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Any processes of heat treatment resulting in a heating of the workpiece up to a certain temperature with subsequent, in most cases slow, cooling, are called annealing.
Since heating must be effected slowly, evenly and drastically, furnaces are granted special favours to be the heat source which are equipped with temperature measuring devices. If there is no possibility to exactly gauge the temperature, it must be estimated by means of the decoloration of the workpiece effected during heating.
Standard values of colours for plain carbon steels:
|
Temper colour |
Temperature |
Temperature |
|
Brownish black |
520 to 580 |
793 to 853 |
|
Brownish red |
580 to 650 |
853 to 923 |
|
Dark-red |
650 to 750 |
923 to 1023 |
|
Dark cherry red |
750 to 780 |
1023 to 1053 |
|
Cherry red |
780 to 800 |
1053 to 1073 |
|
Light cherry red |
800 to 830 |
1073 to 1103 |
|
Light-red |
830 to 880 |
1103 to 1153 |
|
Yellowish red |
880 to 1050 |
1153 to 1323 |
|
Dark-green |
1050 to 1150 |
1323 to 1423 |
|
Light-yellow |
1150 to 250 |
1423 to 1523 |
|
White |
1250 to 1350 |
1523 to 1623 |
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The identification of colour differences requires a darkened
background and experience in estimating! |
Annealing is effected in three working steps:
1st - Heating of the workpiece to annealing temperature
2nd -
Keeping the temperature constant over a certain period of time
3rd - Slow
cooling down of the workpiece
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Workpieces have to be prepared very carefully, rust and scale have to be removed prior to annealing! |
According to the required properties of the workpiece, different annealing processes are used:
Stress-free annealing:
Stresses which may result in distortion of the workpiece are caused by different cooling processes for forged or rolled steel or by cold forming.
Hose stresses can largely be removed by the annealing process.
Proceedings:
- Heating to temperatures between 550°C and 650°C;
- Keeping the temperature constant for 30 to 120 minutes in dependence on material thickness;
- Slow and even cooling down in warmed ash or in a furnace.
Figure 17 - Temperature/time
diagram of stress-free annealing - 1 heating, 2 keeping constant. 3 cooling
down - I Temperature, II Time
Soft annealing:
Hardened materials or materials rich in carbon (above 0.9%) have a bad free-cutting machinability and cannot easily be cold-formed. In order to make those materials better for being machined, they are soft-annealed.
Proceedings:
- Heating to temperatures between 650°C and 750°C; it is also possible to work with changing heating and slight cooling-down processes around 723°C;
- Keeping the temperature range constant for 3 to 4 hours in dependence on the type of material and material thickness;
- Slow cooling down.
Figure 18 - Temperature/time
diagram of soft annealing - 1 heating, 2 keeping constant
(shuttle), 3 slow cooling down - I Temperature, II Time
Normalizing:
When dressing, bending, hammer forging and forging, the material is cold or hot-worked and the structure of the workpiece is distorted and partly solidified, the same goes for areas near welding seams.
The working properties of heavily-stressed workpieces and parts of constructional engineering are therefore unfavourable. By means of normalizing, the structural irregularities will be balanced and a fine-grained structure will be reached.
Proceedings:
- Heating to temperatures between 750°C and 980°C according to the carbon content of the material;
- Keeping the temperature constant according to material thickness;
- Slow cooling down in air.
Figure 19 - Temperature/time
diagram of normalizing - 1 heating, 2 keeping constant, 3 slow cooling down -
I Temperature, II
Time
- In case of a high carbon content, choose low temperatures within the range!
- Holding time for massive workpieces is adjusted ace, to the rule: 20 minutes plus half of material thick ness.
Example for determining the holding time:
A workpiece of 60 mm thickness has to be annealed.
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Rule: |
Holding time = 20 minutes + D/2(without unit) |
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Holding time = 20 minutes + 60/2 |
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Holding time = 20 minutes + 30 |
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Holding time = 50 minutes |
The workpiece has to be annealed for 50 minutes before it is cooled down.
What demands have to be made on workpieces being prepared for
the annealing
process?
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What principle is the annealing process based
on?
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What is the purpose of stress-free
annealing?
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What is the purpose of soft
annealing?
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What is the purpose of
normalizing?
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What holding time has to be considered when a shaft of 84 mm
diameter must be
annealed?
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Annealing defects can be identified by means of the appearance of the broken workpiece. That is why the test workpiece has to be notched after being annealed and broken across to the rolling direction by a short blow of a hammer.
Figure 20 - Control of the
annealing result - 1 notching, 2 breaking
|
Appearance of break |
Kind of material and treatment |
Improvement |
|
criystalline fracture, glistening |
steel of low carbon content, "structural steel" |
no |
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fine-grained, dead grey, smooth |
steel of higher carbon contents, "tool steel" |
no |
|
coarse-grained |
steel kept too long at annealing temperature or heated too much |
normalizing |
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coarse-grained, very glistening over the entire cross-section |
steel heated too much so that carbon is burned out, "burned steel" |
waste because structure is destroyed |
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large- to coarse-grained, glistening edge |
steel heated too long and too much, carbon removed from surface, "decarburized surface" |
decarburized edge zone to be removed or recarburized by inserting -normalizing |
A change of the mechanical properties can first be determined when the workpieces will be worked further.
After what can annealing defects be
determined?
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Which defects do suggest a coarse-grained
structure?
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