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
met-als 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
differ-ences 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
steekworker, 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 com-bustible gas, such as MAPP
(methylacetylene-propadi-ene) 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 lesson 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 pene-tration and speeds up the welding
operation; therefore, you will find that arc welding is often more practical and
economical than gas welding. |
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All arc-welding processes
have three things in com-mon: a heat source, filler metal, and shielding. The source of
heat in arc welding is produced by the arcing of an electrical current between two
contacts. The power source is called a welding machine or simply, a welder. This
should not be confined with the same term that is also used to describe the person who is
performing the welding operation. The welder (welding machine) is either electric- or
motor-powered. In the Naval Construction Force (NCF), there are two main types of
arc-welding processes with which you should become familiar. They are shielded metal arc
welding and gas shielded arc 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, shielding it from contamination;
therefore the process is called shielded metal arc welding. The main
advantages of shielded metal arc welding are that high-quality welds are made rapidly at a
low cost. You will learn more about shielded metal arc welding in a later lesson.
Gas Shielded Arc Welding
The primary difference between shielded metal arc welding and gas shielded arc welding is
the type of shielding used. In gas shielded arc welding, both the arc and the molten
puddle are covered by a shield of inert gas. The shield of inert gas prevents atmospheric
con-tamination, thereby producing a better weld. The pri-mary gases used for this process
are helium, argon, or carbon dioxide. In some instances, a mixture of these gases is used.
The processes used in gas shielded arc welding are known as gas tungsten arc welding
(GTAW). |
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(GTAW) (fig. 3-4) and gas
metal arc welding (GMAW) (fig. 3-5). You will also hear these called “TIG” and
“MIG.” Gas shielded arc welding is extremely useful because it can be used to
weld all types of ferrous and nonferrous metals of all thicknesses.
Now that we have discussed a
few of the welding processes available, which one should you choose? There are no
hard-and-fast rules. In general, the control-ling factors are the types of metal you are
joining, cost involved, nature of the products you are fabricating, and the techniques you
use to fabricate them. Because of its flexibility and mobility, gas welding is widely used
for maintenance and repair work in the field. On the other hand, you should probably
choose gas shielded metal arc welding to repair a critical piece of equipment made from
aluminum or stainless steel.
No matter what welding
process you use, there is some basic information you need to know. The remain-der of this
chapter is devoted to this type of information. Study this information carefully because
it allows you to follow welding instructions, read welding symbols, and weld various types
of joints using the proper weld-ing techniques. |
