Hot and Cold Working of Metals | Deformation | Metallurgy

Hot working is defined as the plastic deformation of a material at a temperature above its recrystallisation temperature, whereas working below this temperature is called cold working. The literal meaning of the words, hot and cold in ordinary sense does not apply here. For example, the working of lead and tin at room temperature is called hot working as their recrystallisation temperature is below the room temperature, but tungsten, when worked at 1200°C, is still called cold worked as its recrystallisation temperature is higher than 1200°C.

When a metal is cold worked, it becomes harder and stronger, whereas, the annealing of cold worked metal causes relaxation and softening that is why, the rate of work-hardening decreases in cold working as the temperature of working increases. For a given -rate of working, there is a temperature of working at which the rate of hardening and the rate of softening just balance, so that a metal could be plastically deformed without any increase of stress (no strain-hardening occurs), i.e., hot working takes place and this temperature is recrystallisation temperature.

If the rate of deformation is increased such as in hammer forging, the rate of hardening then becomes more, and thus, the temperature of working has to be increased to get a balance of hardening and softening. During hot working, the grains are distorted, elongated with increased imperfections, but simultaneously the processes of dynamic recovery, recrystallisation, and even grain growth (if the temperature of piece is very high) take place.

Thus, on a broad basis, hot working is equivalent to cold working and annealing. Actually, a typical process of hot working (such as hot rolling) produces the deformation in a fraction of a second, and in such a process, time in which the recrystallisation is to be completed is shorter, (otherwise the temperature may decrease and cold working may result), the temperature of hot working must be appreciably higher than the usual ‘recrystallisation temperature’. (The processes taking place in recrystallisation are slow at lower temperatures and quick at higher temperatures).

Normally, while doing hot working, much higher temperatures are used initially to promote uniformity in the material (diffusion is faster at higher temperatures to remove chemical heterogeneity. Even inclusions get uniformly and finely dispersed), and even the resulting large grains allow more economical reduction during initial working such as hot rolling of ingots.

As the hot working progresses, the temperature of the material decreases, the newly recrystallised grains every time become smaller and smaller, and become very fine after the last working operation, where the temperature of the material should be just slightly above the recrystallisation temperature.

If at this stage, the temperature is high, the grain size obtained shall be coarser, and if it is low, than cold working takes place without recrystallisation (Actually, the piece may get stuck in the rolls and the mill stops, because the hot-working mill shall not be able to cold-roll as cold-rolling requires high-powered mill for which a hot rolling mill is not designed).

Commonly, it is economical that ingots of metals are initially hot-worked to reduce by large amounts to smaller section, and then it may be cold worked to get exact dimensions and good surface finish.

Characteristics of Hot-Worked State:

There are a number of advantage and limitations of hot working a metal:

1. Lack of Strengthening:

As no strengthening occurs during hot working because of dynamic recovery and recrystallisation, the amount of plastic deformation is almost unlimited. That is why metal ingots are first hot-worked. The last pass is invariably just above the recrystallisation temperature with heavy reductions to produce the finest possible grain size, followed by final cold working for exact dimensions, higher derived strength and good surface finish. Metals like HCP- magnesium is hot worked (warm rolling) as more slip systems become active to improve the ductility to permit larger deformations than possible by cold-working.

2. Removal of Defects:

Some defects present may be eliminated, or their effects are minimised. Porosity can be collapsed and welded. The segregation is reduced, at least the diffusion distance is reduced.

3. Anisotropic Properties:

Properties are invariably anisotropic. Surface has finer grains than the centre. Fibrous structure is obtained as inclusions or second phase particles are elongated in working direction-leading to a texture, or preferred orientation.

4. Surface Finish and Dimensional Accuracy:

Hot working almost always results in surface oxidation. Thus, surface finish is poor. Metals like tungsten and beryllium are hot worked in protective atmospheres. Dimensional accuracy is difficult during hot working as elastic strain and thermal contraction occur.

Characteristics of Cold Worked State:

There are a number of advantages and limitations of strengthening or cold working a metal:

1. There is simultaneous increase of strength and hardness while producing a desired final shape.

2. Excellent dimensional tolerances with excellent surface finish can be obtained.

3. It is a cheaper method to produce small parts in mass. Such parts should be able to be shaped by smaller deformations, otherwise intermittent annealing shall be required.

4. Some metals such as the HCP magnesium being almost brittle at room temperature cannot be given large degree of cold working otherwise it shall fracture.

5. Some properties like ductility, electrical conductivity and corrosion resistance are impaired by cold working process, but these can be recovered by annealing process.

6. Residual stresses and anisotropic behaviour may be introduced during cold working. The latter becomes more prominent if the amount of cold deformation is large. These characteristics may be either harmful or beneficial, depending on how they can be controlled.

7. Some cold working techniques can be accomplished only if cold working occurs. Wire drawing requires pulling a rod through a die to produce a smaller cross-sectional area. For the same draw force Fd, a different stress is developed in the original and the final wire. The stress on the initial wire should be higher than yield strength so that deformation occurs. The stress on the final wire must be less than its yield strength to prevent its failure. This condition can only be accomplished if straining occurs of the wire by drawing.