manufacturing engineering technology in si units, 6...

Manufacturing Engineering Technology in SI Units, 6th Edition PART III:

Forming and Shaping Processes and Equipment

Copyright © 2010 Pearson Education South Asia Pte Ltd

PART III: Forming and Shaping Processes and Equipment

“Forming” indicates changing the shape of an existing

solid body



For forming processes, the starting material may be in

the shape of a plate, sheet, bar, rod, wire, or tubing of

various cross sections

Shaping processes involve the molding and casting of

molten materials and the finished product is near the

final desired shape

Molten metalis cast into individual ingots or continuously

cast into slabs, rods, or pipes

Cast structures are converted to wrought structures by

plastic-deformation processes


Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 13: Metal-Rolling Processes and Equipment

Copyright © 2010 Pearson Education South Asia Pte Ltd Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Outline

Introduction

The Flat-rolling Process

Flat-rolling Practice

Rolling Mills

Various Rolling Processes and Mills


Introduction

Rolling is the process of

reducing the cross section

of a long workpiece by

compressive forces

applied through a set

of rolls



Flat-rolling process is shown

Friction forces act on strip surfaces

Roll force, F, and torque, T, acts on the rolls



As the surface speed of the rigid roll is constant, there

is relative sliding between the roll and the strip along

the arc of contact in the roll gap, L

At neutral point or no-slip point, the velocity of the

strip is the same as that of the roll

The maximum possible draft is defined as the

difference between the initial and final strip thicknesses

From the relationship, higher the friction and the larger

the roll radius, the greater the maximum possible draft

becomes


Rhh fo

2

The Flat-rolling Process:

Roll Force, Torque, and Power Requirements

Rolls apply pressure on the flat strip to reduce its

thickness, resulting in a roll force, F

Roll force in flat rolling can be estimated from

Total power (for two rolls) is


avgLwYF L = roll-strip contact length

w = width of the strip

Yavg = average true stress of the strip

000,33

2hp)(in Power

000,60

2kW)(in Power

FLN

FLN



EXAMPLE 13.1

Calculation of Roll Force and Torque in Flat-rolling

An annealed copper strip 228 mm wide and 25 mm thick is

rolled to a thickness 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.




Solution

Roll-strip contact length is calculated through geometry,

Absolute true strain of the strip is

Average true stress is

The roll force is


223.020

25ln

mm 7.3820253000 fo hhRL

MN 4.171801000250

10007.38 avgLwYF

MPa 1802/28080



Solution

With 100 rpm, the total power is calculated from

Force and the power requirements is difficult to obtained

due to (a) the exact geometry between the roll and the

strip and (b) coefficient of friction and the strength of the

material in the roll gap


W705000,66

100

1000

7.381074.12

000,66

2 6 FLN

Power



Reducing Roll Force

Roll forces can cause deflection and flattening of the

rolls

The columns of the roll stand may deflect under high

roll forces

Roll forces can be reduced by:

1. Reducing friction at the roll–workpiece interface

2. Using smaller diameter rolls

3. Reduce the contact area

4. Rolling at elevated temperatures

5. Applying front and/or back tensions to the strip



Geometric Considerations

Roll forces will bend the rolls elastically during rolling

When the roll bends, the strip has a constant thickness

along its width

The heat generated by plastic deformation cause the

rolls to be slightly barrel shaped (thermal camber)

Roll forces also tend to flatten the rolls elastically



Geometric Considerations

Spreading

Increase in width is called spreading

Spreading increases with:

1. Decreasing width-to-thickness ratio of the entering strip

2. Increasing friction

3. Decreasing ratio of the roll radius to the strip thickness



Vibration and Chatter

Vibration and chatter have effects on product quality

and the productivity of metalworking operations

Chatter defined as self-excited vibration

Occur in rolling and in extrusion, drawing, machining,

and grinding operations

Chatter results from interactions between the structural

dynamics of the mill stand and the dynamics of the

rolling operation

Chatter can be reduced by increasing the roll radius,

strip-roll friction and incorporating dampers in the roll

supports



Initial rolling steps (breaking down) of the material is

done by hot rolling

A cast structure is dendritic and is brittle and porous

Hot rolling converts the cast structure to a wrought

structure with finer grains and enhanced ductility

Product of the first hot-rolling operation is called a

bloom, a slab, or a billet

To improve flatness, the rolled strip goes through a

series of leveling rolls


Flat-rolling Practice:

Defects in Rolled Plates and Sheets

Defects may be present on the surfaces or there may

be internal structural defects

They are undesirable as they compromise surface

appearance and adversely affect strength, formability,

and other manufacturing characteristics

Surface defects may be caused by inclusions and

impurities in the original cast material

Wavy edges on sheets are

the result of roll bending

Cracks are due to poor

material ductility at the

rolling temperature



Other Characteristics of Rolled Metals

Residual Stresses

Residual stresses develop in rolled plates and sheets

due to nonuniform deformation of materials in roll gap



Other Characteristics of Rolled Metals

Dimensional Tolerances

Thickness tolerances for cold-rolled sheets range from

±0.1~0.35 mm

Flatness tolerances are within ±15 mm/m for cold

rolling and ±55 mm/m for hot rolling

Surface Roughness

Cold rolling can produce a very fine surface finish

Cold-rolled sheets products may not require additional

finishing operations


Rolling Mills

Automated mills produce close-tolerance, low cost and

high quality plates and sheets at high production rates


Rolling Mills

Two-high rolling mills are used for hot rolling in initial

breakdown passes (cogging mills) on cast ingots or in

continuous casting

In tandem rolling, the strip is rolled continuously

through a number of stands to thinner gages with each

pass


Rolling Mills

Roll Materials

Basic requirements for roll materials are strength and

resistance to wear

Forged-steel rolls have higher strength, stiffness, and

toughness than cast-iron rolls

Rolls made for cold rolling should not be used for hot

rolling as they may crack from thermal cycling (and

spalling

Lubricants

Hot rolling of ferrous alloys do not need lubricants

Water-based solutions are used to cool the rolls Copyright © 2010 Pearson Education South Asia Pte Ltd


Shape Rolling

Straight and long structural shapes are formed at

elevated temperatures by shape rolling



Roll Forging

Cross section of a round bar is shaped by passing it

through a pair of rolls with profiled grooves



Skew Rolling

Similar to roll forging and used for making ball bearings

Another method is to shear pieces from a round bar

and then upset them in headers between two dies with

hemispherical cavities



Ring Rolling

A thick ring is expanded into a large-diameter thinner

one

Thickness is reduced by bringing the rolls closer

together as they rotate

Short production times, material savings and close

dimensional tolerances



Thread Rolling

Thread rolling is a cold-forming process by which

straight or tapered threads are formed on round rods or

wire

Threads are formed with rotary dies at high production

rates



Thread Rolling

Thread-rolling process has the advantages of

generating threads with good strength without any loss

of material

Internal thread rolling can be carried out with a

fluteless forming tap, produces accurate internal

threads with good strength



Rotary Tube Piercing

Also known as the Mannesmann process

It is a hot-working operation for making long, thick-

walled seamless pipe and tubing

The round bar is subjected to radial compressive

forces while tensile stresses develop at the center of

the bar



Tube Rolling

Diameter and thickness of pipes and tubing can be

reduced by tube rolling, which utilizes shaped rolls


Various Rolling Processes and Mills:

Various Mills

Integrated Mills

Integration process starts from production of hot metal

to the casting and rolling of finished products to

shipping

Minimills

Scrap metal is:

1. Melted in electric-arc furnaces

2. Cast continuously

3. Rolled directly into specific lines of products


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