Manual metal arc welding

The process

Manual metal arc (MMA) welding was invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. In MMA welding, an arc is initiated and maintained between the end of a consumable electrode (the filler metal) and the workpiece. Intense heat from the arc causes the surface of the workpiece to melt and form a weld pool. At the same time, the tip of the electrode melts and small globules of filler metal travel across the arc into the molten weld pool to form a weld.

To initiate the arc, the welder momentarily touches the electrode tip on the workpiece, causing current to flow: The electrode is immediately retracted to give a gap of around 3mm between the electrode tip and workpiece: current continues to flow across the gap, initially in the form of a small spark. This spark rapidly ionises the air in the gap, forming an intense welding arc.

The electrode has a pre-coated, dense layer of dry flux over most of its surface: a short length is left uncoated where it fits into the electrode holder and at the opposite end, the tip where it makes contact with the workpiece to initiate the arc is also bare.

As soon as the arc starts, the rapidly heated flux forms both a slag and gaseous shield to protect the weld from atmospheric contamination. Liquid slag, which appears brighter than the molten metal and is more free- running, forms on top of the solidifying weld metal and the gaseous shield protects the weld pool, hot electrode tip and globules of filler metal from atmospheric contamination.

Consumable electrode Evolved gas shield Weld Metal Slag Flux covering Core Wire Arc Weld Pool Parent metal Electrode angle 75-80˚ to the horizontal Direction of travel

As globules of filler metal transfer to the weld pool, the electrode becomes shorter. The welder continuously compensates for this and keeps the arc length constant by feeding the electrode towards the weld using a carefully controlled hand movement. Most MMA electrodes are fairly short (around 350-450mm in length) which means that relatively short lengths of weld are made before having to install a new electrode, which is a quick and simple job. Although the flux coating around the electrode clearly has significant benefits, including helping to stabilise the arc, it has some disadvantages too. As the weld cools, the slag cools and solidifies and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited), suitable eye protection, eg safety glasses, is essential. This cleaning process is especially important in multi-pass welding where slag may become entrapped, resulting in inclusions, which can weaken the weld.

Types of flux/electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:  Cellulosic.

 Rutile.  Basic.

Cellulosic electrodes

Contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the stovepipe welding technique.

Features

 Deep penetration in all positions.  Suitability for vertical down welding.  Reasonably good mechanical properties.

 High level of hydrogen generated - risk of cracking in the heat affected zone (HAZ).

Rutile electrodes

Contain a high proportion of titanium oxide (rutile) in the coating, which promotes easy arc ignition, smooth arc operation and low spatter. General purpose electrodes with good welding properties and can be used with AC and DC power sources, in all positions, especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features

 Moderate weld metal mechanical properties.

 Good bead profile produced through the viscous slag.

 Positional welding possible with a fluid slag (containing fluoride).  Easily removable slag.

Basic electrodes

Contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. Are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features

 Low hydrogen weld metal.

 Require high welding currents/speeds.

 Poor bead profile (convex and coarse surface profile).  Slag removal difficult.

Power source

Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources; however AC electrodes are normally used on DC.

Welding current

Welding current level is determined by the size of electrode, the normal operating range and current are recommended by manufacturers. As a rule of thumb when selecting a suitable current level, an electrode will require about 40A per millimetre (diameter). Therefore, the preferred current level for a 4mm diameter electrode would be 160A, but the acceptable operating range is 140-180A.