Gas fusion welding

Procedural principle
Gas fusion welding. The burner on the left, the filler metal on the right. The star consists of splashing weld beads.

The gas welding or gas welding is a welding method from the group of fusion welding . The flame of an autogenous welding device serves as the heat source , which also protects the melt against oxygen and thus against oxidation. It is one of the simplest and oldest welding processes, but is rarely used because of the relatively high operating costs and low productivity , especially in the trade and on construction sites because of the low acquisition costs of the required equipment and the high flexibility in use. It is closely related to oxy-fuel cutting , both in terms of the process principle and the equipment.

The torch is used to melt the workpieces to be joined at the joints. In addition, filler material in the form of welding rods is usually held in the flame and melted. It is also possible to apply a layer ( build-up welding ).

Oxy-fuel welding is used for welding sheet metal , car bodies , boilers , pipes and equipment as well as for build-up welding . With the latter, very low degrees of melting of 10% to 5% are possible. The heat output of the flame is relatively low compared to other welding processes, which makes autogenous welding slow and unproductive . However, the low power is an advantage for thin sheets. Other methods are more economical for sheet metal thicknesses of 8 mm or more. Since the filler material is fed separately from the flame output, it is very flexible. In addition, the acquisition costs for the required equipment are low. Components in predicament may very well be welded and it is also suitable for hard to reach places. In contrast to most other welding processes, no electrical energy source is required. The internal stresses introduced are low, but the heat affected zone is large. This leads to a relatively strong distortion of the component and a coarsening of the grain in the material, which leads to lower strengths . Oxy-fuel welding is therefore used in particular in trade and on construction sites . It is one of the oldest and simplest welding processes, but is of little importance because of its low cost effectiveness .

Procedural principle

As with all fusion welding processes, the base material , i.e. the material of the components to be connected, is melted at the joints. Often an additional material is also used, which is also melted and solidifies in the joint between the two components to be connected. In autogenous welding, a flame is used as a heat source. The feed of the filler material, mostly in the form of rods, is independent of this, while in many other welding processes the filler material is fed in via the torch. With oxy-fuel welding, the welder can therefore easily change the supply of heat and filler material and adapt it to the specific requirements.

The flame not only serves as a heat source for melting the materials, but also as a protection of the melt from undesired gases. The fuel gas usually only burns incompletely in the burner. The oxygen, which is required for further combustion, is taken from the environment so that it can no longer oxidize the melt. This is where the term autogenous welding comes from , as the process is autogenous , i.e. it runs automatically. Other fusion welding processes require additional measures to prevent the melt from oxidizing, for example shielding gases for gas- shielded arc welding , the sheathing of the wires for manual arc welding or the powder for submerged arc welding .

Gases used

Combustion gases and oxygen are used.

Fuel gases

In principle, all fuel gases can be used, including methane , propane , hydrogen or natural gas . Almost always, however, acetylene (chemical is acetylene , C 2 H 2 ) is used, because of the high flame performance , and the high flame temperature up to 3200 ° C.

Acetylene is stored in special acetylene bottles . A 40 liter acetylene bottle usually contains 8 kg of acetylene, which corresponds to a volume of about 7 m 3 under normal pressure. At higher pressures and temperatures above 300 ° C it tends to decompose (explosion). Correct handling of the acetylene cylinders is therefore of particular safety-related importance: incorrect handling repeatedly leads to fatalities.

oxygen

Oxygen is also bottled for use on construction sites. However, much higher pressures of up to 200 to 300 bar are possible, so that a single bottle can store significantly more volume than the acetylene bottles. Liquid storage and cold gasification are also used in companies with greater requirements from around 3000 m³.

Schematic representation of the neutral flame: a) cold zone / flame core, b) working zone, hottest point c) scattered flame

There are three variants of the flame:

• In the case of the carburizing flame, only a small amount of oxygen is supplied and the combustion produces carbon , which can get into the material, especially when welding ferrous materials. In the case of steel , this is undesirable and leads to embrittlement. The carbon lowers the melting temperature and leads to a thin liquid melt that sags in the seam . The carburizing flame, however, removes a great deal of oxygen from the environment, so that the melt is very well protected against oxidation. It is used for welding cast iron , for surfacing and for brazing .
• With the neutral flame with a balanced ratio of oxygen to fuel gas, no more carbon is produced. However, the oxygen supplied from the burner is not sufficient to completely burn the fuel gas. It therefore removes oxygen from the environment and thus protects the melt from oxidation
• The oxidizing flame has an excess of oxygen. The fuel gas is completely burned, but the melt is no longer protected from oxidation.

Neutral flame

With the neutral flame, the combustion of acetylene (C 2 H 2 ) takes place in a three-stage process. In the first stage, which takes place in the cold zone, the acetylene is split because of the heat of 400 ° C there and then reacts with the oxygen to form carbon monoxide and hydrogen.

${\ displaystyle C_ {2} H_ {2} + O_ {2} \ longrightarrow 2CO + H_ {2}}$

The ratio of fuel gas to oxygen is therefore 1: 1. In practice, a slight excess of oxygen is set with a ratio of up to 1: 1.2.

The resulting products are further oxidized in the scattered flame, whereby the oxygen required for this is taken from the ambient air and thus no longer reaches the melt.

${\ displaystyle 4CO + 2H_ {2} + 3O_ {2} \ longrightarrow 4CO_ {2} + 2H_ {2} O}$

The gases were thus completely burned. The hottest point of the flame is in the core flame, about 2 to 4 mm in front of the flame cone. Therefore this area is used for welding.

If the core flame is immersed in the melt, it absorbs the gases: the carbon leads to embrittlement, the hydrogen remains as porosity when it cools down quickly, and the oxygen flowing in burns the desired alloying elements (when welding alloyed steel).

Oxidizing flame

If there is an excess of oxygen, the result is an oxidizing flame. It is short, hard and bluish-purple (for acetylene) with a pointed flame cone. The acetylene is already completely burned in the first stage:

${\ displaystyle 4O_ {2} + 2C_ {2} H_ {2} \ longrightarrow 4CO_ {2} + 2H_ {2}}$
${\ displaystyle 2H_ {2} + O_ {2} \ longrightarrow 2H_ {2} O}$

Since more energy is released, the flame is hotter, but there is no protection of the melt against oxidation. When welding steels, the result is a rough seam surface that is slightly oxidized. It also contains slag inclusions. It is used to weld brass.

Carburizing flame

If there is a lack of oxygen, the result is a carburizing flame. The oxygen is then insufficient to burn all of the carbon to carbon monoxide. In the first stage, acetylene is split into pure carbon and hydrogen in the heat:

${\ displaystyle C_ {2} H_ {2} \ longrightarrow 2C + H_ {2}}$

The carbon can get into the material, which is particularly desirable with cast iron.

The heat introduced by the flame can be regulated in different ways:

• Choice of welding insert. This is the front part of the welding torch, which comes in a variety of sizes. They are usually selected according to the sheet thickness. For example, size 1 is for (steel) sheets with a thickness of 0.5 to 1 mm, size 6 for 9 to 14 mm. When welding aluminum, the welding inserts must be chosen two sizes higher, since aluminum materials have a greater thermal conductivity than steel.
• The outflow velocity of the gases. It is usually in the range of 80 m / s (soft flame) to 160 m / s (hard flame)
• Adaptation of the work technique. Depending on whether the flame is held directly on the work site or just close by, there is a different heat input.

Equipment and resources

On the left an oxygen cylinder and on the right an acetylene cylinder with valves, hoses and pressure reducers. The burner is located on the bottom left of the board next to the oxygen bottle. In Germany, the oxygen bottle would be blue, unlike in the picture.

Welding torches are required as operating equipment , as well as valves, fuses and hoses on the cylinders.

Welding torch

A flame cutting nozzle for flame cutting

The welding torch consists of a nozzle , a handle and the gas feed.

Fittings

The burners work with pressures of 0.5 bar for the acetylene and 2.5 bar for the oxygen. There is a significantly higher pressure in the respective bottles, so that pressure reducing valves are required. If the outflow speed of the gases is too low, there is a flashback in the burner, which is why flashback arresters are installed.

Working methods

A distinction is made between welding to the left and welding to the right:

• When welding to the left, the torch is located between the filler metal and the weld seam. If the torch is held in the right hand and the welding rod in the left, then work is done to the left. In this mode of operation, the burner is directed in such a way that a large part of the flame or heat is conducted through the open joint. The heat input is therefore low, which is why this variant is suitable for thin sheets of less than 3 mm. The burner is guided in a straight line and the welding rod is used to dab into the melt, causing it to run into the joint. The degree of melting cannot be observed well in the root area, which can lead to binding errors. Due to the suction effect of the flame, oxygen can be sucked in from the surroundings, which then gets into the melt. The protective effect of the flame is comparatively low.
• With right-hand welding, the torch is aimed directly at the melt and the welding rod is located between the flame and the weld seam. The heat input is greater and the quality of the connection is better, since the flame protects the melt better against oxidation and the roots are melted better. The slower cooling ensures good degassing , but the weld seam becomes wider and can fall through. The right-hand welding enables higher welding speeds. In this variant, the welding rod dips into the melt and stirs it in circular or semicircular movements.

Weldable materials

Training in gas welding of copper pipes

Steels can usually be machined without additional tools. The material properties can be changed in a targeted manner by a suitable choice of filler materials. When there are high demands on the strength of the connection, gas fusion welding is not very suitable because of the coarse grain formation.

Cast iron workpieces are fully heated, otherwise cracks can occur due to the very different local temperatures. Cast iron is welded with a carburizing flame to compensate for the carbon burn-off. The silicon contained in cast iron also burns, which is compensated for by welding rods with silicon.

With most non-ferrous metals , compounds are formed from metals on the one hand and carbon ( carbides ) and oxygen ( oxides ) or hydrogen, which usually have a negative effect on the process, as they usually form a viscous slag. It is mostly dissolved by suitable flux contained in the welding rod. They dissolve the slag and prevent it from forming again, but they are also dangerous for the welder and the environment.

• Copper is welded with a neutral flame, otherwise it will oxidize or absorb hydrogen. The slag formed with the flux is thin.
• Brass is an alloy of copper and zinc . Since zinc has a lower boiling point than the melting point of copper, it tends to evaporate, which is prevented by a viscous slag. It consists of various oxides that are formed by flux and filler metal. Additional oxygen is required to form these oxides, and brass is therefore welded with an oxidizing flame.
• Aluminum alloys , which have a melting point of around 600 ° C, form aluminum oxide with oxygen , which melts at over 2000 ° C and has to be dissolved by flux.

Filler materials: welding rods

The filler metals in gas fusion welding are the welding rods. They usually have a length of 1000 mm, were cast in a calm manner and consist of a material that largely corresponds to the base material.

Abbreviation Chemical composition Welding behavior
C. Si Mn Mon Ni Cr Flow behavior Splash Pore ​​inclination
OZ Any other agreed composition n / A
OI 0.03-0.12 0.02-0.20 0.35-0.65 - - - Thin flowing Much Yes
O II 0.03-0.20 0.05-0.25 0.5-1.20 - - - Less fluid Little Yes
O III 0.05-0.15 0.05-0.25 0.95-1.25 - 0.35-0.8 - Sluggish No No
O IV 0.08-0.15 0.10-0.25 0.90-1.20 0.45-0.65 - -
OV 0.10-0.15 0.10-0.25 0.80-1.20 0.45-0.65 - 0.8-1.20
O VI 0.03-0.10 0.10-0.25 0.40-0.70 0.90-1.20 - 2.0-2.30

Commons : Gas Fusion  - Collection of images, videos and audio files

literature

• Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 142–147.
• Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, pp. 1-10.
• Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, pp. 8–18.

Individual evidence

1. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 142, 146 f.
2. a b Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 1.
3. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 142.
4. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 8.
5. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 2.
6. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 8 f.
7. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 143 f.
8. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 9 f.
9. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 9 f.
10. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 1 f.
11. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 143.
12. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 13.
13. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 142 f.
14. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 14.
15. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 5.
16. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 142.
17. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 7.
18. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, pp. 10–15.
19. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, pp. 10–12.
20. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, pp. 3-5.
21. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 16 f.
22. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 146.
23. a b Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 8f.
24. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 16.