Gas welding parameters. Gas welding modes

During gas welding, various processes occur: physical, associated with heating and melting of the metal, formation of a weld, as well as chemical, caused by combustion, interaction of flux and filler material with the molten metal.

The main tool of a gas welder is a welding flame. It is formed when a flammable gas burns in oxygen. The ratio of the volumes of oxygen and combustible gas in their mixture determines the appearance, temperature and nature of the influence of the welding flame on the molten metal.

Let's consider the structure of the flame (Fig. 7.1). The welding flame has three clearly distinguishable areas: core 7, reduction zone 2 and torch 3.

Rice. 7.1. The structure of an acetylene welding flame and the temperature distribution along the length of the torch: 1 - core; 2 - recovery zone; 3 - torch

Core The flame is a brightly luminous zone, in the outer layer of which hot carbon particles formed during the decomposition of acetylene burn.

Recovery zone, darker, consists of carbon monoxide and hydrogen, which deoxidize the molten metal, removing oxygen from its oxides.

Torch- the peripheral part of the flame - is a zone of complete combustion of hydrocarbons in ambient oxygen.

Depending on the ratio of the volumes of oxygen and acetylene, three main types of welding flame are obtained: normal, oxidizing and carburizing (Fig. 7.2).

Rice. 7.2. Types of welding flame: a - normal; b - oxidative; c - carburizing; 1 - core; 2 - recovery zone; 3 - torch

Normal welding flame is formed when in the burner there is one volume of acetylene per volume of oxygen. In a normal flame, all three zones are clearly visible.

The core has a sharply defined shape, close to a cylinder with a brightly glowing shell. The core temperature reaches 1000 °C.

Welding is carried out in the reduction zone containing products of incomplete combustion of acetylene. The temperature of this zone at a point 3...6 mm from the core is 3150°C. The torch has a temperature of 1200... 2500 °C.

A normal welding flame is used to weld all grades of steel, copper, bronze and aluminum.

Oxidizing welding flame is obtained with an excess of oxygen, when more than 1.3 volumes of oxygen are supplied to the burner per volume of acetylene. The core of such a flame has a shortened, cone-shaped shape. It takes on less sharp outlines and a paler color than a normal flame. The length of the reduction zone decreases compared to a normal flame. The torch has a bluish-violet color. Combustion is accompanied by noise, the level of which depends on the oxygen pressure. The temperature of an oxidizing flame is higher than that of a normal flame, but when welding with such a flame, due to excess oxygen, porous and brittle seams are formed.

Oxidizing flame is used in brass welding and brazing.

Carburizing Welding Flame obtained with an excess of acetylene, when in the burner there is no more than 0.95 volume of oxygen per volume of acetylene. The core of such a flame loses its sharp outline, and a green rim appears at its end, the presence of which indicates an excess of acetylene. The reduction zone is significantly lighter than that of a normal flame and almost merges with the core. The torch becomes yellow in color. With a significant excess of acetylene, the flame becomes smoky. The temperature of the carburizing flame is lower than that of the normal and oxidizing flames.

Cast iron is welded using a lightly carburizing flame and hard alloys are deposited.

The gas welder adjusts and sets the type of welding flame “by eye”.

When performing welding work, it is necessary that the welding flame has a thermal power sufficient to melt the metal being welded.

The flame power during gas welding depends on the acetylene flow rate - the volume of gas passing through the torch in one hour. The power is adjusted by selecting the burner tip and changing the position of the acetylene valve. The flame power is selected in accordance with the thickness of the metal being welded and its thermophysical properties.

The acetylene consumption, dm 3 /h, required to melt a 1 mm thick layer of welded metal is established in practice. Thus, a layer of low-carbon steel 1 mm thick is melted at an acetylene consumption of 100... 130 dm 3 / h. To determine the acetylene consumption when welding a specific part, you need to multiply the consumption corresponding to a unit thickness by the actual thickness of the metal being welded, mm.

Example. When welding low-carbon steel 3 mm thick, the minimum acetylene consumption, dm 3 / h, will be 100x3 = 300, and the maximum - 130x3 = 390.

Gas welding is welding by fusion of metal, which is heated by a torch flame. When heated, the edges of the workpieces being welded melt along with the filler material, which is additionally introduced into the burner flame. After crystallization of the liquid metal, a weld is formed. The advantages of gas welding include the simplicity of the method, uncomplicated equipment, and the absence of a source of electrical energy.

The disadvantages of gas welding include lower productivity, complexity of mechanization, a large heating zone and lower mechanical properties of welded joints than with arc welding. In addition, the disadvantages of gas-flame welding include the low efficiency of the calorific value of the combustible gas, since only 6-7% of the heat released during the combustion of acetylene is spent on metal welding. The rest of the heat is spent on radiation and convection, losses from incomplete combustion of gas, heating of areas adjacent to the seam, metal splashing, etc.

During gas welding, the welder holds the torch in his right hand and the filler wire in his left hand. The burner flame is directed at the metal being welded so that the edges are in the reduction zone of the flame at a distance of 2-6 mm from the end of the core. Do not touch the molten metal with the end of the flame core, as this causes carburization of the weld pool. The end of the filler wire should be in the recovery zone or slightly immersed in the weld pool.

Gas welding modes

Gas welding modes determine:

• welding flame power
• angle of inclination of the filler material and the torch nozzle
• diameter of filler material
• welding speed.

The welding flame must have sufficient thermal power, which is selected depending on the thickness of the metal being welded and its physical properties. The choice of welding modes entirely depends on the thickness of the parts being welded.

The power of the welding flame directly depends on the consumption of combustible gas and for acetylene welding it can be approximately determined by the formula:

Va = k S

Where Va is the flame power, determined by the acetylene consumption, l/hour; S - thickness of the material being welded, mm; k is a proportionality coefficient, the value of which depends on the type of steel.

For example, for low-carbon steel and cast iron k = 100 - 130, and for high-carbon steel k = 75 100. For aluminum and its alloys k = 100 - 15 for copper alloys - 150 - 225. By changing the thermal power of the flame, the welder has a fairly wide range of within limits, it can regulate the heating rate of metal melting, which is one of the advantages of gas flame welding.

The angle of inclination of the welding torch mouthpiece increases with increasing thickness of the metal being welded. The dependence of the inclination angle for welding steels is shown in Fig. 1. If non-ferrous metals whose thermal conductivity is higher than steel are welded, then the angle of inclination of the mouthpiece is slightly increased.

The diameter of the filler material is selected depending on the thickness of the parts being welded and the welding method. Typically, the diameter of the filler wire is equal to half the thickness of the metal being welded. In practice, when the metal thickness is more than 15 m, filler material is taken with a diameter of 6-8 mm.

The welding speed is a value that depends on the thickness of the metal being welded and its properties. The welding speed is determined by the formula:

V = A/S

Where A is a coefficient depending on the properties of the material and for steels of medium thickness it is equal to 12 - 15, S is the thickness of the metal being welded, mm.

Gas welding methods

There are several ways to apply a welding seam. Their use is dictated by the welder's habits and the characteristics of the welded joint.

Left welding(Fig. 2A) - is the most used method for gas welding of metals with a thickness of 4-5 mm. In this method, the torch is moved from right to left, and the filler wire is moved in front of the torch. The welding flame, directed from the seam, warms up the unwelded area and the filler wire well. When the metal thickness is small (less than 8 mm), the burner is moved only along the seam, and when the metal thickness is more than 8 mm, additional oscillatory movements are performed across the axis of the seam. The end of the filler wire is immersed in the weld pool, stirring its spiral in figurative movements.

The good thing about the left method is that the welder can clearly see the seam, which gives him the opportunity to ensure the uniformity of the welding bead. The seam is smooth and beautiful. Welding flame power: with the left-hand welding method, it is taken within the range of 100 - 130 dm3 of acetylene per hour per mm of metal thickness.

Right welding(Fig. 2B) is considered more economical, since the flame is directed directly at the seam. This makes it possible to weld thick metal with a reduced opening angle of the edges. And since the amount of deposited metal is reduced, the likelihood of warping of parts is reduced. With this method, the burner moves from left to right, and the filler material moves after the burner. Since the flame is directed towards the seam, its cooling rate is reduced, and the metal is simultaneously subjected to heat treatment, which helps improve the quality of the seam.

Through bead welding(double bead) are used for vertical welding of butt joints from top to bottom (Fig. 3). To do this, a through hole is melted in the lower part of the joint and, gradually raising the flame upward, the upper part of the hole is melted. By introducing filler material, the lower part of the hole is welded. When welding thick metal, welding is carried out simultaneously from both sides by two welders.

Welding with pools(Fig. 3A) consists of the sequential formation of baths of molten metal and the introduction of several drops of filler material into them. Welding with baths; used for welding metal up to 3 mm thick. With this type of welding, each subsequent; the bath overlaps the previous one by 2/3 of its diameter. This method is used when welding thin; sheets and pipes made of low-carbon steel, butt and corner joints with part thicknesses up to 3 mm, achieving high quality welding seams. To do this, having melted a bath with a diameter of 4-5 mm, the welder inserts the end of the filler wire into it and, having melted a small amount of it, moves the end into the reduction zone of the flame, which reduces the likelihood of metal oxidation. The burner mouthpiece is used to make movements that allow the formation of an adjacent bath, which should overlap the previous one by ⅓ of the diameter. In this case, the flame core should not be immersed in the bath to avoid carburization of the weld metal.

Welding along flanged edges used for welding metal up to 2 - 3 mm thick. This type of welding is used without filler metal, but only due to the oscillatory and spiral movements of the torch.

Welding at different seam positions. Welding in the lower position The seam usually does not cause any problems. Vertical, ceiling and horizontal seams on a vertical surface (Fig. 5) have their own characteristics and require skill in operation.

Welding vertical seams From bottom to top it is better to do it in the left way. Horizontal seams on a vertical plane are performed in the right way. In this case, the gas flame flow is directed towards the seam, preventing the metal from spreading out of the weld pool. Unlike the usual right-hand method, welding is carried out from right to left, creating a slight distortion of the weld pool.

Ceiling seams It is also better to do it in the right way, since with this method the end of the filler wire and the pressure of the gas flow prevent the liquid metal from flowing down.

Types of welded joints and preparation of metal for welding

During the fusion welding process, the base metal and, in most cases, the filler metal are melted. Regulation of the degree of melting of the filler metal during gas welding can be carried out within very wide limits. The degree of melting of the base metal is determined by the flame power, geometric dimensions and thermophysical properties of the metal.

With conventional welding torches it is possible to weld metal of limited thickness in one pass (for steel this thickness is about 15 mm). However, without compromising welding performance, it is better to fusion the base metal to a shallower depth (for example, for steel up to 4-5 mm). At the same time, welding thin metal (less than 0.8-1 mm) is difficult due to its strong melting. Therefore, special edge preparation is used when welding.

The main type of welded joint is butt. In gas welding, in addition to butt joints, end and corner joints are often used (Fig. 44). Flare butt joints and end joints are typically welded without filler metal. Fillet joints with external welds are made with or without filler metal.

T-joints and lap joints are used in gas welding only for small thicknesses, since as the thickness increases, the performance of the welding itself sharply deteriorates due to uneven heating of the edges and significant warping during welding. Welding of such joints is done using fillet welds (Fig. 45). In this case, mainly concave (lightweight) seams are used, which are widely used in the aviation industry as they are more resistant under alternating loads and produce less warping.

In order to obtain high-quality welded joints, the metal at the edges and close to them (up to 30-50 mm) must be cleaned of various contaminants (a thick layer of oxides, grease stains, etc.) before welding. This cleaning is done either by mechanical means (sandblasting, manual or powered steel brushes) or by chemical cleaning. Sometimes, before cleaning parts with a brush, heating is carried out with a gas flame, which separates oxides from the metal and burns a number of other contaminants.

Usually, before welding, together with the assembly, the elements to be welded are secured with various devices, and most often with tack welds (short seams). The general principle of tack placement is shown in Fig. 46.

When welding long seams of loose sheets, in order to avoid unacceptable deformations, the assembly is sometimes performed with an expanding gap (with the ends apart). Edge preparation, assembly and tack welding largely determine the quality of the weld.

Gas welding mode and technique

The effectiveness of the gas welding process is determined by the welding mode (flame power, welding speed, filler metal diameter) and welding technique (including the location of the torch and filler metal in relation to the metal being welded, as well as the movement of the torch and filler metal).

The flame power is determined by the amount of fuel burned per unit time and is usually measured in l/h.

It has been established from practice that the flame power V a required for welding is approximately proportional to the thickness of the metal being welded:

where δ is the metal thickness in mm;

R is the proportionality coefficient (l/h mm), equal to 100-130 for low-carbon steel, 75-100 for cast iron and stainless steel, 100-150 for aluminum, 150-225 for copper.

The average speed of flame movement (υ in m/h) in relation to the metal being welded during manual welding in a steady state of heating and melting of the metal being welded also depends on the thickness:

where A is a coefficient depending on the properties of the metal being welded and, to some extent, on the thickness (for steel of medium thicknesses A = 12-15; for nickel A = 9 - 11).

The diameter of the filler metal (usually in the form of wire rods or cast rods) is selected depending on the thickness of the metal being welded and its thermophysical properties. In most cases, the additive diameter d is taken from δ/2 to δ.

Of great importance for obtaining good quality seams is the welding technique, which allows during welding to correctly introduce and distribute heat in the product being welded, to melt the welded edges and filler metal, and to control the liquid metal of the weld pool.

The distribution of heat introduced into the workpiece being welded and the influence of the mechanical action of the flame depend on the angle of inclination of the flame axis to the surface of the metal being welded (φ). Penetration of the base metal and welding speed also depend on this angle. At a small value of the angle φ, the flame seems to slide along the surface of the metal, melting it little, but by heating the metal in front, it contributes to its thermal preparation for subsequent melting. At a value of φ close to 90°, the penetration depth increases, and the degree of thermal preparation of the still unmelted metal decreases. In this regard, welding of metals of small thickness is carried out at a small value of the angle φ. When welding large thicknesses, the location of the torch is changed, directing the flame more vertically. The following are approximate flame angles when welding steels:

When welding light metals (aluminium, magnesium), the angle of inclination φ should be small to avoid the metal being blown out of the pool by the mechanical action of the flame.

During the welding process, the angle of the flame may change. At the beginning of welding, when the base metal has not yet been heated up, it is necessary to keep the angle φ large, reducing it to a normal value during the welding process as the metal being welded warms up.

An important factor influencing the efficiency of gas welding is the choice of method, determined by the relative position of the flame and filler metal in relation to the welding direction.

There are two welding methods: left and right.

With the left-hand welding method (Fig. 47, a), the flame is directed forward to the not yet welded edges of the base metal and is located between the welded section of the seam and the filler metal. In this case, the edges to be welded are preheated both directly by the flame and by heat distributed as a result of the thermal conductivity of the metal. This method is effective for small thicknesses (for steel at δ<4 мм) и позволяет получить большую скорость сварки.

With the right method (Fig. 47, b), the flame is directed towards the already welded area of ​​the seam, and the filler metal is located between the flame and the welded area of ​​the seam. In this case, the edges in front are not heated by the flame, but the introduction of heat into the weld pool turns out to be more effective, especially if there are edges being cut, since the flame core can be brought closer to the surface of the molten metal. This method is more effective for large metal thicknesses (for steel at δ > 5 mm).

In right-hand welding, the weld metal is washed by the flame during the cooling process and cools somewhat more slowly. This allows, in some cases, to obtain seams with better metal properties than with left-hand welding.

Making seams using the right method is more difficult and requires appropriate welding skills.

To achieve the highest labor productivity with minimal consumption of materials, in particular fuel, it is necessary to strive for the maximum reduction of heat losses. The practice of manual gas welding shows that the productivity of welders, depending on technical techniques, can vary by 30-50%.

Welding techniques (including movements of the torch and additive) depend on the location of the seam in space, the form of edge preparation, the thickness and properties of the metal being welded.

The easiest way to make bottom seams is seams located on the upper horizontal plane of the product being welded. When making a bottom seam with flanged edges (or an end seam), left-hand welding is used, and the trajectory of the torch must be straight, without transverse vibrations. With contaminated metal, to improve fusion, it is sometimes necessary to use longitudinal oscillatory movements of the torch in the vertical plane. When welding left-handed butt welds at δ = 2-3 mm, performed without filler metal, transverse vibrations of the torch are used (Fig. 48, a).

As the metal thickness increases, butt welds are made by left-hand welding using filler metal, as shown in Fig. 48, b (for δ=4-5 mm) and Fig. 48, in (for δ > 5-6 mm).

Right-hand welding with thicknesses of about 5-6 mm is characterized mainly by transverse vibrations of the additive, and with large thicknesses - both the torch and the additive (Fig. 48, d). In the latter case, both the flame and the additive are brought to the edges synchronously, in contrast to left-hand welding, when the flame and the additive are usually located on opposite edges (Fig. 48, b and c).

Vertical seams (i.e. seams located vertically on a vertical plane) are made either from top to bottom (for small b) or from bottom to top. Welding from top to bottom is performed in the right way; Welding from bottom to top is performed using both left and right methods.

When welding thicknesses of 2-8 mm, double bead welding is very effective. With this method, a through hole is melted at the bottom of the joint. The flame, located in this hole and gradually rising from bottom to top, melts the upper part of the hole. This molten and filler metal fills the pool that forms on the lower surface of this hole (Fig. 49).

When welding horizontal seams (seams located horizontally on a vertical plane), the metal of the bath tends to flow to the lower edge. Therefore, welding is usually performed in the right way (using mechanical flame support). In this case, the bath is held asymmetrically (with a skew) in relation to the edges being welded (Fig. 50).

Ceiling seams (seams made on a horizontal plane from below, above the welder’s head) are better formed with right-hand welding.

In all cases, the use of filler metal is very important:

1) to regulate the temperature of the bath, which is carried out by immersing and removing the additive from it;

2) to protect the edges of an already welded seam section from melting during right-hand welding;

3) to support the bath with an additive (when welding horizontal and ceiling seams).

Welding defects associated with welding technique

Most weld defects are related to welding technique. Let's look at the main ones.

Lack of penetration - insufficient fusion or lack of fusion of the edges of the base metal with the weld metal. The reasons for lack of penetration are: incorrect choice of flame power and welding speed; improper heat distribution between the edges, as well as improper cutting of the edges (small bevel angle, large dullness); small gap or significant contamination of the edges with oxides. Types of lack of penetration are shown in Fig. 51.

The undercut (Fig. 52, a) is a consequence of excessive melting of the edges of the base metal with an insufficient amount of deposited filler metal.

Sagging (Fig. 52, b) is caused by insufficient heating and melting of the upper part of the edges; Sagging in some cases is accompanied by hidden lack of penetration of the edges.

In some cases, insufficient melting of the filler metal also leads to a weakening of the weld cross-section (Fig. 52, c), which is unacceptable for most butt welds.

A through burn is a defect that can occur when the base (mainly thin) metal is significantly heated by an insufficiently qualified welder.

Unfilled craters at the ends of seams are a defect caused by the carelessness of the welder.

Sagging, undercuts, insufficient cross-section of seams, unfilled craters (and some types of lack of penetration and burns) can be detected during external inspection and measurements. To detect lack of penetration in most cases, it is also necessary to inspect the seams from the reverse side.

Defects detected during external examination are called external. In addition to external defects, welds may also contain internal defects that are not detected during external inspection.

Internal defects, in addition to some types of lack of penetration, include slag inclusions and porosity.

Slag inclusions appear: when using a flame with excess oxygen; with insufficient mixing of the filler metal bath; when the bath hardens too quickly due to insufficient heating of the metal, etc. In addition, the cause of such inclusions can be significant contamination of the base and filler metal and improper use of fluxes.

The porosity of the weld results from the release of gases during cooling, when they do not have time to be removed from the metal. Porosity in gas welding is caused by improper flame adjustment and excessively rapid cooling of the pool as a result of improper welding technique.

A completely unacceptable defect are cracks caused by the low welding properties of the metal being welded, the quality of the filler metal, in particular its contamination with various impurities, as well as the incorrect technological sequence of assembly and welding operations.

In addition to macrostructure defects, welds made by gas welding sometimes have microstructure defects, the most typical of which are overheating and burnout.

Overheating is associated with prolonged exposure to heat and, as a rule, leads to a very coarse-grained structure of both the weld metal and the heat-affected zone of the base metal. Such coarse-grained metal has worse mechanical properties.

The structure of overheated metal can be corrected by general or local heat treatment.

Burnout is also associated with prolonged heating and, in addition, with the oxidizing effect of the flame, leading to the location of oxide inclusions along the grain boundaries. Overburning sharply worsens the properties of the metal and cannot be eliminated by subsequent heat treatment. If it is detected, the seams must be removed and digested again.

Ways to increase gas welding productivity

In a number of welding applications, automation and mechanization of the process is a fundamentally important area. For gas welding in its modern application, although this path is possible, it is not widely used due to the replacement of gas welding by other processes in mass production, in which the use of specialized automatic machines is justified.

For individual and small-scale work, the use of specialized automatic machines is irrational, so one should consider ways to possibly increase the productivity of manual gas welding, used by advanced welders.

When manual welding, it is possible to use higher flame powers than are usually used. However, this requires highly qualified welders and leads to an increase in labor productivity by about 20% with an increase in flame power of about 50%. The question of the rationality of using this method must be decided in each particular case.

The use of a hard flame (i.e., a flame with increased flow rates of the combustible mixture from the burners) leads to a greater concentration of heat and thereby to an increase in welding productivity. In this case, the flow rate with universal burners can be extremely increased by 20-30% of the normal flow rates. Welding with a hard flame is even more difficult than welding with a high-power flame, due to the increased blowing of metal from the weld pool.

More effective is the use of an “activated” flame, i.e. a flame with a slightly increased amount of oxygen. In this case, simultaneously with an increase in the efficiency of heating and melting, oxidation of the molten metal will also occur. To deoxidize the liquid metal, it is necessary to introduce a sufficient amount of deoxidizers into the bath (when welding carbon steels, usually Si and Mn), which, as a rule, are introduced with the filler metal (for example, for steel, filler wire with a Si content of 0.5-0.8% is used and Mn 0.8-1%). When trying to improve welding productivity, the increased cost of filler metal should be taken into account.

Common forms of increasing the productivity of gas welding are also the use of local or general preheating before welding using cheap fuel (coke ovens, forges, etc.). These methods are especially effective for mass production or welding of defective cast parts.

When welding small parts, some welders, skillfully placing them on a welding (usually rotary) table, use the heat of the exhaust gases of the flame for preheating, which heats the next part when welding the previous one. This results in a 20-40% increase in welding productivity without any increase in material consumption.

Rational methods for increasing the efficiency of gas welding must be sought in each individual case of its application.

Administration Overall rating of the article: Published: 2012.06.02

Gas flame processing of metals is a series of technological processes associated with the processing of metals with a high-temperature gas flame. The most widely used are gas welding and cutting, which, despite the lower productivity and quality of welded joints compared to electric fusion welding methods, continue to retain their importance when welding thin sheet steel, copper, brass, and cast iron. The advantages of gas welding and cutting are especially evident during repair and installation work due to the simplicity of the processes and the mobility of the equipment. In addition to welding and cutting, gas flames are used for surfacing, soldering, metallization, surface hardening, heating for subsequent welding by other methods or thermal straightening, etc.

Gas welding. A gas flame is most often formed as a result of the combustion (oxidation) of flammable gases in technically pure oxygen (purity not lower than 98.5%). When burning flammable gases using air, the temperature of the gas flame is low (not higher than 2000 ° C), since a lot of heat is spent on heating the nitrogen contained in the air. Acetylene, hydrogen, methane, propane, propane-butane mixture, gasoline, and lighting kerosene are used as flammable gases.

Gas welding oxy-acetylene “normal” flame has the shape shown schematically in Fig. 1 . In the inner part of the flame core (zone 1), the gas mixture coming from the nozzle is heated to the ignition temperature. Partial decomposition of acetylene occurs in the outer shell of the core. The released carbon particles are hot, glow brightly, clearly highlighting the outlines of the core shell (the temperature of the gases in the core is low and does not exceed 1500 ° C).

Zone 2 (reduction zone) is the most important part of the welding flame (welding zone). The first stage of acetylene combustion occurs in it due to oxygen entering the nozzle from the cylinder, as a result of which the maximum temperature develops here.

The gases contained in the welding zone have reducing properties in relation to the oxides of many metals, including iron oxides. Therefore, it can be called restorative. The carbon content in the weld metal changes slightly. In zone 3 or the flame torch, the combustion of gases occurs due to oxygen in the air, which reflects the composition of the gases in the torch. The gases contained in the torch and their dissociation products oxidize metals, i.e. this zone is oxidative. The type of acetylene-oxygen flame depends on the ratio of oxygen and acetylene (β) in the gas mixture supplied to the burner.

Rice. 2 Structure of oxygen-acetylene flame:a - normal; b - oxidative; c - carburizing

At β = 1.1 ... 1.2 the flame is normal (Fig. 2, a). The flame core is sharply defined, cylindrical in shape with a smooth rounding, a brightly luminous shell, all three zones are clearly defined.

With an increase in this ratio (for example β = 1.5), i.e., a relative increase in the oxygen content (oxidizing flame), the shape and structure of the flame changes (Fig. 2, b). In this case, oxidation reactions accelerate, and the flame core turns pale, shortens and takes on a conical, pointed shape. In this case, the welding zone loses its reducing properties and acquires an oxidizing character (the carbon content in the weld metal decreases and is burned out).

With decreasing β (for example, β = 0.5), i.e. As the acetylene content in the gas mixture increases, the oxidation reactions slow down. The nucleus lengthens and its outlines become blurred (Fig. 2, c). The amount of free carbon increases, its particles appear in the welding zone. With a large excess of acetylene, carbon particles also appear in the flame. In this case, the welding zone becomes carburizing, i.e. the carbon content in the weld metal increases.

The flame of acetylene substitutes is fundamentally similar to oxygen acetylene and has three zones. Unlike hydrocarbon gases, the hydrogen-oxygen flame does not have a luminous core (there are no luminous carbon particles).

One of the most important parameters that determine the thermal, and therefore technological properties of the flame is its temperature. It is different in its different parts, both in length along its axis (Fig. 1) and in cross section. It depends on the composition of the gas mixture and the degree of purity of the gases used (Fig. 3). The highest temperature is observed along the flame axis, reaching a maximum in the welding zone at a distance of 2 ... 3 mm from the end of the core. This welding zone is the main one for melting the metal. As β increases, the maximum temperature increases and shifts towards the burner mouthpiece. This is explained by an increase in the combustion rate of the mixture with excess oxygen. With an excess of acetylene (β less than 1), on the contrary, the maximum temperature moves away from the mouthpiece and decreases in value.

Rice. 3. Variation of flame temperature of different types

Combustible gases are substitutes for acetylene, cheaper and not in short supply. However, their calorific value is lower than that of acetylene. Maximum flame temperatures are also significantly lower. Therefore, they are used in limited quantities in technological processes that do not require a high-temperature flame (welding of aluminum, magnesium and their alloys, lead, soldering, welding of thin sheet steel, gas cutting, etc.). For example, when using propane and propane-butane mixtures, the maximum flame temperature is 2400 ... 2500 °C. They are used for welding steel up to 6 mm thick, welding cast iron, some non-ferrous metals and alloys, surfacing, gas cutting, etc.

When using hydrogen, the maximum flame temperature is 2100 °C.

Heating of metal by flame is caused by radiant and mainly convective heat exchange between the flow of hot gases and the metal surface in contact with it. When positioned vertically from the flame, its spreading flow forms a heating spot on the metal surface that is symmetrical relative to the center. When the flame is tilted, the heating spot extends along the axis and narrows laterally. The heating intensity in front of the core is higher than behind it.

Heat input into the product during gas welding occurs over a larger area of ​​the heating spot. The heat source is less concentrated than other fusion welding methods. As a result of the extensive heating area of ​​the base metal, the heat-affected zone (heat-affected zone) is large, which leads to the formation of increased deformations of welded joints (warping).

During gas welding, the metal of the weld pool is actively affected by the gas phase of the entire flame and especially the welding zone, containing mainly CO + H 2 and partially water vapor, as well as CO 2, H 2, O 2 and N 2 and some free carbon . The composition of the gas phase is determined by the ratio of oxygen and combustible gas in the gas mixture, the temperature of the flame and is different in its different zones. The metallurgical interactions of the gas phase with the metal of the weld pool depend on this. The main reactions in welding are oxidation and reduction.

The direction of the reaction depends on the oxygen concentration in the gas phase (oxidizing and carburizing flame), the reaction temperature and the properties of the oxide. When welding steels, the main interaction of the gas phase occurs with iron, i.e. formation of its oxides or reduction. Elements that have a greater affinity for oxygen than iron (Al, Si, Mn, Cr, etc.) can be intensively oxidized when iron oxidation reactions do not occur. They are easily oxidized not only in their pure form, but also in the form of alloying additives, and the higher their content, the more intense the oxidation. The oxidation of elements such as Al, Ti, Mg, Si and some others cannot be completely eliminated, and to reduce their waste, in addition to regulating the composition of the gas mixture, fluxes should be used.

Due to the relatively low protective and restorative effect of the flame, deoxidation of the metal in the weld pool when welding steels is achieved by introducing manganese, silicon and other deoxidizers into it through the filler wire. Their action is based on the formation of fluid slags that promote self-fluxing of the weld pool. The slag formed on the surface of the weld pool protects the molten metal from oxygen, hydrogen and nitrogen, the gaseous environment of the flame and sucked air.

The hydrogen contained in the flame can dissolve in the molten metal of the weld pool. When a metal crystallizes, some of the hydrogen that has not yet evolved can form pores. Nitrogen entering the molten metal from the air forms nitrides in it. Structural transformations in the weld metal and heat-affected zone during gas welding are of the same nature as in other fusion welding methods. However, due to slow heating and cooling, the weld metal has a more coarse-crystalline structure with equilibrium, irregularly shaped grains. In it, when welding steels containing 0.15 ... 0.3 carbon with rapid cooling, a Widmanstätten structure can form. The higher the cooling rate of the metal, the finer the grain in it and the higher the mechanical properties of the weld metal. Therefore, welding should be done at the highest possible speed.

The heat affected zone consists of the same characteristic areas as in arc welding. However, its width is much larger (up to 30 mm when welding thick steel) and depends on the gas welding mode.

During the welding process, the base and filler metals melt. Regulation of the degree of their melting is determined by the power of the burner, the thickness of the metal and its thermophysical properties.

Gas welding is used to make welded joints of various types. Metal up to 2 mm thick is joined end-to-end without cutting edges and without gaps or, better, with flanging of edges without filler metal.

Metal 2 ... 5 mm thick with filler metal is butt welded without cutting the edges with a gap between the edges. When welding metal over 5 mm, V- or X-shaped edge preparation is used.

T-joints and lap joints are permissible only for metal up to 3 mm thick. With large thicknesses, uneven heating leads to significant deformations, residual stresses and the possibility of cracks. The welded edges are cleaned of contamination by 30 ... 50 mm using mechanical methods or a gas flame. Before welding, the parts of the welded joint are fixed in an assembly-welding fixture or assembled using short seams - tack welds.

The direction of movement of the torch and its inclination to the metal surface has a great influence on the efficiency of heating the metal, welding productivity and the quality of the seam.

There are two welding methods: right and left (Fig. 4). The appearance of the seam is better with the left-hand welding method, since the welder sees the process of formation of the seam. For metal thickness up to 3 mm, the left-hand welding method is more productive due to preheating of the edges. However, with a large thickness of metal when welding with edge preparation, the bevel angle of the edges with the right welding method is 10 ... 15° less than with the left one. The angle of inclination of the mouthpiece can also be 10 ... 15° less. The result is increased welding productivity. The thermal effect of the flame on the metal depends on the angle of inclination of the flame axis to the metal surface (Fig. 4).

Rice. 4. Right and left gas welding methods

Rice. 5. Applicable burner angles depending on the thickness of the metal

During the welding process, the torch is subjected to oscillatory movements and the end of the nozzle describes a zigzag path. The welder holds the torch in his right hand. When using filler metal, the filler rod is held in the left hand. The filler rod is located at an angle of 45° to the metal surface.

The melted end of the filler rod experiences zigzag vibrations in the direction opposite to the movement of the mouthpiece (Fig. 6). Gas welding can be performed in the lower, vertical and overhead positions. When welding vertical seams “uphill”, it is more convenient to carry out the process using the left method, horizontal and ceiling ones - using the right method. ≥α

Rice. 5 Movements of the torch and wire: a - when welding steel with a thickness of more than 3 mm in the lower position;b - when welding fillet bead welds; 1 - wire movement;2 - burner movement; 3 - places of traffic delays

If it is necessary to use flux, it is applied to the edges to be welded or introduced into the weld pool with the melted end of the filler rod (sticking to it when immersed in the flux). Fluxes can also be used in gaseous form when supplied to the welding zone with flammable gas.

Bibliography

1. Losev V.A., Yukhin N.A. Illustrated welder's manual. M.: Publishing house "Souelo", 2000. 60 p.

Welding mode- a set of process parameters that determine the possibility of welding a given joint from metal of a given grade and thickness in spatial positions determined by the design of the product.

The main parameters of gas welding are the type and power of the flame, the diameter of the filler wire and the welding speed.

Type of flame depends on the material being welded: carbon and alloy steels are welded with a normal flame, cast iron is welded with a carburizing flame, and brass is welded with an oxidizing flame. The choice of the desired type of flame is carried out according to the nature of its glow.

Flame power burner, selected in accordance with the thickness of the metal being welded and its thermophysical properties, is determined by the acetylene consumption required to melt it. The thicker the metal being welded and the higher its thermal conductivity (as, for example, copper and its alloys), the greater the flame power should be. It is regulated stepwise by selecting the burner tip (see subsection 6.6.2) and smoothly by valves on the burner.

Choice filler wire diameter carried out depending on the thickness of the metal being welded and the welding method. When welding low- and medium-carbon steels, the diameter of the filler wire, mm, for the left-hand welding method is determined by the formula

d p ​​= s / 2 + 1,

and for the right -

where s is the thickness of the metal being welded, mm.

Welding speed set by the welder in accordance with the melting speed of the edges of the part.

Welding technique- a set of methods, techniques and manipulations carried out by a welder to form a high-quality seam.

In gas welding, the components of the welding technique are:

• the angle of inclination of the torch mouthpiece to the surface of the edges being welded;
• welding method;
• manipulation of the burner mouthpiece and filler wire as the flame moves along the seam.

Mouthpiece angle The welder selects the torch to the surface of the edges being welded depending on the thickness of the metal and its thermophysical properties. For low-carbon steels, this relationship can be presented as follows:

The greater the thickness of the metal and the higher its thermal conductivity (such as copper and its alloys), the greater the angle of inclination of the burner mouthpiece. Thus, the welder, by changing the angle of inclination of the mouthpiece and thereby the amount of heat supplied to the metal, controls the process of weld formation.

Welding methods are shown in Fig. 9.4.

Rice. 9.4. Welding methods:
a - left; b - right; - burner movement; ---- movement of filler wire; arrows indicate welding directions

The torch in the welder’s hand can only move in two directions:

• from right to left, when the flame is directed at the cold, not yet welded edges of the metal, and the filler wire is fed in front of the flame. This method is called left;
• from left to right, when the flame is directed towards the welded area of ​​the seam, and the filler wire is fed after the flame. This method is called the right one.

The left method is used when welding thin-walled (up to 3 mm thick) structures and low-melting metals and alloys.

The right method is used for welding structures with a wall thickness of over 3 mm and metals with high thermal conductivity.

The quality of the seam with the right welding method is higher than with the left one, since the metal is better protected by the burner flame from exposure to air.

Manipulating the torch mouthpiece(Fig. 9.5), carried out by the welder, contribute to the formation of a high-quality seam. If a filler wire is used, its movements improve the processes of melting, mixing the weld pool and removing oxides.

Rice. 9.5. Manipulation of the torch mouthpiece during welding:
a - with delay at the root of the seam; b - in a spiral; c - “crescent”; z - zigzag

The end of the burner mouthpiece simultaneously performs two types of movements: longitudinal - along the axis of the seam and transverse - in the perpendicular direction. The torch mouthpiece should be moved in such a way that the metal of the weld pool is always protected from exposure to air by the reduction zone of the flame.

The filler wire performs the same oscillatory movements as the mouthpiece, but in the direction opposite to the oscillations of the torch, and the end of the filler wire must always be in the weld pool or the reduction zone of the flame. When welding in the lower position, the “crescent” movement of the filler wire is most often used (see Fig. 9.5, c).