28-12-2012, 04:40 PM
INTRODUCTION TO WELDING
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INTRODUCTION
In the Navy as well as private industry, welding is
widely used by metalworkers in the fabrication, maintenance,
and repair of parts and structures. While there
are many methods for joining metals, welding is one of
the most convenient and rapid methods available. The
term welding refers to the process of joining metals by
heating them to their melting temperature and causing
the molten metal to flow together. These range from
simple steel brackets to nuclear reactors.
Welding, like any skilled trade, is broad in scope and
you cannot become a welder simply by reading a book.
You need practice and experience as well as patience;
however, much can be gained through study. For instance,
by learning the correct method or procedure for
accomplishing a job from a book, you may eliminate
many mistakes that otherwise would occur through trial
and error.
WELDING PROCESSES
Welding is not new. The earliest known form of
welding, called forge welding, dates back to the year
2000 B.C. Forge welding is a primitive process of
joining metals by heating and hammering until the metals
are fused (mixed) together. Although forge welding
still exists, it is mainly limited to the blacksmith trade.
Today, there are many welding processes available.
Figure 3-1 provides a list of processes used in modern
metal fabrication and repair. This list, published by the
American Welding Society (AWS), shows the official
abbreviations for each process. For example, RSW
stands for resistance spot welding. Shielded metal arc
welding (SMAW) is an arc-welding process that fuses
(melts) metal by heating it with an electric arc created
between a covered metal electrode and the metals being
joined. Of the welding processes listed in figure 3-1,
shielded metal arc welding, called stick welding, is the
most common welding process. The primary differences
between the various welding processes are the
methods by which heat is generated to melt the metal.
Once you understand the theory of welding, you can
apply it to most welding processes.
The most common types of welding are oxyfuel gas
welding (OFW), arc welding (AW), and resistance
welding (RW). As a Steelworker, your primary concern
is gas and arc welding. The primary difference between
these two processes is the method used to generate the
heat.
GAS WELDING
One of the most popular welding methods uses a gas
flame as a source of heat. In the oxyfuel gas welding
process (fig. 3-2), heat is produced by burning a combustible
gas, such as MAPP (methylacetylene-propadiene)
or acetylene, mixed with oxygen. Gas welding is
widely used in maintenance and repair work because of
the ease in transporting oxygen and fuel cylinders. Once
you learn the basics of gas welding, you will find the
oxyfuel process adaptable to brazing, cutting, and heat
treating all types of metals. You will learn more about
gas welding in chapter 5.
ARC WELDING
Arc welding is a process that uses an electric arc to
join the metals being welded. A distinct advantage of arc
welding over gas welding is the concentration of heat.
In gas welding the flame spreads over a large area,
sometimes causing heat distortion. The concentration of
heat, characteristic of arc welding, is an advantage because
less heat spread reduces buckling and warping.
This heat concentration also increases the depth of penetration
and speeds up the welding operation; therefore,
you will find that arc welding is often more practical and
economical than gas welding
Shielded Metal Arc Welding
(SMAW)
Shielded metal arc welding (fig. 3-3) is performed
by striking an arc between a coated-metal electrode and
the base metal. Once the arc has been established, the
molten metal from the tip of the electrode flows together
with the molten metal from the edges of the base metal
to forma sound joint. This process is known as fusion.
The coating from the electrode forms a covering over
the weld deposit.
FILLER METALS
When welding two pieces of metal together, you
often have to leave a space between the joint. The
material that you add to fill this space during the welding
process is known as the filler metal, or material. Two
types of filler metals commonly used in welding are
welding rods and welding electrodes.
The term welding rod refers to a form of filler metal
that does not conduct an electric current during the
welding process. The only purpose of a welding rod is
to supply filler metal to the joint. This type of filler metal
is often used for gas welding.
FLUXES
Before performing any welding process, you must
ensure the base metal is clean. No matter how much the
base metal is physically cleaned, it still contains impurities.
These impurities, called oxides, result from oxygen
combining with the metal and other contaminants
in the base metal. Unless these oxides are removed by
using a proper flux, a faulty weld may result. The term
flux refers to a material used to dissolve oxides and
release trapped gases and slag (impurities) from the base
metal; thus the flux can be thought of as a cleaning agent.
In performing this function, the flux allows the filler
metal and the base metal to be fused.
Different types of fluxes are used with different
types of metals; therefore, you should choose a flux
formulated for a specific base metal. Beyond that, you
can select a flux based on the expected soldering, brazing,
or welding temperature; for example, when brazing,
you should select a flux that becomes liquid at the
correct brazing temperature. When it melts, you will
know it is time to add the filler metal. The ideal flux has
the right fluidity at the welding temperature and thus
blankets the molten metal from oxidation.
PARTS OF JOINTS
While there are many variations of joints, the parts
of the joint are described by standard terms. The root of
a joint is that portion of the joint where the metals are
closest to each other. As shown in figure 3-7, the root
may be a point, a line, or an area, when viewed in cross
section. A groove (fig. 3-8) is an opening or space
provided between the edges of the metal parts to be
welded. The groove face is that surface of a metal part
included in the groove, as shown in figure 3-8, view A.
A given joint may have a root face or a root edge. The
root face, also shown in view A, is the portion of the
prepared edge of a part to be joined by a groove weld
that has not been grooved. As you can see, the root face
has relatively small dimensions. The root edge is basically
a root face of zero width, as shown in view B. As
you can see in views C and D of the illustration, the
groove face and the root face are the same metal surfaces
in some joints.
TYPES OF WELDS
Some of the comthe
bead, groove,
resistance.
There are many types of welds.
mon types you will work with are
fillet, surfacing, tack, plug, slot, and
As a beginner, the first type of weld that you learn
to produce is called a weld bead (referred to simply as
a bead). A weld bead is a weld deposit produced by a
single pass with one of the welding processes. An example
of a weld bead is shown in figure 3-12. A weld
bead may be either narrow or wide, depending on the
amount of transverse oscillation (side-to-side movement)
used by the welder. When there is a great deal of
oscillation, the bead is wide; when there is little or no
oscillation, the bead is narrow. A weld bead made without
much weaving motion is often referred to as a
stringer bead. On the other hand, a weld bead made
with side-to-side oscillation is called a weave bead.
WELDED JOINT DESIGN
The details of a joint, which includes both the geometry
and the required dimensions, are called the joint
design. Just what type of joint design is best suited for
a particular job depends on many factors. Although
welded joints are designed primarily to meet strength
and safety requirements, there are other factors that must
be considered.