Forming and rolling class 25 mar ppt

Metal FORMING AND

SHAPING PROCESSES

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Metal FormingLarge group of manufacturing

processes in which plastic deformation is used to change the shape of metal workpieces

The tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by

the geometry of the die


Stresses in Metal Forming Stresses to plastically deform the

metal are usually compressive Examples: rolling, forging, extrusion

However, some forming processes Stretch the metal (tensile stresses) Others bend the metal (tensile and

compressive) Still others apply shear stresses


Essential Material Properties Desirable material properties:

Low yield strength High ductility

These properties are affected by temperature: Ductility increases and yield strength

decreases when work temperature is raised Other factors:

Strain rate and friction


Basic Types of Metal Forming Processes

1. Bulk deformation Rolling processes Forging processes Extrusion processes Wire and bar drawing

2. Sheet metalworking Bending operations Deep or cup drawing Shearing processes


Bulk Deformation Processes Characterized by significant

deformations and massive shape changes

"Bulk" refers to workparts with relatively low surface area‑to‑volume ratios

Starting work shapes are usually simple geometries Examples:

Cylindrical billets Rectangular bars


Bulk Deformation Processes(a) Rolling and (b) forging


Considerations in Choosing a Lubricant

Type of forming process (rolling, forging, sheet metal drawing, etc.)

Hot working or cold working Work material Chemical reactivity with tool and

work metals Ease of application Cost

Metals are worked within the yield point and breaking stress producing the plastic deformation.

Hot working reduces the stress required to produce the yielding.

Working Range

A typical stress-strain curve for mild steel

CLASSIFICATION ON TEMPERATURE BASIS

HOT WORKINGCOLD WORKING

Forming of metals at a temperature above the re-crystallization temperature. Shape is obtained by virtue of their ability to flow plastically in the solid-state without accompanying deterioration of properties.

Advantages and Limitations Porosity in metal is largely eliminated. Impurities in the form of inclusions are

broken up and distributed thought the metal.

Coarse grains are refined; Physical properties improved.

HOT WORKING

HOT WORKINGEnergy required to change shape is lessGood Machinability and weldabilityNo residual stresses developed Ready to use productsRapid oxidation and scaling of the surface lead to poor surface finish and loss of metal Loss of carbon from steel surface- decarburizationWeakening of surface lead to fatigue cracksClose Tolerance cannot be obtained

Recrystallization Temperatures

Hot working Temperatures

Hot working is done above the recrystallization and below the melting temperature.

Depends upon the material properly and sometimes may be at room temperature (Lead, Zinc etc.)

Rolling, Forging, Extruding and Drawing are some major hot working in metals.

Working on metals at a temperature below the re-crystallization temperature, usually at room temp.

Deformation is brought by distortion of lattice about slip planes through slip and twinning mechanism

Advantages and limitations: Improves strength, machinability, dimensional

accuracy and surface finish of metals. Lesser oxidation and scaling cold working

allows thinner sheets to be worked accurately. Many products are cold finished after hot

working to make them commercially acceptable, close to tolerance and remove the scales and oxides.

COLD WORKING

Strength and hardness increases, loss in ductility.

Ideal for increasing hardness of metals which are not responding to heat treatment.

Surface finish is better and no oxidation Close dimensional control is possible Much more Pressure is needed than hot

working and there is no recovery of the distortion (Residual Stress).

COLD WORKING

COLD WORKING Distortion and fragmentation of grain

structure is created. Only applicable to ductile material Stresses are setup in metals that remains

unless removed by subsequent heat treatment

It results in loss of uniformity of metal composition and effect metal properties

Impact strength and elongation are reduced Bending, Drawing, Spinning, Forming,

Embossing, cold extrusion ,cold rolling and seaming are major cold working operations

ROLLİNG

Rolling of Metals

Flat-Rolling and Shape-

Rolling Processes

Flat-Rolling Process

Effects of Hot Rolling

It is the process of reducing the thickess orchanging the cross-section of a long workpiece by compressive forces applied through a set of rolls 90 % of Metals used are rolled sheets The metal moves through the rolling mill by a

friction process. The metal is pushed or drawn through the rolling mill

ROLLING PROCESS

Rolling Process : Introduction

Rolling is the most extensively used metal forming process and its share is roughly 90%

The material to be rolled is drawn by means of friction into the two revolving roll gap

The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area


The geometry of the product depend on the contour of the roll gap

Roll materials are cast iron, cast steel and forged steel because of high strength and wear resistance requirements

Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish


In rolling the crystals get elongated in the rolling direction. In cold rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone

The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough.

In the deformation zone the thickness of the strip gets reduced and it elongates. This increases the linear speed of the at the exit.

Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverses.


When the angle of contact α exceeds the friction angle λ the rolls cannot draw fresh strip

Roll torque, power etc. increase with increase in roll work contact length or roll radius

Hot rollingThe coarse –grained, brittle and porous

structure of the ingot (or continuous cast metal) is broken in wrought structure

Finer grain size, enhanced properties, strength and hardness

Cold rolling Room temperature, hardness and better

surface finishAnisotropic properties-preferential

orientationThis process requires higher energy

Rolling processes

Pressure variation during Rolling

Typical pressure variation along the contact length in flat rolling. The peak pressure is located at the neutral point. The area beneath the curve, representsroll force.

Friction in Rolling Friction in rolling: It depends on

lubrication, work material and also on the

temperature. In cold rolling the value of coefficient of

friction is around 0.1 and in warm working it is around 0.2. In hot rolling it is around 0.4. In hot rolling sticking friction condition is also seen and then friction coefficient is observed up to 0.7.

Rolling force and powerF=LwYavg

F rolling forceYavg is the average

true stressw is the width of the

stripL length of the

contactAdd 20 % for frictionTorque on the roll is the

product of a and F equals in average a=L/2

N revolutions per minute

kWFLNPower000.60

2

W width of the strip Diameter of the contact roll Metal structure Temperature of the metal Rolling speed µ friction coefficient Reduction in thickness Thickness incoming sheet Pull or push at the beginning or at the

end

Parameters that affect Rolling Force

Spreading in Flat Rolling

ROLLING TERMINOLOGY Absolute Draught : Difference between initial and final

thicknesses of the metal being rolled (T1 – T2) Absolute Elongation : Difference between the final and initial

length = ( L2 – L1) Absolute Spread : Difference between the final and initial

width = ( W2 – W1) Relative Draught : Ratio of absolute drawght to initial

thickness = (T1 – T2)/T1 Elongation Coefficient : Ratio of final length to initial length :

L2/L1

ROLLING TERMINOLOGY

Angle of Contact, No slip point, lagging zone, forward slip zone Forward slip : (V2 – V)/ V, V= roll velocity, V2 = velocity at delivery point. It vary from 3 to 10% and increases with the increase in roll diameter and coefficient of friction and also with reduction in thickness of the strip being rolled.

Rolling Mill

Shape Rolling of an H-section part

Two high rolling mills Reversing Mills Reversed direction Four High Mills and Clusters Mills (Sendzimir or

Z mills) : Use for subsequent rolling of slabs Cluster roll mill : used for cold rolling Continuous rolling mill (Tandem Mill) : Material

pass from initial to final stage . Speed of each roll is adjusted properly. Used for mass production

TYPES OF ROLLING MILLS

Roll Arrangements

To control the friction process lubricants have to be used

Low friction has to be used to avoid damage of the equipment

But friction is necessary

Lubrication during rolling

Defects Wavy Edges : Due to roll bending, edge

elongate more than the centre Spread : Due to high width to thickness

ratio, width of material remain constant. With smaller ratio, width increases considerable in the roll gap.

Crocodile crack : Due to weakness at the center, sheet bifurcate into two parts.

Bending of Rolls

Applications

Straight and long structural shapes Solid bars with various cross sections Channels I-beams Railroad rails Typical applications :

300 mm reactors vessels100mm thick tanks 1, 8 mm Boeing aircrafts0,7 mm car body panels 6 µm Al foil for packaging 0,28 mm Al beverage cans

Forming and rolling class 25 mar ppt

Engineering

Transcript of Forming and rolling class 25 mar ppt