Hardening is a widely used heat treatment technology for steel products. Its essence is to heat the metal so that its temperature reaches a critical point, at which changes in the crystal structure occur or the process of phase dissolution begins to occur in the matrix formed at low temperatures of the part. After this, the metal cools sharply. As a result, the steel acquires a needle-type microstructure called martensite. Thanks to this phenomenon, the hardness of the alloy increases and its wear resistance increases.
Hardening temperature
The main criteria on the basis of which hardening modes are divided into types are the heating temperature and the speed of the technical process. There are also differences in such parameters as:
- time interval of exposure at certain temperature indicators;
- speed of the cooling procedure.
In general, based on the “heating temperature,” hardening can be of two types. Let's look at them briefly.
Complete hardening
Hypoeutectoid steel is processed by complete hardening. It is heated so that the final temperature exceeds the critical point Ac3 by 30°-50°. Then the mixture of ferrite and cementite is completely transformed into austenite. With further cooling, a predominantly martensitic structure is formed.
Hardening is incomplete
Tool steels are most often subjected to the incomplete hardening procedure. Carrying out heat treatment of this type has the goal of heating the product until the process of formation of excess phases begins. In this case, the following temperature range must be observed:
Ac1≤Т≤ Ac2, where
- T – heating temperature;
- Ac1, Ac2 – critical points. In the first (+727°C), recrystallization begins - pearlite is transformed into austenite. In the second (+768°C) α-Fe transforms into β-Fe and the steel loses its magnetic properties.
If this temperature range is maintained, the martensite structure will retain a certain amount of ferrite after quenching the steel.
Incomplete hardening of a hypereutectoid alloy is of the highest quality if the product is heated to a level exceeding Ac1 by 20°C-30°C. Then, during the process of heating and cooling, cementite will not be transformed. Because of this, the hardness of martensite will increase. If the heating temperature of the part goes beyond the above range, this characteristic may, on the contrary, worsen.
Cooling rate during quenching
The structure and properties of hardened steel largely depend not only on the heating temperature, but also on the cooling rate. The formation of hardening structures is due to the overcooling of austenite below the PSK line, where its state is unstable. By increasing the cooling rate, it is possible to supercool it to very low temperatures and transform it into various structures with different properties. The transformation of supercooled austenite can occur both during continuous cooling and isothermally, during exposure at temperatures below the Ar1 point (i.e. below the PSK line).
The influence of the degree of supercooling on the stability of austenite and the rate of its transformation into various products is presented graphically in the form of diagrams in temperature-time coordinates. As an example, consider such a diagram for steel of eutectoid composition (Figure 3). Isothermal decomposition of supercooled austenite in this steel occurs in the temperature range from Ar1 (727 °C) to Mn (250 °C), where Mn is the temperature at which the martensitic transformation begins. Martensitic transformation in most steels can only occur with continuous cooling.
Cooling Features
As is known, austenite is least stable at temperatures 550°С≤Т≤650°С. And the structure of martensite is formed when conditions are created for accelerated cooling of the alloy until its temperature index enters precisely this range. When the temperature falls below +240°C, the martensitic transformation is ensured by slow cooling. This technological solution leads to the fact that the stresses that arise in the metal body have time to level out. Moreover, without reducing the hardness of the formed martensite.
Successful heat treatment requires the correct choice of hardening medium. As such, the most commonly used are:
- mineral quenching oil;
- an aqueous solution of table salt (NaCl + H2O) or sodium hydroxide (NaOH);
- actually, water.
It is better to harden steel with alloying additives using oil. It is recommended to carry out this procedure with carbon alloys by cooling with water.
Influence of cooling rate on the structure and properties of steel
It is more convenient to clarify this question using the example of eutectoid steel (C = 0.8%). A series of samples are made from this steel, all of them heated to the austenitic state, i.e. above 727°C and subsequently each sample cools at a different rate (Fig. 38).
a b
Rice. 38. Diagram of isothermal decomposition of supercooled austenite of eutectoid steel with cooling curves superimposed on it:
a – general view; b – resulting structures
The transformation of austenite at temperatures of 550°C and above is called pearlite transformation, at 550°C...МН - martensitic (МН - beginning, МК - end of martensitic transformation).
Pearlite transformation. In the temperature range of pearlite transformation, lamellar structures of ferrite and cementite crystals are formed, which differ in the degree of dispersion of particles F and C.
The dispersity of pearlite structures is assessed by the interlamellar distance S of adjacent ferrite and cementite laminae (Fig. 39).
In order not to confuse cementite with ferrite, a special etchant is used - sodium picrate, which colors cementite black. Ferrite is not colored in this case, i.e. remains light.
Rice. 39. Ferrite-cementite structure
If the transformation occurs at temperatures of 650–670°C, then pearlite is formed, S = 6·10-4 mm.
At transformation temperatures of 640–590°C, sorbitol is formed,
S = 3·10-4 mm.
At transformation temperatures of 580–550°C, troostite is formed, S = 1´10-4 mm.
As can be seen from experience, with an increase in the cooling rate, the grains of the ferrite-cementite mixture are crushed more and more, which dramatically affects the properties. So, for example, for perlite NV 2000, for sorbitol NV 3000, and for troostite NV 4200, MPa.
Intermediate (bainite) transformation. As a result of the intermediate transformation bainite , which is a structure consisting of an a-solid solution somewhat supersaturated with carbon and cementite particles. The bainite transformation combines elements of pearlite and martensite transformations. Volumes enriched and depleted in carbon are formed in austenite. The carbon-depleted areas of austenite undergo a g ® a transformation in a diffusion-free manner (martensitic). In volumes of austenite enriched with carbon, at t = 400–550°C, cementite particles precipitate. At t < 400°C, cementite particles precipitate in a-phase crystals.
Bainite formed at temperatures of 400–550°C is called upper bainite; it has a feathery structure with worse mechanical properties (lower sv, KCU and d).
At lower temperatures (below 400°C), lower bainite is formed; it has a needle-like structure with better mechanical characteristics (higher sв, KSU and d).
Martensitic transformation of austenite. Martensite is a supersaturated solid solution of interstitial carbon in Feα
Martensite is formed only from austenite as a result of strong supercooling of the latter at a rate not less than the critical quenching rate (Vcr = - tangent to the diagram, see Fig. 38, a).
Martensitic plates (needles) are formed almost instantly, at a speed of more than 1000 m/s, only within the austenite grain and do not cross the boundary between grains. Therefore, the size of martensite needles depends on the size of austenite grains. The finer the austenite grains, the smaller the martensite needles and the structure is characterized as coarse-needle or fine-needle martensite. The martensite lattice is tetragonal, i.e. periods c > a (Fig. 40).
Rice. 40. Microstructure and crystal lattice of martensite
The mechanism of martensitic transformation is that at temperatures below MH, the austenite lattice, which dissolves carbon well (up to 2014% C), turns out to be unstable and is reconstructed into the Feα lattice , the ability of which to dissolve carbon is very low (up to 0.02%).
Due to the high cooling rate, all the carbon located in austenite (fcc lattice) remains fixed in Feα (bcc lattice), where there is no room for its placement. Therefore, excess carbon distorts the lattice, causes the appearance of large internal stresses and, as a result, hardness and strength increase, while toughness and ductility decrease.
The austenitic-martensitic transformation is accompanied by an increase in volume. All steel structures can be arranged (from maximum volume to minimum) in the following row: martensite – troostite – sorbitol – pearlite – austenite.
Difference from pearlite transformation:
1) high conversion rate;
2) the transformation is diffusion-free, i.e. without preliminary release of carbon and formation of Fe3C;
3) the transformation begins at the MN point and ends at the MC point, and the position of these points depends only on the chemical composition of the alloy;
4) in the structure of martensite there is always a small amount of residual untransformed austenite (up to 4%);
5) martensite lattice is tetragonal (a = b ¹ c).
Types of heat treatment. Heat treatment is a technological operation in which, by heating the alloy to a certain temperature, holding it at this temperature and subsequent cooling, structural changes occur, causing changes in the properties of the metals.
Heat treatment is usually carried out in cases where:
1) polymorphic transformations;
2) limited and variable (increasing with temperature) solubility of one component in another in the solid state;
3) a change in the structure of the metal under the influence of cold deformation.
The main parameters of heat treatment modes are: temperature and heating rate, duration of exposure at a given temperature, cooling rate.
The heating temperature of steel depends on the position of critical points, the type of heat treatment and is assigned based on an analysis of the state diagram of the alloy.
The heating rate depends on the chemical composition of the alloy, the size and shape of the parts being processed, the mass of the charge, the nature of the arrangement of the parts in the furnace, the type of heating device, etc.
Holding at a given temperature is necessary to complete the phase transformations occurring in the metal, equalizing the concentration throughout the entire volume of the part. The heating time (40) is the sum of the intrinsic heating time tн(2) and the holding time tв:
ttot = tn + tv (40)
where tв is taken equal to 1 min per 1 mm of thickness for carbon steels and 2 min for alloy steels.
tн = 0.1D K1 K2 K3(41)
where D is the size of the largest section (dimensional characteristic); K1 – medium coefficient (for gas – 2, salt – 1, metal – 0.5); K2 – shape coefficient (for a ball – 1, cylinder – 2, plate – 4, parallelepiped – 2.5); K3 – coefficient of uniform heating (universal – 1, one-sided – 4).
The cooling rate depends mainly on the degree of stability of austenite, i.e. on the chemical composition of the steel, as well as on the structure that needs to be obtained.
Depending on the cooling rate of carbon steel, the following structures are obtained: ferrite with pearlite, pearlite, sorbitol, trostite, martensite.
According to the Fe-Fe3C phase diagram, the temperature points forming the PSK line are designated A1; line GS – A3; line ES – Ast. if the heating process is considered, then the letter C (AC1, AC3) is placed in front of the digital index, and if in the case of cooling, r(Аrз, Ar1).
Carbon steels are subjected to the following types of heat treatment: annealing, normalizing, hardening and tempering.
Annealing steel. Annealing purpose:
1) correction of the structure after hot processing (forging, casting);
2) reduction of hardness to facilitate cutting;
3) relieving internal stress;
4) preparing the structure for subsequent heat treatment and cold stamping;
5) reduction of chemical heterogeneity.
During complete annealing, the steel is heated above the AC3 line by 30–50°C, maintained for the required time at this temperature and then slowly cooled, as a rule, along with the furnace (Fig. 41).
When heated above the AC3 point, recrystallization occurs, as a result of which the grains are crushed, internal stresses are eliminated, and the steel becomes soft and viscous. Hypoeutectoid steels are predominantly subjected to full annealing.
When these steels are heated below AC3, some of the ferrite grains remain in the same form as they were before annealing (large sizes, plate-like shape), which leads to a decrease in the toughness of the steel.
During incomplete annealing, the steel is heated above the AC1 line by 30–50°C and, after holding, it slowly cools along with the furnace. With incomplete annealing, only partial recrystallization occurs (pearlite-austenite). This type is used for hypereutectoid steels.
Heating these steels above the Acm line (austenitic state) is impractical, since cementite dissolved in austenite during subsequent cooling will precipitate along the boundaries of pearlite grains in the form of a network, which sharply reduces ductility and makes the steel brittle.
Diffusion annealing (homogenization) is used to level out chemical heterogeneity across a crystal object in large castings. It is carried out at a temperature of 1050–1150°C and with longer exposures (10–18 hours).
Recrystallization annealing is used to remove cold hardening and internal stresses in steel after cold pressure treatment (rolling, stamping, drawing, etc.). For carbon steels, this type of annealing is carried out at a temperature of 650–690°C. As a result, hardness decreases and ductility increases.
Rice. 41. Optimal heating temperatures for various types of annealing
Normalization of steel. A type of heat treatment consisting of heating steel 30–50°C above the GSE line (Асз and Аcm), holding at this temperature and subsequent cooling in still air is called normalization .
The purpose of normalization is grain refinement, improvement of mechanical properties, preparation of the structure for final processing (hardening and tempering).
Compared to annealing, the cooling rate during normalization is much higher, so the decomposition of austenite occurs at high supercooling, which leads to the formation of fine-grained decomposition products - ferrite and pearlite, and, consequently, an increase in hardness.
Normalization, as a type of heat treatment, is used mainly for low-carbon construction steels. Normalization eliminates the cementite network in hypereutectoid steels when preparing them for hardening.
Hardening of steel. A type of heat treatment consisting of heating steel to a temperature above the line AC3 (hypoeutectoid steel) or AC1 (hypoeutectoid steel) by 30–50°C, holding at this temperature and subsequent rapid cooling in water or oil (Fig. 42) is called hardening .
Rice. 42. Optimal heating temperatures for hardening carbon steels
The purpose of hardening is to increase the hardness, strength and wear resistance of steel by obtaining a martensite structure, which has a characteristic needle-like structure.
To transform austenite into martensite, the cooling rate must be greater than the critical quenching rate Vcr.
The critical quenching rate is the lowest cooling rate at which all austenite is supercooled to the point of martensitic transformation. If the cooling rate is less than Vcr , austenite decomposes into a ferrite-cementite mixture (cane, sorbite, pearlite, see Fig. 38).
The required cooling rate is ensured by selection of the cooling medium. In industrial practice, water, mineral oils, aqueous solutions of salts, and alkalis are used for hardening. The main advantage of oils compared to water is slow cooling in the martensitic region (below 300°C), as a result of which quenching in oil produces less deformation, stress and the tendency to form quenching cracks.
A distinction is made between complete and incomplete hardening of steel. When fully hardened, the steel heats up 30–50°C above the critical point AC3. Hypoeutectoid steels must be fully hardened, i.e. heated until the ferrite-pearlite structure completely transforms into austenite. With subsequent cooling at a rate above critical, the steel acquires a martensite structure.
Underheating of hypoeutectoid steel to the AC3 point leads to the retention of a certain amount of ferrite in the structure of the hardened steel along with martensite and, consequently, to reduced mechanical properties after hardening. Such hardening is called incomplete, and for hypoeutectoid steel it is a defect (Fig. 43, b).
Rice. 43. Structural transformations in hypoeutectoid steel during hardening:
a – full hardening; b – incomplete hardening
When hypereutectoid steel is incompletely quenched (heating above the AC1 point, but below the Acm point), the remaining undissolved cementite increases the hardness of the steel after quenching, since it is a strengthening phase. If hypereutectoid steel is heated above the Acm line, then its structure will contain coarse-needle martensite with an increased amount of retained austenite (see Fig. 44, b). Thus, if for hypoeutectoid steels incomplete hardening is a defect, then for hypereutectoid steels it is the main type of hardening.
Steel tempering. A type of heat treatment consisting of heating martensite-hardened steel to a temperature below the PSK line (AC1), holding at this temperature and subsequent cooling is called tempering .
Heating steel during tempering facilitates the transition from a metastable state of a supersaturated a-solid solution to a more stable one. When tempering with increasing temperature in hardened steel, carbon is released from martensite, which is accompanied by a decrease in the crystal lattice, the formation and coagulation (coarsening) of cementite particles.
With increasing tempering temperature, tensile strength and hardness decrease, while ductility and toughness increase.
Rice. 44. Structural transformations in hypereutectoid steel during hardening:
a – incomplete hardening; b – full hardening
In accordance with the technical requirements for products in practice, the following types of tempering are used: low, medium and high.
Low tempering is characterized by low heating temperatures (150–250°C) and is used for products that require high hardness (HRC 56–64) and wear resistance. Low tempering slightly reduces the hardness of hardened steel and increases toughness, relieving internal stresses in products. The properties of steel after tempering depend not only on the heating temperature, but also on the duration of exposure. Low tempering is used for cutting tools, chisels, rolls, gauges, templates, and products made of case-hardening steels.
The structure of steels after low tempering is tempered martensite.
Medium tempering is characterized by heating hardened steel to a temperature of 300–400°C and provides relatively high hardness HRC 40–54 and a maximum elastic limit with sufficient strength. This type of tempering is used for springs, springs, dies, impact tools, etc. The structure of products after medium tempering is tempered.
High tempering is carried out by heating hardened steel to 500–600°C and is used for products made of structural steels exposed to high dynamic, alternating or static stresses. The structure of steel after high sorbitol tempering.
Characteristics of steel
In the context of this topic, steel has two important characteristics.
Hardenability
This characteristic reflects the fact how capable the steel is of becoming hard after undergoing the hardening procedure. There are alloys whose properties practically do not change as a result of this heat treatment, that is, the hardness remains at an insufficient level. They say about such a metal: “does not accept hardening.”
Metallurgy explains the high hardness of carbon-containing martensite by the distortion of its crystalline cells. This factor makes plastic deformation of the material difficult. The hardness index increases with increasing amount of carbon. In numbers it looks like this: the value of this parameter, established using the Rockwell method with the content of the element carbon (C) in steel at the level:
- 0.1%, equal to 30HRC;
- 0.7% is 64HRC.
But a further increase in the amount of carbon in the alloy does not lead to a significant increase in the hardness index. All this is displayed on the graph.
The following designations are used on it:
- pos. “1” – the heating temperature exceeds the Ac3 point;
- pos. “2” – the heating temperature of the product is 770°C, which is higher than only point Ac1;
- pos. “3” is an indicator of martensite hardness.
Typically, alloys with a carbon content of less than 0.3% are not subjected to the hardening procedure due to their low degree of hardening.
Hardenability
This characteristic indicates the depth of hardening of the steel. During this process, the core of the part cools more slowly than its surface. This phenomenon is explained by direct contact of the outer layer with a cooling substance that absorbs thermal energy. The situation is different with the central fragment of the product. Its heat is transferred through the thickness of the metal to the surface area, and there it is absorbed by the same cooling substance.
Hardenability is a characteristic derived from the critical hardening rate. This is understood as the lowest rate of supercooling of all austenite to the martensitic structural transformation. The depth of hardening is inversely proportional to this parameter. That is, the lower the speed of the above process, the deeper the hardening of the metal occurs. This is clearly manifested in alloys with large and small grains. The former are calcined to a greater depth than the latter, since they have a low critical speed.
Types of hardening
Quite a lot of methods for hardening metal have been developed today. When choosing a specific one, you need to consider:
- chemical composition of the material;
- design features of the product;
- a given hardness indicator of the final product;
- cooling process conditions.
Quenching in one environment
To better understand the features of the hardening procedure, consider the image below. It shows graphs of cooling lines characteristic of various methods of such heat treatment.
The progress of hardening in one environment is shown by curve “1”. This method can be implemented without any particular difficulties. But it does not apply to all steel products. In particular, problems may arise with parts with variable cross-sections. An accelerated decrease in their temperature indicators leads to:
- formation of internal stresses;
- temperature unevenness.
The combination of these factors usually causes warping and cracking of such products.
When performing this hardening method, cracks can also form in parts made from alloys with a high content of the element carbon. In this case, volumetric transformations of structural stresses cannot be excluded. For hardening in one environment, products with a simple configuration made from hypereutectoid steels are better suited.
Hardening in two environments
This method is represented by curve “2” in the above figure. Hardening in two environments is most often applied to tools made from steels with a high carbon content. This heat treatment method is implemented in 2 stages:
- the product is first immersed in water, where its temperature will enter the range of 300°C≤T≤400°C;
- then the part is moved into an oil-cooling working environment. There the product remains until it cools completely.
Step hardening
The features of stepwise hardening are shown by curve “3”. This method is done like this:
- First, the steel product is placed in a bath of molten salts. Here it is necessary to control that the temperature of the coolant exceeds the martensitic transformation temperature (this range is 240°C≤T≤250°C);
- then the part is cooled in oil or in open space under natural environmental conditions.
With step hardening, the likelihood of warping or cracking of the product is zero. Workpieces with a cross-section of no more than 30 mm, made from steels with alloying additives, as well as products with a cross-section not exceeding 8 - a maximum of 10 millimeters, made of carbon alloys, are subject to such heat treatment.
Isothermal hardening
In the above figure, heat treatment of this type corresponds to curve number 4. The technique for performing it is similar to the previous method. The difference lies in the length of time the alloy is kept in a bath of molten salts. For isothermal hardening, this time interval is longer.
This technological solution ensures comprehensive decomposition of austenite. On the graph, the shutter speed is shown by points “a” and “b” on the S-shaped line. There are no restrictions on the cooling rate of the alloy subjected to isothermal hardening - it can take values from any range. This heat treatment method has another advantage: the metal of the final product acquires toughness.
Light hardening
Carrying out light hardening requires the use of a specially equipped furnace. It must contain a protective environment. To obtain a light surface for the workpiece, which also does not have visible flaws, it is recommended to use step hardening. Upon completion, the steel must be cooled in a molten substance with the following chemical formula: NaOH is a caustic alkali. Before the hardening procedure, the product is heated in equipment called a salt bath filled with sodium chloride. The temperature should exceed the Ac1 point by 20°C-30°C. During cooling, the temperature of the medium is maintained in the range of 180°C-200°C. It includes:
- caustic soda (NaOH) – 25%;
- caustic potassium (KOH) – 75%.
This mixture is diluted with water in an amount of about six to eight percent of the total mass of alkaline components.
Hardening with self-tempering
This method is used in the production of tool steel. The essence of the technology is the removal of a steel product from the cooling environment until it cools completely. After this operation, thermal energy is retained in the thickness of the metal. At her expense, in fact, further vacation is carried out.
But you need to perform subsequent actions related to this procedure while monitoring the temperature of the part. Only when this characteristic reaches the value required for tempering is the product moved to a quenching environment, where it is finally cooled.
Control of the tempering itself is carried out on the basis of tarnish colors. They represent a spectrum of different shades that appear on the surface of the alloy when an oxide film forms on it. This phenomenon occurs when the metal temperature varies in the range 220°С≤Т≤330°С.
Self-tempering hardening is used in the manufacture of hammers for masons and mechanics; chisels of all kinds, from scarpels to cross-mixers; sledgehammers, both sharp- and blunt-nosed. In general, for tools that require high surface hardness without sacrificing toughness.
Cooling methods
When hardening steel parts, which is carried out with accelerated cooling, the likelihood of significant internal stresses occurring is very high. For this reason, metal warping occurs. Even cracking is possible. Preventing these negative phenomena is possible by cooling products in an oil environment, of course, if their production technology allows this.
A different approach is relevant for carbon steels. They cannot be cooled in oil. Therefore, this operation must be performed in water.
In addition to the cooling medium, the method of immersing workpieces into it is important from the point of view of the formation of internal stresses. In this case, the following rules should be followed:
- It is necessary to immerse parts, the design of which includes thin and thick fragments, into the hardening substance, starting with the larger one;
- drills, tools with which internal threads are cut - taps - in general, products characterized by an elongated configuration should be immersed without allowing their longitudinal axis to deviate from the vertical. Then they won't warp.
There are cases when it is necessary to harden only part of the part. This problem is solved by using local heat treatment. Only the required fragment of the product is heated, and the entire part is subject to immersion in the quenching liquid.
Possible defects during hardening
During the hardening process, some defects may appear in the workpieces. Only the most significant ones are described below.
Insufficient hardness
Insufficient hardness in a product that has undergone a hardening procedure most often appears when:
- the temperature of the heat treatment performed was incorrectly selected;
- the cooling rate was lower than that specified in the flow chart.
For example, during hardening of hypoeutectoid steels, this defect usually occurs due to the retention of ferrite in the structure of the alloy. This phenomenon occurs due to a violation of technology. In this case, the quenching temperature was simply not brought to the value corresponding to point Ac3.
Continuing the conversation about hypoeutectoid alloys, it is necessary to note another possible reason for the insufficient hardness of the material. This is overheating. As a result, martensite is formed, characterized by a coarse-needle structure. This structure not only reduces the hardness of the metal, but also reduces its impact strength. By the way, overheating manifests itself in a similar way in hypereutectoid steels.
Formation of soft spots
The reasons for the formation of soft spots are as follows:
- heterogeneity of the alloy structure;
- during the cooling process, the products came into contact with each other;
- uneven cooling;
- the presence of grease stains on the surface of the parts.
To correct this defect, the product is hardened again. Elimination of structural heterogeneity is carried out by preliminary normalization.
Carbon oxidation and combustion
Decarburization (the so-called burnout of carbon during hardening) and oxidation occur as a result of the interaction of the surface layer of the product with molten salts or furnace gases. The combination of these defects poses a particular danger to cutting tools. Its durability decreases significantly.
Such a defect in heat treatment cannot be corrected. The only thing that can save the situation is a sufficient allowance. Then the defective layers are removed by mechanical processing, and sometimes only grinding is sufficient.
Burnout
Burnout occurs when the heating temperature approaches the melting point of the metal. For this reason, what happens:
- penetration of oxygen into the thickness of the steel, accompanied by the formation of oxides at the grain boundaries;
- melting the material along the grain boundaries. This phenomenon, although rare, does happen.
As a result, the continuity of the alloy is broken, which puts it in the category of irreparable defect. That is, it is unsuitable for use.
Hardening cracks
The reasons for the appearance of hardening cracks are as follows:
- a part in the design of which there were sharp changes in the configuration of sections was subjected to heat treatment. It is in these places that significant internal stresses are formed, causing cracking;
- cooling was carried out extremely quickly;
- heating was carried out unevenly and also unnecessarily accelerated.
Another possible option for the appearance of cracks is that the product was subjected to a tempering procedure with some delay (not directly after hardening), due to which the internal stresses were not leveled in a timely manner.
Warping and deformation
Distortion of the product configuration - warping - causes uneven cooling. The change in volumetric characteristics - deformation - is associated with structural transformations that occur during heat treatment. These defects in the hardened alloy are due to the difference in the specific volumes of the formed structures. In particular, the value of this parameter of pearlite is less than that of martensite. In addition, thermal and structural stresses have different effects on the change in shape of different products.
To prevent the formation of these defects, the cooling procedure must be carried out at a slow rate in the temperature range of martensitic transformation using both isothermal and step hardening methods.
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.
