Heat treatment of steel and metals

The basic properties and qualities of a metal are determined by its structure. Heat treatment is the most common method of influencing a material, which is used to change its structure and, consequently, properties. How is heat treatment of steel and metals carried out - the main types of technological process, and for what purposes is this type of treatment used? All this knowledge can be obtained by familiarizing yourself with the basics of metal technology - a branch of science that studies the techniques and methods of creating and processing metal materials.

A little history

Even in ancient times, blacksmith masters used the most primitive hardening methods. To do this, a red-hot piece of iron was immersed in water, oil or wine. But time passed, and along with experience, methods of hardening metal also developed.

At the beginning of the 19th century, brittle cast iron was placed in a container with ice and covered with sugar. After the heating process lasted for 20 hours, the cast iron became soft and easy to forge.

The middle of the 19th century is significant in that the Russian inventor, metallurgist D.K. Chernov, made an outstanding discovery. He found that when the temperature changes, the metal changes its properties.

Dmitry Konstantinovich Chernov became the founder of the science that studies the properties of metals - materials science.

What methods of heat treatment of metal exist?

To change the technical characteristics of a metal, you can create an alloy based on it and add other components to it. However, there is another way to change the parameters of a metal product - heat treatment of the metal. With its help, you can influence the structure of the material and change its characteristics.

Features of heat treatment

Heat treatment of metal is a series of processes that allow you to remove residual stress from a part, change the internal structure of the material, and improve performance. The chemical composition of the metal does not change after heating. When the workpiece is uniformly heated, the grain size of the material structure changes.

Story

The technology of heat treatment of metal has been known to mankind since ancient times. During the Middle Ages, blacksmiths heated and cooled sword blanks using water.

By the 19th century, people learned to process cast iron. The blacksmith placed the metal in a container full of ice and poured sugar on top. Next, the process of uniform heating begins, lasting 20 hours.

After this, the cast iron billet could be forged.

In the mid-19th century, Russian metallurgist D.K. Chernov documented that when a metal is heated, its parameters change. From this scientist came the science of materials science.

Why is heat treatment needed?

Equipment parts and communication units made of metal are often subjected to severe loads. In addition to exposure to pressure, they may be exposed to critical temperatures. To withstand such conditions, the material must be wear-resistant, reliable and durable.

Purchased metal structures are not always able to withstand loads for a long time. To make them last much longer, metallurgy masters use heat treatment.

During and after heating, the chemical composition of the metal remains the same, but the characteristics change. The heat treatment process increases the corrosion resistance, wear resistance and strength of the material.

How does it work. Heat treatment

Heat treatment of non-ferrous alloys

The presented types of heat treatment of metals are not suitable for various types of alloys and non-ferrous metals. For example, when working with copper, recrystallization annealing is carried out. Bronze heats up to 550 degrees. They work with brass at 200 degrees. Aluminum is initially hardened, then annealed and aged.

Heat treatment of metal is considered a necessary process in the manufacture and further use of structures and parts for industrial equipment, cars, aircraft, ships and other equipment. The material becomes stronger, more durable and more resistant to corrosion processes. The choice of technological process depends on the metal or alloy used.

What are the advantages of heat treatment?

When carrying out heat treatment, the properties of the metal are improved, which is very valuable on the scale of modern industrial production. The main advantages of heat treatment include:

increasing wear resistance, which means extending the shelf life of processed metal products;
significant reduction in the percentage of defective products;
saving money and resources in production as a result of increasing the strength and improving the quality characteristics of industrial equipment parts.

The essence of heat treatment is to follow a certain sequence of technological operations for heating, holding and cooling the metal.

Due to this, materials acquire different physical and mathematical properties due to the effects of temperatures and changes in the structure of the metal.

Aging

Aging is a heat treatment of alloys that causes the decomposition of supersaturated metal after hardening. The result of aging is an increase in the limits of hardness, fluidity and strength of the finished product. Not only cast iron, but also non-ferrous metals, including easily deformable aluminum alloys, undergo aging. If a metal product subjected to hardening is kept at normal temperature, processes occur in it that lead to a spontaneous increase in strength and a decrease in ductility. This is called natural aging of metal. If the same manipulation is performed under conditions of elevated temperature, it will be called artificial aging.

Processing principles

The basic principle is that the total heat treatment time for a workpiece is equal to the time required to heat it to the required temperature, the time the metal is kept at the required temperature and the cooling method.

The time and degree of heating of the material is determined individually; they depend on several factors:

size of the workpiece;
type of metal;
the type of furnace in which the workpiece is processed;
speed of transformation of material properties.

You can get acquainted with the main types and methods of heat treatment using the example of a metal such as steel. In modern industry, steel is the most popular type of metal. It is used in the manufacture of both massive structures and in the creation of ultra-precise instruments.

The invention of this material was made possible by obtaining an alloy of iron and carbon. The carbon content in the steel alloy is no more than 2.1%. How is heat treatment of steel products carried out?

Heat treatment of metals

The principle of heat treatment of metals

The results that can be obtained from heat treatment depend largely on the structure of the metal and how the structure changes as the metal is heated and cooled. For pure metals, heat treatment is ineffective, since their structure changes little when heated. On the other hand, most alloys are heat treatable precisely because their structure changes during heating and cooling.

The original alloy can be in the following forms:

Solid solution;
Mechanical mixture;
Combinations of solid solution and mechanical mixture.

When an alloy is in solid solution form, the elements and compounds that form the alloy dissolve into one another, much like salt dissolves in a glass of water. The constituent parts cannot be identified even under a microscope. When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed.

The mechanical mixture can be compared to a mixture of sand and gravel in concrete, when both sand and gravel are clearly visible. Just as sand and gravel are held together by the cement mixture, so the other alloy constituents are immersed in the mixture formed by the base metal.

An alloy, which is in the form of a mechanical mixture at ordinary temperatures, can turn into a solid solution when heated. When cooled to normal temperature, the alloy can return to its original structure. At the same time, it can remain a solid solution or form a combination of a solid solution and a mechanical mixture.

An alloy consisting of a combination of a solid solution and a mechanical mixture at normal temperatures can turn into a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution.

Thus, all types of heat treatment of steel are represented as a chain of interconnected events/cycles. These cycles include:

Heating (usually slow to ensure structural uniformity);
Holding the metal at a given temperature for a certain period of time;
Cooling (or returning) a metal to room temperature, sometimes quickly, sometimes slowly.

In the heating cycle, temperature uniformity is of paramount importance. If one part of a part heats up faster than another, the resulting uneven expansion often causes the part to warp or crack.

The heating rate of a part depends on several factors. One of the important ones is the thermal conductivity of the metal. A metal that conducts heat easily can heat up at a faster rate than a metal in which the heat cannot be quickly absorbed by the entire part. The condition of the metal also affects the rate of its heating. For example, the heating rate of hardened tools and parts should be lower than that of metals that are not in a stressed state.

Size and cross-section have an important influence on the heating rate. Parts with a developed cross-section require slower heating than thin parts. This is necessary so that the internal space is heated to the same temperature as the surface. Heating of such workpieces is difficult, but they are less prone to cracking or excessive deformation.

The purpose of heat treatment is to change the properties of the metal. To do this, the metal must be heated to a temperature at which internal structural changes occur. These changes occur when the metal's constituents go into solution. However, every metal has the property of thermal inertia. This means that once the metal is heated to the proper temperature, it must be kept at that same temperature until the metal is completely heated.

The holding time depends on the chemical composition of the metal and the mass of the part. If steel parts are not uniform in cross section, then the holding time is determined by the largest cross section.

The metal temperature rarely rises from room temperature to final temperature in a single operation. Therefore, the steel is slowly heated to a temperature below the point at which a solid solution is formed, and then maintained at this temperature until the heat is absorbed by the metal. This stage is called preheating, after which heating can be carried out faster. Preheating helps achieve uniform temperatures throughout the part, reducing the risk of warping and cracking.

Once heated to the proper temperature, the metal must be returned to room temperature, completing the heat treatment process. The metal is cooled by direct contact with a gas, liquid, or a combination of both. The solid, liquid or gaseous substance used to cool a metal is called a "coolant". The metal cooling rate depends on:

Type of metal
Desired properties;
Characteristics of the cooling medium.

The choice of cooling medium has an important influence on the resulting properties. More often than others, oil and water are used. Water (and water-based solutions) cools faster and should only be used for metals that require rapid cooling. Oil cools more slowly and is more suitable for metals that are easily damaged by rapid cooling. Carbon steels are often cooled in water, while alloy steels are often cooled in oil.

Classification and types of heat treatment

There are several types of heat treatment of steel:

thermal - characterized exclusively by the temperature effect on the properties of metals;
thermomechanical processing – a combination of the effects of temperature and plastic deformation of the workpiece;
chemical-thermal treatment – is a combination of temperature effects with chemicals.

Depending on the structure of the steel, types of heat treatment are divided as follows:

annealing process;
normalization;
hardening;
cold treatment;
vacation.

Heat treatment of steel is carried out to impart to the metal properties necessary for industrial use of products, for example, increased strength. And also in technological processes when heat treatment is an intermediate operation and not a final one.

This is required when it is necessary to reduce the hardness of steel for subsequent processing. A reduction in hardness is required when processing initial steel workpieces. To process finished parts, processes are used to increase their strength, wear resistance and hardness.

Heat treatment of steels and metals, what is it - the main types of heat treatment of alloys

14Nov

articles

Varieties of metallic substances have varying degrees of strength, susceptibility to corrosion and other chemical reactions.

Using heating, you can achieve the required properties from the workpiece, improve wear resistance, and prepare it for further procedures during metalworking.

In this article we will talk about heat treatment of steel parts - what it is, what are the main types of heat treatment of metals.

Purpose of the technological process

You can work with both blanks and finished products. For the former, internal stress is relieved after various types of casting and stamping, the material becomes more plastic, and it is much easier to work with, especially cutting it. If a whole part is processed, then the following goals are pursued:

increased strength;
protection against premature rusting;
increased resistance to temperature changes, the upper and lower temperature thresholds at which the item can be used become larger;
extension of potential service life.

Benefits of technology

This process is used universally in many enterprises - every second production of metal products requires thermal exposure. This is due to the advantages:

You can work with steel, non-ferrous metals and alloys - a wide range.
Increased product shelf life.
Reduced level of abrasive wear.
The percentage of defects in production shops is becoming much lower.
Cost savings, since it is easier to carry out a number of manipulations with a heat-treated workpiece.

Processing principles

The main rule is that the time spent on one part is equal to the duration of heating of the material, depending on its maximum temperature, holding period and cooling. The total count allows you to calculate the total time value. Each of these points depends on:

workpiece dimensions;
type of metal subjected to heat treatment;
oven power.

All this determines how quickly the transformations will take place.

Classification

All varieties are used for different purposes, with different materials. For this, the technology remains the same - heating, holding, cooling, but the time of each stage changes. Features are presented in the video:

Short

The main task is to increase viscosity at the same hardness. This is achieved by giving the internal microstructure a needle or plate type. Often used for heat treatment of cutting parts and medical instruments. The workpiece is heated within 150-250 degrees. Leave for at least one and a half hours, and then cool with air or oil.

Average

Here, martensite (the type of structure described above) is transformed into trustite, which is characteristic of cast iron. Feature – high dispersion. At the same high viscosity, the hardness also increases. This is very important for elements that will be subject to large elastic loads. Temperature limits – from 340 to 500, air cooling.

High

Crystallization occurs with the appearance of sorbitol. Thanks to it, the stress inside the alloy is completely eliminated. This method is used for structures that are very important - in aircraft construction, in the construction of space objects. Heating temperature – from 450 to 650 degrees.

General definition and types

During casting or other primary processing processes, in addition to stress, defects appear. You can remove these changes and achieve a homogeneous structure of the crystal lattice using the following algorithm of actions:

heating - it is necessary to slightly exceed the critical level for this type of steel;
a certain period is required to maintain a stable temperature;
The workpiece should be cooled slowly along with the oven.

Annealing has the following varieties.

Homogenization

Refers to the first kind, when changes are considered minor. The purpose of such manipulation is to remove the heterogeneity of the structure and bring it to uniformity. In this case, the product should be heated at a temperature range of 1000 to 1150 degrees, then held for about 8-15 hours and gradually reduced heating, cooling the workpiece with oxygen.

Recrystallization

Also a type of annealing phase 1. The purpose of the procedure is to bring all the crystals into a single form, as well as relieve the internal stress of the metal. There are two subspecies:

softening - usually used as a final treatment, implies an improvement in plastic characteristics;
strengthening – increases elasticity, especially important for hardening springs.

The temperature is selected depending on the alloy, usually 100-200 degrees higher than the recrystallization point. It is necessary to maintain the temperature for an hour or two, then allow it to cool slowly.

Isothermal annealing

The goal is to achieve high-temperature face-centered modification of iron (decomposition of austenite) to soften it. This results in a more uniform structure of the product. More often, this type of metalworking is used for small stampings, because they can be subjected to rapid cooling without problems. Process:

heating is 20-30 degrees more than the material limit;
short aging;
rapid cooling is an advantage over other subspecies.

To eliminate stress

This is an operation of removing, removing the negative internal state of excessive hardness, due to which the metal becomes brittle and short-lived. It quickly deforms from external physical influences. The process involves temperatures from 700 to 750, then a slight cooling to 600 and holding for up to 20 hours, then slowly cooling under the influence of air.

Complete annealing

Used to create a plastic, uniform fine-grained structure. The most typical method of intermediate impact on rolled metal is after casting, forging, stamping and before cutting by any method. Stages:

heating is 30-50 more than the steel limit;
holding;
very slow cooling with the oven - no more than 50-150 degrees in 60 minutes.

Incomplete

There are no significant transformations at the level of the crystal lattice, but hardness is imparted to previously plastic materials. This is especially necessary for structures formed by welded joints, as well as tools that require special strength. The method involves a temperature of about 700, and after 20 hours gradual cooling.

Hardening as the main type of heat treatment of steel

A very common method of heat treatment, as it allows you to make the product less susceptible to compression, shear, and also give it strength and durability, immunity to external physical influences. This occurs by imparting a needle-like structure to the metal. The substance hardens with “needles” due to a lack of alloying materials.

The workpiece is heated up strongly, and then cooled as quickly as possible, using external sources - water, oil, a solution with added salt. Due to the speed, diffusion processes do not have time to occur in the semi-molten alloy. It is cheapest to create water baths, but cracks may appear on the surface; an oil medium is the most preferable.

Cryogenic heat treatment

Another thermal method of exposure, but without heating. The product is placed in a refrigeration unit, sometimes an entire workshop is used for large-sized structures. Low temperatures and subsequent warming reduces the risk of corrosion, extends service life, increasing strength.

Thermo-mechanical effect

TMO was used by blacksmiths in ancient times. These are any plastic deformations (impacts, compressions) produced by heating the entire product or element. It is usually combined with hardening, that is, after deformation it is quickly cooled.

Hardenability and hardenability of steel

These indicators determine the results of all of the above procedures. The first term is hardness, which is directly related to the amount of carbon, and the second is the depth of hardening, that is, which top layer has been subjected to changes.

Cooling methods

There are several environments in which temperature can be taken:

air;
liquid;
molten salt;
oil;
brine;
a combination of the above substances.

Selected depending on the type of heat treatment.

Conclusion

This is one of the most common metalworking methods in production; without it, hot stamping and cutting are often not started. We have listed all the main types of heat treatment of metals and alloys, their features, and to complete the article, watch a few videos:

To clarify the information you are interested in, please contact our managers by phone;; 8 (800) 707-53-38. They will answer all your questions.

General definition and types of annealing

During the process of casting, forging and other operations used to make workpieces, the metal acquires a heterogeneous structure and internal stresses appear.

Inhomogeneity in the chemical composition of castings causes defects and an annealing process is used to eliminate it. The principle of this method is that the workpiece or part is heated to a certain temperature, and then a slow cooling process is carried out.

Annealing is also divided into several modes:

annealing of the 1st type - diffusion, recrystallization, reducing metal stress;
annealing of the 2nd type - complete, incomplete, isothermal.

Objectives and effects of heat treatment

When heated to temperatures above the recrystallization temperature, the microstructure of most steel grades becomes equilibrium, and the properties become more uniform in all directions.
However, such benefits are not preserved with natural cooling. Moreover, coarse crystallites are formed, banding and other defects increase, worsening the performance properties of the metal. To preserve positive characteristics, it would be convenient to record favorable phase-structural changes at room temperatures. The easiest way is to heat the product to the desired temperature and then quickly cool it. Most heat treatment technologies are based on this principle.

When we talk about changes in mechanical properties, we mean shear strength, impact strength, tensile strength and hardness of steel. Taking into account this mechanical change in properties allows bed products to be more efficient in everyday tasks and more resistant to wear even in the most severe conditions. Proper heat treatment of steel reduces the labor intensity of other important production steps. For example, by wisely using the capabilities of heat treatment processes, stress can be relieved, facilitating the assembly or welding of structural elements, and the dimensions of their sections can be optimized.

Gears, shafts, bearings and other critical machine components benefit greatly from these heat treatment processes, increasing their wear resistance and overall service life. In particular, increasing fatigue strength allows steel products to more effectively resist alternating and impact loads.

The role of heat treatment processes in tool production is also great. Hard steels and alloys are often used as cutting and stamping parts where it is necessary to maintain sharp shapes and edges. In this case, it is possible to achieve the required balance between the surface hard layers of the tool and a more viscous, plastic core.

To summarize, it can be noted that as a result of heat treatment the material:

Becomes more durable and hard (or, conversely, softer and more flexible!);
Increases its fatigue strength;
Improves weldability;
Provides the necessary microstructure;
In some cases, it changes the chemical composition of the surface.

In many cases, heat treatment is reversible, which allows you to change the properties of steel if for some reason the resulting characteristics do not meet production requirements.

Description of annealing of the 1st kind

The purpose of carrying out thermal operations related to type 1 of annealing is to eliminate the heterogeneity and disequilibrium of the steel structure that arose as a result of previous technological treatments. Based on the state of the workpiece, the following processes can be applied to it:

relieving internal stress;
recrystallization;
homogenization (diffusion annealing).

Annealing of the 1st type is applied to any type of metal or alloy; its implementation does not entail any phase transformations. The decisive factors for this method of heat treatment of steel are: high heating temperature and the time the metal is kept at this temperature.

Thermo-mechanical treatment

A new method of processing alloys combines the processing of metals at high temperatures with mechanical deformation of products in a plastic state. Thermomechanical treatment (TMT) can be of three types according to the method of implementation:

Low-temperature TMT consists of two stages: plastic deformation followed by hardening and tempering of the part. The main difference from other types of TMT is the heating temperature to the austenitic state of the alloy.
High-temperature TMT involves heating the alloy to a martensitic state in combination with plastic deformation.
Preliminary - deformation is carried out at t 20 followed by hardening and tempering of the metal.

Diffusion annealing or homogenization

The meaning of diffusion annealing is to heat the workpiece to a temperature of at least 1000˚C, hold it at high temperatures for 8 to 15 hours and gradually cool it. As a result of prolonged exposure to heating, diffusion processes are accelerated, due to which the metal structure becomes more homogeneous.

When processing alloy steel with this method, it is possible to achieve its ductility, which greatly facilitates its further machining.

The disadvantages of the diffusion method include the possibility of the following side effects:

deterioration of the mechanical properties of steel due to grain growth;
the appearance of secondary heterogeneity and porosity;
the occurrence of coagulation of excess phases.

For this reason, homogenization is considered a pre-treatment.

After this, it is recommended to carry out complete annealing or normalization of the steel.

Annealing

Annealing is a production process in which metals and alloys are heated to a given temperature, and then, together with the furnace in which the procedure took place, they cool very slowly naturally. As a result of annealing, it is possible to eliminate inhomogeneities in the chemical composition of the substance, relieve internal stress, achieve a grain structure and improve it as such, as well as reduce the hardness of the alloy to facilitate its further processing. There are two types of annealing: annealing of the first and second kind.

Annealing of the first kind involves heat treatment, as a result of which changes in the phase state of the alloy are insignificant or absent altogether. It also has its own varieties: homogenized - the annealing temperature is 1100-1200, under such conditions the alloys are kept for 8-15 hours, recrystallization (at t 100-200) annealing is used for riveted steel, that is, deformed when it is already cold.

Second-order annealing leads to significant phase changes in the alloy. It also has several varieties:

Full annealing is heating the alloy 30-50 above the critical temperature characteristic of a given substance and cooling at a specified rate (200 / hour - carbon steels, 100 / hour and 50 / hour - low-alloy and high-alloy steels, respectively).
Incomplete - heating to a critical point and slow cooling.
Diffusion – annealing temperature 1100-1200.
Isothermal - heating occurs in the same way as with full annealing, but after this it is rapidly cooled to a temperature slightly below critical and left to cool in air.
Normalized - complete annealing followed by cooling of the metal in air rather than in a furnace.

Annealing by recrystallization

During cold plastic deformation, inhomogeneity may occur in the steel structure, as well as changes in the size and shape of crystals and an increase in the internal stress of the metal.

To eliminate such phenomena, the recrystallization annealing method is used. Recrystallization annealing can be of two types: strengthening and softening.

The softening method is often used as a final treatment to improve the ductility properties while maintaining sufficient strength of the metal.

Strengthening annealing is used to improve the elasticity of parts such as membranes or springs.

In industry, the recrystallization type of annealing is used as a pre-treatment before processing metal using the cold pressure method, as well as for final processing of parts to secure the necessary properties.

Reducing metal stresses (low annealing)

Residual metal stress is a by-product of casting, forging, or certain types of heat or mechanical processing and can cause metal failure. Low annealing is used to completely or partially relieve these stresses.

The method consists of annealing at temperatures below 700˚C for approximately 20 hours. This time is sufficient for almost complete elimination of residual stresses.

Industrial heat treatment

Features of annealing of the 2nd kind

When processing steel using type 2 annealing methods, a complete or partial change in the structure of the material occurs. This process occurs due to double recrystallization, due to which the grain sizes are reduced, and internal stresses are also eliminated.

In industrial production, this type of annealing is used during preliminary or final processing of the workpiece.

There are the following types of annealing of the 2nd kind:

full;
incomplete;
spheroidizing annealing;
isothermal.

Full annealing value

This technology is used to create a fine-grained structure in steel blanks produced by forging, casting or hot stamping. As a result of processing, the material becomes plastic and internal stress disappears. The steel acquires a uniform fine-grained structure.

The method of complete annealing is used to process steel intended for subsequent cutting and hardening of the product.

When carrying out complete annealing, the heating temperature exceeds the established critical indicators by 40–50˚C.

Partial Annealing Process

With this type of heat treatment of steel, phase transformations, as a rule, are absent or appear in quantities that do not have any effect on the result. Products or workpieces made of steel are heated at temperatures above the lower critical level. After being kept in a heated state for a certain time, the metal slowly cools.

Annealing on granular pearlite (spheroidization)

Spheroidizing annealing is widely used for heat treatment of carbon and alloy tool steel. The metal is heated to approximately 30˚C above the critical point and held for a set amount of time. Up to 600˚C the cooling process takes place very slowly in the furnace, then the steel cools in air. Thanks to this processing method, it is possible to obtain a granular (rounded) shape of perlite, which greatly facilitates the processing of cutting the workpiece.

Isothermal annealing

The essence of isothermal annealing of steel is to heat the metal, quickly cool it to a certain temperature level and hold it until austenite decomposes.

Next, cooling continues in the open air.

The structure of steel when using this method becomes more homogeneous, as with complete annealing. The advantage of the isothermal method is that, compared to full annealing, the entire technological process takes less time. Isothermal processing is used mainly for annealing small products - stampings, blanks for tools.

Equipment used

Thermal furnaces can be divided into two main types: batch and continuous. The fundamental difference between them lies in how the workpieces being processed are placed in the units, and how they interact with the atmosphere inside the furnaces.

The main energy sources for heating equipment are natural gas and electricity. Alternative energy sources, such as fuel oil, are used less frequently.

Furnaces in which metals are heat treated are classified according to the upper limit of heating temperature. The commonly used temperature range is from 600 to 8000C. Convection heating is mainly used, based on the circulation of air, combustion products or inert gas located inside the furnace.

Batch installations, as a rule, process workpieces in batches, and the heating of each batch can last several hours (and sometimes even days). In a batch kiln, the workload is usually stationary so that it interacts with changes in the kiln atmosphere under near-equilibrium conditions. Types of batch furnaces:

Bell-shaped;
Box-shaped;
Heating wells;
With a movable hearth;
Fluidized bed;
Mine;
Vacuum.

Continuous furnaces differ in the method of movement of the processed workpieces and the characteristics of the working environment (air, inert gas or vacuum).

Types of continuous furnaces:

Chamber;
Tape
Monorails
Push
With roller/rotating hearth;
Vibrating hearth furnaces;
Vacuum ovens;
With walking beams.

The best control of heating parameters is provided by electric furnaces.

Steel normalization

The process involves heating steel, briefly holding it at a certain temperature and then cooling it in air rather than in a furnace.

Widely used as an intermediate treatment of steel to improve the structure of the metal before hardening, and to soften it before cutting. At its core, normalization resembles the annealing process.

The normalization process is mainly used for heat treatment of carbon steels. As a result, there is no need to harden steel with a medium carbon content.

During processing, complete recrystallization of the steel occurs and the coarse grain structure is eliminated. Normalization is also often used for heat treating low-carbon steel instead of full annealing. For steel alloys with high carbon content, complete annealing is necessary.

Features of heat treatment of non-ferrous alloys

Most alloys can be subjected to two types of heat treatment - hardening and aging. The last type is a tempering carried out at temperatures of 120...2000C, with cooling at room temperature (natural aging) or with the supply of an air stream (artificial aging).

However, there is a large scatter between many combinations of metals and non-ferrous alloys in the rate of strain hardening, which makes it difficult to systematize the heat treatment processes of non-ferrous alloys.

Key Features:

Alloys of the copper-nickel system are effectively amenable to mechanical-thermal treatment, during which the structure becomes fine-grained, but the hardness increases.
All types of non-ferrous alloys can be annealed, and the type of heating does not matter, since the intensity of scale formation is low. Time has less influence on the efficiency of annealing than temperature.
Hardening of non-ferrous alloys is much less effective. With the exception of titanium, the commonly used alloys of aluminum, copper and magnesium are not allotropic; thus, they do not react in the same way as steel when they are heated and cooled.
Many alloys such as bronzes cannot be heat treated at all, since for these alloys the solid solutions formed at elevated temperatures remain completely stable at room temperature or below.
Heat treatment temperature and time cycles cover a wide range, which depends not only on the composition of the alloy, but also on whether the alloy is in a wrought or cast state.

Non-ferrous metals are rarely preheated because it increases the grain size and degrades the alloy structure.

Steel hardening

Hardening is a method of heat treatment of steel, during which the metal is heated to approximately 900˚C, held for a certain period of time, and then cooled very quickly. Thanks to this technology, the strength and wear resistance of the alloy is increased, and its other physical and mechanical characteristics are improved.

For successful heat treatment, the correct choice of quenching medium is of great importance.

Most often used for hardening:

water;
saline solutions;
caustic alkaline materials;
technical oils.

Oil is one of the materials used for hardening metal

Summary

Heat treatment of metals and alloys is the main technological process in both ferrous and non-ferrous metallurgy. Modern technologies have a variety of heat treatment methods that make it possible to achieve the desired properties of each type of processed alloys. Each metal has its own critical temperature, which means that heat treatment must be carried out taking into account the structural and physicochemical characteristics of the substance. Ultimately, this will allow not only to achieve the desired results, but also to significantly streamline production processes.

Hardenability and hardenability of steel

The following indicators are characteristic of hardening - hardenability and hardenability of the material:

Hardenability determines the hardness that steel acquires after hardening. Hardness is directly dependent on the carbon content in the metal being processed. For example, hardening is not applied to a material with a carbon content below 0.3% due to its ineffectiveness.
Hardenability determines the depth to which the hardening region extends. This indicator depends on the chemical composition of the steel, as well as on the cooling rate. The faster the metal cools, the deeper the workpiece is calcined. Carbon content also has an impact on this indicator - the higher its content, the greater the degree of calcination. The size of the workpiece or part is another factor that determines the depth of processing - larger parts require more time to cool, therefore, the calcination will spread to a shallower depth.

The essence of heat treatment processes

The objectives of various heat treatment technologies are:

Ensuring the most favorable microstructure of steels and alloys;
Obtaining the required level of hardness: either in a thin surface (or subsurface) zone, or over the entire cross-section of the workpiece;
Correction of the chemical composition in the grains of macrostructures of various alloys.

In the first case, it is necessary to ensure the maximum degree of homogeneity of the properties of metals, which is important, for example, for subsequent mechanical or - especially - processing that deforms them. As a result, the conditions for changing the shape of the workpiece along all three coordinate axes are the same, and rejects of the final part are eliminated.

Heat treatment of metal

In addition, the alignment of micro and macrostructures for metal forming processes is necessary in order to increase the degree of deformation of semi-finished products, ultimately bringing the shape of the workpiece closer to the shape of the finished product. Moreover, in the smallest number of transitions, and using the minimum required equipment effort.

The change in hardness (as a result of heat treatment) is intended to improve the performance of parts. Since operating conditions can be very different, the complex of physical and mechanical properties is selected strictly individually: there are no universal heat treatment processes for alloys with different compositions.

A change in the chemical composition in the grains of the microstructure, due to the formation of new compounds, in most cases not only increases the hardness indicators, but also increases the wear resistance of parts that must be operated at increased friction, temperature, or increased specific loads compared to usual.

Quenching-tempering

The first group of heat treatment technologies for various alloys, including steel, includes annealing and tempering. The second is hardening, normalization, improvement, aging, cold treatment. Third - all types of thermochemical treatment.

Annealing

The essence of the processes occurring in the structure of most alloys subjected to annealing is to provide the most balanced structure of the workpiece, in which there are either no internal stresses, or their level is sufficiently low, and therefore does not affect the subsequent workability of the metals/alloys.

Annealing furnaces manufactured by BOSIO

The initial structure of almost all alloys and steels consists of fairly large grains, between which inclusions and impurities, mainly sulfur and phosphorus, are located. This increases the fragility of the metal, which can be important when shaping products of complex configurations from an ingot (or wire rod). Therefore, it is necessary to reduce the grain size and give it an optimal ellipsoidal shape, in which the mechanical properties will be approximately the same along all three coordinate axes.

Annealing of non-ferrous metals

For this purpose, the initial workpiece must be heated to a temperature 50...700C above the temperature at which the austenitic transformation begins. Its result is the formation of small and well-oriented austenite grains between the grains of the main structural components of steel - ferrite and cementite. Austenite is formed from pearlite, a structure that has the largest grains, which contributes to the increased fragility of any ingot. The austenitic transformation for most alloys proceeds quite slowly, so annealing is a long procedure that should last at least an hour.

Metal annealing

The second important task of annealing is to relieve the internal stresses that form in the workpiece during cold processing. The fact is that any deformation is accompanied by fragmentation of the grains of the original structure of steels and alloys. As a result, there are more grains, the resistance to deformation increases, which not only requires increased deformation force, but also causes the destruction of the semi-finished product, the degree of deformation of which has exceeded the critical value for a given metal.

Accordingly, to implement the first task, high-temperature annealing technology is used (for steels, depending on the carbon content, it ranges from 550...7500C), and in the second - low-temperature annealing (180...2200C).

High temperature annealing methods

Heating occurs slowly, followed by holding the product at a given temperature, followed by slow cooling. For alloy steels and alloys, such cooling is carried out at a particularly low speed, in the furnace itself where the annealing took place.

Vacation

Tempering technology resembles annealing, but is performed not with a workpiece, but with a finished product, and therefore has other goals - to relieve internal stresses after heat treatment, which was carried out to increase the hardness of the part.

Metal release

Tempering is not an independent heat treatment process. Unlike annealing, tempering is sometimes performed in several stages: in most cases this applies to products for the production of which various types of high-alloy steel were used.

Hardening

Scheme of structural transformations of martensite in U-8 steel during heating.
Quenching consists of rapid heating of the workpiece to the end temperature of the austenitic transformation (900...11000C for low-carbon steels, 750...8500C for high-carbon steels) and subsequent rapid cooling in special quenching media. The latter are used water (for products of low importance) or oil.

Hardening modes are characterized by the greatest variety. The main factor determining the effectiveness of hardening is the intensity of formation of martensite in the structure - a high-temperature component that gives the metal or alloy increased hardness.

The conditions for the formation of martensite are determined by the following circumstances:

Grades of steels or alloys.

Heating temperature range for hardening carbon steels
The original structure.
Required final hardness.
The need for the presence of a number of compounds in the microstructure, which are formed only at elevated temperatures.

Accordingly, for each grade of steel or alloy, individual hardening modes have been developed, which differ:

The rate of heating of the workpiece to the required temperatures (permissible error for some types

The steel hardening mode, depending on the grade of
high-alloy alloy, can be 20...300C, which forces the use of hardening furnaces with automatically controlled temperatures in the working space);
The duration of exposure of the product in the oven at a given temperature;
The intensity of cooling of the product;
The number of quenching and subsequent tempering cycles.

Particular care is taken to harden steels and alloys with a complex composition, including several alloying elements (in particular, cobalt, molybdenum). In the process, these metals form intermetallic compounds along the grain boundaries of the main structure, which significantly increase the hardness and strength of steels (in particular, tool steels). The shape and concentration of intermetallic compounds depend only on the accuracy of the hardening technology.

The presence of molybdenum or tungsten in steel increases heat resistance, hardenability and reduces the tendency to reversible brittleness

Types of hardening are determined by the equipment on which it is performed. For example, for products such as gears, shafts, guide columns, where an optimal combination of high surface hardness and a relatively viscous core is required, surface hardening with high frequency currents is used.

High frequency hardening, steel hardening, hardening temperature

To do this, the product is placed in an induction coil through which a high-frequency (up to 15,000...25,000 Hz) current is passed. Penetrating to a limited depth, this current helps to increase the surface strength of steels or alloys. As a result, the fatigue strength of parts that operate under cyclically varying tension-compression stresses increases noticeably.

A more intense change in the hardness of the surface of a part can be obtained by using high-energy heat sources for hardening - a spark or arc discharge. Discharges must be excited in a liquid medium where the workpiece or part being processed is placed.

Heat treatment modes of carbon tool steels during hardening and after tempering

After hardening, in the vast majority of cases, tempering is necessary, otherwise the excessive final hardness of the part becomes the cause of increased brittleness under impact loads.

The influence of cooling methods on hardening

Depending on the method of cooling the steel, hardening is classified as follows:

Hardening in one environment is the simplest and most commonly used heat treatment method in industry. Its main disadvantage is the possibility of internal stresses in the metal.
Quenching in two media - using this method, the material is cooled alternately in two liquids. Water and oil can be used for the process.
Isothermal hardening - the principle of this method is similar to step hardening. Molten salt or oil is used to cool the material. This type of hardening is widely used for hardening small parts - washers, springs, bolts.
Step hardening – the product is cooled using a salt solution having a temperature of 200–300˚C. After a certain period of exposure, the steel is finally cooled in the open air. Step hardening helps relieve internal stresses and reduces the possibility of cracks.

Application of heat treatment of steel: main types, pros and cons

Heat treatment of metal is an important part of the production process in non-ferrous and ferrous metallurgy.
After this procedure, the materials acquire the necessary characteristics. Heat treatment has been used for quite some time, but it was imperfect. Modern methods allow you to achieve better results with less effort and reduce costs. To impart the desired properties to a metal part, it is subjected to heat treatment. During this process, a structural change occurs in the material .

Metal products used in the household must be resistant to external influences. To achieve this, the metal must be strengthened by exposure to high temperature. This treatment changes the shape of the crystal lattice, minimizes internal stress and improves its properties.

Types of heat treatment of steel

Heat treatment of steel comes down to three stages: heating, holding and rapid cooling. There are several types of this process, but the main stages remain the same.

The following types of heat treatment are distinguished:

Technical (tempering, hardening, cryogenic treatment, aging).
Thermo-mechanical, which uses not only high temperature, but also physical impact on the metal.
Chemical-thermal involves heat treatment of the metal followed by exposure of the surface to nitrogen, chromium or carbon.

Annealing

This is a manufacturing process of heating metal to a predetermined temperature, and then slowly cooling, which occurs naturally. As a result of this procedure, the heterogeneity of the metal is eliminated, internal stress is reduced, and the hardness of the alloy is reduced, which greatly facilitates its processing. There are two types of annealing: first and second kind.

During first-order annealing, the phase state of the alloy changes slightly. It has varieties:

Homogenized - the temperature is 1100−1200 °C, the metal is kept for 7−14 hours in such conditions.
Recrystallization - annealing temperature 100−200 °C, this procedure is used for riveted steel.

During second-order annealing, a phase change in the metal occurs. The process has several types:

Full annealing - the metal is heated 25−40 °C above the critical value for this material and cooled at a special speed.
Incomplete - the alloy heats up to a critical point and takes a long time to cool down.
Diffusion - annealing is carried out at a temperature of 1100−1200 °C.
Isothermal - heating of the metal occurs as during complete annealing, but cooling below the critical temperature, cooling in the open air.
Normalized - the metal is completely annealed and cooled in air.

Hardening

This is the process of manipulating metal to achieve martensitic transformation, which provides increased strength and reduced ductility of the product. During quenching, the alloy is heated to a critical value, as during annealing, but the cooling process is much faster, and a liquid bath is used for this. There are several types of hardening:

Quenching in one liquid, oil is used for small parts, and water for large parts.
Intermittent hardening - lowering the temperature occurs in two stages: sharp cooling to a temperature of 300 ° C using water, and then the product is placed in oil or in open air.
Stepped - when the metal reaches the required temperature, it is cooled in molten salts and then in the open air.
Isothermal - similar to stepwise, differs in exposure time.
Quenching with self-tempering, the alloy is not completely cooled, leaving a warm area in the middle. As a result, the metal gains increased strength and high toughness. This combination is great for percussion instruments.

Incorrectly done hardening can lead to the following defects:

decarbonization;
cracks;
warping or leashes.

The reason for the leash and cracks is an uneven change in the size of the part during cooling or heating. They can also occur with a sharp increase in strength in certain places. The best way to avoid these problems is to slowly cool the metal to the martensitic transformation point.

Leash and warping occurs when curved parts are cooled unevenly. These defects are quite minor and can be corrected by sanding. Preliminary annealing of parts and their gradual and uniform heating will help avoid warping.

Decarburization of metal occurs as a result of carbon burning out during prolonged heating. The intensity of the process depends on the heating temperature; the higher it is, the faster the process. To correct the part, it is heated in a neutral environment (muffle furnace).

Scales on the metal surface lead to waste and deformation of the product. This reduces the heating rate and makes machining more difficult. Scales are removed chemically or mechanically.

In order to avoid their appearance, you need to use a special paste (100 g of liquid glass, 25 g of graphite, 75 g of refractory clay, 14 g of borax, 100 g of water, 30 g of carborundum).

The composition is applied to the products and left until completely dry, and then heated as usual.

Vacation

It softens the effects of hardening, relieves stress, reduces fragility, and increases toughness. Tempering is carried out by heating a part hardened to a critical temperature.

Depending on the temperature value, the states of trostite, martensite, and sorbitol can be obtained. They differ from similar hardened states in properties and structure, which is more point-specific. This increases the ductility and strength of the alloy.

Metal with a dot structure has higher impact strength.

Depending on the temperature, the following types of holiday are distinguished: low, medium, high.

To accurately determine the temperature, use a color table. The film of iron oxides gives the metal different colors. It appears if the product is cleaned of scale and heated to 210 °C; with increasing temperature, the film thickness increases.

During low tempering (temperatures up to 300 °C), martensite remains in the alloy, which changes the structure of the material. In addition, iron carbide is released. This increases the toughness of the steel and reduces its hardness. During low tempering, the metal is cooled in salt and oil baths.

High tempering significantly improves the mechanical properties of steel, increases toughness, ductility, and strength. It is widely used for the manufacture of springs, engine connecting rods, forging dies, and car axles. For fine-grained alloy steel, tempering is carried out immediately after normalization.

To increase the workability of the metal, it is normalized at a high temperature (970 °C), which increases its hardness. To reduce this parameter, make a high tempering.

Cryogenic treatment

Changes in the structure of the metal can be achieved not only by high temperature, but also by low temperature. Alloy processing at temperatures below 0 °C is widely used in various industries. The process occurs at a temperature of 195 °C.

Advantages of cryogenic processing:

Reduces the amount of austenite, which gives stability to the dimensions of parts.
Does not require subsequent tempering, which shortens the production cycle.
After this treatment, the parts are better suited for grinding and polishing.

Chemical-thermal treatment

Chemical-thermal treatment includes not only high temperature exposure, but also chemical exposure. The result of this procedure is increased strength and wear resistance of the metal, as well as making it fire and acid resistant.

There are the following types of processing:

Cementation.
Nitriding.
Nitrocarburization.
Borating.

Steel carburization is a process of additional processing of metal with carbon before hardening and tempering. After the procedure, the product’s endurance during torsion and bending increases.

Before cementation begins, the surface is thoroughly cleaned, after which it is coated with special compounds. The procedure is carried out after the surface has completely dried.

There are several types of cementation: liquid, solid, gas. In the first type, a special bath oven is used, into which 75% soda, 10% silicon carbide, 15% sodium chloride are poured. After which the product is immersed in a container. The process takes place for 2 hours at a temperature of 850 °C.

Hard cementation can be conveniently performed in a home workshop. For it, a special paste is used based on soda ash, soot, sodium oxalate and water. The resulting composition is applied to the surface and waits to dry. After this, the product is placed in an oven for 2 hours at a temperature of 900 °C.

Gas cementation uses mixtures of gases containing methane. The procedure takes place in a special chamber at a temperature of 900 °C.

Nitriding of steel is the process of saturating the metal surface with nitrogen by heating to 650 °C in an ammonia atmosphere. After processing, the alloy increases its hardness and also becomes resistant to corrosion.

Nitriding, unlike carburization, allows you to maintain high strength at high temperatures. And also the products do not warp when cooled.

Metal nitriding is widely used in industry to impart wear resistance to the product, increase hardness and protect against corrosion.

Nitrocarburization of steel involves treating the surface with carbon and nitrogen at high temperatures, followed by hardening and tempering. The procedure can be carried out at a temperature of 850 °C in a gas environment. Nitrocarburization is used for tool steels.

When boriding steel, a layer of boron is applied to the surface of the metal. The procedure takes place at a temperature of 910 °C. This treatment is used to increase the durability of die and drilling tools.

Thermo-mechanical treatment

This method involves high temperature and plastic deformation. There are the following types of thermomechanical treatment:

High temperature.
Low temperature.
Preliminary.

During high-temperature processing, metal deformation occurs after heating. The alloy is heated above the recrystallization temperature. After which hardening and tempering are carried out.

High temperature metal processing:

Increases viscosity.
Eliminates temper brittleness.

Structural, tool, carbon, spring, and alloy steels are subjected to this treatment.

In low-temperature processing, the workpiece after cooling is kept at a temperature below the recrystallization value and above the martensitic transformation. At this stage, plastic deformation is performed. Such processing does not provide stability to the metal during tempering, and its implementation requires powerful equipment.

To carry out thermomechanical processing, it is necessary to use special devices for pressure, heating and cooling of the workpiece.

Non-ferrous metals differ in their properties from each other, so different types of heat treatment are used for them. To equalize the chemical composition of copper, it is subjected to recrystallization annealing. Brass is processed at low temperature (200 °C). Bronze is annealed at a temperature of 550 °C. Magnesium is hardened, annealed and aged, and aluminum is subjected to a similar treatment.

In ferrous and non-ferrous metallurgy, various types of heat treatment of metals are widely used. They are used to obtain the desired properties of alloys, as well as to save money. For each procedure and metal, its own temperature values are selected.

What is the steel tempering process?

Tempering is a type of final stage of thermal finishing of steel, during which the final formation of the structure of the material occurs. The tempering process consists of heating to a temperature below the critical point, followed by cooling.

The process itself is divided into three types:

Low tempering – occurs at a temperature of 150–250˚C. During the low tempering process, the internal stresses and brittleness of the metal decrease, and the toughness of the steel increases slightly. The hardness remains practically unchanged.
Medium tempering - characterized by the fact that the process takes place at a temperature of 350 to 450 ˚C. The difference from other types of tempering is that the hardness of the part decreases, and the viscosity increases significantly. Used for processing parts that experience moderate shock loads during operation.
High tempering – is carried out in compliance with the temperature range from 500 to 650˚C, followed by gradual cooling. Internal stresses of the material are practically eliminated. Strength and ductility in this type of processing have high characteristics in combination with sufficient hardness of the metal. High tempering is used for carbon and alloy types of workpieces intended for the manufacture of shafts and gears.

Vacation

To normalize the characteristics of steel workpieces after hardening, it is recommended to temper it. Its essence lies in thermal exposure to temperatures at which phase transformation does not occur. The result of this operation will be the homogeneity of the steel structure.

Types of tempering for metal blanks:

Short. Used for carbon steel grades. The maximum exposure temperature is +200°C. As a result, the fragility index decreases and the tension in the structure decreases.
Average. Heat treatment occurs at +400°C. Technology is needed to remove excess carbon. In this case, the crystal lattice becomes cubic.
High. Processing temperature – up to +650°С. It is used to achieve optimal characteristics of strength, viscosity and ductility.

The defining indicator for this process is temper brittleness. It indicates the degree to which impact strength drops during sudden temperature changes.

Cryogenic treatment

Cold finishing also refers to heat treatment methods. The operation is carried out after hardening by cooling in special cryogenic chambers at subzero temperatures for a set time. After this, the state of the part returns to room temperature. Cryogenic finishing increases wear resistance and strength of products, and also increases resistance to corrosion.

From all of the above, one important conclusion follows - heat treatment of steel is an integral part of modern industry.

Hardness parameters and its indicators

Hardness is one of the most interesting indicators for assessing the properties of a material and metal structures and parts. Based on hardness, strength, machinability parameters, and wear resistance can be calculated.

The last indicator is the most important, since it is responsible for the service life and safety of a metal or alloy product. Several types of hardness testing of products have proven themselves in the metallurgical industry:

Rockwell hardness. This is an option for a fast, automated testing method. In this case, a specific tool of conical or spherical shape is used, made of ultra-strong materials, in particular diamond or carbide. This tool applies pressure to a sample of the part being tested. First, a test amount of force is applied to the sample, and then an additional force is applied for the required period of time. After this, the additional impact is removed and the hardness is calculated based on the penetration depth and the numerical indicators N and S.
Brinell hardness. This method is used in a wide variety of designs, for metal from low to medium hardness. In this case, the tool chosen is a hardened steel ball. The final value depends on the force applied, the diameter of the ball, as well as the diameter of the resulting print.
Vickers hardness. Method of application regardless of the hardness of the metal. Applies to structures that have undergone chemical and thermal hardening. The test tool is a diamond pyramid with an apex angle of 136°
Knoop hardness. This method is very similar to the Vickers method, but the resulting print has the shape of an elongated diamond. For the calculation, indicators of the applied force and parameters of the large diagonal of the rhombus are required.
Ball print hardness. In this case, the method is more suitable not for metal, but for products made of hard rubber. The tool used is a hardened steel ball with a diameter of 0.5 cm. The test sample should not have a thickness less than the diameter of the ball.
According to Martens. This evaluates plastic and elastic deformation by penetrating a pyramid-shaped tool into the test sample.
Scleroscope. This method helps to establish the hardness of bulky and large metal structures.

Regardless of the method of establishing strength indicators, after proper qualified heat treatment the metal becomes stronger.