Mechanical Working of Metals (Rolling and Forging)

7/29/2019 Mechanical Working of Metals (Rolling and Forging)

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Rolling and Forging

Mechanical Working of Metals


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Working Processes

Mechanical

Working

Cold Working Hot Working

Done at temp < Trec Done at temp > Trec

Recrystallization temperature :

temperature at which recrystallization occurs

0.3 to 0.6 Tm


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The Effect of Temperature on

Working Processes

Cold Working

metal strain hardens

after deformation

metal hardens

needs high stress to

deform metal

very good surfacefinish

very good

dimensional tolerance

Hot Working

metal recrystallizes

after deformation

metal stays soft

needs little stress to

deform metal

forms SCALES atthe skin

poor dimensional

tolerance


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Rolling

Process of reducing the thickness of

metals by allowing it to pass through a

pair of rotating rolls

More metal is rolled

than the total of all theother working processes


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Products of Rolling


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Roll Stand


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Rolls for I-beams


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Two-high

mill

Two-high

Reversing mill

Three-high

mill

Types of Rolling Mills


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Types of Rolling Mills

Four-high

mill

Six-high

mill

Cluster mill

(Sendzimir Mill)


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Cluster or Sendzimir Mill


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Planetary Mill


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Rolling Sequence


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Rolling Sequence


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Rolling Defects

ALLIGATORING


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Misaligned Rolls


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Wavy Edges


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Edge Cracking


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Rolling Load Calculation

Ideal Rolling Load No friction involved

Actual Rolling Load Friction involved

Friction normally increases the required

load


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Terminologies used in Rolling


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Rolling Load Analysis

h = draft = ho - hfVo = entry speed

Vf = exit speed

R = roll radius

= angle of bite

= angle between

entrance plane and

centerline


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Ideal Rolling Load

Pressure = Load

Area

Rolling Load = Roll Pressure x Area

hRWRL y =

RL = Rolling Load

y = flow stress (yield stress)W = Width

R = Roll Radius

h = draft


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Considerations on

Actual Rolling Load

At any point in

the arc of contact,

2 forces act on

the metal:

Pr = Rolling Load

F = Friction Load

For unaided entry: Fcos = Prsin = tan

and F = Pr


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Maximum Reduction Possible

Rh

hR

hR =

=

2

tan

= tan

(h)max = 2R


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Sample Problem

Determine the maximum reduction possible

on a piece of metal when using 500 mm

diameter rolls and during cold rolling when = 0.1 and hot rolling when = 0.6


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Determine the final height of an aluminumsheet with initial thickness 60 mm if

subjected to cold rolling ( = 0.2) using

300 mm diameter rolls and assumingmaximum reduction (also consider 3%elastic rebound).

Sample Problem


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Observations in Actual Rolling

Exit speed, Vf > Entry speed, Vo

Assuming constant volume passing through

one point at any given time

Vroll

Vo

Vf

A B

Neutral Point

orNo-Slip

point


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Friction Hill

At the Neutral

Point, there is no

slipping betweenthe metal and the

Rolls

At the Neutral

Point, Friction is

at a maximum!

N


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Actual Rolling Load

Condition Condition

Plain Strain 1.155 < 0.5Slipping Friction

HomogenousDeformation

1.0 > 0.5Sticking Friction

1/2

Plane Strain

when

Slipping Frictionis found in cold

working

Sticking Frictionis found in hot

working

+=

22

1

1

hh

hRhRWRL yActual

5>

hR

W


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Sample Problem

Calculate the rolling load required to reducesteel 600 mm wide and 30 mm thick by

20%. The roll diameter used is 800 mm,

= 0.15 and flow stress of steel is 150 MPa.


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Sample ProblemExplain the Design of Cluster Mill.


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Forging

The act of

shaping metal by

hammering or

pressing

Forging is done inspecial molds

called "dies"


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Forging Products
http://www.sbf.net.cn/html/..%5Cimages%5Ccp-a22_r2_c2.jpg


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Dies used in Forging


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Types of Forging

Press Forging Impact (Smith)

Forging


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Types of Forging

OPEN-DIE

FORGING

CLOSED-DIE

FORGING


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Closed-Die Forging


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Examples of Forging

UPSETTING COGGING

SWAGING


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FULLERING

EDGING

HEADING

Examples of Forging


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Precision Forging


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Precision Forging Operation


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Forging of Bevel Gear


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Forging Defects


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Defects in Forging

Laps Formed in

Forged Metal


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Calculation of Forging Load

UPSET FORGING

P


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Ideal vs Actual

HOMOGENOUS

DEFORMATION

NONHOMOGENOUS

DEFORMATION


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Nonhomogenous Deformation

BARRELING


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Grain Texture in Forged Metal

Eff t f F i ti F i


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Effect of Friction on Forging

Pressure

Pressure builds-up at

the center creating a

pressure hill!

Eff t f F i ti F i


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Effect of Friction on Forging

Load

The build-up ofpressure also creates

a Friction Hill

-Forging Load Calculation-


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Forging Load Calculation

The SLAB METHOD

ASSUME PLANE STRAIN (WIDTH IS CONSTANT)

b = tool bite

Analysis of Stresses using


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Equilibrium of forces in the horizontal direction:


Slab Method

( ) 02 =++ hdxhd xyxx

02

=+ dxh

dy

x

Since both x and y are unknown, another equation must beobtained using yield criteria; specifically, the distortion-energy

criterion for plane strain is used.

YYyx

==3

2

Henceyx dd =

and



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The two equations for the stressescan now be solved:

( ) hxay eYp

/2

==

hxy

y

yCedx

h

d/2

2

==

The value of the constant, C, is found

using the boundary conditions. At x=a,

x=0 and y= Y`ha

eYC

/2

=The value of the constant, C, is found using the boundaryconditions. At x=a, x=0 and y= Y`. The pressure is then


Slab Method

Si lifi ti f F i


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Simplification of Forging

Pressure Equation

( ) hxay eYp

/2 ==

If x


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Sample Problems

A copper slab was upset to give a final

dimension of 60 x 20 x 10 mm (L x W x H). If

the yield stress of copper is 40 MPa and the

coefficient of friction is 0.15, determine theForging Pressure at the following points on

the billet:

i. At the middle

ii. At the ends

iii.(1/3)a distance from the middle

P


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The average pressure canbe approximated from the

mean of the maximum and

the minimum pressure

+h

aYpavg

1

The forging load, F, is

( )( ) ( )widthapF

avg2=

Average Forging Pressure, Pavg


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Reminders on P and Pavg

+

h

aYpavg

1]

)(21['

h

xaYp y

+==

Condition FlowStress Condition

PlaneStrain

Y = 1.155 < 0.5Slipping Friction

HomogenousDeformation

Y > 0.5Sticking Friction

1/2

Plane Strain W/b > 4

when

Slipping Frictionis found in cold

working

Sticking Frictionis found in hot

working


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Sample Problems

A copper slab was upset to give a final

dimension of 60 x 20 x 10 mm (L x W x H). If

the yield stress of copper is 40 MPa and the

coefficient of friction is 0.15, determine theaverage forging pressure of this process.

P


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Sample Problem

Determine the Forging Load required to

reduce the thickness of half of a steel billet

100x40x10 mm (LxWxH) by 40%(assume plane strain). Assume that the

yield stress of steel is 240 MPa and =0.65.

F

Mechanical Working of Metals (Rolling and Forging)

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