Drawing Print

8/7/2019 Drawing Print

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CONTENTS:

Introduction

Properties of hot and cold drawing

process

Advantages & Disadvantages

Bibliography


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Introduction:

Drawing is a metalworking process which uses tensile

forces to stretch metal. It is broken up into two types: sheet

metal drawing and wire, bar, and tube drawing. The specific

definition for sheet metal drawing is that it involves plastic

deformation over a curved axis. For wire, bar, and tube drawing

the starting stock is drawn through a die to reduce its diameter

and increase its length. Drawing is usually done at room

temperature, thus classified a cold working process, however it

may be performed at elevated temperatures to hot work large

wires, rods or hollow

sections in order to

reduce forces.

The resistance ofmetals to plastic

deformation generally

falls with temperature.

For this reason, larger

massive sections are always worked hot by forging, rolling, or

extrusion. Metals display distinctly viscous characteristics atsufficiently high temperatures, and their resistance to flow

increases at high forming rates. This occurs not only because

it is a characteristic of viscous substances, but because the rate

of recrystallization may not be fast enough.

Hot Drawing:


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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.

Cold Drawing:

Plastic deformation which is carried out in a temperature

region and over a time interval such that the strain hardening is

not relieved is called cold work. Considerable knowledge on

the structure of the cold-worked state has been obtained. In

the early stages of plastic deformation, slip is essentially on

primary glide planes and the dislocations form coplanar arrays.

As deformation proceeds, cross slip takes place. The cold-

worked structure forms high dislocation density regions that

soon develop into networks. The grain size decreases with

strain at low deformation but soon reaches a fixed size. Cold

working will decrease ductility.

Hot working refers to the process where metals are

deformed above their recrystallization temperature and

strain hardening does not occur. Hot working is usually


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performed at elevated temperatures. Lead, however, is hot-

worked at room temperature because of its low melting

temperature. At the other extreme, molybdenum is cold-

worked when deformed even at red heat because of its high

recrystallization temperature.

Properties of Hot & Cold Drawing

Process:

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.

The porosity of the steel ingot can be eliminated to a

greater extent.

Great latitude in shape and size of form is possible due to

reduction of elastic limit.

A uniformity is established either by squeezing other

impurities into fiber sla gs or distributing them throughout

the mass.

Directional property resulting from a fiber structure is

obtained.


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Due to refinement of grains, mechanical properties such

as toughness, ductility, elongation and reduction in area

are improved.

The power required to finish the part ingot is less.

It can be used on most of the metals, because it is a rapid

and economical process.

Advantages & Disadvantages of

Drawing Process :

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.


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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 productionrate and better quality of products.


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Bibliography:

www.wikipedia.com

ASM Publications
http://www.wikipedia.com/http://www.wikipedia.com/

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