A
ABB Group .............. .38
Ace Micromatic Group ...... .61
ACMA ................ 32,62
Ampco Metal ............. .64
Automation Industry Association .............. .26
B
Bajaj Motors ............. .63
Batliboi Ltd .............. .61
BFW Ltd ................ .61
Bharat Electronics Ltd ...... .63
Bharat Heavy Electricals ..... .63
BorgWarner Turbo Systems
Worldwide Headquarters .... .52
Bosch Ltd ............... .61
Boschert GmbH + Co KG .... .64
Brakes India Ltd ........... .52
C
Carl Zeiss India (Bangalore) Pvt Ltd ................. .64
Claas .................. .38
Cummins India ........... .38
D
Danfoss ................. .38
Delcam Software .......... .61
Deutsche Messe. . . . . . . . . . . .24
DMG MORI ............ 26,76
DRDO .................. .63
G
Grind Master Machines ..... .64
GW Precision Tools India .... .61
H
HAESL .................. .46
Hexagon Metrology India .... .61
Hypertherm (India) Thermal
Cutting ................. .64
I
IMTMA ........ .8,10,22,60,62
Indian Railways ........... .63
International Institute of
Welding ................ .56
Inverto ................. .38
Isgec Heavy Engineering Ltd . . .65
ISRO ................... .63
This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. The metallic mold can be reused many times before it is discarded or rebuilt.
The molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness. The mold is made in two halves in order to facilitate the removal of casting from the mold. It may be designed with a vertical parting line or with a horizontal parting line as in conventional sand molds.
The mold walls of a permanent mold have thickness from 15 mm to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. For faster cooling, fins or projections may be provided on the outside of the permanent mold. This provides the desirable chilling effect.
Advantages
(i) Fine and dense grained structure is achieved in the casting. Because of rapid rate of cooling, the castings possess fine grain structure.
(ii) No blow holes exist in castings produced by this method. Good surface finish and surface details are obtained. defects observed in sand castings are eliminated.
(iii) The process is economical for mass production. Fast rate of production can be attained.
(iv) Close dimensional tolerance or job accuracy is possible to achieve on the cast product.
(v) Manpower required is less.
Disadvantages
(i) The cost of metallic mold is higher than the sand mold. The process is impractical for large castings.
(ii) The surface of casting becomes hard due to chilling effect.
(iii) Refractoriness of the high melting point alloys.
Applications
(i) This method is suitable for small and medium sized casting such as carburetor bodies, oil pump bodies, connecting rods, pistons etc.
(ii) It is widely suitable for non-ferrous casting.
PRESSURE DIE CASTING
Molten metal is forced into metallic mold or die under pressure in pressure die casting. The pressure is generally created by compressed air or hydraulically means. The pressure varies from 70 to 5000 kg/cm 2 and is maintained while the casting solidifies. The application of high pressure is associated with the high velocity with which the liquid metal is injected into the die to provide a unique capacity for the production of intricate components at a relatively low cost. This process is called simply die casting in USA. The die casting machine should be properly designed to hold and operate a die under pressure smoothly. There are two general types of molten metal ejection mechanisms adopted in die casting set ups which are:
(i) Hot chamber type
(a) Gooseneck or air injection management
(b) Submerged plunger management
(ii) Cold chamber type
Die casting is widely used for mass production and is most suitable for non-ferrous metals and alloys of low fusion temperature. The casting process is economic and rapid. The surface achieved in casting is so smooth that it does not require any finishing operation. The material is dense and homogeneous and has no possibility of sand inclusions or other cast impurities. Uniform thickness on castings can also be maintained.
The principal base metals most commonly employed in the casting are zinc, aluminum, and copper, magnesium, lead and tin. Depending upon the melting point temperature of alloys and their suitability for the die casting, they are classified as high melting point (above 540°C) and low melting point (below 500°C) alloys. Under low category involves zinc, tin and lead base alloys. Under high temperature category aluminum and copper base alloys are involved.
Applications
1. Carburetor bodies
2. Hydraulic brake cylinders
3. Refrigeration castings
4. Washing machine
5. Connecting rods and automotive pistons
6. Oil pump bodies
7. Gears and gear covers
8. Aircraft and missile castings, and
9. Typewriter segments
10.2 Heating and Pouring
10.2.1 Heating the Metal
10.2.2 Pouring the Molten Metal
10.2.3 Engineering Analysis of Pouring
10.2.4 Fluidity
10.3 Solidification and Cooling
10.3.1 Solidification of Metals
10.3.2 Solidification Time
10.3.3 Shrinkage
10.3.4 Directional Solidification
10.3.5 Riser Design
The starting work material is either a liquid or is in a highly plastic condition, and parts are created through solidification of the material. Casting and molding processes dominate this category of shaping operations.
Casting is a process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. Casting also means the part that is made by the casting process.
11. METAL CASTING PROCESSES
Chapter Contents
11.1 Sand Casting
11.1.1 Patterns and Cores
11.1.2 Molds and Mold Making
11.1.3 The Casting Operation
11.2 Other Expendable-Mold Casting Processes
11.2.1 Shell Molding
11.2.2 Vacuum Molding
11.2.3 Expanded Polystyrene Process
11.2.4 Investment Casting
11.2.5 Plaster-Mold and Ceramic-Mold Casting
11.3 Permanent-Mold Casting Processes
11.3.1 The Basic Permanent-Mold Process
11.3.2 Variations of Permanent-Mold Casting
11.3.3 Die Casting
11.3.4 Squeeze Casting and Semisolid Metal Casting
11.3.5 Centrifugal Casting
11.4 Foundry Practice
11.4.1 Furnaces
11.4.2 Pouring, Cleaning, and Heat Treatment
Engineering materials used to manufacture of articles or products, dictates which manufacturing process or processes are to be used to provide it the desired shape. Sometimes, it is possible to use more than one manufacturing processes, then the best possible process must be utilized in manufacture of product. It is therefore important to know what materials are available in the universe with it usual cost. What are the common characteristics of engineering materials such as physical, chemical, mechanical, thermal, optical, electrical, and mechanical? How they can be processed economically to get the desired product. The basic knowledge of engineering materials and their properties is of great significance for a design, manufacturing and industrial engineers. The elements of tools, machines and equipments should be made of such a material which has properties suitable for the conditions of operation. In addition to this, a product designer, tool designer and design engineer should always be familiar with various kinds of engineering materials, their properties and applications to meet the functional requirements of the design product. Product industrial engineers also need to have this knowledge. They must understand all the effects which the manufacturing processes and heat treatment have on the properties of the engineering materials.
CLASSIFICATION OF ENGINEERING MATERIALS
A large numbers of engineering materials exists in the universe such as metals and non metals (leather, rubber, asbestos, plastic, ceramics, organic polymers, composites and semiconductor). Leather is generally used for shoes, belt drives, packing, washers etc. It is highly flexible and can easily withstand against considerable wear under suitable conditions. Rubber is commonly employed as packing material, belt drive as an electric insulator. Asbestos is basically utilized for lagging round steam pipes and steam pipe and steam boilers because it is poor conductor of heat, so avoids loss of heat to the surroundings. Engineering materials may also be categorized into metals and alloys, ceramic materials, organic polymers, composites and semiconductors. The metal and alloys have tremendous applications for manufacturing the products required by the customers.
Metals and Alloys
Pure metals possess low strength and do not have the required properties. So, alloys are produced by melting or sintering two or more metals or metals and a non-metal, together. Alloys may consist of two more components. Metals and alloys are further classified into two major kind namely ferrous metals and non-ferrous metals.
(a) Ferrous metals are those which have the iron as their main constituent, such as pig iron, cast iron, wrought iron and steels.
( b ) Non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminium, brass, bronze, tin, silver zinc, invar etc.
Ferrous metals are iron base metals which include all variety of pig iron, cast iron wrought iron and steels. The ferrous metals are those which have iron as their main constituents. The ferrous metals commonly used in engineering practice are cast iron, wrought iron, steel and alloy steels.
Main Types of Iron
1. Pig iron
2. Cast iron
(A) White cast iron
(B) Gray cast iron
(C) Malleable cast iron
(D) Ductile cast iron
(E) Meehanite cast iron
(F) Alloy cast iron
3. Wrought iron
4. Steel
(A) Plain carbon steels
1. Dead Carbon steels
2. Low Carbon steels
3. Medium Carbon steels
4. High Carbon steels
(B) Alloy steels
1. High speed steel
2. Stainless steel
Grey Cast Iron: Applications
The grey iron castings are mainly used for machine tool bodies, automotive cylinder
blocks, pipes and pipe fittings and agricultural implements. The other applications involved
are
(i) Machine tool structures such as bed, frames, column etc.
(ii) Household appliances etc.
(iii) Gas or water pipes for under ground purposes.
(iv) Man holes covers.
(v) Piston rings.
(vi) Rolling mill and general machinery parts.
(vii) Cylinder blocks and heads for I.C. engines.
(viii) Frames of electric motor.
(ix) Ingot mould
(x) General machinery parts.
(xi) Sanitary wares.
(xii) Tunnel segment.
White Cast Iron: Applications
(i) For producing malleable iron castings.
(ii) For manufacturing those component or parts which require a hard, and abrasion resistant surface such as rim of car.
(iii) Railway brake blocks.
Malleable cast iron
Malleable cast iron are generally used to form automobile parts, agriculture
implementation, hinges, door keys, spanners mountings of all sorts, seat wheels, cranks,
levers thin, waned components of sewing machines and textiles machine parts.
Wrought Iron: Applications
It is used for making chains, crane hooks, railway couplings, and water and steam pipes. It has application in the form of plates, sheets, bars, structural works, forging blooms and billets, rivets, and a wide range of tubular products including pipe, tubing and casing, electrical conduit, cold drawn tubing, nipples and welding fittings, bridge railings, blast plates, drainage lines and troughs, sewer outfall lines, weir plates, sludge tanks and lines, condenser tubes, unfired heat exchangers, acid and alkali process lines, skimmer bars, diesel exhaust and air brake piping, gas collection hoods, coal equipment, cooling tower and spray pond piping.
Aluminium: Applications
It is mainly used in aircraft and automobile parts where saving of weight is an advantage.
The high resistance to corrosion and its non-toxicity make it a useful metal for cooking
utensils under ordinary conditions. Aluminium metal of high purity has got high reflecting
power in the form of sheets and is, therefore, widely used for reflectors, mirrors and telescopes.
It is used in making furniture, doors and window components, rail road, trolley cars, automobile
bodies and pistons, electrical cables, rivets, kitchen utensils and collapsible tubes for pastes.
Aluminium foil is used as silver paper for food packing etc. In a finely divided flake form,
aluminium is employed as a pigment in paint. It is a cheap and very important non ferrous
metal used for making cooking utensils.
Copper: Applications
Copper is mainly used in making electric cables and wires for electric machinery, motor
winding, electric conducting appliances, and electroplating etc. It can be easily forged, casted,
rolled and drawn into wires. Copper in the form of tubes is used widely in heat transfer work
mechanical engineering field. It is used for household utensils. It is also used in production
of boilers, condensers, roofing etc. It is used for making useful alloys with tin, zinc, nickel
and aluminium. It is used to form alloys like brass, bronze and gun metal. Alloys of copper
are made by alloying it with zinc, tin, and lead and these find wide range of applications.
Brass, which is an alloy of copper and zinc, finds applications in utensils, household fittings,
decorative objects, etc. Bronze is an alloy of copper and tin and possesses very good corrosion
resistance. It is used in making valves and bearings. Brass and bronze can be machined at
high speeds to fine surface finish.
Red Brass: Applications
Red brass is mainly utilized for making, heat exchanger tubes, condenser, radiator cores,
plumbing pipes, sockets, hardware, etc.
Muntz metal: Applications
It is utilized for making for making tubes, automotive radiator cores, hardware fasteners, rivets, springs, plumber accessories and in tube manufacture.
Admiralty Brass: Applications
Admiralty brass is utilized for making condenser tubes in marine and other installations. It is used for making plates used for ship building. It is utilized also for making bolts, nuts, washers, condenser plant and ship fittings parts, etc.
Timber is obtained from trees by cutting the main body of tree in the suitable sizes after the full growth of tree. The timber structure is consisting of annual rings, heartwood, sapwood, pith, cambium layer, bast, medullary rays and bark. Commercial timbers are commonly classified into hardwoods and softwoods. Hardwoods comprises of oak and beech that have a broad leaf. Whereas softwoods include pine and spruce which have narrow needle like leaf.
Conversion means sawing of timber logs into different commercial sizes. A notable feature in conversion is to provide an adequate allowance for shrinkage that takes place during
seasoning of sawn or converted wood. The shrinkage of wood usually varies between 3.2 mm
to 6.4 mm, according to the type of wood and its time of cutting.
Seasoning of wood is the reduction of the moisture or sap content of it to the point where, under normal conditions of use, no further drying out will take place. The main objective of seasoning is to reduce the unwanted amount of moisture from the timber
Good timber is free from knots, insects attack, excessive moisture, discoloration, twisted fibers,
cup and ring shake, sound, bright and free from any discoloration. It is solid with annual rings
but not hallow in the center. Timber should be well seasoned for easily workable specific use.
It should possess straight fibers and high fire resistance. It should not split when nails are
driven in it. It should not clog with the saw teeth during the sawing operation. Timber should
be highly suitable for polishing and painting.
The factors influencing the selection of timber involve the quality of timber in terms of its durability, workability, weight, hardness, cohesiveness, elasticity, type of texture, type of grains, resistance to fire, resistance to various stresses, ability to retain shape, suitability for polishing and painting.
Metal forming is also known as mechanical working of metals. Metal forming operations are employed either to produce a new shape or to improve the properties of the metal. Metal forming is an intentional and permanent deformation of metals plastically beyond the elastic range of the material. The main objectives of metal forming processes are to provide the desired shape and size, under the action of externally applied forces in metals. The process improves required mechanical properties in the metal and reduces any internal voids or cavities present and thus make the metal dense.
The plastic deformation of a metal takes place when applied forces reaches the yield point.
Plasticity, ductility and malleability are the properties of a material, which retains the deformation produced under applied forces permanently and hence these metal properties are important for metal working processes.
Mechanical working/forming processes which are done above recrystallisation temperature of the metal are know as hot working processes. If the hot working is completed just above the recrystallisation temperature then the resultant grain size would be fine. For any hot working process the metal should be heated to such a temperature below its solidus temperature, that after completion of the hot working its temperature will remain a little higher than and as close as possible to its rccrystalisation temperature
HOT WORKING PROCESSES
1. Hot rolling 2. Hot forging 3 . Hot extrusion 4. Hot drawing 5. Hot spinning 6. Hot piercing or seamless tubing 7. Tube Forming and 8. Hot forming of welded pipes
Hot Rolling
Rolling is the most rapid method of forming metal into desired shapes by plastic deformation through compressive stresses using two or more than two rolls. It is one of the most widely used of all the metal working processes. The main objective of rolling is to convert larger sections such as ingots into smaller sections which can be used either directly in as rolled state or as stock for working through other processes.
The coarse structure of cast ingot is convened into a fine grained structure in rolling. Significant improvement is accomplished in rolled parts in their various mechanical properties such as toughness, ductility, strength and shock resistance. The crystals in parts are elongated in the direction of rolling, and they start to reform after leaving the zone of stress.
The majority of steel products are being converted from the ingot form by the process of rolling. Hot rolling process is being widely used in the production of large number of useful products such as rails, sheets, structural sections, plates etc. There are different types of rolling mills.
________________
________________
Two-High Rolling Mill
A two-high rolling mill has two horizontal rolls revolving at the same speed but
in opposite direction. The rolls are supported on bearings housed in sturdy upright side
frames called stands. The space between the rolls can be adjusted by raising or lowering the
upper roll. Their direction of rotation is fixed and cannot be reversed. The reduction in the
thickness of work is achieved by feeding from one direction only. However, there is another
type of two-high rolling mill, which incorporates a drive mechanism that can reverse the
direction of rotation of the rolls. A Two- high reverse arrangement is also there.
In a two-high reversing rolling mill, there is continuous rolling of the workpiece through
back-and-forth passes between the rolls.
Three-High Rolling Mills
It consists of three parallel rolls, arranged one above the other. The directions of rotation of the upper and lower rolls are the same but the intermediate roll rotates in a direction opposite to both of these. This type of rolling mill is used for rolling of two continuous passes in a rolling sequence without reversing the drives. This results in a higher rate of production than the two-high rolling mill.
Four-High Rolling Mill
It is essentially a two-high rolling mill, but with small sized rolls. Practically, it consists of four horizontal rolls, the two middle rolls are smaller in size than the top and bottom rolls. The smaller size rolls are known as working rolls which concentrate the total rolling pressure over the work piece. The larger diameter rolls are called back-up rolls and their main function is to prevent the deflection of the smaller rolls, which otherwise would result in thickening of rolled plates or sheets at the centre. The common products of these mills are hot or cold rolled plates and sheets.
Cluster Mill
It is a special type of four-high rolling mill in which each of the two smaller working rolls are backed up by two or more of the larger back-up rolls. For rolling hard thin materials, it may be necessary to employ work rolls of very small diameter but of considerable length. In such cases adequate support of the working rolls can be obtained by using a cluster-mill. This type of mill is generally used for cold rolling work.
Continuous Rolling Mill
It consists of a number of non reversing two-high rolling mills arranged one after the other, so that the material can be passed through all of them in sequence. It is suitable for mass production work only, because for smaller quantities quick changes of set-up will be required and they will consume lot of time and labor.
Applications of Rolling
Rolling mills produce girders, channels, angle irons and tee-irons. Plate mill rolls slabs into plates. The materials commonly hot rolled are aluminium, copper magnesium, their alloys and many grades of steel.
Industrial Engineering and Productivity Management of Hot Rolling
The rolled product quality depends on the quality of the charge, the construction of a rolling machine, setting of the rolls, a kind and state of armament, temperature and a way of heating as well as the level of training a worker and his experience. Other significant quality parameters which need to be addressed are; Raw Material Inspection and Approval Process, Finished Product quality Approval Process, Geometrical Parameter Test, Physical Parameter Test, Chemical Test.
Productivity and Quality Improvement through Setting Parameters in
Hot Rolling Mill
International Research Journal of Engineering and Technology (IRJET)
Volume: 05 Issue: 04 | Apr-2018 https://www.irjet.net/archives/V5/i4/IRJET-V5I4239.pdf
Hot piercing is also known as seamless tubing or roll piercing process. . It is used for making thin-
walled round objects. Seamless tube forming is popular and economical process in comparison to machining because it saves material wasted in boring of parts.
Hot piercing includes rotary piercing to obtain formed tube by piercing a pointed mandrel through a billet in a specially designed rolling mill. The rotary piercing can be performed either on a two-high rolling mill or on a three-high rolling mill. In the former, the two rolls are set at an angle to each other. The billet under the rolls is deformed and a cavity formation is initiated at the centre due to tensile stressing. The carefully profiled shape of the mandrel assists and controls the formation of cavity. In a three-high rolling mill, the three shaped rolls are located at 1200 and their axes are inclined at a feed angle to permit forward and rotary motion of the billet. The squeezing and bulging of the billet open up a seam in its center pass makes a rather thick-walled tube which is again passed over plug and through grooved rolls in a two-high roll mill where the thickness is decreased and the length is increased. While it is still up to a temperature, it is passed on to a reeling machine which has two rolls similar to the piercing rolls, but with flat surfaces. If more accuracy and better finish are desired, the run through sizing dies or rolls. After cooling, the tubes are used in a pickling bath of dilute sulphuric acid to remove the scale.
It is the process of enclosing the heated billet or slug of metal in a closed cavity and then
pushing it to flow from only one die opening so that the metal will take the shape of the
opening. The pressure is applied either hydraulically or mechanically. Extrusion process is
identical to the squeezing of tooth paste out of the tooth paste tube. Tubes, rods, hose, casing,
brass cartridge, moulding-trims, structural shapes, aircraft parts, gear profiles, cable sheathing
etc. are some typical products of extrusion. Using extrusion process, it is possible to make
components, which have a constant cross-section over any length as can be had by the rolling
process. The intricacy in parts that can be obtained by extrusion is more than that of rolling,
because the die required being very simple and easier to make. Also extrusion is a single pass
process unlike rolling. The amount of reduction that is possible in extrusion is large. Generally
brittle materials can also be easily extruded. It is possible to produce sharp corners and re-
entrant angles. It is also possible to get shapes with internal cavities in extrusion by the use
of spider dies, which are explained later.
The extrusion setup consists of a cylinder container into which the heated billet or slug of
metal is loaded. On one end of the container, the die plate with the necessary opening is fixed. From
the other end, a plunger or ram compresses the metal billet against the container walls and the
die plate, thus forcing it to flow through the die opening, acquiring the shape of the opening. The
extruded metal is then carried by the metal handling system as it comes out of the die.
The extrusion ratio is defined as the ratio of cross- sectional area of the billet to that
of the extruded section. The typical values of the extrusion ratio are 20 to 50. Horizontal
hydraulic presses of capacities between 250 to 5500 tonnes are generally used for conventional
extrusion. The pressure requirement for extrusion is varying from material to material. The
extrusion pressure for a given material depends on the extrusion temperature, the reduction
in area and the extrusion speed.
Methods of Hot Extrusion
Hot extrusion process is classified as
1. Direct or forward hot extrusion
2. Indirect or backward hot extrusion
3. Tube extrusion
Different methods of extrusion Each method is described as
under.
Direct or Forward Hot Extrusion
In this method, the heated metal billet is placed in to the die chamber and the pressure is applied through ram. The metal is extruded through die opening in the forward direction, i.e. the same as that of the ram. In forward extrusion, the problem of friction is prevalent because of the relative motion between the heated metal billet and the cylinder walls. To reduce such friction, lubricants are to be commonly used. At lower temperatures, a mixture of oil and graphite is generally used. The problem of lubrication gets compounded at the higher operating temperatures. Molten glass is generally used for extruding steels.
In indirect extrusion, the billet remains stationary while the die moves into the billet by the hollow ram (or punch), through which the backward extrusion takes place. Since, there is no friction force between the billet and the container wall, therefore, less force is required by this method. However
this process is not widely used because of the difficulty occurred in providing support for the extruded part.
Tube Extrusion
This process is an extension of direct extrusion process where additional mandrel is needed to restrict flow of metal for production of seamless tubes. Aluminium based toothpaste and medicated tubes are produced using this process.
HOT DRAWING
Drawing is pulling of metal through a die or a set of dies for achieving a reduction in a diameter. The material to be drawn is reduced in diameter. Fig. is another method used in hot drawing or shaping of materials where the heated blank is placed over the die opening the punch forces the blank through the die opening to form a cup or shell. The multiple dies are also used to accomplish the stages in drawing process. Kitchen utensils and components of food processing industries are manufactured by this process.
Hot spinning is a process in which pressure and plastic flow is used to shape material. Spinning is generally carried over a spinning lathe. The metal is forced to flow over a rotating shape by pressure of a blunt tool. The amount of pressure of the blunt tool against the disc controls the generated
heat, which helps in forming processes.
EFFECT OF HOT WORKING ON MECHANICAL PROPERTIES OF METALS
1. Raising the metal temperature lowers the stresses required to produce deformations
and increases the possible amount of deformation before excessive work hardening
takes place.
2. In hot working processes, compositional irregularities are ironed out and non-metallic impurities are broken up into small, relatively harmless fragments, which are uniformly dispersed throughout the metal instead of being concentrated in large stress-raising metal working masses.
3. Hot working such as rolling process refines grain structure. The coarse columnar dendrites of cast metal are refined to smaller equiaxed grains with corresponding improvement in mechanical properties of the component.
4. Oxidation and scaling take place and hence surface finish of hot worked metal is not nearly as good as with cold working.
5. The temperatures at which hot work is started and stopped affects the properties to be introduced in the hot worked metal.
6. Too high a temperature may cause phase change and overheat the steel whereas too low temperature may result in excessive work hardening.
7. Defects in the metal such as blowholes, internal porosity and cracks get removed
or welded up during hot working.
8. During hot working, self-annealing occurs and recrystallization takes place immediately following plastic deformation. This self-annealing action prevents hardening and loss of ductility.
HOT WORKING - MERITS
1. As the material is above the recrystallisation temperature, any amount of working
can be imparted since there is no strain hardening taking place.
2. At a high temperature, the material would have higher amount of ductility and
therefore there is no limit on the amount of hot working that can be done on a
material. Even brittle materials can be hot worked.
3. In hot working process, the grain structure of the metal is refined and thus mechanical
properties improved.
4. Porosity of the metal is considerably minimized.
5. If process is properly carried out, hot work does not affect tensile strength, hardness,
corrosion resistance, etc.
6. Since the shear stress gets reduced at higher temperatures, this process requires
much less force to achieve the necessary deformation.
7. It is possible to continuously reform the grains in metal working and if the temperature and rate of working are properly controlled, a very favorable grain size could be achieved giving rise to better mechanical properties.
8. Larger deformation can be accomplished more rapidly as the metal is in plastic state.
9. No residual stresses are introduced in the metal due to hot working.
10. Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal.
11. Mechanical properties, especially elongation, reduction of area and izod values are
improved, but fibre and directional properties are produced.
12. Hot work promotes uniformity of material by facilitating diffusion of alloy constituents and breaks up brittle films of hard constituents or impurity namely cementite in steel.
DEMERITS OF HOT WORKING
1. Due to high temperature in hot working, rapid oxidation or scale formation and surface de-carburization take place on the metal surface leading to poor surface finish and loss of metal.
2. On account of the loss of carbon from the surface of the steel piece being worked the surface layer loses its strength. This is a major disadvantage when the part is put to service.
3 . The weakening of the surface layer may give rise to a fatigue crack which may ultimately result in fatigue failure of the component.
4. Some metals cannot be hot worked because of their brittleness at high temperatures.
5. Because of the thermal expansion of metals, the dimensional accuracy in hot working is difficult to achieve.
6. The process involves excessive expenditure on account of high cost of tooling. This however is compensated by the high production rate and better quality of components.
7. Handling and maintaining of hot working setups is difficult and troublesome.
Cold working of a metal is carried out below its recrystallisation temperature. Normal room temperatures are ordinarily used for cold working of various types of steel. But temperatures up to the recrystallisation range are sometimes used in certain applications.
COLD WORKING PROCESSES
1. Rolling 2. Extrusion 3. Wire drawing 4. Forging
5. Cold spinning 6. Shot peening
Cold working processes are also similar to hot working processes except for the temperature at which work is done.
COLD-ROLLING
Cold rolling process setup is similar to hot rolling. Bars of all shapes such as rods, sheets and strips are commonly finished by rolling. Foil is made of the softer metals in this way. Cold-rolling metals impart smooth bright surface finish and in good physical and mechanical properties to cold rolled parts.. Cold rolling also improves machinability in the cold rolled part by conferring the property of brittleness, a condition, which is conducive to smooth tool, finishes with broken chips.
The preliminary step to the cold-rolling operation, the sheets of pre hot-rolled steel are immersed in an acid solution to remove the washed in water and then dried. The cleaned steel is passed through set of rolls of cold rolling process thereby producing a slight reduction in each the required thickness is obtained.
The arrangement of rolls in a rolling mill, also called rolling stand, varies depending on the application. The various possible configurations of rolls are similar to hot rolling. Internal stresses are set up in cold rolled parts which remain in the metal unless they are removed by proper heat-treatment. This process needs more power for accomplishing the operation in comparison to hot rolling.
COLD EXTRUSION
Principle of cold extrusion is similar to that of hot extrusion. Impact extrusion is also a cold extrusion process. It is used for making small components from ductile materials. Impact extrusion of material is accomplished where the work blank is placed in position over the die opening the punch forces the blank through the die opening causing material to flow plastically around the punch. The outside diameter of the tube is same as diameter of the die, and the thickness is controlled by the clearance between punch and die. Collapsible medicare tubes and toothpastes etc. are produced using this impact extrusion.
WIRE DRAWING
The process of producing the wires of different diameters is accomplished by pulling a wire through a hardened die usually made up carbide. However a smaller diameter wires are drawn through a die made of diamond. The larger diameter oriented wire is first cleaned, pickled, washed and then lubricated. It is normally done by acid pickling. After picklng, it is washed in water and coated with lime and other lubricants. To make for an easier entrance of wire into the die, the end of the stock is made pointed to facilitate the entry. A pointed or reduced diameter at the end of wire duly lubricated is pushed or introduced through the die which is water cooled also. This pointing is done by means of rotary swaging or by simple hammering. It is then gripped and pulled for attaching it to a power driven reel. The wire diameter is reduced in die because of the ductility property of the material to the smaller diameter through one set of die. For more reduction in diameter of the wire, various sets of dies can be used in line for subsequent reduction in diameter at each stage. The reduction in each pass through the die range about 10% for steel and 40% for ductile materials such as copper.
The drawing of the wire starts with a rod or coil of hot rolled steel, which is 0.8 to 1.6 mm larger than the final size required. The material should be sufficiently ductile since it is pulled by the tensile forces. Hence, the wire may have to be annealed properly to provide the necessary ductility. Further, the wire is to go through the conical portion and then pulled out through the exit by the gripper. To carry the lubricant input through the die, special methods such as gulling, coppering, phosphating and liming are used.
For very thin wires, electrolytic coating of copper is used to reduce friction. The dies used for wire drawing are severely affected because of high stresses and abrasion.
The various die materials that are used are chilled cast iron, tool steels, tungsten carbide and diamond. The cast iron dies are used for small runs. For very large sizes, alloy steels are used in making the dies. The tungsten carbide dies are used most commonly for medium size wires and large productions. The tungsten carbide dies arep referred because of their long life that is 2 to 3 times that of alloy steel dies. For very fine wires, diamond dies are used. Wire drawing improves the mechanical properties because of the cold working. The material loses its ductility during the wire drawing process and when it is to be repeatedly drawn to bring it to the final size, intermediate annealing is required to restore the ductility.
Cold Drawing
Like hot drawing, it also involves the forcing of a metal through by means of a tensile force applied to the exit side of the drawing die. Most of the plastic flow is accomplished by the compressive force which arises from the reaction of metal with die. It is the operation in which the metal is made to flow plastically by applying tensile stresses to the metal. The blank of calculated diameter is placed on a die and held of it by a blank holder and bottom is pressed into the die by a punch and the walls are pulled.
This process is generally used for making cup shaped parts from the sheet blanks, without excessive wrinkling, thinning and fracturing. It can undertake jobs of nearly any size. It is a process of managing a flat precut metal blank into a hollow vessel. Utensils of stainless steel are generally made by this process.
Efficiency of operation
The efficiency of operation depends upon blank size, reduction factor, drawing pressure, blank holding pressure, punch and die diameters, type of lubricant, die material etc.
SHOT PEENING
It is a process of increasing the hardness and fatigue strength on parts surfaces. The process comprises of throwing a blast of metal shot on to the surface of a component requiring shot peening. It is used to set up a superficial state of surface compression stress, causing the interior of the member to assume an opposite tensile stress. Blast may be thrown either by air pressure or with help of a wheel revolving at high speed. This high velocity blast of metal shot provides a sort of compression over the components surface and increases hardness and strength of the surface and also its fatigue resistance.
