Construction
Materials
At the end of this lesson, students
should know
Properties and applications of common
engineering materials.
·
Types and uses of ferrous metals such as cast iron,
wrought iron and steel.
·
Types and uses of some common non-ferrous metals.
·
Types and uses of some non-metals.
·
Important mechanical properties of materials.
1.2.1 Introduction
Choice of materials for a machine
element depends very much on its properties, cost, availability
and such other
factors. It is
therefore important to
have some idea of
the common engineering
materials and their
properties before learning the details of design procedure.
This topic is in the domain of material science or metallurgy but some relevant
discussions are necessary at this stage.
Common engineering materials are
normally classified as metals and non- metals. Metals may conveniently be
divided into ferrous and non-ferrous metals. Important ferrous metals for the
present purpose are:
(i)
cast iron (ii) wrought iron
(iii) steel.
Some of the important non-ferrous
metals used in engineering design are:
(a) Light metal
group such as
aluminium and its
alloys, magnesium and manganese alloys.
(b) Copper based alloys such as brass
(Cu-Zn), bronze (Cu-Sn).
(c) White metal
group such as
nickel, silver, white
bearing metals eg.
SnSb7Cu3, Sn60Sb11Pb, zinc etc.
Cast iron, wrought iron and steel will
now be discussed under separate headings.
1.2.2
Ferrous materials
Cast
iron- It is
an alloy of
iron, carbon and
silicon and it
is hard and
brittle. Carbon content may be within 1.7% to 3% and carbon may be
present as free carbon or iron carbide Fe3C. In general the types of cast iron
are (a) grey cast iron and (b) white cast iron (c) malleable cast iron (d)
spheroidal or nodular cast iron (e) austenitic cast iron (f) abrasion resistant
cast iron.
(a) Grey cast iron- Carbon here is
mainly in the form of graphite. This type of cast iron
is inexpensive and
has high compressive
strength. Graphite is an
excellent solid lubricant and this makes it easily machinable but brittle. Some
examples of this
type of cast
iron are FG20,
FG35 or FG35Si15.
The numbers indicate ultimate
tensile strength in MPa and
15 indicates 0.15% silicon.
(b) White cast
iron- In these
cast irons carbon
is present in
the form of
iron carbide (Fe3C) which
is hard and
brittle. The presence
of iron carbide increases hardness
and makes it
difficult to machine.
Consequently these cast irons are
abrasion resistant.
(c) Malleable cast
iron- These are
white cast irons
rendered malleable by annealing. These are tougher than grey
cast iron and they can be twisted or bent
without fracture. They
have excellent machining
properties and are inexpensive. Malleable cast iron are used
for making parts where forging is expensive such as hubs for wagon wheels,
brake supports. Depending on the method of processing they may be designated as
black heart BM32, BM30 or white heart WM42, WM35 etc.
(d) Spheroidal or
nodular graphite cast
iron- In these
cast irons graphite
is present in the
form of spheres
or nodules. They
have high tensile
strength and good elongation
properties. They are
designated as, for
example, SG50/7, SG80/2 etc where the first number gives the tensile
strength in MPa and the second number indicates percentage elongation.
(e) Austenitic cast iron- Depending on
the form of graphite present these cast iron can be classified broadly under
two headings:
Austenitic flake graphite iron
designated, for example, AFGNi16Cu7Cr2
Austenitic spheroidal or nodular
graphite iron designated, for example, ASGNi20Cr2. These are alloy cast irons
and they contain small percentages of silicon, manganese, sulphur, phosphorus
etc. They may be produced by adding alloying elements viz. nickel, chromium,
molybdenum, copper and manganese in sufficient quantities. These elements give
more strength and improved properties. They are used for making automobile
parts such as cylinders, pistons, piston rings, brake drums etc.
(f)
Abrasion resistant cast iron- These are alloy cast iron and the alloying
elements render abrasion resistance. A typical designation is ABR33 Ni4 Cr2
which indicates a tensile strength in kg/mm2
with 4% nickel and 2%
chromium.
Wrought iron- This is a very pure iron
where the iron content is of the order of 99.5%. It is produced by re-melting pig iron and some small
amount of silicon, sulphur, or phosphorus may be present. It is tough, malleable and ductile and can easily
be forged or welded. It cannot however
take sudden shock. Chains, crane hooks, railway couplings and such other
components may be made of this iron.
Steel-
This is by
far the most
important engineering material
and there is an
enormous variety of steel to meet the wide variety of engineering requirements.
The present note is an introductory discussion of a vast topic.
Steel is basically an alloy of iron and
carbon in which the carbon content can be less
than 1.7% and
carbon is present
in the form
of iron carbide
to impart hardness and strength.
Two main categories of steel are (a) Plain carbon steel and (b) alloy steel.
(a) Plain
carbon steel- The properties of plain carbon steel depend mainly on the carbon
percentages and other alloying elements are not usually present in more than
0.5 to 1% such as 0.5% Si or 1% Mn etc.
There is a large variety of plane carbon steel and they are designated as C01,
C14, C45, C70 and so
on where the
number indicates the
carbon percentage.
Following categorization of
these steels is
sometimes made for convenience:
Dead mild steel- upto 0.15% C
Low carbon steel or mild steel- 0.15 to
0.46% C Medium carbon steel- 0.45 to 0.8% C.
High carbon steel- 0.8 to 1.5% C
Detailed properties of these steels may
be found in any standard handbook but in general higher carbon percentage
indicates higher strength.
(b) Alloy steel-
these are steels
in which elements
other than carbon
are added in sufficient
quantities to impart
desired properties, such
as wear resistance, corrosion
resistance, electric or
magnetic properties. Chief alloying elements
added are usually
nickel for strength
and toughness, chromium for
hardness and strength,
tungsten for hardness
at elevated temperature, vanadium
for tensile strength, manganese for high strength in hot rolled and heat
treated condition, silicon for high elastic limit, cobalt for hardness and
molybdenum for extra
tensile strength. Some
examples of alloy steels are
35Ni1Cr60, 30Ni4Cr1, 40Cr1Mo28, 37Mn2. Stainless steel is one such alloy steel
that gives good corrosion resistance. One important type of stainless steel is
often described as 18/8 steel where chromium and nickel percentages
are 18 and
8 respectively. A
typical designation of a
stainless steel is 15Si2Mn2Cr18Ni8 where carbon percentage is 0.15.
1.2.4 Non-ferrous metals
Metals containing elements other than iron
as their chief constituents are usually
referred to as
non-ferrous metals. There
is a wide
variety of non-metals
in practice. However, only a few exemplary ones are discussed below:
Aluminium- This is the white metal
produced from Alumina. In its pure state it is weak and
soft but addition
of small amounts
of Cu, Mn,
Si and Magnesium makes it hard and strong. It is
also corrosion resistant, low weight and non-toxic. Duralumin- This is an alloy
of 4% Cu, 0.5% Mn, 0.5% Mg and aluminium. It
is widely used in automobile and aircraft components.
Y-alloy- This is an alloy of 4% Cu,
1.5% Mn, 2% Ni, 6% Si, Mg, Fe and the rest is Al. It gives large strength at high temperature. It
is used for aircraft engine parts such
as cylinder heads, piston etc.
Magnalium- This
is an aluminium
alloy with 2
to 10 % magnesium. It
also contains 1.75% Cu.
Due to its
light weight and
good strength it
is used for aircraft and automobile components.
Copper alloys
Copper is one of the most widely used
non-ferrous metals in industry. It is soft, malleable and
ductile and is
a good conductor
of heat and
electricity. The following two
important copper alloys are widely used in practice:
Brass (Cu-Zn alloy)- It is fundamentally
a binary alloy with Zn upto 50% . As Zn percentage increases,
ductility increases upto
~37% of Zn
beyond which the ductility falls. This is shown in figure-1.2.4.1. Small amount
of other elements viz. lead or tin imparts other properties to brass. Lead
gives good machining quality
and
tin imparts strength.
Brass is highly
corrosion resistant, easily
machinable
and therefore a good bearing material.
Ductility Zn (%)
Bronze (Cu-Sn alloy)-This is mainly a
copper-tin alloy where tin percentage may vary between 5 to 25. It provides
hardness but tin content also oxidizes resulting in brittleness.
Deoxidizers such as
Zn may be
added. Gun metal
is one such alloy where 2% Zn is added as
deoxidizing agent and typical compositions are
88%
Cu, 10% Sn,
2% Zn. This
is suitable for
working in cold
state. It was originally made for casting guns but used
now for boiler fittings, bushes, glands
and other such uses.
1.2.5
Non-metals
Non-metallic materials
are also used
in engineering practice
due to principally their low cost, flexibility and
resistance to heat and electricity. Though there are many suitable non-metals,
the following are important few from design point of view:
Timber- This is a relatively low cost
material and a bad conductor of heat and electricity. It has also good elastic
and frictional properties and is widely used in foundry patterns and as water
lubricated bearings.
Leather- This is widely used in
engineering for its flexibility and wear resistance. It is widely used for belt
drives, washers and such other applications.
Rubber-
It has high
bulk modulus and
is used for
drive elements, sealing, vibration isolation and similar
applications.
Plastics
These are synthetic materials which can
be moulded into desired shapes under pressure with or without application of
heat. These are now extensively used in various industrial applications for
their corrosion resistance, dimensional stability and relatively low cost.
There are two main types of plastics:
(a) Thermosetting plastics-
Thermosetting plastics are
formed under heat and
pressure. It initially
softens and with
increasing heat and
pressure, polymerisation takes place.
This results in
hardening of the
material. These plastics cannot
be deformed or
remoulded again under
heat and pressure. Some
examples of
thermosetting plastics are
phenol formaldehyde
(Bakelite), phenol-furfural (Durite),
epoxy resins, phenolic resins etc.
(b) Thermoplastics- Thermoplastics do
not become hard with the application of heat and pressure and no chemical change
takes place. They remain soft at elevated
temperatures until they
are hardened by
cooling. These can be
re-melted and remoulded
by application of
heat and pressure. Some examples of thermoplastics are
cellulose nitrate (celluloid), polythene, polyvinyl acetate,
polyvinyl chloride ( PVC) etc.
