Singh R. Introduction to Basic Manufacturing Processes and Workshop Technology

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118 Introduction to Basic Manufacturing Processes and Workshop Technology

4. Resistivity

It is the property of a material by which it resists the flow of electricity through it.

5. Thermoelectricity

If two dissimilar metals are joined and then this junction is heated, a small voltage (in

the milli-volt range) is produced, and this is known as thermoelectric effect. It is the base of

the thermocouple. Thermo -couples are prepared using the properties of metals.

7.1.5 Magnetic Properties

Magnetic properties of materials arise from the spin of the electrons and the orbital motion

of electrons around the atomic nuclei. In certain atoms, the opposite spins neutralize one

another, but when there is an excess of electrons spinning in one direction, magnetic field

is produced. Many materials except ferromagnetic material which can form permanent magnet,

exhibit magnetic affects only when subjected to an external electro-magnetic field. Magnetic

properties of materials specify many aspects of the structure and behavior of the matter.

Various magnetic properties of the materials are magnetic hysteresis, coercive force and

absolute permeability which are defined as under.

1. Magnetic Hysteresis

Hysteresis is defined as the lagging of magnetization or induction flux density behind the

magnetizing force or it is that quality of a magnetic substance due to energy is dissipated in

it on reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the rising

temperature at which the given material ceases to be ferromagnetic, or the falling temperature

at which it becomes magnetic. Almost all magnetic materials exhibit the phenomenon called

hysteresis.

2. Coercive Force

It is defined as the magnetizing force which is essential to neutralize completely the

magnetism in an electromagnet after the value of magnetizing force becomes zero.

3. Absolute Permeability

It is defined as the ratio of the flux density in a material to the magnetizing force

producing that flux density. Paramagnetic materials possess permeability greater than one

whereas di-magnetic materials have permeability less than one.

7.1.6 Optical Properties

The main optical properties of engineering materials are refractive index, absorptivity,

absorption co-efficient, reflectivity and transmissivity. Refractive index is an important optical

property of metal which is defined as under.

Refractive Index

It is defined as the ratio of velocity of light in vacuum to the velocity of a material. It

can also be termed as the ratio of sine of angle of incidence to the sine of refraction.

7.1.7 Mechanical Properties

Under the action of various kinds of forces, the behavior of the material is studied that

measures the strength and lasting characteristic of a material in service. The mechanical

properties of materials are of great industrial importance in the design of tools, machines and

Porperties and Testing of Metals 119

structures. Theses properties are structure sensitive in the sense that they depend upon the

crystal structure and its bonding forces, and especially upon the nature and behavior of the

imperfections which- exist within the crystal itself or at the grain boundaries. The mechanical

properties of the metals are those which are associated with the ability of the material to resist

mechanical forces and load. The main mechanical properties of the metal are strength, stiffness,

elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, castability

and weldability. These properties can be well understood with help of tensile test and stress

strain diagram. The few important and useful mechanical properties are explained below.

1. Elasticity

It is defined as the property of a material to regain its original shape after deformation

when the external forces are removed. It can also be referred as the power of material to

come back to its original position after deformation when the stress or load is removed. It

is also called as the tensile property of the material.

2. Proportional limit

It is defined as the maximum stress under which a material will maintain a perfectly

uniform rate of strain to stress. Though its value is difficult to measure, yet it can be used

as the important applications for building precision instruments, springs, etc.

3. Elastic limit

Many metals can be put under stress slightly above the proportional limit without

taking a permanent set. The greatest stress that a material can endure without taking up

some permanent set is called elastic limit. Beyond this limit, the metal does not regain its

original form and permanent set will occurs.

4. Yield point

At a specific stress, ductile metals particularly ceases, offering resistance to tensile

forces. This means, the metals flow and a relatively large permanent set takes place without

a noticeable increase in load. This point is called yield point. Certain metals such as mild steel

exhibit a definite yield point, in which case the yield stress is simply the stress at this point.

5. Strength

Strength is defined as the ability of a material to resist the externally applied forces with

breakdown or yielding. The internal resistance offered by a material to an externally applied

force is called stress. The capacity of bearing load by metal and to withstand destruction

under the action of external loads is known as strength. The stronger the material the

greater the load it can withstand. This property of material therefore determines the ability

to withstand stress without failure. Strength varies according to the type of loading. It is

always possible to assess tensile, compressive, shearing and torsional strengths. The maximum

stress that any material can withstand before destruction is called its ultimate strength. The

tenacity of the material is its ultimate strength in tension.

6. Stiffness

It is defined as the ability of a material to resist deformation under stress. The resistance

of a material to elastic deformation or deflection is called stiffness or rigidity. A material that

suffers slight or very less deformation under load has a high degree of stiffness or rigidity.

For instance suspended beams of steel and aluminium may both be strong enough to carry

the required load but the aluminium beam will “sag” or deflect further. That means, the steel

120 Introduction to Basic Manufacturing Processes and Workshop Technology

beam is stiffer or more rigid than aluminium beam. If the material behaves elastically with

linear stress-strain relationship under Hooks law, its stiffness is measured by the Young’s

modulus of elasticity (E). The higher is the value of the Young’s modulus, the stiffer is the

material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of

elasticity”; in shear, the modulus of rigidity, and this is usually 40% of the value of Young’s

modulus for commonly used materials; in volumetric distortion, the bulk modulus.

7. Plasticity

Plasticity is defined the mechanical property of a material which retains the deformation

produced under load permanently. This property of the material is required in forging, in

stamping images on coins and in ornamental work. It is the ability or tendency of material

to undergo some degree of permanent deformation without its rupture or its failure. Plastic

deformation takes place only after the elastic range of material has been exceeded. Such

property of material is important in forming, shaping, extruding and many other hot or cold

working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel

is plastic at forging temperature. This property generally increases with increase in temperature

of materials.

8. Ductility

Ductility is termed as the property of a material enabling it to be drawn into wire with

the application of tensile load. A ductile material must be strong and plastic. The ductility is

usually measured by the terms, percentage elongation and percent reduction in area which

is often used as empirical measures of ductility. The materials those possess more than 5%

elongation are called as ductile materials. The ductile material commonly used in engineering

practice in order of diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin

and lead.

9. Malleability

Malleability is the ability of the material to be flattened into thin sheets under applications

of heavy compressive forces without cracking by hot or cold working means. It is a special

case of ductility which permits materials to be rolled or hammered into thin sheets. A

malleable material should be plastic but it is not essential to be so strong. The malleable

materials commonly used in engineering practice in order of diminishing malleability are

lead, soft steel, wrought iron, copper and aluminium. Aluminium, copper, tin, lead, steel, etc.

are recognized as highly malleable metals.

10. Hardness

Hardness is defined as the ability of a metal to cut another metal. A harder metal can

always cut or put impression to the softer metals by virtue of its hardness. It is a very

important property of the metals and has a wide variety of meanings. It embraces many

different properties such as resistance to wear, scratching, deformation and machinability etc.

11. Brittleness

Brittleness is the property of a material opposite to ductility. It is the property of

breaking of a material with little permanent distortion. The materials having less than 5%

elongation under loading behavior are said to be brittle materials. Brittle materials when

subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron,

brass and ceramics are considered as brittle material.

Porperties and Testing of Metals 121

12. Creep

When a metal part when is subjected to a high constant stress at high temperature for

a longer period of time, it will undergo a slow and permanent deformation (in form of a crack

which may further propagate further towards creep failure) called creep.

13. Formability

It is the property of metals which denotes the ease in its forming in to various shapes

and sizes. The different factors that affect the formability are crystal structure of metal, grain

size of metal hot and cold working, alloying element present in the parent metal. Metals with

smal1 grain size are suitable for shallow forming while metal with size are suitable for heavy

forming. Hot working increases formability. Low carbon steel possesses good formability.

14. Castability

Castability is defined as the property of metal, which indicates the ease with it can be

casted into different shapes and sizes. Cast iron, aluminium and brass are possessing good

castability.

15. Weldability

Weldability is defined as the property of a metal which indicates the two similar or

dissimilar metals are joined by fusion with or without the application of pressure and with

or without the use of filler metal (welding) efficiently. Metals having weldability in the

descending order are iron, steel, cast steels and stainless steels.

7.2 RECOVERY, RECRYSTALLISATION AND GRAIN GROWTH

When metal is subjected to hot working and cold working processes, plastic deformation

occurs which is an important phenomenon. Plastic deformation of metal distorts the crystal

lattice. It breaks up the blocks of initial equiaxed grains to produce fibrous structure and

increases the energy level of metal. Deformed metal, during comparison with its un-deformed

state, is in non-equilibrium, thermodynamically unstable state. Therefore, spontaneous

processes occur in strain-hardened metal, even at room temperature that brings it into a

more stable condition. When the temperature of metal is increased, the metal attempts to

approach equilibrium through three processes: (i) recovery, (ii) recrystallisation, and (iii) grain

growth. Fig.7.1 reflects the recovery, recrystallisation and grain growth and the main property

changes in each region.

7.2.1 Recovery

When a strain-hardened metal is heated to a low temperature, the elastic distortions of the

crystal lattice are reduced due to the increase in amplitude of thermal oscillation of the

atoms. This heating will decrease the strength of the strain-hardened metal but there is an

increase in the elastic limit and ductility of metal, though they will not react the values

possessed by the initial material before strain-hardening. No changes in microstructure of

metal are observed in this period. The partial restoration of the original characteristics,

produced by reducing the distortion of the crystal lattice without remarkable changes in

microstructure, is called recovery. At the initial state, the rate of the recovery is fastest and

it drops off at longer times at given temperature. Hence the amount of recovery that occurs

in a practical time increases with increasing temperature. The individual characteristic recover

at different rates and gain various degrees of completion in a given cold worked metal.

122 Introduction to Basic Manufacturing Processes and Workshop Technology

7.2.2 Recrystallisation

Formation of new equiaxed grains in the heating process of metal, instead of the oriented

fibrous structure of the deformed metal, is called recrystallisation. The process of

recrystallisation is illustrated through Fig. 7.1. The first effect of heating of metal is to form

new minute grains and these rapidly enlarge until further growth is restricted by grain

meeting another. The original system of grains go out of the picture and the new crystallized

structure is formed in the metal. Recrystallisation does not produce new structures however

it produces new grains or crystals of the same structure in the metal. It consists in having

the atoms of the deformed metal overcome the bonds of the distorted lattice, the formation

of nuclei of equiaxed grains and subsequent growth of these grains due to transfer of atoms

from deformed to un-deformed crystallites. Finer grains get refined and acquire a shape

resembling fibres. The temperature at which crystallization starts, that is new grains are

formed, is called recrystallisation temperature. Recrystallisation temperature is also

defined as that temperature at which half of the cold worked material will recrystallise in 60

minutes.

Internal

residual

stress

Strength

hardness

ductilityGrain growth

Strength

Ductility

Hardness

Cold worked and reworked

Recovery

New grains

Recrystallization Grain growth

Fig. 7.1 Recovery, recrystalisation and grain growth

7.2.3 Grain Growth

On recrystallisation of metal, the grains are smaller and somewhat regular in shape. The

grains in metal will grow if the temperature is high enough or if the temperature is allowed

to exceed the minimum required for recrystallisation and this growth of grain is the result

of a tendency to return to more stable and larger state. It appears to depend primarily on

the shape of the grain. For any temperature above the recrystallization temperature, normally

there is practical maximum size at which the grains will reach equilibrium and cease to grow

significantly. However, there are certain kinds of abnormal grains growth in metal that occur

as a result of applied or residual gradients of strain due to non-uniform impurity distribution,

and which permits growing very large single grain in metal.

7.3 TESTING OF METALS

Metal testing is accomplished for the purpose of for estimating the behavior of metal under

loading (tensile, compressive, shear, tortion and impact, cyclic loading etc.) of metal and for

Porperties and Testing of Metals 123

providing necessary data for the product designers, equipment designers, tool and die designers

and system designers. The material behavior data under loading is used by designers for

design calculations and determining weather a metal can meet the desired functional

requirements of the designed product or part. Also, it is very important that the material

shall be tested so that their mechanical properties especially their strength can be assessed

and compared. Therefore the test procedure for developing standard specification of materials

has to be evolved. This necessitates both destructive and non-destructive testing of materials.

Destructive tests of metal include various mechanical tests such as tensile, compressive,

hardness, impact, fatigue and creep testing. A standard test specimen for tensile test is

shown in Fig. 7.2. Non-destructive testing includes visual examination, radiographic tests,

ultrasound test, liquid penetrating test and magnetic particle testing.

Shoulder

Len

Gau

Len

R = Fillet Radius

D = Diameter

Fig. 7.2 Tensile test specimen

7.3.1 Tensile test

A tensile test is carried out on standard tensile test specimen in universal testing machine.

Fig. 7.3 shows a schematic set up of universal testing machine reflecting the test

specimen griped between two cross heads. Fig. 7.4 shows the stress strain curve for

ductile material. Fig. 7.5 shows the properties of a ductile material. Fig. 7.6 shows the

stress strain curves for wrought iron and steels. Fig. 7.7 shows the stress strain curve

for non ferrous material.

Upper Cross

Head

Test

Specimen

Operatin

Hand Wheel

Lockin

Liver

Adjustable

Cross Head

Square Threaded

Screws

Operatin

Hand

Wheel

Strain Gau

e or

Extensionmeter

Column

Stress

Strain

A – Limit of proportionalit

B – Elastic limit

C – Yield point

D – Maximum stress point

E – Breakin

of fracture point

Fig. 7.3 Schematic universal testing machine Fig. 7.4 Stress strain curve for ductile material

124 Introduction to Basic Manufacturing Processes and Workshop Technology

Elastic

Soft

Hard

Brittle Tou

Stron

Weak

Stress

Strain

Fig. 7.5 Properties of a ductile material

Tool Steel

Crucible Steel

Medium Steel

Mild Steel

Wrou

ht Iron

Stess

Strain

Hard Brass

Annealed Brass

Rolled Aluminium

Strain

Stress

Rolled Annealed Copper

Aluminum Bronze

Fig. 7.6 Stress strain curves for wrought Fig. 7.7 Stress strain curves for non-ferrous material

iron and steel

7.3.2 Compression Test

Compression test is reverse of tensile test. This test can also be performed on a universal

testing machine. In case of compression test, the specimen is placed bottom crossheads. After

that, compressive load is applied on to the test specimen. This test is generally performed for

testing brittle material such as cast iron and ceramics etc. Fig. 7.8 shows the schematic

compression test set up on a universal testing machine. The following terms have been

deduced using figures pertaining to tensile and compressive tests of standard test specimen.

Hook’s Law

Hook’s law states that when a material is loaded within elastic limit (up to proportional

limit), stress is proportional to strain.

Strain

Strain is the ratio of change in dimension to the original dimension.

Porperties and Testing of Metals 125

Lockin

Lever

Lower

Cross-

Head

Column

Square Threaded

Screw

Compression

Plates

Beds

TableTest Piece

Hand

Wheel

Fig. 7.8 Schematic compression test set up on a universal testing machine

Tensile Strain

The ratio of increase in length to the original length is known as tensile strain.

Compressive Strain

The ratio of decrease in length to the original length is known as compressive strain.

Modulus of Elasticity

The ratio of tensile stress to tensile strain or compressive stress to compressive strain is

called modulus of elasticity. It is denoted by E. It is also called as Young’s modulus of elasticity.

E = Tensile Stress/Tensile Strain

Modulus of Rigidity

The ratio of sheer stress to shear strain is called modulus of rigidity. It is denoted by G.

G = Shear Stress/Shear Strain

Bulk Modulus

The ratio of direct stress to the volumetric strain (ratio of change in volume to the

original volume is known as volumetric strain) is called Bulk modulus (denoted by K).

K = Direct stress/volumetric strain

Linear and Lateral Strain

When a body is subjected to tensile force its length increases and the diameter decreases.

So when a test specimen of metal is stressed, one deformation is in the direction of force

which is called linear strain and other deformation is perpendicular to the force called lateral

strain.

126 Introduction to Basic Manufacturing Processes and Workshop Technology

Poisson’s Ratio

The ratio of lateral strain to linear strain in metal is called poisson’s ratio. Its value is

constant for a particular material but varies for different materials.

Proof Resilience

The maximum amount of energy which can be stored in an elastic limit is known as

proof resilience.

Modulus of Resilience

The proof resilience per unit volume of a material is modulus of resilience or elastic toughness.

7.3.3 Testing of Hardness

It is a very important property of the metals and has a wide variety of meanings. It embraces

many different properties such as resistance to wear, scratching, deformation and machinability

etc. It also means the ability of a metal to cut another metal. The hardness of a metal may

be determined by the following tests.

(a) Brinell hardness test

(b) Rockwell hardness test

(d) Shore scleroscope

Fig. 7.9 shows Rockwell hardness testing machine.

1. Indicator.

2. Indentor holder.

3. Indentor.

4. Screw.

5. Screw wheel.

6. Wei

ht.

7. Load.

Fig. 7.9 Rockwell hardness testing machine

7.3.4 Testing of Impact Strength

When metal is subjected to suddenly applied load or stress, it may fail. In order to assess the

capacity of metal to stand sudden impacts, the impact test is employed. The impact test

measures the energy necessary to fracture a standard notched bar by an impulse load and

as such is an indication of the notch toughness of the material under shock loading. Izod test

and the Charpy test are commonly performed for determining impact strength of materials.

These methods employ same machine and yield a quantitative value of the energy required

to fracture a special V notch shape metal. The most common kinds of impact test use notched

specimens loaded as beams. V notch is generally used and it is get machined to standard

specifications with a special milling cutter on milling machine in machine shop. The beams

may be simply loaded (Charpy test) or loaded as cantilevers (Izod test). The function of the

Porperties and Testing of Metals 127

V notch in metal is to ensure that the specimen will break as a

result of the impact load to which it is subjected. Without the

notch, many alloys would simply bend without breaking, and it

would therefore be impossible to determine their ability to absorb

energy. It is therefore important to observe that the blow in Charpy

test is delivered at a point directly behind the notch and in the

Izod test the blow is struck on the same side of the notch towards

the end of the cantilever. Fig. 7.10 shows the impact testing set

up arrangement for charpy test. The specimen is held in a rigid

vice or support and is struck a blow by a traveling pendulum that

fractures or severely deforms the notched specimen. The energy

input in this case is a function of the height of fall and the weight

of the pendulum used in the test setup. The energy remaining

after fracture is determined from the height of rise of the pendulum

due to inertia and its weight. The difference between the energy

input and the energy remaining represents the energy absorbed by

the standard metal specimen. Advance testing setups of carrying out

such experiments are generally equipped with scales and pendulum-

actuated pointers, which provide direct readings of energy absorption.

7.3.5 Testing of Fatique

Material subjected to static and cyclic loading, yield strength is the main criterion for product

design. However for dynamic loading conditions, the fatigue strength or endurance limit of

a material is used in main criteria used for designing of parts subjected to repeated alternating

stresses over an extended period of time. Fig 7.11 shows a fatigue test set up determining

the fatigue strength of material. The fatigue test determines the stresses which a sample of

material of standard dimensions can safely endure for a given number of cycles. It is performed

on a test specimen of standard metal having a round cross-section, loaded at two points as

a rotating simple beam, and supported at its ends. The upper surface of such a standard test

specimen is always in compression and the lower surface is always in tension. The maximum

stress in metal always occurs at the surface, halfway along the length of the standard test

specimen, where the cross section is minimum. For every full rotation of the specimen, a

point in the surface originally at the top centre goes alternately from a maximum in compression

to a maximum in tension and then back to the same maximum in compression. Standard test

specimens are tested to failure using different loads, and the number of cycles before failure

is noted for each load. The results of such tests are recorded on graphs of applied stress

against the logarithm of the number of cycles to failure. The curve is known as S-N curve.

Test Specimen

Ball

Bearin

Shaft

Shut Off Switch

Wei

hts

Motor

Flexible Couplin

Revolution

Counter

Fig. 7.11 Schematic fatigue test setup

Specimen

Fig. 7.10 Schematic impact

testing machine setup