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The following requirements are made on devices designed to heat the workpieces:
- Temperature must be reached in the shortest possible time
- Temperature has to be controlled as easy as possible
- Temperature has to be kept constant at the required figure
- The workpiece must evenly be heated in all spheres.
Smith's hearth (open fire):
It is used for heating small parts and tools; in most cases, it is heated by means of charcoal. Pit coal is not suitable due to its sulphur content, fresh timber does not generate enough heat and it bums unevenly. Temperature must be estimated by the temper colours of the workpiece; thus, it is unsafe to assess.
Therefore, assessment can only be made perfectly in darkened rooms (windows are coated with blue paint).
Figure 3 - Smith's hearth -
1 flue, 2 fire case, 3 blower, 4 air supply, 5 frame, 6 nozzles and
slag lever, 7 explosion chamber
Smith's hearth is equipped with firing appliances: coal shovel, swatter and fire rake.
Figure 4 - Firing appliances -
1 coal shovel, 2 swatter, 3 fire rake
Gas burner:
When tempering small parts and tools, gas burners of the Bunsen type (fuel gas: e.g. propane) are used. In case of flame hardening, special burners designed for gas/oxygen mixtures and, in most cases, equipped with mechanical feeders onto which water sprinklers are mounted subsequently, are applied.
Figure 5 - Gas burner - 1
gas and oxygen burner, 2 Bunsen burner
Furnaces:
They are used for evenly heating any workpieces; gas, oil or electric power are used as fuel. In furnaces equipped with temperature controllers there is no danger that the workpieces are overheated or changed in their chemical composition. In order to contact the workpieces not directly with the heat source they may be arranged in inserts
(muffles) made of fire clay, graphite or heat-resistant cast iron. Thus, the workpieces are evenly heated as a whole.
Figure 6 - Muffle furnace -
1 chimney flue, 2 insert (muffle), 3 work-piece, 4 air supply,
5 fuel gas supply, 6 combustion chamber
Apart from the simple muffle furnace operating at nominal temperatures around 1200°C, the double-chamber furnace equipped with two differently heated chambers (nominal temperatures in the chambers: 1300°C and 900°C, ensuring timesaving hardening and tempering, are applied. Hardening temperature can be controlled by means of electrical or optical measuring instruments. Modern furnaces used in industrial large-series production are designed in such a way that they guarantee an uninterrupted heat treatment. For that purpose, they may be equipped with multi-purpose chambers for annealing, tempering and quenching, ace, to type. They have chambers with certain gas atmospheres and different transport mechanisms.
Melting baths (hot baths):
They are used to evenly heat sensitive workpieces (measuring and testing means, cutting tools). By applying fuels such as gas, oil or electric power, metals, metal alloys or salt mixtures are heated, which melt within a limited temperature range; the workpieces are entirely or partly suspended into the melting baths.
Overheating is avoided by means of automatic temperature controllers. In a short time, the workpieces are heated up to bath temperature, scaling of surfaces cannot be affected in the bath.
Whereas lead melting baths can be heated up to 800°C, melting baths can be heated up to 1400°C.
Figure 7 - Melting bath - 1
hood, 2 electrode, 3 workpiece, 4 melt
Other possibilities for heating:
- When tempering, heated steel plates, heated sand baths or hot oil baths are especially used which ensure a slow and even heating of the work-pieces.
Figure 8 - Heat steel plate -
1 workpiece, 2 heated steel plate, 3 gas burner
- In case of casehardening, sheet-metal boxes filled with sand and carbon-containing powder and equipped with cover are required. They are sealed by means of clay.
Figure 9 - Box for
casehardening - 1 fireproof box with cover, 2 workpiece, 3 carbon-containing
powder, 4 sand, 5 clay
Which requirements have to be made on the devices designed to
beat the
workpieces?
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What are the advantages of melting baths compared to the smith's
hearth?
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Hooks and spears:
They are used for operating the smith's hearth and the furnaces, in order to turn and displace the workpieces.
Figure 10 - Hooks and spears
Tongs:
They grip the workpieces when being inserted into the furnaces and when being removed as well as when being suspended into the quenching baths. Their bit-type shape can be formed variously in order to use them properly according to the shapes of workpieces.
Figure 11 - Tongs
Self-tightening claws:
Large and heavy workpieces can be lifted beyond a lifting device, the clamping jaws are clamped due to workpiece's weight.
Figure 12 - Self-tightening
claw
Hangers:
Heavy workpieces which cannot be gripped by means of claws are inserted into the hangers and moved by means of a lifting device.
Figure 13 - Hanger
Sieves:
Small workpieces may be placed onto the sieve and moved for being heated in melting baths and for being quenched in quenching tanks, in large quantities.
Figure 14 - Sieves
Baskets:
Very small workpieces may be poured into baskets in large quantities and moved for being heated in melting baths and for being quenched in quenching tanks.
Figure 15 -
Baskets
Quenching tank:
In order to quickly cool down the workpieces, the tank is filled with water or oil, for example, and it must constantly be kept cool. That is ensured by means of cooling coils, but also by the regular feed of coolant (with simultaneous discharge). Workpieces have to be moved in quenching tanks so that cooling down is effected as quick as possible; thus, steam bubbles quickly come off the workpiece.
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The quicker cooling is effected, the higher is the degree of hardness! |
As to mild cooling processes, heated quenching media can also be applied in heated tanks (hot baths).
Figure 16 - Quenching tank
Quenching media:
According to the desired properties of the workpieces, rough or mild quenching media are used.
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Rough quenching media |
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great strength and little elasticity of the workpiece to be treated |
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Mild quenching media |
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little strength and higher elasticity of the workpiece to be treated |
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Quenching media |
Effect |
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Acidified water |
very rough |
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Salt-containing water (10% salt) |
rough |
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Ice-cold water (2° C) |
very powerful |
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Pure water (20° C) |
powerful |
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Soluble oil emulsion or hot water (40° C) |
less powerful |
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Petroleum, oil, train oil |
mild |
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Compressed air |
very mild |
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Air |
very mild |
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The rougher the quenching medium, the harder is especially the case of the workpiece! |
Temperature data may be given in various units. Apart from the units "degree centigrade" used herein, the units "Kelvin" or "Fahrenheit" may be used.
Conversion table for temperature data
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Centigrade |
Kelvin |
Fahrenheit |
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0°C = |
273 K = |
32 F |
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100°C = |
373 K = |
212 F |
Which requirements are made on quenching
tanks?
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What are the effects of the use of a rough quenching medium onto
the
workpiece?
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