Difference Between Hot Working and Cold Working

Difference Between Hot Working and Cold Working Comparison Between Hot Working and Cold Working Summary: The Major Difference Between Hot Working and Cold Working is that both are the metal forming processes. Forming is the metalworking process of fashioning metal parts and objects through mechanical deformation. Hot Working is done at temperatures above the recrystallization temperature of the metal, and Cold Working is done at temperatures below the recrystallization temperature of the metal. Comparison Chart HOT WORKING COLD WORKING Hot working is carried out above the recrystallization temperature but below the melting point, hence deformation of metal and recovery takes place simultaneously Cold working is carried out below the recrystallization temperature and as such, there is not the appreciable recovery of metal During the process, residual stresses are not developed in the metal During the process, residual stresses are developed in the metal Because of higher deformation the temperature used, the stress required for deformation is less The stress required to cause deformation is much higher Hot working refines metal grains, resulting properties. Cold working leads to distortion of grains No hardening of metal takes place. Metal gets work hardened. If the process is properly performed, it does not affect ultimate tensile strength, hardness, corrosion and fatigue resistance of the metal It improves ultimate tensile strength, yield and fatigue strength but reduces the corrosion resistance of the metal It also improves some mechanical properties like impact strength and elongation During the process, impact strength and elongation are reduced Due to oxidation and scaling, poor surface finish is obtained Cold worked parts carry better surface finish. Close dimensional tolerances cannot be maintained Superior dimensional accuracy can be obtained Hot working is most preferred where heavy deformation is required Cold working is preferred where work hardening is required Hot Working  Hot working is accomplished at a temperature above the recrystallization temperature but below the melting or the burning point of the metal, because above the melting or the burning point, the metal will burn and become unsuitable for use.  Every metal has a characteristic hot working temperature range over which hot working may be performed.  The upper limit of working temperature depends on the composition of the metal, prior deformation, and impurities within the metal.  The changes in structure from hot working improves mechanical properties such as ductility, toughness, resistance to shock and vibration, % elongation, % reduction in area, etc. Hot Working Advantages  Due to hot working, no residual stresses are introduced in the metal.  Hot working refines the grain structure and improves the physical properties of the metal.  Any impurities in the metal are disintegrated and distributed throughout the metal.  The porosity of the metal is minimized by the hot working.  During hot working, as the metal is in a plastic state, larger deformation can be accomplished and more rapidly.  Hot working produces raw material which is to be used for subsequent cold working operations. Hot Working Disadvantages  Due to the loss of carbon from the surface of the steel piece being worked, the surface layer loses its strength.  This weakening of the surface layer may give rise to fatigue crack which results in failure of the part.  Close tolerances cannot be obtained.  Hot working involves excessive expenditure on account of high tooling costs. Cold Working  The working of metals at temperatures below their recrystallization temperature is called as cold working.  Most of the cold working processes are performed at room temperature.  Unlike hot working, it distorts the grain structure and does not provide an appreciable reduction in size.  Cold working requires much higher pressure than hot working.  If the material is more ductile, it can be more cold worked.  Cold Working results in Strain Hardening, distortion of grains and the crystallographic structure. Cold Working Advantages  Better dimensional control is possible because there is not much reduction in size.  The surface finish of the component is better because no oxidation takes place during the process.  Strength and hardness of metal are increased.  It is an ideal method for increasing the hardness of those metals which do not respond to heat treatment. Cold Working Disadvantages  The ductility of the metal is decreased during the process.  Only ductile metals can be shaped through cold working.  Over-working of metal results in brittleness and it has to be annealed to remove this brittleness.  To remove the residual stresses set up during the process, subsequent heat treatment is mostly required.