FORGING
INTRODUCTION
In forging the parts are shaped by heating them in an open fire or hearth and shaping them through applying compressive forces using hammers. Forging is defined as the plastic deformation of metals at elevated temperatures into a predetermined size or shape using compressive forces exerted through some means of hand hammers, small power hammers, die, press or upsetting machine.
Forging consists essentially of changing or altering the shape and section of metal by hammering at a temperature of about 980°C, at which the metal is entirely plastic and can be easily deformed or shaped under pressure. The shop in which the various forging operations are carried out is known as the smithy or smith’s shop. Forging processes may be classified into hot forging and cold forgings and each of them possesses their specific characteristics, merits, demerits and applications.
Black-smithy is a process by which metal may be heated and shaped to its requirements by the use of blacksmith tools either by hand or power hammer. In smithy small parts are shaped by heating them in an open fire or hearth. Shaping is done under hand control using hand tools.
Forging by machine involves the use of forging dies and is generally employed for mass production of accurate articles. In drop forging, closed impression dies are used and there is drastic flow of metal in the dies due to repeated blow or impact which compels the plastic metal to conform to the shape of the dies. The final shape of the product from raw material is achieved in a number of steps.
Advantages of forging
Some common advantages of forging:
1. Forged parts offer great resistance to impact and fatigue loads.
2. Forging refines the structure of the metal.Forging distorts the previously created unidirectional fiber as created by rolling and increases the strength by setting the direction of grains.
3. It results in considerable saving in time, labor and material as compared to the production of similar item by cutting from a solid stock and then shaping it.
4. The reasonable degree of accuracy may be obtained in forging operation.
5. The forged parts can be easily welded.
Disadvantages of forging
1. Rapid oxidation in forging of metal surface at high temperature results in scaling which wears the dies.
2. Forging is limited to simple shapes and has limitation for parts having undercuts etc.
3. The initial cost of forging dies and the cost of their maintenance is high.
Applications of forging
Almost all metals and alloys can be forged.
The low and medium carbon steels are readily hot forged without difficulty, but the high-carbon and alloy steels are more difficult to forge and require greater care. Forging is generally carried out on carbon alloy steels, wrought iron, copper-base alloys, Stainless steels in ferrous materials.
In non ferrous group, alumunium alloys, and magnesium alloys. nickel-based super-alloys are forged. Titanium components are forged especially for aerospace uses.
Producing of crank shaft of alloy steel is a good example which is produced by forging. Forging processes are among the most important manufacturing techniques utilized widely in manufacturing of small tools, rail-road equipments, automobiles and trucks and components of aeroplane industries. These processes are also extensively used in the manufacturing of the parts of tractors, shipbuilding, cycle industries, railroad components, agricultural machinery etc.
FORGEABILITY
The ease with which forging is done is called forgeability. The forgeability of a material can also be defined as the capacity of a material to undergo deformation under compression without rupture. Forgeability increases with temperature up to a point at which a second phase, e.g., from ferrite to austenite in steel, appears or if grain growth becomes excessive. Certain mechanical properties are also influenced by forgeability. Metals which have low ductility have reduced forgeability at higher strain rate whereas highly ductile metals are not so strongly affected by increasing strain rates. The pure metals have good malleability and thus good forging properties. The metals having high ductility at cold working temperature possesses good forgeability.
The main alloys for cold forging or hot forging are mostly aluminium and copper alloys, including the relatively pure metals. Aluminium alloys are forged between 385°C and 455°C or about 400°C. Aluminium alloys do not form scale during hot forging operations, die life is thus excellent.
Copper and brasses with 30% or less zinc have excellent forgeability in cold working operations.
High zinc brasses can be cold forged to a limited extent but are excellent hot forging alloys.
Magnesium possessing hexagonal close packed (HCP) structure has little ductility at room temperature but is readily hot forged. Magnesium alloys are forged on presses at temperature above 400°C. At higher temperatures, magnesium must be protected from oxidation or ignition by an inert atmosphere of sulphur dioxide.
Carbon steels with 0.25 % carbon or less are readily hot forged or cold-headed. High carbon and high alloy steels are almost always hot forged.
FORGABLE MATERIALS
Forgeable metals are purchased as hot-rolled bars or billets with round or rectangular cross the sections. Forgeable materials should possess the required ductility and proper strength. Some forgeable metals are given as under in order of increasing forging difficulty.
1. Aluminium alloys
2. Magnesium alloys
3. Copper alloys.
4. Carbon and low alloy steels
5. Martensitic stainless steels
6. Austenitic stainless steels
7. Nickel alloys
8. Titanium alloys
9. Columbium alloys
10. Tantalum alloys
11. Molybdenum alloys
12. Tungsten alloys
13. Beryllium.
ADVANTAGES OF FORGING IN COMPARISON TO CASTING AND
MACHINING
Because of inherent improvement in the grain size and introduction of un-interrupted grain flow in the structure of finished forged component forging has the following advantages in comparison to casting and machining. Some of such advantages are given as under.
(i) Greater strength and toughness.
(ii) Reduction in weight of the finished part.
(iii) Saving in the material.
(iv) Elimination of internal defects such as cracks, porosity, blowholes, etc.
(v) Ability to withstand unpredictable loads during service.
(vi) Minimum of machine finish to be carried out on the component especially when it
is forged in dies.
EFFECT OF FORGING ON METAL CHARACTERISTICS
A continuous and uninterrupted grain flow in a forged component results in higher strength and toughness. In a cast part, there is no grain flow. Cast part is having random orientation of grains so it has weak crystalline structure.
The original crystals are deformed during forging operation and many of the constituents
are precipitated at high temperatures which again become soluble in the solid iron on freezing,
thus increasing the local homogeneity of the metal. The properties, like elastic limit, tensile
strength of metal are improved due to the grain flow.
Forging is generally employed for those components which require high strength and
resistance to shock or vibrations. It provides fine crystalline structure to the metal, improves
physical properties, closes all voids and forms the metal to shapes. It enhances the mechanical
properties of metals and improves the grain flow which in turn increases the strength and
toughness of the forged component.
But there may be certain defects also, like scale inclusions on the surface, misalignment
of the dies, crack, etc. These defects can be controlled.
All forgings are covered with scale and hence they require cleaning operation. It is done by
pickling in acid, shot peening or tumbling depending upon the size and composition of the
forgings. If some distortion has occurred in forging, a sizing or straightening operation may
be required. Controlled cooling is usually provided for large forgings. Heat treatment may
also be required to provide certain physical properties.
Choice of Forging Process
Parts made using cast iron tend to need to be bulky and are used where they will not be subjected to high stresses. Typical examples are machine bases, cylinder blocks, gear-box housings etc.
Besides other factors, cost is a major consideration in deciding whether to cast a component or to forge it. An I.C. engine connecting rod is a very good example of where a forging will save machining time and material, whereas the cylinder block of the same engine would be very expensive if produced by any process other than casting. Big or small complex shapes can easily be cast. Small parts can directly be machined out from regular section materials economically.
A part machined out from the rolled steel stock definitely possesses better mechanical properties than a conventionally cast part. Sometimes the shape and size of a part would mean removing a large amount of material by machining. It is sometimes more economical to forge the part, thereby reducing the machining time and the amount of material required.
Forging Process - Heating for Forging
Hearths and Furnaces for Heating
Forgeable metals are heated either in a hearth or in a furnace. The hearths are widely used for heating the metals for carrying out hand forging operations.
Furnaces are also commonly used for heating metals for heavy forging. The forging job is always heated to the correct forging temperature in a hearth or in a furnace located near the forging arrangements.
Gas, oil or electric-resistance furnaces or induction heating classified as open or closed hearths can be used. Gas and oil are economical, easily controlled and mostly used as fuels.
The formation of scale, due to the heating process especially on steel creates problems in forging. A non-oxidizing atmosphere should, therefore, be maintained for surface protection. Special gas-fired furnaces have been developed to reduce scaling to minimum. Electric heating is the most modern answer to tackle scaling and it heats the stock more uniformly also.
In some cases, coal and anthracite, charcoal containing no sulphur and practically no ash are the chief solid fuels used in forging furnaces.
Forge furnaces are built raise temperatures up to 1350°C in their working chambers. They
should be sufficiently large to allow proper combustion of the fuel, and to obtain uniform
heating of the forging jobs.
Each heating furnace consists of parts including firebox, working chamber, chimney, flues, re-cuperator or regenerator, and various auxiliary arrangements.
Fuels used in forging shop
The fuels used in forging shop are classified as solid, liquid and gaseous fuels which are discussed as under.
Solid fuels: Wood, coal, anthracite, peat, charcoal, coke, pulverized fuel etc.
Liquid fuels: Crude oil, petroleum, kerosene, tar oil etc.
Gaseous fuels: Natural gas and some artificially produced gases are used generate heat.
CONTROL OF HEATING DEVICES
For good control of heating devices such as hearth or forging furnace, the following points are should always be considered.
1. The nozzle pointing into the centre of the hearth is called the tuyre and is used to direct a stream of air into the burning coke. The air is supplied by centrifugal blower.
2. As the hottest part of the fire is close to the tuyre opening, therefore, the tuyre is provided with a water jacket to prevent it from burning away.
3. The hood provided at the top of hearth collects smoke, fumes etc., and directs them away from the workplace through the chimney in form of exhaust.
4. The fuel for the fire may be either black-smithing coal or coke. To light the fire, either use paper and sticks or preferably a gas poker.
5. Impurities will collect as clinker and must be removed from the bottom of the fire when the fire cools.
6. The blowers are used to control the air supply using forced draught. Regulators control the draught and the temperature of the fire.
7. Blower delivers to forge adequate supply of air at proper pressure which is very necessary for the combustion of fuel.
8. A centrifugal blower driven by an electric motor is an efficient means of air supply in forging hearth.
9. Fire tools such as rake, poker and slice are generally used to control or manage the fire and theses tools are kept nearby the side of the hearth. Rake is used to take heated workpiece out of the fire. Poker is a steel rod which is used to poke (stir) fire in the hearth.
10. The place of the metal to be heated should be placed just above the compact centre of a sufficiently large fire with additional fuel above to reduce the heat loss and atmospheric oxidation.
FORGING TEMPERATURES
The temperature to start the forging for soft, low carbon steels is 1,250 to 1,300°C, the
temperature to finish forging is 800 to 840°C. The corresponding temperatures for high carbon
and alloy steels which are hard in nature are 1100 to 1140°C and 830 to 870°C. Wrought iron
is best forged at a temperature little below 1,290°C. Non ferrous alloys like bronze and brass
are heated to about 600 to 930°C, the aluminium and magnesium alloys to about 340 to 500°C.
The temperature of heating steel for hand forging can be estimated by the color of
heat and which color of the light emitted by the heated steel. For accurate determinations of
forging temperatures of the heated part, the optical pyrometers are generally used.
FORGING METHODS
The forging methods are generally classified into two categories namely hand forging and power forging.
Hand forging
Hand forging is performed in the black smithy shop. The job is heated at the forging temperature in hearth and it is then brought on anvil using tong. It is then forged using hand hammers and other hand forging tools for imparting specific shape.
COMMON HAND FORGING TOOLS
For carrying out forging operations manually, certain common hand forging tools are employed.
These are also called blacksmith’s tools, for a blacksmith is one who works on the forging
of metals in their hot state. The main hand forging tools are as under.
1 .Tongs
2.Flatter
3. Swage
4. Fuller
5. Punch
6. Rivet header
7. Hot chisel
8. Hammers
9. Anvil
10. Swage block
11. Drift
12. Set-hammer
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14. Brass scale
15. Brass
16. Black smith’s gauge
17. Heading tool
The applications of some of the hand forging tool are described as under.
Tongs
The tongs are generally used for holding work while doing a forging operation.
1. Flat tongs are used for mainly for holding work of rectangular section.
2. Straight-lip fluted tongs are commonly used for holding square, circular and hexagonal bar stock.
3. Rivet or ring tongs are widely used for holding bolts, rivets and other work of circular section.
4. Gad tongs are used for holding general pick-up work, either straight or tapered.
Flatter
Flatter is is commonly used in forging shop to give smoothness and accuracy to articles which have already been shaped by fullers and swages.
Swage
Swage is used for forging work which has to be reduced or finished to round, square or hexagonal form. It is made with half grooves of dimensions to suit the work being reduced. It consists of two parts, the top part having a handle and the bottom part having a square shank which fits in the hardie hole on the anvil face.
Fuller
Fuller is used in forging shop for necking down a forgeable job. It is made in top and bottom tools as in the case of swages. Fuller is made in various shapes and sizes according to needs, the size denoting the width of the fuller edge
Punch
Punch is used in forging shop for making holes in metal part when it is at forging heat.
Rivet header: Rivet header is used in forging shop for producing rivets heads on parts.
Chisels
Chisels are used for cutting metals and for nicking prior to breaking. They may be hot
or cold depending on whether the metal to be cut is hot or cold. A hot chisel generally used
in forging shop. The main difference between the two is in the edge. The edge of a cold chisel is hardened and tempered with an angle of about 60°, whilst the edge of a hot chisel is 30° and the hardening is not necessary. The edge is made slightly rounded for better cutting action.
Hand hammers
There are two major kinds of hammers are used in hand forging: (1) the hand hammer
used by the smith himself and (2) the sledge hammer used by the striker. Hand hammers
may further be classified as (a) ball peen hammer, ( b ) straight peen hammer, and
(c) cross peen hammer. Sledge hammers may further be classified as (a) Double
face hammer, ( b ) straight peen hammer, and (c) cross peen hammer. Hammer heads are made
of cast steel and, their ends are hardened and tempered. The striking face is made slightly
convex. The weight of a hand hammer varies from about 0.5 to 2 kg where as the weight of
a sledge hammer varies from 4 to 10 kg.
Set hammer
A set hammer generally u is used for finishing corners in shouldered work where the flatter would be inconvenient. It is also used for drawing out the gorging job.
Anvil
An anvil is a most commonly tool used in forging shop. It acts as a support for blacksmith’s work during hammering. The body of the anvil is made of mild steel with a tool steel face welded on the body, but the beak or horn used for bending curves is not steel faced. The round hole in the anvil called pritchel hole is generally used for bending rods of small diameter, and as a die for hot punching operations. The square or hardie hole is used for holding square shanks of various fittings. Anvils in forging shop may vary up to about 100 to 150 kg and they should always stand with the top face about 0.75 mt. from the floor. This height may be attained by resting the anvil on a wooden or cast iron base in the forging shop.
Swage block
Swage block generally used in forging shop. It is mainly used for heading, bending, squaring, sizing, and forming operations on forging jobs. It is 0.25 mt. or even more wide. It may be used either flat or edgewise in its stand.
Drift
Drift generally used in forging shop. It is a tapered rod made of tool steel. Holes are opened out by driving through a larger tapered punch called a drift.
Hardie
Hardie is a type of chisel used in forging shop. Its taper head is fixed into the hardie hole of the anvil, the cutting edge being upward. The part to be cut is kept over the cutting edge of the fixed hardie on anvil and another chisel is placed over the job and the cutting is performed by hammering.
Shovel
Shovel generally used in forging shop to place coal or coke in the furnace. It is also used to set coal pieces in furnace and remove ash from furnace.
Poker
Poker is employed for removing clinker from the furnace and to loose the
compact coal pieces in the furnace.
Rake
Rake is used to put coal pieces on tuyres.
Beak Iron
Beak iron is also known as small anvil and is made of forged steel. Its upper front end consists of horn and upper back end comprises of flat tail. Its taper shank is inserted into the hardie hole of the anvil. It is commonly used as anvil for small forge work.
The hand forging operations are:
1. Upsetting 2. Bending
3. Drawing down 4. Cutting
5. Setting down 6.Punching
7. Flattening 8. Fullering
9. Forge Welding 10. Swaging
(i) Drawing out
Drawing out is used to reduce the thickness of a bar and to increase its length. It may be carried out by working the metal over the horn the anvil , then by hammering it on the anvil face.
The rounded horn of the anvil acts as a blunt edge, which forces the metal to flow lengthwise when struck by the hammer. For drawing down very heavy work, fuller may be used for drawing down a bar over the horn (round portion) of anvil.
(ii) Fullering
Fullering operation involves heating the stock in the black smith hearth. Then heated stock is placed on the fuller fixed on anvil. A fuller is put over the sock and hammering is done to reduce the cross section of job at required point.
(iii) Upsetting
Upsetting is also known as jumping operation which is carried out to increase the
thickness (or diameter) of a bar and to reduce its length. Generally, the increase in thickness
is only local, for example, when forming a bolt head. This operation is an operation just
opposite to drawing and involves increasing the cross-sectional area usually by hammering or
pressing in a direction parallel to the ingot axis. The length of the ingot decreases and
following the path of least resistance it spreads out. The required shape is given the ingot
by spreading it between two dies. Only that portion of the bar which is to be upset is heated
locally. Or, the whole bar is heated and except for the portion to be upset, the rest is quenched
in water so that upset will form only on the hot portion of the bar. In one method of upsetting,
the bar is held in the tong and supported vertically on the anvil. The top edge of the bar is
then hammered to form the upset on the bottom hot end of the bar. For upsetting, the blow
of the hammer must be in line with the bar to prevent bending of the bar.
(iv) Bending
Bending is a very commonly used forging operation in forging shop to give a turn to a
metal rod or plate. It is accompanied by spreading of the metal in the inside of the bend and
narrowing at outside. The simplest method of bending a piece of metal in hand forging is to
support it on the anvil and to strike its free end with a hammer When bent, the metal of
the workpiece thins out round bend causing weakness. This can be overcome by upsetting the
bar prior to bending.
(v) Cutting
Cutting is a main forging operation to cut out a metal rod or plate into two pieces with
the help of a chisel and hammer when the metal is in red hot condition. A hot or cold cut
(chisel) is used for cutting heated metal bars in a smithy shop. The hot set does not require
hardening and tempering. Its cutting edge is keener than that of a cold set. Hot sets are
manufactured from a tough variety of steel in order that they may cut through relatively soft
red-hot metal with ease. While cutting, it is best to cut half through the workpiece to turn
it over and cut through from the other end.
(vi) Punching
Punching is a main forging operation used for producing hole in metal plate by using a
tool known as punch. The metal plate is placed over the hollow cylindrical die and punch is
placed above it at required location where hole is being made. For punching a hole, the metal
job must be at near welding heat and the punch is driven part way through the job with
hammer blows. The work is then turned over and the hole is completed from the other side.
The above said practice is adopted for thicker jobs.
(vii) Forge Welding
It is a process of joining two metal pieces to increase the length by pressing or hammering
them when they are at forging temperature. It is performed in forging shop and hence
sometimes it is called as forge welding.
Power Forging
To have heavy impact or blow for more plastic deformation, power hammers generally classified as spring hammer and drop hammers are used. The capacity of these hammers is given by the total weight. A 100 kg hammer will be one of which the falling pans weigh 100 kg. The heavier these parts and greater the height from which they fall, the higher will be intensity of blow the hammer will provide. Power hammers are of different types e.g. spring power hammers, pneumatic power hammers etc. These hammers are named due to their construction, according to their way of operation.
Spring Hammer
Spring hammer is commonly used for small forgings. It is light type of power hammer. The oscillation of the spring is responsible for the up and down movement of the tup thus, the required blows are provided on the job to be forged. A hand lever is also equipped with this mechanical kind of hammer to adjust the stroke of the connecting rod and, hence the intensity of blows.
Eccentric type of spring hammer is the one in which a rotating eccentric disc is used for
producing vibrations in the spring. It can be operated by means of a foot ring, known as
treadle provided at the bottom and is connected to the shaft at the top through a vertical bar
having a clutch at its end. The shaft at the top of hammer carries a pulley and a solid disc
at the end. The pulley is driven by means of a belt from the line shaft or an electric motor.
The solid disc, at the, end of the shaft, carries a crank connected eccentrically to it which has
a laminated spring at its lower end. The nip carrying the weight is suspended on a toggle joint
connecting the two ends of the laminated spring. When the foot treadle is pressed the clutch
engages with the shaft and the disc carrying the crank starts rotating which in turn produces
fluctuations in the toggle joint of the machine. It makes the tup to move and down in vertical
direction. The speed of blows entirely depends upon the speed of the driving pulley.
Spring hammers may be made available in various capacities having the tup weights
from 30 to 250 kg. Those having top weights 50 to 100 kg and speed of blows up to 300 per
minute are in generally used in forging shop. These hammers have a common drawback in
their springs getting broken very frequently due to severe vibrations during forging of the
jobs in the forging shop.
Drop Hammers
Drop hammers are operated hydraulically and are widely used for shaping parts by drop hammering a heated bar or billet into a die cavity. A drop forging raises a massive weight and allows it to fall under gravity on close dies in which forge component is allowed to be compressed. The die incorporates its shape on to the hot work piece. Drop hammers are commonly used for forging copper alloys and steel.
HEAT TREATMENT OF FORGING
Heat treatment is carried out for releasing the internal stresses arising in the metal during forging and cooling of work piece. It is used for equalizing the granular structure of the forged metal and improving the various mechanical properties. Generally forged parts are annealed, normalized and tempered to obtain the desired results.
