©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
III 3.2.1
Functions
of Electrode
Coatings - The ingredients
that are commonly used in coatings
can be classified physically in a broad manner as liquids and solids. The
liquids are generally
sodium silicate or potassium silicate. The solids are powdered or granulated
mate- rials that
may be found free in nature, and need only concentration and grinding to
the proper particle
size. Other solid materials used are produced as a result of chemical reactions,
such as alloys
or other complex synthetic compounds. 3.2.1.1
The particle size of the solid material
is an important factor. Particle size may be as coarse
as fine sand, or as minute as sub-sieve size. 3.2.1.2
The physical structure of the coating
ingredients may be classified as crystalline, fibrous
or amorphous (non-crystalline). Crystalline materials such as rutile, quartz
and mica are commonly
used. Rutile is the naturally occurring form of the mineral titanium
dioxide and is
widely used in electrode coatings. Fibrous materials such as wood fibers,
and non-crystal- line
materials such as glasses and other organic compounds are also common coating
ingre- dients.
3.2.1.3 The
functions of the coating on covered electrodes are as follows:
a) Shielding
of the
Weld Metal
- The most important function of a coating is to shield
the weld metal from the oxygen and nitrogen of the air as it is being transferred
across the arc,
and while it is in the molten state. This shielding is necessary to ensure
the weld metal will
be sound, free of gas pockets, and have the right strength and ductility. At
the high tem- peratures
of the arc, nitrogen and oxygen combine readily with iron to form iron nitrides
and iron oxides
that, if present in the weld metal above certain minimum amounts, will cause brittle-
ness and porosity. Nitrogen is
the primary concern since it is difficult to control its effect once it
has entered the deposit. Oxygen
can be counteracted by the use of suitable deoxidizers. In
order to avoid contamination from the
air, the stream of molten metal must be protected or shielded
by gases that exclude the surrounding atmosphere from the arc and the molten weld
metal. This is accomplished by
using gas-forming materials in the coating that break down
during the welding operation and produce
the gaseous shield. b) Stabilization
of the
Arc - A stabilized arc is one that starts
easily, burns smoothly
even at low amperages, and can be maintained using either a long or a short arc
length. c)
Alloying
Additions to
Weld Metal
- A variety of elements such as chromium, nickel,
molybdenum, vanadium and copper can be added to the weld metal by including them
in the coating composition. It
is often necessary to add alloys to the coating to balance the
expected loss of alloys of the core
wire during the welding operation, due to volatization and
