5. MMA PROCESS.
Welding with coated electrode, also known as MMA (Manual Metal Arc), SMAW or 111 process, forms part of the
welding processes by electric arc. Therefore, before starting the requested welding jobs, we must bear in mind the
basic principles of the process.
Electric arc welding with coated electrode is a process
whereby the metal between the part and a coated metal
electrode is melted.
As the electrical current circulates through the electrode,
heat increases at the end of the electrode that produces an
arc that melts the core or rod of the electrode, burning its
coating. Thus obtaining the appropriate atmosphere to
transfer the molten metal from the core of the electrode to
the melt bath in the base material.
These drops of molten metal fall coated with molten slag from the melting of the arc coating. The slag floats on the
surface and forms, above the welding bead, a protective layer of cast metal, controlling the bead cooling speed and
avoiding the oxidation of the filler metal.
APPLICATIONS
This welding process is especially recommended for repair and maintenance welding, manufacture and installation
of pipes, as well as outdoor assembly work. Production and repair welding in naval construction, storage tanks,
structures, pressure containers, oil refineries, boilers and any type of piping, are some of its application sectors.
The main characteristics of the process are its simplicity and its low price, making it a practical and excellent
procedure for offshore use or outdoor work.
However, the welding procedure with coated electrode is not suitable for its automation or semi-automation; its
application is essentially manual.
If electrodes are used in less favourable conditions (with humidity, with no preheating, etc.) we can improve the
welding features by adjusting the control parameters in manual process.
5.1. MMA SYSTEM INSTALLATION.
When welding with coated electrode we must connect the electrode-holder clamp (F) and the earth clamp (G)
according to the polarity advised by the electrode manufacturer.
Normally, the majority of the electrodes must be placed with straight polarity; that is, the welding electrode-holder
clamp in the negative pole and the earth clamp in the positive pole. However, welding with basic or special
electrodes is usually carried out with reverse polarity; that is, the electrode-holder clamp in positive pole and the
earth clamp in negative pole. We will follow the electrode manufactuer’s instructions in each case. Equipment
preparation for the working mode is drawn in the figure; in this case, observe that the polarity used is inverse,
meaning that the welding electrode-holder is connected to the positive pole.
Installation for COATED ELECTRODE welding.
Summary of Contents for Smart 140 MMA
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