Induction equipment for heat treatment of welds and connections of pipelines, pipes, metals (steel, cast iron)

During large-scale fabrication of metal structures and pipeline connections, additional processing is carried out, which novice welders forget about. One of these technological processes is the heat treatment of welds. It allows you to improve the characteristics of the finished fastener and extend its service life.

Heat treatment of welds

Heat treatment equipment

Heat treatment equipment can have various capabilities that facilitate specific processes.
This applies to the maximum temperature created in it, the amount of material processed simultaneously, and the type of processing performed. Equipment for heat treatment at various enterprises is presented:

• shaft furnaces;
• chamber furnaces;
• ovens with a retractable hearth;
• vacuum furnaces;
• melting presses;

Shaft furnaces have high productivity and can process materials of large dimensions. With their help, it is possible to carry out heat treatment for the operation of hardening, annealing, tempering, and normalizing non-ferrous metals. The application is optimal for enterprises that do not focus on the accuracy of operations.

Today, various enterprises produce shaft furnaces that have electric and gas heating. Installations of this type can be used in endogas, nitrogen, air, vacuum and hydrogen environments. Their main application is the heat treatment of large steel elements. These include steel parts and assemblies, large-sized castings and forgings. In addition, they are normalized and rolled out.

Chamber heat treatment furnaces have smaller dimensions and are therefore used to change the properties of small objects. Installations of this type are popular in various types of production. They can be used either separately or in conjunction with automated systems.

The set of equipment for heat treatment may include:

• heating furnaces;
• hardening tank;
• washing chambers;
• vacation cameras;

Tempering chambers in some installations are combined with a cooling container to avoid temper brittleness. Chambers are often used in which the elements are treated with cold, this helps reduce retained austenite. The automated complex may include a rail transport system for loading and unloading.

Furnaces with a retractable hem are the optimal tool for heat treatment of parts or assemblies that have large dimensions. To carry out loading and unloading, cranes and overhead cranes are used. Among the disadvantages, large heat loss can be eliminated. This is due to their size. They are used to perform austenitization and annealing. Often used to heat metal before the forging procedure. Small manipulators and robots can be used to load items. The workspace can be heated by gas or electricity.

Chemical treatment

The best results when processing welded joints are achieved with a combination of mechanical and chemical means. Two methods of working with seams are used: etching and passivation.

Etching is performed before mechanical grinding. To carry out this operation, chemical compounds are used to provide a uniform coating that prevents corrosion processes. In addition, etching allows you to eliminate areas affected by tarnish. The fact is that in such places there is an accumulation of nickel and chromium oxides, as a result of which the steel becomes rusty.

In small areas of welded joints, it is recommended to carry out etching by directly applying the composition to the surface to be treated. If the part is large enough or has a complex configuration, it should be placed in a container with an etching solution. The time the metal remains in the etching flow is calculated individually in each situation.

When etching is completed, passivation begins. The process involves applying a special composition to the metal, resulting in the formation of a film. This protective coating prevents corrosion. From a chemical point of view, passivation can be explained as follows: oxidants, interacting with steel, remove free metal from the surface, thereby activating the formation of a protective film.

The chemical treatment is completed by cleaning the welded joints from reagents. The flushed water contains many toxic substances, heavy metals and acids. Acids are neutralized with alkalis, and then the remaining liquid is filtered. Waste material must be disposed of only in specially designated areas in accordance with environmental protection legislation.

Weld processing - overview of methods

Welds are responsible for the integrity of the metal structure. In particular, the connection must be strong enough, resistant to rust and moisture. Weld seam processing is designed to ensure the fulfillment of these tasks.

Processing methods

There are three methods by which welded joints are protected:

1. Heat treatment. Thanks to this method, it is possible to remove residual stresses in the material resulting from welding work. Heat treatment is carried out using one of two technologies: local, when only the connection itself is heated or cooled, or general - the entire part is subject to temperature treatment.
2. Mechanical restoration. In this case, the task is to remove slag residues and check the reliability of the connection. A typical example of machining is tapping a seam with a hammer or sanding it. If the slag is not removed, corrosion may develop.
3. Chemical treatment. Applying protective coatings to joints is one of the ways to combat corrosion processes. The most affordable option for chemical protection is treating the seam with a primer paint and varnish material.

Heat treatment

In addition to reducing residual stresses in the metal, heat treatment allows you to achieve the following goals:

• make the structure of the seam and heat-affected zones more adapted to external factors;
• optimize the physical and operational properties of the material, in particular, increase rust resistance, heat resistance, etc.

Heat treatment of welded joints involves heating the welded joint or the entire metal to a given temperature for a certain time. Next comes artificial cooling, which is also carried out according to a certain scenario.

Heat treatment equipment

Four types of technological equipment can be used for heat treatment of joints:

1. Induction devices. Induction heating is often used during pipeline installation. The essence of this method is the use of copper inductors, including multi-core air-cooled copper cable. When installing the inductor on the pipeline, the distance between the pipe and the inductor must be taken into account. The general rule is: the larger the gap between objects, the worse the equipment's power is used.
2. Flexible resistance heaters. This method is considered one of the most convenient and affordable methods for processing welds.
3. Muffle furnaces. When working with this type of equipment, special attention must be paid to the uniform heating of the connection, which is achieved by non-centered installation of the part in the furnace.
4. Heating using gas-flame equipment. When using gas flame heating, welding and special multi-flame gas burners are used. Gas heaters produce thermal energy resulting from the combustion of a mixture of combustible gas and oxygen.

Heating equipment is selected based on installation conditions, availability of a particular type of device and other circumstances. Heating equipment must meet certain requirements: be clearly connected to the welds, not have too much mass, and ensure uniform heating of the joint both in width and length.

To reduce heat loss, all kinds of heat insulators are used when heat treating welded joints.

Thermal insulation must be heat-resistant with low thermal conductivity, durable, but at the same time flexible, wear-resistant and safe to use.

Heat treatment methods

Several methods of heat treatment of welded joints are known:

1. Preheating. It is used both before welding and during welding of parts. This type of heat treatment is used when welding structures made of low-carbon steel. The metal warms up to 150-200 degrees Celsius.
2. High holiday. The technique consists of heating the material to 650-750 degrees Celsius (the specific temperature depends on the type of steel). The temperature is maintained for 5 hours. The technology makes it possible to reduce stress by 80%, as well as increase the material’s resistance to mechanical stress and increase its elasticity.
3. Normalization. Applicable to carbon and low-alloy steel grades. Such heat treatment of the compound is carried out at temperatures from 950 degrees Celsius. At the end of heating, exposure and cooling are carried out under ambient conditions. Normalization makes it possible to reduce the grain size of the metal, reduce stress, and also increases the strength of the weld.
4. Austenization. It is the hardening of a welded joint by heating it to a temperature of 1070 degrees and above. The part is heated for 60 minutes, and then rapid artificial cooling is performed. The technique is widely used for hardening austenitic steels. The result of austenitization is increased elasticity of the welded joint.
5. Stabilization. Stabilizing annealing differs from austenitization by a lower temperature and a shorter period of holding the metal.
6. Thermal rest. The technology consists of heating the weld to 250-300 degrees Celsius. Then the metal is kept in a heated state. As a result of the procedure, the level of diffuse hydrogen in the welded joint is reduced and internal stresses are reduced.

The choice of the method by which heat treatment of welded joints will be carried out depends on the physical and chemical characteristics of the steel (determined by its grade). Fulfillment of technological requirements is of particular importance, otherwise the quality of the welded joint will deteriorate.

Key parameters to consider when performing local heat treatment:

• width of the heated area;
• uniform heating across the wall thickness and width of the heated area;
• holding period;
• cooling intensity.

Mechanical restoration

Mechanical elimination of welding defects is carried out using a wire brush. You can greatly simplify the task and improve the quality of sanding if you use a portable sander or an angle grinder with a flap attachment. Instead of a nozzle, you can also use an abrasive wheel.

Mechanical cleaning allows you to remove the following weld defects:

• scale;
• burrs;
• oxides;
• consequences of discoloration.

Despite the simplicity and low cost of the technology, there are a number of nuances regarding the choice of nozzle, knowledge of which will allow you to perform the job better:

1. First of all, you need to choose a grinding wheel from a suitable material. Aluminum zirconate circle is best suited for mechanical cleaning. The advantage of this material is that, firstly, it provokes corrosion processes, and secondly, aluminum zirconate is stronger than aluminum oxide, from which some types of nozzles are also made.
2. The blades of the grinding wheel should be on the fabric component. The fabric is more reliable and more resistant to heavy loads compared to paper, which is sometimes used on petals as a base. However, such attachments cost much more than their paper-based counterparts. The higher cost of fabric attachments is completely justified and will pay off for work that is aggressive towards the material, such as sanding seams.
3. The size of the abrasive grain depends on the type of work being performed. Very often, when cleaning joints, nozzles with different grain sizes may be needed. Therefore, it is recommended to purchase several types of attachments at once.
4. If you need to properly clean a seam, then different grain sizes are simply necessary, since grinding is carried out with a gradual change of attachments to smaller grain sizes. For example, large scales are removed with coarse-grained nozzles, but fine grinding is done with fine-grained nozzles. Finishing penetration is carried out with the finest grains. Nozzles should be changed sequentially - no more than one size may be missed. However, when it comes to creating a mirror-like shine on a welded joint, no size can be skipped.
5. To process seams located in hard-to-reach places (cavities, edges, holes), special devices are used - burrs installed in a grinding machine. Burrs come in a wide variety of sizes and shapes, so choosing the right configuration is easy.

Processing methods

There are three methods by which welded joints are protected:

1. Heat treatment. Thanks to this method, it is possible to remove residual stresses in the material resulting from welding work. Heat treatment is carried out using one of two technologies: local, when only the connection itself is heated or cooled, or general - the entire part is subject to temperature treatment.
2. Mechanical restoration. In this case, the task is to remove slag residues and check the reliability of the connection. A typical example of machining is tapping a seam with a hammer or sanding it. If the slag is not removed, corrosion may develop.
3. Chemical treatment. Applying protective coatings to joints is one of the ways to combat corrosion processes. The most affordable option for chemical protection is treating the seam with a primer paint and varnish material.

Below we will look at weld protection technologies in more detail.

Equipment for heat treatment of welds and connections of pipelines, pipes, metals

Induction heat treatment of welded joints and seams of steel, most often pipes or pipelines, is an innovative post-welding procedure. This technology is widely used in a variety of fields: oil refining, energy, chemical.

Induction heat treatment of pipes and pipelines can be either local, when it directly concerns the weld or joint, or complete, when the entire structure, including joints, is heated. Regardless of the scale of the operation, there are three main stages of weld heat treatment: first, heating occurs to the desired temperature at a certain speed, then the welded joint is held for some time, and finally, cooling, which also takes place at a predetermined speed.

Induction equipment for heat treatment of metal and welds from the PROMINDUKTOR company is completely air-cooled, has an easy control interface, and a built-in industrial controller with the ability to program heat treatment modes:

1. Preheat

– a simple method of heating a material to a certain temperature and maintaining this temperature for some time;

2. Annealing

ADVANTAGES

• Heating occurs much faster than with gas-flame or resistive methods;
• Even at low ambient temperatures, heating occurs with high efficiency, compared to resistive and gas-flame methods;
• The heating rate is controlled within specified limits and is characterized by a high degree of stability;
• Heating occurs uniformly and is localized across the entire width of the welded joint zone and along the entire circumference of the pipe;
• Eliminates the risk of damage to the factory pipe insulation;
• There are no open flames or live elements;
• There is no need to store large quantities of gas reserves, and therefore cylinders, as with the gas-flame method.

SPECIFICATIONS

PURPOSE

The installation is designed to:

• Preliminary and concomitant (interlayer) heating of the edges of metal pipes before welding and during the welding process;
• Heat treatment of the weld zone;
• Heating the weld joint area before applying protective insulation.

The installation can be used for the repair of field and main pipelines, gas pipelines (condensate pipelines) and their branches made of pipe steels.

The installation is intended for operation in field conditions and in production workshop conditions.

Vacuum ovens

Vacuum furnaces are the optimal means to produce high-quality tools, high-speed steels, titanium alloys, copper, refractory metals and structural steels. Vacuum furnaces carry out all processes with high technological precision parameters. The temperature in them cannot deviate by more than 5 degrees. They are used as components of heat treatment lines.

Vacuum furnaces can use nitrogen, helium, or air. Moreover, their operation does not require the use of water quenching tanks. This makes it difficult to harden low-carbon and low-alloy steels in them. To make the inner surface of a vacuum furnace, molybdenum sheets are used, heating elements are graphite, ceramics, and powder materials.

Installations that have high power are capable of creating a pressure in a vacuum furnace of 0.00005 mbar. The maximum ambient pressure level will be 20 mbar and the temperature will be 1350 degrees. Water is used as a coolant.

Vacuum chambers are equipped with various vacuum pumps, receivers with a gas cooling medium and installations that provide reverse water cooling. The degree of automation of this heat treatment equipment can vary between 0.7-0.9.

Vacuum furnaces are expensive because they require much more money to develop and manufacture. However, they have one drawback, which is due to the fact that the surface of the alloys is dealloyed if high temperatures are used in them.

Heat treatment technology

When carrying out heat treatment, the length of the seam is taken into account, uniform heating of the joint and adjacent areas is maintained, a suitable heating speed and temperature is selected, the heating duration time is set (also called dwell time) and the cooling rate is set.

Heat treatment of pipeline welds begins with the seam being insulated using heat-insulating material. For example, when using a gas-flame burner, the seam is wrapped with a layer of sheet asbestos 2-3 centimeters thick. Only then does the installation of the burner itself take place. The same principle applies when welding with induction devices or heating elements.

To ensure that welds do not lose heat, insulating materials must be strong and heat-resistant at the same time. At the same time, they should be light in weight and bend easily. The table below describes the main thermal insulation materials used in heat treatment. The temperature range is also indicated.

Processing of the weld seam is entrusted only to specialists. The specialist undergoes preliminary training and only after that begins work. In this case, the process should be led by a senior master. The specialist must not only select and install the heating equipment correctly, but also check how well the mechanics have prepared the metal. Heat treatment of pipeline welds will not begin without careful preparation.

After treatment, thermal rest can be carried out. Let the part cool down. Then the welds are ground with a grinder. Cleaning the weld after welding is necessary to remove unnecessary inclusions formed during welding. For example, slag.

Basic set UINT-50-2.4

The questionnaire can be downloaded here

COMPOUND

• Frequency converter;
• Transformer-matching device;
• Set of inductors (depending on the purpose of the installation);
• Heating control panel;
• Pyrometric temperature sensor (measurement range 0 ÷ 400°C) – when heating is performed;
• Thermocouple temperature sensor (measurement range 0 ÷ 1200°C) – during heat treatment;
• Device for welding thermocouples (optional);
• Inductor water cooling station (optional);
• Specialized software for maintaining heating protocols;
• Set of connecting cables and sleeves.

Equipment used

The following types of equipment are used for heat treatment of welds:

1. Induction. The operating principle is based on heating the metal by eddy currents created by an induction coil (inductor) connected to a high-frequency generator. The heated area is preliminarily covered with asbestos. On top of it, coil turns are wound with a flexible wire in increments of 2.5 cm at a distance of 25 cm on both sides of the joint. Pads with wires located inside are also used as an inductor. The technology ensures fast, uniform heating of the joint area, regardless of the position of the parts.
2. Radiation. Heating is carried out by heat from nichrome wires through which electric current passes. Flexible heating elements are convenient for processing complex shaped joints. Radiation equipment is more effective than induction equipment when working with metals with low electromagnetic characteristics.
3. Gas is advantageous for use because it does not require electricity. However, it takes a long time to heat up. Therefore, the equipment is used on small structures. To ensure uniform heating of the joint, the work is performed with two multi-flame acetylene torches simultaneously on both sides.
4. Muffle furnaces are used to work with small parts. They are also used on small diameter pipelines.

Heat treatment

Heat treatment of welded joints is often used in the process of constructing pipelines. To create them, pipes are used that have a large internal diameter and significant wall thickness. This will ensure the strength and reliability of the system during operation. But this creates a new problem - it is difficult to carry out high-quality joining of pipes that have such geometric characteristics.

During the welding process, a small area of ​​the product is heated. As a result, this leads to the emergence of a number of physical processes that adversely affect the operation of the structure.

The heat treatment of welded joints itself is carried out in several stages:

• Preparation of products for welding. The strength of the connection depends on how well this work is done.
• Heat treatment of the joint of products during welding.
• Treatment of seams after welding.

Heat treatment is needed in order to improve the welding properties of metal structures. But, above all, products and seams can be annealed and highly tempered.

Why is it needed?

Welding is carried out under the influence of an electric arc, as well as filler material from the electrode. In this case, the temperature is 1500-50000C. As a result of such heating, various negative phenomena arise on thick metal, which are worth considering more carefully:

• Great overheating occurs at the junction of the weld and filler material. As a result, the crystallization of the metal, which has a large structure, increases. This significantly reduces its ductility. Also, during the heating process, manganese and silicon burn out. A section of a metal product becomes rigid and loses its original technical characteristics.
• There is a hardening zone near the seam, which also experiences heating. Of course, it is affected by a lower temperature than in the environment of direct connection of the filler and base material. As a result, certain elements are pierced in the metal. The area loses its original plasticity and becomes harder. The impact strength properties of the material also change.
• The softening zone is located at a remote distance from the place of direct welding of metal products. It is susceptible to the moderate temperature emitted by the electron arc. Due to the short duration of this process, the material retains its plasticity. But, as for strength, it decreases somewhat.

As a result of metal welding, residual stress is applied to the metal, which can lead to deformation. This may cause some difficulties when installing volumetric structures. This is especially true for places where new units will be installed.

The danger with residual stress is that it can cause cracks to form in the future. This is especially true for the welding area. This is unacceptable, since in the future it will lead to the destruction of the connection.

The situation worsens when, during the operation of the structure, high temperatures are applied to the welding site. This leads to a decrease in the metal’s resistance to corrosion processes and cyclic strength. This also applies to the ability of the metal to resist brittleness that occurs under the influence of low temperatures.

Features of the event

Heat treatment of welded joints is carried out at high temperatures, the value of which ranges from 700 to 10000C. Thanks to this, it is possible to eliminate the consequences of uneven heating that was carried out during arc welding. This is especially true for metal products that have significant thickness. As a result of processing, the seam is given a structure similar to the rest of the material.

Heat treatment of welded joints is carried out in three stages:

1. The area near the connection is heated. For this, special equipment is used, which we will talk about later.
2. An area or the entire product is kept at a certain temperature for a certain time.
3. At the final stage, the material is systematically cooled to normal temperature.

Thanks to this process, it is possible to eliminate residual effects after arc welding, align the structure of the metal, and also relieve stress, which is often the cause of its deformation. The process is carried out in various ways. The technology for carrying it out depends on the thickness and type of material. Processing is not always carried out, but there are cases when it is simply necessary.

In the video: how the heat treatment process occurs.

Advantages and disadvantages

Processing the joint using the thermal method has certain advantages and disadvantages. Among the advantages are:

• As a result of the process, welded joints acquire new properties. As a result, the parts will become more suitable for use in certain conditions. This is especially true for protecting metal from corrosion.
• Processing allows you to eliminate some of the negative aspects that arose as a result of welding work.
• Heat treatment relieves residual stress that occurs during the welding process.

Of course, to achieve such a result, it is necessary to approach the processing correctly. This especially applies to compliance with certain rules. As for the disadvantages of this processing method, among them are:

• The process must be performed by an experienced specialist. This is because heat treatment is irreversible. This means that it is almost impossible to eliminate the mistakes made.
• To carry out the processing, you may need special equipment, which requires certain skills to operate. Especially when protecting pipeline welds.
• The procedure must be carried out exactly according to the requirements.
• In each case, its own processing parameters are selected.

If you follow certain rules, then no problems will arise during the process of stripping the joint.

What is being processed?

Heat treatment is often used in the construction of pipelines for various purposes. This primarily applies to pipes that have a diameter of more than 10 cm and a wall thickness of 1 cm or more. The process is carried out using induction heating with a current whose frequency is 50 Hz.

Heat treatment of pipes is quite simple. For this purpose, muffle furnaces and special electric heating wires, which have sufficient flexibility, are used. If the product has a thickness of no more than 2.5 cm, then gas-flame heating is used for processing. The main thing here is to evenly distribute the temperature in the area of ​​the connecting seam.

Heat treatment is carried out not only on circumferential seams, but also on joints that have a different shape.

When processing welds, it is worth taking into account the thickness and characteristics of the metal. So, for example, if the pipeline is made of steel pipes whose wall thickness is 45 mm, then the process must be carried out immediately after welding. In this case, the cooling of the material should not reach 3000C. This also applies to products with a thickness of 25 mm.

If it is not possible to carry out processing, then the seam is protected with heat-insulating material. Clean up at the first opportunity. The process must be carried out within 3 days from the date of welding.

Process parameters

Features of heat treatment directly depend on the type and thickness of steel. Thus, in the case of chromium-molybdenum steel and its alloys, the process is carried out using induction or radiation methods.

So, depending on the thickness of the material and the method used, the processing process will take the following time:

If you analyze the table, you can note that metal processing using the induction method takes less time. This is explained by the peculiarities of the process.

What equipment is used?

Heat treatment of welds is carried out using various means. When choosing, the thickness of the metal and the possibility of using this or that equipment in a certain place are taken into account. Today there are three main methods of heating the heat-affected area. Let's look at each of them.

Induction

A special device is installed on site, which generates high-frequency alternating voltage. A heating element is connected to it, which uses a flexible wire. It is wound onto a welding joint, which is previously wrapped in thermal insulating asbestos. This technology is used to process horizontal and vertical seams.

The wire is wound close to the insulator. In this case, a gap of 2.5 cm thick is left between the turns. As a result, 25 cm of the product is covered on both sides of the seam. When the coils are installed in accordance with all requirements and standards, the device turns on. This takes into account the operating time of the equipment. This directly depends on the thickness of the metal. During operation of the device, voltage passes through the turns, which creates induction, as well as heating the metal.

To perform processing, special belts that contain a certain number of wires are also widely used. This allows you to effortlessly and quickly prepare the product for cleaning after welding.

On video: induction heating of a pipe.

Radiation

The radiation method of processing welds is no less popular. Special nichrome wires are used as a heating element. Voltage is passed through them, which causes them to heat up. It is worth noting here that induction does not take part in the process. The material is heated using a hot wire. The heating elements are laid on a base made of heat-insulating material.

Gas-flame

This method is the cheapest. For heat treatment of welded areas, an acetylene mixture and oxygen are used. The method is used to process materials whose thickness is no more than 10 cm. A mouthpiece with large holes is installed on a burner filled with a combustible mixture. To ensure uniform heat supply to the surface being treated, an asbestos funnel is placed on the nozzle. This allows the flame to be distributed over a width of 25 cm.

When using this method, it is worth considering some features. So, in order to perform high-quality heat treatment of heat-affected areas, it is necessary to heat them simultaneously. This means that two burners take part in the process at once.

Types of heat treatment

Thermal treatment of a welded joint can be performed in several ways. The purpose of the process is taken into account. Among the main methods are:

• Thermal rest. In this case, the material is heated to 3000C. This temperature is maintained for two hours. As a result of the process, hydrogen in the weld is reduced and residual stress is relieved. This method is often used for materials that have thick walls, and also where it is not possible to use other technologies.
• High holiday. With this technology, the product can be heated at temperatures up to 7000C. This treatment lasts about three hours. The duration of heat action on a material directly depends on its thickness. This method allows you to remove residual stress by almost 90%. When it comes to processing low-alloy steel, the result is destruction of the hardening structure and the carbides become larger. In this way, an increase in ductility and toughness can be achieved. This technology is often applied to pearlitic steels.
• Normalization. This method involves simultaneous heating of the material and the weld to a temperature of 8000C. In this case, heat treatment should not exceed 40 minutes. Using normalization, residual stress can be partially relieved. But the main advantage is that the processing results in a homogeneous and fine-grained structure. This in turn improves the mechanical properties of the seam and heat-affected areas. Normalization is often used on materials that are thin.
• Austenization. The material is heated to a temperature of 11000C. The thermal effect continues for two hours. After this, the material is cooled in air. It is not recommended to perform forced cooling, as this will lead to a decrease in the strength of the metal and, as a result, to the appearance of cracks. Austenization is used on high-alloy steels. With its help, the plasticity of the material increases and the residual stress decreases.
• Stabilizing annealing. The method is used for processing materials with a superimposed seam. It is heated at a temperature of 9700C for three hours. After this time, the material is allowed to naturally cool in air. Stabilizing annealing can prevent the occurrence of intercrystalline corrosion. The technology is often used on high-alloy steels. This will protect the areas from corrosion.

Heat treatment is used on products made of various metals. With its help, their service life is increased. To carry out the process correctly, it is necessary to carefully select the operating temperature, heating method, and time of heat treatment. Cleaned products must be handled very carefully so as not to damage them.

Which method should I choose?

The choice of technology for heat treatment of welded joints directly depends on the physical and chemical characteristics of the material. This is evidenced by the grade of steel. Experts recommend paying particular attention to meeting technological requirements. Otherwise, the quality of the weld is significantly reduced, which in the future can lead to its complete destruction.

When performing heat treatment of a welded joint, it is worth considering the following parameters:

• the width of the area that will be processed;
• uniform thermal effect on the material, both in thickness and width;
• heating duration;
• Features of cooling the material after heat treatment.

If you take into account all these features, you can choose a heat treatment method that will improve the quality of the connection. This is especially true for its strength.

Temperature control

As already mentioned, during the heat treatment process it is necessary to carefully monitor the heating temperature. To achieve this goal, special tools are used, such as thermal pencil and thermal paint. When they reach a certain temperature, they suddenly change color. Depending on the principle of operation, such thermostats can be chemical or melting.

When a certain temperature is reached, chemical temperature regulators change color as a result of a reaction between the components. The measurement accuracy is directly affected by the time of thermal exposure of the material, as well as pressure fluctuations.

If heating is carried out for 3 minutes, then the measurement error will be no more than 100C. It is worth noting that a change in the shade of heat controllers occurs at critical temperatures.

In pencils and paints of the second type, the change in shade occurs as a result of the melting of a substance that is very sensitive to increased temperature. Unlike chemical temperature indicators, these agents change regardless of the duration of heat exposure. This allows you to more accurately set the heating temperature. In this case, the error is no more than 20C.

Thermal melting indicators have many advantages, among which it is worth highlighting their inertness to variable temperature, vacuum, solar radiation, sea fog and other negative environmental factors that can affect the accuracy of measurements.

Thermal pencils and thermal melting paints are divided into two types:

• Adsorbent. The indicator consists of a pigment in a binder solution and a suspension of a substance that is sensitive to temperature increases during heating of the material. As a result of thermal exposure, the heat-sensitive substance melts, after which it is adsorbed by the color pigment.
• Melting varnishes. They have a certain melting point. The substance is applied directly to the surface to be treated. It dries quickly, after which a rough surface is formed. As a result of reaching a certain temperature, it will become glossy.

Today, paint and varnish factories are engaged in the production of thermal indicators. At the same time, they provide information about the critical melting points of a particular substance. This allows you to significantly improve the quality of heat treatment of welds.

Treatment of the weld after welding

Welding is the most reliable, fastest and economical way to create permanent joints of metals and their alloys. During the welding process, the metal is heated to its melting point. This causes internal tension in him. In addition, slag remains on the surface of the weld. To remove slag and relieve internal stresses in the metal, various weld processing methods are used.

Basic methods of processing welded joints

The following weld processing methods have become the most common:

• Thermal. Used to eliminate internal residual stresses. It is carried out by local or general heating.
• Mechanical. Cleaning welds after welding removes slag and scale from the joint surface.
• Chemical. Consists of degreasing and coating with a protective layer. Prevents the formation of corrosion foci. The method is used to process materials susceptible to corrosion that will have to work in active environments.

The method of exposure is chosen based on the technical requirements for the structure and its operating conditions. Often all three methods are used sequentially.

Heat treatment

Heat treatment of welded joints must be carried out after welding thin-walled products that are especially susceptible to deformation under the influence of internal stresses. Such structures include pipelines, various containers, and pressure vessels.

Heat treatment of the weld

Heat treatment is also carried out for most critical structures, such as nuclear and chemical reactor vessels.

Heat treatment consists of heating the part and then cooling it according to a strictly specified temperature schedule.

Why is it needed?

During welding, a small area of ​​the part near the seam is heated. Uneven heating leads to the emergence of internal stresses that can deform or even destroy the part. In addition, in the zone of uneven heating, the structure of the metal crystal lattice changes, which leads to a deterioration in its physical, mechanical and chemical properties.

Near the weld seam there is a hardening zone in which the strength is increased and the elasticity, on the contrary, is decreased. It is surrounded by a zone of softening, in which ductility is preserved, and the strength becomes lower than it was before welding.

Heat treatment of welded joints is designed to restore the internal structure of the metal and its properties, restore the characteristics of strength, ductility and corrosion resistance

Types of heat treatment

Heat treatment of welded joints can be carried out in several ways. The most effective include:

1. Heating parts before welding. Used when working with low-carbon steels. The welder heats the working surfaces to 200 degrees. After welding, the structure should cool at room temperature.
2. Release of metal. This involves heating parts to critical temperatures. The blanks are kept in this mode for up to 5 hours. The material is then slowly cooled.
3. Thermal rest. The workpieces are heated to 300 degrees. They are kept at this temperature for up to three hours. Gradually, residual loads disappear, and the seam becomes stronger.
4. Normalization. It is carried out to reduce large grains of the material structure and increase strength indicators.
5. Austenization. Before welding, the parts are heated to 1100 degrees. The exposure at this temperature is 90 minutes. The cooling process takes place in the fresh air. Mechanical properties are improved and residual stress disappears.
6. Stabilizing annealing. The finished seam is heated to 800 degrees. For three hours the temperature is maintained at the same level. Reduces the risk of rust formation.

The heat treatment method depends on the material used.

Application of heating elements