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Hot and cold working
This is the temperature at which atomic mobility can repair the damage caused by the working process.
Working above the recrystallization temperature is called "hot working" even if the actual temperature involved is not particularly hot by human standards..
The advantages of hot working are
- Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;
- The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;
- Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.
- Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals (see DeGarmo Fig. 17-4 below).
- Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).
- If the recrystallisation temperature of the worked metal is high e.g. if we are talking about steel, specialised methods are needed to protect the machines that work the metal. The working processes are also dangerous to human operators and very unpleasant to work near (see picture below for some idea why).
- The surface finish of hot worked steel tends to be pretty crude because (a) the dies or rollers wear quite rapidly; (b) there is a lot of dimensional change as the worked object cools; and (c) there is the constant annoying problem of scale formation on the surface of the hot steel.
Cold working refers to plastic deformation that occurs usually, but not necessarily, at room temperature.
Hot working refers to plastic deformation carried out above the recrystallization temperature.
Warm working: as the name implies, is carried out at intermediate temperatures. It is a compromise between cold and hot working.
Cold Working (Work Hardening)
As stated before, cold working refers to plastic deformation that occurs usually, but not necessarily, at room temperature.
For example: Deforming lead at room temperature is a hot working process because the recrystallization temperature of lead is about room temperature.
Cold and hot are relative terms.
Plastic deformation is a deformation in which the material does not return to its original shape; this is the opposite of an elastic deformation.
Effects of Cold Working:
The behavior and workability of the metals depend largely on whether deformation takes place below or above the recrystallization temperature.
Deformation using cold working results in:
· Higher stiffness, and strength, but
· Reduced malleability and ductility of the metal.
Hot working is the deformation that is carried out above the recrystallization temperature.
In these circumstances, annealing takes place while the metal is worked rather than being a separate process. The metal can therefore be worked without it becoming work hardened.
Hot working is usually carried out with the metal at a temperature of about 0.6 of its melting point.
Effects of hot working
· At high temperature, scaling and oxidation exist. Scaling and oxidation produce undesirable surface finish. Most ferrous metals needs to be cold worked after hot working in order to improve the surface finish.
· The amount of force needed to perform hot working is less than that for cold work.
· The mechanical properties of the material remain unchanged during hot working.
· The metal usually experiences a decrease in yield strength when hot worked. Therefore, it is possible to hot work the metal without causing any fracture.
Quenching is the sudden immersion of a heated metal into cold water or oil. It is used to make the metal very hard. To reverse the effects of quenching, tempering is used (reheated of the metal for a period of time)
To reverse the process of quenching, tempering is used, which is the reheat of the metal.
2.3 Warm WorkingDefinition:
As mentioned before, warm working: as the name implies, is carried out at intermediate temperatures. It is a compromise between cold and hot working.
The temperature ranges for cold, warm and hot process are given in table 2.1.
The range for warm working is between 0.3 and 0.5
Effects of Warm Working
The range for warm working is between 0.3 and 0.5 .
The effects of this type of working depend on how close is the warm process to be a cold or hot process.
This means that the effects and characteristics get similar to the ones in a cold process when the warm process gets closer to 0.3 . This also applies for hot processes. The effects and characteristics get similar to the ones in a hot process when the warm process gets closer to 0.5 .
The type of process chosen depends on the physical and mechanical properties needed for the product, meaning the product itself and its uses.
2.4 Methods used for Cold, Hot working
The initial breaking down of an ingot or of a continuously cast slab is done by hot rolling. A cast structure includes coarse and non-uniform grains. This structure is usually brittle and may contain porosities. Hot rolling converts the cast structure to a wrought structure. This structure has finer grains and enhanced ductility, both resulting from the breaking up of brittle grain boundaries and the closing up of internal defects, especially porosity.
The product of the first hot rolling operation is called bloom or slab. A bloom usually has a square cross-section, at least 150 mm (6in) on the side; a slab is usually rectangular in cross section. Blooms are processed further, by shape rolling, into structural shapes, such as I-beams and railroad rails. Slabs are rolled into planes and sheet.
Billets are usually square, with a cross-sectional area smaller than blooms; they are later rolled into various shapes, such as round rods and bars, by the use of shaped rolls. Hot-rolled round rods are used as the starting material for rod and wire drawing. They are called wire rods.
Forging is a process in which the workpiece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations. Most forgings require a set of dies and a press or a forging hammer.
Unlike rolling operations, which generally produce continuous plates, sheets, strip, or various structural cross-sections, forging operations produce discrete parts.
Typical forged products are bolts and rivets, connecting rods, shafts for turbines, gears, hand tools, and structural components for machinery, aircraft, railroads and a variety of other transportation equipment.
In the extrusion process, a billet (generally round) is forced through a die in a manner similar to squeezing toothpaste from a tube. Almost any solid or hollow cross-section may be produced by extrusion, which can create essentially semi-finished parts. Because the die geometry remains the same throughout the operation, extruded products have a constant cross-section. This can be done with cold, warm or hot working.
Typical products made by extrusion are railings for sliding doors, tubing having carious cross-sections, structural and architectural shapes, and door and windows frames.
Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, screws.
Drawing also improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment.
Figure 2.4.3: Drawing Process (Ref 1)
2.4.5. Sheet Forming
Examples of products made by sheet-metal forming are desks, file cabinets, appliances, car bodies, aircraft fuselages, beverage cans etc.
For aerospace applications, the common sheet materials are aluminum and titanium, and they are usually cold worked, but sheet forming in general can be done by hot or cold working.
There are many processes used for sheet-metal forming. The main ones are:
· Roll Forming: This process is used for forming continuous lengths of sheet metal and for large production runs. The metal strip is bent in stages by passing it through a series of rolls. The parts are then usually sheared and stacked continuously.
Stretch Forming: the sheet is clamped along its edges and then stretched over a die or form block, which moves upward, downward, or sideways, depending on the particular machine. Stretch forming is used primarily to make aircraft wing skin panels, automobile door panels, and window frames.
Drawing: A round sheet-metal blank is placed over a circular die opening and is held in place with a blank holder. The punch travels downward and forces the blank into the die cavity, forming a cup
1. The terms cold, warm and hot working are relative. The difference between them is the temperature at which the process is carried out.
2. There is not a “best” process. It all depends on what will be the use of the product, or what is the product going to be. The geometry of the product has a great influence on this decision.
Cold workingAs explained above, when we work a metal below the recrystallisation temperature, there is accumulation of a kind of material damage at the atomic level, through the pile-up of dislocations.
However this is not necessarily a bad thing. Many useful engineering objects are deliberately cold-worked as part of the manufacturing process to achieve improved properties. One common example is fencing wire. It is cold-drawn in the final stages, before being galvanised (plated with zinc) and coiled ready for sale.
The cold working stages increase the yeild stress of the wire, meaning we can pull harder on the wire before it deforms plastically (stretches). That's helpful when you are stringing a fence. However the cold working does not increase the ultimate strength of the material.
So in a sense, cold working uses up some of the safety margin of the material. If a very strongly cold worked material is overloaded, it could well just break like a brittle material with no warning. So we try to design cold working as a compromise. A little bit can be good: too much could be dangerous.
The advantages of cold working are
- A better surface finish may be achieved;
- Dimensional accuracy can be excellent because the work is not hot so it doesn't shrink on cooling; also the low temperatures mean the tools such as dies and rollers can last a long time without wearing out.
- Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for example) can make such scale come off the surface of a previously hot-worked object.
- Controlled amounts of cold work may be introduced.
- As with hot working, the grain structure of the material is made to follow the deformation direction, which can be good for the strength of the final product.
- Strength and hardness are increased, although at the expense of ductility.
- OH & S problems related to working near hot metal are eliminated.
- There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations. There are ways to get around this problem, see below.
- Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines (= $$$).
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