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| ANNEALING OR RECRYSTALLIZATION | ||
| We have discussed in very brief manner the theory of cold working of copper in the light of what happens within the metal during the process of drawing, rolling, or other means of cold deformation. In general, it was pointed out that cold working increases hardness, and that this increase in hardness was due to distortion of the orderly molecular arrangement resulting in increased stresses between the molecules. In other words, hardness is due to increased stress. It was pointed out too, that it is seldom commercially practical to cold work the metal to the utmost degree, but rather it was usually desirable after a certain degree of cold work—to soften the metal by relieving the stresses set up and begin over again. This softening process is called annealing and is brought about by subjecting the metal to moderate or high temperatures, say 5000 to 1600°F. depending upon alloy. Let us consider a piece of copper that has had some cold work done upon it. It is hard. The molecules are not in orderly arrangement. There are stresses between them. The crystals, which are the result of orderly molecular arrangement, have been broken up and no longer exist as such. The molecules would, if they could, return to their original arrangement. However, in the cold condition they do not have sufficient freedom of movement to do this. If we can increase their energy sufficiently, they will have sufficient force and freedom of movement to rearrange themselves. Heat is energy, nothing more. It may be considered an indication of molecular movement. At absolute zero (a low temperature we have never reached) matter is believed to be absolutely inert —in other words, there is no molecular movement. As we increase the energy of the molecules (by heating) the molecules have motion. At first they merely “vibrate” in the same relative position, but as we increase their energy (their temperature) they have more and more freedom of movement —finally becoming a liquid —or even a gas. Returning now to our hard copper we apply heat. As we do so, the molecules move faster and faster—finally reaching a point at which they have enough energy to overcome their relatively fixed position and they can now realign themselves in the way they choose—namely, in the orderly crystalline arrangement which gives freedom from stress or to form soft metal. The temperature at which the system may be restored to minimum energy is always fixed for a given substance with a given degree of internal stress (degree of cold work). This point is called the recrystallization temperature and is just what the name implies—the minimum point at which the molecules will rearrange themselves and form new crystals. There is nothing mysterious about it—it is just a given temperature for a certain substance that has been worked to a given degree. However, certain complications arise. We find that we can control the size of the crystals into which the molecules form themselves by controlling conditions. If recrystallization temperature is just barely reached and held for only a short time, the crystals formed will be small. If that same temperature is held for long extended periods of time, the crystals will become relatively large. On the other hand, if the recrystallization temperature is considerably exceeded, the molecules have great freedom of movement and in a comparatively short time form into large crystals. Therefore, if we wish to control the crystal size we must control (a) the amount of previous cold work, (b) the temperature and (c) the time at that temperature. Note that we say the ‘time at that temperature” because the time in the furnace itself will vary a great deal according to its type. For example, in a strand anneal for strip the actual furnace temperature may be several hundred degrees hotter than the metal ever gets because the strip travels fast enough to prevent overheating. The same sort of thing occurs in continuous furnaces of the belt or conveyor type used for tubing. On the other hand the batch or “bell” furnaces do not use such high “temperature heads”. All types of furnaces, however, must be calibrated after installation so that mill operators will know what combinations of furnace load, temperature and time will produce the desired result for various metals and alloys. Perhaps we should digress at this point to show the reasons for controlling grain size. We have previously mentioned that finish tempers are controlled by giving a certain percentage of cold working on a soft material. There are, however, degrees of softness. A material with a small grain (say .010 mm.) is not as soft as one with a large grain (say .100 mm.). Perhaps it is too obvious to point out here that with a given degree of cold work we will obtain a harder temper from the material, which is not as soft in the first place. Therefore, to control temper we must control the degree of softness of the material we cold work to finish. In mill terminology we control the ready-finish grain size. A second reason for control may be taken from the customer’s viewpoint. If he wishes to do severe work on the material he must have very soft material (a large grain). On the other hand, if appearance is an essential item a small grain is indicated to avoid an “orange peel” or grainy appearance on any bent or stretched surfaces. This is particularly true if polishing or plating is done. In this instance the customer probably wants as soft a material as he can get for ease in working—and yet the grain must be fairly small. A compromise, therefore, is indicated and close control is necessary. Previously we mentioned the amount of cold work affecting the annealing temperature; the greater the amount of cold work prior to annealing the lower the temperature at which softening takes place. It also varies from one metal to another so that Cupro-Nickel or Aluminum-Bronze anneals at several hundred degrees higher than copper, for example. Small amounts of some elements may have very powerful effects on annealing temperature; for example, silver-bearing copper has from 8 oz./ton up to 60 oz./ton of silver added. This is only .027% to .2% silver, yet annealing temperatures may be increased several hundred degrees. Note that this also means that material will not soften readily when soldered and will resist, without softening, higher operational temperatures. Conversely various other elements may occur as impurities, e.g. iron, tin, silicon, nickel and the like, which also raise annealing temperatures. Consequently, control of impurities is a necessary part of manufacturing techniques. To sum up the simple annealing properties of simple alloys: |
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| Thus far we have discussed only the simple annealing of cold worked materials to restore ductility for further working on our part or for further fabricating by our customers. There are other forms of heat treatment, however, which are related to particular types of alloys. For example, there are a large number of alloys of aluminum, steel and some in copper, which are “precipitation hardened”. That means we have added an element, or elements, which dissolve in the base metal at elevated temperatures, and will be retained in solution only if we cool suddenly to room temperature. If we then reheat to a lower temperature for a certain length of time, the elements will ‘precipitate out”. In other words, a number of very fine particles of compound will not be dissolved any more and will be so placed in the crystal structure so as to produce a marked strengthening effect. In copper alloys it will also increase the electrical conductivity from that in the unheat-treated state. Notable examples are Beryllium Copper (170, 172, 175), Chromium Copper (184, 185), Zirconium Copper (150). (In Aluminum, those alloys containing both magnesium and silicon are the most common). Note that in brasses containing less than 64% copper we have a second phase, called beta. We can retain the beta at room temperature if we cool rapidly (water quench) from the formation temperature. However, it is not in our best interest to do so because beta brass is considerably harder than alpha brass and does not provide as good cold-working properties as all alpha brasses. This is not precipitation-hardening, however, but only the control of phases which will or will not be present after fast cooling. There is no reheating to ‘precipitate out” the hardening constituent. Another phenomena, as a result of cold working and annealing, is “preferred orientation”; which can occur with copper metals as well as aluminum alloys. With certain combinations of cold work and annealing the crystals are so aligned that elongation in one direction of the strip is different than in another. Its effects are seen in deep drawing so that a cup-shaped piece will have a wall higher in spots than others, These “ears” occur in copper as 4 in number at 90° to each other and 6 in number in brass with less than 64% copper. Both copper metals and aluminum may be ordered as “non-earing”. |
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