Heat treatment. What is good and what is bad.
As a rule, when buying a knife, a typical customer will definitely ask two questions:
1. What steel is the knife made of?
2. What is the hardness?
That is, even a non-specialist somewhere in the depths of his soul understands that iron glands are different and can be processed in different ways. The latter, however, is obviously not for everyone.
Which method of heat treatment to choose (nitriding, hardening, TVF, cementation)?
It all depends on the product and its use.
- Cementation is used for increased loads of parts, gears with high revolutions. But not all steel was cemented.
- TVC (Thermal Vacuum Condensation Hardening) - local heat treatment, performed on parts that need to be hardened some part of the neck, sprocket teeth, etc.
- Volume hardening - the entire part is hardened, but there are possible leaks after heat treatment, it all depends on the selected material.
- Nitriding of metals - gives a high hardness of more than 60 HRC, they withstand friction well, but the nitriding layer is relatively small, not all materials are suitable for nitriding.
Cementation of steel is a type of chemical and thermal treatment , which consists in surface diffusion saturation of low-carbon steel with carbon in order to increase hardness and wear resistance . Cementation with subsequent heat treatment simultaneously increases the endurance limit .

Low-carbon and alloy steels are subjected to cementation . The process in the case of using a solid carburetor is carried out at temperatures of 900...950 °С , in case of gas cementation (gaseous carburetor) - at 850...900 °С.
After cementation, the products are subjected to heat treatment ( tempering ), which leads to the formation of the martensitic phase in the surface layer of the product (martensite hardening), followed by tempering to relieve internal stresses .
Very often you can see statements like "I just bought a knife with 95X18 - it's a complete threshing floor, it crumbles on sausage, it's dull on oil." And then - "But you're driving, I've sorted out my three boars and at least henna." In general, the degree of user satisfaction with a knife is an extremely multifaceted issue, but it also includes steel and its maintenance. Which can be different. Sometimes strongly.
Hardening using TVC (Thermal Vacuum Condensation Hardening) is the most common type of surface hardening.
Due to the rapid heating, the hardness on the surface of the part is 2-4 units higher than during volume hardening.

The raw, viscous core helps reduce fragility.
The depth of the hardened layer ranges from 1 to 5 mm.
tvch vtulka
The TVC installation consists of a high-frequency generator, a transformer and the inductor itself.
The inductor is made of a copper tube. The type of inductor depends on the shape and dimensions of the part.
Most often, the appearance of the most common inductor resembles a coil, which can have one or more turns.
Volume-surface hardening (of metals) is a hardening in which the depth of annealing is regulated by the chemical composition of the steel and the mode of induction heating; the depth of the semi-martensitic layer at the same time is 015-025 of the diameter or thickness of the product.

OPZ technology has a number of advantages over similar Russian and foreign analogues, in many of which it outperforms the nearest analogues by 3-10 times.
The main advantages are:
- Extremely long service life of products.
- The hardening cycle from the standard 12-20 hours to 1-5 minutes.
- There are no thermal deformations of the parts after their heat treatment.
- The cost of one low-incandescence steel is much lower than alloy steel.
- Reduction in the need for electricity by 10-12 times.
- The possibility of adjusting the specified parameters in different places of the same product.
- The possibility of adjusting the service life of products.
- Absolute environmental friendliness.
The main areas of application of OPZ technology: strengthening of parts of drilling equipment, parts of the ball-bearing group, gears, shafts, springs, etc.
Nitriding of metals (sometimes nitriding, but not to be confused with nitriding in organic chemistry) is a type of chemical-thermal treatment, which consists in saturating the surface layer of metal products with nitrogen to increase hardness, resistance to cracking, durability and corrosion resistance in various aggressive environments.

High-strength nitriding of products is carried out at a temperature of 500...600 °C in chamber, shaft, container or cap furnaces, into which a stream of dry ammonia is fed. In the furnace, ammonia breaks down into hydrogen and nitrogen. Alloying elements (aluminum, molybdenum, vanadium, chromium) form stable chemical compounds with nitrogen - nitrides, which give products high hardness (1200 according to Vickers). The thickness of the nitrided layer is 0.3...0.6 mm when kept in the furnace for 24 to 90 hours.
Nitriding increases the hardness of the surface layer, wear resistance, durability and corrosion resistance of steel products. The nitrogenized layer has a hardness of 700...1200 HV, (58...72 HRC), which is maintained at working temperatures up to 600 °C.
Engine cylinders and valves, gear teeth, valve seats, spindles and lead screws of high-speed machines, etc., are subjected to nitriding. Nitriding also increases the stability of measuring tools used in mechanical engineering (threaded plugs and rings, flat calipers, staples, templates, etc.). Products are nitrided after mechanical and heat treatment.
Carbon steel products are also subjected to anti-corrosion nitriding (temperature 600...700 °C) with the formation of a nitrided layer 0.01...0.04 mm thick when kept in the furnace for 15 minutes for small parts and up to 10 hours for large ones.
So what is heat treatment and what is it eaten with?
Well, it is already clear from the name that this term describes many methods of processing materials based on changing their structure (and, accordingly, properties) under the influence of temperatures. Often when applied to the finished product, all of this is often referred to as "tempering", although the actual tempering is only one of the stages. Sometimes, including hot deformation, all this is called TMO (thermomechanical treatment), which is mostly fundamentally incorrect. Heat treatment usually includes several stages (sometimes several dozen). They all have different goals and different modes. Adding to the confusion is the fact that in the theory of heat treatment and in practice quite often individual processes have different names depending on the purpose and place in the technological cycle. We will not go into the slums, the main stages and their regimes are more important to us from the point of view of the impact on the final result.
I think that it will be easier to analyze it on the example of a typical blade production technology (with an indication of the main technological processes), which is used by the vast majority of Russian (and global) manufacturers. Consider a typical scheme used by private craftsmen and small-scale manufacturers.
FORGING is one of the types of pressure treatment of metals, which is based on the ability of metals to be plastically deformed - to change shape without breaking under the influence of external forces. Metals and alloys with fairly high plasticity and low resistance to deformation are subjected to forging. During forging, the workpiece is struck multiple times with a special tool. It is usually installed loosely on the anvil or in a padded stamp.
1. Normalization (sometimes + high leave)
(cutting blanks)
2. Annealing or TCO.
3. Hardening with MKO
4. High holiday
5. Hardening
6. Cryo treatment
7. Resulting holiday
(Rough grinding)
8. Holiday after grinding
(clean grinding and proofing)
In the event that it is processed by cutting, there may be additional vacations (or dropped).
In mass production of identical parts, stamping or pressing is more often used. The main operations during forging:
During deposition, the material is flattened - the width and length are increased, and the thickness is reduced until it is given the required size and shape;
When planting, thickening is created due to a decrease in length. In the process of manual or machine forging. use basic, supporting and measuring tools.
- протягання,
- rolling out,
- прошивання,
- cutting,
- bending and straightening.
Among the main tools for manual forging are sledgehammers, hammers, hand tools, chisels, chisels, crimpers, and breakers; among the main machine tools are punches (flat, shaped, rounded, combined), plates, piercing. Pliers and risers are used as supporting tools for manual forging, and pliers and chucks for machine forging. Blanks for forging can be ingots, graded and profiled rolled products, sheets, etc. Before forging, steel blanks are preferably heated in flame or various types of electric furnaces to a temperature of 1100–1250 °C.
Advantages of metal processing by forging compared to cutting:
- lower (about 30%) consumption of metal per product;
- higher productivity of the equipment;
- the metal becomes denser in its structure.
Forging without heating was known even in the late Neolithic times, with the help of which metal nuggets (copper, gold, silver, meteorite iron) were processed.
Let's consider the influence of individual stages on the properties and quality of products.
1. Normalization (sometimes + high leave) - allows you to bring the structure of the steel "to a common denominator" from which you can dance further, relieve tension, grind the grain, in some cases remove the carbide mesh or obtain the necessary hardness for processing. It is carried out in the form of heating to temperatures above the temperature of phase transformations (often to temperatures that cause significant dissolution of carbides) and cooling in still air. At the same time, many steels are able to be roasted and obtain high hardness - in this case, a high release is added.
2. Annealing or TCO – Allows to grind grain, reduce hardness to minimum values (for cutting or cold deformation), remove residual stress. It is carried out by heating to temperatures slightly above the temperature of phase transformations (in some cases - the intercritical region) and slow cooling to the temperature at which pearlite decomposition ends. It is often beneficial to replace annealing with thermocycling - repeated heating-cooling cycles to temperatures above/below the phase transformation temperatures, respectively. Such processing allows you to significantly grind the grain to a greater extent and, as a result, to obtain noticeably the best fur. Characteristics.
3. Hardening with MKO. It allows to significantly reduce the leash and warping of parts, thanks to the closure of micropores, in some cases, it slightly increases hardness and fur. Steel indicators. It is performed as "soft" quenching from the intercritical region, as a rule, by cooling in oil.
4. High vacation (from the point of view of maintenance theory - pre-critical annealing) - relieves tension after fur. processing, which prepares the steel structure for hardening, in some cases reduces the steel hardness to minimum values.
5. Hardening - The main stage of maintenance. It consists in heating to temperatures above the phase transformation temperatures and, as a rule, causing significant dissolution of carbides, which create the necessary saturation of the solid solution with carbon and alloying elements, and rapid cooling (at a speed above the critical), which fixes this solid supersaturated solution.
6. Cryotreatment - cooling the product to low temperatures (usually -78 - 196C). It is intended to enable a more complete transformation of residual austenite, which increases hardness, resistance to crumpling and reduces the risk of austenite transformation during operation, but may reduce viscosity.
7. The resulting vacation - forms the final properties of the blade. Heating is usually carried out to relatively low temperatures (sometimes medium temperatures). When hardening on Tue
8. Vacation after grinding - relieves grinding stresses and sometimes stabilizes the austenite formed during grinding.
Not all stages are always necessary, some can partially or completely replace each other - it all depends on the steel and the technological cycle. In the case of the purchase of semi-finished products, a significant part of maintenance has already been done at the enterprise - manufacturers.
Maintenance stages are usually divided into preliminary and resulting maintenance. The resulting MOT forms the properties of the finished product (as a rule, this is all, starting from the last high-temperature stage - hardening), the task of MOT is to ensure the necessary technological properties and prepare the structure for the resulting MOT.
Of course, it is the resulting maintenance that most affects the "basic" properties of the steel, but it is the maintenance that often allows you to "squeeze" the maximum of what it is capable of from the steel.
Of course, there are no free cakes. As maintenance becomes more complicated, labor costs, equipment loading, etc. increase. Which inevitably leads to an increase in the price of products. Often reusable. Therefore, it is too optimistic to look for diamonds in the middle of the thicket. On the other hand, trying to squeeze the maximum can lead to such costs that the product acquires the status of "exclusive" with a corresponding price. We have to stop somewhere. Where exactly - each manufacturer decides for himself. More precisely, where his target buyer stops.
Let's consider the main options.
1. Shackled, heated in a furnace until bright yellow-hot, put in oil. I held it over the coal for 5 minutes - that's all... In this case, it is quite optimistic to count on at least an average result for this steel. With great experience, everything is possible...
2. Gave it to "some thermist" from the defense plant. What and how he did with the railway - this secret is big... The result - from a complete mess to very good, although with a noticeable advantage of the first. Everyone decides on personnel.
3. There is a stove, there is a "data sheet", there is a strip of bourgeois steel. Knowledge and understanding of what and how - no. If you don't mow particularly hard, you can get a good result. Especially with modern steels - they are usually quite error tolerant.
4. The same + minimal ideas about what, where and why. As a rule, by accumulating and understanding one's own and other people's experience and personal responsibility, it is possible to obtain consistently good results.
5. Have a clear understanding of the subject and/or vast personal experience. Plus interest in the result and personal responsibility. These are prerequisites for obtaining stable results that are significantly above average. The author's maintenance schemes often allow you to squeeze much more out of the steels than what is expected of them.
6. Blades - champions also require some luck.
Let's consider the main errors during maintenance and their impact on product quality.
1. Insufficient hardness - as a rule, the result of underheating during hardening (rarely - overheating) or excessively high relaxation. In moderate forms, it is found on inexpensive knives as a compensator for overly simplified maintenance.
2. Excessive hardness and fragility of "Perecal". But here everything is more complicated. It is often not a question of high hardness, but of overheating during hardening (or not carried out PTO), when the steel gets too large a grain. Actually, hardness is not the only indicator of the quality of maintenance - the same hardness can be reached in different ways and with different results. So statements like "A knife higher than 58HRc is as fragile as glass" should be taken with healthy skepticism.
3. Carbonless layer. In the absence of protective atmospheres/coatings or vacuum equipment, there is almost always. When etched, it usually looks noticeably lighter than the background. With proper planning of the technical process, this layer is removed, but in some cases (for example, when hardening a thinly reduced workpiece or performing a knife with "chisel" sharpening without removing the decarburized layer), it can appear on the RK, with the saddest consequences for the latter. Sometimes it can cause errors regarding the hardness - there it will be noticeably less than on the body of the blade and RK.
4. Cracks. They can appear at various stages of production, most often during forging, hardening or grinding. It is an unconditional irreparable marriage. The sale of such a blade (with the exception of VERY rare cases on multilayer swords or damascus) is a direct indication of the manufacturer's attitude to the matter. Crazy attitude.
5. Floods and warping. At long range they are practically inevitable, at short sword they are permissible to a certain extent.
In conclusion, some real stories about different knives.
1. During tempering, blacksmith A screws several dozen blanks with pins, throws them into the furnace, and goes to drink vodka. After a few hours, he returns, throws the "sandwich" into the oil tank, and goes to drink vodka. He doesn't take vacations - why is there anyway 58...
2. For many years, blacksmith B has been forging X12MF at temperatures 50 degrees higher than optimal. To a reasonable question about the reasons - "I always do this, people don't complain."
3. Enthusiast B decided to carry out cryoprocessing by quenching the pre-heated workpiece in liquid nitrogen. At the suggestion to first find the value of the heat of vaporization for liquid nitrogen in two days, he thoughtfully expressed "Blah".
4. Blacksmith G forges each workpiece differently. At the same time, he himself does not feel them and does not systematically collect reviews. Looking for a person...
5. When tempering EACH blade, in addition to the author's maintenance and testing for hardness, master D always controls breakage - just in case. This is a request for a responsible attitude to the matter, which manifests itself in other issues and finds its mark in the price of the products.
So, choosing a MOT, you choose a MANUFACTURER. Different craftsmen may have different views on maintenance, but a responsible and self-respecting and consumer manufacturer will never sell a product with properties below a certain minimum. And in the case of marriage (which does not happen), he will make every effort to resolve the situation.
Processes of heat treatment of steel
Steels can be heat treated to obtain a wide range of microstructures and properties. Generally, heat treatment uses a phase transformation during heating and cooling to change the microstructure of the solid state. In heat treatment, the treatment is usually thermal and only changes the structure of the steel. In the case of thermomechanical processing of steels, the shape and structure of steel components also change. In the case of thermochemical treatment of steels, the chemical composition of the surface and the structure of the steel are modified. Thermomechanical and thermochemical treatment processes are also important approaches to the heat treatment of steel and are discussed in the field of heat treatment. The process of heat treatment requires careful control of all factors affecting the heating and cooling of steel. The atmosphere of the heating furnace also affects the condition of the steel subjected to heat treatment.
All heat treatment processes consist of subjecting steel to a certain temperature-time cycle. This time-temperature cycle consists of three components, namely (i) heating, (ii) holding in a certain temperature range (soaking), and (iii) cooling. Individual cases may differ, but certain fundamental goals are the same.
The heating rate of the part depends on several factors. These factors are: (i) the thermal conductivity of the steel, (ii) the condition of the steel, and (iii) the size and cross-section of the steel. An important factor is the thermal conductivity of steel. Steel with high thermal conductivity heats up faster than steel with low thermal conductivity. The rate of heating is not particularly important unless the steel is under severe stress, for example, as a result of severe cold working or pre-hardening. In such cases, the heating rate should be slow. This is often nearly impossible as the furnaces used for heating can be at operating temperature and placing cold steel in a hot furnace can cause warping or even cracking. This hazard can be minimized by using a preheating furnace maintained at a temperature below temperature A1 on the iron-carbon phase diagram (Fig. 1). The steel, preheated for a sufficient time, is then moved to the furnace, which is at operating temperature. This procedure is also useful when machining steels with significant variations in profile thickness or very low thermal conductivity.

Rice. 1. Iron-carbon phase diagram
After the steel section is heated to the proper temperature, it is held at that temperature until the desired internal structural changes occur. This process is called "soaking". The period of exposure at the proper temperature is called the "soaking period". The task of soaking is to ensure uniformity of temperature throughout the volume. Obviously, thin sections don't need to be soaked as long as thick sections, but if the same piece of steel is of different thicknesses, the time it takes to heat the thickest section evenly will decide the time at temperature. As a rule, about 30 minutes of soaking is necessary for a slice with a thickness of 25 mm.
After the steel profile is soaked, it must be returned to room temperature to complete the heat treatment process. A refrigerant can be used to cool the metal. The cooling medium can consist of gas, liquid, solid, or a combination thereof. The cooling rate of the steel profile depends on the steel and the desired properties. The cooling rate depends on the cooling medium, and therefore the choice of cooling medium has an important effect on the required properties. The microstructure and properties of the steel depend on the cooling rate of the steel, which in turn is determined by factors such as mass, tempering medium, etc. It should be understood that the thicker the steel profile, the slower the cooling rate regardless of the cooling method used, with the exception of operations such as induction hardening.
The different types of heat treatment processes are similar in that they all involve heating and cooling the steel. However, the processes differ in the heating temperatures and cooling rates used and the final results. The normal processes used to heat treat steel are (i) annealing, (ii) normalizing, (iii) hardening and (iv) tempering.
Annealing
Annealing is a heat treatment process that involves heating and cooling. The process is usually used to soften steel. The term also refers to treatments aimed at changing mechanical or physical properties, creating a specific microstructure, or removing gases. The operating temperature and cooling rate depend on the type of steel being annealed and the purpose of the treatment. The different types of annealing processes are described below.
Full annealing is a softening process in which a steel section is heated to a temperature above the austenitic transformation range and, after holding for a sufficient time at this temperature, is slowly cooled to a temperature below the transformation range. Steels are generally allowed to cool slowly in the furnace, although it can be removed and cooled in some kind of cooling medium. Since transformation temperatures are affected by the carbon content of the steel, it is obvious that high carbon steels can be fully annealed at lower temperatures than low carbon steels. The microstructure of pre-eutectoid steels, which are formed after full annealing, consists of ferrite and pearlite. Eutectoid and near-eutectoid steels are often partially or completely spheroidized upon full annealing.
Process annealing – Process annealing is also often referred to as stress relief annealing. The process is typically used to cold work low carbon steel (up to about 0.25% carbon) to soften the steel sufficiently for further cold working. Steel is usually heated close to the A1 temperature, but below it. If the steel is not to be further cold worked, but internal stress relief is required, then a lower temperature range (approximately 540 °C) is sufficient. The cooling rate is not important here. This type of annealing causes recrystallization and softening of the cold-worked ferrite grains, but usually does not affect the relatively small amount of cold-worked pearlite.
Spheroidization
Spheroidization is a process of heating and cooling steel that forms a rounded or spherical shape of carbide in a ferrite matrix. This is usually accomplished by prolonged heating to temperatures just below temperature A1, but can be facilitated by alternating heating to temperatures just above temperature A1 and cooling to temperatures just below A1. The last step, however, consists of holding at a temperature just below the critical temperature A1. The cooling rate is not critical after slow cooling to about 540 °C. The rate of spheroidization is influenced by the initial structure. The finer the pearlite, the easier spheroidization occurs. The martensitic structure is very susceptible to spheroidization. This treatment is usually used for high carbon steels (0.60% carbon and above). The purpose of this treatment is to improve the machinability of steel. The process is also used to condition high carbon steel for cold drawing of wire.
Normalization
Normalized treatment is often applied to steel to achieve any one or more of these objectives, namely (i) to improve the grain structure, (ii) to obtain a uniform structure, (iii) to reduce residual stresses, and (iv) to improve the machinability of the steel .
Normalizing is a process in which steel is heated to a temperature above A3 or Acm and then cooled in ambient air. The purpose of the normalizing treatment is to eliminate the effects of any previous heat treatment (including the coarse grain structure that sometimes results from high forging temperatures) or cold working. The normalizing process is performed to ensure uniform austenite when reheated for quenching or full annealing. The resulting structures are pearlitic or pearlitic with an excess of ferrite or cementite, depending on the composition of the steel.
Structures after normalization differ from structures obtained after annealing and steels with the same carbon content in the near-eutectoid or near-eutectoid ranges, there is less excess ferrite or cementite, and pearlite is finer. These are the results of faster cooling. Since the type of structure, and therefore the mechanical properties, is affected by the cooling rate, significant variations can occur in normalized steels due to differences in the cross-sectional thickness of the forms being normalized.
Hardening
Steels can be hardened by the simple means of heating the steel to a temperature above the A3 transformation temperature, holding it long enough to ensure that a uniform temperature is reached and the carbon dissolves in the austenite, and then rapidly cooling the steel (quenching). Complete hardening depends on cooling so quickly that austenite that does not decompose on cooling through the A1 temperature and is maintained at relatively low temperatures. When this is achieved, the austenite begins to transform to martensite on cooling below the Ms temperature (about 220 °C) and completely transforms to martensite below the Mf temperature. Rapid cooling is necessary only for the temperature of the steel to drop significantly below the upper part of the S-curve (Fig. 2). When this is achieved, slow cooling from this point, either in oil or in air, is useful to avoid warping and cracking. Special processing techniques, such as time hardening and tempering, are designed to achieve these conditions. Since martensite is quite brittle, steel is rarely used in the hardened state, that is, without tempering. The maximum hardness that can be achieved in fully hardened low-alloy steels and simple carbon structural steels depends primarily on the carbon content.

Rice. 2 Time-temperature transformation curve
Effect of mass - the mass of steel affects the formation of martensite. It can be seen that even with relatively small sample sizes, the rate of heat removal is uneven. Heat is always removed from the surface layers faster than from the inner potion. In this cooling environment, the cooling rate of both the surface and the interior decreases as the sample size increases, and the possibility of exceeding the critical cooling rate becomes less. To overcome this, the steel must be quenched in an environment that has a very high heat removal rate, such as a chilled brine, but even so, many steels have a physical limit to the maximum size that is sensitive to full quenching regardless. extinguishing media. A noticeable effect of mass on the hardness of hardened steel can be demonstrated by measuring the hardness distribution of different sized blanks of the same steel hardened in the same environment.
Effect of carbon – The carbon content of plain carbon and low alloy steels affects the Ms transformation temperature. As the carbon content increases, the Ms temperature decreases (Fig. 3).
Rice. 3 The influence of carbon on the temperature of Ms
Hardening
Annealing (sometimes called drawing) is the process of reheating hardened (martensitic) or normalized steel to a temperature lower than the A1 temperature. The cooling rate does not matter, except for some steels that are sensitive to temper embrittlement. As the tempering temperature increases, the martensite of hardened steel passes through the stages of tempered martensite and gradually transforms into a structure consisting of spheroids of cementite in a matrix of ferrite (formerly called sorbite). These changes are accompanied by a decrease in hardness and an increase in impact viscosity.
The tempering temperature depends on the desired properties and the purpose for which the steel will be used. If significant hardness is required, the tempering temperature should be low. On the other hand, if significant viscosity is required, the quenching temperature must be high. Proper tempering of hardened steel requires some time. At any selected vacation temperature, the hardness initially drops rapidly, gradually decreasing more slowly as time progresses. Short curing periods are generally undesirable and should be avoided. Good practice calls for at least 30 minutes (or preferably 1-2 hours) at tempering temperature for any hardened steel.
It is impossible to overestimate the necessity of tempering steel immediately after hardening. If fully hardened steel cools to room temperature during quenching, there is a risk of steel cracking. Carbon steels and most low-alloy steels should be tempered as soon as they cool enough to handle comfortably. Steels should not be tempered before they have cooled to this temperature because some steels have a low Mf temperature and untransformed austenite may be present. A portion of all this residual austenite transforms to martensite on cooling from the tempering temperature, so that the final structure consists of both tempered and untempered martensite. Brittle untempered martensite, together with the internal stresses caused by its formation, can easily cause the fracture of a heat-treated steel part. If it is possible that such a condition exists, a second tempering (double tempering) must be carried out to temper the fresh martensite that has formed during cooling after the initial tempering.
If structural steels are to be used in the normalized condition, the normalizing operation is often accompanied by heating to a temperature of about 650 °C to 700 °C. The purpose of this treatment, which is also referred to as tempering, is to release the internal stresses that occur during cooling from the normalizing temperature and to improve the ductility of the steel.
Strengthening the case
Cement hardening is the process of hardening ferrous alloys in such a way that the surface layer or shell becomes significantly harder than the inner layer or core. The chemical composition of the surface layer changes during treatment by adding carbon, nitrogen, or both. The most common carburizing processes are carburizing, cyanidation, carbonitriding, and nitriding.
Carburizing is a process that introduces carbon into a hard ferrous alloy by heating the metal in contact with the carbonaceous material to a temperature greater than the A3 steel temperature and holding it at that temperature. The depth of carbon penetration depends on the temperature, the time spent at the temperature and the composition of the cementing agent. As a rough estimate, a carburizing depth of about 0.75 mm to 1.25 mm can be achieved in about 4 hours at 930 °C, depending on the type of carburizing agent, which can be solid, liquid or gaseous. Since the main purpose of carburizing is to obtain a hard shell and a relatively soft, strong core, use only low carbon steels (about 0.25% carbon maximum), with or without alloying elements (nickel, chromium, manganese). or molybdenum) are commonly used. After cementation, the steel has a high-carbon body that transitions into a low-carbon core.
A variety of heat treatments can be used after carburizing, but they all involve hardening the steel to strengthen the carburized surface layer. The simplest processing consists in hardening the steel directly from the cementation temperature. This treatment strengthens both the body and the core (as far as the core can be hardened). Another simple treatment, and probably the most commonly used, is to slowly cool from the carburizing temperature, reheat to above A3 body temperature (about 775 °C) and quench. This treatment only strengthens the case. A more complex treatment is double tempering, consisting of first tempering above the A3 temperature of the core (about 900 °C for low carbon steel) and then from the temperature above A3 of the body (about 775 °C). This treatment cleans the core and strengthens the case. Plain carbon steels are almost always quenched in water or brine, while alloy steels are usually quenched in oil.
Although post-hardening tempering of case-hardened steel is sometimes omitted, low temperature tempering at around 300 °C is good practice. Sometimes it is desirable to carburize only certain parts of the surface. This can be done by covering the surface to be protected from carburizing with some material that prevents the passage of the carburizing agent. The most common method is copper plating of the surfaces to be protected. Several proprietary solutions or pastes are also available that are quite effective in preventing carbonization. Commercial mixes commonly used for pack (hard) cementing contain mixtures of carbonate (usually barium carbonate), coke (thinner) and hardwood charcoal with oil, resin or molasses as a binder. Mixtures of charred skin, bone, and charcoal are also used. The carbonizing effect of these compounds decreases with use, so new material must be added before the mixture is reused. Adding one part of the unused mixture to three to five parts of the used mixture is a common practice.
Parts to be carburized are packed in boxes (or other suitable containers) made of heat-resistant alloys, although rolled or cast steel can also be used where the durability of the box is not important. The top cover of the box should be covered with fireclay or other refractory material to prevent the leakage of carbonizing gas, which is generated at the carbonization temperature. The depth and uniformity of the body is affected by the method of packaging and the design of the container. Liquid carburizing consists of hardening steel or cast iron in molten salt baths containing mixtures of mainly cyanides (highly poisonous), chlorides and carbonates. The casing obtained by this method contains both carbon and nitrogen, but mainly the former. The temperatures used range from about 850 °C to 900 °C or higher, depending on the composition of the bath and the desired casing depth. At 900 °C, a case depth of approximately 0.25 mm to 0.4 mm can be achieved in 1 hour, and approximately 0.5 mm to 0.75 mm can be achieved in 4 hours. Much deeper hulls can be obtained at higher temperatures for longer periods of time.
After carburizing, the steel must be quenched in the same way as during hard carburizing, but this is usually done directly from the molten bath. In all modern commercial gas carburizing processes, two or more hydrocarbons are used in combination to introduce carbon into the steel. Methane, ethane, propane and oil vapor are used as hydrocarbons. Steel parts are placed in sealed containers through which carbonizing gases circulate. The temperatures used are around 925 °C. The average expected value of the casing depth in carburized steel is shown in fig. 4. After carburizing, the steel must be hardened.
Rice. 4. Relationship of time and temperature with carbon penetration during gas carburization
Cyanide - A hard, surface case can be quickly obtained on low carbon steels by cyanide. This process involves the introduction of both carbon and nitrogen into the surface layers of the steel. The steel to be cyanided is usually heated in a molten bath of cyanide-carbonate-chloride salts (usually containing 30% to 95% sodium cyanide) and then quenched in brine, water, or mineral oil. The operating temperature is usually in the range of 850 °C to 875 °C. The depth of the case depends on the time, temperature and composition of the cyanide bath. Immersion times are relatively short compared to carburizing, typically ranging from 15 minutes to 2 hours. The maximum depth of the case rarely exceeds about 0.5 mm, and the average depth is much less.
Steels can also be cyanidated by heating to a suitable temperature and immersion in a powdered cyanide mixture or by sprinkling the powder on the steel followed by quenching. The case formed in this way is extremely thin. Cyanide salts are extremely poisonous if they come into contact with scratches or wounds. When ingested, they are fatally poisonous. When cyanides come into contact with acids, deadly poisonous vapors are released. Cyanide baths should be equipped with a cap for the removal of gases released during heating, and the working room should be well ventilated. Molten cyanide should never be allowed to come into contact with the sodium or potassium nitrates commonly used for tempering baths, as the mixtures are explosive. Also, care must be taken in the preparation of the salt bath, and the steel must be completely dry before being placed in the molten bath. When operating and maintaining salt baths, it is necessary to strictly follow the recommendations of salt manufacturers.
Carbonitriding – Carbonitriding is also known as gas cyaniding, dry cyaniding, and ni-carbing. This is the process of hardening a steel part in a gas atmosphere that contains ammonia in controlled percentages. Carbonitriding is used mainly as an inexpensive substitute for cyanidation. As in the cyanide process, carbon and nitrogen are added to the steel. The process is performed above the A1 steel temperature and is practical up to 925 °C. Hardening in oil is fast enough to achieve maximum surface hardness. This moderate cooling rate minimizes distortion. The depth to which carbon and nitrogen penetrate depends on temperature and time. Carbon penetration is approximately the same as in gas carburization (Fig. 3).
Nitriding – The nitriding process consists of exposing machined and heat-treated steel without surface decarburization to a nitrogenous medium, usually ammonia gas, at a temperature of approximately 500 °C to 540 °C. A very hard surface is obtained by this process. The effect of surface strengthening is due to the absorption of nitrogen, so further heat treatment of steel is omitted. The required time is relatively long, usually 1 to 2 days. The case, even after 2 days of nitriding, is usually less than 0.5 mm. And the highest hardness exists in the surface layers to a depth of only a few hundredths of a millimeter.
Special low-alloy steels have been developed for nitriding. These steels contain elements that easily combine with nitrogen to form nitrides. The most favorable of these elements are aluminum, chromium and vanadium. Molybdenum and nickel are used in these steels to add strength and toughness. Typically, the carbon content is between 0.20% and 0.50%, although in some steels where high core hardness is required, it can be as high as 1.3%. Stainless steels can also be nitrided.
Since nitriding is carried out at a relatively low temperature, it is advisable to use hardened and tempered steel as the main material. This produces a strong, durable core with an intensely hard wear-resistant body that is indeed much harder than can be obtained by quenching carbonized or cyanidated steel. Although distortion is not a problem during nitriding, steels do increase in size slightly during this treatment. This growth can be taken into account in the finished product. Protection against nitriding can be achieved by tinning, copper plating, bronzing or the application of some paints.
Surface strengthening
It is often necessary to strengthen only the surface of steels without changing the chemical composition of the surface layers. If the steel contains sufficient carbon to respond to quenching, it is possible to harden the surface layers only by very rapid heating for a short period of time, thus preparing the surface for hardening by quenching.
Induction hardening – during induction hardening, a high-frequency current is passed through a coil surrounding the steel, the surface layers of which are heated by electromagnetic induction. The depth to which the heated zone extends depends on the frequency of the current (lower frequencies give greater depth) and on the duration of the heating cycle. The time required to heat the surface layers to a temperature above A3 is surprisingly short and often only a few seconds. Selective heating (and therefore strengthening) is achieved by the appropriate design of coils or inductor blocks. At the end of the heating cycle, the steel is usually hardened by water jets passing through the inductor coils. Precise methods of controlling the operation, i.e. energy consumption, heating duration and cooling rate, are required. These functions are included in induction hardening equipment, which is usually fully automatic.
Flame quenching is a process of heating the surface layers of steel above the transformation temperature using a high-temperature flame followed by quenching. During this process, the gas flame falls directly on the steel surface to be hardened. The heating speed is very high, although not as fast as with induction heating. Plain carbon steels are usually water-spray quenched, while the cooling rate of alloy steels can vary from rapid water quenching to slow air quenching depending on the composition. Any type of hardened steel can be flame hardened. For best results, the carbon content should be at least 0.35%, the usual range being 0.40% to 0.50%.
Special processing processes
Special machining processes typically include quenching, tempering, and cold working
Austempering – Austempering is the trade name for a proprietary heat treatment process. It basically consists of heating the steel to a transformation temperature above A3 and then quenching in a hot bath maintained at a temperature below that at which fine pearlite is formed (end of the S-curve, Fig. 2) but above the Ms temperature. as shown in fig. 3. The product of isothermal decomposition of austenite in this temperature range is bainite. This component combines relatively high viscosity and hardness.
The vacation process has certain limitations that make it impractical for use with many steels. To ensure a uniform structure (and therefore uniform properties), it is important that the entire cross-section of the steel cools fast enough that even the center avoids the transformation at the end of the S-curve. In carbon steels, the time required for transformation to begin at the end of the S-curve is extremely short, so that only relatively small sections (about 10 mm maximum thickness) can be successfully heat-hardened in Ausgart baths. The time required to transform the austenite of alloy steels to fine pearlite is generally longer and therefore larger sections can be successfully austempered (about 25 mm maximum). However, the time required to transform to bainite often becomes prohibitively long for many alloy steels, and therefore the tempering process is usually not feasible for these steels.
Quenching – Quenching consists of heating the steel to a transformation temperature above A3 and then quenching in a bath maintained at a temperature approximately equal to the Ms temperature. The steel is kept in a hot bath until its temperature becomes uniform, and then it is cooled in air. During hardening, strong internal stresses occur in steel. As the steel cools, it contracts but undergoes significant expansion as the austenite transforms to martensite. Because hardened steel must be cooled from the surface inward, different parts turn at different times. Thus, the steel is subjected to a variety of different expansions and contractions, resulting in significant internal stresses. By equalizing the temperature throughout the section before transformation and then slowly cooling through the martensite range (Ms-Mf), internal stresses are greatly reduced, which is also the main purpose of tempering.
Cold Work - The Mf temperature of many alloy steels is so low that complete transformation of austenite to martensite does not occur during quenching to room temperature or cooling after quenching. This retained austenite can be partially or completely transformed by cooling below atmospheric temperature, and such treatment is called "cold working". The beneficial effects of cold treatment have not been fully studied. It is known that residual austenite of high-alloy steels is often difficult to transform. Cooling these steels to low temperatures (to the temperature of solid CO2 or even lower) immediately after quenching is sometimes effective in transforming residual austenite, but with the associated risk of cracking. When cold working is applied after tempering, the retained austenite is much more resistant to transformation. If cold working is used, the steel must always be quenched afterwards.
Alternate heating to a temperature slightly below that used in quenching and cooling to subzero temperatures in a chilled brine with ice, carbon dioxide, liquid air, or liquid nitrogen are commonly used to transform residual austenite (dimensional stabilization) of steel gauges, especially ball bearing type