3. HEAT TREATMENT
It is defined as the process of heating and cooling
of metals and alloys in their solid state for certain
predetermined period to produce certain desired
properties.
4. PURPOSE OF HEAT TREATMENT
To improve mechanical properties such as
tensilestrength,hardness,toughness,ductility,
shock-resistance,etc.
To improve machinability
To increase resistance to heat,wear and
corrosion of materials
5. PURPOSE OF HEAT TREATMENT
To remove internal stresses of the object produced
by casting and hot or cold working
To change the internal structure and grain size
To improve electrical and magnetic properties
To soften the cold-worked metals or alloys which
get hardened due to working
6. PRINCIPLE OF HEAT TREATMENT
A metal or alloy experiences change in grain
structure when heated above a certain
temperature.It undergoes again a change in
structure when cooled to room temperature
Cooling rate is an important factor in developing
different structures.Slow cooling gives soft
structure and rapid cooling gives hard structure.
7. HEAT TREATMENT PROCESSES
Annealing
Normalizing
Hardening
Tempering
Case hardening or Surface heat treatment
9. HEAT TREATMENT FURNACES
A heat treatment furnace is a refractory lined
chamber,in which the work piece is heated to the
required temperature.
10. What is a Furnace?
A furnace is an equipment to melt metals for
casting or heat materials for change of
shape ( rolling, forging etc) or change of
properties (heat treatment).
11. FURNACE
It is a device which converts chemical energy of
fuel into heat energy or electrical energy to heat
energy.
The heat energy is then transferred to materials to
be heated which is placed inside the furnace for
that purpose.
13. Furnace Energy Supply
The products of flue gases directly contact the stock,
so type of fuel chosen is of importance.
For example, some materials will not tolerate sulphur
in the fuel. Also use of solid fuels will generate
particulate matter, which will interfere the stock place
inside the furnace.
Hence, majority of the furnaces use liquid fuel,
gaseous fuel or electricity as energy input.
Melting furnaces for steel, cast iron use electricity in
induction and arc furnaces. Non-ferrous melting
utilizes oil as fuel.
14. FURNACE AUXILIARIES
The important furnace auxiliaries are
–Damper
–Fans
–Control valve
–Chimney (stack)
15. DAMPER
Prime instrument in controlling furnace economy
It regulate furnace draft
It is made to work on auto mode modern furnace
It should move easily in its seating and be readily
capable of fine adjustment
Quite small movement may means waste of ton of
fuel.
16. Fan
Mechanical draft is indispensable with the
installation of WHR system in the down stream of
furnace.
A recuperator/regenerator requires a high velocity
air flow for adequate heat transfer causing
increase in system pressure drop.
17. CONTROL VALVE
Regulate fuel entering the furnace
Control valve be regulated automatically
depending upon the heat load of the furnace
Any departure from ideal rate of fuel supply
results in wastage of fuel.
All control valves to be cleaned,checked and
adjusted regularly to ensure their good working.
18. BURNER
The essential function of burner is to mix the fuel
and air in the required rate and proportion.
Burner provides the desired flame shape, length
and penetration in the furnace.
It should have a good turn down ratio
Proportionating control is preferred instead of
on/off control.
19. Types and classification of furnaces
Furnace
classification
Recuperative
Regenerative
According
to mode of
heat transfer
According
to mode of
charging
Mode of heat
recovery
Open fire place furnace
Heated through liquid medium
Periodical
Forging
Re-rolling
(Batch / continuous
pusher)
Pot
Continuous
Glass tank
melting
(regenerative /
recuperative)
Based on the method of generating heat: combustion type (using fuels) and electric type
21. TYPES OF HEAT TREATMENT
FURNACES
According to source of heat
1.Gas fired
2.Oil fired
3.Electric furnaces
According to type of work
1.Batch work
2.Continuous work
22.
23. INDUSTRIAL FURNACE
Also called as “Metal heating furnaces”.
Heating operation that involves no chemical
change and no change of state.
Used for operation such as Annealing, Case
hardening, Reheating, Forging, Rolling extrusion,
Hardening of steel.
24. PROCESS FURNACES
Used for operations such as
– Melting of metals
– Coking of coal
– Nitriding
– Hot dip galvanizing
25. KILNS
Used for operations such as
– Burning and nitrification of ceramic
products.
– Calcination of lime stone and dolomite.
26. OTHER TYPES
The other types of furnaces are
– Boiler furnace
– Electrical furnace
– Resistance heating furnace
– Induction heating furnace
27. Oil Fired Furnace
Furnace oil is the major fuel used in reheating and heat
treatment furnaces
LDO is used in furnaces where presence of sulphur is
undesirable.(No problom with sulphur )
Furnaces operate with efficiencies as low as 7% as against
upto 90% achievable in other combustion equipment such as
boiler.
This is because of the high temperature at which the furnaces
have to operate to meet the required demand. For example, a
furnace heating the stock to 1200oC will have its exhaust gases
leaving atleast at 1200oC resulting in a huge heat loss through
the stack. However, improvements in efficiencies have been
brought about by methods such as preheating of stock,
preheating of combustion air and other waste heat recovery
systems.
28. TYPICAL FURNACE SYSTEM
FORGING FURNACES
Used for preheating billets and ingots to attain a ‘forge’
temperature.
The furnace temperature is maintained at 1200 to 1250oC.
Forging furnaces, use an open fireplace system and most
of the heat is transmitted by radiation.
The typical loading in a forging furnace is 5 to 6 tones
with the furnace operating for 16 to 18 hours daily.
The total operating cycle can be divided into (i) heat-up
time (ii) soaking time and (iii) forging time.
Specific fuel consumption depends upon the type of
material and number of ‘reheats’ required.
29. Rerolling Mill Furnace
Batch type furnace:
Used for heating up scrap, small
ingots and billets weighing 2 to
20 kg. for batch type rerolling.
Charging and discharging of the
‘material’ is done manually and
the final product is in the form of
rods, strips etc.
Operating temperature is1200
oC.
Total cycle time can be
categorized into heat-up time and
rerolling time.
Specific fuel consumption
varies from 180 to 280 kg. of coal
/ tonne of heated material.
Continuous Pusher Type:
The process flow and operating
cycles of a continuous pusher
type is the same as that of the
batch furnace.
Operating temperature
is1250oC.
The material or stock recovers
a part of the heat in flue gases as
it moves down the length of the
furnace.
Heat absorption by the material
in the furnace is slow, steady and
uniform throughout the cross-
section compared with batch
type.
30. CONTINUOUSCONVEYOR FURNACE
A chain conveyor is used to carry the work pieces
automatically from the charging end through the
heating chamber to the quenching bath.
The conveyor chain is made up of heat resisting
alloy steel.
32. CONTINUOUS CONVEYOR
FURNACE
The chain conveyor carrying the work pieces,moves at a
slow speed
The speed is controlled ,so the work pieces inside the
heating chamber attains the required heat treatment
temperature.
Heating is done by electrical or by burning fuel.
After heating ,the work pieces move to the quenching tank.
33. USES OF CONTINUOUS
CONVEYOR FURNACE
It is used for tempering and
hardening treatments in continuous
mass production industries
34. OPEN HEARTH FURNACE
Furnace consist of saucer shaped floor,directly
exposed to flames.
Furnace is lined with refractory brick.
Air and fuel(gas,oil)are heated below the hearth
and they are ignited to high temperature.
35. MUFFLE FURNACE
Muffle retards heat transfer and accordingly
assists temperature equalization in the charge.
The principle purpose of the muffle is to protect
the charge from the effect of the products of
combustion.
Alloy steel and silicon carbide are the most
suitable for muffle walls
36. MUFFLE FURNACE
The heating capacity of a muffle that surrounds
the stock depends on
– Surface area and emissivity of stock
– Area of muffle wall and emissivity of
muffle wall
– Temperature of muffle wall
37. MUFFLE FURNACE
Muffle walls are made as thin as practically
possible with material having high thermal
conductivity and durability against high
temperature.
Muffles are seldom used for heating material more
than 1000°c because of cost of maintenance.
38. BLAST FURNACE
Iron oxides can come to the blast furnace plant in the
form of raw ore, pellets or sinter. The raw ore is
removed from the earth and sized into pieces that
range from 0.5 to 1.5 inches.
This ore is either Hematite (Fe2O3) or Magnetite
(Fe3O4) and the iron content ranges from 50% to
70%.
This iron rich ore can be charged directly into a blast
furnace without any further processing. Iron ore that
contains a lower iron content must be processed or
beneficiated to increase its iron content.
39. BLAST FURNACE
Pellets are produced from this lower iron content ore.
This ore is crushed and ground into a powder so the
waste material called gangue can be removed.
These fine materials are proportioned to obtain a
desired product chemistry then mixed together. This
raw material mix is then placed on a sintering strand,
which is similar to a steel conveyor belt, where it is
ignited by gas fired furnace and fused by the heat
from the coke fines into larger size pieces that are
from 0.5 to 2.0 inches.
40. BLAST FURNACE
These fine materials are proportioned to
obtain a desired product chemistry then
mixed together.
This raw material mix is then placed on a
sintering strand, which is similar to a steel
conveyor belt, where it is ignited by gas fired
furnace and fused by the heat from the coke
fines into larger size pieces that are from 0.5
to 2.0 inches.
41. BLAST FURNACE
The iron ore, pellets and sinter then become the
liquid iron produced in the blast furnace with any of
their remaining impurities going to the liquid slag.
The coal is crushed and ground into a powder and
then charged into an oven. As the oven is heated
the coal is cooked so most of the volatile matter
such as oil and tar are removed.
The cooked coal, called coke, is removed from the
oven after 18 to 24 hours of reaction time. The coke
is cooled and screened into pieces ranging from one
inch to four inches.
42. BLAST FURNACE
The coke contains 90 to 93% carbon, some
ash and sulfur but compared to raw coal is
very strong. The strong pieces of coke with a
high energy value provide permeability, heat
and gases which are required to reduce and
melt the iron ore, pellets and sinter.
The final raw material in the iron making
process in limestone. The limestone is
removed from the earth by blasting with
explosives.
43. BLAST FURNACE
It is then crushed and screened to a size that ranges
from 0.5 inch to 1.5 inch to become blast furnace flux .
This flux can be pure high calcium limestone, dolomitic
limestone containing magnesia or a blend of the two
types of limestone.
Since the limestone is melted to become the slag
which removes sulfur and other impurities, the blast
furnace operator may blend the different stones to
produce the desired slag chemistry and create
optimum slag properties such as a low melting point
and a high fluidity.
44. BLAST FURNACE
All of the raw materials are stored in an ore field and
transferred to the stock house before charging. Once
these materials are charged into the furnace top, they
go through numerous chemical and physical
reactions while descending to the bottom of the
furnace.
The iron ore, pellets and sinter are reduced which
simply means the oxygen in the iron oxides is
removed by a series of chemical reactions.
45. BLAST FURNACE
These reactions occur as follows:
1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4 Begins at 850° F
2) Fe3O4 + CO = CO2 + 3 FeO Begins at 1100° F
3)FeO+CO=CO2+Fe
Begins at 1300 ° F
FeO + C = CO + Fe
46. BLAST FURNACE
At the same time the iron oxides are going through
these purifying reactions, they are also beginning to
soften then melt and finally trickle as liquid iron
through the coke to the bottom of the furnace
The coke descends to the bottom of the furnace to
the level where the preheated air or hot blast enters
the blast furnace.
The coke is ignited by this hot blast and immediately
reacts to generate heat as follows:
C + O2 = CO2 + Heat
47. BLAST FURNACE
Since the reaction takes place in the presence of
excess carbon at a high temperature the carbon
dioxide is reduced to carbon monoxide as follows:
CO2+ C = 2CO
The product of this reaction, carbon monoxide, is
necessary to reduce the iron ore as seen in the
previous iron oxide reactions.
The limestone descends in the blast furnace and
remains a solid while going through its first reaction
as follows:
CaCO3 = CaO + CO2
48. BLAST FURNACE
This reaction requires energy and starts at about
1600°F. The CaO formed from this reaction is used to
remove sulfur from the iron which is necessary before
the hot metal becomes steel. This sulfur removing
reaction is:
FeS + CaO + C = CaS + FeO + CO
The CaS becomes part of the slag. The slag is also
formed from any remaining Silica (SiO2), Alumina
(Al2O3), Magnesia (MgO) or Calcia (CaO) that
entered with the iron ore, pellets, sinter or coke. The
liquid slag then trickles through the coke bed to the
bottom of the furnace where it floats on top of the
liquid iron since it is less dense.
49.
50. BLAST FURNACE
Another product of the ironmaking process, in
addition to molten iron and slag, is hot dirty
gases. These gases exit the top of the blast
furnace and proceed through gas cleaning
equipment where particulate matter is removed
from the gas and the gas is cooled.
This gas has a considerable energy value so it is
burned as a fuel in the "hot blast stoves" which
are used to preheat the air entering the blast
furnace to become "hot blast".
51.
52. BLAST FURNACE
Any of the gas not burned in the stoves is sent to the
boiler house and is used to generate steam which
turns a turbo blower that generates the compressed
air known as "cold blast" that comes to the stoves.
In summary, the blast furnace is a counter-current
realtor where solids descend and gases ascend. In
this reactor there are numerous chemical and
physical reactions that produce the desired final
product which is hot metal.
54. ELECTRIC ARC FURNACE
The electric arc furnace operates as a batch melting
process producing batches of molten steel known
"heats". The electric arc furnace operating cycle is
called the tap-to-tap cycle and is made up of the
following operations:
Furnace charging
Melting
Refining
De-slagging
Tapping
Furnace turn-around
55.
56. FURNACE CHARGING
The first step in any tap-to-tap cycle is "charging" into the
scrap. The roof and electrodes are raised and are swung to
the side of the furnace to allow the scrap charging crane to
move a full bucket of scrap into place over the furnace. The
bucket bottom is usually a clam shell design - i.e. the
bucket opens up by retracting two segments on the bottom
of the bucket. The scrap falls into the furnace and the scrap
crane removes the scrap bucket.
The roof and electrodes swing back into place over the
furnace. The roof is lowered and then the electrodes are
lowered to strike an arc on the scrap. This commences the
melting portion of the cycle. The number of charge buckets
of scrap required to produce a heat of steel is dependent
primarily on the volume of the furnace and the scrap
density.
57.
58. FURNACE CHARGING
Most modern furnaces are designed to operate with a
minimum of back-charges. This is advantageous
because charging is a dead-time where the furnace
does not have power on and therefore is not melting.
Minimizing these dead-times helps to maximize the
productivity of the furnace.
In addition, energy is lost every time the furnace roof
is opened. This can amount to 10 - 20 kWh/ton for
each occurrence. Most operations aim for 2 to 3
buckets of scrap per heat and will attempt to blend
their scrap to meet this requirement. Some operations
achieve a single bucket charge. Continuous charging
operations such as CONSTEEL and the Fuchs Shaft
Furnace eliminate the charging cycle.
59.
60. MELTING
The melting period is the heart of EAF operations. The EAF has
evolved into a highly efficient melting apparatus and modern
designs are focused on maximizing the melting capacity of the
EAF. Melting is accomplished by supplying energy to the
furnace interior. This energy can be electrical or chemical.
Electrical energy is supplied via the graphite electrodes and is
usually the largest contributor in melting operations. Initially, an
intermediate voltage tap is selected until the electrodes bore into
the scrap. Usually, light scrap is placed on top of the charge to
accelerate bore-in.
61.
62. MELTING
Chemical energy is be supplied via several sources including oxy-fuel
burners and oxygen lances. Oxy-fuel burners burn natural gas using
oxygen or a blend of oxygen and air. Heat is transferred to the scrap
by flame radiation and convection by the hot products of combustion.
Heat is transferred within the scrap by conduction.
Large pieces of scrap take longer to melt into the bath than smaller
pieces. In some operations, oxygen is injected via a consumable pipe
lance to "cut" the scrap. The oxygen reacts with the hot scrap and
burns iron to produce intense heat for cutting the scrap. Once a molten
pool of steel is generated in the furnace, oxygen can be lanced directly
into the bath. This oxygen will react with several components in the
bath including, aluminum, silicon, manganese, phosphorus, carbon
and iron. All of these reactions are exothermic
63. MELTING
Once enough scrap has been melted to accommodate the second
charge, the charging process is repeated. Once the final scrap charge is
melted, the furnace sidewalls are exposed to intense radiation from the
arc. As a result, the voltage must be reduced. Alternatively, creation of
a foamy slag will allow the arc to be buried and will protect the
furnace shell. In addition, a greater amount of energy will be retained
in the slag and is transferred to the bath resulting in greater energy
efficiency.
Once the final scrap charge is fully melted, flat bath conditions are
reached. At this point, a bath temperature and sample will be taken.
The analysis of the bath chemistry will allow the melter to determine
the amount of oxygen to be blown during refining. At this point, the
melter can also start to arrange for the bulk tap alloy additions to be
made. These quantities are finalized after the refining period.
64. REFINING
Refining operations in the electric arc furnace have traditionally
involved the removal of phosphorus, sulfur, aluminum, silicon,
manganese and carbon from the steel. In recent times, dissolved
gases, especially hydrogen and nitrogen, been recognized as a
concern. Traditionally, refining operations were carried out
following meltdown i.e. once a flat bath was achieved.
These refining reactions are all dependent on the availability of
oxygen. Oxygen was lanced at the end of meltdown to lower the
bath carbon content to the desired level for tapping. Most of the
compounds which are to be removed during refining have a higher
affinity for oxygen that the carbon. Thus the oxygen will
preferentially react with these elements to form oxides which float
out of the steel and into the slag.
65. DESLAGGING
De-slagging operations are carried out to remove impurities
from the furnace. During melting and refining operations,
some of the undesirable materials within the bath are oxidized
and enter the slag phase.
It is advantageous to remove as much phosphorus into the
slag as early in the heat as possible (i.e. while the bath
temperature is still low). The furnace is tilted backwards and
slag is poured out of the furnace through the slag door.
Removal of the slag eliminates the possibility of phosphorus
reversion.
66. DESLAGGING
During slag foaming operations, carbon may be injected
into the slag where it will reduce FeO to metallic iron and
in the process produce carbon monoxide which helps foam
the slag. If the high phosphorus slag has not been removed
prior to this operation, phosphorus reversion will occur.
During slag foaming, slag may overflow the sill level in
the EAF and flow out of the slag door.
67. TAPPING
Once the desired steel composition and temperature are
achieved in the furnace, the tap-hole is opened, the furnace
is tilted, and the steel pours into a ladle for transfer to the
next batch operation (usually a ladle furnace or ladle
station). During the tapping process bulk alloy additions are
made based on the bath analysis and the desired steel grade.
De-oxidizers may be added to the steel to lower the oxygen
content prior to further processing. This is commonly
referred to as "blocking the heat" or "killing the steel".
Common de-oxidizers are aluminum or silicon in the form
of ferrosilicon or silicomanganese. Most carbon steel
operations aim for minimal slag carry-over. A new slag
cover is "built" during tapping. For ladle furnace operations,
a calcium aluminate slag is a good choice for sulfur control.
Slag forming compounds are added in the ladle at tap so that
a slag cover is formed prior to transfer to the ladle furnace.
Additional slag materials may be added at the ladle furnace
if the slag cover is insufficient.
68. FURNACE TURN-AROUND
Furnace turn-around is the period following completion of tapping
until the furnace is recharged for the next heat. During this period, the
electrodes and roof are raised and the furnace lining is inspected for
refractory damage. If necessary, repairs are made to the hearth, slag-
line, tap-hole and spout. In the case of a bottom-tapping furnace, the
taphole is filled with sand. Repairs to the furnace are made using
gunned refractories or mud slingers. In most modern furnaces, the
increased use of water-cooled panels has reduced the amount of
patching or "fettling" required between heats. Many operations now
switch out the furnace bottom on a regular basis (2 to 6 weeks) and
perform the hearth maintenance off-line. This reduces the power-off
time for the EAF and maximizes furnace productivity. Furnace turn-
around time is generally the largest dead time (i.e. power off) period
in the tap-to-tap cycle. With advances in furnace practices this has
been reduced from 20 minutes to less than 5 minutes in some newer
operations.
69. SELECTION OF FURNACE
The selection of a furnace depends on:
– Purpose for which it is required
– Furnace capacity
– Type of operation
– Probable fuel consumption
70. IMPORTANT FACTORS IN
FURNACE SELECTION
Robustness of the furnace structure
Ease of control
Ease of maintenance
Insulation
Provision of adequate draft
Furnace atmosphere
Waste heat recovery
