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Mechanical working of

metals

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Introduction:Mechanical working of metals is the

shaping of metals in either a hotor a cold

state by some mechanical means and does

not include the shaping of metal by

machining, casting and grinding.


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Metals are generally subjected to mechanical

working for the following purposes:

i. To reduce the original block or ingot to the finished

dimensions of the part, thereby saving materials

machining costs and the time.

ii. To improve the mechanical properties of the metal

through:

1. refinement of grain structure.

2. directional control of flow lines.

3. Break up and distribution of unavoidable

inclusions, particularly in steel.

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Mechanical working of metals can be carried out inthree forms :

Hot working:working above recrystallisation temperature

(lower critical temperature) but below the burning points.

Cold working: plastic deformation at room temperature

(residual stresses are produced) .

Worm working: plastic deformation above room

temperature but lower than hot working temperature.


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Effect of Structure of Metals

The property of any material is directly related to its

structure (arrange of atoms within metals).Metals are composed of crystals (grains), three basic

structures being:

body centered cube (BCC).

face centered cube (FCC).

Hexagonal close-backed (HCB).

These crystals are not perfect and contain various defects

and imperfection, such as dislocation, vacancies, impurities,

inclusions and grain boundaries.Plastic deformation in metals takes place by means of a

slip mechanism.

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The stress required to continue deformation increases with

deformation and is known as work hardening or strainhardening .It results from entanglement of dislocations

with each other and with impurities grain boundaries.

Recovery of effects of Plastic deformation occurs at a

certain temperature range below the recrystallisation

temperature of the metals and the stresses in the highly

deformed region are relived.

Recrystallisation: the process in which, at a certain

temperature range, new strain-free grains are formed and

replacing the older grains.If temperature is raised further, the grains begin to grow

(grain growth) and it adversely affects the mechanical

properties slightly. Large grain produce a rough surface

appearance on sheet metal, calledorange peel.


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Mechanical properties:

Properties like strength and ductility are important both in

manufacturing processes and to the service life ofcomponent.

Mechanical properties of metals help in estimating the

forces required in forming processes and also predict about

the behavior of metals in shaping processes

Mechanical properties depend on a number of variables

like: (1) temperature , (2) rate of deformation, (3) surface

condition, (4) environment, (5) presence of flaws, inclusions, or

impurities in metals etc

Presence ofresidual stress in a component is significantboth in subsequent processing and in its service life.

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Wrought products:

In mechanical working the wrought forms can be:i. long-length forms, like drawn bar, rolled section, sheet

plate, tube, wire and extruded sections.

ii.individual component forms like forging of connecting

rod, crankshaft, etc.

Bars. Bars in several cross-sections, like round, square,

rectangular, and hexagonal are the common rolling-mills

products. Mild steel, medium carbon steel and alloy steel

are commonly supplied in the form ofblackbar (black

surface occurs due to oxidation of steel by atmosphere

during hot working). The corners are generally rounded

in case if black bars. Bars are sometimes cut off to

smaller lengths to produce billets, suitable for forging.


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Bright-drawn bars. These are produced by drawing bars

through shaped dies. These have smooth surface and are

of accurate size. Such a bar is used on capstan, turret orautomatic lathes. The drawing operation work-hardens

the surface of bright-drawn bar, the depth of hardening

being proportional to the severity of the drawing. Work-

hardening is not desirable If the bar is to be heated for

joining purposes. To avoid distortion, such bars should be

subjected to bright annealing.

Rolled sections.These include angles, channels, joists are

used for construction work.

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Sheet.Hot-rolled and cold rolled sheet is available in a range

of metals and alloys, and over a wide range of sizes. Tin-coated steel sheet is used for food canning and fancy goods.

The finish is rust-proof and can be easily soldered.

Galvanized sheet is commonly used. It can not be worked

or heated for bending.

Plate.Mild steel plate is hot-rolled to a thickness between 3

mm and 50 mm. Plates are used in shipwork, boilers, heavy

tanks and vessels etc.

Tube. It is produced by solid drawing for heavy duty jobs,

seam welded and closed jointed.

Wire. It is usually cold-drawn through dies. Cold-drawing

work hardens the material and as such wire should be

annealed if it to be bent at a later stage.

Extruded sections.Simple as well as complicated section are

extruded.


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Hot working:

In hot working process the metals are given desired shape

by subjecting them to forces, which cause them to undergoplastic deformation when above the recrystallisation

temperature.

The recrystallisation temperature of steel is 800 C (50

100 C above the lower critical temperature) and for lead,

tin and zinc it is at or near room temperature.

To get best results in each shaping operation important

points such as : (1) amount of energy consumed, (2) the pest

temperature, (3) rate of shaping, (4) design of equipment, (5)

crystal structure and properties of the finished productmust be

given due consideration.

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Hot working advantages:

1. Above the recrystallisation temperature, the metal becomesplastics and causes the growth of grains. By hot working,

the grains are broken up and the parts are deformed into

small and more numerous crystals or in other words the

refinement of grain occurs.

2.Metals possess little elasticity and low load is required to

shape the metal as the strength and hardness decrease at

elevated temperature.

3.The porosity of the steel ingot can be eliminated to greater

extent.

4.Greater latitude in shape and size of form is possible due to

reduction of elastic limit.

5. A uniformity is established either by squeezing other

impurities into fibre slag or distributing them throughout

the mass.


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6. Directional properties resulting from a fibre structure is

obtained.

7. Due to refinement of grains, mechanical properties such astoughness, ductility, elongation and reduction in area are

improved .

8. The power required to finish the part from ingot is less.

9. It can be used in most metals, because it is a rapid and

economical process.

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Hot working limitations:

1.There is rapid oxidation or scaling of the surface due tohigh temperature of the metal which results in poor

surface finish

2.Closed tolerances can not be maintained ; oxide films are

a surface characteristic.

3.Tooling and handling costs are high.

4.The life of the tools used is less as they have to work at

high temperature .


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Methods of Hot working :

The various methods of hot working are as follows:

i.Rolling, ii.Forging,

iii.Extruding, iv.piercing,

v.Drawing or Cupping,

vi. Spinning.

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Hot Rolling:

In rolling, a metal is formed through a pair or revolvingrolls with plain or grooved barrels. It is a more economical

method of deformation than forging when metal is required

in long lengths of uniform cross-section.


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Forging:

Forging can be define as the controlled plastic deformation

of metals at elevated temperatures into a predetermined sizeor shape using compressive forces exerted through some

type of die by a hammer, a press or an upsetting machine.

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Extrusion:

Extrusion is define as the process of pushing the heatedbillet or slug of metal through an orifice provided into a die,

thus forming an elongated part of uniform cross-section

corresponding to the shape of the die orifice. The pressure is

applied either hydraulically or mechanically. Aluminum,

nickel and their alloy are the metals used for extrusion

directly at elevated temperatures.


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piercing:

This process is employed for the production of seamlesstubes. It consists of passing the hot rolled billet at 1100 C

between two conical rollers and over a mandrel which helps

in piercing and controlling the size of bore as the billet is

forced over it.


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Drawing or Cupping and Bending:

Drawing is defined as a process of making cup-shaped parts

from sheet metal blanks by pulling it into dies with the helpof punch. Sheet-blanks are heated to working temperatures

to provide sufficient plasticity.

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Hot Spinning:

This process is used to form a dish or head form circular,heavy steel plates. It is also used to neck or closed the ends

of tubes. The operation is performed on a lathe which

imparts rotational motion of job.


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Manufacture of steel Pipes and Tubing by welding

process:

According to the production method there are two type of

tubes and pipes:

1. seamless (in which there is not joint and manufacturing by

piercing).

2. fabricated or welding edge.

In fabricated type pipes and tubes are made by rolling steel

strip and plate to the proper size. The edges of the sheet are

rolled in such a way that they either meet in a butt joint or a

lap joint. Thus the sheet (called skulp) is bent into a tube and

the ends are welded.

The general practice is for lap-joint weld as it can withstand

high pressures and is also satisfactory for many types of

services.

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Cold working of metals:Metal is said to be cold worked if it mechanically processed

below the recrystallisation temperature of the metal.

Most of cold working processes are performed at room

temperature

(1) The grain structure is distorted and resistance to working

keeps on increasing due to the lattice distortion .

The effects of cold working on metals are:

(2) Residual stresses are set up in the metal which remain

unless they are removed by subsequent heat treatment.

When reheating is done below the recrystallisation

temperature, the residual stresses are removed without

appreciable change in physical properties of grainstructure. Further heating into the recrystalline range

eliminates the effect of cold working and restores the

metal to its original conditions

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(3) Cold working results in loss of ductility and increase of

strength and hardness of metals.(4) Surface finish is improved and closed tolerances can be

maintained(5) Re-crystallisation temperature for steel is raised.


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Cold working limitations:

(1) Only small sized component can be easily cold worked as

greater force are required and expensive capitalequipment for large sections.

(2) The grain structure is not refined and residual stresses

have harmful effects on certain properties of metal.

(3) Many of the metals having less ductility, e.g., carbon steel

and certain alloy steels cannot be cold worked at room

temperature. It is, therefore. Limited to ductile metals and

the range of shapes produced is not as wide as can be

obtained by machining.

(4) Tooling costs are high and as such it is used when largequantities of similar components are required.

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Metals which can be suitably cold worked in the form of

sheets or other forms are:1. Mild carbon steel of low carbon content.

2. Copper and its alloys including brass and bronze.

3. Aluminum bronze having aluminum less than 7%.

4. Nickel brass.

5. Monel metal.

5. Stainless steel.

3. Duralumin and several other aluminum alloys.


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Cold working Processes:

1. Blanking

Shearing

2. Punching 3. Perforating4. Trimming 5. Notching 6. Slitting

7. Lancing 8. Broaching 9. Burnishing

10. Dinking 11. Bar cropping

1.Blank drawing

Drawing

2. Bulging 3. Tube drawing

4.Wire drawing 5. Spinning 6.Embossing

7.Stretch drawing 8.Drop hammer forming

9.Press-brake forming 10. Cold-roll forming

11. Wire forming

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1. Cold rollingSqueezing

2. Sizing 3.Swaging or cold forging

4. Coining 5. Stamping 6. Hobbing

7. Crimping 8. Tread rolling 9. Knurling

10. Riveting 11. Impact extrusion

1.Bending

Bending

2. Roll forming 3. Seaming

4.Angle bending

12. Planishing

13. Shot peening

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