41

3. PROCESS PARAMETERS

The data obtained in a uniaxial tensile test has limited use for metal forming calculations as

metal forming conditions are far different from those available in uniaxial tensile test.

Stressstrain characteristic are affected by two types of factors.

1. Factors related to deformation process

(i) amount of deformation

(ii) strain rate

(iii) operating temperature (t)

2. Factors not related to deformation process

(i) chemical composition and metallurgical structure

(ii) average grain diameter

(iii) pre strain history of materials.

I. AMOUNT OF DEFORMATION

Flow stress increases with amount of deformation and relationship between these is defined by

Hollomon equation and Ludwik equation.

………. Hollomon equation

……… Ludwik equation

42

Fig. 1

n = 1 is valid for metals which are heavily deformed.

Larger the amount of deformation, greater will be value of flow stress.

Table 1

Typical values for k and ‘n’ at room temperature.

Material K(MPa) N

Aluminium

11000

2024T4

60610

180

690

200

0.2

0.17

0.2

43

Brass

70/30 annealed

85/15 cold rolled

copper, annealed steel

900

580

315

0.5

0.33

0.55

Low Carbon

4135 annealed

4135 cold rolled

304 stainless steel (annealed)

410 stainless steel (annealed)

C 60 quenched/temnp

530

1015

1100

1275

960

1600

0.25

0.18

0.14

0.45

0.1

0.1

Table 2

1. Strain hardening exponent ‘n’ is useful in determining the behaviour of materials during

many working operations.

2. When deformation exceeds abilities of a material to undergo uniform straining, strain get

localized and necking takes place. The strain hardening exponent is a measure of materials

abilities to distribute strain uniformly and resist localization of strain and thereby delays

necking.

44

Materials which have a high work hardening exponent such as copper and brass (n

can be given a large plastic deformation move easily than those which have smaller

n, such as heat treat steel . Materials having higher n value are desirable in wire

drawing.

3. ‘n’ represent limiting strain for uniform and homogenous deformation.

4. High value of 302 austenitic steel (n = 0.3) n is an indication of poor machinability. This is

because the cutting action of the tool causes strain hardening a head of the tool. Due to the

high ‘n’ value, this causes a large increase in strength and hardness. Thus the cutting tool is

always working against higher strength material, reaviring larger cutting forces.

5. In contrast, a high value is desirable for sheet formation, in which resistance to local

necking or reduction in sheet thickness is necessary. When a high ‘n’ value material begins

to neck, the deforming region rapidly strain hardens, causing subsequent plastic

deformation to occur in the surrounding softer metal. This produces a long diffusion neck.

In contrast, neaking in a material with a low value occurs move locally, causing failure at a

lower strain.

Fig. 2

Comments on Hollomon equation

1. It is found that the work hardening of many metals approximates to a parabolic form. For

annealed metals with cubic lattice, the stressstrain relation is well defined by Hollomon

45

equation. For pre-worked materials, this equation gives less accurate results. The values of

stress are valid from yield point to maximum load point.

2. Strain hardening exponent for many metals range between 0 to 0.5. It is zero for non

hardening rigid plastic metals. The higher the value n, more pronounced is the strain

hardening of the material.

3. The stretching capacity [strain forming, deep drawing] of metals is related to its ability to

delay or resist necking. One measure of this resistance to neck is strain-hardening

exponent. The higher the value of ‘n’, larger is the uniform elongation and greater is

resistance to necking. A higher value of n improves the ability of metal to resist

localisation of strain in presence of stress gradient. This generates move uniform

distribution of strain and permits more effective utilisation of available metal.

A high value of n indicates good stretch formability and cold formed grade steels have n

value in the range of 0.22 to 0.5. On the other hand, hot rolled materials can have n values

as low as 0.1 and may undergo excessive thinning or fracture in severely strained region of

processing.

Time does not enter as a parameter and ordinarily, the deformation characteristics is taken

as independent of time.

4. A simple guide for calculating n for steels is

n =

5. Many a times, strain hardening exponent is considered as strain hardening rate, which is

not true.

Taking derivative

=

46

strain hardening rate

strain hardening rate is not simply equal to n but equal to .

6. Limitations of Hollomon equation:

i. This equation is quite reliable when induced strain is greater than 0.04 but less than the

strain at which necking begins.

ii. Use of this equation to predict initial yield strength of the metal should be avoided.

Instead, a method such as offset should be used.

iii. Most metal working operations imparts strain far in excess of 0.04 and the exclusion of

the elastic and transition strain region leads to little error in this regard.

II. EFFECT OF TEMPERATURE ON FLOW STRESSFlow stress in influenced by temperature. Flow stress decreases with increase in temperature.

Ductilities also increases with temperature. This characteristic is fully utilised in hot working

where large reduction are obtained at relatively lower flow stresses.

Fig. 3 : Effect of temperature on flow stress.

3. Strain rate

Strain rate does not influences flow stress in cold working. It is a significant factor in

determing flow stress in hot working. Flow stress increases with strain rate and dependence of

flow stress on strain rate increases with temperature.

47

Fig. 4

where m = strain rate sensitivity exponent

c = strain rate strength constant

48

As the temperature increases the slope of the curve increases. Thus the strength becomes move

and more sensitive to strain rate as temperature increases. Slope is relatively flat at room

temperature

m

Cold working upto 0.05

Hot working 0.05 to 0.4

Super plastic 0.3 to 0.85

High the value of m, more the capacity of the material to delay the necking and more stretching

is possible at elevated temperature before failure.

2. When strain rate is of high order, the stressstrain curve change as a function of strain rate

in accordance with equation

m = 0 stress is independent of strain rate

m = 0.2 common metals

= 0.4 to 0.9 superplastic metals

Fig, 5

The dependence of flow stress on strain rate increases with temperature.

Larger the value of m, more postponement of necking will be. The reason for this is that as soon

as necking starts in some region, the strain rate increases locally resulting in a rapid increase

49

in the stress required to cause further deformation in that region. The deformation then shits to

other region of the material, where there is no necking. Here the strain rate and hence the stress

to cause deformation is smaller stainless steel, aluminium and titanium alloys exhibit

superplastic behaviour.

Principal effects of strain rate in metal forming: -

1. Flow stress of metal increases with strain rate, especially at temperatures above

recrystallisation temperature.

2. At higher strain rates, the temperature of workpiece increases abnormally as there is little

time available for heat transfer. It is adiabatic heating.

3. Lubrication at the tool-work interface improves as long as lubrication is maintained – At

higher strain rates there is possibility of breakage of lubrication film, resulting in poor

surface finish.

Deformation velocity of commercial equipments used in metal forming is much higher than

observed in tension test. For wire drawing at speed of 40 m/s result in strain rate of

However, there is a group of newer metal working processes which utilises velocities as high as

200 m/s to carry out forging, sheet forming, extrusion etc. They are known as High Velocity

Forming [HVF] and High Energy Rate Forming processes. These processes have strain rate

much higher than the conventional methods. For many materials elongation limit (upto fracture)

increases with strain rate, upto limit. Beyond this limit of strain rate ductility falls sharply, this

limit is called as critical strain rate.

At the other extreme of strain rate spectrum, there is superplastic forming. Materials having high

strain rate sensitivity index ‘m’ (0.3 <m exhibits pronounced resistance

for necking. Superplasticity behaviour is observed when operating temperature is above 0.4 Tm

and strain rates are below

Very fine grain (grain size <10 m)

High operating temperature (T > 0.4 Tm)

Very low strain rate (

The chief advantages of superplasticity

50

(i) Very low How stress 5 – 30 MPa

(ii) Very large deformations can be obtained.

The effect of m on hot deformation behaviour is somewhat analogous to that of strain hardening

exponent for cold deformation. A high ‘m’ value causes a considerable increase in strength and

hardeness of the material at high strain rates, leading to a requirement for higher forming forces.

Alternatively, for high m materials a slow strain rate is necessary for forming. This can lead to

unacceptably long and often, uneconomical forming time. An important advantage of a high m

value, like a high n value is improved formability resulting from forming a diffuse neck rather

than a local neck.

A positive value of ‘m’ reduces localisation of strain and thus necking. A large and positive

value of ‘m’ opposes rapid localisation of strain and causes neck to be more diffused. In reverse

way, a negative value of ‘m’ promotes localisation of strain and thus generate severe strain

gradient. Thus both sign and magnitude of ‘m’ is important.

In sum a positive value of strain rate sensitivity index.

1. Higher stresses are required to form part at higher strain rates.

2. At a given forming rate, the material resists further deformations in regions that are being

strained more rapidly than adjacent region by increasing the flow stress in these regions.

This helps in distributing strain more uniformly.

In many forming operation, need for higher stress for deformation is not a major consideration

but ability to distribute strain uniformly is one. Generally, metal have value – 0. 01 to 0. 06.

Metals in superplastic region have high ‘m’ values, which is one to two orders higher than

typical steel. High ‘m’ and ‘n’ values is of little use in deep drawing as they strengthen wall as

well as flange which make it harder to draw.

Mean strain rate for various metals forming processes:

1. For up setting and tension test

51

v = cross head velocity

h =

instantaneous height of the specimen.

2. For extrusion and wire drawing

V = extrusion velocity

d billet diameter

extrusion diameter

R = extrusion ratio.

3. For rolling

thickness of slab before rolling

thickness of slab after rolling.

Superplastic behaviour of metals and alloys

Conventioned metals and alloys exhibit elongation in the range of 10 to 60%. Some metals and

alloys exhibit very large deformations, more than 100% and even upto 3000% with out fracture.

This behaviour of metals is termed as superplastic behaviour. It has enabled the economical

production of large, complex shaped products with compound curves. Deep or complex shapes

can now be made as one piece [rather than joining /welding many pieces into a assembly], single

operation pressing rather than multistip conventional pressings. Precision is excellent and fine

details or surface texture can be reproduced accurately. Springback and residual stresses are non

existent and products have a fine, uniform grain size.

Main characteristics of superplastic behaviour are:

i. Very large deformations are obtained without fracture. It permits forming of metals as if

they are polymer or glass.

ii. The other characteristics normally observed concurrently is substantial reduction in flow

stress. The flow stress is about 5 to 50% of the conventional method flow stress.

Following are requirements if the material is to exhibit superplstic behaviour:

52

1. The operating temperature should be more than the half of melting point.

T > 0.5 Tm Tm = melting point k.

2. Small and stable grain size. Grain size should be less than 10 m. The presence of second

phase particles inhibit grain growth at high operating temperature. Further, the strength of

second phase should be similar to that of matrix to avoid excessive cavity formation.

3. Flow stress of superplastic material is very sensitive to the strain rates. High value of strain

rate sensitivity index is necessary. Strain rate sensitivity index should be in the range of

0.3 to 0.9

i.e. 0.3 < m < 0.9

4. Superplasticity is observed within specified range of strain rates. Ductility reduces

dramatically on either side strain rate range is of this range.

Superplastic deformation occurs predominantly by grain boundary sliding and grain

rearrangement. Both of these mechanisms require a large grain area and hence the need for a

small grain size. They are accommodated by grain boundary diffusion which is a temperature

activated process and hence the requirement for elevated temperature.

The strain rate sensitivity index (m) itself is function of strain rates and strain rate range should

be Within this range of strain rate, m is sufficiently large for superplastic

behaviour of metal. It is observed that the high value of m is obtainable with fine grain

microstructure or m value increases with decrease in grain size. Such a fine grain structure is

easily obtained and maintained in eutectic and eutectoid alloys. For these materials different

microstructure/phases are formed simultaneously at subeutectic or subeutectoid temperatures and

then precipitate in a fine dispersion. It is, however, demonstrated that superplastic behaviour is

not confined to these two phase structures, but even fine grained pure metals also exhibit the

same.

53

A characteristics feature of superplastic deformation is that large macroscopic elongations are

possible without significant elongation of individual grains. In superplstic alloy, grain boundary

sliding constitutes the greatest contribution to deformation in superplastic region.

Fig. 6 : Dependence of m on strain rate.

Superplastic alloys

Small and stable grain size is requirement for superplastic behaviour of metal and therefore not

all commercially available alloys are superplastic. Following alloy exhibit superplastic

behaviour.

Alloy Temperature m % elongation

Titanium

Ti – 6 Al – 4V

Ti – 6 Al – 4V- 2Ni

850

815

0.75

0.85

750 – 1150

720%

Aluminium

Al – 33 Cu 450 0.8 400 – 100%

Zn 78% + 22% Al - 0.5

Al 67% + 33% Cu - 0.9

Cu 90 + 10% Al 500 - 0.5

54

Advantages

1. Commercial development of superplastic materials has made it possible the production of

large, complex shaped products often in limited avantity economical. Deeper or complex

shaped can be made from one piece in a single operation rather than multistep conventional

pressing or multipiece assemblies.

2. Drastically deduced flow stress of the material.

3. Because of the low forming pressures, forming blocks can be used in place of die set and

hence tooling is relatively inexpensive.

4. It requires shorter production lead time.

5. Many applications of superplasticity eliminates a considerable number of subsequent

operations. The weight of the products can be reduced and there are fewer fastening holes.

These holes generally initials cracks on repeated loads.

6. Precision obtained is excellent and fine details are produced products have fine and

uniform grain size.

Limitation

A major limitations to superplastic forming is the low forming rate that is required to maintain

superplastic forming, cycle time range from 2 min to 2 hours per part is too long compared to

several seconds that is typical of conventional press work. As a result, application tend to be

limited to low volume products such as those common to aerospace industry. By making

products larger and eliminating assembly operations, the weight of products can be reduced.

There are fewer fastening holes to initiate fatigue cracks, tooling and fabrication costs are

reduced.

Strain hardening

Strain hardening is a phenomenon whereby the yield stress of a metal increases with increasing

deformation (strain). It occurs at low temperature, below recrystallisation temperature which is

55

about 0.5 Tm (Tm is melting point temperature K). This applies to forming temperatures that are

so low that thermally activated processes play no significant role. Strain hardening results in

higher forming force and forming energy requirement, thus increasing load/stresses acting on the

tool. Strain hardening increases strength and hardness but cause decrease in ductility and

therefore in many cases, annealing become necessary to restore ductility and formability to

obtain the required deformation. To negate the effect of strain hardening, forming can be carried

out at elevated temperatures, the accuracy and surface quality obtained would be inferior to the

one obtained in cold forming.

Besides these undesirable side effects of strain hardening there is an increase in the strength

values of the finished components through forming which is very desirable. Strain hardening

can be used for practically all metals and alloys to increase hardness and strength. The increased

strength permits the use of materials with lower initial strength compared to components

produced by machining. Moreover, in many cases heat treatment (costly and time consuming) is

unnecessary because of strain hardening.

Strain, hardening is a result of a large number of dislocations participating simultaneously.

During metal forming the dislocation density increases by several order from

By this zones of higher dislocation density emerge, which represent a hindrance for moving

dislocations. Presence of dislocations lowers the shear stress required to cause slip. But

dislocations can:

(i) become entangled and interfere with each other.

(ii) be impeded by barriers, such as grain boundaries and inclusions in the material.

As dislocation density increases, the stress required for moving dislocation increases due to

interfering effect of stress fields surrounding the dislocations. Entanglement and impediments

increase the shear stress required for slip. The effect is an increase in shear stress that causes an

increase in the overall strength of the metal, known as strain hardening or work hardening. This

phenomenon forms basis for work hardening, described by means of equation:

The shear stress to move a dislocation increases with increasing dislocation density e,

according to the following equation.

56

where = base stress to move the dislocation in the crystal in the absence of other dislocations.

A = constant

This equation describes the work hardening behaviour. For a soft crystal, the CRSS (critically

resolved shear stress) for initiation of plastic deformation is typically 0.5

A = 0.01 N/mm

low dislocation density =

CRSS for annealed crystal =

= 1.5 N/mm

heavily cold worked, the dislocation density increases to

= 100

Strain hardening depends on :

1. Lattice structure. Strain hardening rate is greater for cubic crystal.

(i) Strain hardening rate in FCC metals is affected more than for HCP crystals by stacking

fault energy. Therefore, copper, nickel, austenitic steel harden more rapidly than

aluminium.

(ii) HCP (Hexagonal close packed) metals subjected to “twinning” and strain hardening rate is

much rapid as there is only one plane for easy glide.

(2) Strain hardening rate increases with complexity of structure impurities, grain size, second

phase particles etc. Therefore, final strength of the cold worked solid solution alloy is

always greater than that of pure metal cold worked to the same extent.

57

Strain hardening capacity

Two measures of strain hardening capacity

i.

ii. uniform elongation eu which is the elongation at max load.

n = log (1 + eu)

value of ‘n’ is highest when the material is normalised. It is lowered by cold working. A

typical low carbon steel has ‘n’ value 0.2 to 0.22. A value of 0.25 is considered high for

these steels while those below 0.18 are considered to have low ductility and poor strain

hardening capacity.

Strain hardening is an important industrial process used for strengthening and hardening metals

[Cu, Al] or their alloys which do not respond to heat treatment process. Of course, the product

must not be used at temperature that will anneal the metals.

Strain hardening has important place in industry.

i. It alters the properties of metal. Cold working improves strength and hardness of metal, but

reduces elongation.

Fig. 7

Cold work reduces the amount of plastic deformation that a metal can undergo

subsequently during shaping operation. The hardened, less ductile, cold worked metal

require more power for further working and is subjected to cracking. Therefore cold work

anneal cycles are used to assist production i.e. formation limit is extended through

58

annealing. The loss of ductilities has useful side effect improvement in machinability by

25% in lowmedium carbon steel. With less ductility, chips break move readily thus

facilitating cutting operation. Cold work stock is used for machining screw stock.

ii. The preferential orientation of second phase particles and structural discontinuities [voids,

inclusions, segregations] in the principal direction of deformation give rise to mechanical

fibering. A important consequence of mechanical fibering is that mechanical properties in

different directions. In general, tensile strength, ductility,

fatigue strength is more in longitudinal direction than transverse. But shear strength is

greater in transverse direction.

FRICTION IN METALS

Friction is defined as the resistance to relative motion between two bodies in contact, under a

normal load. Friction plays an important role in metalworking and manufacturing processes

because of the relative motion and forces that are always present on tools, dies, and workpieces.

Friction dissipates energy, thus generating heat, which can have detrimental effects on an

operation. Furthermore, because friction impedes free movement at interfaces, it can

significantly affect the flow and deformation of materials in metalworking processes. On the

other hand, friction is not always undesirable; without friction, for example, it would be

impossible to roll metals, clamp workpieces on machines, or hold drill bits in chucks.

Friction plays a great role in all engineering applications whenever solid surfaces are in sliding

contact with each other. This is particularly true in metal working processes where the sliding

pair of surfaces are metals and where plastic deformation of the softer of two metals usually

takes place. Friction conditions between the deforming tool and workpiece in metal working of

greatest importance as it decides force required and mode of deformation, properties of the

finished specimen, resulting surface finish etc.

The friction stress, is measured in force per unit area. The surface area of contact is a

boundary of the deformed metal.

I. Coulomb’s Friction Law

59

F = N

F = Frictional force

A = Apparent area of contact

Fig. 8

The tangential stress, Tat any point on that surface is proportional to the pressure p between the

two bodies and is directly in opposite direction to the relative motion between these bodies. The

coefficient of friction ‘’ is taken as constant for given die and the workpiece (under constant

surface and temperature conditions) and is said to be independent of the velocity, applied load,

and area of contact.

Fig. 9

P > k

It is expected that this relation is sufficient to describe the conditions until the product of and P

(P) becomes higher than the yield stress in pure shear (k) of the material. The material will then

T= P

60

stick to the tool and yielding takes place in the interior of the material. Modern theory of friction

is based on premise that flat surface is not flat but consists of numerous peaks and valleys.

61

Apparent Actual surface

Fig. 10

A commonly accepted theory of friction is the adhesion theory, developed by F.P. Bowden

(1903 1968) and D. Tabor (1913 ). The theory is based on the observation that two clean and

dry surfaces, regardless of how smooth they are, contact each other (junction) at only a fraction

of their apparent area of contact (Fig. 8). The maximum slope of the hills on these surfaces

ranges typically between

In such a situation, the normal (contact) load, N, is supported by the minute asperities (small

projections from the surface) that are in con tact with each other. The normal stresses at these

asperities are, therefore, high; this causes plastic deformation at the junctions. Their contact

creates an adhesive bond: the asperities form microwelds. Cold pressure welding is based on this

principle.

Sliding motion between two bodies which have such an interface is possible only if a tangential

force is applied. This tangential force is the force required to shear the junctions; it is called the

friction force, F. The ratio of F to N (see Fig. 8) is the coefficient of friction, .

62

Fig. 11: Schematic illustration of the interface of two bodies in contact, showing real areas of

contact at the asperities. In engineering surfaces, the ratio of the apparent to real areas of

contact can be as high as 4 5 orders of magnitude.

In addition to the force required to break these junctions by shearing, a plowing (or ploughing)

force can also be present if one surface scratches the other (abrasive action). This force can

contribute significantly to friction at the interface. Plowing may (a) cause displacement of the

material and/or (b) produce small chips or slivers, as in cutting and abrasive processes.

Depending on material and processes involved, coefficients of friction in manufacturing vary

significantly, as is obvious in Table 1.

Table 1: Range of Coefficients of Friction in Metalworking Processes

Coefficient of friction ()

Process Cold Hot

Rolling 0.05 0.1 0.2 0.7

Forging 0.05 0.1 0.1 0.2

Drawing 0.03 0.1

Sheetmetal forming 0.05 0.1 0.1 0.2

Machining 0.5 2

Almost all of the energy dissipated into heat (a small fraction becomes stored energy in the

plastically deformed regions; raising the interface temperature. The temperature increases with

friction, sliding speed, decreasing thermal conductivity, and decreasing specific heat of the

63

sliding materials. The interface temperature may be high enough to soften and even melt the

surfaces, and sometimes to cause microstructural changes.

Temperatures also affects the viscosity and other properties of lubricants, causing their

breakdown. Note, for example, how butter and oil burn and are degraded when temperatures are

excessive. These results, in turn, adversely affect the operations involved, and cause surface to

the object.

When two surfaces are in touch, sufficient contact is established to support the applied load.

A = Apparent area of contact After application of

load

Ar = real area of contact

Fig. 12

Ar, real area of contact increases with increases in load.

yield strength of weaker material in contact.

Large stresses and plastic deformation causes the tear off upper contaminated layers present on

the surfaces. Then the real materials come in contact. This results in welding of the asperity

junction and the sliding of one body above the other will be possible only after these asperity

junctions are sheared:

Force required to shear T = shear strength of weak

64

From this, depends only on material in contact. For better results, should be taken for

alloy formed at the junction due to heavy cold work and welding. But is valid only when

As Ar attains the value of A (apparent area), the force required to slide will not

increase even if N is increased. Under such situation, mechanism other than welding of asperity

junctions become active making friction phenomenon quite complex. One such mechanism is

locking of asperities.

Resistance to sliding motion is due to:

i. The force necessary to plough the peaks of harder material through softer material.

ii. The force required to break welding weldaments.

Ar < A

Fig. 13

2. When real area of contact approximates apparent area, movement w.r.t. each other

continues but due to subsurface shearing in softer and weaker material. The frictional

force, then, equals the shear strength of the material.

65

Fig. 14

Such type of friction is called “sticking friction”.

(As per Von miser) yield strength of softer material [Aluminium].

P = normal pressure.

A very low value of ‘’ = 0.05 is possible with highly polished tool surface and flood lubrication

with soluble oils. ‘’ between 0.05 to 0.15 are usually found in cold working operations such as

wire drawing, tube drawing and extrusion while rolling thin strips

with mirror finish rolls, a value of 0.05 is common. However, in cold rolling with good

lubrication can range between 0.07 0.15. In hot rolling, presence of scale increases the value

of ‘’and depends on rolling temperature. A value of 0.4 is quite common in rolling at

Amonton’s law is a good approximation for ordinary sliding where

66

But in metal forming, areal area of contact approximates apparent area of contact.

Fig. 15

Zone I:

Plastic deformation is confined to asperties only and thus Amonton’s law is obeyed.

Zone II:

Ar increases with deformation but ‘’ decreased.

Zone III:

T does not vary with normal pressure and become independent of normal stress.

Sliding Friction Sticking Friction

1. One surface slides over the other

and friction exist there is no welding

of two surfaces at the contact

interface.

1. In sticking friction the metal surface

adjacent to the tool surface does not slide,

instead it moves due to shearing.

Fig.

SubSurface shearing occur in aluminium.

67

2. The definition of coefficient of

friction implies that the frictional

force is directly proportional to

normal force and there must be

relative movement between the

workpiece and die surfaces.

Interfaces at which these conditions

exists undergo sliding friction.

It is valid when

P << k

2. Under such conditions the coulomb’s law

ceases to apply and the magnitude of shear

stress is controlled by the shear strength of

the work material. For the case of sticking

friction the frictional stress

yield stress of the material in tension.

Due to high interface pressure P, the frictional

shear stress is greater than shear

stress required for shearing the workpiece

(workpiece has lower shear strength

compared to die metal). If this occurs, less

tangential force is required for metal to shear

within the body of the workpiece than for the

workpiece to move relative to the die. This is

referred as sticking friction, through no actual

sticking together of the die and workpiece

necessarily occur. (shear strength

of workpiece) when this inequality is satisfied,

‘’, the coefficient of friction becomes

meaningless. Hence sticking

friction represents an upper limit to the

interface frictional stresses.

The interface pressure developed in the most

metal working process is at least equal to

uniaxial yield stress and may appreciably

exceed K (shear strength). As K is

independent of applied pressure, is not

correct to describe sticking friction, leads to

misinterpretation.

3. Usually present in cold working. 3. Usually present in hot working.

Sticking deformation is undesirable.

i. Deformation with sticking friction requires greater energy.

68

ii. Because localised internal shearing of the workpiece occurs, results in less deformation

homogeneity compared to sliding friction.

Q. Show that the maximum meaningful friction coefficient is 0.5.

Soln:

The interface shear stress is The interface pressure P is the same as the normal

interface stress

From the Tresca or maximum shear stress yield criterion, The internal

shear stress cannot exceed because when yielding of the workpiece in

shear will occurs. Therefore the maximum = 0.5. A coefficient

of friction above 0.5 is not attainable as the shearing of the workpiece will have occurred.

Effect of friction in metal forming:

1. Force required for deformation increases with friction.

Ram travel

1. Direct extrusion

2. Indirect extrusion

3. Hydrostatic extrusion

Fig. 16

69

Fig. 17

In extrusion, friction exists between billet and die and between billet and container.

Friction between die and container is eliminated in indirect extrusion but between die and

billet exists. In hydrostatic extrusion, friction is eliminated altogether and lower amount of

force is required for extrusion.

2. Excess load capacity is provided and load perse is not important aspect from practical point

of view. Effective lubrication decreases the load requirement but it also serves other more

important functions.

i. prevention of pickup:

If too much surface contact occurs, metal pickup on the tooling can damage product

finish and size.

ii. minimizes tool wear and thus better control over size and lower maintenance cost.

iii. Frictional energy is converted into heat and raises the temperature of workpiece. To

keep the temperature at lower level, extrusion velocity is reduced in hot extrusion of

M.S.

3. Friction plays positive role in rolling. Rolling is not possible without adequate friction

between the rolls and bloombillet.

For rolling = angle of bite.

i.

ii.

for cold rolling

70

for hot rolling.

4. Proper distribution of friction improves the deformation process.

i. Deep drawing

The coefficient of friction around punch corner should be high and that around die

corner should be low. It prevents thinning of the sheet metal.

ii. Tube drawing

If the friction between tube and mandrel is high, then drawing load is shared by the

mandrel and thereby reducing the stress in the tube wall. This allows for large

reduction ratios.

Fig. 18

5. Friction causes more wastage of material especially in extrusion. The dead zone formed in

direct extrusion is result of friction and redundant work. It causes 2030 % wastage of

material which is quite large compared to indirect extrusion where friction between

container and billet is eliminated.

71

Lubrication

The surfaces of tools, dies and workpieces are subjected to

(i) forces and contact pressure, ranging from very values multiples of the yield stress of the

workpiece material.

(ii) relative speeds, from very low to very high.

(iii) temperatures, which range from ambient to melting.

Metal working fluids should be applied to reduce friction and wear effectively working

temperature. Lubrication is the process of applying these fluids and solids. There are four types

of lubrication:

Lubrication mechanism

1. Boundary 2. Thick Film 3. Thin Film 4. Mixed

Lubrication (full film lubrication) Lubrication Lubrication

Fig. Fig. Fig.

A thin layer of low shear

strength material adheres at

surface interface. Adherence

may be physical or chemical

[Sulphur, chlorine] or both.

Completely prevent

metal to metal contact.

Some metal to metal

contact occurs.

film thickness is about 10

Ra (Ra = surface

roughness)

film thickness is about 5

Ra

72

(a) Thick film

(b) Thin film

(c) Mixed

(d) Boundary

Fig. 19: Types of lubrication generally occurring in metalworking operations.

a. In thickfilm lubrication, the two mating surfaces are completely separated by a fluid film

as in hydrodynamic lubrication, and lubricant viscosity is the important factor. Such films

can develop in some regions of the workpiece in highspeed operations, and can also

develop from highviscocity lubricants that become trapped at dieworkpiece interfaces.

The film thickness is 10 Ra.

There is no metal to metal contact and therefore no wear of parts. The coefficient of friction is

between 0.001 to 0.02 and depends on viscosity of lubricant and contact pressure. Coefficient of

friction increases with normal pressure [double with every increase in pressure by 35 MPa] and

decreases with increase in temperature [every rise of temperature by decreases it to half].

Conditions favourable for thick film lubrication are somewhat rare in metal working but

they do occur at higher sliding speed. High speed wire drawing and rolling are best

examples this.

A thick lubricant film results in a dull, grainy surface appearance on the workpiece,

whereby the degree of roughness depends on grain size (Fig. 19). In operations such as

73

coining and precision forging, trapped lubricant are undesirable because they prevent

accurate shape generation.

b. Thin film lubrication

As the load between the die and workpiece increases or as the speed and the viscosity of

the metal working fluid decrease, the lubricant film becomes thinner (thin film lubrication).

The film thickness decreases to 3 to 5 Ra. This causes some metal to metal contact and

raises the friction at the sliding interfaces and result in slight wear.

c. Mixed lubrication

In most liquid lubricated metal working operations. Some asperity contact is unavoidable.

A significant part of the load is carried by the metal to metal contact and rest is carried

pockets of liquid in the valleys of the asperities. This mixed film lubrication is common in

metal working. Boundary lubricants with extreme pressure (EP) are added to the fluid at

the points of metal contact.

d. Boundary lubrication

In boundary lubrication, the load is supported by contacting surfaces covered with a

boundary film of lubricant. These are thin organic films physically adhered to the metal

surfaces or chemically adsorbed on the metal surface, thus preventing direct metal to

metal contact of the two bodies and hence reducing wear. The boundary lubricants are

typically polar substances, natural oils, fats, fatty acids, esters or

soaps. These films are firmly adhered to the metallic surfaces and have lower shear

strength, thereby sliding surfaces takes place easily.

Solid lubrication

Product resulting from the reaction of liquid lubricant and parent metal can be termed as solid

lubricant because solid phase formed of it is effective as lubricant.

Sometimes interposing film of molybdenum disulphide, graphite, teflon is used as lubricant.

These films have lower shear strength which is temperature and pressure dependent. Boundary

film forms rapidly and as the film thickness decreases, metal to metal contact occurs. Depending

74

on the boundary film shear strength and its thickness, the coefficient of friction varies between

0.1 to 0.4.

Boundary films can break as a result of:

i. desorption caused by high temperatures at the sliding interfaces

ii. being rubbed off during sliding.

Deprived of this protective film, the metal surfaces may then wear and score severely.

Other Considerations

The valleys in the surface roughness of the contacting bodies can serve as local reservoirs or

pockets for lubricants, thereby supporting a substantial portion of the load. The workpiece, not

the die, should have the rougher surface; otherwise, the rougher and harder die surface, acting as

a file, may damage the workpiece surface. The recommended surface roughness on most dies is

about 0.4 .

The overall geometry of interacting bodies is also an important consideration in ensuring proper

lubrication. The movement of the workpiece into the deformation zone, as during

wire drawing, extrusion, and rolling, should allow a supply of lubricant to be carried into the

dieworkpiece interface. With proper selection of process parameters, a relatively thick lubricant

film can be entrained and maintained.

METALWORKING FLUIDS

The functions of a metalworking fluid are to:

a. reduce friction, thus reducing force and energy requirements, and temperature rise.

b. reduce wear, seizure, and galling.

c. improve material flow in tools, dies, and molds.

d. act as a thermal barrier between the workpiece and tool and die surfaces, thus preventing

workpiece cooling in hotworking processes.

e. act as a release or parting agent, a substance which helps in the removal or ejection of

parts from dies and molds.

75

Several types of metalworking fluids are now available which fulfill these requirements and

which have diverse chemistries, properties, and characteristics. This section describes the general

properties of the most commonly used lubricants.

General properties of most commonly used lubricants

Oils have high film strength on the surface of the metal and it is effective in reducing friction and

wear. Oils have lower thermal conductivity and specific heat. Thus they are not effective in

conductivity away the heat generated by friction and plastic deformation in metal forming

operations. Oils are difficult to remove from component surfaces that are subsequently be

painted or welded, and they are difficult to dispose off. The source of oils are mineral

(petroleum), animal, vegetable and fishoils may be compounded with variety of additives or

with other oils to impart special properties.

Metal working oils are blended with several additives. Important additives are sulphur, chlorine,

and phosphorus known as EP [Extreme Pressure] additives and used singly or in combination,

they react chemically with metal surfaces and forms adherent films of metallic sulphides and

chlorides. These films have lower shearstrength and good antiweld properties and thus

effectively reduce friction and wear. While EP additives are important in boundary lubrication,

these lubricant attack the cobalt binder in tungsten carbide tools and dies, causing increase in

their roughness and integrity.

Emulsions [or water soluble fluids] are two types: direct and indirect. In direct emulsions,

mineral oil is dispersed in water as very small droplets. Direct emulsions are important fluids

because the presence of water gives them high cooling capacity and useful in high speed cutting.

Oils maintain high film strength on the surface of a metal, as we can observe when trying to

clean an oil surface. Although they are very effective in the reduction of friction and wear, oils

have low thermal conductivity and low specific heat. Consequently, they do not effectively

conduct away the heat generated by friction and plastic deformation. In addition, it is difficult

and costly to remove oils from component surfaces that are to be painted or welded, and it is

difficult to dispose of them.

76

The sources of oils can be mineral (petroleum), animal, vegetable, or fish. Oils may be

compounded with any number of additives or with other oils; this process is used to change such

properties as viscositytemperature behavior and surface tension, heat resistance, and boundary

layer characteristics. Mineral (hydrocarbon) oils with or without additives, used undiluted, are

known as neat oils.

Oils can be contaminated by the lubricants used for the slideways and various components of the

machine tools and metalworing machinery. These oils have different characteristics than those

used for the process itself, and thus can have adverse effects. When present in the metalworking

fluid itself, these oils are known as tramp oil.

Emulsions

An emulsion is a mixture of two immiscible liquids, usually of oil and water in various

proportions, along with additives. Emulsifiers are substances that prevent the dispersed droplets

in a mixture from joining together (hence the term immiscible).

Milky in appearance, emulsions are also known as watersoluble oils or waterbase coolants,

and are of two types: direct and indirect. In a direct emulsion, mineral oil is dispersed in water in

the form of very small droplets. In an indirect emulsion, water droplets are dispersed in the oil.

Direct emulsions are important fluids because the presence of water gives them high cooling

capacity. They are particularly effective highspeed machining where temperature rise has

detrimental effects on tool life, the surface integrity of workpieces, and the dimensional accuracy

of parts.

Synthetic and Semisynthetic Solutions

Synthetic solutions are chemical fluids that contain inorganic and other chemicals dissolved in

water; they do not contain any mineral oils. Various chemical agents are added to a particular

solution to impart different properties. Semisynthetic solutions are basically synthetic solutions

to which small amounts of emulsifiable oils have been added.

Soaps, Greases, and Waxes

Soaps are typically reaction products of sodium or potassium salts with fatty acids. Alkali soaps

are soluble in water, but other metal soaps are generally insoluble. Soaps are effective boundary

77

lubricants and can also form thick film layers at dieworkpiece interfaces, particularly when

applied on conversion coatings for cold metalworking applications.

Greases are solid or semisolid lubricants and generally consist of soaps, mineral oil, and various

additives. They are highly viscous and adhere well to metal surfaces. Although used extensively

in machinery, greases are of limited use in manufacturing processes.

Waxes may be of animal or plant (paraffin) origin; compared to greases, they are less “greasy”

and are more brittle. Waxes are of limited use in metalworking operations, except as lubricants

for copper and, as chlorinated paraffin, as lubricants for stainless steels and hightemperature

alloys.

Additives

Metalworking fluids are usually blended with various additives, such as the following:

a. oxidation inhibitors.

b. rustpreventing agents.

c. foam inhibitors.

d. wetting agents.

e. odorcontrolling agents.

f. antiseptics.

Sulfur, chlorine, and phosphorus are important oil additives. Known as extremepressure (EP)

additives and used singly or in combination, they react chemically with metal surfaces and form

adherent surface films of metallic sulfides and chlorides.

These films have low shear strength and good antiweld properties and, thus, effectively reduce

friction and wear. On the other hand, they may preferentially attack the cobalt binder in tungsten

carbide tools and dies (selective leaching), causing in the surface roughness and integrity of

tools.

78

SOLID LUBRICANTS

Because of their unique properties and characteristics, several solid materials are used as

lubricants in manufacturing operations. Described below are four of the most commonly used

solid lubricants.

Graphite

Graphite is weak is shear along its basal planes and thus has a low coefficient of friction in that

direction. It can be effective solid lubricant, particularly at elevated temperatures.

However, friction is low only in the presence of air or moisture. In a vacuum or an inert gas

atmosphere, friction is very high; in that, graphite can be abrasive in these situations. We can

apply graphite either by rubbing it on surfaces or by making it part of a colloidal (dispersion of

small particles) suspension in a liquid carrier such as water, oil, or an alcohol.

There is a more recent development in carbon called fullerenes or Buckyballs. These are carbon

molecules in the shape of soccer balls. When placed between sliding surfaces, these molecules

act like tiny ball bearings. They perform well as solid lubricants, and are particularly effective in

aerospace applications as bearings.

Molybdenum Disulfide

This is a widely used lamellar solid lubricant; it is somewhat similar in appearance to graphite.

However, unlike graphite, it has a high friction coefficient in ambient environment. Oils are

commonly used as a carriers for molybdenum disulphide and are used as lubricant at room

temperature. Molybdenum disulfide can be rubbed onto the surfaces of a workpiece.

Metallic and Polymeric Films

Because of their low strength, thin layers of soft metals and polymer coatings are also used as

solid lubricants. Suitable metals include lead, indium, cadmium, tin, silver, polymers such as

PTFE (Teflon) polyethylene, and methacrylates. However, these coatings have limited

79

applications because of their lack of strength under high contact stresses and at elevated

temperatures.

Soft metals are also used to coat highstrength metals such as steels, stainless steels, and

hightemperature alloys. Copper or tin, for example, is chemically deposited on the surface of the

metal before it is processed. If the oxide of a particular metal has low friction and is sufficiently

thin, the oxide layer can serve as a solid lubricant, particularly at elevated temperatures.

Glasses

Although it is a solid material, glass become viscous at elevated temperatures and, hence, can

serve as a liquid lubricant. Viscosity is a function of temperature, but not of pressure, and

depends on the type of glass. Poor thermal conductivity also makes glass attractive, since it acts

as a thermal barrier between hot workpieces and relatively cool dies. Glass lubrication is

typically used in such applications as hot extrusion and forging.

CONVERSION COATINGS

Lubricants may not always adhere properly to workpiece surfaces, particularly under high

normal and shearing stresses. This property has the greatness effects in forging, extrusion, and

wire drawing of steels, stainless steels, and hightemperature alloys.

For these applications, the workpiece surfaces are first transformed through chemical reaction

with acids (hence the term conversion). The reaction leaves a somewhat rough and spongy

surface, which acts as a carrier for the lubricant. After treatment, any excess acid from the

surface is removed using borax or lime. A liquid lubricant, such as a soap,

is then applied to the surface. The lubricant film adheres to the surface and cannot be scraped off

easily.

Zinc phosphate conversion coatings are often used on carbon and lowalloy steels. Oxalate

coatings are used for stainless steels and hightemperature alloys.

Friction and lubrication in rolling

80

The friction between the roll and the metal surface is of great importance. Frictional force is

required to pull the metal into the rolls. A large fraction of rolling load comes from the frictional

forces. High friction results in high rolling load, a steep friction hill and great tendency for edge

cracking. The friction varies from point to point along contact arc of the rolls. However, it is

very difficult to measure this variation in .

i. For cold rolling with lubricant varies from about 0.05 to 0.1.

ii. But for cold rolling friction coefficient from 0.2 upto the sticking conditions are common

[0.5 to 0.6].

Lubrication in hot rolling

1. Hot rolling of ferrous alloys is usually carried out without lubricant, although graphite may

be used, waterbased solution are used to cool the rolls and break up the scale on rolled

material. Nonferrous alloys are hot rolled with variety of compounded oils, emulsions and

fatty acids.

2. Cold rolling is carried out with water soluble oils or low viscosity lubricants such as

mineral oils, emulsions, paraffins and fatty oils.

3. Heating medium used in heat treating billets and slabs also act as lubricant. e.g. residual

salts from molten salt baths.

Lubrication in forging

Forging

1. Lubricants greatly influence friction, wear, force required and metal flow in the cavities.

They also act as a thermal barrier between the hot workpiece and relatively cool dies,

slowing the rate of cooling of the workpiece and improves the metal flow.

2. It acts as a parting agent, that is, one which inhibits the forging from sticking to the dies

and helps in its release from the die.

3. For hot forging graphite, and sometimes glass are used as lubricant. In hot forging,

the lubricant is usually applied directly to the dies.

4. For cold forging, mineral oils and soaps are commonly used as lubricants, applied after

conversion coatings of the blanks. In cold forging lubricant is directly applied to the WP.

5. The method of application and uniformity of the lubricant’s thickness blank is important.

81

HOT WORKING

Temperature of work material is resultant of following contributing factors:

1. Initial temperature of work and toolings.

2. Heat generated due to plastic deformation which is proportional to strain induced.

3. Heat generation due to friction at worktool interface

4. Heat transfer from work material to tool and surrounding.

Effect of temperature on metal forming processes:

1. Flow stress decreases with increase in work material temperature and therefore power

required deformation decreases. For this reason, hot working is used for

primary forming processes [rolling, extrusion, forging] where primary objective is to reduce

cross sectional area and refine grain size.

2. High operating temperature increases the deformation limit before fracture. True strain of

fracture of tungsten is increased from 1% at room temperature is 55% at 1000C.

Very large reductions are possible at elevated temperatures because recovery process neutralises

the strain hardening effect of deformation.

are limiting strain at and temp.

Fig. 20

Hot working refers to deformation carried out under conditions of temperature and strain rate

such that recovery process occurs substantially and is predominant over the strain hardening,

thereby neutralising the effect of the later.

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It is important to note that distinction between cold working and hot working does not depends

on any arbitrary temperature of deformation. Hot working is usually carried out at temperature

above recrystallisation temperature and approx. equals to.

0.6 where = melting point temperature at in K

Typically hot working temperature range

Material temperature C

M.S. 9001200

Aluminium 350450

zinc, lead, Tin room temperature

Hot working temperature is high for commercial metals, but lead, tin, zinc recrystallise rapidly at

even room temperature and therefore working of these metals even at room temperature

constitutes hot working. In contrast, working of tungsten even at 1100C, a hot working

temperature for steel, constitutes cold working because this high melting point metal has

recrystallisation temperature above this working temperature.

Recrystallisation Temperature

Recrystallisation does not occur unless the degree of cold work is sufficient and temperature is

sufficiently high. The minimum degree of cold work necessary for recrystallisation to occur is

called the critical degree of cold work. This is of the order of 2 to 3% for most of the metals land

alloys. The temperature at which a metal or alloy with normal degree of cold work completely

(i.e. 95 % or above) recrystalizes in a reasonable period (usually 1 hour) is called as the

recrystallisation temperature of metal. Recrystalisation temperature of a metal is dependent on

following factors:

1. Degree of cold work

Recrystallisation is a process of nucleation and formation crystals. Deformed grains have more

free energy than undeformed one. The free energy difference between the cold worked

deformed crystals and the undeformed crystals is driving force behind recrystallization.

Therefore, applied deformation prior to recrystallization should be more than the crystal degree

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of cold work. Further, the deformed grains and grain boundaries provide preferred sites for

nucleation. Larger the degree of cold work, lower is recrystalisation temperature. For example,

Amount of cold work Recrystallization temp time

20 % 3200 C 1 hour

60 % 2800 C 1 hour

2. Melting Point

Higher the melting point, higher is the recrystallisation temperature. It has been observed that

the recrystallisation temperature of most of metals and alloys are in the range of 0.4 to 0.6 of

their melting point temperatures in degree kelvin

Tcr = 0.5 Tm 0K , where Tm = melting point temperature in Kelvin.

Metal Recrystallization

temperature

Melting point 0 C

Tin Below Room Temp 232 0.6

Lead Below Room Temp 327 0.5

Zinc Below Room Temp 419 0.43

Aluminium 150 660 0.45

Magnesium 200 650 0.51

Silver 200 960 0.38

Gold 200 1063 0.41

Copper 200 1083 0.35

Iron/Steel 450 1539 0.40

Platinum 450 1760 0.35

Nickel 600 1452 0.51

Tungsten 1200 3410 0.40

3. Purity of metal

Presence of soluble impurities or addition of small amount of soluble alloying elements raise the

recrystallisation temperature.

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(i) Six nine (99.9999 %) pure aluminium has recrystallization temperature below room

temperature and therefore cannot be strain hardened at room temperature. But

commercially pure aluminium has recrystallization temperature of 150 0C.

(ii) Recrystallization temperature of commercial purity copper is 150 0C but addition of 0.5 %

impurity of arsenic raises recrystallization temperature above 500 0C. Therefore, arsenic

copper is used for high temperature applications such as boiler tubes.

4. Grain size

Finer the original grain size, lower is the recrystallization temperature.

5. Heating time

It has little influence on recrystallization temperature. Recrystallization temperature is far more

important than recrystallisation time. One expects such results as flux of atoms is proportional to

diffusivity which in turn is strongly dependent on temperature.

Time and temperature required for recrystallization of 75 % cold worked aluminium alloy.

Time Temperature

1 minutes 350 0C

60 minutes 300 0C

40 days 250 0C

The effect of degree of cold work on recrystallized grain size.

85

Fig. 21

The grain size of the material obtained at the end of recrystallisation depends temperature of

heating, time, degree of prior cold work and level of impurities. Lower temperature of heating

(above Tcr), higher degree of prior cold work and insoluble and fine impurity particles give

smaller grain size.

LIMITS OF TEMPERATURE

1. The lower limit of temperature for hot working is the lower temperature for hot working is

the lowest temperature at which recrystallisation rate is rapid enough to eliminate strain

hardening effect. This temperature depends on (i) the amount of prior deformation. The

greater the amount of prior deformation, lower is the recrystallisation temperature. (ii)

duration for which the material is held at that temperature.

2. The upper limit of hot working is determined by the temperature at which melting or

excessive oxidation occurs. Generally the maximum working temperature is limited to 50C

below the melting point to avoid hot shortness.

Hot Shortness

It is caused by local melting of a constituent or an impurity in the grain boundary at a

temperature below the melting point of a metal itself. When subjected to plastic deformation at

elevated temperature, the piece of metal crumbles and disintegrates along grain boundaries.

Examples of impurities.

antimony in copper

sulphur in steel

lead leaded steel

leaded brass

Beneficial effects of hot working

1. Lower flow stress and hence lower power requirement of deformation machines.

2. Large deformations are possible.

86

3. Removal of structural inhomogeneity. Rapid diffusion at hot working temperatures

decreases chemical inhomogeneity of the cast ingot structures. Blowholes and porosity are

eliminated by welding together these discontinuities and the coarse columnar grains of the

case ingot are broken down and refined into smaller equiaxed recrystalled grains. These

changes improves the ductility, toughness and strength of the material.

PROBLEMS WITH HOT WORKING

1. Additional heating facilities are required

2. Material handling in hot condition becomes difficult and risky.

3. As high temperature is involved, surface reactions between the metal and the furnace

atmosphere becomes a problem. Ordinarily, hot working is done in air, oxidation results

and considerable amount of material is lost. The work metal tends

to oxidise. Scaling of steel and copper alloys causes loss of metal and roughned surfaces

while processing under inert atmosphere is possible, it is prohibitively expensive and is

avoided in the case of very reactive metals. Reactive metal like titanium are severely

embrittled by oxygen and therefore they must be hot worked in an inert atmosphere, which

is prohibitively expensive. Surface decarburisation of hot worked steel can be serious

problem and frequently extensive finish machining is required to remove the decarburised

layer. Because of pilling of oxide scale leaving pits and embedding rollinoxide in the

surface finish obtained is poor and require further machining operation.

4. The dimensional control on hot worked products is poor. The dimensional tolerances has to

be large to take into account the expansion and contraction of metal.

5. Lubrication is more difficult. Lubricants used for hot working should be stable at elevated

operating temperature and should bot decompose. Viscous glasses are used in hot extrusion

of steels. Although molybdenum di sulphide, graphite is used as lubricants for hot working,

much hot working is done without lubrication.

6. Tool life is shortened because of heating, the presence of abrasive scales, and the lack of

lubrication. Sometimes, scale breakers are employed and rolls are cooled by water spray to

minimize tool damage.

7. Poor surface finish and loss of precise gauge control result from the lack of adequate

lubrication, oxide scales, and roughened tools.

87

8. The lack of work hardening is undesirable where the strength of cold worked product is

needed.

METALLURGICAL ASPECTS OF METAL WORKING

1. Composition

Resistance to deformation and ductility depends on chemical composition. Alloys have always

better strength than the pure constituents taken singly. For steel, increase in carbon content or

alloying constituent increases resistance to deformation and decreases ductility and thereby

decreases deformability. Carbon content should not exceed 0.5% for steels to be used in cold

extrusion and 1.5% for cold forging.

(ii) Impurities such as sulphur, oxygen, phosphorous and nitrogen decreases deformability.

(iii) The strength of a cold worked solid solution alloy is always greater than that for a pure

metal, cold worked to the same extent.

2. Microstructure also affects deformability

Soft annealing [spherodising] of steel prior to cold working improves its cold workability. The

large grains are easier to deform.

(i) Crystal structure

Formability depends upon the ductility which in turn is dependent on crystal structure. Crystal

structure which provides more number of slip planes have more ductility. Strain hardening rate

is more for cubic crystals than for HCP.

FCC has greatest opportunity for slip as it has four (4) distinct non parallel planes and three

directions for slip. Therefore materials with FCC structure are relatively weak and possess

excellent ductility.

Aluminium Platinum

Copper Silver

Gold

88

iron

BCC structure has fewer slip planes and therefore more difficult to deform metals with this

structure generally possess high strength with moderate ductility.

Molybdenum Vanadium

Tungsten

Chromium

HCP: Metals with HCP structure tend to have a low ductility and are often appear brittle.

Example Magnesium, Zinc, Zirconium.

(ii) Grain size

Grain size significantly influences mechanical properties of metal. At room temperature, a large

grain size is generally associated with low strength, low hardners and low ductility. Large

grains, particularly in sheet metals, also cause a rough surface appearance on stretching e.g.

orange peel.

Grain boundaries are more reactive than the grains themselves, because the atoms along the grain

boundaries are packed less efficiently and are more disordered. As a result, they have higher

energy than the atoms in the orderly lattice within the grains. Size of grain is not that important

as the length of the grain boundary. Grain boundaries restrict the amount of slip that can occur

fine-grained materials have greater length of grain boundaries than coarse-grained material.

Thus coarse grained material has lower strength and yield point. This relationship can be

expressed in the form of equations:

yield strength

= the frictional stress of crystal lattice for dislocation movement.

k = locking parameter which measures relative hardening contribution.

d = average grain diameter

k = 0.1 BCC crystal

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= 0. 7 FCC crystal

For ductile materials, hardness and strength are related.

For steel 0.36 BHN kgf / Large grained metals are more ductile.

Direction of slip changes more rapidly in smaller grain and therefore it is difficult to propagate

crack and difficult to break.

Grain refinement

Grain size is an outcome of relative rates of nucleation and grain growth combination of high

nucleation rate and slow grain growth yield fine grain size. The driving force behind the

solidification is reduction in free energy from liquid to solid rate.

Small grained material Large grained material

Fig. 1: a, b.

Nucleation can be homogenous or heterogeneous.

Homogenous nucleation Heterogeneous nucleation

1. Degree of undercooling is essential

homogenous nucleation.

1. Nucleation starts at preferred sites.

These sites are provided by

Fig.

(i) container walls.

(ii) intensional inclusions.

Boron (Bo) and titanium are added

in aluminium alloys for grain

refinement

90

cooling curve for homogenous nucleation.

Tm = melting point temperature

when liquid approaches melting point

temp, atoms begins to join together to

form unit cells and lattice formation

takes place. Undercooling causes

forming of stable lattice. The energy

released during solidification raises

the temp to the true melting point.

Undercooling is needed to start

nucleation.

Solid nuclei grow in all direction with

result that number of crystals compete

the same space and perfect external

shape is difficult to obtain. This

results in formation of crystals which

do not have regular external shape.

Ferrosilicon is added to cast Iron.

Zirconium in Magnesium alloys.

In hot working processes, fine grain size is obtained by a low finishing temp and a

rapid cooling rate. Grain size determines strength and fracture toughness. In processing

steel which undergo a structural transformation on cooling from the finishing temperature, the

ferrite grain size obtained depends on achieving a fine austenite grain size.

GRAIN SIZE AND YIELD STRENGTH

Hall petch equaiton: The equation defines relationship between yield strength of the material and

its average grain diameter.

91

where, = yield stress for a crystal of the same material where there are no grain boundaries.

It measures resistance of the material to dislocation motion due to effects other than grain

boundary.

k = constant

d = average grain diameter.

Material

Aluminium 16 2.2

Copper 26 3.5

Zinc 33 7.0

Iron 48 22.44

Grain boundaries provide obstacles to dislocation movement. In addition, the crystals are

separated by a thin noncrystaline region, which is the characteristics structure of a large angle

grain boundaries. Hence dislocations are stopped by a grain boundary and pile up against it.

The smaller the grain size, the more frequent is the pile up of dislocations.

‘k’ value for BCC iron is 22.44 (N/ ) where as only 2.2. and 3.5 for FCC matts such as

Aluminium and Copper. So for a given amount of grain refinement greater strengthening effect

is produced from BCC metals than FCC.

At temperatures below 0.5 Tm (Tm is melting point in k) and at higher strain

rates, grain boundaries increases rate of dark hardening and increases the

strength. At high temperatures and slow straining rates, deformation is localised

at the grain boundaries. Thus a grain boundary sliding and stress induced

migration takes place. Finally fracture takes place at the grain boundaries.

92

ASTM Number

The ASTM grain size number (n) is related to the number of grains (N) per square inch at a

magnification level of 100x

The grain dia can be approximately calculated from the ASTM specification for grain size. For

an ASTM number (n). The number of grains per square inch at a magnificaiton of

100x is given by

Grain size number ASTM 1 corresponds to 1 grain per square inch at a magnification of 100x

If, d is average grain diameter, and N is number of grains

Fig. 23

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Grain sizes between 5 and 8 are generally considered fine. A grain size of 7 is generally

acceptable for sheet metal forming car bodies, appliances and kitchen utensils.

A distinction between a coarse grained steel and fine grained steel is not clearly define.

However, a grain size number below ASTM number 3 represents a definitely coarse grained

steel and above As TM number 6 represents a reasonably fine grained steel. Steels having

ASTM grain size number greater than 8 are called ultra fine grained steels.

Fig. 24

1. The yield strength of a polycrystalline material increases from 120 on decreasing

the average grain diameter from 0.04 mm to 0.01 mm. fine the yield strength for a grain

size of 0.025 mm.

Solution:

Hall petch equation states the relationship between yield strength of a material and grain size.

.

Therefore Hall petch equation for this polycrystalline material is

When average grain diameter is 0.025 mm

.

94

The yield strength of the given material at average diameter is 146.5 .

2. A yield strength of a polycrystalline material increases from 120 MPa to 220 MPa on

decreasing the grain diameter from 0.04 to 0.01 mm. Find the yield strength for a grain

size of ASTM 9.

Solution:

(1)

(2)

(1) (2)

100 =

For ASTM No. 9, n = 9

95

Yield strength of ASTM 9 material will be 170

3. Estimate the yield strength of polycrystalline Fe 3% si alloy when grain size is ASTM 1,

4, 8 respectively.

Assume,

Solution:

(i) n 1

.

(ii) n = 4

(iii) n = 8