5. STRAIGHT CHAMBER FURNACE
Straight-chamber furnaces are continuous type furnaces.
Working Principle
“The material is introduced from the charging end on moving
trays or slabs with or without fixtures and after completion heat
treating process it is removed from the other end.”
6. TYPES OF STRAIGHT CHAMBER CONYINUOUS FURNACES
• Pusher-type furnaces
• Walking-beam furnaces
• Conveyor-type furnaces that use rollers or belts
• Continuous furnaces with tumbling or inertia action of the
parts for movement
7. PUSHER FURNACE
“A pusher mechanism pushes a solid row of trays from the charge end
until a tray is properly located and proven in position at the discharge
end for removal.”
8. CONSTRUCTION & FEATURES
• Construction usually consists of a gastight welded shell with radiant
tubes for heating.
• Cycle time through the furnace is varied only by changing the push
intervals.
• Pusher-type furnaces are quite versatile and, depending on the size and
shape of parts.
• parts may be loaded randomly and free quenched in an elevator-type
quench tank.
9. CONSTRUCTION & FEATURES
• Multi-track designs offer very high production flexibility, because
each track can move work pieces at different pusher speeds (cycle
times) to allow for varying case hardening depths
• Alternatively, parts may be removed individually from the furnace
trays for plug or press quenching.
• Circulating fans are almost always used for more uniform temperature
and carburization.
• In many instances, washing and tempering equipment is incorporated
to provide a fully automated heat-treating line.
• Most pusher-type furnaces are built with purging (purification)
vestibules (small bodily cavity) at the charge and discharge ends to
reduce contamination of the atmosphere by air.
12. TEMPERATURE
• The operating temperature is about 1250 C.
• Generally, the furnace capacity is 50-250 tones per hour.
REFRACTORY
• Pre-fired, dense quality shapes containing 35-58 percent alumina
(Al203)
• The trays or rails are cast or fabricated from Nickel-Chrome heat-
resisting alloy.
• Because of the ever-increasing cost of nickel-chrome alloys, alloy skid
rails are being replaced where possible by less expensive silicon
carbide refractory rails.
13. HEATING SYSTEMS
• Single-ended and U-type radiant tube systems mounted
horizontally or vertically are compatible with either fossil
fuel burners or electric elements.
• Large-diameter radiant tubes increase physical strength and
permit lower heat dissipation rates for longer service life.
• Piloted or spark ignited burner systems available.
• Internal and external recuperators can be added to reduce fuel
consumption by as much as 30%.
14. WALKING-BEAM FURNACE
A walking-beam furnace has movable rails that lift and advance parts
along stationary rails inside the hearth. With this system, the moving
rails lift the work from the stationary rails, move it forward, and then
lower it back onto the stationary rails. The moving rails then return to
the starting position and repeat the process to advance the parts again.
16. TEMPERATURE
• The operating temperature is about 1150 C.
• Typical furnace capacities range from 5 to 80 tones per hour.
REFRACTORY
• Pre-fired, dense quality shapes containing 35-58 percent alumina (Al203)
• The trays or rails are cast or fabricated from Nickel-Chrome heat-resisting
alloy.
• Because of the ever-increasing cost of nickel-chrome alloys, alloy skid
rails are being replaced where possible by less expensive silicon carbide
refractory rails.
HEATING SYSTEM
• Double walking beam furnaces are being heated increasingly by
recuperation burners in addition to hot-air burners.
17. CONVEYOR-TYPE FURNACES
• Rollers move the work-piece through a heating zone with powered,
shaft-mounted rollers that contact the work-pieces or trays.
18. TEMPERATURE
• The operating temperature is about 1100 C.
• Typical furnace capacities range from 5 to 80 tones per hour.
REFRACTORY
• Pre-fired, dense quality shapes containing 35-58 percent alumina (Al203)
HEATING SYSTEM
• Conveyer type furnaces are being heated increasingly by recuperation
burners in addition to hot-air burners.
19. FURNACE ATMOSPHERES
• A controlled Atmosphere is defined as the furnace atmosphere which
protects the metal from oxidation and maintain the desired properties
at the surface of metal during heat treatment.
• Broadly speaking controlled atmosphere is either Protective or
Chemically Active.
PROTECTIVE ATMOSPHERE
• The aim of Protective atmosphere to prevent oxidation,
decarburization and other unwanted chemical changes on the surface
of the metal.
• Bright annealing, normalizing of ferrous and non-ferrous metals and
alloys are the examples of Protective atmosphere.
20. CHEMICALLY ACTIVE ATMOSPHERE
• The aim of this atmosphere is to bring about chemical changes on the
surface of metal and alloy through out its whole cross-section.
• Carburizing, Carbonitriding, Decarburization, Nitrididing, Chromizing
are the examples of chemically active atmosphere.
CONTROL OF FURNACE ATMOSPHERE
• INFRARED CONTROLLER
Used to measure CO, Carbon dioxide and methane content
• CHROMATOGHRAPHY CONTROLLER
Estimate the relative proportion the component gases in furnace.
22. CARBURIZING
CARBURIZING is a case-hardening process in which carbon is
dissolved in the surface layers of a low-carbon steel part at a temperature
sufficient to render the steel austenitic, followed by quenching and
tempering to form a martensitic microstructure.
Carburizing Process Variables
• Temperature
• Time
• Atmosphere composition
23. CARBON CONTENTS AND DISTANCE
• as we move from the surface to core
the carbon decreases. And the strength
of specimen also decreases.
• Carbon contents and distance are
reciprocal to each other
25. CARBURIZING METHODS
There are three different methods of carburizing
• Pack carburizing
• Gas carburizing
• Liquid carburizing
GAS CARBURIZING
• Components are heated in carburized atmosphere by gas.
• Controlled carburizing atmospheres are produced by blending a
carrier gas with an enriching gas, which serves as the source of
carbon.
• In commercial practice carrier gas is used, which is enriched with
hydrocarbon.
26. • Chances of decarburizing of high carbon contents steel are always
present. So it should be avoided.
• This diffusion rate increases greatly
with increasing the rate of
carbon addition at 925 C
(1700 F) is about 40%
greater than at 870 C (1600 F).
• Carburizing temperature for low
carbon steels is at 870-940 C
(1600-1720 F).
• Hydrogen 40%, N 40%, CO 20%,
CO2 0.3%,H2O 0.8%, CH4 0.5%.
Table showing case depths at
different temperature
27. CARBONITRIDING
• CARBONITRIDING is a modified form of gas carburizing. The
introduction of ammonia into the gas carburizing atmosphere to add
nitrogen to the carburized case as it is being produced.
• Nascent nitrogen forms at the work surface by the dissociation of
ammonia in the furnace atmosphere; the nitrogen diffuses into the steel
simultaneously with carbon.
• Case is generally from 0.05 to 0.75 mm (0.003 to 0.030in.) deep.
• A carbonitrided case has better hardenability than a carburized case
(nitrogen increases the hardenability of steel).
• Steels such as 4140, 5130, 5140, 8640, and 4340 for applications like
heavy-duty gearing are treated by this method at 845 C (1550 F).
28. • Components are generally quenched in oil.
• Molten salt is also available to minimize distortion.
• Quenching is followed by tempering.
• Typical mixture contains 15% ammonia, 5% methane and
80% carrier gases.
• The addition of the carbonitrided surface improves contact
fatigue resistance.
• It is carried out at low temperatures 800 C -870 C.
29. TEMPERING
“The process in which previously hardened or normalized steel
is usually heated to a temperature below the lower critical
temperature and cooled at a suitable rate, primarily to increase
ductility and toughness, but also to increase the grain size of
the matrix.”
Principal Variables
• Tempering temperature
• Time at temperature
• Cooling rate from the tempering temperature
• Composition of the steel, including carbon content, alloy
content, and residual elements
31. ANEALING
“It consists of heating to and holding at a suitable temperature followed
by cooling at an appropriate rate, primarily for the softening of metallic
materials.”
32. NEUTRAL HARDENING
• Chemical composition of the surface of the parts is not
intended to be changed during the process.
• Temperature range is 675-870 C (1250-1600 F)
• Process description in three steps
1- Heating
2- Holding
3- Cooling
33. PRESS QUENCHING
• Quenching presses are designed for controlled quenching of
ring gears and other round, flat, or cylindrical parts to permit
heat treating with minimum distortion.
• In press quenching, before the quenching start the die
contacts the heated part, and the pressure of the press aligns
the part mechanically. While the part is still above critical
temperature.
• The machine and dies then force the quenching medium into
contact with the part in a controlled manner.
• Pneumatic or hydraulic in design.
35. ADVANTAGES OF A PUSHER FURNACE
• High operational safety through sturdy design
• Very even distribution of the protective atmosphere because of the
excellent recirculation
• Optimum repeatability
• Cost-efficient solution for treating large throughput volume
• Multi-row systems for flexible processing
• Designed for bottom loading front door to save energy
• Floor mounted oil quench tanks eliminate the need for costly pits
36. DISADVANTAGES OF PUSHER TYPE FURNACES
• Frequent damage of refractory hearth and skid marks on material
• Top and bottom fired furnaces have a detrimental effect on energy use.
• Discharge must be accompanied by charge.
• Stock sizes and weights and furnace length are limited by friction and
the possibility of stock pile-ups.
• All round heating of the stock is not possible.
37. ADVANTAGES OF WALKING-BEAM FURNACES
The typical advantages of walking-beam furnaces are:
• Only the work being processed has to be heated because normally
trays or fixtures are not needed
• Friction is reduced for heavy loads because the work is never skidded
• The system can be loaded or unloaded automatically
• A part can be picked from a specific spot and placed in a specific spot
by using the walking-beam mechanism
• Equipment is self-emptying on shutdown
38. DISADVANTAGES OF WALKING-BEAM FURNACES
The typical disadvantages of walking-beam furnaces are:
• Mechanisms are usually more expensive than for pusher-type systems
• On large high-temperature slab or billet reheat furnaces, there is a
dramatic increase in thermal holding losses and related fuel
consumption due to the water-cooled insulated walking-beam rail
system
• Walking-beam mechanisms are not commonly used where protective
atmospheres are required in the furnace chamber.
39. ADVANTAGES OF CONVEYOR-TYPE FURNACES
• This type of furnace might be used to advantage in heating much
longer slabs than would be practical in a pusher-type or walking-beam
furnace.
• minimum temperature difference at end of soaking time.
• maximum heating-up rate
• easy to maintain
DISADVANTAGE OF CONVEYOR-TYPE FURNACES
• Wear and tear of ceramics rollers may occur.
40. APPLICATIONS OF STRAIGHT CHAMBER FURNACE
• Walking beams traditionally have been used in steel mills in
reheat furnace hearth systems for slabs and billets.
• Walking beam furnaces are principally used to re-heat
unfinished product such as rebar, wire, bar, structural shapes
and tube to an even temperature prior to entering the mill area
of a facility.
• Conveyor furnace is used to improve the wear resistance and
prevent deformation under impact on gears, crank
shafts, camshafts, etc.
• These furnaces are also used for re-heating of forging parts.
41. CONCLUSION
The success of heat treatment depends upon:
• The proper choice of heat treating furnace and the type of atmosphere
maintained in it.
• Providing protective atmosphere which is necessary to ensure that
surface deterioration doesn’t take place in reactive metals during heat
treatment
• Maintaining constant temperature and raising the temperature at
desired rate
