It is proposed to deal with the effect of mechanical work on the structure and macro-properties of metals and to follow this with a classification of the processes used for mechanical working.
Effects of Mechanical Work on Metals
During the process of shape change which accompanies mechanical working the volume of the mass remains constant and an increase in length such as in rolling is accompanied by a decrease in thickness.
As deformation is applied to a structure consisting of one kind of deformable grains, they will become elongated. At the same time mechanical properties become directional and the structure and properties are anisotropic. The behavior of a duplex structure is very similar except that the two phases or types of grains, α and β, are likely to react differently to the deformation process. α may be soft and ductile, whilst β may be hard and brittle, will therefore tend to fracture and appear as orientated fragments or stringers in the longitudinal direction. A duplex structure will tend to become more anisotropic than a single-phase structure.
At very high degrees of deformation the structure appears fibrous because the grains have been so elongated as to lose their individual characteristics.
Deformation also affects mechanical properties, in that the hardness, ultimate tensile and yield stresses all increase to a maximum, whilst the ductility falls to a very low value. The toughness, as measured by the Izod or Charpy test, increases with working up to a maximum and then gradually decreases. It is found in practice that the hardness and strength of most metals increase by 2.5 to 3 times the annealed value as a result of cold working.
All structural metals have approximately the same ductility as measured by percentage elongation. An annealed metal will have approximately 35% elongation; whilst a metal which has been cold worked 80% will have only approximately 2% elongation before failure in a tensile test.
The best combination of properties is usually found in the longitudinal direction, and the worst in the short transverse direction.
The Effect of Heat on Cold-Worked Metals
A metal sample which has been cold worked 80% will be hard and brittle, the grains will be elongated and there will be a considerable degree of anisotropy. If the sample is heated, a temperature will be reached at which new nuclei begin to form in the distorted grains. This occurs due to the fact that the thermal energy supplied allows the atoms to diffuse to sites and form stable nuclei. How much thermal energy is needed depends upon the amount of prior cold work carried out on the metal.
Cold work increases the internal energy of the metal, and the greater the cold work the higher the residual internal energy. This means that less thermal energy is required to nucleate a heavily cold-worked metal than a lightly cold- worked one.
It is important to understand the mechanism of nucleation and the factors which control the number of nuclei formed. It is recognized that nucleation will occur in those regions with the highest residual stresses, and these occur at multiple boundary intersections.
The longer the time that the worked sample is held at a nucleating temperature, the greater the number of atoms that will diffuse to the nuclei and occupy positions of minimum energy. The volume around each nucleus will grow to visible size and after some time further growth will be prevented by the interference of one growth volume with another. These growth volumes become grains and the interstititial zones of distorted atomic pattern are the grain boundaries. The grains will be softer and much larger than the worked grains and the atomic orientation will be random as between grains, replacing the common forced orientation in the worked material.
Each nucleus has grown to form one grain and this gives the recrystallised grain size. The greater the degree of cold works the smaller the recrystallised grain size. With no cold work there are no high stress centers so no recrystallisation on heating. With the critical amount of cold work there are a few and these grow excessively to give very large grains.
If the metal is held at the recrystallisation temperature after it has completely recrystallised, diffusion of atoms still occurs and some grains grow at the expense of others. This is called grain growth. It is quite possible in an industrial process that quite an appreciable amount of grain growth occurs so that the final or annealed grain size is much coarser than the recrystallised grain size.
Grain growth occurs by a diffusion process and all such processes are affected by time and temperature. It has been seen that diffusion is a linear function of time, but increasing temperature has a far more critical effect on diffusion since the rate is an exponential function of temperature. Increasing the temperature by 10°C doubles the diffusion rate, and if the sample is heated to a temperature substantially above the recrystallisation temperature the grain growth will result in a coarse structure.
Final grain size after cold working and annealing is very important in industrial processes. If the grains are too coarse the metal will exhibit a rough surface finish on machining and an "orange peel" effect after pressing. Grain size also affects the toughness. The best structure for further working consists of small, uniform equiaxed grains. The most important factor in the industrial process is the final temperature in the furnace. This should be as low as possible, whilst ensuring complete recrystallisation in adequate time.
Hot Working of Metals
Cold working followed by annealing can be compared to working at above the recrystallisation temperature. This is described as hot working and deformation of the grains is followed by instantaneous recrystallisation. The effects of deformation on structure and properties are therefore instantly removed. This is the idealized situation in hot working. In practice the effects of deformation are instantaneous but recrystallisation requires time and unless the hot deformation system is slow enough to allow complete recrystallisation, then evidence of working persists at the end of the process.
This concept gives us the true definition of hot and cold working:
An interesting example of the application of the above principles to hot working is in the hot rolling of steel strip for deep drawing. The final material should consist of small equiaxed grains exhibiting the minimum amount of mechanical anisotropy.
Hot working is working at such a temperature and strain rate that recrystallisation keeps pace with deformation.
Cold working is working under conditions such that recrystallisation does not keep pace with deformation.
Steel consists of two phases, ferrite and cementite. The two phases, however, have totally different recrystallisation temperatures. The ferrite recrystallises at around 600°C whereas the cementite requires temperatures between 700°C and 900°C, depending upon the carbon content. Even if the ferrite recrystallises after hot working, the presence of the cementite in an oriented formation prevents the development of equiaxed grains and mechanical anisotropy persists giving "pan caked" grains. Two precautions are taken to avoid this, firstly the carbon content is limited to 0.1% maximum to reduce the amount of second phase, and secondly the form of the second phase is controlled so that it is in an innocuous form.
Cementite may appear as a massive form on the ferrite grain boundaries, if either the cooling after hot rolling is very slow or the finishing temperature is very high. A lamellar form, i.e. pearlite, is obtained if the cooling rate is intermediate, resulting from a relatively high finishing temperature. Finally, it can occur as fine dispersed spheroids if the cooling rate is an optimum value achieved by a relatively low hot-working temperature.