COLD WORK AND HOT WORKThis is a featured page




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.
The temperature ranges for these 3 categories of plastic deformation are given in the next table in term of a ratio, where T is the working temperature and Tm is the melting point of the metal, both on the absolute scale. Although it is a dimensionless quantity, this ratio is known as the homologous temperature.


Definition:
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.
· Anisotropy
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Hot Working

Definition
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.


Methods used for Cold, Hot working


ROLLING -- FORGING ------



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).
The main problems, however, are
  • 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.
Of course smart people have found ways to minimise the problems or work around them - more below - and as a result hot working is a very common and useful process. We just have to be aware of its limits and follow the hot working operations with other types of manufacturing process that can fix the problems that occur.

Cold working

As 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.

However
  • 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 (= $$$).

A neat trick: cold work then normalise

Cold working has many advantages and is very much the more common type of metal forming. However if a large overall deformation is desired, how can we do it using only cold working? The answer is: do some cold work, then put the object through a heat-treatment cycle to relieve the atomic-scale damage caused by the cold work. This is called annealing or normalising the metal. It is done by heating the metal object above the recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of course we have to pay for the energy to do the heating.
This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who shape copper tube on a building site. When a piece of tube has to bent sharply, it is done in easy stages with a proper annealing between each stage (usually done using a hand-held gas flame). This ensures that metal won't crack during the bending operations.
Think about working with a sheet of lead on a nice warm day in the Australian sun. The lead will likely be above its recrystallisation temperature, with no special heating required. This can actually be very useful. It means you can shape your sheet of lead for hours - bend it back and forth, hammer it out, whatever - and it will probably accept all the deformation with cracking. This is one of the reasons lead sheet was so popular in ancient times as a roofing/guttering material (for those who could afford it). Any strange shape needed could be hammered out of a sheet or even a lump, right on site, and with no special furnaces or other technology.




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).
The main problems, however, are
  • 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.
Of course smart people have found ways to minimise the problems or work around them - more below - and as a result hot working is a very common and useful process. We just have to be aware of its limits and follow the hot working operations with other types of manufacturing process that can fix the problems that occur.

Cold working

As 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.
However
  • 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 (= $$$).

A neat trick: cold work then normalise

Cold working has many advantages and is very much the more common type of metal forming. However if a large overall deformation is desired, how can we do it using only cold working? The answer is: do some cold work, then put the object through a heat-treatment cycle to relieve the atomic-scale damage caused by the cold work. This is called annealing or normalising the metal. It is done by heating the metal object above the recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of course we have to pay for the energy to do the heating.
This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who shape copper tube on a building site. When a piece of tube has to bent sharply, it is done in easy stages with a proper annealing between each stage (usually done using a hand-held gas flame). This ensures that metal won't crack during the bending operations.
Think about working with a sheet of lead on a nice warm day in the Australian sun. The lead will likely be above its recrystallisation temperature, with no special heating required. This can actually be very useful. It means you can shape your sheet of lead for hours - bend it back and forth, hammer it out, whatever - and it will probably accept all the deformation with cracking. This is one of the reasons lead sheet was so popular in ancient times as a roofing/guttering material (for those who could afford it). Any strange shape needed could be hammered out of a sheet or even a lump, right on site, and with no special furnaces or other technology.


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