manufacturing week 2a- rolling of metals

8/10/2019 manufacturing Week 2a- Rolling of Metals

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MCB3073 Manufacturing Technology 2

Week 2

Metal Forming-Rolling of Metals

Lecturer:

Dr. Turnad Lenggo Ginta


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Course Outcome

Students should be able to:

Understand the process of rolling of metals.

Calculate the rolling force during manufacturing


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Rolling

Process of reducing the thickness of long workpiece by

compressive forces applied through a set of rolls.

Carried out at elevated temperatures where cast metal is

broken down with finer grain size and improvedproperties.


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The Flat-Rolling Process


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The Flat-Rolling Process

The maximum possible draft is defined as the difference

between the initial and final strip thicknesses. It can be shown

that this is a function of the coefficient of friction, between the

strip and the roll and the roll radius, R, by the following

relationship:

Thus, as expected, the higher the friction and the larger the roll

radius, the greater the maximum possible draft becomes.

1.132Rhhfo


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13.2.1 Roll force, torque and power requirement

The rolls apply pressure on the flat strip in order to reduce its

thickness, resulting in a roll force, F.

The roll force in flat rolling can be estimated from the formula

where L is the roll-strip contact length, w is the width of the

strip, and Yavgis the average true stress

2.13avg

LwYF


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13.2.1 Roll force, torque and power requirement

The total power (for two rolls) in S.I. units is

where F is in newtons, L is in meters, and N is the

revolutions per minute of the roll.

In traditional English units, the total power can be

expressed as

3.13kW000,60

2 FLNPower

4.13hp000,33

2 FLNPower


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Example 13.1 Calculation of roll and torque in flat rolling

An annealed copper strip, 250 mm wide and 25 mmthick, is rolled to a thicknes of 20 mm in one pass. The

roll radius is 300 mm, and the rolls rotate at 100 rpm.

Calculate the roll force and the power required in this

operation.


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Solution The roll force is determined from Eq. (13.2) in

which L is the roll-strip contact length. It can be shown

from simple geometry that this length is given

approximately by

The average true stress for annealed copper is

determined as follows. First note that the absolute value

of the true strain that the strip undergoes in thisoperation is


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Referring to Fig. 2.6, note that annealed copper has a

true stress of about 80 MPa in the unstrained condition,

and at a true strain of 0.223, the true stress is 280 MPa.

Thus, the average true stress is (80+280)/2=180 MPa.

We can now define the roll force as

The total power is calculated from Eq. (13.3), noting that

N = 100 rpm. Thus,


MN74.1MPa1801000

250

1000

7.38

avgLwYF

kW705

000,60

100

1000

7.381074.12

000,60

2Power 6

FLN


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Fig. 2.6


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Defects in rolled products

Successful rolling practice requires material properties,

process variables and lubrication.

Surface defects result from inclusions and impurities in

the material. Structural defects affect the integrity of the rolled

product.

Wavy edges are caused by bending of the rolls.

Wavy edges Zipper cracks Edge cracks Alligatoring


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Defects in rolled products

Residual stresses

Develop due to inhomogeneous plastic deformation in

the roll gap.

Generates compressive residual stresses on the surfacesand tensile stresses in the bulk.


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13.5 Various rolling processes and mills

Shape rolling

Straight and long structural shapes (such as channels, I-

beams, railroad rails, and solid bars) are formed at

elevated temperatures by shape rolling (profile rolling), in

which the stock goes through a set of specially designed

rolls.

Cold shape rolling also can be done with the starting

materials in the shape of wire with various cross-sections.

Fig 13.12 shows the Steps in the shape rolling of an I-beampart. Various other structural sections, such as channels

and rails, also are rolled by this kind of process.


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Roll Forging

In this operation (also called cross rolling), the cross-

section of a round bar is shaped by passing it through

a pair of rolls with profiled grooves.

Fig 13.13 shows two examples of the roll-forgingoperation, also known as cross-rolling. Tapered leaf

springs and knives can be made by this process.


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Ring Rolling

In ring rolling, a thick ring is expanded into a large-diameter

thinner one.

The ring is placed between sets of two rolls, one of which is

driven while the other is idle.

Fig 13.15 (a) shows schematic illustration of a ring-rolling

operation. Thickness reduction results in an increase in the part

diameter. (b) through (d) Examples of cross-sections that can

be formed by ring rolling.

Typical applications of ring rolling are large rings for rockets

and turbines, jet engine cases, gearwheel rims, ball-bearing and

roller-bearing races, flanges, and reinforcing rings for pipes.


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Thread Rolling

Thread rolling is a cold-forming process by which

straight or tapered threads are formed on round rods

or wire by passing them between dies.

Threads are formed on the rod or wire with eachstroke of a pair of flat reciprocating dies.

Fig 13.16 shows Thread rolling processes: (a) and (b)

reciprocating flat dies; (c) two-roller dies; (d) A

collection of thread-rolled parts made economically at

high production rates.


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Thread Rolling

Thread rolling is superior to the other methods of

manufacturing threads, because machining the threads

cuts through the grain-flow lines of the material, whereas

rolling the threads results in a grain-flow pattern thatimproves the strength of the thread.

Fig 13.17 (a) shows features of a machined or rolled

thread. Grain flow in (b) machined and (c) rolled threads.

Unlike machining, which cuts through the grains of themetal, the rolling of threads imparts improved strength

because of cold working and favorable grain flow.


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Thread Rolling

Lubrication is important in thread-rolling operations in

order to obtain a good surface finish and surface

integrity and to minimize defects.


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Tube Rolling

The diameter and thickness of pipes and tubing can be

reduced by tube rolling, which utilizes shaped rolls.

Fig 13.19 shows the schematic illustration of various

tube-rolling processes: (a) with a fixed mandrel; (b)with a floating mandrel; (c) without a mandrel; and (d)

pilger rolling over a mandrel and a pair of shaped rolls.

Tube diameters and thicknesses also can be changed

by other processes, such as drawing, extrusion, and

spinning.


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13.5.1 Various mills

Integrated mills

These mills are large facilities that involve complete

integration of the activitiesfrom the production of

hot metal in a blast furnace to the casting and rolling

of finished products ready to be shipped to thecustomer.


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End of class

manufacturing week 2a- rolling of metals

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