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Class Notes From Rajan Mital

HOT & COLD WORKING
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HOT & COLD WORKING

Q.1. What is hot working ? State its principles,

advantages and disadvantages.

Ans. Mechanical working of metals above its

recrystallisation temperature is known as hot working.

Advantages :

1. Larger deformation can be accomplished and more rapidly

by hot working since the metal is in plastic state.

2. Porosity of the metal is considerably minimised.

3. Concentrated impurities, if any in the metal are

disintegrated and distributed throughout the metal.

4. Grain structure of the metal is refined and physical

properties improved.

Disadvantages :

1. Due to high temperature a rapid oxidation or scale

formation takes place on the metal surface, leading

to poor surface finish and loss of metal.

2. On account of the lost of carbon from the surface of

the steel piece being worked the surface layer loses

its strength, which is a disadvantage when the part

is put to service.

3. This weakening of the surface layer may give rise to

a latigue crack which may ultimately result in

fatigue failure of the part.

4. Close tolerances cannot be maintained.

5. It involves excessive expenditure on account of high

cost of tooling. This, however, is compensated by the

high production rate and better quality of products.

Q.2. Describe the procedure of hot extrusion of tubes.

Ans. A common method of hot extrusion of tubes is shown in

figure. It is actually a forward extrusion method using a

mandrel to form the bore of the tube. First the mandrel is

pushed through the centre of the billet and the die,

followed by applying pressure on the billet by advancing

the plunger. The metal is forced to flow through the

opening between the die and the mandrel. The operation is

performed quite rapidly. Most of the metals and alloys are

hot extruded, although some of these are cold extruded

also, for production of seamless tubes.

Q.3. Write short notes on

Ans. 1. Hot Spinning :-

The process consist of heating the metal to forging

temperature and then forming it into the desired shape on a

spinning lathe which is similar to an engine lathe.

Usually shapes of circular cross-section which are

symmetrical about the axis of rotation, are formed by this

process. The workpiece is shaped over a formed revolving

metal holding device, called chuck, with the help of

spinning tools. It very well compares with drawing of

stamping in so far as the production in small quantities is

concerned, since the cost of dies for such small quantities

will lead to uneconomical production through the latter

methods. Hot spinning is generally used for thicker plates

and sheets which cannot be shaped through cold spinning.

In operation it is similar to cold spinning and, therefore,

the details of equipment, tools and procedure, etc., will

be given later under 'cold spinning’.

2. Hot Forging

These process basic ally consists of heating the metal

to plastic state and then applying pressure to form it into

different shapes and sizes. Unlike rolling, the pressure

in this case is not continuous but intermittent. The hot

metal piece may be compressed along its length to increase

its cross-section, along its cross-section to increase its

length, within a closed cavity to acquire the shape of that

cavity or in different directions to bend it into different

shapes. The pressure may be applied by hand hammer called

hand or smith forging, by power hammers, called hammer

forging, by presses (press forging) or upset forging

machines.

3. Hot drawing

This process is widely used for the production of

thicker walled seamless tubes and cylinders. It is usually

performed in two stages. The first stage consists of

drawing a cup shape out of a hot circular plate with the

help of a die and a punch. The second stage consists of

reheating the drawn cup and drawing is further to the

desired length having the required wall thickness. The

second drawing operation is performed through a number of

dies, which are arranged in a descending order of their

diameters, so that the reduction in wall thickness is

gradual in various stages. The farther end of the drawn

object is always blind, which may be cut off to produce a

through hole, if required.

Q.4. What are the specific advantages and limitations of

cold working?

Ans. Advantages and limitations:

1.Better dimensional control than hot working is possible

because the reduction in size is not much.

2.Surface finish of the component is better because no

oxidation takes place during the process.

3.Strength and hardness of the metal are increased.

4.It is an ideal method for increasing hardness of those

metals which do not respond to the heat treatment.

5.Only ductile metals can be shaped through cold working.

6.Over-working of metal results in brittleness and it has

to be annealed to remove the same.

7. Subsequent heat treatment is mostly needed to remove the

residual stresses set up during cold working.

Q.5. What are the main characteristics of the hot working

of metals or compared with cold working process?

Ans. (1) Above the re-crystallisation temperature, the

metal becomes plastic and causes the growth of grains. By

hot working, the grains are broken up and their parts are

deformed into small and more numerous crystals or in other

words the refinement of grain occurs. Metals possess

little elasticity and low load is required to shape the

metal as the strength and hardness decrease at elevated

temperatures.

(2)The porosity of the steel ingot can be eliminated to a

greater extent.

(3)Great latitude in shape and size of form is possible due

to reduction of elastic limit.

(4)A uniformity is established either by squeezing other

impurities into fiber sla gs or distributing them throughout

the mass.

(5)Directional property resulting from a fiber structure is

obtained.

(6)Due to refinement of grains, mechanical properties such

as toughness, ductility, elongation and reduction in area

are improved.

(7)The power required to finish the part ingot is less.

(8)It can be used on most of the metals, because it is a

rapid and economical process.

Q.6.Why forging generally preferred for those components

which require high strength and resistance to shock?

Explain briefly.

Ans. Characteristics of the Forged Parts

(1)It refines the structure of metal by closing up the

cavities and by smashing up large grain formations.

(2)Forged parts have directional properties and hence have

good strength.

(3)Mechanical properties such as percentage elongation,

percentage reduction of area and resistance to shock and

vibration are improved.

(4)Cracks and blow holes are minimised.

Q.7. For various methods of forging known to you give the

application areas.

Ans.

(1) Hand forging :- Hand forging is emplayed only to shape

a small number of light forgings chiefly in repair

shops.

(2) Hammer forgings :- Usually used for small item forging.

(3) Press forging :- Usually used for heavy item forging.

(4) Machine forging :- For medium sized and large articles

requiring very heavy blows.

(5) Drop forging :- For mass production of identical parts.

Q.8. Define up setting, edging, drawing, piecing and

punching operations as applied to not working of metals.

Ans.

Upsetting :- This is just opposite to drawing and involves

increasing of the cross-sectional area usually by pressing

or hammering in a direction parallel to the original ingot

axis. In the process of upsetting, the shaft or rod is

generally gripped in dies, and the head or flange upset

either by a plain flattened ram or with further dies,

shaped to give the desired contour.

Edging :-

Drawing :- Refer – (Hot drawing)

Piercing :- This process is employed for the production of

seamless tubes. It offers the most economical mechanical

working, process for the manufacture of seamless tubes. It

consists of passing the hot rolled billet at 11000C. 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.

Q.9.Define the following terms related to cold working of

metals.

Ans.

(1) Blanking :- This is the operation carried out on presses

and consists of cutting the outside contour of a

stamping. Production of sheet-metal blanks of flat

shapes requires a single -action press equipped with

tools comprising a punch, a corresponding die, a

stripper to keep the sheet from following the punch on

its upstroke and means for aligning the sheet or strip

of material and for spacing successive cuts. Cutting

inside contours, i.e. holes and slots is called

piercing. All these operations will be dealt in detail

under the chapter of presses. However pressed-metal

parts, or stampings are recommended for mass production.

Stampings combine the virtues of lightness, a high

degree of uniformity, and surfaces well adopted to

receive protective and decorative finishes.

(2) Swaging :- This operation consists of applying

compressive or impact forces on the metal below the

recrystallisation temperature. It causes the metal to

flow in the predetermined shape according to the design

of the dies. Rotary swaging and cold heading are the

two important processes of swaging.

(3) Lancing :- It is a special form of piercing operation in

which the entire contour is not cut, the blanked

material remains attached with the sheet. It is

achieved by bending down one side of partially punched

hole.

(4) Embossing :- It is also like a drawing or stretching

operation and does not require much pressure like

drawing and coining. It consists of producing, projected

or raised designs in relief on a surface of sheet. It

is done with the help of two mating dies. The sheet is

first blanked and then little more force is applied by

the punch which forces the metal against a mating die

conforming to the same configuration as the punch. In

this way very little metal is squeezed in the operation

and the words are printed on the sheet in projected

form.

Q.10. Explain extrusion process as used for production of

tubes. Draw sketches for various stages in it.

Ans.

1.Tube drawing

2.Roll piercing

3.Tube manufacturing by lap welding.

4.Tube extrusion.

Roll piercing :-

It is a method of producing seamless tubes. The

piercing machine used in the process consists of two

tapered rolls called piercing rolls. Round heated billet

of steel is passed between these rolls over a mandrel.

Both the rolls rotate in the same direction. The billet is

centre punched or provided with a small drilled hole at one

end and heated to proper temperature. It is then pushed

forward into the rolls. The rolls grip the billet and pull

it further into them. The axes of the rolls are crossed,

therefore they revolve the billet as well as draw it

forward to force it on to the mandrel. The mandrel can

also revolve in its own position. This combination of the

revolving motions of the billet, and mandrel, together with

the axial advancement of the billet, provide a helical

rolling effect on the material. Production of a 12 meter

length of upto 150 mm diameter rough tubing will take about

10 to 30 seconds through this method. If tubing of larger

bore (say upto 350 mm) are to be made a second piercing

operation is necessary after the first. Still larger sizes

will need a third piercing operation.

The rough tubing produced as above is further

subjected to rolling, reeling and sizing etc. to bring it

to the correct shape and size and to provide a fine surface

finish. Such tubes are produced in various metals and

alloys like steel alloys, aluminum, brass and copper

etc.

Q.13. What do you mean by cold working methods ? Explain

any two cold working process with neat sketch.

Ans. The processes which are done on the metals by

keeping the temperature below recrystallisation temperature

are known as cold working processes. The force required for

these processes is high as compared to hot working

processes. Cold working processes are also applied for soft

materials otherwise the work piece will crack. In cold

working processes strength & hardness is increased but

grain structure is distorted.

Q.14. Write short notes on

1.Hot spinning (described earlier)

2.Hot drawing (described earlier)

3.Roll piercing (described earlier)

Q.15. Explain principle of rolling showing its effect on

granular structure.

Ans. The process of rolling basically consists of

passing the hot ingot through two rolls rotating in

opposite direction at a uniform peripheral speed. The space

between the rolls is adjusted to conform to the desired

thickness of the rolled section, and the same is always

less than the thickness of the ingot to reduce its crosssection

and increase its length.

The process is illustrated in figure, which shows the

changes that take place in the grain structure of the metal

as it passes through the rolls. As a result of squeezing

the grains are elongated in the direction of rolling and

the velocity of material at exit is higher than that at the

entry. After crossing the stress zone the grains start

refining. But this is the case only in hot rolling. In cold

rolling they tend to retain the shape acquired by them

during rolling.

Q.16.Describe the types of rolling mills. Explain their

arrangements, specific uses in detail.

Ans. The different types of rolling mills are

described below:

1)Two high mill-

It consists of two heavy horizontal rolls, placed

exactly one over the other. The rolls are supported on

bearings housed in sturdy upright side frames, called

stands. The space between the rolls can be adjusted by

raising or lowering the upper roll. The position of the

lower roll is fixed. Both the rolls rotate in opposite

directions to one another, as shown in figure. Their

direction of rotation is fixed and cannot be reversed.

Thus, the work can be rolled by feeding from one direction

only. There is another type of two high mill which

incorporates a drive mechanism that can reverse the

workpiece continuously through back-and-forth passes

between the rolls. This type of rolling mill is known as a

two-high

reversing mill. They are normally employed for the initial

rolling of an ingot.

2)Three high rolling mills-

It consists of three horizontal rolls, positioned

directly one rotation of the upper and lower rolls are the

same, but the intermediate roll rotates in a direction

opposite to both of these. All the three rolls continuously

revolve in the same fixed directions and are never

reversed. The work piece is fed in one direction between

the upper and middle rolls and in the reverse direction

between the middle and lower rolls. Many pieces may be

passed through the rolls simultaneously. This results in a

higher rates of production than the two-high mill. This

mill may be used for blooming, billet rolling or finish

rolling.

3)Four-high rolling mills -

It consists four horizontal rolls, two of smaller

diameter and two of larger diameter, arranged directly one

over the other as shown in figure. The larger diameter

rolls are called back-up rolls and their main function is

to prevent the deflection of the smaller rolls, which

otherwise would result in thickening of rolled plates or

sheets at the centre. The smaller rolls are known as

working rolls and they are the rolls which concentrate the

total rolling pressure over the metal. These mills are

generally used for subsequent rolling of slabs. The common

products of these mills are hot or cold rolled sheets and

plates.

4)Cluster mill-

It consists of two working rolls of smaller

diameter and four or more back-up rolls of larger diameter.

The arrangement of rolls for this mill is shown in figure.

The number of back-up rolls may go up as high as 20 or

more, depending upon the amount of support needed for the

working rolls during the operation. This type of mill is

generally used for cold rolling.

5)Continuous rolling mills-

It consists of a number of nonreversing two-high

mills arranged one after the other, so that the material

can be passed through all of each successively. The

millstand rotate at a faster speed than that of the

preceding rolls in order to accommodate the increasing

length of the metal piece being rolled. This arrangement

facility facilitates a very rapid production, because the

component passes continuously from one stand to the other

until it reaches the final pass. But it is suitable for

mass production work only, because for smaller quantities

quick changes of set-up will be required and they will

consume a lot of time and labour. As the speed of rolls on

each successive stand varies it is necessary that their

respective surface speeds should be properly calculated and

adjusted.

Q.17.Write short notes on

1)Cold spinning

2)Roll bending

Ans. Cold Spinning-

The process is similar to hot spinning, described

earlier. The metal is pressed on to the surface of a wooden

or metallic form, called chuck, attached to the lathe

spindle. An adapter fitted in the barrel of the tailstock

holds the work against the form. The tools used, called

spinning tools, are provided with blunt edges and are

supported on the tool rest fitted on the cross slide, as

shown in figure.

Aluminum and other soft metals are best suited for cold

spinning. A few commonly used spun articles out of

Aluminum and its alloys are processing kettles, cooking

utensils, liquid containers and light reflectors etc.

Various other ductile metals, particularly non-ferrous

ones, are quite successfully spun into various typical

shapes. The thickness of the metal is reduced during

spinning, and proper allowance should be made for this in

the blank to be cut. At the end of the operation the

unwanted material is trimmed off to bring the product to

required size. The form or chuck may be in single piece or

composed of many sections attached together, depending upon

the possibility of the spun part from it. Where the space

of the part does not permit its removal from chuck, off centre

chuck are used.

This process of spinning is generally used for:

1.Limited production.

2.Keeping the tool costs low.

3.Production such peculiar shapes which are difficult to be

produced through other forming methods.

2) Roll bending-

It is also s kind of roll forming operation

through which metal sheets and bent into cylindrical

shapes. The roll bending machines carries three rolls; two

being fixed and the third adjustable. Diameter of all the

same. By adjusting the position of the adjustable roll the

plates or sheets can be bent in different curvatures.

Q.18. Define cold bending.

Ans. It is employed for bending into desired shapes

various stock materials like rods, wires, bars, pipes,

tubes and various structural shapes. Formed dies are used

for bending these articles and the operation is usually

performed in many stages. Well designed fixtures are also

used where mass bending of such components is required.

Q.20. Sketch and describe the types of rolls used in

rolling mills.

Ans. The rolls used in rolling mills essentially

consists of three parts, namely body, neck, and wabbler.

The main rolling operation is performed by the body. It is

therefore made to have different shapes on its periphery,

according to the desired shapes of the rolled products,

such as smooth for flat and grooved for other sections. The

rolls vary in diameters from a few centimeters to about 1.5

meters. The extent to which the reduction in cross-section

can be made in a single pass depends upon the weight and

size of the rolls. The neck is that part of the roll on

which it rotates in the bearing. The wabbler is the starshaped

construction at both ends of the roll which engages

the hollow cylinder to connect it to the driving shaft to

receive power. The advantage of providing the wabblers is

that the main body of the roll is not directly connected

with the driving shaft. Thus, in case of too heavy

loading, if there is any damage, it will occur only in the

wabbler, which is a much weaker section, and the main body

will remain absolutely free from it. The rolls are

generally made from cast or forged steel or cast iron.

Q.21. Describe the process of cold spinning stating its

advantages and specific uses.

Ans. Advantages of cold spinning

(1) This is very cheap process.

(2) It does not require much investment.

(3) The production of in cylindrical shape by spinning is

less economical.

Specific uses of cold spinning:

Cold-spinning process is frequently used in the

making of bells on musical instruments and also for light

fixtures, kitchen-ware, reflectors, funnels, and large

processing kettles.

Q.22. What is continuous rolling mill? What are its

advantages?

Ans. This consists of several stands of two high mills

arranged one after the other. As the metal comes out of

one set of rolls, it enters second, third and so on and

finally comes out in required size and shape. The speed of

every stage goes on increasing in comparison to preceding

roll in order to accommodate the increasing length of the

metal and thereby making it a continuous process. The

operation is thus very fast and space requirement for mill

is less. The production is economical and at the same time

mass production is possible.

Q.23. Write short notes on

Ans. Cold rolling:

Cold rolling is generally employed for providing

a smooth and bright surface finish to the previously hot

rolled steel. It is also used to finish the hot rolled

components to close tolerances and improve their toughness

and hardness. The items generally subjected to cold

rolling for this purpose are bars, rods, sheet plates,

strips and wires etc. Before being put to cold rolling the

hot rolled articles are cleaned through pickling and other

operations. The same types of rolling mills, described

earlier in connection with hot rolling, are used in cold

rolling. In order to obtain a smooth surface finish the

roll surfaces are polished and scratches, if any, removed.

The part being rolled is usually annealed and pickled

before the final pass is made, so as to bring it to

accurate size and obtain a perfectly clean surface.

Q.24.What is hot working? What are the common hot working

processes?

Ans. The principal hot working processes generally

applied to various metals, are the following:

1.Hot rolling.

2.Hot forging.

3.Hot spinning.

4.Hot extrusion.

5.Welded pipe and tube manufacturing.

6.Roll piercing.

7.Hot drawing.

Q.25.Explain the following cold working process.

Ans. Cold hobbing:

It is a process for producing cavities of various

shades in a blank of soft metal by pressing a hardened

steel form into it. This form is known as a hob. The

operation may require several pressings and annealings in

between. Hydraulic presses are used for providing the

required pressure. The main advantages of this process is

the economical production of identical cavities in large

number. Moulds for plastic moulding can be produced

through this method.

Q.26. What principles are involved in hot rolling? Explain.

Q.27. What is hot extrusion? In how many ways it can be

performed?

Ans. The process of extrusion consists of compressing

a metal inside a chamber to force it out through a small

opening called die. Any plastic material can be extruded

successfully. Most of the presses used for extruding

metals are hydraulically operated horizontal presses. A

large number of extruded shapes are in common use such as

tubes, rods, structural shapes and lead covered cables

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