©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
III chemical
reaction. Mild steel electrodes require small amounts of carbon, manganese
and silicon in
the deposit to give sound welds of the desired strength level. A portion
of the carbon and
manganese is derived from the core wire, but it is necessary to supplement it
with ferromanganese
and in some cases ferrosilicon additions in the coating.
d) Concentration
of the
Arc Stream
- Concentration or direction of the arc stream
is attained by having a coating crater form at the tip of the electrodes as discussed
earlier. Use of the proper binders
assures a good hard coating that will maintain a crater and
give added penetration and better direction
to the arc stream. e) Furnish
Slag for
Fluxing - The function
of the slag is (1) to provide additional protection
against atmospheric contamination, (2) to act as a cleaner and absorb impurities
that are floated off and trapped by
the slag, (3) to slow the cooling rate of the molten metal to
allow the escape of gases. The
slag also controls the contour, uniformity and general appear-
ance of the weld. This is particularly
true in fillet welds. f)
Characteristics
for Welding
Position -
It is the addition of certain ingredients, primarily
titanium compounds, in the coating that makes it possible to weld out-of-position
, vertically,
and overhead. Slag characteristics, primarily surface tension and freezing
point, determine
to a large degree the ability of an electrode to be used for out-of-position work.
g) Control
of Weld
Metal Soundness
- Porosity or gas pockets in weld metal can
be controlled to a large extent by the coating composition. It is the balance
of certain ingredients
in the coating that have a marked effect on the presence of gas pockets in the
weld metal. The proper balance
of these is critical to the soundness that can be produced.
Ferromanganese is probably the most
common ingredient used to attain the correctly bal- anced
formula. h) Specific
Mechanical Properties
to the
Weld Metal
- Specific mechanical properties
can be incorporated into the weld metal by means of the coating. High impact
values at low temperature, high ductility,
and increases in yield and tensile properties can be attained
by alloy additions to the coating. i)
Insulation
of the
Core Wire
- The coating acts as an insulator so that the core
wire will not short-circuit when welding
in deep grooves or narrow openings; coatings also serve
as a protection to the operator when changing electrodes.
3.2.2
Classification
of Coating
Ingredients -
Coating materials can be classified into the
following 6 major groups:
