Hot Working: Mechanical working processes which are done above recrystallisation temperature of the metal are know as hot working processes. Some metals, such as lead and tin, have a low recrystallisation temperature and can be hot-worked even at room temperature, but most commercial metals require some heating. However, this temperature should not be too high to reach the solidus temperature; otherwise the metal will burn and become unsuitable for use. In hot working, the temperature of completion of metal working is important since any extra heat left after working aid in grain growth. This increase in size of the grains occurs by a process of coalescence of adjoining grains and is a function of time and temperature. Grain growth results in poor mechanical properties. If the hot working is completed just above the recrystallisation temperature then the resultant grain size would be fine. Thus for any hot working process the metal should be heated to such a temperature below its solidus temperature, that after completion of the hot working its temperature will remain a little higher than and as close as possible to its rccrystalisation temperature.
EFFECT OF HOT WORKING ON MECHANICAL PROPERTIES OF METALS:
- This process is generally performed on a metal held at such a temperature that the metal does not work-harden. A few metals e.g., Pb and Sn (since they possess low crystallization temperature) can be hot worked at room temperature.
- Raising the metal temperature lowers the stresses required to produce deformations and increases the possible amount of deformation before excessive work hardening takes place.
- Hot working is preferred where large deformations have to be performed that do not have the primary purpose of causing work hardening.
- Hot working produces the same net results on a metal as cold working and annealing. It does not strain harden the metal.
- In hot working processes, compositional irregularities are ironed out and nonmetallic impurities are broken up into small, relatively harmless fragments, which are uniformly dispersed throughout the metal instead of being concentrated in large stress-raising metal working masses.
- Hot working such as rolling process refines grain structure. The coarse columnar dendrites of cast metal are refined to smaller equiaxed grains with corresponding improvement in mechanical properties of the component.
- Surface finish of hot worked metal is not nearly as good as with cold working, because of oxidation and scaling.
- One has to be very careful as regards the temperatures at which to start hot work and at which to stop because this affects the properties to be introduced in the hot worked metal.
- Too high a temperature may cause phase change and overheat the steel whereas too low temperature may result in excessive work hardening.
- Defects in the metal such as blowholes, internal porosity and cracks get removed or welded up during hot working.
- During hot working, self-annealing occurs and recrystallization takes place immediately following plastic deformation. This self-annealing action prevents hardening and loss of ductility.
MERITS OF HOT WORKING:
- As the material is above the recrystallisation temperature, any amount of working can be imparted since there is no strain hardening taking place.
- At a high temperature, the material would have higher amount of ductility and therefore there is no limit on the amount of hot working that can be done on a material. Even brittle materials can be hot worked.
- In hot working process, the grain structure of the metal is refined and thus mechanical properties improved.
- Porosity of the metal is considerably minimized.
- If process is properly carried out, hot work does not affect tensile strength, hardness, corrosion resistance, etc.
- Since the shear stress gets reduced at higher temperatures, this process requires much less force to achieve the necessary deformation.
- It is possible to continuously reform the grains in metal working and if the temperature and rate of working are properly controlled, a very favorable grain size could be achieved giving rise to better mechanical properties.
- Larger deformation can be accomplished more rapidly as the metal is in plastic state.
- No residual stresses are introduced in the metal due to hot working.
- Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal.
- Mechanical properties, especially elongation, reduction of area and izod values are improved, but fibre and directional properties are produced.
- Hot work promotes uniformity of material by facilitating diffusion of alloy constituents and breaks up brittle films of hard constituents or impurity namely cementite in steel.
DEMERITS OF HOT WORKING:
- Due to high temperature in hot working, rapid oxidation or scale formation and surface de-carburization take place on the metal surface leading to poor surface finish and loss of metal.
- On account of the loss of carbon from the surface of the steel piece being worked the surface layer loses its strength. This is a major disadvantage when the part is put to service.
- The weakening of the surface layer may give rise to a fatigue crack which may ultimately result in fatigue failure of the component.
- Some metals cannot be hot worked because of their brittleness at high temperatures.
- Because of the thermal expansion of metals, the dimensional accuracy in hot working is difficult to achieve.
- The process involves excessive expenditure on account of high cost of tooling. This however is compensated by the high production rate and better quality of components.
- Handling and maintaining of hot working setups is difficult and troublesome.