METAL PROPERTIES

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METAL PROPERTIES
There is no simple definition of metal; however,
any chemical element having "metallic properties" is
classed as a metal. "Metallic properties" are defined
as luster, good thermal and electrical conductivity, and
the capability of being permanently shaped or
deformed at room temperature. Chemical elements
lacking these properties are classed as nonmetals.

A few elements, known as metalloids, sometimes behave
like a metal and at other times like a nonmetal. Some
examples of metalloids are as follows: carbon,
phosphorus, silicon, and sulfur.

Although Steelworkers seldom work with pure
metals, we must be knowledgeable of their properties
because the alloys we work with are combinations of
pure metals. Some of the pure metals discussed in this
chapter are the base metals in these alloys. This is true
of iron, aluminum, and magnesium. Other metals
discussed are the alloying elements present in small
quantities but important in their effect. Among these are
chromium, molybdenum, titanium, and manganese.

Very rarely do Steelworkers work with elements
in their pure state. We primarily work with alloys and have
to understand their characteristics. The characteristics
of elements and alloys are explained in terms of
physical, chemical, electrical, and mechanical
properties. Physical properties relate to color, density,
weight, and heat conductivity. Chemical properties
involve the behavior of the metal when placed in
contact with the atmosphere, salt water, or other
substances. Electrical properties encompass the
electrical conductivity, resistance, and magnetic
qualities of the metal. The mechanical properties
relate to load-carrying ability, wear resistance,
hardness, and elasticity.

When selecting stock for a job, your main
concern is the mechanical properties of the metal.
The various properties of metals and alloys were
determined in the laboratories of manufacturers and
by various societies interested in metallurgical
development. Charts presenting the properties of a
particular metal or alloy are available in many
commercially published reference books.

The charts provide information on the melting point,
tensile strength, electrical conductivity, magnetic
properties, and other properties of a particular metal
or alloy. Simple tests can be conducted to determine
some of the properties of a metal; however, we
normally use a metal test only as an aid for
identifying apiece of stock. Some of these methods
of testing are discussed later in this chapter.

MECHANICAL PROPERTIES
Strength, hardness, toughness, elasticity, plasticity,
brittleness, and ductility and malleability are
mechanical properties used as measurements of how
metals behave under a load. These properties are
described in terms of the types of force or stress that
the metal must withstand and how these are resisted.
Common types of stress are compression, tension,
shear, torsion, impact, 1-2 or a combination of these
stresses, such as fatigue. (See fig. 1-1.)
Compression stresses develop within a material
when forces compress or crush the material. A column
that supports an overhead beam is in compression, and
the internal stresses that develop within the column are
compression.

Tension (or tensile) stresses develop when a
material is subject to a pulling load; for example, when
using a wire rope to lift a load or when using it as a
guy to anchor an antenna. "Tensile strength" is defined
as resistance to longitudinal stress or pull and can be
measured in pounds per square inch of cross section.

Shearing stresses occur within a material when
external forces are applied along parallel lines in
opposite directions. Shearing forces can separate
material by sliding part of it in one direction and the
rest in the opposite direction.

Some materials are equally strong in compression,
tension, and shear. However, many materials show
marked differences; for example, cured concrete has a
maximum strength of 2,000 psi in compression, but
only 400 psi in tension. Carbon steel has a maximum
strength of 56,000 psi in tension and compression but
a maximum shear strength of only 42,000 psi;
therefore, when dealing with maximum strength, you
should always state the type of loading.

A material that is stressed repeatedly usually fails
at a point considerably below its maximum strength in
tension, compression, or shear. For example, a thin
steel rod can be broken by hand by bending it back and
forth several times in the same place; however, if the
same force is applied in a steady motion (not bent back
and forth), the rod cannot be broken. The tendency of
a material to fail after repeated bending at the same
point is known as fatigue.

Strength Rockwell "C" number. On nonferrous metals, that are
Strength is the property that enables a metal to resist
deformation under load. The ultimate strength is the
maximum strain a material can withstand. Tensile
strength is a measurement of the resistance to being
pulled apart when placed in a tension load.

Fatigue strength is the ability of material to resist
various kinds of rapidly changing stresses and is expressed
by the magnitude of alternating stress for a
specified number of cycles.

Impact strength is the ability of a metal to resist
suddenly applied loads and is measured in foot-pounds
of force.

Hardness
Hardness is the property of a material to resist
permanent indentation. Because there are several methods
of measuring hardness, the hardness of a material is
always specified in terms of the particular test that was
used to measure this property. Rockwell, Vickers, or
Brinell are some of the methods of testing. Of these tests,
Rockwell is the one most frequently used. The basic
principle used in the Rockwell testis that a hard material
can penetrate a softer one. We then measure the amount
of penetration and compare it to a scale. For ferrous
metals, which are usually harder than nonferrous metals,
a diamond tip is used and the hardness is indicated by a
softer, a metal ball is used and the hardness is indicated
by a Rockwell "B" number. To get an idea of the
property of hardness, compare lead and steel.

Lead can be scratched with a pointed wooden stick but steel
cannot because it is harder than lead.
A full explanation of the various methods used to
determine the hardness of a material is available in
commercial books or books located in your local library.

Toughness
Toughness is the property that enables a material to
withstand shock and to be deformed without rupturing.
Toughness may be considered as a combination of
strength and plasticity. Table 1-2 shows the order of
some of the more common materials for toughness as
well as other properties.

Elasticity
When a material has a load applied to it, the load
causes the material to deform. Elasticity is the ability of
a material to return to its original shape after the load is
removed. Theoretically, the elastic limit of a material is
the limit to which a material can be loaded and still
recover its original shape after the load is removed.

Plasticity
Plasticity is the ability of a material to deform
permanently without breaking or rupturing. This property
is the opposite of strength. By careful alloying of
metals, the combination of plasticity and strength is used
to manufacture large structural members. For example,
should a member of a bridge structure become overloaded,
plasticity allows the overloaded member to flow
allowing the distribution of the load to other parts of the
bridge structure.

Brittleness
Brittleness is the opposite of the property of plasticity.
A brittle metal is one that breaks or shatters before
it deforms. White cast iron and glass are good examples
of brittle material. Generally, brittle metals are high in
compressive strength but low in tensile strength. As an
example, you would not choose cast iron for fabricating
support beams in a bridge.

Ductility and Malleability
Ductility is the property that enables a material to
stretch, bend, or twist without cracking or breaking. This
property makes it possible for a material to be drawn out
into a thin wire. In comparison, malleability is the
property that enables a material to deform by compressive
forces without developing defects. A malleable
material is one that can be stamped, hammered, forged,
pressed, or rolled into thin sheets.

CORROSION RESISTANCE
Corrosion resistance, although not a mechanical
property, is important in the discussion of metals. Corrosion
resistance is the property of a metal that gives it
the ability to withstand attacks from atmospheric,
chemical, or electrochemical conditions. Corrosion,
sometimes called oxidation, is illustrated by the rusting
of iron.

Table 1-2 lists four mechanical properties and the
corrosion resistance of various metals or alloys. The first
metal or alloy in each column exhibits the best characteristics
of that property. The last metal or alloy in each
column exhibits the least. In the column labeled "Toughness,"
note that iron is not as tough as copper or nickel;
however, it is tougher than magnesium, zinc, and aluminum.
In the column labeled "Ductility," iron exhibits a
reasonable amount of ductility; however, in the columns
labeled "Malleability" and "Brittleness," it is last.
1-4

METAL TYPES
The metals that Steelworkers work with are divided
into two general classifications: ferrous and nonferrous.

Ferrous metals are those composed primarily of iron and
iron alloys. Nonferrous metals are those composed primarily
of some element or elements other than iron.

Nonferrous metals or alloys sometimes contain a small
amount of iron as an alloying element or as an impurity.

FERROUS METALS
Ferrous metals include all forms of iron and steel
alloys. A few examples include wrought iron, cast iron,
carbon steels, alloy steels, and tool steels. Ferrous metals
are iron-base alloys with small percentages of carbon
and other elements added to achieve desirable properties.

Normally, ferrous metals are magnetic and nonferrous
metals are nonmagnetic.

Iron
Pure iron rarely exists outside of the laboratory. Iron
is produced by reducing iron ore to pig iron through the
use of a blast furnace. From pig iron many other types
of iron and steel are produced by the addition or deletion
of carbon and alloys. The following paragraphs discuss
the different types of iron and steel that can be made
from iron ore.

PIG IRON.- Pig iron is composed of about 93%
iron, from 3% to 5% carbon, and various amounts of
other elements. Pig iron is comparatively weak and
brittle; therefore, it has a limited use and approximately
ninety percent produced is refined to produce steel.
Cast-iron pipe and some fittings and valves are manufactured
from pig iron.
WROUGHT IRON.- Wrought iron is made from
pig iron with some slag mixed in during manufacture.
Almost pure iron, the presence of slag enables wrought
iron to resist corrosion and oxidation. The chemical
analyses of wrought iron and mild steel are just about
the same. The difference comes from the properties
controlled during the manufacturing process. Wrought
iron can be gas and arc welded, machined, plated, and
easily formed; however, it has a low hardness and a
low-fatigue strength.

CAST IRON.- Cast iron is any iron containing
greater than 2% carbon alloy. Cast iron has a
high-compressive strength and good wear resistance; however,
it lacks ductility, malleability, and impact strength.

Alloying it with nickel, chromium, molybdenum, silicon, or
vanadium improves toughness, tensile strength, and
hardness. A malleable cast iron is produced through a easily
as the low-carbon steels. They are used for crane
prolonged annealing process. hooks, axles, shafts, setscrews,
and so on.

INGOT IRON.- Ingot iron is a commercially pure
iron (99.85% iron) that is easily formed and possesses
good ductility and corrosion resistance. The chemical
analysis and properties of this iron and the lowest carbon
steel are practically the same. The lowest carbon steel,
known as dead-soft, has about 0.06% more carbon than
ingot iron. In iron the carbon content is considered an
impurity and in steel it is considered an alloying element.

The primary use for ingot iron is for galvanized
and enameled sheet.

Steel
Of all the different metals and materials that we use
in our trade, steel is by far the most important. When
steel was developed, it revolutionized the American iron
industry. With it came skyscrapers, stronger and longer
bridges, and railroad tracks that did not collapse. Steel
is manufactured from pig iron by decreasing the amount
of carbon and other impurities and adding specific
amounts of alloying elements.

Do not confuse steel with the two general classes of
iron: cast iron (greater than 2% carbon) and pure iron
(less than 0.15% carbon). In steel manufacturing, controlled
amounts of alloying elements are added during
the molten stage to produce the desired composition.

The composition of a steel is determined by its application
and the specifications that were developed by the
following: American Society for Testing and Materials
(ASTM), the American Society of Mechanical Engineers
(ASME), the Society of Automotive Engineers
(SAE), and the American Iron and Steel Institute (AISI).
Carbon steel is a term applied to a broad range of
steel that falls between the commercially pure ingot iron
and the cast irons. This range of carbon steel may be
classified into four groups:

HIGH-CARBON STEEL/VERY HIGH-CARBON

STEEL.- Steel in these classes respond well to
heat treatment and can be welded. When welding, special
electrodes must be used along with preheating and
stress-relieving procedures to prevent cracks in the weld
areas. These steels are used for dies, cutting tools, mill
tools, railroad car wheels, chisels, knives, and so on.
LOW-ALLOY, HIGH-STRENGTH, TEMPERED
STRUCTURAL STEEL.- A special lowcarbon
steel, containing specific small amounts of
alloying elements, that is quenched and tempered to get
a yield strength of greater than 50,000 psi and tensile
strengths of 70,000 to 120,000 psi. Structural members
made from these high-strength steels may have smaller
cross-sectional areas than common structural steels
and still have equal or greater strength. Additionally,
these steels are normally more corrosion- and abrasionresistant.

High-strength steels are covered by ASTM
specifications.
NOTE: This type of steel is much tougher than
low-carbon steels. Shearing machines for this type of
steel must have twice the capacity than that required for
low-carbon steels.

STAINLESS STEEL.- This type of steel is classified
by the American Iron and Steel Institute (AISI)
into two general series named the 200-300 series and
400 series. Each series includes several types of steel
with different characteristics.

The 200-300 series of stainless steel is known as
AUSTENITIC. This type of steel is very tough and
ductile in the as-welded condition; therefore, it is ideal
for welding and requires no annealing under normal
atmospheric conditions. The most well-known types of
steel in this series are the 302 and 304. They are commonly
called 18-8 because they are composed of 18%
chromium and 8% nickel. The chromium nickel steels
Low-Carbon Steel . . . . . . . . 0.05% to 0.30% carbon are the
most widely used and are normally nonmagnetic.

Medium-Carbon Steel . . . . . . 0.30% to 0.45% carbon The 400
series of steel is subdivided according to
High-Carbon Steel . . . . . . . . 0.45% to 0.75% carbon their
crystalline structure into two general groups.

One Very High-Carbon Steel . . . . . 0.75% to 1.70% carbon group
is known as FERRITIC CHROMIUM and the

other group as MARTENSITIC CHROMIUM.
LOW-CARBON STEEL.- Steel in this classifi- Ferritic Chromium.
- This type of steel contains
cation is tough and ductile, easily machined, formed, 12% to
27% chromium and 0.08% to 0.20% carbon.

and welded. It does not respond to any form of heat These
alloys are the straight chromium grades of staintreating,
except case hardening.

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Warmly,
Pat Mitchell
http://www.weldingsecrets.net/main.html