HEAT TREATMENT LAB PRESENTATION

STRAIGHT CHAMBER CONTINUOUS FURNACES

PRESENTED BY

TALHA NAFEES

2011-MM-55

SUHAIB SHAFI

2011-MM-74

AHMAD ADAM

2011-MM-78

CONTENTS

• CLASSIFICATION OF FURNACES

• STRAIGHT CHAMBER CONTINUOUS FURNACES

• TYPES OF STRAIGHT CHAMBER CONTINUOUS FURNACES

• ATMOSPHERES IN THE FURNACE

• HEAT TREATMENT PROCESSES

• ADVANTAGES AND DISADVANTAGES

• APPLICATIONS

• CONCLUSION

CLASSIFICATION OF FURNACES

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.”

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

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.”

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.

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.

PUSHER FURNACE

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.

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%.

WALKING-BEAM FURNACEA 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.

WALKING BEAM FURNACE

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.

CONVEYOR-TYPE FURNACES

• Rollers move the work-piece through a heating zone with powered, shaft-mounted rollers that contact the work-pieces or trays.

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.

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.

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.

HEAT TREATMENT PROCESSES IN STRAIGHT CHAMBER FURNACES

• GAS CARBURIZING

• CARBONITRIDING

• NEUTRAL HARDENING

• TEMPERING

• ANEALING

• PRESS QUENCHING

CARBURIZINGCARBURIZING 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

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

TIME & CASE DEPTH

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.

• 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

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).

• 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.

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

EFFECT OF TEMPERING ON HARDNESS

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.”

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

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.

ADVANTADES AND DISADVANTAGES

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

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.

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

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.

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.

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.

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