and nonferrous metals, as well as their alloys, respond
Q6-61.
DC reverse polarity in a welding machine
to some form of heat treatment. Almost all metals have
causes a greater concentration of heat in
a critical temperature at which the grain structure
what location?
changes. Successful heat treatment, therefore, depends
Q6-62.
Directional and distributional control of the
largely on knowledge of these temperatures as well as
shielding gas is provided by what
the time required to produce the desired change.
component?
PRINCIPLES OF HEAT TREATMENT
Q6-63.
What is the most common type of shielding
gas used in the tungsten-arc welding process?
The results that may be obtained by heat treatment
Q6-64.
When striking an arc, you should hold the
depend, to a great extent, on the structure of the metal
electrode what distance above the work
and the manner in which the structure changes when the
piece?
metal is heated and cooled. A pure metal cannot be
hardened by heat treatment because there is little
Q6-65.
The process that uses a consumable wire
change in its structure when heated. On the other hand,
electrode is known as what type of welding?
most alloys respond to heat treatment because their
Q6-66.
What determines the melting rate of the filler
structures change with heating and cooling.
wire?
An alloy may be in the form of a solid solution,
Q6-67.
When you use the GMA welding method to
mechanical mixture, or a combination of a solid
weld aluminum, what is the preferred
solution and a mechanical mixture. When an alloy is in
shielding gas?
the form of a solid solution, the elements and
Q6-68.
When you use the GMA welding method, why
compounds that form the alloy are absorbed, one into
should you pull the gun back quickly when
the other, in much the same way that salt is dissolved in
contact is made between the electrode and the
a glass of water. The constituents cannot be identified
work piece?
even under a microscope.
Q6-69.
If you hear a loud crackling sound while you
When two or more elements or compounds are
are GMA welding, what direction should you
mixed, but can be identified by microscopic
move the wire-feed speed dial to correct this?
examination, a mechanical mixture is formed. A
mechanical mixture might be compared to the mixture
Q6-70.
When using GMA welding equipment, where
of sand and gravel in concrete. The sand and gravel are
should you attach the ground connection?
both visible. Just as the sand and gravel are held
Q6-71.
When GMA welding equipment is not in use,
together and kept in place by the mixture of cement, the
what should you do to the power source?
other constituents of an alloy are embedded in the
mixture formed by the base metal.
HEAT TREATMENT OF METALS
An alloy that is in the form of a mechanical mixture
at ordinary temperatures may change to a solid solution
LEARNING OBJECTIVE: Recognize the
when heated. When cooled back to normal
principles of heat treatment. Identify the most
temperature, the alloy may return to its original
common forms of heat treatment.
structure. On the other hand, it may remain a solid
This text covers the forms and principles of heat
solution or form a combination of a solid solution and
treatment in general. Ferrous and nonferrous heat
mechanical mixture. An alloy that consists of a
treatment of metals is covered. Covered information is
combination of a solid solution and mechanical mixture
for training purposes only. When actually performing
at normal temperatures may change to a solid solution
heat treatment tasks, you must refer to the applicable
when heated. When cooled, the alloy may remain a
technical publications.
solid solution, return to its original structure, or form a
complex solution.
Heat treatment is a series of operations involving
the heating and cooling of a metal or alloy in the solid
Heat treatment involves a cycle of events. These
state for the purpose of obtaining certain desirable
events are heating, generally done slowly to ensure
characteristics. The rate of heating and cooling
uniformity; soaking, or holding the metal at a given
determines the crystalline structure of the material. In
temperature for a specified length of time; and cooling,
general, both ferrous metals (metals with iron bases)
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