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  • Difference between Hot Working and Cold Working

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    saurabhjain

    They both are the metal forming processes. When plastic deformation of metal is carried out at temperature above the recrystallization temperature the process, the process is known as hot working. If this deformation is done below the recrystallization temperature the process is known as cold working. There are many other differences between these processes which are described as below.

    Difference between Hot Working and Cold Working:

    S.No.
     
    Cold working
     
     
    Hot working
     
    1
     
    It is done at a temperature below the recrystallization temperature.
     
    Hot working is done at a temperature above recrystallization temperature.
     
    2.
     
    It is done below recrystallization temperature so it is accomplished by strain hardening.
     
    Hardening due to plastic deformation is completely eliminated.
     
    3.
     
    Cold working decreases mechanical properties of metal like elongation, reduction of area and impact values.
     
    It increases mechanical properties.
     
    4.
     
    Crystallization does not take place.
     
    Crystallization takes place.
     
    5.
     
    Material is not uniform after this working.
     
    Material is uniform thought.
     
    6.
     
    There is more risk of cracks.
     
    There is less risk of cracks.
     
    7.
     
    Cold working increases ultimate tensile strength, yield point hardness and fatigue strength but decreases resistance to corrosion.
     
    In hot working, ultimate tensile strength, yield point, corrosion resistance are unaffected.
     
    8.
     
    Internal and residual stresses are produced.
     
    Internal and residual stresses are not produced.
     
     
    9.

    Cold working required more energy for plastic deformation.

    It requires less energy for plastic deformation because at higher temperature metal become more ductile and soft.
     
    10.

    More stress is required.

    Less stress required.
     
    11.

    It does not require pickling because no oxidation of metal takes place.

    Heavy oxidation occurs during hot working so pickling is required to remove oxide.
     
    12.

    Embrittlement does not occur in cold working due to no reaction with oxygen at lower temperature.

    There is chance of embrittlement by oxygen in hot working hence metal working is done at inert atmosphere for reactive metals.


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    In a previous job we used A36 L sections in an unusual application. I believe these sections are hot rolled. When we machined the outer surfaces of the two legs, making the surfaces flat and square to each other, the material would move in unpredictable ways. As a side note the shop in Texas purchased a vibration table that relieved the internal stresses. After this operation the shop did not have as much movement in the sections during machining. 

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    Good comment, JAG. Structural shapes are always hot rolled, in my experience. Vibration is one of the common means of stress relieving a part, and certainly would be applicable to hot rolled sections. I presume they did the stress relief before machining the surfaces? Otherwise, it would be too late and the damage already done.

    DrD

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