Recrystallization annealing
| temperature | colour |
|---|---|
| 550 ° C | Dark brown |
| 630 ° C | Brownish red |
| 680 ° C | Dark red |
| 740 ° C | Dark cherry red |
| 780 ° C | Cherry red |
| 810 ° C | Light cherry red |
| 850 ° C | Bright red |
| 900 ° C | Good bright red |
| 950 ° C | Yellow Red |
| 1000 ° C | Light yellow bread |
| 1100 ° C | yellow |
| 1200 ° C | Light yellow |
| > 1300 ° C | Yellow white |
Under recrystallization annealing is meant a glow without phase change at a temperature in the recrystallization (if steel is usually 550-700 ° C) after cold working . Recrystallization annealing is mainly used after (and, if necessary, between) the individual deformation stages in cold rolling or drawing of sheets and wires.
Through cold forming (drawing, rolling , pressing , upsetting ) the structure of metals is stretched in the forming direction, the strength increases. However, the deformability decreases. This is known as work hardening. After a certain degree of deformation (depending on the material), a recrystallization annealing must now be carried out in order to restore the original structural condition.
Procedure
Three different methods of annealing steel strip are used in industry.
Hood annealing
During hood annealing , several coils are placed in a closed furnace. The glow times can be up to several days, but the heating and cooling speeds are limited. The possible temperatures for recrystallizing hood annealing range from 620 to approx. 700 ° C, and significantly higher with co-wrapping of wire - in this case, however, the edge of the steel strip must then be cut off and scrapped.
Continuous annealing
In the continuous annealing line , the strip is unwound and runs continuously through a furnace several 100 m long, which is divided into several zones with different temperatures. The overall length of the furnace limits the glow time to a maximum of 10 minutes. The furnace temperature can be up to 950 ° C, and above that when manufacturing electrical steel.
Before about 1940 the annealing process was carried out in continuous furnaces under a normal atmosphere, the result was a more or less strong scaling of the surfaces. The annealed parts had to be descaled in a pickling bath. Even today, steel strips are still partially annealed in a normal atmosphere. These contour lines are usually coupled with pickling in order to remove the scale that has developed in the furnace in the same pass.
Nowadays, bright annealing lines are mostly used for high-quality flat steels - especially if their surface requires a special gloss or a particularly high degree of reflection.
Bright annealing
A modern process is bright annealing. There is no need for pickling and the associated roughening of the surface. The bright annealing also takes place in a continuous furnace, which is, however, under a protective gas atmosphere. The protective gas is obtained from partially burned gas or forming gas and is under a slight overpressure in the furnace, which reliably prevents the entry of oxygen. These continuous ovens are heated by gas (in radiant tubes), less often electrically. Radiant heat pipes are airtight systems with air and gas supply in relation to the furnace atmosphere.
The combustion takes place in the combustion pipe, the exhaust gas is routed through the exhaust pipe. The combustion air is preheated by the exhaust gas via an integrated recuperator. After passing through a bright annealing system, consisting of an input station, furnace, cooling section and output station, the material is available for further processing (further drawing or rolling processes) or as a finished product. The recrystallization or bright annealing is not limited to steel, but also applies to non-ferrous metal alloys, e.g. B. Brass.
Recrystallization annealing of steel
The recrystallization annealing eliminates the consequences of cold forming without, however , causing an α-γ transformation ( α ferrite - austenite transformation) of the crystal lattice . With a degree of deformation of 5 to 15% (critical degree of deformation), a coarse grain is created due to the low number of germs. Here is a rather normalizing the workpiece recommended. If the degree of deformation is more than 20%, the desired fine grain is created . The recrystallization annealing is used for fine graining well above the critical degree of deformation of cold-formed parts and is usually just below the A1 temperature:
- with thin parts around 700 ° C (but below A1) for approx. 10 minutes
- for thicker parts at 600–650 ° C for one hour.
High temperatures are dangerous when there are degrees of deformation around the critical degree of deformation, since secondary recrystallization can result in rapid grain growth.
See also
- Stress relief annealing
- Solution annealing of austenitic steels
Individual evidence
- ^ Ulrich Fischer: Metal table book . 41st edition. Verlag Europa-Lehrmittel Nourney, Vollmer, 2001, ISBN 3-8085-1721-2 , p. 128B.
- ↑ Bleck: Material Science Steel
- ↑ http://www.jfe-21st-cf.or.jp/chapter_3/3c_5.html
- ↑ http://grz.g.andritz.com/c/com2011/00/01/32/13208/1/1/0/815895841/me-brochure_ssab_cal.pdf
- ^ Bargel, Schulze: Material Science. 10th edition, Springer, 2008, p. 165