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COATED ELECTRODES

Types of electrodes

What happens when the arc is struck? The electrode is part of an electronic circuit.

To strike an arc the electrode must first touch the workpiece. This action causes a shortcircuit, and when the electrode is withdrawn slightly, the arc is formed.

Mains electricity supply

Welding machine

Electrode holder

Arc Electrode

Welding cable Return cable Return clamp

Workpiece

The Arc

A Weld deposit

B Penetration

C Heat affected zone (HAZ)

Gas shield Slag

A

Core rod Coating

B C Weld pool

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The actual transfer of metal from the electrode to the workpiece is in the form of molten globules of different sizes depending on the type of electrode used. Some electrodes produce globules that are so large that they actually shortcircuit the arc for a moment.

In these cases it is of special importance that the welding power source has a fast dynamic response so that extreme currents and spatter are avoided.

High speed films have been taken of the process in the arc from a coated electrode. These films indicate that the globules actually explode. When a drop leaves the electrode, the extremely high temperature behind it causes an explosion-like expansion effect and the metal is drawn towards the molten pool. This so-called pinch force is stronger than the force of gravity, and overhead welding is thereby made possible.

3.01

Metal globule

The arc will immediately melt some of the coating and core wire of the electrode tip, and the gas shield shown in the figure is formed. At the temperatures found in the arc (approx. 7000 °C) the gas will be ionized, providing good electrical conductivity in the arc.

As soon as the globule leaves the electrode, it is covered by slag material from the electrode coating. This drop is then carried through the arc to the molten pool. At this point the slag separates from the metal, floats up and forms a coating on top of the weld.

The core rod in the electrode melts faster than the coating, and the resulting crater in the end of the electrode aids directional stability of the weld.