Message on the topic of gas welding modes. Gas welding technique
Welding mode - a set of process parameters that determine the possibility of welding a given joint from a 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.
The type of flame depends on the material being welded: carbon and alloy steels are welded with a normal flame, cast iron is carburized and brass is oxidized. Choice desired type flame is carried out according to the nature of its glow.
The flame power of the burner, selected in accordance with the thickness of the welded metal and its thermophysical properties, is determined by the consumption of acetylene necessary for its melting. The thicker the metal to be 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 and smoothly - by valves
For this type of work, I choose a low-power injection burner GS-2, since it is used for welding thin metal. The burner is produced complete with four tips (0,1,2,3). It is equipped with acetylene and oxygen needle valves that provide precise gas regulation.
Tip number 2, since a torch with this tip can weld metal with a thickness of 1.0 -2.0 mm. The mouthpiece number is also 2, this mouthpiece is suitable for this tip.
The working pressure of oxygen should be 0.2 - 0.5 MPa. But if it is more than this, then the flame will be hard and the metal will melt very quickly and burn holes in the metal, and if the pressure is less than this, then the flame will be soft, it will heat up longer, there will be frequent pops and back blows. The working pressure of acetylene should be 1-7 kPa. If it is less, then there will be frequent claps and reverse blows, and if it is more, then the flame will be hard.
The diameter of the hoses is selected depending on the type of burner, since the diameters of the fittings and the nipples screwed into them are different for burners of different capacities. This burner requires hoses with an internal diameter of 6.3 mm.
To melt the gap between the edges of the metal to be welded and to form a weld bead, a filler wire of the same composition as the metal to be welded is introduced into the weld pool. Do not weld metal with wire of an unknown brand. Before welding, the wire must be cleaned of moisture, dirt, rust, oil, paint.
The choice of the diameter of the filler wire is carried out depending on the thickness of the metal to be welded and the welding method. When welding low and medium carbon steels filler wire diameter, mm, for the left welding method is determined by the formula:
and for the right
where s is the thickness of the welded metal, mm.
The welding speed is set by the welder in accordance with the rate of melting of the edges of the part.
Welding technique
Welding technique is a set of methods, techniques and manipulations carried out by a welder to form a high-quality seam.
For gas welding constituent elements welding techniques are:
* the angle of inclination of the mouthpiece of the burner to the surface of the welded edges;
* way of welding;
* manipulation of the torch mouthpiece and filler wire as the flame moves along the seam.
The angle of inclination of the torch mouthpiece to the surface of the welded edges is chosen by the welder, depending on the thickness of the metal and its thermophysical properties. For low-carbon steels, this relationship can be represented as follows:
Table 1.
The dependence of the angle of inclination of the mouthpiece of the burner on the thickness of the metal
The torch in the welder's hand can only move in two directions:
* from right to left, when the flame is directed to the cold, not yet welded metal edges, and the filler wire is fed ahead of the flame. This method is called the left;
* from left to right, when the flame is directed to the welded area of the seam, and the filler wire is fed after the flame.
This is called the right way.
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 more than 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 flame of the burner from exposure to air.
Before lighting the burner, it is necessary to check it for injection. The process of checking the burner for injection includes: first, you need to remove the acetylene hose from the burner, then open the oxygen valve, oxygen goes through the central hole of the injector and accelerates, thereby creating a vacuum in the side channels of the injector and due to this, acetylene is sucked from these channels. After the oxygen valve is open, we put our finger to the burner fitting and if the finger sticks, it means that the burner is working and welding can be done.
The burner should be ignited in the following order. First, oxygen is opened half a turn, and then acetylene, but in no case vice versa, since the flame will smoke and acetylene will not completely burn out.
For welding various metals and alloys, a certain type of flame is required. For mild steel welding, the flame should be normal. A normal flame is where 1.1 - 1.3 volumes of oxygen enters 1 volume of acetylene. The core of a normal flame has cylindrical shape. There is no free oxygen and carbon in the reduction zone.
The angle of inclination of the mouthpiece and the surface of the welded metal is approximately 30°. This is done so that the metal does not burn through.
Low-carbon steels contain up to 0.25% carbon.
Difficulty in welding. Welding does not cause any particular difficulties. The steel has good weldability over a wide range of flame heat outputs.
Flame characteristics. The type of flame is normal. Its thermal power with the left method of welding is selected based on the consumption of acetylene 100 ... 130 dm3 / h per 1 mm of the thickness of the welded metal, and with the right method - 120 ... 150 dm3 / h.
Technological features. Welding is carried out without flux using the following grades of welding wire as filler material:
* Sv-08 and -08A - for non-critical structures;
* Sv-08G, -08GA, -10GA and -14GS - for critical structures.
T h e n i k a s w a r k i. Welding is performed in both left and right ways.
ADDITIONAL MEASURES. To seal and increase the ductility of the deposited metal after welding, forging and subsequent heat treatment of the weld are used. Forging is recommended to be carried out at a light red heat temperature (800...850 °C) and finished at a dark red heat temperature.
Responsible and thick-walled structures are subject to heat treatment after welding.
For welding mild steel with a thickness of 1.5 mm, it is necessary to adjust the normal flame, the flame power based on the consumption of acetylene 150 ... 200 m3 / h for the left welding method, the diameter of the filler wire is 1.7 mm.
Seams 800 mm long are welded using the reverse step welding method. To do this, the seam is divided into sections of 100-200 mm, since there is more deformation during gas welding, tacks are first made, the length of the tacks is about 10 mm, and the distance between them is about 80 mm. Welding is carried out according to the scheme in sections 1, 2, 3 in one direction, and the seam increases, grows in the opposite direction. All this is done in order to more evenly heat the seam along the entire length and reduce deformation during welding.
Since the thickness of the welded metal is 1.5 mm, a single-layer seam is performed. The gap between two sheets should be minimal to avoid burns.
With this method, the welder clearly sees the welded seam, therefore appearance the seam is better than with the right method.
welding mode- a set of process parameters that determine the possibility of welding a given joint from a 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 carburized and brass is oxidized. 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 welded metal and its thermophysical properties, is determined by the consumption of acetylene necessary for its melting. The thicker the metal to be 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 the valves on the burner.
Choice filler wire diameter is carried out depending on the thickness of the welded metal and the welding method. When welding low- and medium-carbon steels, the diameter of the filler wire, mm, for the left welding method is determined by the formula
d p \u003d s / 2 + 1,
and for the right
where s is the thickness of the welded metal, mm.
Welding speed set by the welder in accordance with the rate of melting 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 constituent elements of welding technology are:
- the angle of inclination of the mouthpiece of the burner to the surface of the welded edges;
- welding method;
- manipulation of the torch mouthpiece and filler wire as the flame moves along the seam.
Mouthpiece angle burners to the surface of the welded edges are selected by the welder, depending on the thickness of the metal and its thermophysical properties. For low-carbon steels, this relationship can be represented as follows:
The greater the thickness of the metal and the higher its thermal conductivity (as, for example, with copper and its alloys), the greater the angle of inclination of the burner mouthpiece. Thus, the welder, by changing the angle of the mouthpiece and thereby the amount of heat supplied to the metal, controls the process of forming the seam.
Welding methods shown in fig. 9.4.
Rice. 9.4. Welding methods:
a - left; b - right; - movement of the burner; ---- movement of filler wire; arrows show welding directions
The torch in the welder's hand can only move in two directions:
- from right to left, when the flame is directed to the cold, not yet welded metal edges, and the filler wire is fed ahead of the flame. This method is called the left;
- from left to right, when the flame is directed to the welded area of the seam, and the filler wire is fed after the flame. This is called the right way.
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 more than 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 flame of the burner from exposure to air.
Torch mouthpiece manipulation(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 of the weld pool and removal of oxides.
Rice. 9.5. Manipulation of the torch mouthpiece when welding:
a - with a delay at the root of the seam; b - in a spiral; in - "crescent"; g - zigzag
The end of the mouthpiece of the burner performs two types of movements simultaneously: longitudinal - along the axis of the seam and transverse - in a perpendicular direction. The torch mouthpiece must be moved in such a way that the metal of the weld pool is always protected from exposure to air by the reducing zone of the flame.
The filler wire makes the same oscillatory movements as the mouthpiece, but in the opposite direction to the oscillations of the burner, and the end of the filler wire must always be in the weld pool or flame reduction zone. When welding in the lower position, the movement of the filler wire in a “crescent” is most often used (see Fig. 9.5, c).
Gas welding has the following advantages: the welding method is relatively simple, does not require complex and expensive equipment, as well as a source of electricity. By changing the thermal power of the flame and its position relative to the place of welding, the welder can control the rate of heating and cooling of the metal being welded over a wide range.
The disadvantages of gas welding include a lower heating rate of the metal and a large zone of heat effect on the metal than in arc welding. In gas welding, the heat concentration is less, and the warping of the parts to be welded is greater than in arc welding. However, with a correctly selected flame power, skillful regulation of its composition, the proper grade of filler metal and the appropriate qualifications of the welder, gas welding provides high-quality welded joints.
Due to the relatively slow heating of the metal by the flame and the relatively low heat concentration during heating, the productivity of the gas welding process decreases significantly with an increase in the thickness of the metal being welded. For example, with a steel thickness of 1mm, the gas welding speed is about 10m/h, and with a thickness of 10mm, only 2m/h. Therefore, gas welding of steel with a thickness of more than 6 mm is less productive compared to arc welding and is used much less frequently.
The cost of combustible gas (acetylene) and oxygen in gas welding is higher than the cost of electricity in arc and resistance welding. As a result, gas welding is more expensive than electric welding.
The gas welding process is more difficult to mechanize and automate than the electric welding process. Therefore, automatic gas welding with multi-flame linear burners is used only when welding shells and pipes made of thin metal with longitudinal seams; gas welding is used for:
Manufacture and repair of products from thin-sheet steel (welding of vessels and tanks of small capacity, welding of cracks, welding of patches, etc.);
welding of pipelines of small and medium diameters (up to 100 mm) and fittings for them;
repair welding of cast iron, bronze and silumin products;
welding of products made of aluminum and its alloys, copper, brass, lead;
surfacing of brass on parts made of steel and cast iron;
welding of forged and ductile iron using brass and bronze filler rods, low-temperature welding of cast iron.
With the help of gas welding, almost all metals used in engineering can be welded. Metals such as cast iron, copper, brass, lead are easier to gas welding than arc welding. If we also take into account the simplicity of the equipment, then it becomes clear that gas welding is widely used in some areas of the national economy (at some engineering plants, agriculture, repair, construction and installation works, etc.).
For gas welding it is necessary:
1) gases - oxygen and combustible gas (acetylene or its substitute);2) filler wire (for welding and surfacing);
3) related equipment and apparatus, including:
a. oxygen cylinders for storing oxygen;
b. oxygen reducers for reducing the pressure of oxygen supplied from cylinders to the burner or cutter;
in. acetylene generators for producing acetylene from calcium carbide or acetylene cylinders in which acetylene is under pressure and dissolved in acetylene;
G. welding, surfacing, hardening and other burners with a set of tips for heating a broom of various thicknesses;
d. rubber sleeves (hoses) for supplying oxygen and acetylene to the burner;
4) accessories for welding: glasses with dark glasses (light filters) to protect the eyes from the bright light of the welding flame, a hammer, a set of keys for the torch, steel brushes for cleaning metal and the weld;
5) Welding table or fixture for assembling and fixing parts during tacking, welding;
6) fluxes or welding powders, if required for welding this metal.
Materials used in gas welding.
Oxygen Oxygen at atmospheric pressure and ordinary temperature is a colorless and odorless gas, somewhat heavier than air. At atmospheric pressure and a temperature of 20 gr. the mass of 1m3 oxygen is 1.33 kg. The combustion of combustible gases and vapors of combustible liquids in pure oxygen occurs very vigorously at a high rate, and a high temperature occurs in the combustion zone.To obtain a welding flame with a high temperature, it is necessary to quickly melt the metal at the welding site, a combustible gas or vapor of a combustible liquid is burned in a mixture with pure oxygen.
If compressed gaseous oxygen occurs with oil or fats, the latter may ignite spontaneously, which may cause a fire. Therefore, when handling oxygen cylinders and equipment, care must be taken to ensure that even slight traces of oil and grease do not fall on them. A mixture of oxygen from combustible liquids at certain ratios of oxygen and combustible substance explodes.
Technical oxygen is extracted from atmospheric air, which is subjected to processing in air separation plants, where it is purified from carbon dioxide and dried from moisture.
Liquid oxygen is stored and transported in special vessels with good thermal insulation. For welding, technical oxygen is produced in three grades: the highest, with a purity of at least 99.5%
1st grade purity 99.2%
2nd grade with a purity of 98.5% by volume.
The rest 0.5-0.1% is nitrogen and argon
Acetylene As a combustible gas for gas welding, acetylene is a compound of oxygen with hydrogen. At normal to and pressure, acetylene is in a gaseous state. Acetylene is a colorless gas. It contains impurities of hydrogen sulfide and ammonia.
Acetylene is an explosive gas. Pure acetylene is capable of exploding at an excess pressure of over 1.5 kgf/cm 2 , upon rapid heating to 450-500C. A mixture of acetylene with air explodes at atmospheric pressure if the mixture contains from 2.2 to 93% acetylene by volume. Acetylene for industrial purposes is obtained by the decomposition of liquid combustibles by the action of an electric arc discharge, as well as by the decomposition of calcium carbide with water.
Gas substitutes for acetylene. When welding metals, other gases and vapors of liquids can be used. For effective heating and melting of the metal during welding, it is necessary that the to of the flame be approximately twice as high as the melting to of the metal being welded.
Combustion of various combustible gases requires a different amount of oxygen supplied to the burner. Table 8 shows the main characteristics of combustible gases for welding.
Gas substitutes for acetylene are used in many industries. Therefore, their production and extraction on a large scale and they are very cheap, this is their main advantage over acetylene.
Due to the lower flame t of these gases, their use is limited to certain processes of heating and melting metals.
When welding steel with propane or methane, it is necessary to use a welding wire containing an increased amount of silicon and manganese used as deoxidizers, and when welding cast iron and non-ferrous metals, fluxes are used.
Gases - substitutes with low thermal conductivity are uneconomical to transport in cylinders. This limits their use for flame treatment.
Table 8 Main gases used in gas welding
Welding wires and fluxes
In most cases, in gas welding, a filler wire is used that is close in its chemistry. composition of the metal to be welded.Do not use random wire of an unknown brand for welding.
The surface of the wire must be smooth and clean, free of scale, rust, oil, paint and other contaminants. The melting point of the wire must be equal to or somewhat lower than the melting point of the metal.
The wire should melt calmly and evenly, without strong spattering and boiling up, forming a dense homogeneous metal during solidification without foreign inclusions and other defects.
For gas welding of non-ferrous metals (copper, brass, lead), as well as stainless steel, in cases where there is no suitable wire, as an exception, strips cut from sheets of the same grade that welds the metal are used.
Fluxes Copper, aluminum, magnesium and their alloys, when heated during welding, react vigorously with oxygen in the air or welding flame (when welding with an oxidizing flame), forming oxides that have a higher melting point than metal. Oxides cover droplets of molten metal with a thin film and this greatly complicates the melting of metal particles during welding.
To protect the molten metal from oxidation and remove the resulting oxides, welding powders or pastes called fluxes are used. Fluxes previously applied to the filler wire or rod and the edges of the metal to be welded melt when heated and form fusible slags that float to the surface of the liquid metal. A slag film covers the surface of the molten metal, protecting it from oxidation.
The composition of the fluxes is chosen depending on the type and properties of the metal to be welded.
Calcined borax, boric acid are used as fluxes. The use of fluxes is necessary when welding cast iron and some special alloy steels, copper and its alloys. When welding carbon steels are not used.
Apparatus and equipment for gas welding.
Water safety locks Water seals protect the acetylene generator and piping from backfire from the welding torch and torch. Backstroke is the ignition of an acetylene-oxygen mixture in the channels of a burner or cutter. The water lock ensures the safety of work during gas welding and cutting and is the main part of the gas welding station. The water lock must always be kept in good condition and filled with water up to the level of the control tap. A water seal is always included between the torch or torch and the acetylene generator or gas pipeline.
Figure 17 Scheme of the device and operation of the medium pressure water seal:
a - normal work shutter, b - reverse flame blow
Cylinders for compressed gases
Cylinders for oxygen and other compressed gases are cylindrical steel vessels. A hole with a conical thread is made in the neck of the cylinder, into which a shut-off valve is screwed. Seamless cylinders for high pressure gases are made of carbon and alloy steel pipes. Cylinders are painted from the outside in word colors, depending on the type of gas. For example, oxygen cylinders in blue, acetylene in white, hydrogen in yellow-green for other combustible gases in red.The upper spherical part of the cylinder is not painted and the passport data of the cylinder is embossed on it.
The cylinder at the welding post is installed vertically and secured with a clamp.
Cylinder valves
Valves for oxygen cylinders are made of brass. Steel for valve parts cannot be used because it corrodes strongly in compressed moist oxygen.Acetylene valves are made of steel. It is forbidden to use copper and alloys containing more than 70% copper, since acetylene can form an explosive compound with copper - acetylene copper.
Reducers for compressed gases
Reducers are used to reduce the pressure of the gas taken from the cylinders (or gas pipeline), and to maintain this pressure constant, regardless of the decrease in gas pressure in the cylinder. The principle of operation and the main parts of all gearboxes are approximately the same.By design, there are single-chamber and two-chamber gearboxes. Double chamber gearboxes have two reduction chambers working in series, give a more constant operating pressure and are less prone to freezing when big expenses gas.
Oxygen and acetylene reducers are shown in fig. eighteen.
Figure 18 Reducers: a - oxygen, b - acetylene
Sleeves (hoses) are used to supply gas to the burner. They must have sufficient strength, withstand gas pressure, be flexible and not restrict the movements of the welder. The hoses are made of vulcanized rubber with fabric gaskets. Sleeves for acetylene and oxygen are issued. For gasoline and kerosene, gasoline-resistant rubber hoses are used.
Welding torches
The welding torch serves as the main tool for manual gas welding. In the burner, oxygen and acetylene are mixed in the required quantities. The resulting combustible mixture flows out of the burner mouthpiece channel at a given speed and, when burned, gives a stable welding flame, which melts the base and filler metal at the welding site. The burner also serves to regulate the thermal power of the flame by changing the flow of combustible gas and oxygen.Burners are injector and non-injector. Serve for welding, soldering, surfacing, heating of steel, cast iron and non-ferrous metals. The most widely used burners are injection type. The burner consists of a mouthpiece, a connecting nipple, a tip tube, a mixing chamber, a cap nut, an injector, a body, a handle, a nipple for oxygen and acetylene.
Burners are divided by flame power:
1.
Micro-low power (laboratory) G-1;
2.
Low power G-2. Consumption of acetylene from 25 to 700 l. per hour, oxygen from 35 to 900 l. at one o'clock. Are completed with tips No. 0 to 3;
3.
Medium power G-3. Consumption of acetylene from 50 to 2500 l. per hour, oxygen from 65 to 3000 l. at one o'clock. Tips #1-7;
4.
High power G-4.
There are also burners for acetylene substitute gases G-3-2, G-3-3. Are completed with tips from No. 1 on No. 7.
Gas welding technology.
Welding flame. External, type, temperature and influence of the welding flame on the molten metal depend on the composition of the combustible mixture, i.e. the ratio of oxygen to acetylene. By changing the composition of the combustible mixture, the welder changes the properties of the welding flame. By changing the ratio of oxygen and acetylene in the mixture, it is possible to obtain three main types of welding flame, fig. nineteen.
Figure 19 Types of acetylene-oxygen flame a - carburizing, b-normal, c - oxidizing; 1 - core, 2 - recovery zone, 3 - torch
For welding most metals, a normal (recovery) flame is used (Fig. 19, b). An oxidizing flame (Fig. 19, c) is used in welding in order to increase the productivity of the process, but it is imperative to use a wire containing an increased amount of manganese and silicon as deoxidizers, it is also necessary when welding brass and hard soldering. A flame with an excess of acetylene is used for surfacing hard alloys. A flame with a slight excess of acetylene is used for welding aluminum and magnesium alloys.
The quality of the deposited metal and the strength of the weld are highly dependent on the composition of the welding flame.
Metallurgical processes in gas welding. Metallurgical processes in gas welding are characterized by the following features: small volume bath of molten metal; high temperature and heat concentration at the welding site; High speed of melting and cooling broom; intensive mixing of the metal of a smooth bath with a gas flow of a flame and a filler wire; chemical interaction of molten metal with flame gases.
The main reactions in the weld pool are oxidation and reduction reactions. Magnesium and aluminum, which have a high affinity for oxygen, are most easily oxidized.
The acids of these metals are not reduced by hydrogen and carbon monoxide, so special fluxes are needed when welding metals. Iron and nickel oxides, on the contrary, are well reduced by carbon monoxide and flame hydrogen, therefore, fluxes are not needed for gas welding of these metals.
Hydrogen is able to dissolve well in liquid iron. With the rapid cooling of the weld pool, it can remain in the seam in the form of small gas bubbles. However, gas welding provides slower cooling of the metal compared to, for example, arc welding. Therefore, when gas welding carbon steel, all hydrogen has time to leave the weld metal and the latter will turn out to be dense.
Structural changes in metal during gas welding. Due to slower heating, the zone of influence in gas welding is larger than in arc welding. The base metal layers directly adjacent to the weld pool are continuous and acquire a coarse-grained structure. In the immediate vicinity of the seam boundary there is a zone of incomplete melting. A base metal with a coarse structure characteristic of an unheated metal. In this zone, the strength of the metal is lower than the strength of the weld metal, therefore, the destruction of the welded joint usually occurs here.
Next is a section, non-recrystallization is also characterized by a coarse-grained structure, for which t of metal melting is not higher than 1100-1200C. Subsequent sections are heated to lower temperatures and have a fine-grained, normalized steel structure.
To improve the structure and properties of the weld metal and the heat-affected zone, hot forging of the weld and local heat treatment by heating with a welding flame or general heat treatment with heating in a furnace are sometimes used.
An illustration of gas welding methods is shown in fig. 20.
Figure 20
Features and modes of welding of various metals.
Welding of carbon steels
Low carbon steels can be welded by any gas welding method. The flame of the burner should be normal, with a power of 100-130dm 3 / h when welding on the right. When welding carbon steels, a wire made of mild steel Sv-8 Sv-10GA is used. When welding with this wire, part of the carbon, manganese and silicon burns out, and the weld metal receives a coarse-grained structure and its tensile strength is that of the base metal. To obtain a deposited metal of equal strength to the main one, Sv-12GS wire is used, containing up to 0.17% carbon; 0.8-1.1 manganese and 0.6-0.9% silicon.Alloy steel welding
Alloy steels are less efficient conductors of heat than mild steels and therefore warp more when welded.Low-alloy steels (for example, XCHD) are well welded by gas welding. When welding, use a normal flame and wire SV-0.8, SV-08A or SV-10G2
Chrome-nickel stainless steels are welded with a normal flame with a power of 75 dm 3 of acetylene per 1 mm of metal thickness. Apply wire SV-02X10H9, SV-06-X19H9T. When welding heat-resistant stainless steel, a wire containing 21% nickel 25% chromium is used. For welding stainless steel containing 3% molybdenum, 11% nickel, 17% chromium.
Cast iron welding
Cast iron is welded when correcting casting defects, as well as restoring and repairing parts: welding cracks, shells, when welding breakaway parts, etc.The welding flame must be normal or carburizing, since the oxidizing flame causes local burnout of silicon, and grains of white cast iron are formed in the weld metal.
Copper welding
Copper has a high thermal conductivity, so when it is welded to the place of metal melting, a large amount of heat has to be carried out than when welding steel.One of the properties of copper that makes welding difficult is its increased fluidity in the molten state. Therefore, when welding copper, no gap is left between the edges. Pure copper wire is used as filler metal. Fluxes are used to deoxidize copper and remove slag.
Welding brass and bronze
Brass welding. Gas welding is widely used for welding brass, which is more difficult to weld with an electric arc. The main difficulty in welding is the significant evaporation of zinc from brass, which begins at 900C. If the brass is overheated, then due to the evaporation of zinc, the seam will turn out to be porous. During gas welding, up to 25% of the zinc contained in brass can evaporate.To reduce the evaporation of zinc, welding of brass is carried out with a flame with an excess of oxygen up to 30-40%. Brass wire is used as filler metal. As fluxes, calcined borax or gaseous flux BM-1 is used.
bronze welding
Gas welding of bronze is used in the repair of cast bronze products, the surfacing of friction surfaces of parts with a layer of antifriction bronze alloys, etc.The welding flame must have a restorative character, since the burnout of tin, silicon, and aluminum from bronze increases with an oxidizing flame. As a filler material, rods or wire are used that are close in composition to the metal being welded. For deoxidation, up to 0.4% silicon is introduced into the filler wire.
To protect the metal from oxidation and remove oxides into slags, fluxes of the same compositions are used as in the welding of copper and brass.
It includes a good preparation of parts for welding, the choice of the desired method of gas welding, the choice of gas welding modes (the required power of the welding torch), the diameter of the filler wire and the correct execution of the gas welding technique. All these points must be taken into account in order to obtain a good welding quality.
The diameter of the welding wire is selected based on the thickness of the metal to be welded and on the selected welding method. More details about the choice of filler materials are given on the page: "Filling materials for gas welding. Selection of welding wire".
Preparation of weld edges for gas welding
The preparation of welded edges includes their cleaning from oil films, paint and varnish coatings, from scale, from dirt and dust, rust, as well as cutting for welding and their tacking with short seams.
Cleaning of welded edges for gas welding
For gas welding, not only the welding edges themselves are cleaned, but also the areas in their immediate vicinity. The width of the cleaned area is 20-30mm on each side of the joint.
The flame of a welding torch is well suited for cleaning. When heated with a burner, the scale moves away from the metal, and paint and varnish coatings and oil burn out. After that, the surface of the welded edges and nearby areas is carefully cleaned with metal brushes or sandpaper. Cleaning is carried out until a metallic sheen appears on the surfaces to be welded. Often, for cleaning, welded parts are subjected to shot-blasting or sandblasting.
In the case when it is impossible to remove dirt with brushes (for example, removal of oxide films is difficult), the weld edges and areas near them are cleaned with special acid-based pastes or pickled in acid. After pickling, the edges must be rinsed and dried.
Cutting edges for gas welding
Welded edges are cut, depending on the type of welded joint. The type of welded joint is determined by the relative position of the parts to be joined. For gas welding, butt welded joints are most characteristic.
Metals of small thickness (up to 2 mm) are butt welded with edge flanging and without the use of filler material (scheme a) in the figure) or without edge flanging and without a gap (scheme b) in the figure), in this case filler material is used.
Metal with a thickness of 2 mm to 5 mm is butt welded without cutting the edges, but leaving a gap between them (diagram c) in the figure). With a weld metal thickness of more than 5 mm, a V-shaped or X-shaped groove is used (scheme d) in the figure). The total angle of opening of the edges should be 70-90° to ensure good penetration of the weld root.
Cutting edges in the parts to be welded can be done manually, with a pneumatic chisel, on milling machines, or on special edge-cutting machines. But an economically viable method is oxygen cutting (manual or mechanized). In this case, scale and slag after cutting must be cleaned to a metallic sheen.
Tacking the edges of the parts to be welded before gas welding
Gas welding technology provides for tacking of parts before welding in order to prevent changes in the position of parts or the appearance of gaps between them during the welding process.
The length of the tacks and the distance between them are determined by the thickness of the metal, the shape and length of the weld. When welding parts of small thickness and with a small length of the weld, tacks are made 5-7 mm long at a distance of 70-100 mm from each other.
In the case of welding of metal of great thickness and at large lengths welds, the length of tacks is 20-30mm, and the recommended distance between tacks is 300-500mm.
Choice of gas welding modes
When choosing gas welding modes guided by the brand of metal or alloy being welded and its thickness. As well as the type and purpose of the welded product. The main characteristics of the gas welding mode include: the power of the welding torch, the type of gas flame, the brand and diameter of the filler rod or wire, the method of gas welding and the welding technique.
Welding torch power selection
The thermal power of the welding torch is determined by the consumption of acetylene passing through it. The required consumption of acetylene can be determined by the formula:
Q=AS, where Q is the consumption of acetylene, l/h; S is the thickness of the welded metal, mm; A is a coefficient that is calculated empirically. When welding carbon steels, the coefficient A \u003d 100-130 l / (h * mm); when welding copper A=150 l/(h*mm), when welding aluminum A=75 l/(h*mm).
The recommended flame power with the right method of gas welding is determined by the consumption of acetylene 120-150 l / h, and with the left welding method, the consumption of acetylene is determined at the rate of 100-130 l / h per millimeter of the thickness of the metal being welded.
It must be borne in mind that an increase in the consumption of acetylene leads to an increase in the power of the welding torch. But with excessive power, there is a risk of burning through the metal. Power must be optimal and this must be taken into account.
The power of the gas flame is regulated by interchangeable tips that come with welding torches.
Gas welding technique. How to cook with gas welding?
From correct gas welding techniques depends on and , and its performance. Welding technique includes both the position of the welding torch and the direction of its movement. Next, we will analyze both of these points in order to understand how to cook with gas welding.
Welding torch position for gas welding
The position is determined by its angle of inclination with respect to the surface of the parts to be welded. The angle of inclination of the torch mouthpiece is affected by the thickness of the parts to be welded and the thermal conductivity of the metal being welded. With a large thickness of the metal and with its high thermal conductivity, it is recommended to increase the angle of inclination of the burner.
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A large angle of inclination of the burner allows you to concentrate the heating of the metal in one place due to the supply a large number heat in a small area. Changing the angle of the burner allows you to change the rate of heating of the metal.
The figure on the right shows the recommended angles of the torch mouthpiece, depending on the thickness of the metal to be welded. The angles recommended in the chart are given for . With , especially when welding copper and when welding aluminum, the recommended angle should be slightly increased (approximately 15 °), because. these metals have high thermal conductivity.
At the very beginning of the welding process, the torch is set at the maximum angle in order to ensure good heating of the metal, then the angle is reduced to the recommended value. At the end of the welding process, it is recommended to gradually reduce the angle of inclination in order to better build up the crater and eliminate possible metal burns.
Gas torch movement during welding
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When , the mouthpiece of the welding torch in two directions: transverse (this direction is perpendicular to the axis of the seam) and longitudinal (along the axis of the seam). The main movement of welding is the longitudinal movement. The transverse movement is auxiliary, but it is necessary in order to evenly heat the edges to be welded and ensure the desired width of the weld.
The lateral movement methods are shown in the figure on the left:
a) movement with a separation of the burner;
b) spiral movement;
c) crescent movement;
d) wavy way of moving.
The deposition of metal using a gas flame flow has not become widespread due to the appearance of large ones. Gas flame surfacing has been used in surfacing with cast hard alloys.
ANSWER
Gas welding modes
Gas welding modes are determined by: the power of the welding flame
the angle of inclination of the filler material and the mouthpiece of the torch
filler material diameter
welding speed. The welding flame must have sufficient thermal power, which is selected depending on the thickness of the metal to be welded and its physical properties. The choice of welding modes depends entirely on the thickness of the parts to be welded
Gas welding methods
There are several ways to apply a weld. Their use is dictated by the habits of the welder and the characteristics of the welded joint.
Left welding(Fig. 2A) - is the most used method for gas welding of metals, 4-5 mm thick. With 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 away from the seam warms up the unwelded area and the filler wire well. With a small thickness of the metal (less than 8 mm), the burner is moved only along the seam, and with a metal thickness of more than 8 mm, additional oscillatory movements are performed across the axis of the seam. The filler wire is immersed at the end of the weld pool, mixing it with a spiral in figurative movements.
The left method is good because the welder sees the seam well, 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 welding method, acetylene is taken in the range of 100 - 130 dm3 per hour per one mm of metal thickness.
Right welding(Fig. 2B) is considered more economical, since the flame is directed directly to the seam. This makes it possible to weld thick metal with a reduced edge opening angle. And since the amount of deposited metal is reduced, the probability of warping of parts is reduced. The burner in this method moves from left to right, and the filler material is moved after the burner. Since the flame is directed at the seam, its cooling rate is reduced, the metal is simultaneously subjected to heat treatment, which improves the quality of the seam.
Vrpros No. 2 The device and principle of operation of the welding unit
Welding units are self-contained power supplies welding arcs, which include a DC generator and a drive gasoline or diesel engine (sometimes electric). The generator and engine are mounted on a common frame and connected by a coupling. There is also a rheostat for regulation welding current, rechargeable batteries, fuel tank, control panel, hood with roof and curtains.
The following types can be distinguished welding units:
o by type of generator - with a collector or valve generator;
o by type of drive - with a gasoline, diesel or electric engine;
o according to the installation method - mobile or stationary.
Aggregates gasoline engines are cheaper in cost, but they require more expensive fuel. Aggregates with diesel engine have a higher cost, but operate on cheaper fuel, are easier to operate and are more reliable in operation at low temperatures
Question number 3 Chemical thermal processing of metals
Answer
Chemical-thermal treatment(HTO) - heating and holding of metallic (and in some cases non-metallic) materials at high temperatures in chemically active media (solid, liquid, gaseous). In the vast majority of cases, chemical-thermal treatment is carried out in order to enrich the surface layers of products with certain elements. They are called saturating elements or saturation components. As a result of CTO, a diffusion layer is formed, i.e. the chemical composition, phase composition, structure and properties of the surface layers change. Change chemical composition causes changes in the structure and properties of the diffusion layer
Depending on the saturating element, the following chemical-thermal treatment processes are distinguished:
· one-component : cementation - saturation with carbon; nitriding - saturation with nitrogen; aluminizing - saturation with aluminum; chromium plating - saturation with chromium; boriding - saturation with boron; siliconizing - saturation with silicon;
· multicomponent : nitrocarburizing (cyanidation, carbonitration) - saturation with nitrogen and carbon; boron and chromium aluminizing - saturation with boron or chromium and aluminum, respectively; chromosiliconation - saturation with chromium and silicon and