of hypoeutectoid steels, along with martensite, part of the ferrite, which reduces the hardness in the quenched state and worsens the mechanical properties after tempering. This is understandable, since the hardness of ferrite is 80HRC, and the hardness of martensite depends on the carbon content and can be more than 60HRC. Therefore, these steels are usually heated to temperatures 30–50 °C above A3 (full hardening). In theory, incomplete hardening of steels is not permissible and is considered a defect. In practice, in some cases, incomplete quenching can be used to avoid quenching cracks. Very often this concerns hardening with high frequency currents. With such hardening, it is necessary to take into account its feasibility: type of production, annual program, type of product responsibility, economic justification. For hypereutectoid steels, quenching at temperatures above A1 but below Acm produces excess cementite in the structure, which increases the hardness and wear resistance of the steel. Heating above the temperature Acm leads to a decrease in hardness due to the dissolution of excess cementite and an increase in retained austenite. In this case, the austenite grain grows, which also negatively affects the mechanical characteristics of the steel.

Thus, the optimal quenching for hypoeutectoid steels is quenching from a temperature 30–50 °C above A3, and for hypereutectoid steels – at 30–50 °C above A1.

The cooling rate also affects the hardening result. The optimal cooling medium is one that quickly cools the part in the temperature range of minimum stability of supercooled austenite (in the range of the nose of the c-curve) and slowly in the temperature range of martensitic transformation.

Cooling stages during hardening

The most common quenching media are water of various temperatures, polymer solutions, alcohol solutions, oil, molten salts. When hardening in these environments, several cooling stages are distinguished:

— film cooling, when a “steam jacket” is formed on the surface of the steel;

- nucleate boiling, which occurs with the complete destruction of this steam jacket;

— convective heat transfer.

More details about the cooling stages during quenching can be found in the article “Characteristics of quenching oils”

In addition to liquid quenching media, cooling in a gas flow of different pressures is used. It can be nitrogen (N2), helium (He) and even air. Such quenching media are often used in vacuum heat treatment. Here it is necessary to take into account the fact of the possibility of obtaining a martensitic structure - the hardenability of steel in a certain environment, i.e. the chemical composition of the steel on which the position of the c-curve depends.

Cooling media for hardening

Cooling media for hardening

Cooling during hardening should ensure the formation of a martensite structure within a given cross-section of the product (certain hardenability) and should not cause hardening defects: cracks, deformation, warping and high tensile residual stresses in the surface layers.
A high cooling rate (above: the critical quenching rate) in the temperature range A

1 –
Mn
to suppress the decomposition of supercooled austenite in the region of pearlitic and intermediate transformations and slow cooling in the temperature range of the martensitic transformation
Mn
–
Mk
. A high cooling rate in the martensitic temperature range is undesirable, since it leads to a sharp increase in the level of residual stresses and even to formation of cracks.
At the same time, too slow cooling in the temperature range Mn
-
Mk
can lead to partial tempering of martensite and an increase in the amount of retained austenite due to its stabilization, which reduces the hardness of the steel.

Typically, boiling liquids are used for hardening - water, aqueous solutions of salts and alkalis, oils. When hardening in these environments, three periods are distinguished:

1) film boiling, when a “steam jacket” forms on the surface of the steel; during this period the cooling rate is relatively low;

2) nucleate boiling, which occurs when the vapor film is completely destroyed, observed when the surface is cooled to a temperature below the critical temperature; during this period, rapid heat removal occurs;

3) convective heat exchange, which corresponds to temperatures below the boiling point of the coolant; heat removal during this period occurs at the lowest rate.

Table 2 shows the approximate nucleate boiling temperature range and the relative cooling rate in the middle of this range for various cooling media.

When hardening carbon and some low-alloy steels, which have low stability of supercooled austenite, water and aqueous solutions of NaCl or NaOH are used as a cooling medium.

Water as a cooling medium has significant disadvantages. A high cooling rate in the temperature range of martensitic transformation often leads to the formation of hardening defects; with increasing temperature, the hardening ability sharply deteriorates (Table 1). When quenching products in hot water, due to their slow cooling at high temperatures and rapid cooling at low temperatures, thermal stresses are low, and the most dangerous structural stresses are high, which can cause the formation of cracks. Cold 8–12% aqueous solutions of NaCl and NaOH, which have proven themselves well in practice, have the highest and most uniform cooling ability.

When quenching in aqueous solutions, the steam jacket is destroyed almost instantly, and cooling occurs more uniformly and mainly occurs at the nucleate boiling stage. An increase in cooling capacity is achieved by using jet or shower cooling, which is widely used, for example, in surface hardening.

A further improvement in cooling methods was the use of water-air mixtures supplied through nozzles. Water-air media are used for cooling large forgings, rails, etc.

For alloy steels that have high resistance to supercooled austenite during quenching, mineral oil (usually petroleum) is used.

Oil as a quenching medium has the following advantages: a low cooling rate in the martensitic temperature range, which reduces the occurrence of quenching defects and the constancy of the quenching ability over a wide range of medium temperatures (20–150°).

Disadvantages include increased flammability (flash point 165–300 °C), insufficient stability and low cooling capacity in the region of pearlite transformation temperatures, as well as increased cost.

The oil temperature during quenching is maintained within 60–90 °C, when its viscosity is minimal.

In recent years, instead of water or oil, aqueous solutions of detergents containing surfactants, liquid silicate, and especially synthetic substances, such as aqua-plast, have begun to be used.

Aqua-plast is a concentrate in water of a highly viscous transparent liquid containing water-soluble plastic and a corrosion-protective inhibitor. Solution concentration 0.5–0.7%,

Quenching polymers and detergents in aqueous solutions leads to a decrease in the cooling rate at temperatures below M

n, and as a consequence of this, the formation of cracks is eliminated and warping is reduced.
Table 1
- Relative cooling capacity of quenching media

Factors influencing the position of c-curves:

- Carbon. Increasing the carbon content to 0.8% increases the stability of supercooled austenite, and accordingly the c-curve shifts to the right. When the carbon content increases above 0.8%, the c-curve shifts to the left;

— Alloying elements. All alloying elements increase the stability of austenite to varying degrees. This does not apply to cobalt; it reduces the stability of supercooled austenite;

— Grain size and homogeneity. The larger the grain and the more uniform its structure, the higher the stability of austenite;

— An increase in the degree of distortion of the crystal lattice reduces the stability of supercooled austenite.

Temperature affects the position of c-curves through all of the above factors.

Methods of hardening steels

In practice, various cooling methods are used depending on the size of the parts, their chemical composition and the required structure (diagram below).

Diagram: Cooling rates for different methods of hardening steels

Continuous hardening of steel

Continuous hardening (1) is a method of cooling parts in one environment. After heating, the part is placed in a quenching medium and left there until completely cooled. This technology is the most common and is widely used in mass production. Suitable for almost all types of structural steels.

Hardening in two environments

Quenching in two environments (speed 2 in the figure) is carried out in different quenching environments, with different temperatures. First, the part is cooled in the temperature range, for example, 890–400 °C, for example in water, and then transferred to another cooling medium - oil. In this case, the martensitic transformation will already occur in an oil environment, which will lead to a decrease in the leash and warping of steel. This hardening method is used for heat treatment of stamping tools. In practice, the opposite technological technique is often used - first the parts are cooled in oil and then in water. In this case, the martensitic transformation occurs in oil, and the parts are moved into water for faster cooling. This saves time on implementing the hardening technology.

Step hardening

During stepwise quenching (speed 3), the product is cooled in a quenching medium having a temperature higher than the martensitic transformation temperature. In this way, a certain isothermal holding is obtained before the transformation of austenite into martensite begins. This ensures uniform temperature distribution over the entire cross-section of the part. This is followed by final cooling, during which the martensitic transformation occurs. This method produces hardening with minimal internal stress. Isothermal holding can be done just below the temperature Mn, after the start of the martensitic transformation (speed 6). This method is more difficult from a technological point of view.

Isothermal hardening of steels

Isothermal hardening (speed 4) is done to obtain the bainitic structure of the steel. This structure is characterized by an excellent combination of strength and plastic properties. During isothermal hardening, parts are cooled in a bath of molten salts, which have a temperature 50–150 °C above the martensite point Mn, maintained at this temperature until the end of the transformation of austenite into bainite, and then cooled in air.

When hardening onto bainite, it is possible to obtain two different structures: upper and lower bainite. Upper bainite has a feathery structure. It is formed in the range of 500-350°C and consists of lath-shaped ferrite particles <1 µm thick and 5-10 µm wide, as well as thin cementite particles. The structure of upper bainite is characterized by higher hardness and strength, but lower ductility. Lower bainite has a needle-like martensite-like structure and is formed in the range of 350-200 °C. Lower bainite consists of fine particles of ε-carbides located in ferrite platelets. The bainite transformation never goes to completion. The structure always contains martensite and retained austenite. More preferable, in terms of performance characteristics, is the lower bainite structure. Products with such a structure are used in car construction, where parts experience shock-tensile stresses. The bainite hardening technology requires special hardening equipment. Additional materials on this technology can be found in the article “Technology of hardening for bainite.”

Cold treatment (5) is used for steels in which the temperature of the end of the martensitic transformation Mk is below room temperature.

High-speed steels, cemented parts, measuring instruments, and other particularly precise products are subjected to cold treatment. You can read more about this non-standard method of heat treatment in the article “Cold processing of steel parts”

Cooling modes during hardening

The most studied issues in materials science are the relationship between the chemical composition and the structure of the metal at certain temperatures. The most poorly studied area in hardening technology is methods, conditions and cooling modes. Meanwhile, it is in cooling that large reserves for controlling the structure and properties of the metal in finished products lie.

The main question of hardening is how intensely to cool? It would seem that if you cool it as quickly as possible, you will get maximum hardness, but at the same time, increased internal stresses will lead to the formation of cracks in the parts. The so-called hardening cracks, which are well known to all thermal experts. By cooling slowly, you will not achieve the required hardness and the part will need to be annealed and then undergo repeated heat treatment. Each brand has its own “critical” cooling rate, which ensures maximum hardness and will not lead to cracking. For example, 45X steel, depending on the type of coolant, can be hardened to HRC 45 or HRC 60. In order to “squeeze out” the maximum hardness, it is necessary to cool at a rate as close as possible to the critical speed, for a specific steel grade and workpiece geometry. From this we can draw a simple conclusion that the intensity of temperature reduction must be adjustable. There are only two widely used speeds: the cooling speed in water and in oil. Even taking into account that the intensity, in a small range, can be adjusted by temperature and circulation, the critical hardening rate may still not be obtained.

Water and oil media may only provide "critical" hardening rates in some applications. In addition, while working with water is relatively simple, oil hardening has specific features and disadvantages:

• insufficient cooling intensity for some brands;
• the ability to ignite, emit harmful vapors, smoke, coke on the walls of air ducts, etc.,
• good wetting of surfaces and, as a result, large removal of oil from workpieces, evaporation;
• changes in chemical composition under the influence of high temperatures;
• the need to wash the workpieces in detergents with further regeneration of oil films.

The disadvantages of traditional hardening options have contributed to the search for more optimal hardening mixtures and hardening methods, at least for some variants of workpieces and alloys. As a result, several options for hardening technologies and compositions have emerged that are better suited for certain types of products. The most widely used are liquid polymer concentrates combined with water. This technology first appeared in the Soviet Union in 1980.

Characteristics of water-polymer media

These compositions are a mixture of water and polymers in certain proportions

. Polymers are chemical compounds formed by long chains of macroparticles obtained by combining microparticles - monomers. This reaction is called polymerization. Mixing allows you to obtain a stable liquid with adjustable heat capacity, and therefore cooling capacity.

The basis of the composition of the liquid is water, even with altered properties. Therefore, there are restrictions on the use of water-polymer liquids. These environments are not recommended for hardening high-alloy tool and die steel grades, as well as parts of complex shapes or with variable cross-sections.

Polyacrylic iron salt is used as the initial polymer concentrate

brand PK-M. This polymer turned out to be cheap and had advantages over other polymers of similar composition. Polymer coolants were originally designed to replace oil to eliminate flammability. Soon they developed materials that were superior to oil in efficiency for some products. Other advantages of water-polymer media have also been discovered.

Average cooling results in various environments

Dependence of martensite hardness on carbon content

The hardness of steel after quenching depends on the hardness of martensite, which in turn depends on the carbon content. As the carbon content increases, the hardness after hardening of the steel also increases. The graphical dependence is shown in the figure.

Graph of martensite hardness versus carbon content
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