Methane is used for gas welding of metals. Elements of the gas mixture. Welding gases

There are many types of welding. The division is based on the method of obtaining a high-temperature weld pool (type of energy). For example, welding with an electric arc, ultrasound, gas flame and others. This torch can cut and weld any kind of metal. The edges of the metal parts being welded literally melt and, joining together, form a new single structure in the place of the alloy, called the weld.

Welding gases include, first of all, acetylene for welding, which is released as a result of the reaction of calcium carbide with water. Mixing with oxygen, it allows a flame temperature of over three thousand degrees to be obtained.

Also propanes, butanes, liquefied MAFs (new gases that replaced acetylene), benzenes, kerosene and others are considered welding. An important feature the use of welding gases will be the mandatory presence of oxygen as a combustion catalyst. Moreover, the developed temperature also depends on the quality (purity) of the oxygen supplied to the burner.

What should be the gas mixture for welding?

Gas mixture for welding with the use of technically pure oxygen, it gives a very intensive and complete combustion of the mixture itself or the vapors of combustible substances, since it provides very high combustion temperatures. The amount of oxygen in a flame will determine its oxidizing or reducing properties.

On the other hand, the use of technical (pure) oxygen requires special cylinders for its storage and supply. In a mixture with such oxygen, some gases or compounds can be explosive (due to the extremely high rate of their combustion in such a catalyst).

They can often be dangerous in and of themselves due to their toxicity. For example, acetylenes, cyanides, etc.

The use of oxygen contained in the atmospheric air makes welding gas mixtures less effective. Their combustion is slowed down, which sharply lowers the flame temperature. The reason is that oxygen in the air makes up no more than a fifth of it; other gases are mostly present, the same nitrogen, for example.

In addition to the above, welding under conditions of using atmospheric oxygen often does not give the required geometry of the joint, changes the properties of the metal in this zone, which ultimately affects the quality of the joint.

Shielding gases - what are they and what are they used for?

Industrial gases are used not only in welding. Shielding gases are also widely used for electric arc welding, etc. The use of various inert (helium, argon) or active (nitrogen, CO2, hydrogen, oxygen) gases as a shielding medium for the weld pool significantly improves the quality of the result, increases the speed of work, allows you to obtain the desired parameters of the seam, etc.

Welding principle in the environment protective gases simple. The required composition is fed into the arc action zone through the nozzle of a special burner under pressure, creating this very protective environment. Popular semiautomatic welding is based on this principle.

Such welding is available not only in the factory, it is widely used in workshops and even in private garages. Most often, the gas for welding with a semiautomatic device is a mixture of inert and carbon dioxide (in various proportions). Of the inert, helium and argon are more applicable. In practice, the use of argon is accepted, therefore, CO2 and argon are present in the composition.

In general, an inert gas for welding is needed to protect the molten pool from external exposure to air, as well as if it is necessary to carry out high-quality welding work on stainless steels, titanium and its alloys, non-ferrous metals (nickel, copper, aluminum and alloys), etc. This electrode can be any: classic melting, not changing its shape and structure (serving to create an arc), etc.

The choice of the gas required for welding is influenced by the type of metal used in the work. The same mixture of CO2 and argon when welding steel elements contains more carbon dioxide (about 18%). And when welding stainless steels, argon predominates (98%), CO2 is only two percent.

Thus, what gas is used for welding is determined by the metal, its brand, the required properties of the seam, the types of welding equipment, the requirements for chemical composition and even the shape of the seams, working conditions, etc.

There are several types of welding, differing from each other by the method of obtaining a high temperature weld pool intended for cutting or joining various metals and alloys: electric arc, ultrasound, gas flame. The principle of operation is that the edges of the structures to be reunited melt, and a completely new structural unit is formed at the place of their joint - the weld.

Welding gases

The temperature indicator depends primarily on the type of gas used for welding. For example, as a result of the reaction of water with calcium carbide, acetylene is released, which, interacting with oxygen, makes it possible to obtain a flame temperature of more than 3000 degrees. Welding gases include all types of propane, butane, MAF (alternative to acetylene), benzene, kerosene and others.

The presence of oxygen, which is a combustion catalyst, is mandatory during the use of any welding gases. The oxygen supplied to the burner must be of high quality and purity. Depends on these indicators Maximum temperature received during operation.

Gas mixture elements

Regardless of what gas is used for welding, it is important to remember that to obtain high temperature combustion and certain properties of the flame are possible only when using commercially pure oxygen in a mixture. The completeness of combustion or evaporation of combustible elements depends on the quality of the component, and the properties obtained by the flame: reducing or oxidizing, depend on the quantity.

Special requirements apply to storage and serving conditions. The use of special separate cylinders is necessary in this case:

• technical oxygen is a powerful catalyst;
• many of the gases used for welding are highly toxic.

If atmospheric oxygen is used, then it is unlikely that it will be possible to obtain perfectly smooth seams. This will change the properties of the metal acquired in the process of melting and joining, which will significantly reduce the quality of the weld made.

Not considered efficient use in the welding gas mixture with normal oxygen in the atmosphere. The presence of impurities existing in the air significantly reduces the rate of combustion of the elements, which is reflected in the flame temperature.

Welding gases

Regardless of what gas is used for welding, inert or carbon dioxide, remember to respect the proportions in the mixture. The exact choice depends on the metal. For example, when welding steel structures, carbon dioxide predominates (about 18%), and when working with stainless steel, argon (almost 98%).

Inert and active gases are used for welding. Inert elements, as a rule, are not poisonous, do not interact with metal and do not dissolve in them.

• Argon (Ar) is colorless and odorless, does not burn, and is one and a half times heavier than air. There are two varieties. The highest grade is used for welding loaded metal structures from active and rare metals and alloys. The first grade is required for aluminum and steel products.
• Helium (He) - colorless, odorless, lighter than air. There are two types: high purity and technical element. The rare use of helium is due to its high cost. Recommended for pure and active metals, aluminum and steel.
• Nitrogen (N2) - colorless, odorless. Suitable for copper and copper alloys. There are 4 types of nitrogen with different substance content.

Active gases protect the welding zone from air, enter into chemical interaction with metals and dissolve in them.

• Oxygen (O2) - has no color, smell, taste. Oxygen does not burn, but is a powerful combustion catalyst. It is used in mixtures with inert or active ingredients.
• Carbon dioxide (CO2) has an odor and pronounced oxidizing properties. It dissolves perfectly in water and weighs 1.5 times heavier than air. There are 3 types of substances that are used to join cast iron, low and medium-carbon metals and alloys, low-alloy structures and corrosive steels. It should be remembered that carbon dioxide is not used in welding.

Improve the process and weld quality by using welding mixtures. The most demanded mixtures are:

• helium and argon;
• argon and oxygen;
• carbon dioxide and argon;
• oxygen and carbon dioxide;
• argon, carbon dioxide, oxygen.

When choosing which gas for welding is suitable in a particular case, it is necessary to take into account several factors: the characteristics of the metal being joined, the type of equipment, the desired shape of the seam, the conditions for performing the work, and the properties of the chemical composition.

Gas welding is more expensive than electric, because the price of acetylene and oxygen is much higher than the cost of electric energy required for resistance or arc welding.

Gas welding is a type of welding work where parts need to be heated to a molten state by means of a high-temperature flame. This method is widely used in the creation of structures based on thin carbon steel, in the repair of cast iron products, as well as when it is necessary to weld defects in various products obtained by casting from non-ferrous or ferrous metals.

What gases are used?

Gas welding uses flammable gases - natural, acetylene, gasoline vapors, hydrogen. These gases are distinguished by good combustion in air, without developing a high temperature; an oxygen jet is sufficient for combustion. Gas welding is most often carried out on the basis of acetylene, which is created on the basis of water and calcium carbide. It burns at a temperature of 3200-3400 degrees.

What are the features?

The advantages of gas welding include the following:

1. Simple technology.
2. No need for a welding power source.
3. The simplicity of the equipment on the basis of which gas welding is performed.

On the other hand, this process is not very productive. Welding is carried out only by hand, and the mechanical and operational properties of finished products are not always of high quality.

Oxygen reducer

When welding, oxygen comes from a special cylinder - it turns blue or blue. To ensure normal operation, oxygen must enter the burner evenly and at low pressure. It is for these purposes that there is a reducer on the cylinders - it regulates the gas supply. In this case, gas welding sleeves - acetylene and oxygen - are supplied to the torch. Oxygen is supplied to the central channel, where the jet is discharged more, sucks in acetylene, which enters the burner under low pressure. The gases are mixed in the chamber and then escaped from the tip.

Technology features

When performing gas welding, it is important to obtain a high-quality joint, therefore, great attention is paid to careful preparation of the edges to be welded, the choice of the method of joining the metal, the installation of the torch in the desired position and the determination of the required parameters of the torch power. Gas welding technology assumes that the edges must be thoroughly cleaned of various contaminants. The bevel is performed using a manual or pneumatic chisel, and sometimes special machines are involved. Scale and slag can be removed with a wire brush. Tacking of edges prevents changing their position during welding.

Welding methods

Gas welding can be carried out in several ways. The first is left-handed welding, which is the most common. It is used when working with thin and low-melting metals. The torch moves from right to left and the filler wire is guided in front of the flame towards the unwelded seam. Right-hand welding traces the torch from left to right and the filler wire trails behind the torch. With this method, the heat of the flame is dissipated to a lesser extent, therefore the seam opening angle is not 90 degrees, but less - 60-70.

It is advisable to use right-hand welding to join metal with a thickness of 3 mm and more, as well as metals with a high level of thermal conductivity. It is recommended to use a filler wire with a diameter equal to half the thickness of the metal to be welded.

Gas welding technology also involves a process that is carried out by a through bead. In this case, the sheets are installed vertically to the gap - it is equal in size to half the sheet thickness. The torch melts the edges to form a round hole. Then it is melted on all sides until the seam is welded. This method is good in that the sheets to be welded have a tight seam without pores and slag inclusions.

Pot welding is good for welding joints and corners of metals that have a maximum thickness of 3 mm. As soon as a bath is formed on the seam, the end of the filler wire is introduced into it, which slightly melts, then the end of the wire is moved to another section of the seam. The peculiarity of this approach is in the seam of excellent quality, especially if thin sheets and pipes of steel (low-carbon and low-alloy) were welded.

Gas welding and cutting can be carried out on the basis of multi-layer welding. This method has a number of features:

• the heating zone is small;
• the underlying layers are simply annealed during the surfacing of the subsequent ones;
• each seam can be hammered before the next is applied.

This has the effect of improving the quality of the seam. On the other hand, this method is characterized by low productivity, requires a high gas consumption in comparison with single-layer welding, therefore it is used when it is necessary to create critical and high-quality products.

Features of welding various seams

To work with horizontal seams, the right method is used, which makes it easy to form a seam, and the metal of the bath itself does not drain. Welding of vertical and inclined seams is carried out in the left way, and if the metal thickness is more than 5 mm, a double bead is used. Welding overhead seams involves heating the edges until they melt, then a filler wire is introduced into the bath - its end quickly melts. The process itself is conducted in the right way.

What kind of equipment?

Gas welding equipment for gas welding is a wide range of devices that allow you to carry out a number of works. This type of welding is considered simple, and the equipment itself is quite concise and easy to operate. Depending on the type of fuel, gas welding devices are propane-oxygen or acetylene-oxygen, gasoline or kerosene-oxygen. Most often, welding is carried out on the basis of propane-oxygen and acetylene-oxygen welding, since the flame of these gases has the highest temperature.

Gas welding equipment for gas welding is also a generator that is complemented by different kinds gas. Also, when working, you will need an oxygen cylinder and reducers. The most common are acetylene generators for gas welding, which allow you to directly obtain acetylene by mixing calcium carbide and water. This type of generator is presented in five types, which allows you to choose the best option for a specific material.

Safety gates play an important role when working with welding, their task is to ensure safety during welding. With their help, the backfire of the flame that occurs during welding is prevented. In addition, check valves prevent gas backflow into rubber hoses during flame processing of metals and when working with compressed gases.

Gas cylinders

Gas welding equipment includes cylinders and valves for them. A balloon is a cylindrical vessel that has a threaded hole in the neck, into which a shut-off valve is screwed. It is created from alloy or carbon steel, and each such product has its own color depending on the gas it contains. Cylinder valves are made of brass because steel is not corrosion resistant.

Gearboxes: types and features

A gas pressure regulator is a device that constantly lowers or keeps the gas pressure at a certain level. Gas welding and metal cutting are carried out on the basis of different types of gearboxes:

1. Oxygen are used in gas welding and metal welding. Such a gearbox is made with blue markings. It can be used in aggressive environments, as it is made of corrosion-resistant metals.
2. Acetylene reducers are widely used in gas welding. They are marked in white, fastened to the cylinder with a collar clamp. This type of reducer has two pressure gauges, one of which keeps the gas pressure in the cylinder under control, the second - the gas pressure in the working chamber.
3. Carbon dioxide reducers are widely used in industry - food, chemical. They have one or two gauges and can only be connected to a vertical gauge.

In argon-arc welding, argon reducers are widely used, which can also work with non-combustible gases.

Features of gas burners

Gas welding of steels is a process that requires the use of a variety of devices. Gas burners are an integral part of equipment that is used in various industries. By design, the products are approximately the same: each burner consists of a body. Several elements are attached to it at once: a tip, a valve that regulates the fuel supply, and a lever that regulates the height of the flame. The connection to the cylinder is made by a reducer, while the burner itself can often be supplemented with piezo ignition, flame windscreen and other components.

The propane gas welding torch is safe to operate with a high flame temperature that is sufficient for a wide range of applications. Many types of welding are carried out on the basis of acetylene torches, which work on a mixture of acetylene and oxygen.

Types of gas cutters

Gas cutters are different types: acetylene, propane and working on substitutes for gas or liquid fuel. The design of the products includes a handle, nipples to which the gas hoses are attached, a body, an injector, a mixing chamber, a tube, a torch head and a tube with a valve. Gas welding of metals and its quality depend on how well the torch is selected.

The essence of its work is as follows: oxygen from the cylinder enters the reducer, the sleeve, and then enters the body - here the cutter branches into two channels. Some of the oxygen passes through the valve and is directed to the injector. The gas exits from here at a high speed, and during this process combustible gas is sucked in. When combined with oxygen, it forms a combustible mixture, which is directed into the space between the mouthpieces and burns out. As a consequence, a heating flame appears. Oxygen, which was directed through the second channel, enters the tube, due to which a cutting jet is formed. It is he who processes the metal section.

Features of pipe welding

Welding of gas pipes is carried out in several stages. First, the metal is prepared, that is, the marking is carried out, the pipes are cut and assembled. Due to the circular cross-section of the pipes, cutting is performed with a thermal cutter. Most of the work on welding is the assembly of parts for it, when you need to take into account many parts - from a series of products to their diameter and other factors. The assembly is carried out with welding tacks, which prevent possible displacement of the pipe sections, which affects the appearance of cracks during cooling.

An arc is ignited. This is done in a variety of ways. Then the melting of metals begins - basic and electrode. For a high-quality weld, it is important to pay attention to the angle of inclination of the electrode.

Flame cutting technology

Oxy-fuel cutting is carried out using metals and their alloys, which burn in a jet of commercially pure oxygen. This type of cutting is done in two ways - separating or superficial. The first method allows you to cut blanks, cut metal, cut the edges of the seam for welding. With the help of surface cutting, surface metal is removed, grooves are cut, and surface defects are removed. This procedure is performed on the basis of special cutters.

Safety engineering

Gas welding is a process that requires careful attention. Dangerous situations can arise in several cases:

1. Welding should not be carried out near flammable and flammable materials (gasoline, kerosene, tow, shavings).
2. If welding is carried out in an enclosed space, workers should periodically go out into fresh air.
3. Work should be carried out in a well-ventilated area.
4. If flame processing of metal is carried out, the room must be ventilated to remove harmful gases.
5. Cutting and welding are carried out at a distance of up to 10 m from bypass ramps, acetylene generators.
6. Do not overfill the loading box sections with carbide.
7. The generator body must be constantly filled with the correct amount of water.
8. It is forbidden to work with an oxygen cylinder, the pressure of which is below normal.
9. The burner flame is directed in the direction opposite to the gas supply source.

Welding work should be carried out with the maximum observance of safety rules and using only high-quality equipment. This will make the process safe and the metal bonding reliable.

This method of joining metal parts, such as gas welding, is more than a hundred years old. During this time, this technology has continued to improve successfully, although other welding methods that use an electric arc are developing more actively and are replacing welding using a gas torch.

Pros and cons of gas welding

A method of joining metals, such as gas welding, involves melting the materials to be joined, resulting in a homogeneous structure. The combustion of the gas, due to which the heating and melting of the metal is carried out, is ensured by introducing pure oxygen into the gas mixture. This method of joining metals has a number of advantages.

• This welding method does not require the use of sophisticated equipment (welding inverter or semi-automatic machine).
• Everything expendable materials to carry out such welding is easy to purchase.
• Gas welding (respectively, and gas welding of pipes) can be carried out even without a powerful source of energy and sometimes without special protective equipment.
• The process of such welding lends itself well to regulation: you can set the required power of the burner flame, control the degree of heating of the metal.

This method also has disadvantages.

• Metal heats up very slowly, as opposed to using an electric arc.
• The heat zone that is formed by a gas burner is very wide.
• It is very difficult to concentrate the heat generated by a gas burner, it is more diffused compared to the electric arc method.
• Gas welding can be classified as a rather expensive method of joining metals when compared with. The cost of oxygen and acetylene consumed significantly exceeds the cost of electricity consumed for welding similar parts.
• When welding thick metal parts, the speed of the joint is significantly reduced. This is due to the fact that the concentration of heat when using a gas burner is very low.
• Gas welding is difficult to automate. Only the gas welding process can be mechanized thin-walled pipes or tanks, which is performed using a multi-flame burner.

Gas welding materials

Gas welding technology involves the use of various types of gases, the choice of which depends on a number of factors.

One of the gases used for welding is oxygen. This gas is characterized by the absence of color and odor, it acts as a catalyst, activating the processes of melting of the material to be joined or cut.

In order to store and transport oxygen, special cylinders are used in which it is contained under constant pressure. Oxygen can ignite on contact with technical oil, therefore, the very possibility of such contact should be excluded. Oxygen cylinders must be stored in rooms protected from sources of heat and sunlight.

Welding oxygen is obtained by separating it from ordinary air, for which special devices are used. Depending on the degree of its purity, oxygen is of three types: the highest (99.5%), the first (99.2%) and the second (98.5%) grade.

For various manipulations with metals (welding and cutting), the colorless gas acetylene C2H2 is also used. Under certain conditions (pressure exceeding 1.5 kg / cm2 and temperatures above 400 degrees), this gas can explode spontaneously. Acetylene is obtained by the interaction of calcium carbide and water.

The advantage of using acetylene in metal welding is that its combustion temperature allows this process to be carried out without problems. Meanwhile, the use of cheaper gases (hydrogen, methane, propane, kerosene vapors) does not make it possible to obtain such a high combustion temperature.

Wire and flux for welding

To carry out welding of metals, in addition to gas, are also needed. It is due to these materials that the weld is created, all of its characteristics are formed. The wire used for welding must be clean, free from corrosion and paint on its surface. In some cases, a strip of the same metal that is being welded can be used as such a wire. In order to protect the weld pool from external factors, it is necessary to use a special flux. Boric acid and borax are often used as such flux, which are applied directly to the surface of the metal to be welded or to the wire used for welding. Gas can be performed without flux, and when joining parts made of aluminum, copper, magnesium and their alloys, such protection is necessary.

Gas welding equipment

Gas welding technology involves the use of certain equipment.

Water seal

A water seal is necessary to ensure the protection of all elements of the equipment (acetylene generator, pipes) from the back draft of the fire from the burner. Such a seal, in which the water must be at a certain level, is placed between the gas burner and the acetylene generator.

Such cylinders are painted with different paints, depending on what kind of gas is planned to be stored in them. Meanwhile, the upper part of the can is not painted in order to exclude contact of the gas with the paint components. It should also be borne in mind that copper valves must not be installed on cylinders in which acetylene is stored, as this can lead to a gas explosion.

It is used to reduce the pressure of the gas leaving the cylinder. Reducers can be of direct or reverse action, and for liquefied gas, models with ribbing are used, which exclude its freezing at the exit.

Gas welding cannot be performed without the use of special hoses, through which both gas and flammable liquids can be supplied. Such hoses are divided into three categories, marked with 1) a red stripe (operate at pressures up to 6 atmospheres), 2) a yellow stripe (for supplying flammable liquids), 3) a blue stripe (operate at pressures up to 20 atm).

Mixing of gases and their combustion is ensured through the use of a burner, which can be injector and non-injector type. Burners are also classified according to their power, which characterizes the amount of gas passed per unit of time. So, there are burners of large, medium, small and micro-low power.

Gas welding is carried out at a specially equipped place, which is called a post. In fact, such a place is a table, which can be with a swivel or fixed table top. This table, equipped with exhaust ventilation and everything necessary for storing auxiliary tools, greatly facilitates the work of the welder.

Features of performing gas welding

The flame parameters are adjusted using a reducer, which allows you to change the composition of the gas mixture. With the help of a reducer, three main types of flame can be obtained: reducing (used for welding almost all metals), oxidizing and with an increased amount of combustible gas. When welding metals in a molten pool, two processes occur simultaneously - oxidation and reduction. At the same time, when welding aluminum and magnesium, oxidation processes are more active.

The welding seam itself and the area adjacent to it are characterized by different parameters. So, the metal section adjacent to the seam is characterized by minimal strength, it is he who is most prone to destruction. The metal adjacent to this zone has a structure with large grains.

To improve the quality of the seam and the area adjacent to it, additional heating or the so-called thermal forging of the metal is performed.

Welding technologies for various metals have their own nuances.

• Gas is performed using any gas. As a filler material for welding such steels, a wire made of steel containing a small amount of carbon is used.
• Welding methods are selected depending on their composition. So, heat-resistant stainless steels are cooked using wire containing chromium and nickel, and some grades require the use of filler material additionally containing molybdenum.
• Cast iron is cooked with a carburizing flame that prevents silicon pyrolysis and the formation of brittle white cast iron grains.
• For copper welding, a higher power flame must be used. In addition, due to the increased fluidity of copper, parts from it are welded with a minimum gap. Copper wire is used as a filler material, as well as a flux, which contributes to the deoxidation of the weld metal.
• When there is a risk of zinc volatilization from its composition, which can lead to increased porosity of the weld metal. To avoid this, more oxygen is fed into the burner flame, and brass wire is used as an additive.
• Bronze welding is carried out with a reducing flame, which does not burn out tin, aluminum and silicon from this alloy. As an additive, a bronze wire of a similar composition is used, which additionally contains silicon, which contributes to the deoxidation of the weld metal.

Inert gases, active gases and their mixtures are used as shielding gases in fusion welding.

Inert gases... Gases are called inert if they are not capable of chemical reactions and practically insoluble in metals. These are monatomic gases, the atoms of which have outer electron shells filled with electrons, which explains their chemical inertness. Of inert gases, argon, helium and their mixtures are used for welding.

Argon- non-flammable and non-explosive gas. It does not form explosive mixtures with air. Being heavier than air, argon provides good gas shielding of the weld pool. Pure gaseous argon is supplied in accordance with GOST 10157-62 of three grades: A, B and C (Table 7-40). The moisture content for gaseous argon of all three grades should not exceed 0.03 g / m 3.

Argon grade A is recommended for welding and melting of active and rare metals (titanium, zirconium and niobium) and alloys based on them, as well as for welding critical items from other materials at the final stages of manufacturing. Argon grade B is intended for welding and melting with a consumable and non-consumable tungsten electrode of alloys based on aluminum and magnesium, as well as other alloys sensitive to impurities of gases soluble in the metal. Argon grade B is recommended for welding and melting of chromium-nickel corrosion-resistant and heat-resistant alloys, alloy steels of various grades and pure aluminum.

Argon should be stored and transported in steel seamless cylinders that meet the requirements of GOST 949-57. The cylinder at a pressure of 150 atm contains about 6 m 3 of gaseous argon. The pure argon storage cylinder is painted black at the bottom and white at the top. On the top of the cylinder, the inscription “Pure Argon” is inscribed in black letters. Argon is mainly obtained from air, in which it is contained in a relatively small amount (1.28% by weight). Argon production is carried out in oxygen plants with argon attachments. In these attachments, raw argon is purified to the required degree of purity from nitrogen and oxygen. Helium, like argon, is chemically inert, but unlike it, it is much lighter. Helium is lighter than air, which complicates the protection of the weld pool and requires a higher flow rate of shielding gas. Compared to argon, helium provides more intense heating of the weld zone, which is caused by a large voltage drop gradient in the arc.

Helium supply according to MRTU 51-77-66 of two grades - helium of high purity and technical helium. Helium is stored and transported in steel seamless cylinders at pressures up to 150 atm. Helium cylinders are painted in Brown color with the inscription in white letters "Helium". The cylinders must meet the requirements of GOST. Helium is extracted from natural hydrocarbon gases by cooling them in special installations. In this case, gaseous methane, ethane and other hydrocarbons are liquefied, and helium remains in a gaseous state, since it has a very low temperature liquefaction (-269 ° C).

Table 7-40. Composition of gaseous argon (GOST 10157-62), vol. %

Natural gases in the USA are especially rich in helium, which determines the widespread use of helium for welding in this country. A small amount of helium is contained in the air and, like argon, is produced as a by-product in oxygen plants.

Inert gas mixtures usually consist of argon and helium. Having a higher density than helium, such mixtures better protect the weld pool metal from air. Particularly good protective properties are possessed by an inert gas mixture consisting of 70 vol.% Argon and 30 vol.% Helium. The density of such a mixture is close to that of air. For welding chemically active metals, an inert mixture containing 60-65 vol. % helium, and the rest is argon. Although inert gas mixtures are noticeably more expensive than argon, they surpass it in terms of the intensity of the electric arc heat release in the welding zone. This is essential when welding metals with high thermal conductivity.

Inert gas mixtures of the required composition are usually obtained by mixing gases coming from two separate cylinders using special mixers. Some foreign companies supply ready-made argon-helium mixture of the required composition in cylinders.

Mixtures of inert and active gases are increasingly used in consumable-electrode welding of steels of various classes due to their technological advantages: lower intensity of chemical action on the weld pool metal compared to active gases, high stability of the arc process, and favorable nature of the transfer of electrode metal through the arc. Compared to pure argon, mixtures of inert and active gases have advantages when welding structural steels. It is known that with a consumable electrode, the best performance of the welding process is usually achieved with a direct current of reverse polarity. However, when welding steel, the use of pure argon as a shielding gas is accompanied by the instability of the position of the cathode spot on the surface of the product. The result is poorly formed welds.

The addition of a small amount of oxygen or other oxidizing gas to argon significantly increases the arc stability and improves the quality of the formation of welds. The presence of oxygen in the arc atmosphere promotes finer droplet transfer of the electrode metal. This is due to the surface-active action of oxygen on iron and its alloys. Dissolving in liquid metal and accumulating mainly on the surface, oxygen significantly reduces its surface tension. As a result, the formation of individual metal droplets is facilitated, and their size is reduced. Therefore, for welding steel, not pure argon is used, but mixtures with oxygen and carbon dioxide Ar-O 2, Ar-CO 2, Ar-CO 2 -O 2.

Rice. 7-37. Form of penetration when welding in shielding gases:
a - in argon;
b - in carbon dioxide

For consumable electrode welding of austenitic steels, it is recommended to use argon with an addition of 1 vol. % oxygen. This gas mixture ensures a stable welding process and at the same time weakly oxidizes the metal of the weld pool. It is advisable to use a mixture of argon with 2 or 5 vol.% Oxygen in welding ferritic steels when a jet transfer of the electrode metal is required. When welding in such gas mixtures, the quality of the formation of the seams is high, and the spatter of the electrode metal is very small. The disadvantages of the mentioned mixtures of argon with oxygen are intense arc radiation and finger-like penetration of the base metal, which is characteristic of argon.

In this respect, mixtures containing carbon dioxide are much better. When welding in mixtures of Ar + 20% CO 2 and Ar + (15 - = - 30%) CO 2 + 5% O 2, the intensity of the arc column radiation is relatively low, and the form of penetration of the base metal is the same as that of carbon dioxide (Fig. . 7-37). At the same time, these mixtures are close to carbon dioxide in terms of chemical action on the weld pool metal.

Adverse changes in the chemical composition of the weld pool metal occur when protected by carbon dioxide. The metal is carburized, and the content of manganese, niobium and silicon is markedly reduced. As a result, the corrosion resistance of the weld metal, characterized by the ratio of the concentrations of niobium and carbon, is reduced. The weld metal welded in a mixture of Ar + 1% O 2 differs the least in chemical composition from the original welding wire. Seams welded in gas mixtures containing carbon dioxide occupy an intermediate position in this respect. At the same time, the oxidation of the metal of the weld pool by the active gases included in the gas mixtures also has a positive value.

An argon-hydrogen mixture (up to 20 vol.% H 2) is used in microplasma welding. The presence of hydrogen in the mixture provides compression of the plasma column, makes it sharper and more concentrated. In addition, hydrogen creates a reducing atmosphere in the welding zone, which is required in some cases.

When transporting gas cylinders and working with them, you must follow the rules for handling cylinders high pressure... Due to the high internal pressure, the walls of the cylinders are in a stressed state and any local increase in stresses can cause the destruction of an insufficiently high-quality cylinder. Therefore, compressed gas cylinders must not be thrown or subjected to shock or heat. Special care should be taken in winter. At the workplace, the cylinder must be placed vertically and must be secured.

Active gases... Active shielding gases are gases that are capable of protecting the welding zone from air access and at the same time chemically reacting with the metal being welded or physically dissolving in it. In arc welding of steel, carbon dioxide is used as a protective medium. Due to its chemical activity in relation to tungsten, welding in this gas is carried out only with a consumable electrode. The use of carbon dioxide ensures reliable protection of the welding zone from contact with air and prevents nitriding of the weld metal. Carbon dioxide has an oxidizing and carburizing effect on the weld pool metal. Of the alloying elements of the bath, aluminum, titanium and zirconium are most strongly oxidized, while silicon, manganese, chromium, vanadium, etc. are oxidized less intensely.

An obstacle to the use of carbon dioxide as a protective medium was previously the pores in the seams. The pores were caused by the boiling of the solidifying metal of the weld pool from the release of CO due to its insufficient deoxidation. The use of welding wires with a high silicon content eliminated this disadvantage, which made it possible to widely use carbon dioxide in welding production.

Carbon dioxide(carbon dioxide) colorless, non-toxic, heavier than air. At a pressure of 760 mm Hg. Art. and a temperature of 0 ° C, the density of carbon dioxide is 1.97686 g / l, which is 1.5 times the density of air. Carbon dioxide is highly soluble in water. Liquid carbon dioxide is a colorless liquid, the density of which varies greatly with temperature. As a consequence, it is supplied by weight and not by volume. When 1 kg of liquid carbon dioxide evaporates under normal conditions (760 mm Hg, 0 ° C), 509 liters of carbon dioxide are formed. On an industrial scale, carbon dioxide is produced in special installations by extracting it from flue gases generated during fuel combustion, from fermentation gases in the alcohol industry and gases obtained during limestone burning. Carbon dioxide is transported in a liquid state in steel cylinders or insulated containers. In steel cylinders, carbon dioxide is under pressure up to 50 atm, from where it is taken in a gaseous state. The cylinders must comply with the requirements of GOST 949-57, be painted black with the inscription "CO 2 welding", applied with yellow oil paint. In an ordinary standard cylinder with a capacity of 40 liters, 25 kg of carbon dioxide is poured, during the evaporation of which 12 600 liters of gas are formed.

Carbon dioxide should not contain mineral oils, glycerin, hydrogen sulfide, hydrochloric, sulfuric and nitric acids, alcohols, ethers, organic acids and ammonia. In addition, there should be no water in the cylinders with welding carbon dioxide. Due to the scarcity of grade I welding carbon dioxide, grade II welding carbon dioxide and food grade are used for welding. An increased content of water vapor in such carbon dioxide can lead to the formation of pores in the seams during welding and reduce the plastic properties of the welded joint.

The moisture content of the gas rises at the beginning and end of the gas extraction from the cylinder, therefore, in these cases, defects in the seams most often appear. To reduce the moisture content of the carbon dioxide entering the welding to a safe level, a dehumidifier is installed in its path. The desiccant is filled with calcium chloride, silica gel or other moisture absorbers to trap moisture. When gas is released from the cylinder, due to the throttling effect and heat absorption during the evaporation of liquid carbon dioxide, the gas is significantly cooled. With intensive gas extraction, the reducer may become clogged with frozen moisture contained in carbon dioxide, as well as dry ice. To avoid this, it is recommended to heat up the carbon dioxide escaping from the cylinder.

For welding can be applied and hard carbon dioxide(dry ice) supplied in accordance with GOST 12162-66 of two grades - food grade and technical grade. In terms of the content of impurities, food grade carbon dioxide meets the requirements for liquid welding carbon dioxide, while technical carbon dioxide is contaminated with mineral oils. Dry ice is produced in the form of cylindrical or rectangular blocks.

The gas mixture of carbon dioxide with oxygen (CO 2 + + O 2) finds industrial application in welding low-carbon and low-alloy structural steels. Apply a mixture containing 30 vol. % oxygen, in Japan - a mixture with less oxygen (no more than 20 vol.%). A mixture of CO 2 + O 2 has a more intense oxidizing effect on the liquid metal than pure carbon dioxide. This increases the fluidity of the metal, which improves the formation of the seam and reduces the weldability of metal droplets to the surface of the product. In addition, oxygen is cheaper than carbon dioxide, which makes the mixture cost effective. A mixture of CO 2 + 30% Oa is made from pure carbon dioxide and oxygen using special mixers.

Oxygen is a part of gas mixtures CO 2 + O 2 and Ar + O 2. It is a colorless, odorless gas that supports combustion. Oxygen gas is obtained from atmospheric air by deep cooling or by electrolysis of water. Oxygen gaseous technical and medical supply in accordance with GOST 5583-68. Depending on the content of oxygen and impurities, technical gaseous oxygen is produced in three grades. The oxygen content in the first grade must be at least 99.7 vol. %, in the second - not less than 99.5 vol. % and in the third - not less than 99.2 vol.%. The content of water vapor in industrial oxygen of all three grades should not exceed 0.005 g / m 3, which corresponds to a dew point of 63 o C. Industrial gaseous oxygen obtained by electrolysis of water should not contain more than 0.7 vol. % hydrogen. Gaseous oxygen is supplied in steel cylinders under a pressure of 150 or 200 kgf / cm 2. The cylinders must comply with the requirements of GOST 949-57, be painted blue with the inscription "Oxygen" applied in black paint. On cylinders with oxygen obtained by electrolysis of water, the inscription "Electrolysis oxygen" should be displayed.

Hydrogen used in atomic hydrogen welding. Hydrogen is colorless, odorless and combustible. Due to the fact that mixtures of hydrogen with air or oxygen are explosive, when working with it, it is necessary to observe fire safety rules and special safety rules. Depending on the method of obtaining technical hydrogen, four grades are produced: A, B, C and D. The purest is technical hydrogen of grade A, obtained by electrolysis of water. The hydrogen content in it must be at least 99%, the rest is mainly oxygen. Technical hydrogen is supplied in steel cylinders at pressures up to 150 atm, rubber-fabric gas holders and through pipelines. The hydrogen cylinders are dark green with three red stripes around the circumference.

Nitrogen- colorless gas, odorless, does not burn and does not support combustion. Nitrogen does not dissolve in molten copper and does not interact with it, and therefore can be used in copper welding as a shielding gas. According to GOST 9293-59, nitrogen is supplied in four grades: gaseous electric vacuum, gaseous 1st grade, gaseous 2nd grade and liquid. The nitrogen content in these grades must be, respectively, not less than vol.%: 99.9; 99.5; 99 and 96. The main impurity is oxygen.

Nitrogen is obtained from atmospheric air by its liquefaction and rectification. Gaseous nitrogen is transported in steel cylinders under pressure up to 150 atm. The cylinders are painted gray with a brown stripe and the inscription in yellow letters "Nitrogen" on the upper cylindrical part. Liquid nitrogen is transported in metal Dewars and transport containers. Under normal conditions (pressure 760 mm Hg and temperature 20 ° C) 1 kg of liquid nitrogen corresponds to 0.86 m 3 of gaseous nitrogen.