Study Notes - Power Engineering 4th Class - Basic Properties of Engineering Materials

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Engineering Materials
Unit 10
Chapter 46

Learning Outcome:
When you complete this chapter you will be able to…
Describe the mechanical properties of ferrous and non-ferrous engineering
materials, plus the effects and purposes of various alloys.
Learning Objectives:
Here is what you will be able to do when you complete each objective
1. Discuss the mechanical properties of materials.
2. Describe the various types of ferrous materials with respect to their
mechanical properties.
3. Describe the various types of non-ferrous materials with respect to their
mechanical properties.

Lecture Outline
1. Introduction
2. Mechanical Properties of Metals
I. Hardness
a. The Brinnel Tester
b. The Rockwell Tester
II. Brittleness
III. Ductility
IV. Plasticity
V. Elasticity
VI. Malleability
VII. Toughness
3. Types of Engineering Materials
I. Iron
II. Cast Iron
III. Wrought Iron
IV. Steel
a. Carbon Steels
V. Alloy Steels
4. Nonferrous Metals
I. Copper
II. Aluminum
III. White Metal

Introduction
The properties of a metal are the characteristics by which it can be accurately
identified, or by which its range of usefulness can be determined. Power
engineers are concerned with the applications and properties of two main
groups of metals; ferrous and nonferrous.
The word ferrous is derived from Latin, meaning iron. Ferrous metals include
pure iron, as well as alloys of iron. Alloys are a metals which are made by
combining two or more elements (such as iron and nickel). Alloys are created
to result in a metal which has greater strength, resistance to corrosion, or to
achieve other properties not possible by a “pure” non-alloy metal.
Nonferrous metals contain no iron, or only trace amounts. Examples of
nonferrous metals are copper, lead, aluminum, zinc, nickel, antimony, tin and
magnesium Nonferrous metals can also be combined into alloys, such as brass
(copper + zinc), and bronze (copper + tin)
Serviceability and safety are the ultimate criteria in choosing metals.
Knowledge of types, their performance, and preservation are absolutely
essential. By knowing to what extent each property exists in a metal, the metal
can be used with the assurance that it will meet requirements for a specific
application.

Mechanical Properties of Metals
Hardness:
Hardness is the ability to resist wear, abrasion, cutting, and indentation. This may
be a property which is uniform throughout the metal, or it may be a surface
condition of a metal, such as when a metal is subjected to case-hardening. Case-
hardening is a process which increases the hardness of the exterior face of a metal,
while leaving its interior at its natural softness - this is often done to allow the
metal object to absorb stresses. Increasing hardness throughout would also
increase its brittleness.
The hardness of a metal is often measured using its resistance to indentation. This
may be done by a hardened steel or diamond ball, cone or pyramid pressed into a
metal sample to be tested. Another common test is to strike the sample with the
indenter, and measure how much the indenter rebounds (a softer metal will
experience a greater indentation, absorbing kinetic energy from the indenter,
resulting in a smaller rebound).
Two common tests to measure hardness are the Brinnel, and the Rockwell tests.

Hardness:
The Brinnel test can be performed on materials with a more coarse structure than
the Rockwell test is capable of, and is often used to test the hardness of large
metal parts.
In the USA and Canada, Brinnel tests are typically performed on ferrous metals
using 10 mm carbide balls as the indenter, with a standard force of 3,000 kg. Tests
on softer, nonferrous metals are performed with a 5 or 10 mm carbide ball, with a
pressure of 500 kg. In Europe, Brinnel tests are performed with a much wider
range of ball sizes and pressures.
The Brinnel Test

Test Method:
The carbide ball (indenter) is pressed into
the metal sample at the pressure of 3,000 kg
(for ferrous metals) or 500 kg (for nonferrous)
for a dwell time of 10-15 seconds.
A low-powered microscope is then used to measure the diameter of the
indentation made, and then the surface area of the indentation is calculated from
its diameter, or a chart is referenced. The hardness value is calculated with this
formula:
𝑩𝒓𝒊𝒏𝒏𝒆𝒍 𝑯𝒂𝒓𝒅𝒏𝒆𝒔𝒔 𝑵𝒖𝒎𝒃𝒆𝒓 𝑩𝑯𝑵 =
𝑳𝒐𝒂𝒅 (𝒌𝒈)
𝑨𝒓𝒆𝒂 𝒐𝒇 𝒕𝒉𝒆 𝒎𝒂𝒓𝒌 (𝒎𝒎 𝟐)
Hardness: The Brinnel Test

In the Rockwell test, a steel ball, or
diamond cone is placed on a metal
sample under a minor load (typically
10 kg). The position of the indenter at
this point is set as the zero, reference
point. A major load (typically a total
of 100 kg for a steel ball, or 150 kg for
very hard materials being tested with
a diamond cone) is then placed onto
the indenter for a dwell time.
The difference in depth between the indenter while under the minor load to its
depth under the major load is measured and used as it’s hardness value.
Readings made with a steel ball are referred to as a Rockwell B measurement,
whereas readings made with a diamond cone are Rockwell C measurements.
Hardness: The Rockwell Test

Brittleness:
Brittleness is a property of a metal which permits no permanent deformation
before breaking. Brittle materials generally break instantly, without any
intermediate stage of bending (lack of plasticity). An example of a brittle material is
cast iron.
The opposite of brittleness is ductility and plasticity.

Ductility:
Ductility is a material's ability to deform under tensile stress; this is often
characterized by the material's ability to be stretched into a wire.
Malleability, a similar property, is a material's ability to deform under compressive
stress; this is often characterized by the material's ability to form a thin sheet by
hammering or rolling.
Both of these mechanical properties are aspects of plasticity.

Plasticity:
A material is said to exhibit plasticity, or to be plastic, if it is very soft and easily
deformed. Examples of plastic materials include wax, lead, and babbitt. Plastic
materials have very little elasticity; that is, they do not return to their original shape
after the deforming force has been removed.
Materials with high plasticity are not brittle.

Elasticity:
Elasticity is the ability of a material to return to its original shape after any force
acting on it has been removed. Elasticity is one of the most important properties for
power engineers to be concerned with, as pipes, pressure vessels, and valves must
reliably return to their original shape and position after being altered slightly from
pressures or changes in heat.
Materials which are tough and ductile, such as wrought iron, typically have high
elasticity.
Materials which are hard and brittle, such as cast iron, have very little elasticity.

Malleability:
Malleability is that property which allows a material to be hammered or rolled into
other sizes and shapes. Copper is an example of metal which is very malleable.
Brittle materials are not malleable,
The malleability of most materials will increase significantly when the material is
heated, such as when iron or steel are heated before forging.

Toughness:
Toughness is the property that determines whether or not a material will break
under a sudden impact. This property is also referred to as “impact strength”, and
impact tests are used to determine the toughness of a material.
A common test used to determine
toughness is the Izod Impact Test, in
which a sample is notched, secured at
one end, and a swinging pendulum is
struck against the other end. The
amount of joules expended by the
pendulum swing necessary to break
the sample is used to determine the
material’s toughness.

Types of Ferrous Engineering Materials
Iron:
To produce iron, iron ore is placed in a blast
furnace. Coke and limestone are added to the
iron ore and the furnace’s contents are heated
to melt the pure iron out of the ore.
Coke is a fuel (often made from coal) with very
few impurities. It also produces carbon
monoxide during combustion, which prevents
much of the iron from forming into iron
oxides.

Iron:
Limestone combines with the impurities in the
iron ore to form slag, which floats on top of the
molten iron, and is removed and discarded.
The purified molten iron is then ladled off into
molds to cool and form pig iron.
Pig iron is very brittle and hard, and not very
machinable at cold or hot temperatures, and is
virtually useless in any real world application.
For this reason, pig iron is refined further into
two classes of materials: steel and cast iron

Cast Iron:
Cast iron is produced by melting pig iron together
with some scrap iron in a cupola furnace, which is
similar to a small blast furnace. The resulting molten
iron contains 2%-4% carbon.
If most of the carbon is combined chemically with the
iron, performed by rapidly cooling it within molds,
white cast iron is produced. White cast iron is very
hard and brittle, and is used for machinery parts
which are subjected to excessive wear, such as crusher
jaws and grinding mill balls and liners.
White Cast Iron:

Cast Iron:
If the molten iron is cooled slowly, it causes the
carbon to disassociate from the iron and form into
graphite. This mechanically combined product (as
opposed to being chemically combined as above), is
known as grey cast iron.
Grey cast iron is softer, with good compressive
strength, and is widely used for machinery bases and
supports.
Grey Cast Iron

Cast Iron:
If the molten metal is annealed (a process of heating
and cooling at a controlled rate), malleable cast iron is
produced. This form of cast iron has increased
toughness and ductility, and is used for some farming
implements, automobile parts, pipe fittings and tools.
Malleable Cast Iron

Wrought Iron:
Wrought iron is produced by a process known as a
puddling. By melting pig iron, and allowing the carbon
and other impurities to oxidize while it melts, which
is then removed, the remaining iron forms a plastic
mass which is formed into a ball by use of a puddling
bar.
The ball is then removed from the furnace and
squeezed and rolled to remove most of the slag. The
result is wrought iron.
Wrought iron is very ductile, and resistant to
corrosion. At one time, it was used extensively for
boiler tubes, piping and bolts, however steel has
largely replaced wrought iron for these purposes.

Steel:
Steels are alloys of iron and carbon containing less than 2% carbon. If the
carbon content is greater than 2%, then the alloy is cast iron. Steels are divided
into plain carbon and alloy steels. Plain carbon steels are alloys of iron and
carbon only.
Approximately 90% of the steel manufactured in North America is produced
by the open hearth method, with most of the remainder produced by the
electric furnace method.

Steel:
The open hearth furnace is charged with
scrap iron, pig iron, and ore, and the heat is
supplied by the combustion gas or oil with
preheated air. The very hot gases pass from
the furnace through a brickwork checker
chamber at one end of the furnace, while the
air for combustion enters through another
brickwork checker chamber at the other end
of the furnace.
Every fifteen minutes, the gas and air flows are reversed with the air being heated
as it passes through the checker chamber, which in the previous fifteen minutes
had been exposed to the hot gases.
As the charge in the furnace is heated and melted, the carbon content is reduced to
the proper point by oxidation, and in this way carbon steel is produced.
If alloy steel is desired, then the required alloying materials are added to the molten
carbon steel.

Steel:
Special high alloy steel is frequently produced in electric furnaces where the
heat is furnished by electrical arcs.

Steel:
Carbon steels are grouped according to their carbon content
low, medium, high, and very high carbon.
• Hardness, strength and often brittleness increase with increasing carbon
content.
• Impurities such as phosphorus or sulphur will lower the ductility, malleability,
and welding qualities of a steel.
• High and very high carbon steels respond well to heat treatments.
• Most of these materials may, in the annealed state, be readily machined.
Carbon Steels

Steel
Alloy steel is steel that is alloyed with a variety of elements in total amounts
between 1.0% and 50% by weight to alter its mechanical properties.
Some of the alloying elements combine with the carbon to form compounds; other
elements do not form compounds but remain in solution in the iron. "In solution"
means that the alloy and carbon elements do not combine chemically with each
other, but are held suspended as crystals in the ferrite.
The main advantages of alloy steels are the ability to respond to heat treatment,
improved corrosion resistance, improved properties at high and low temperatures,
and combination of high strength with good ductility.
Most alloy steels may be welded, provided the carbon content is within welding
range. Generally, these steels require heating before, during, and after welding in
order to avoid residual stresses.
Alloy Steels

Steel
An example of an alloy steel is Specification SA-335-P22 which is a chrome
molybdenum steel used for high temperature steam piping. This material is
suitable for severe service because of its high creep strength and resistance to
oxidation and corrosion at high temperatures (above 500°C).
Creep is slow, permanent stretching of a material under stress, at high
temperatures.
Alloy Steels

Steel
Some of the more important elements which are added to steel to produce alloy
steel, with their effect on the properties of the steel, are the following:
• Nickel
Nickel is a tough, silvery element of about the same density as copper. It has
excellent resistance to corrosion and oxidation even at high temperatures. It
improves toughness, and prevents brittleness at low temperatures.
Nickel steels are especially suitable for the case hardening process for such
applications as roller bearings and gears. These steels provide strong, tough
cases that are resistant to wear and fatigue.
Alloy Steels

Steel
• Chromium
Chromium resists oxidation caused by hot gases, maintains high strength at
elevated temperatures, and increases hardness and abrasion resistance. When
chromium is present in amounts in excess of 4.0%, corrosion resistance is greatly
increased. With a minimum of 12% chromium, the steel is called stainless steel.
• Molybdenum
Molybdenum increases hardness and endurance limits of steel, and decreases
the tendency towards creep. It also increases the steel’s resistance to corrosion.
Alloy Steels

Steel
• Vanadium
Vanadium produces a fine grain structure during heat treatment, promotes
hardening ability, and increases ductility.
• Copper
Copper readily combines with many other elements and improves the
atmospheric corrosion resistance qualities of the steel.
• Lead
Lead improves machinability.
Alloy Steels

Steel
• Manganese
Manganese increases strength and hardness, promotes high impact strength,
and offers excellent resistance to wear by abrasion.
• Tungsten
Tungsten produces a fine grain structure. The alloy retains hardness and
strength at high temperatures.
Alloy Steels

Copper
• Obtained from copper ore which is smelted and then further refined by
electrolysis.
• Commonly made into castings, wire, bars, sheets, plates, tubes, etc.
• The properties which make copper desirable as an engineering material are its:
• high electrical conductivity
• high heat conductivity
• high corrosion resistance
• high ductility and toughness.
In a power plant, it is used primarily for electrical equipment, and as an alloy in the
materials used for heat exchanger tubes, valves, and fittings.
Types of Nonferrous Engineering Materials

Copper
Copper alloys are stronger, easier to machine, and have better corrosion resistance.
The most commonly used copper alloys are various brasses and bronzes. Brasses
and bronzes find use as condenser tubes, piping, valves, fittings, and bearing shells.
• Brass
An alloy of copper and zinc (up to 40%). Frequently, small amounts of other
metals such as lead, tin, nickel, aluminum, and manganese are also included in
the mixture.
• Bronze
An alloy of copper and tin, sometimes also containing zinc to ensure non-
porous castings, and lead to improve machining qualities. Additions of up to 1%
phosphorus produces bronzes, or bearing bronzes, which are hard but not
abrasive. Bronze is approximately equal to pure copper in its resistance to
corrosion.
Copper Alloys

Aluminum
Produced by electrolysis of bauxite ore. Being only 1/3 as heavy as an iron or steel,
its low density is one of the most valuable properties of aluminum. It is also an
important material because of its:
• Good conduction of electricy
• Excellent conduction of heat
• High resistance to corrosion
A disadvantage in its pure form is its low tensile strength. Aluminum is usually
alloyed with other materials such as copper, silicon, manganese, zinc, nickel,
magnesium, and chromium in order to improve its properties.
Aluminum alloys are used for internal combustion engine parts, aircraft parts,
tubing, water jackets, etc.

White Metal
White metal is the name given to alloys made up primarily of lead and tin. It often
contains small amounts of other elements, such as antimony, bismuth, silver, or
zinc.
For a power engineer, white metal is chiefly found in bearing materials, because of
the following properties:
• Easily melted and cast into the bearing shell
• Sufficient strength and ductility not to crack or squeeze out under heavy loads
• Soft enough to wear to conform to the shape of the shaft
• Good thermal conductivity to carry heat away from the bearing surface
Different combinations of the alloying elements are used to produce white metals
suitable for various applications. Babbit, a common tin-based white metal, is
composed of 89% tin, 7.5% lead, and 3.5% copper, and is used for high speed and
light load applications.
A lead-based white metal used for slower speeds and heavier loads is composed of
10% tin, 15% antimony, and 75% lead.

Study Notes - Power Engineering 4th Class - Basic Properties of Engineering Materials

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