Forging - Introduction

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

?

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.

Ch.14 Rajinder Singh

WYMAN-GORDON 50,000-TON FORGING PRESS. 1955. NORTH ... The Mesta Machine Co. of Pittsburgh was contracted to build the press.

https://www.asme.org/wwwasmeorg/media/ResourceFiles/AboutASME/Who%20We%20Are/Engineering%20History/Landmarks/89-wyman-gordon-50000-ton-hydraulic-forging-press.pdf

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5.MANUFACTURINGPROCESSES

Product Design Guide For Forging

5. FORGING - MANUFACTURING PROCESSES
    5.1 Forging Machinery
        5.1.1 Hammers
        5.1.2 Presses
    5.2 Forging Processes
        5.2.1 The Open Die Process
        5.2.2 The Impression Die Process
            5.2.2.1 Conventional Impression Die Forging
            5.2.2.2 Flashless (Enclosed Impression Die) Forging
            5.2.2.3 Net and Shape Forging
            5.2.2.4 Hot Die and Isothermal Forging
        5.2.3 The Ring Rolling Process
        5.2.4 The Cold Forging Process
            5.2.4.1 Alloys Used for Cold Forging
            5.2.4.2 Cold Forging Processes
            5.2.4.3 Product Advantages of Cold Forging
        5.2.5 The Warm Forging Process
    5.3 Secondary Operations
        5.3.1 Heat Treating
        5.3.2 Machining
        5.3.3 Finishing Operations

https://www.forging.org/forging/design/5-manufacturing-processes-0.html

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Cost Drivers for Forging

https://www.forging.org/forging/design/331-materials-cost.html

https://www.forging.org/forging/design/332-tooling-costs.html

https://www.forging.org/forging/design/333-manufacturing-cost.html

More in:

https://www.forging.org/forging/design/3-design-and-development-products-made-forgings.html

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3.THEDESIGNANDDEVELOPMENTOFPRODUCTSMADEFROMFORGINGS

3. THE DESIGN AND DEVELOPMENT OF PRODUCTS MADE FROM FORGINGS

3.1 Concurrent Engineering
3.2 Design Parameters for Forgings

• 3.2.1 Service Loads
• 3.2.2 Service Temperatures
• 3.2.3 Alloys Forged
• 3.2.4 Interfacing Structural Components
• 3.2.5 Accidental Unanticipated Conditions

3.3 Cost Drivers

• 3.3.1 Material Cost
• 3.3.2 Tooling Cost
• 3.3.3 Manufacturing Cost
• 3.3.4 Secondary Operations
• 3.3.5 Quantities Produced

3.4 Process Tradeoffs

• 3.4.1 A Comparison of Open Die, Impression Die, Rolled Ring and Cold Forging Processes
• 3.4.2 Forging Processes Compared With Alternate Processes
• 3.4.2.1 Sheet Metal Stamping
• 3.4.2.2 Weldments
• 3.4.2.3 Foundry Casting
• 3.4.2.4 Investment Casting
• 3.4.2.5 CNC Machined Bar and Plate
• 3.4.2.6 Powder Metallurgy
• 3.4.2.7 Reinforced Plastics and Composites

3.5 Designing Products Made from Forgings

• 3.5.1 Development of a Typical Shape
• 3.5.2 Selecting a Forging Company
• 3.5.3 Selecting the Optimum Forging Alloy
• 3.5.4 Product design Guidelines
• 3.5.4.1 Design Rules for Parts Made From Impression Die Forgings
• 3.5.4.2 Design Rules For Parts Made From Upset Forgings
• 3.5.4.3 Design Rules for Parts Made From Open Die Forgings
• 3.5.4.4 Design Rules for Parts Made From Rolled Rings
• 3.5.4.5 Design Rules for Parts Made by Cold and Warm Forging

3.6 Predicting, Developing and Maintaining Properties in Forgings

• 3.6.1 Physical Properties
• 3.6.2 Mechanical Properties
• 3.6.3 Developing and Maintaining Product Performance

3.7 Specifying Heat Treating

• 3.7.1 Heat Treating Steel
• 3.7.1.1 Annealing Processes for Steel
• 3.7.1.2 Hardening Treatments for Steel
• 3.7.2 Heat Treating Stainless Steels
• 3.7.3 Heat Treating Aluminum Alloys
• 3.7.4 Heat Treating Titanium Alloys
• 3.7.5 Heat Treating Heat Resistant Alloys

3.8 Prototyping

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Industrial Engineering and Productivity Improvement of Forging Operations

https://www.researchgate.net/publication/312316702_Productivity_Improvement_in_Forging_Industry_Using_Industrial_Engineering_Techniques

https://www.springerprofessional.de/en/industrial-engineering-and-ergonomics/manufacturing/scientists-are-developing-ergonomic-forging-tongs/16909142

About forging on IISE site
https://www.iise.org/details.aspx?id=2712

Enhancing tool availability in the forging industry by adjusting PPC and tool maintenance
2011 IEEE International Conference on Industrial Engineering and Engineering Management
https://ieeexplore.ieee.org/document/6117886


Forging Companies

https://www.bharatforge.com/worldwide/national

UD 21 May 2021
Pub 12 August 2019

Metal Forming - Hot Working - Cold Working

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.

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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

Analysing quality and productivity improvement in steel rolling industry in central India
International Conference on Advances in Engineering & Technology – 2014 (ICAET-2014)
PP 06-11
http://iosrjournals.org/iosr-jmce/papers/ICAET-2014/me/volume-7/2.pdf?id=7622

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

HIGH-PERFORMANCE HOT ROLLING MILLS
Electrics and Automation
Good information on electrical drives, control of quality parameters
https://www.sms-group.com/press-media/downloads/download-detail/15493/

 Hot Piercing or Seamless tubing

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.

Seamless steel pipe manufacturing process

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https://www.youtube.com/watch?v=ewA1v-s0Dp4

HOT EXTRUSION

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.

Aluminum Extrusion

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https://www.youtube.com/watch?v=iiGlq7408ME

Indirect or Backward Hot Extrusion

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.

EJP Chain Draw Bench Line DB 120

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https://www.youtube.com/watch?v=INgJSHOgioU

HOT SPINNING

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.

Hot spinning machine for CNG cylinder

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https://www.youtube.com/watch?v=Z39GZYZ5l9U


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.