Manual arc welding

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A welder with a helmet but without special work clothes works on a container. Cloudy sky in the background
Repair work on a container using electric welding

The manual metal arc welding , and electrode welding or MMA welding is a manual variant of arc welding that for fusion welding counts. In this welding process , an arc burns between the workpiece and an electrode, which melts and thus also serves as a filler material . There are stick electrodes used, the enclosure performs many tasks in the process. Above all, the burning cover forms protective gases and slag , which protect the melt from chemical reactions with the ambient air. It is one of the oldest and simplest welding processes that is very flexible, but relatively unproductive, so it is rarely used.

In the EN ISO 4063 standard, manual arc welding is carried out as process 111.

Procedural principle

Manual arc welding:
1 wire electrode with coating
2 core wire
3 shielding gas
4 molten pool
5 base material
6 weld metal
7 slag
Connections (4) on the workpiece (1) and electrodes (3) in the pliers (2) (held in the hand)

A welding arc burns between the workpiece and the electrode , which reaches temperatures of 4500 to 5000 Kelvin. This is significantly more than the flame in gas fusion welding (3400 Kelvin), but significantly less than in plasma welding with 20,000 Kelvin. Since the arc acts on a much smaller area than the flame in gas fusion welding, the power density in manual arc welding is significantly higher, which enables much higher welding speeds.

The workpiece is connected to one pole of a welding power source, the electrode to the other pole. The electrode is usually negatively polarized, but the process also works with alternating current (except for electrodes with a basic coating) and with positive polarity of the electrode. The arc melts both the base material of the workpieces and the electrode. This thus simultaneously functions as a filler material which, together with the melted base material, forms the weld pool and the subsequent weld seam. The material transfer from the electrode into the melt always takes place in the form of drops. This principle is also used by submerged arc welding and inert gas welding , but with wire electrodes that are not coated.

The covering of the stick electrodes is of particular importance in manual arc welding; Bare electrodes and carbon electrodes were only used in the early days of the process. The envelope forms protective gases that flow around the melt and thus shield it from the ambient air, and also forms slag. This is lighter than the melt and therefore floats on top and thus also protects it from the environment. In addition, this cools the melt and solidifies the seam more slowly. The arc is ignited by what is known as contact ignition, in which the electrode touches the workpiece (brushing or dabbing). Because of the short circuit, a very high current flows, which melts the electrode at the tip and ignites the arc. The length of the arc corresponds roughly to the thickness of the electrode (1.5 to 6 mm, sometimes up to 8 mm). The welding power sources used have falling characteristics so that the current is independent of the length of the arc.

Areas of application, productivity, flexibility

Schematic representation of underwater welding: The entire system including the welding power source

Manual arc welding is a relatively unproductive and slow process. But it is very simple and flexible. In addition, the acquisition costs for the required equipment are low, so that it is mainly used in workshops and on construction sites. In terms of productivity, it only outperforms gas fusion welding. In return, it is very flexible and is suitable for a large number of applications. It is dependent on electrical energy, but this can also be generated on construction sites with generators and does not have to be obtained from the grid. It is suitable for all welding positions (overhead, downward, etc.) and many materials. The metallurgical composition of the weld seam can be specifically influenced by a suitable choice of the sheath and the core wire. With alkaline coated electrodes, weld seam qualities (especially strengths ) can be achieved that exceed those of other processes. Even poorly accessible joints can be reached relatively easily with electric hand welding. The influence of wind on the protective effect of the protective gases and slag is relatively small, so that it can be used well on construction sites. It can even be used underwater, which is used, for example, for repairs to ships or oil rigs.

  • Underwater welding on a ship

  • Underwater welding of a chain with strong shielding gas development

The main areas of application are of steel construction of the building construction and mechanical engineering and the production of large-scale facilities, bridges , tanks , apparatuses and pipes .

  • Use in the US military

  • Welding of manhole covers by the police before a Castor transport

  • Welding gas pipes

  • Repair work on a tank, 1969.

  • Application in container construction

  • On a construction site

The disadvantage is the low melting rate of the process, which leads to long processing times. The minimum sheet thickness is 1 mm, 1.5 mm or 2 mm depending on the literature. With smaller thicknesses, the melt falls through the joint. In principle, any thicknesses upwards can be welded by applying several layers; from thicknesses of 20 mm to 25 mm, only significantly more productive processes are used in practice ( electron beam welding , submerged arc welding ). In addition, as with all manual welding processes, the quality of the weld depends on the welder and is not constant.

Range of materials

The manual metal arc welding is available for all steels suitable in principle suitable for welding are. This also includes cast steel . Cast iron can be processed as a hot weld with preheating to 600 ° C. It is only suitable to a limited extent for most other materials. Aluminum , copper and the highly reactive materials titanium , tantalum , zirconium and molybdenum can be processed much better with TIG welding or plasma welding , as the gas uptake is significantly lower. Manual arc welding is also rarely used with nickel . It is basically suitable for copper and nickel, while problems arise with aluminum.

equipment

Part of the equipment: cables and electrode clamps, brushes, electrodes, gloves and protective screen.

The costs for the required equipment are low. This includes the electrode tongs , the cables and a welding power source . It can be a transformer or a rectifier . The required currents are 20 to 500 amperes with arc voltages of 15 to 35 volts . Welding inverters are mainly used on construction sites because of their low weight . Rotating converters with a generator are also possible . These can be operated independently of the mains. The current sources must have a falling characteristic curve so that the current intensity in the arc remains approximately constant, regardless of its length. Basic coated electrodes cannot be used with transformers that supply alternating current , since the arc is extinguished when the current crosses zero.

The equipment for the workplace consists of protective clothing against splashes, heat, UV radiation and smoke and consists of a protective screen or helmet , gloves , flame-retardant clothing and special shoes that do not conduct electricity. A slag hammer and wire brush are used to remove the slag .

Stick electrodes

Stick electrodes with covering

The electrodes used in manual arc welding are the stick electrodes. They melt during welding and thus also serve as filler material. They are always wrapped in various materials that perform numerous tasks. The lengths are between 250 mm and 450 mm. The thicknesses are 1.5 mm to 6 mm, rarely up to 8 mm. Thick electrodes allow a higher current and thus a higher performance.

The wrapping should

  • Form protective gases,
  • Form slag and
  • stabilize the arc. To this end, various easily ionizable substances are added that increase the electrical conductivity of the gas between the workpiece and the electrode. Besides, she should
  • metallurgically change the weld metal (alloy or deoxidize).

The method can therefore be adapted to many applications and materials by means of a suitable covering.

The main wrappings are the following:

  • Acid-coated stick electrodes. They contain various iron and manganese oxides and burn alloying elements in the steel, which is why they cannot be used for higher-alloy steels. They lead to a fine droplet transfer of material and a thin liquid melt. The strength of the connection is relatively low, which is why this variety is rarely used.
  • Rutile- coated varieties. They are the most widely used and there are numerous mixed types with other varieties. The arc burns steadily and is easy to (re) ignite. The burn-off on alloy elements is low. The melt is more viscous than the acid-coated one, so that it cannot fall through in root layers. The strength of the seam is good to very good.
  • Basic-coated electrodes: With them, seams of particularly high quality can be produced, especially those with high impact strength . They are used for higher-alloy and higher-carbon (over 0.25% C) steels. Handling this type of electrode is complex and requires special training. They can only be used with direct current and positive electrode polarity.
  • Cellulose-coated electrodes: They are particularly suitable for working in constrained positions .

Work technique

The exact way of working depends on many factors. These include the type of electrode, the base material, the type of current, the welding position, the seam shape and the seam structure.

The seam should be as even as possible, as this avoids faulty seams.

Thicker sheets are welded using the pendulum technique, in which the tip of the rod executes a semicircular movement in order to melt the material over the entire thickness of the sheet.

The pull-bead technology is used for thinner sheets. Here the electrode only moves along the joint. The welder carries out a slow movement in which the electrode is held above the joint at an angle of approximately 45 ° to the sheet metal, approximately in the middle between the parts, when the parts are at an angle to one another. If the parts are in one plane with one another, the electrode is held significantly steeper.

Occasionally, a pushing movement is carried out in the direction of the melt so that the pull-up beads arise.

literature

  • Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 147–166.
  • Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, pp. 11-31.
  • Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, pp. 32–43.

Individual evidence

  1. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 142, 147.
  2. ↑ Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 32 f.
  3. ^ A b Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 147.
  4. ↑ a b Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 11.
  5. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 148.
  6. ↑ Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 11 f.
  7. ↑ a b c d Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 166.
  8. ↑ a b Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 15.
  9. ↑ dvs-server.de SLV-Hannover underwater welding
  10. ↑ a b c d e Ulrich Dilthey: Welding Production Processes 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 16.
  11. ↑ Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 33.
  12. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 147, 166.
  13. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 155 f.
  14. ↑ Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 34.
  15. ↑ Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 12 f.
  16. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, pp. 156–159.
  17. ↑ Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 35 f.
  18. ↑ Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 41 f.