Fundamentals of Hot and Cold Working Processes

Introduction 

“Hot working and cold working” are to be done for the process of “metal formation”. Through the “mechanical deformation process” “metal formation” of objects and parts is to take place. “Hot working” has to be carried out in general at the temperature above “recrystallization temperature”. But carried out below the “Melting point” of metal. Whereas “cold working process” has to be carried out below the “recrystallization temperature”. 

“Cold working”

“Cold working” is a process executed for the “metal formation process”. According to its name, this metal formation has to be executed below “recrystallization temperature”.  At room temperature hot working is to be done. In this case, expected recovery of metal cannot be gained. This process contrasts with the “hot working” process as it disfigures the gained structure and is unable to provide satisfaction in decreasing size. Unlike “hot working”, in “cold working process”, much more pressure needs to be applied. If metal is more ductile or flexible, then the process has to be done through a “cold working process”. “Cold working processes” cause “strain hardening”, “grain deformation”, and “crystal structure”. “Cold working process” is to be done for reducing flexibility of metal. 

Cold working processes

“Cold working process” is also known as “cold rolling process”. In the cold working process, at first, the material is heated and then the “plastic deformation process” is applied. After the “cold working process” is done, the grains are stretched in materials that’s why this process is carried out below the temperature of “recrystallization”. During the “cold working process” takes place, residual stresses started developing in the metal. For example, during the bending operation of metal, the upper portion of the bent part goes through tension, and the lower part experiences compression. Therefore, the upper portion experiences “tensile stress” and the lower portion experiences “compressive stress”. After load removal, in such a case, the metal will remain in the same structure as it was but residual stresses started developing in the bending part. The upper part of the metal goes through residual compression and the lower part experiences residual tension. In the case of “cold working process,” this residual stress takes place in material. Due to the “higher deformation process” taking place in this case stress needed for deformation is comparatively high. In the “plastic deformation process”, the force has to be applied in high proportion. As this process is carried out below “recrystallization temperature”, more forces are required. “Cold working processes” lead to distortion of grains. After executing the “cold working process”, the grains are elongated. It means distortion of grains takes place. In “cold working process”, metal gets work hardened. Hardening is a “metallurgical metalworking process” used for increasing the hardness of a metal. That means after “cold working process” an increase of metal size can be observed after hardening. Because of this process ultimate “tensile strength” and “fatigue strength” increases but “corrosion resistance” decreases. Sothis process is done under “recrystallization temperature” then the grains get distorted and elongated and due to which these properties increase. But due to this distortion, the chance of getting more corrosion increases after a “cold working process” is applied.

During cold working process

During “cold working processes” impact strength and elongation of the material reduces. “Impact strength” is defined as the significant energy that a material can resist until it applies on it.“Impact strength” can be measured by “Charpy test configuration” and “Izod test configuration”. Due to the cold working process, worked parts carry a better surface finish. There is less chance of oxidation and scaling as the whole process is carried out below “recrystallization temperature”. In “cold working process”, “superior dimensional accuracy” can be obtained. Close dimensions; corrosion can be obtained through this process. This process is applicable where “work hardening”needs to be performed. It means whenever hardness of the material needs to be increased, “cold working process” is under consideration. 

“Hot working”

“Hot working” is performed above the temperature of “recrystallization”. Yet this process is said to be accomplished above  “melting point” for recovery and deformation both can be achieved at the same time. After applying heat to the “metal-plastic deformation”the structure of the metal changes at the end. During this development, no residual stress takes place. Due to increased deformation takes place in this process the temperature used; the stress needed for deformation is less. In this case, very less force is required to be applied to metal. Then the metal gets plastically deformed. Due to “hot working process: metal grains are refined which results in properties of metal.

Conclusion 

The conclusion is drawn based on the fact that both processes are used for metal formation. For “hot working processes”, large deformations can be repeated continuously because the metal remains flexible and soft. After a hot working process, the hardness of material cannot be controlled. Hardness is generally lower in cold working processes and the needed energy for deformation is high. Cold working process can help in achieving ideal metal qualities.