SINTERING AND POST SINTERING PROCESS FOR STEEL INDUSTRY
Chapter 01
INTRODUCTION
Then technique of forming metal parts from powders by pressing and sintering dates back to
the beginning of human civilization. Almost every metal or ceramic material was initially
made using the powder route. Modern applications of sintering in materials technology are
widespread powder-technological production of structural steel parts, self-lubricating
bearings, porous metals for filtering, tungsten wires for lamp filaments, soft and hard
magnetic materials, electrical contacts, composite packages for highly integrated electronic
devices, oxide-dispersion strengthened super alloys for high temperature motors, amalgams
for dental applications, metallic and ceramic materials for medical applications, cemented
carbides for cutting tools and a large variety of ceramic components are only a few of the
many technical production processes involving sintering as an important step. The
consolidation of powders and densification of porous solids is possible by pressing and
subsequent pressure less heat-treatment that is solid-state sintering, by simultaneous
application of pressure and heat that is hot-pressing or pressure-sintering or with the aid of a
limited amount of melt known as liquid-phase sintering.
1.1 MANUFACTURING PROCESS
Manufacturing process is a collection of technologies and methods used to define how
products are to be manufactured. It is production method that creates goods by combining
supplies, ingredients or raw materials using a formula. During the manufacturing process,
these raw materials are modified to deliver the finished goods. There is, obviously, not one
manufacturing process to take you from beginning to end. There are many. Some processes
are intermediate and make components that undergo another manufacturing process to build
the finished product.
1.1.1
TYPES OF MANUFACTURING PROCESSES
Manufacturing of a product includes different processes and operations like Machining
process, Assembly or Joining Process, Moulding Process. To get a final product from raw
material it may undergo into machining process, joining or assembly process, moulding
process. Every process have their advantages and disadvantages, some of the processes
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required additional machining like surface smoothness, finishing. All this type of machining
process is to get a final product with accurate and with quality. Shaping a required product
from a raw material includes different fields including finding the material properties like
strength, hardness, temperature resistance, corrosion resistance. The shaping of a product
can be done through different process like solidification, cutting, deformation, surface
processing, etc.
Machining process
Machining is a process that includes different types of process to complete product
manufacturing. There are some additional works required under the machining process even
they are manufactured through different methods like Centrifugal casting, Investment
casting, Die casting, cold chamber die casting, moulding or casting like cutting, shaping,
finishing.
Assembly or joining process
This is a method to join the different parts to obtain a final product. In this type of process,
it includes permanent joints and fastenings. Permanent Joints are welding‟s, soldering,
brazing. Welding is the strong joints which are used to join both heavy and light metals,
Welding are only used to join metals. Weld joints can work under high temperatures, lifting
heavy loads but soldering and brazing are not such a kind of joining, these are used to join
non-metals also and this cannot work under high temperatures. There is also a fastening
joint which is not a permanent joining type, they are Thread fasteners and rivet joints. Nut
and bolts come under thread fasteners they can be removed whenever required but rivets are
not supposed to remove as easy a nuts, rivets should be destroyed if we to remove the joints.
Nuts and bolts are reusable but rivets are non-reusable type.
Moulding process
Moulding is a casting process which is used for manufacturing a required product by
solidification process. In this type of process, the raw material gets subjected to heat
treatment and converted into the liquid state at this stage the temperature of the molten
material is higher than the required pouring temperature before the molten material getting
into the mould the molten material should maintain the required pouring temperature. After
pouring the molten material into the mould it gets into a solidification process after
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completing the solidification process, we can remove the mould to obtain a product from it.
Moulding contains different process like Injection moulding, Compression moulding, Blow
moulding.
Forming process
Forming Process also known as Metal Forming is a large set of manufacturing process by
which a raw material converted into a product. In this process, we apply stresses like
tension, compression, shear, to deform the raw material. The example of forming processes
are sheet metal manufacturing, forging, rolling, extrusion, wire drawing, thread rolling,
rotary swinging.
Fig.1.1 Manufacturing processes
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1.2 POWDER METALLURGY
Powder metallurgy is a metal forming manufacturing process performed by heating
compacted metal powders just below their melting point. Powder metallurgy is a term
covering a wide range of ways in which materials or components are made from metal
powders. Powder Metallurgy processes can reduce or eliminate the need for subtractive
processes in manufacturing, lowering material losses and reducing the cost of the final
product. Powder metallurgy is also used to make unique materials impossible to get from
melting or forming in other ways. A very important product of this type is tungsten carbide.
Since the advent of industrial production scale metal powder based additive manufacturing
(AM) in the 2010s, selective laser sintering and other metal AM processes are a new
category of commercially important powder metallurgy applications.
The powder metallurgy press and sinter process generally consists of three basic steps:
powder blending also known as pulverisation, die compaction, and sintering. Compaction is
generally performed at room temperature, and the elevated-temperature process of sintering
is usually conducted at atmospheric pressure and under carefully controlled atmosphere
composition. Optional secondary processing such as coining or heat treatment often follows
to obtain special properties or enhanced precision. The several other Powder metallurgy
processes include:
Powder forging
A "preform" made by the conventional "press and sinter" method is heated and then hot
forged to full density, resulting in practically as-wrought properties.
Hot isostatic pressing
Here the powder i.e., normally gas atomized, spherical type is filled into a mould, normally
consisting of a metallic "can" of suitable shape. The can is vibrated, then evacuated and
sealed. It is then placed in a hot isostatic press, where it is heated to a homologous
temperature of around 0.7, and subjected to an external gas pressure of ~100 MPa i.e., 1000
bar, 15,000 psi, for several hours. This results in a shaped part of full density with aswrought or better, properties.
Metal injection moulding
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Here the powder, normally very fine and spherical, is mixed with plastic or wax binder to
near the maximum solid loading, typically around 65vol%, and injection moulded to form a
"green" part of complex geometry. This part is then heated or otherwise treated to remove
the binder to give a "brown" part. This part is then sintered, and shrinked to give a complex
part.
Electric current assisted sintering
Technologies rely on electric currents to densify powders, with the advantage of reducing
production time dramatically, not requiring a long furnace heat and allowing near
theoretical densities but with the drawback of simple shapes. Powders employed in electric
current assisted sintering can avoid binders. Moulds are designed for the final part shape
since the powders densify while filling the cavity under an applied pressure thus avoiding
the problem of shape variations caused by non-isotropic sintering and distortions caused by
gravity at high temperatures. The most common of these technologies is hot pressing, which
has been under use for the production of the diamond tools employed in the construction
industry. Spark plasma sintering and electro sinter forging are two modern, industrial
commercial electric current assisted sintering technologies.
Additive manufacturing
It is a relatively novel family of techniques which use metal powders among other
materials, such as plastics to make parts by laser sintering or melting. Powder Metallurgy
process is perhaps uncertain at the stage. Processes include 3D printing, selective laser
sintering, selective laser melting, and electron beam melting.
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Fig.1.2 powder forming process
Fig.1.3 Hot isostatic pressing (HIP)
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Fig.1.4 Electric current assisted sintering
Fig.1.5 Metal injection molding
Fig.1.6 Additive manufacturing process
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1.2.1
POWDER METALLURGY PROCESS
Powder metallurgy contains the following process given below:
Powder Preparation
This is a first and basic step for producing an object by powder metallurgy process. Any
material can convert into powder. There are various processes of producing powder such as
atomization, grinding, chemical reaction, electrolysis process, etc.
Mixing and Blending
As the name implies, this step involves the mixing of two or more material powder to
produce a high strength alloy material according to the product requirement. This process
ensures even distribution of powder with additives, binders, etc. Sometimes lubricants also
added in the blending process to improve flow characteristic of powder.
Compacting
Compacting means compressed the prepared powder mixture into pre-defined dies. This
step ensures to reduce voids and increase the density of the product. The powder is
compacted into the mould by the application of pressure to form a product which is called
green compact. It involves pressure range from 80 to 1600 MPa. This pressure depends on
the properties of metal powder and binders. For soft powder compacting pressure is about
100 – 350 MPa. For steel, iron, etc. the pressure is between 400 – 700 MPa.
Sintering
The green compact, produced by compressing, is not very strong and can‟t be used as a final
product. This step involves heating of green compact at an elevated temperature which
ensures a permanent strong bond between adjacent particles. This process provides strength
to green compact and converts it into a final product. The sintering temperature is generally
about 70 to 90 per cent of the melting temperature of metal powder.
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1.2.2 SECONDARY OPERATION
The sintered object is more porous compared to fully dense material. The density of the
product depends upon press capacity, sintering temperature, compressing pressure.
Sometimes, the product does not require high density and the sintered product is directly
used as a final product. But sometimes, a highly dense product is required for example
manufacturing bearing. Where a sintered product cannot be used as a finished product.
That‟s why a secondary operation required obtaining high density and high dimensional
accuracy. The most common secondary operation used is sizing, hot forging, coining,
infiltration, and impregnation.
Fig.1.7 Powder metallurgy process
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1.3 GENERAL SINTERING PROCESS
Sintering is the thermal treatment of a powder or compact material at a temperature below
the melting point of the main constituent, for the purpose of increasing its strength by
bonding the particles together. Sintering comes under the powder metallurgy. Because
sintering can enhance material properties such as electrical and thermal conductivity,
strength, and translucency, it has uses in a range of industries and applications. The process
of creating metal parts by pressing powders dates back many centuries and has been used to
make items from almost every type of ceramic or metal. Modern uses include the creation
of structural steel parts, porous metals for filtering, tungsten wiring, self-lubricating
bearings, magnetic materials, electrical contacts, dental products, medical products, cutting
tools and more. There are several types of sintering, depending on the material being joined
or the specific sintering process, as follows
Ceramic Sintering
Sintering is used in the manufacture of ceramic objects including pottery. Because some
ceramic raw materials have a lower plasticity index and affinity for water than clay, they
need organic additives adding ahead of sintering. The process is associated with material
shrinkage as the glass phases flow once the transition temperature has been reached and the
powdery structure of the material consolidates, reducing the material porosity. The process
is driven through the use of high temperatures, although this can be coupled with other
forces such as pressure or electrical currents. Pressure is the most common additional factor,
although „pressure less sintering‟ is possible with graded metal-ceramic composites along
with a nanoparticle sintering aid and bulk moulding technology. Hot isostatic pressing is a
variant of sintering that is used for creating 3D shapes.
Metallic Powder Sintering
Most metals can be sintered, particularly pure metals in a vacuum where surface
contamination cannot occur. When sintering a metal powder, such as iron powder, under
atmospheric pressure a protective gas should be used. Sintering can cause a reduction in the
overall volume of material as the density increases and material fills voids before the final
stages see metal atoms travel along crystal boundaries and smooth out the pore walls due to
surface tension.
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Liquid state sintering
When at least one but not all of the materials are in a liquid state. Still considered powder
metallurgy, this technique is used to make tungsten carbide and cemented carbide. Sintered
metal powder is used for a range of applications from making bearings and jewellery to heat
pipes and even shotgun shells.
Plastic Sintering
Plastic items that need specific material porosity are formed by sintering, including for
applications such as filtration units and the control of fluid and gas flows. Other
applications for sintered plastics include inhaler filters, lining on packaging materials and
the nibs for whiteboard markers. Sintered plastics are also used as the base materials in skis
and snowboards.
Liquid Phase Sintering
This process is used for materials that are difficult to sinter. Liquid phase sintering involves
the addition of an additive to the powder to be sintered. This additive melts and the liquid is
pulled into the pores and cause the grains to be rearranged into a more favourable packing
arrangement. Where the capillary pressures are high and the particles are close together, the
atoms go into solution and precipitate into areas of lower chemical potential in what is
called „contact flattening.‟ This is similar to grain boundary diffusion in solid state
sintering. To be effective, the additive needs to melt before the sintering occurs.
Permanent Liquid Phase Sintering
This process is similar to regular liquid phase sintering, except it promotes capillarity to
attract the liquid into open pores leading to grain movement and improved packing.
Transient Liquid Phase Sintering
This bulk material forming process is used for ceramics, metals and metal matrix-ceramic
materials. These materials need to be mutually soluble with the liquid wetting the solid and
creating a high diffusion rate.
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Electric Current Assisted Sintering
This process uses electric currents to drive or enhance sintering. The process was developed
further over the ensuing years, including combining electric currents with pressure, which
was found to be beneficial for sintering refractory metals and conductive nitride and carbide
powders.
Spark Plasma Sintering
This type of sintering uses pressure and an electric field to enhance the density of ceramic
and metallic powder compacts. By using the electric field and hot pressing to improve
densification, this process allows lower sintering temperatures and less time for the process.
Electro Sinter Forging
This electric current-assisted sintering technology is used to produce diamond metal matrix
composites and is derived from capacitor discharge sintering. The process is being
investigated for use with a range of metals and is characterised by a low sintering time.
Pressure less Sintering
As mentioned above, this technique involves sintering without the use of applied pressure,
avoiding density variations in the final product as a result. Ceramic powder compacts can be
created through cold isostatic pressing, injection moulding or slip casting, following which
they are pre-sintered and machined to a final shape before heating.
Microwave Sintering
This process can be used to generate heat within the material rather than through the surface
from an external heat source. It is suited for small loads where it can offer faster heating,
less energy expenditure and improvements in product properties.
The sintering process takes place in following steps:
1. Mixing of raw powdered materials thoroughly and blending them.
2. Raw materials are heated at high temperature but below melting point of materials.
3. Compressing the heated materials by applying pressure from all the sides.
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Fig.1.8 The general sintering process
1.4 WHAT HAPPENS DURING SINTERING?
At the sintering temperature new crystallites form at the point of contact so those originals
inter particles boundaries disappear, or become recognisable merely as gain boundaries this
process is called recrystallization. During the sintering process atomic diffusion takes place
and during compaction welded areas are formed and grow until eventually they might be
lost completely. Powder particles are brought together and deformed at the point of contact
in the pressing operation. At elevated temperature the sintering temperature, the atoms can
move more easily and quickly along the particle surfaces the technical term for this process
is Diffusion. The primary driving force for sintering is not fusion of materials but formation
and growth of bonds between the particles. The stages in sintering are as follows

Adhesion without shrinkage.

Densification and grain growth stage.

Final stage with closed pores or elimination of the last isolated rounded pores.
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1.4.1
STAGES IN SINTERING PROCESS
Initial Neck Growth
Sintering initially causes the particles that are in contact to form grain boundaries at the
point of contact through diffusion. This is the point contact stage and does not result in any
dimensional changes. The greater the initial density of compaction, the higher the degree of
coherency in the material in this initial stage of sintering, necks begin to form at the contact
points between adjacent particles. This stage is referred to as the "neck growth" stage .No
change in the dimensions is observed nor does porosity decreases. Neck formation is driven
by the energy gradient resulting from the different curvatures of the particles and the neck,
Surface diffusion is usually the dominant mass-transport mechanism during the early stages
of neck growth, as the compact is heated to the sintering temperature.
Intermediate Stage sintering
Intermediate stage sintering begins when adjacent necks begin to imping upon each other.
Densification and grain growth occur during this stage. The packing density and
coordination number of the green packing are important during this stage. A high green
packing density produces rapid sintering with relatively few pores in the final object .The
intermediate stage is pore channel closure where interconnected pore charnels are closed off
isolating porosity. One of the causes of pore channel closure is neck growth. Another cause
is the creation of new contact points by pore shrinkage within the pore itself. Very low
green packing densities which are also associated with low coordination numbers, can lead
to coarsening without densification. In extreme cases, this may lead to open-pore structures
lacking in structural integrity. At the beginning of the intermediate stage, the pores form a
network of interconnected cylindrical pores broken up by necks. By the end, the pores are
smoother and begin to pinch off and become isolated from each other. Bulk transport
mechanisms, such as grain boundary diffusion and volume diffusion, dominate the sintering
process during this stage. As stated previously, these bulk transport mechanisms cause
material to migrate from inside the particles to the surface, resulting in contact flattening
and densification.
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Final Stage Sintering
Final stage sintering begins when most of the pores are closed. As sintering proceeds, the
pores, which during intermediate stage sintering form a network, have become isolated from
each other. Final stage sintering is much slower than the initial and intermediate stages. As
grain size increases, the pores tend to break away from the grain boundaries and become
spherical. Pore shrinkage is the most important stage in sintering. For this stage to occur;
solids must be transported into the pores and a means must exist by which the gas in the
pores can escape to the surface. The resultant effect is to decrease the volume of the
sintering mass smaller pores are eliminated, while larger pores can grow, a phenomenon
called Ostwald ripening. In some cases, pore growth during final stage sintering can lead to
a decrease in density, as gas pressure in the larger pores tends to inhibit further
densification.
Fig.1.9 Stages in sintering process
1.5 BENEFITS OF SINTERING PROCESS
While the different methods and materials offer a range of benefits, there are a number of
general advantages associated with sintering

Sintering offers high levels of purity and uniformity in the starting materials, which
can be maintained due to the simple fabrication process.

Controlling the grain size during input allows for highly repeatable operations.

Create materials with a uniform, controlled porosity.

Sintering can create nearly net-shaped objects.
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
Sintering can create high strength items such as turbine blades.

The sintering process improves the mechanical strength for handling.

Sintering allows you to work with materials that cannot be used with other
technologies, such as metals with very high melting points.
1.6 WHAT DO YOU MEAN BY POST-SINTERING PROCESS?
There are some commonly used post-sintering processes used on sintered metal parts they
are as follows:
Coining and resizing
While most parts are nearly finished after the sintering process step, some parts do require
sizing or coining operation in order to improve its structural and dimensional aspects. Sizing
is able to decrease dimensional variations whereas coining can increase the parts density
and thus its strength.
Steam treatment
Steam treatment is capable of increasing the resistance to corrosion, surface hardness, and
resistance to wear, reduces porosity, and improves the material density.
Heat treatment
Sintered metal parts are heat treated in order to increase the material hardness and strength,
as well as increases the material‟s resistance to wear.
Vacuum or oil impregnation
It makes sintered metal bearing self-lubricating.
Structural infiltration
It improves strength, reducing porosity and improves both ductility and machinability.
Resin or plastic impregnation
It can be used to improve machinability, prepare the surface of the part for plating processes
and to seal the part, making it liquid or gas tight.
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Machining
It includes threading, boring, milling, drilling, turning, tapping and broaching.
Grinding
Grinding on sintered metal parts includes homing, lapping and polishing.
Surface finishing
It includes plating, tumbling, coating, deburring, and vibratory processes.
Fig.1.10 Coining operation
Fig.1.11 Sizing operation
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Fig.1.12 Threading process
Fig.1.13 Milling process
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Chapter 02
LITERATURE REVIEW
H. Danninger, Ch. Harold, Ch. Gierl, H. Ponemayr, M. Daxelmueller, F. Simancik and K.
Izdinsky in their paper said that heat treatment resulted into chemically homogeneous and
fine-grained microstructure.
Sachin in his paper A Review on Powder Metallurgy of Iron Oxide and Iron said that,
Powder metallurgy is impressive method for the fabrication of the different composites of
improved mechanical properties and microstructure. Mean size of powders for blending,
pressure to which the mixture is pressed, sintering temperature of the green are the
considerable governing factors for the fabrication of composites by powder metallurgy.
According to Narasimhan the growth of ferrous powder powder metallurgy over the past
four decades reflect the advancements in the materials, compactation, sintering technologies
and other related fields. There are many other fields in which ferrous powder metallurgy
parts are being used such as lawn and garden structural parts, hand tools, hydraulic
application applications. Satisfy close dimension tolerance requirements for parts with
complex geometries says james and west in their paper. Powder perform forging involves
the fabrication of a perform by the conventional powder metallurgy processing technique.
D.Bubesh Kumar1, B Selva babu2, K M Aravind Jerrin3, N Joseph3, Abdul Jiss in their
paper said that The spark plasma sintering (SPS) technique is a sintering technique in which
the plasma is produced when the sintering is conducted. This plasma is used for the
sintering of the powders. This paper deals with the various variables with effect the plasma
sintering process. First the effect of pressure on SPS Process and its advantages. The review
also highlights the research conducted by researchers on various materials and various
sintering pressures.
Kablana, Jhajjar in their paper said that process parameters affect the sintered process. The
research gap is microscopic mechanism of electro plastic have to be analysed. The electro
plastic effect in metals is the defining of dislocations from the paramagnetic obstacles by
the magnetic field induced by the electric current. When dislocations are more the electro
plastic effect is less. Finding optimal parameters for the plasma arc sintering process is the
key to the strong sintered products. [5].
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Tadayuki Tsutsui in their review paper 2 “Recent technology of powder metallurgy and
applilications” said that over 90% of powder metallurgy products are used in transport
market. Automotive industry is in the trend of post-oil because of its increasing
environmental concerns, and technologies reducing fuel consumptions has been developed
for long. The lightweight technology and engine downsizing for environmental friendly
transportation vehicles. Author of the review paper said that powder metallurgy can be seen
in 5 areas, those are alloys created from high melting point metals, that includes tungsten,
molybdenum, and tantalum, metals or non metal composite materials are represented by
cemented carbide, and friction metals can be created, composite materials doesn‟t disslove
into each other, such as high thermal conducting materials, high density materials , porous
materials, like oil Impregnated bearing, powder metallurgy has excellent economic
efficiency because are formed by pressing powders. He has also explained the materials
processing technology in his review paper where he has explained that at first the main
materials mixed with additives are sent to crushers and then to mixing chamber where they
are mixed thoroughly, later that mixtured is compacted and sent to sintering i.e, heating of
that material to high temprature but not upto the melting point of that material, then the post
treatments are carried out.[9]
D. Fernandez-Gonzalzez and other authors of the research paper of theirs said that sintering
is a thermal agglomeration process that is applied to a mixture of iron ore fines and
additives. The purpose of sintering is manufacturing a product with the suitable
characteristics that are to be fed to the blast furnace. The process is widely studied and
researched in iron and steel making industries to know the best parameters that allows one
to obtain best sinter product quality. Sinter mixture is partially melted at a temperature
between 1300-1480 degree celsius and undergoes a series of reaction that gives or forms
sinter cakes and further secondary operations are carried out to form steel pellets or plates .
Authors of paper said that now a days Dwight-lloyd is typically used in main sinter plants.
Sintering process is perfomed after the granulation process, and it allows obtaining a
product that is greater than 20 mm that is to be used in blast furnace as burden material.
Sintering process has large influence on sinter bed structure.[7]
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Dharm Jeet Gavel, Allert Adema and others in their paper said that by series of semlting and
quenching experiments the physiochemical behaviour of the pellets, sinters and its mixtures
are experimented. For all ferrous raw materials beds, three distinct stages of bed shrinkage
occurs due to indirect reduction, softening and melting. In mixed ferrous bed the first and
third stages are found to be controlled by pellets and sinters. The behaviour of second stage
is initially observed to be closed to pellet and later to the sinter. The interaction between the
pellet and sinter is limited to the interface. The sinter slag controls the melting and dripping
properties of the mixed bed. The blast furnace ironmaking process is energy energy
essential for the sustainablilty. The significant resistance to the gas flow occurs due to
softening and melting of ferrous raw materials at the cohezive zone of blast furnace. To
minimze the resistanc, a small difference between softeneing and melting tempature of the
ferrous burden is desired and that can be achieved by the proper selection of raw materials
for metal production. Mixture of two to three types of ferrous materials are used in the blast
furnace for the production of metals. The chemical balance and economic balance are
checked for the proportions pf raw materials going to be used. The iron ore sinter making is
the best medium to recycle the burden left out in the blast furnace generated during the steel
making process. The iron ore sinter is added so that physical and chemical property is
improved. The experiments were performed on the reduction softening and melting
apparatus under simulated blast furnace conditions says the authors. They further conclude
by saying that pellet bed contraction envloves through three distinct stages. The sinter bed
also has three stages. Using high temperature to sinter the particle the sinter particles form
semi-fused mass to cause very high resistance to gas flow. In pellet and sinter mixed bed
contraction is also realised through three stages. Sintering among the mixed ferrous burden
is observed to restrict the gas flow causing the loss of permeability in the bed.[10].
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Chapter 03
SINTERING PROCESS IN STEEL INDUSTRY
Sintering is a process by which a mixture of iron ores, fluxes and coke is agglomerated in a
sinter plant to manufacture a sinter product of a suitable composition, quality and
granulometric to be used as burden material in the blast furnace. The iron ores that form part
of the mineral mix which, once granulated, is loaded onto the sinter strand where it is
partially melted at a temperature of between 1250-1350 °C and undergoes a series of
reactions that give rise to the formation of sinter, a material of a suitable composition and
strength to be loaded into the blast furnace to produce pig iron. The sintering process is used
to agglomerate a mix of iron ores, return fines, fluxes and coke, with a particle size, so that
the resulting sinter, with a screened size, can withstand pressure and temperature conditions
in the blast furnace.
The first stage of sintering is granulation of the raw mix, which consists of its
homogenisation in a mixing drum for several minutes with the addition of water. The
granulated mix is then loaded onto the permeable sinter strand grate. The bed top is heated
to high temperature by oil or gas burners and air is drawn through the grate. After a short
ignition time, heating of the bed top is discontinued and a narrow combustion zone (flame
front) moves downwards through the bed, heating each layer successively. In the bed the
granules are heated to achieve their softening and then partial melting. In a series of
reactions a semi-molten material is produced which, in subsequent cooling, crystallises into
several mineral phases of different chemical and morphological compositions mainly
hematite, magnetite, ferrites and gangue composed mostly of calcium silicates. Ahead of the
combustion zone, water evaporates and volatile substances are driven off. In the combustion
zone, reactions take place which give rise to the formation of strong agglomerate. Most of
the heat from the gases leaving the combustion zone is absorbed for drying, calcination and
preheating of lower layers of the bed. When the combustion zone reaches the base of the
bed, the process is complete and the sinter cake is tipped from the grate, roughly broken up
and screened.
Sintering is a continuous process. The sinter strand is formed by a series of pallets, each
of which has side walls and permeable grates, which are loaded with the granulated sinter
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mix, pass under the ignition hood, are subjected to downdraught suction, tipped, and then
return to the loading position. After being tipped off the pallets, the sinter is hot screened
and the fine fraction is recycled to be mixed with the raw materials while the coarse fraction
is cooled and sent to the blast furnace hoppers. The wind boxes below the strand are
connected to a fan via a gas scrubbing system.
3.1 STEPS IN MAKING IRON ORE SINTER FOR STEEL SINTERING
PROCESS
The steps during sinter making are as follows
1. Raw material preparation
2. Mixing
3. Feeding
4. Combustion
5. Sintering
6. Screening
Briefly description of the above steps is as follows
3.1.1 RAW MATERIAL PREPARATION
The sinter process can use a variety of material generated as waste. The main components in
raw material are
1. Iron ore fines
2. Coke breeze as fuel
3. Flux
4. Waste fines as micro nodule
3.1.2
MIXING
The various ingredients are fed to a mixing drum with water and rotated. After mixing the
sinter mix, it may be further rotated in another drum to agglomerate for better bed
permeability.
3.1.3 FEEDING
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The wet sinter mix is fed on the hearth layer. The bed height is regulated by a levelling bar.
3.1.4
COMBUSTION
When the green mix reaches below the ignition hood, it is exposed to burner flame and also
suction from bottom located wind box. The coke breeze on the top layer gets ignited.
3.1.5 SINTERING
Once the top layer is ignited, the sintering begins. As the grate advances, the suction of air
makes the combustion front move downwards. The progress of sintering on a moving bed
with sintering time starts from ignition hood. The top most layer of friable sinter as it does
not get sufficient time to fuse and get stronger due to cooling by incoming air.
Fig.3.1 Iron ore sinter used in steel sintering process
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Fig.3.2 Sintered ore stock
Fig.3.3 sinter making process for steel making
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3.2 RAW MATERIALS USED IN STEEL SINTERING PROCESS
The raw mix that forms the sinter bed is comprised mainly of iron ores, coke, and fluxes and
return fines. The behaviour of the raw mix during sintering and the quality of the
manufactured sinter depends largely on the chemical, granulometric and mineralogical
composition of the iron ores. Understanding the impact of ore characteristics on sintering
behaviour is important when it comes to selecting the most suitable raw mix for a given set
of operating conditions. The influence of the raw mix composition on sinter phases has
determines the influence of basicity (CaO/SiO2), temperature, thermal regime and Al2O3
and MgO contents on the ferrites content, total hematite, reoxidised hematite oxidised from
magnetite, reducibility index (RI), reduction degradation index (RDI) and tumbler index
(TI), porosity and coke rate. Iron ore fines, coke breeze, limestone and dolomite along with
recycled metallurgical wastes are converted into agglomerated mass at the Sinter Plant,
which forms 70-80% of iron bearing charge in the Blast Furnace.
Fluxes and sag-forming elements are the mineral gangue and the coke ashes have a
high melting point that is 1700-2000 degree Celsius so they would have problems to be
melted in the blast furnace process. For that reason fluxes and slag-forming elements are
added with the purpose of achieving slags with low melting point and suitable viscosity,
which also absorb undesirable elements that can contaminate the pig iron. Typical fluxes
and slag forming elements added to the furnace charge to lower the melting point and drawn
impurities into the slag are limestone, lime, dolomite, soda, fluorspar, bauxite. Quality and
chemical technical standards of the slag forming elements and fluxes are important to be
controlled in order to avoid undesirable elements in the process By-products In the iron and
steel industry, a huge amount of by-products are generated. The recycling and utilization of
these by-products have long been promoted in the iron and steel-making industry as a
consequence of environmental policies, energy saving and use of wastes with high iron
content. Most of the solid by-products can be employed once again in the sintering process
while others are used by other industries such as blast furnace slags in cement industry.
Gases are used as energy source in the steelmaking factory or in thermal power stations
The coke raw material is used for the supply of coal for coke production is fundamental
in the iron and steelmaking. The reason is that coke provides reluctant gas and energy for
the process. Coal reserves are higher than those of other fossil fuel ones. The main problem
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is that coking coal is only available in some regions. Coke is the best fuel for iron ore
sintering.
3.3 MATERIALS USED IN STEEL SINTERING
A great number of sintering powders can be used in the metal sintering process to
manufacture a number of sintered parts and components, ranging from iron and carbon steel
components and parts, to sintered tungsten and sintered aluminium parts. Table of common
sintering materials, some of the materials or powders used in metal sintering includes:
1. Iron and Carbon Steels.
2. Iron-Copper and Copper Steels
3. Iron-Nickel and Nickel Steels
4. Low Alloy Steels
5. Sintered Hardened Steels
6. Diffusion Alloyed Steels
7. Copper Infiltrated Steels
8. 300 Series Stainless Steel
9. 400 Series Stainless Steels
10. Soft Magnetic Alloys
11. Copper and Copper Alloys
3.4 WHAT IS AGGLOMERATE?
There are four types of agglomerating processes
1. Briquetting
2.
Nodulizing
3.
Sintering
4.
Pelletizing.
3.4.1 BRIQUETTING
Briquetting is the simplest and earliest applied process. Fine grained iron ores are pressed in
to pillow shaped briquettes with the addition of some water or some other binder under high
mechanical compressive pressure.
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3.4.2 NODULIZING
In the nodulizing process, fines or concentrate along with carbonaceous material are passed
through inclined rotary kiln heated by gas or oil. The temperature inside the kiln is
sufficient to soften but not high enough to fuse the ore. The nodules vary considerably in
composition and are too dense, slaggy, lack required porosity and hence this process could
not find great favour. Briquetting and nodulizing are cold binding processes and mostly
used for the recycling of recovered iron ore wastes in the steel plant. Sintering and
pelletizing are the processes of major importance for the iron production.
3.4.3 SINTERING
Middle of nineteenth century, small sintering pot used to be constructed in the copper
mining in England. The origin of sintering process goes back to 1887 when F. Haberlein
and T. Huntington of England invented the process of agglomeration for sintering of
sulphide ores. In this process, the sintering was carried with the sintering bed being blown
with air from bottom upwards. The process was also known as up-draft sintering process.
3.4.4 PELLETIZING
Pelletizing differs from sintering in that a green unbaked pellet or ball is formed and then
hardened by heating. During the development of the sintering process, initial attempts were
in the direction of further improving the process for using micro fines ores. This has led to
the development of a process which was an alternative to sintering. This process was named
pelletizing process. In Sweden and Germany, use of major amounts of fines in the sinter
mix led to limited productivity, and thus brought about the first phase of the development in
the pelletizing process.
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3.5 PROCESS OF SINTERING
Sintering process is developed mainly to utilize under size of lump ore called iron ore fines
which otherwise, could not be charged directly in blast furnace. In order to conserve these,
otherwise waste material, they are compacted together and made into lumps by a process
known as sintering. During the sintering process, iron ore fine particles agglomerate into a
porous compact heterogeneous lumpy mass called SINTER by incipient fusion caused by
the heat produced during the combustion of the solid fuel within the moving bed of loosely
particles.
The coke at the top of the blend is ignited by gas burners that can be fuelled by coke
oven gas, blast furnace gas, or natural gas. As the sinter bed moves, air is sucked from the
top through the mixture, enabling combustion through the entire layer and complete
sintering where the temperatures may reach 1300 – 1480 degree Celsius. At the end of the
strand, the material is cooled by air and finished sinter is size-screened. As per given
burden, raw materials are collected on a common conveyor from the respective bunkers
through weigh feeders and then mixed homogeneously in mixing drums by adding required
water 7 to 8 % and then feed on sinter machine. Generally, raw mix bed height is 550 mm
and is adjusted based on quality of the raw material. The bed in running motion condition is
taken to ignition front. The raw mix undergoes through the ignition furnace and there is a
negative suction from bottom. As soon as suction takes place, hot products of combustion
are sucked through the bed and transfer its heat to the next layer of the bed keeping it ready
for the combustion. These flue gases are let out from chimney through ESP. After
completion of the sintering process, sinter cake will be crushed and screened after discharge
from the machine. Sinter having size > 5 mm will go to the cooler and then it will go to BF.
Sinter with size < 5 mm size fines will be re-cycled in the process.
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Fig.3.4 Sintering process for steel making
Granulation
Granulation consists of the homogenization of iron ore mixture in a rotation drum with 78% water for a few minutes with the finality of obtaining a pre-agglomerated product. The
process has duration between 30 minutes to 1.0 hour, including the addition of moisture,
granulation and insertion in the sintering machine. This process has a fundamental
importance for the iron ore sintering because a good granulation ensures a suitable sinter
bed permeability and hence the productivity of the sinter plant. That is as a consequence of
that a good sinter bed permeability determines the rate at which the sintering process
progresses. Nippon Steel Corporation defined the term quasi-particle in their first studies on
the structure of granulated raw mixes. A quasi-particle consists of an iron ore nuclei
surrounded by fine ore grains with silica gangue in the presence of high basicity
(CaO/SiO2).
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3.6
ADVANTAGES
AND
DISADVANTAGES
OF
SINTERING
PROCESS
3.6.1 ADVANTAGES

Significant reduction of the machining cost, up to its full exclusion.

Energy saving technology is applied. Raw material is used at 97%, and for most of
the processes this coefficient can reach 100%.

Production of parts by this method allows usage of the different many-component
mixtures. When non-metals are mixed with metals and other substances, it is
possible to obtain self-lubricating bearings, filters with different porosity, parts with
adjustable permeability.

Parts produced by this method have better characteristics that is performance
characteristics, technical, economical, if it is compared with the similar products,
produced by traditional technologies.
3.6.2 DISADVANTAGES

High cost of raw materials.

Necessity to maintain whole process in special atmosphere.

Complexity of production of big parts.

To receive parts without admixtures it is required to have clean (100%) powder.
Fig.3.5 Sintering process
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Chapter 04
POST-SINTERING PROCESSES IN STEEL
INDUSTRY
Post-sintering process is producing the final product, a sinter, is a small, irregular nodule of
iron mixed with small amounts of other minerals. The process, called sintering, causes the
constituent materials to fuse to make a single porous mass with little change in the chemical
properties of the ingredients. The purpose of sinter is to be used converting iron into steel.
Post-sintering processes include assembling of parts, heat treatment, densification and
finishing. There is various post-sintering process used in steel industry they are as follows:
The sizing operation, which is carried out at moderate compacting pressures, serves to
improve the dimensional accuracy of the product. For a small batch of components, the
primary pressing or compacting die can be used to carry out the sizing of the compact.
Large batches of compacts are normally sized in a special die using an inexpensive sizing
press. The coining operation serves two purposes: improving the mechanical properties of
the product and improving the dimensional tolerances. The mechanical properties can be
improved only by increasing the density of the compact, which means high compacting
pressures (higher than or equal to the primary compacting pressures). Thus, in general,
coining requires a special die for the purpose, often of a higher quality than the primary die,
because of the higher pressures and the adverse wear conditions. When coining is involved,
the sintering process carried out between the primary compacting and the coining operation
is often incomplete and takes the form of pre-sintering for a short time and at a temperature
considerably below the normal sintering temperature but sufficient to anneal the compact.
After coining, the compact is fully sintered, producing a component with excellent
mechanical properties and dimensional tolerances. If the requirements of the product are
exceptionally high, a sizing operation may follow the coining operation.
Steam treatment is a thermal process that creates a thin controlled oxide layer on the
surface of an iron based metal component. Steam treatment can provide a component with
increased corrosion resistance, hardness, density and magnetic properties. It can also be
used to seal the porosity and improve its wear characteristic. Steam treatment is a batch
process with minimal inputs and has been proven to be a cost effective solution for many
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applications. Components transferred to steam treat must be kept clean and dry as it is
necessary to avoid contaminants or residue on or in the structure prior to processing because
it will impact of how well the oxide layer forms on the surface.
As in most thermal treatments, time, temperature and atmosphere are controlled to
provide the optimal conditions for the expected finish. The desired properties of the
component will dictate what time and process parameters are used for a given part. During a
typical steam treatment process, parts are placed in a steam treat unit and heated to
approximately 1000° F. Once the component is at temperature, steam is introduced and the
water vapour reacts with the iron to form the oxide layer. After a designated period of time
the component is removed from the unit and allowed to cool. The oxide appears on the
component surface as a blue/black finish. Heat treatment of steel involves the heating and
cooling of the material. The metal or alloy is heated to a specific temperature. Then, cooling
occurs to harden the heated material. The process aims towards changing the microstructure
of the metal. Also, it helps to bring out desired mechanical, chemical, and physical
characteristics. The alteration of these properties benefits the working life of the component
involved. For example, there may be increased ductility, strength, surface hardness, or
temperature resistance. Heat treatment is one of the essential aspects of the metal
manufacturing process. This is because it helps to improve a metal part to withstand wear
and tear better. The general definition of heat treatment may be the heating and cooling of
metals. However, the heat treatment process is more controlled. While the heating and
cooling processes are in place, the shape of the working metal remains intact. During this
process, the structural and physical properties of the material change to serve the desired
purpose. It could also be for further metal works. Heat treatment of steel or metals plays
important role in various manufacturing stages.
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Chapter 05
CASE STUDIES
5.1 BOKARO STEEL PLANT
Bokaro steel plant is the fourth integrated plant in the public sector- started taking shape in
1965 in collaboration with the Soviet Union. It was originally incorporated as a limited
company on 29th January 1964, and was later merged with SAIL, first as a subsidiary and
then as a unit, through the public sector iron and steel companies Act 1978.
Bokaro is designed to produce flat products like hot rolled coil, hot rolled plates, hot
rolled sheets, cold rolled coils, cold rolled sheets, tin mill black plates and galvanised plain
and corrugated sheets. Bokaro steel has provided a strong raw material base for a variety of
modern engineering industries including automobile, pipe and tube, LPG cylinder, barrel
and drums.
5.1.1 RAW MATERIALS AND MATERIAL HANDLING PLANTS
Raw materials and material handling plant receives blends, stores and supplies different raw
materials to blast furnace, sinter plant and refractory materials plants as per their
requirements. It also maintains a buffer stock to take cake of any supplies interruptions.
Different raw materials iron ore fines and lumps, lime stones, dolomite lumps and chips and
manganese ore are handled here every year.
5.1.2 SINTERING PLANT
Sinter is produced in sintering machines from iron ore fines, coke fines, flux and other
metallurgical wastes generated in the plant like mill scale, fuel dust etc.,. There are 3
sintering machines at sintering plant in Bokaro steel plant. Two machines have hearth area
of 252 m2 each and one has 276 m2. Annual capacity of gross sinter production is 6.9 MT.
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Fig 5.1 Process flow diagram of Bokaro steel plant
Fig.5.2 Bokaro steel plant
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5.2 BHILAI STEEL PLANT
Bhilai Steel Plant (BSP) is India's largest producer & supplier of world class rails for Indian
Railways including world‟s longest 130 metre rails in single piece and 260 metre long rail
welded panels, and a major producer of large variety of wide and heavy steel plates and
structural steel. The plant also specializes in other products such as wire rods and merchant
products. The entire range of TMT products like Bars & Rods produced by the Plant is of
earthquake-resistant grade and superior quality. The plant also produces heavy structural
including channels and beams. The production capacity of Hot Metal, Crude Steel &
Saleable Steel after completion of Modernisation & Expansion Programme is as under:
Hot Metal – 7.5 MT (Million Tonnes)
Crude Steel – 7 MT
Saleable Steel – 6.56 MT
5.2.1 MAIN TECHNOLOGY
BSP uses Blast Furnace technology for iron making and CCS route of steel making. The
Plant produces continuously cast steel slabs, blooms and billets through Slab, Bloom &
Billet Casters. The CONCAST route of steel making is also equipped with secondary
refining units like Vacuum Arc Degassing (VAD), Ladle Furnace & RH Degasser to
produce the clean steel. These cast products are rolled into long and flat products through
Rolling Mills which include Rail and Structural Mill (RSM), Plate Mill (P Mill), Wire Rod
Mill (WRM) and Merchant Mill (M Mill) that were established during the 1 to 4 MT stage.
The technology has been upgraded and updated with every modernization and expansion
and also through continual assimilation of state of the art technology for product and
process improvements. The Plant produces cleanest steel with Hydrogen in rail steel less
than 1.6 ppm. World-class long rail manufacturing complex at RSM has sophisticated
technologies viz. Online Eddy Current & Ultrasonic Testing Machines for Rails, Laser
Straightness Measurement, and Laser Controlled Presses for Rails, etc. Plate Mill also has
advanced facilities for ensuring high product quality such as – Online Ultrasonic Testing
Machine, Hydraulic Automatic Gauge Control (HAGC), Plan View Rolling (PVR),
Normalizing Furnaces, etc.
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As part of the Plant‟s 7 MT Modernization & Expansion (MODEX) programme,
cutting edge technologies for expanding product profile, improvement in productivity,
yield, quality, cost competitiveness, energy efficiency and environmental protection have
been installed. New Modex units include Blast Furnace No 8, Steel Melting Shop 3, and
Universal Rail Mill & Bar & Rod Mill.
5.2.2 SINTER PLANT
Sinter Plant 2 has 3 machines of 75 square metre hearth areas & 1 machine of 80 square
metre hearth areas. Sinter Plant 3 has 1 machine of 320 square metre hearth areas & 1
machine of 360 square metre hearth areas.
Fig 5.3 Process flow diagram of Bhilai steel plant
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Fig.5.4 Bhilai steel plant
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Chapter 06
RESULTS AND DISCUSSIONS
Sintering is a production procedure of second grade steel and has the properties which is
different from the normal or conventional steels. Mild steel has the shear strength of 345
MPa to 525 MPa and ultimate strength, or stress of mild steel is almost around 800 MPa to
840 MPa, and factor of safety is four. Sintered steel and normal steel have different
properties and are used in different applications. It is not accurate to say that one is better
than other.
Sintered steel is made by compacting and heating powered metal under high pressure in
a sintering furnace. This process creates a material that is more porous and less dense than
normal steel, but also more resistance to wear, corrosion, temperature changes. Sintered
steel is commonly used in applications that require high strength and durability. Such as
automotive parts, bearing, cutting tools.
Normal steel, on other hand, is made by melting iron and adding various alloys and
elements to create different grades and properties. Normal steel can have various and wide
range properties depending upon its composition that includes strength, ductility, corrosion
resistance, and weld ability. Normal steel is used in a wide range of applications, from
construction and infrastructure to manufacturing and consumer products.
Sintered steel and normal steel has different properties due to their different
manufacturing processes. The comparison of main properties of normal and sintered steel is

Density
Normal steel has higher density than sintered steel due to its more compact structure.
The density of normal steel ranges from 7.7 to 8.1 g/cm3, while the density of
sintered steel typically ranges from 6.8 to 7.8 g/cm3.

Strength
Normal steel can have a wide range of strength depending on its composition and
manufacturing process. Generally, normal steel has higher strength than sintered
steel. However sintered steel can be designed to have high strength and toughness,
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making it suitable for certain applications such as automotive parts and aerospace
parts.

Corrosion resistance
Normal steel is susceptible to corrosion and rust, especially when exposed to
moisture and air. Sintered steel, on other hand, can be designed to have improved
corrosion resistance due to its more uniform structure and controlled porosity.

Machinability
Normal steel is generally easier to machine and shape than sintered steel. Sintered
steel is more brittle and may require specialized machining techniques and tools.

Cost
Sintered steel can be more expensive than normal steel due to additional processing
steps involved in its manufacturing. However, the improved properties of sintered
steel may make it more cost effective in certain applications where durability and
performance are critical.
Normal steel and sintered steel have different properties that make them suitable for
different applications. The choice between the two materials depends on specific
requirements of the application, including strength, corrosion resistance, machinability and
cost.
Fig 6.1 Effect of compaction on density of sintered steel
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In the last decades, sintered iron-based alloys have been extensively used as structural
parts in mechanical components due to their good balance between ductility and tensile
strength, low cost, high performance, flexibility of manufacturing, good magnetic properties
and corrosion resistance. Consequently, such components have emerged as an effective
alternative for replacing machined parts, castings and forgings in many engineering
applications. However, continued efforts are demanded for obtaining optimum combination
of properties to withstand various service conditions.
There are several ways to achieve desired strength properties with iron-based sintered
materials. The most important parameters of influence are

Density

Sintering conditions

Alloying elements

Heat-treating conditions
These parameters should be controlled within the closest possible limits, because even
small variations may cause unacceptably wide scatter of dimensional changes during
sintering and thus spoil the dimensional stability of the sintered parts. Density is of prime
importance with respect to the physical properties of sintered structural parts, because
tensile strength and fatigue strength increase in approximate linear proportion, elongation
and impact strength exponentially, with sintered density.
In the study of C. Teisanu, S. Sontea, M. Mangra, I. Ciupitu, A. Tudor, the compacted
samples were sintered in different conditions in dry hydrogen atmosphere and the effect of
the powder additions on the mechanical characteristics was evaluated. The tribological
behaviour of the different iron based materials has been studied by pin on disc tests and the
coefficient of friction and wear rate have been analysed in order to identify the effect of
base material composition. Also, the microstructure of the wear surface was investigated.
Using conventional PM technologies a new iron based antifriction material containing Cu,
Sn, Pb and MoS2 has been developed in order to meet specific working conditions.
As experimental materials, iron powder produced by Ductile S.A. Buzau (DP 200 –
HD), electrolytic copper powder, tin, lead and molybdenum disulphide powder, which was
added as solid lubricant, were used. Elemental powders of Fe, Cu, Sn, Pb and MoS2 were
weighed to required proportions and mixed in a spatial homogenization device for two
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hours. The powder mixtures were compacted at a pressure of 500 MPa obtaining 10 mm
cylindrical specimens and sintered at 800°C, 850°C and 900°C for 50 minutes in a uniform
heating furnace. The sintering atmosphere was dry hydrogen with a flow rate of 1 l/min.
The samples were furnace cooled by switching off the power and maintaining the same flow
rate of the hydrogen gas.
Fig 6.2 Increase of sintered properties with sintered density.
Here in the above figure schematically, a = compacting + sintering. a‟ = warm
compacting b = compacting + sintering + re-pressing + re-sintering c = powder forging.
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Chapter 07
CONCLUSION
Sintering works through the diffusion of atoms across particle boundaries before fusing
together into one piece under the influence of pressure and/or heat. While this process can
occurs naturally for mineral deposits, it is also widely used by a range of industries to
manufacture items from materials including ceramics, metals and plastics. Sintering occurs
at heats below the melting point of the materials, making it useful for creating items from
metals that have high melting points. There are a range of different techniques depending on
factors such as the use of electrical currents, pressure and heat sources as well as the actual
materials being sintered. Sintering, which is also „frittage‟, is the process of forming a solid
mass of material. Material differences that effect how long the process includes the mobility
of the atoms, the self-diffusion co-efficient, melting temperature, and level of thermal
conductivity. In addition, field assisted techniques reduces sintering times while selective
laser sintering is slower than the traditional oven process is still slower. Sintering increases
the strength of material by bonding material together it reduces porosity and enhancing
electrical conductivity. Sintering is used to create structural steel parts, porous metals for
filtering, tungsten wiring, self-lubricating bearing, magnetic materials, and electrical
contact.
Department Of Mechanical Engg. K.L.E It, Hubballi
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SINTERING AND POST SINTERING PROCESS FOR STEEL INDUSTRY
BIBLOGRAPGY
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