GENERAL
PROPERTIES OF METALS
The internal reactions of a metal
known as mechanical properties. The
to external forces are
mechanical properties
are directly related to each other. A change in one property
usually causes a change in one or more additional properties.
For example, if the hardness of a metal is increased, the
brittleness usually increases and the toughness usually
decreases. Following is a brief explanation of the mechanical
properties and how they relate to each other.
TENSILE STRENGTH
Tensile strength is the ability of a metal to resist being pulled
apart by opposing forces acting in a straight line (Figure 2-1).
It is expressed as the number of pounds of force required to
pull apart a bar of the material 1 inch wide and 1 inch thick.
SHEAR STRENGTH
Shear strength is the ability of a metal
fractured by opposing forces not acting in
to resist being
a straight line
(Figure 2-2). Shear strength can be controlled by varying the
hardness of the metal.
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COMPRESSIVE STRENGTH
Compressive strength is the ability of a metal to withstand
pressures acting on a given plane (Figure 2-3).
ELASTICITY
Elasticity is the ability of metal to return to its original size
and shape after being stretched or pulled out of shape (Figure
2-4).
DUCTILITY
Ductility is the ability of a metal to be drawn or stretched
permanently without rupture or fracture (Figure 2-5). Metals
that lack ductility will crack or break before bending.
MALLEABILITY
Malleability is the ability of a metal to be hammered,
rolled, or pressed into various shapes without rupture or
fracture (Figure 2-6).
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SAE 2330
2- Type of steel (nickel).
3- Percent of alloy (3-percent nickel).
30- Carbon content (0.30-percent carbon).
SAE 71650
7- Type of steel (tungsten).
16- Percent of alloy (16-percent tungsten).
50- Carbon content (0,50-percent carbon).
SAE 50100
5- Type of steel (chromium).
0- Percent of alloy (less than l-percent chromium).
100- Carbon content (1-percent carbon).
AA Code
A system similar to the SAE classifications for steel and
alloys has been developed by the Aluminum Association (AA)
for wrought aluminum and aluminum alloys.
This identification system of aluminum, as shown in Figure
2-10, consists of a four-digit number which indicates the type
of alloy. control over impurities, and the specific alloy. The
first number indicates the type of alloy. For example, 2 is
copper, 3 is manganese, 4 is silicone, and so forth. The second
number indicates the control that has been used. The last two
numbers usually indicate an assigned composition. Thus, AA-
2024 means:
2 - Type of alloy (copper).
O - Control of impurities.
24 - Exact composition (AA number 24).
Aluminum alloys vary greatly in their hardness and
physical condition. These differences are called “temper,”
Letter symbols represent the different tempers, In addition to a
letter, one or more numbers are sometimes used to indicate
further differences. The temper designation is separated from
the basic four-digit identification number by a dash; for ex-
ample, 2024-T6. In this case there is an aluminum alloy, 2024,
with a T6 temper (solution heat treated and then artificially
aged). Figure 2-11 shows the numerals 2 through 10 that have
been assigned in the AA system to indicate specific sequences
of annealing, heat treating, cold working, or aging.
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A stencil
METHODS OF MARKING
Stenciling
and white or black paint, whichever shows up
Stamping
Stamping the specification number into the metal should be
better on the metal being marked, should be used when the
used when it is impossible to use the stencil method. It is
size of the metal piece permits. The federal or military
usually necessary to cut or eliminate the marked portion of the
specification numbers should be stenciled on the metal in
metal prior to using the material for work stock. Therefore, the
vertically or hoizontally aligned rows. The distance between
marking should be located where waste will be held to a
the vertical rows should not exceed 36 inches, and the distance
minimum. Gothic style numerals and letters should be used;
between the horizontal rows should not exceed 10 inches.
the height may be 1/16 inch, 1/8 inch, or 1/4 inch, depending
GENERAL
upon the size of the material being marked.
FERROUS METALS
CAST IRON
Ferrous metals are those that contain iron as the base metal.
The properties of ferrous metals may be changed by adding
Cast iron is a metal that is widelv used.
It is a hard, brittle
various alloying elements. The chemical and mechanical
.
metal that has good wear resistance. Cast iron contains 2 to 4
properties need to be combined to produce a metal to serve a
percent carbon. White cast iron is very hard and is used
specific purpose. The basic ferrous metal form is pig iron. Pig
mostly where abrasion and wear resistance is required. White
iron is produced in a blast furnace that is charged with an iron
cast iron may be made into malleable iron by heating it; then
ore, coke, and limestone. The four principal iron ores are
hematite, limonite, magnetite and faconite.
cooling it very slowly over a long period of time. Malleable
iron is stronger and tougher than white cast iron; however, it is
much more expensive to produce. Gray iron is another form of
cast iron. It is used mostly for castings because of its ability to
flow easily into complex shapes.
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WROUGHT IRON
Wrought iron is an iron that has had most of its carbon
removed. It is tough; however, it can be bent or twisted very
easily. Wrought iron is used mostly in ornamental ironwork,
such as fences and handrails, because it is welded or painted
easily and it rusts very slowly.
STEEL
Steel is an alloy of iron and carbon or other alloying
elements. When the alloying element is carbon, the steel is
referred to as carbon steel. Carbon steels are classified by the
percentage of carbon in “points” or hundredths of 1 percent
they contain.
Low Carbon Steel
(Carbon content up to 0.30 percent or 30 points).
This steel is soft and ductile and can be rolled, punched,
sheared, and worked when either hot or cold. It is easily
machined and can be readily welded by all methods. It does
not harden to any great amount; however, it can be easily
case- or surface-hardened.
Medium Carbon Steel
(Carbon content from 0.30 to 0.50 percent or 30 to 50 points).
This steel may be heat-treated after fabrication. It is used for
general machining and forging of parts that require surface
hardness and strength. It is made in bar form in the cold-rolled
or the normalized and annealed condition. During welding, the
weld zone will become hardened if cooled rapidly and must be
stress-relieved after welding.
High Carbon Steel
(Carbon content from 0.50 to 1.05% or 50 to 105 points)
This steel is used in the manufacture of drills, taps, dies,
springs, and other machine tools and hand tools that are heat-
treated after fabrication to develop the hard structure
necessary to withstand high shear stress and wear. It is
manufactured in bar, sheet, and wire forms, and in the
annealed or normalized condition in order to be suitable for
machining before heat treatment. This steel is difficult to weld
because of the hardening effect of heat at the welding joint.
Tool Steel
(carbon content from 0.90 to 1.70 percent or 90 to 170 points)
This steel is used in the manufacture of chisels, shear blades,
cutters, large taps, woodturning tools, blacksmith’s tools,
razors, and other similar parts where high hardness is required
to maintain a sharp cutting edge. It is difficult to weld due to
the high carbon content.
High-Speed Steel
High-speed steel is a self-hardening steel alloy that can
withstand high temperatures without becoming soft. High-
speed steel is ideal for cutting tools because of its ability to
take deeper cuts at higher speeds than tools made from carbon
steel.
Tungsten Carbide
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LEAD
Lead is used mainly in the manufacture of electrical
equipment such as lead-coated power and telephone cables
and storage batteries. Zinc alloys are used in the manufacture
of lead weights, bearings, gaskets, seals, bullets, and shot.
Many types of chemical compounds are produced from lead.
Among these are lead carbonate (paint pigment) and tetraethyl
lead (antiknock gasoline). Lead is also used for X-ray
protection (radiation shields). Lead has more fields of
application than any other metal. It can be cast, cold worked,
welded, and machined. Lead has low strength with heavy
weight.
TIN
The major use of tin is in coating steel. It is the best
container for preserving perishable food. Tin, in the form of
foil, is often used in wrapping food products. A second major
use of tin is as an alloying element. Tin is alloyed with copper
to produce bronze, with lead to produce solder, and with
antimony and lead to form babbitt. Tin can be die cast, cold
worked, machined, and soldered; however, it cannot be
welded.
NICKEL
Nickel is used in making alloys of both ferrous and
nonferrous metals. Chemical and food processing equipment,
electrical resistance heating elements, ornamental trim, and
parts that must withstand elevated temperatures are all
produced from nickel containing metal. Alloyed with
chromium, it is used to make stainless steel. Nickel alloys are
readily welded by either gas or arc methods and can be
machined, forged, cast, and easily formed.
COBALT-CHROMIUM-TUNGSTEN
MOLYBDENUM WEAR-RESISTANT
ALLOYS
These alloys feature a wear resistance which makes them
ideal for metal-cutting operations. Their ability to retain
hardness even at red-heat temperatures also makes them
especially useful for cutting tools. Common cutting tools will
lose their edge at high heat, whereas this alloy group is
actually tougher at red heat than it is when cold; as a result,
higher speeds and feeds may be used when machining with
tools made with these alloys.
PRECIOUS METALS
These include silver, gold, platinum, palladium, iridium,
osmium, rhodium, and ruthenium, and their alloys. These
alloys are produced under technical and legal requirements.
Gold alloys used for jewelry are described in karats. The karat
is the content of gold expressed in twenty-fourths. An 18-karat
gold alloy would contain 18/24 gold (75 percent by weight).
Other than jewelry, there are many industrial uses for precious
metals.
HEAT TREATMENT OF METALS
Heat treatment is any one of a number of controlled heating
and cooling operations used to bring about a desired change in
the physical properties of a metal. Its purpose is to improve
the structural and physical properties for some particular use
or for future work of the metal. There are five basic heat
treating processes: hardening, case hardening, annealing,
normalizing, and tempering. Although each of these processes
bring about different results in metal, all of them involve three
basic steps: heating, soaking, and cooling.
HEATING
Heating is the first step in a heat-treating process. Many
alloys change structure when they are heated to specific
temperatures. The structure of an alloy at room temperature
can be either a mechanical mixture, a solid solution, or a
combination solid solution and mechanical mixture.
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A mechanical mixture can be compared to concrete. Just as
the.sand and gravel are visible and held in place by the
cement. The elements and compounds in a mechanical
mixture are clearly visible and are held together by a matrix of
base metal. A solid solution is when two or more metals are
absorbed, one into the other, and form a solution. When an
alloy is in the form of a solid solution, the elements and
compounds forming the metal are absorbed into each other in
much the same way that salt is dissolved in a glass of water.
The separate elements forming the metal cannot be identified
even under a microscope. A metal in the form of a mechanical
mixture at room temperature often goes into a solid solution or
a partial solution when it is heated. Changing the chemical
composition in this way brings about certain predictable
changes in grain size and structure. This leads to the second
step in the heat treating process: soaking.
SOAKING
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ANNEALING
Once a metal part has been heated to the temperature at
which desired changes in its structure will take place, it must
remain at that temperature until the entire part has been evenly
heated throughout. This is known as soaking. The more mass
the part has, the longer it must be soaked.
COOLING
After the part has been properly soaked, the third step is to
cool it. Here again, the structure may change from one
chemical composition to another, it may stay the same, or it
may revert to its original form. For example, a metal that is a
solid solution after heating may stay the same during cooling,
change to a mechanical mixture, or change to a combination
of the two, depending on the type of metal and the rate of
cooling. All of these changes are predictable. For that reason,
many metals can be made to conform to specific structures in
order to increase their hardness, toughness, ductility, tensile
strength, and so forth.
HEAT TREATMENT OF FERROUS METALS
All heat-treating operations involve the heating and
cooling of metals, The common forms of heat treatment for
ferrous metals are hardening, tempering, annealing,
normalizing, and case hardening.
HARDENING
A ferrous metal is normally hardened by heating the metal
to the required temperature and then cooling it rapidly by
plunging the hot metal into a quenching medium, such as oil,
water, or brine. Most steels must be cooled rapidly to harden
them. The hardening process increases the hardness and
strength of metal, but also increases its brittleness.
TEMPERING
Steel is usually harder than necessary and too brittle for
practical use after being hardened. Severe internal stresses are
set up during the rapid cooling of the metal. Steel is tempered
after being hardened to relieve the internal stresses and reduce
its brittleness. Tempering consists of heating the metal to a
specified temperature and then permitting the metal to cool.
The rate of cooling usually has no effect on the metal structure
during tempering. Therefore, the metal is usually permitted to
cool in still air. Temperatures used for tempering are normally
much lower than the hardening temperatures. The higher the
tempering temperature used, the softer the metal becomes.
High-speed steel is one of the few metals that becomes harder
instead of softer after it is tempered.
Metals are annealed to relieve internal stresses, soften them,
make them more ductile, and refine their grain structures.
Metal is annealed by heating it to a prescribed temperature,
holding it at that temperature for the required time, and then
cooling it back to room temperature. The rate at which metal
is cooled from the annealing temperature varies greatly. Steel
must be cooled very slowly to produce maximum softness,
This can be done by burying the hot part in sand, ashes, or
some other substance that does not conduct heat readily
(packing), or by shutting off the furnace and allowing the
furnace and part to cool together (furnace cooling).
NORMALIZING
Ferrous metals are normalized to relieve the internal stresses
produced by machining, forging, or welding. Normalized
steels are harder and stronger than annealed steels. Steel is
much tougher in the normalized condition than in any other
condition. Parts that will be subjected to impact and parts that
require maximum toughness and resistance to external stresses
are usually normalized. Normalizing prior to hardening is
beneficial in obtaining the desired hardness, provided the
hardening operation is performed correctly. Low carbon steels
do not usually require normalizing, but no harmful effects
result if these steels are normalized. Normalizing is achieved
by heating the metal to a specified temperature (which is
higher than either the hardening or annealing temperatures),
soaking the metal until it is uniformly heated, and cooling it in
still air.
CASE HARDENING
Case hardening is an ideal heat treatment for parts which
require a wear-resistant surface and a tough core, such as
gears, cams, cylinder sleeves, and so forth. The most common
case-hardening processes are carburizing and nitriding.
During the case-hardening process, a low-carbon steel (either
straight carbon steel or low-carbon alloy steel) is heated to a
specific temperature in the presence of a material (solid,
liquid, or gas) which decomposes and deposits more carbon
into the surface of a steel. Then, when the part is cooled
rapidly, the outer surface or case becomes hard, leaving the,
inside of the piece soft but very tough.
HEAT TREATMENT OF NONFERROUS
METALS
Two types of heat-treating operations can be performed on
nonferrous metals. They are annealing and solution heat
treating.
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ANNEALING
Most nonferrous metals can be annealed. The annealing
process consists of heating the metal to a specific temperature,
soaking, and cooling to room temperature. The temperature
and method of cooling depend on the type of metal. Annealing
is often accomplished after various cold working operations
because many nonferrous metals become hard and brittle after
cold working. Also, annealing is used to remove the effects of
solution heat treatment so that machining or working qualities
can be improved.
SOLUTION HEAT TREATMENT
The tensile strength of many nonferrous alloys can be
increased by causing the materials within the alloy to go into a
solid solution and then controlling the rate and extent of return
to an altered mechanical mixture. This operation is called
solution heat treatment. After an alloy has been heated to a
specified temperature, it is “quenched” or cooled rapidly,
which traps the materials in the solid solution attained during
the heating process. From this point, the process varies greatly
depending on the metal. To be sure the materials in the alloy
do not revert to their original configuration after a period of
time, a process of aging or precipitation hardening must
follow. In this process the materials in the alloy are allowed to
change or to precipitate out of the solid solution.
This process occurs under controlled conditions so that the
resultant grain structure will produce a greater tensile strength
in the metal than in its original condition. Depending on the
alloy, this precipitation process can also consist of simply
aging the alloy at room temperature for a specified time and
then air-cooling it; this is called artificial aging.
Aluminum alloys can be obtained in various conditions of
heat treatment called temper designations. Figure 2-11, on
page 2-9, shows the various temper designations and the
process to which they apply. The term “strain-hardened”
refers to aging or hardening that has been brought about by
coldworking the alloy. “Stabilizing” refers to a particular
aging process that freezes or stops the internal changes that
normally would take place in the alloy at room temperature.
Magnesium alloys can be subjected to all of the nonferrous
heat treatments, but the different alloys within the series
require different temperatures and times for the various
processes. Copper alloys are generally hardened by annealing.
The nickel alloys can also be annealed and certain types can
be hardened by heat treatment. Likewise, titanium may be
annealed (mostly relieve machining or cold-working stresses)
but is not noticeably affected by heat treatment.
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