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  • Difference between Hot Working and Cold Working

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    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:

    Cold working
    Hot working
    It is done at a temperature below the recrystallization temperature.
    Hot working is done at a temperature above recrystallization temperature.
    It is done below recrystallization temperature so it is accomplished by strain hardening.
    Hardening due to plastic deformation is completely eliminated.
    Cold working decreases mechanical properties of metal like elongation, reduction of area and impact values.
    It increases mechanical properties.
    Crystallization does not take place.
    Crystallization takes place.
    Material is not uniform after this working.
    Material is uniform thought.
    There is more risk of cracks.
    There is less risk of cracks.
    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.
    Internal and residual stresses are produced.
    Internal and residual stresses are not produced.

    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.

    More stress is required.

    Less stress required.

    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.

    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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       4 of 4 members found this review helpful 4 / 4 members

    To my understanding, item #5 is exactly backward. Cold rolled material is much more uniform that hot rolled. Hot rolling is much more similar to forging; both work the material is a hot state.

    Item #8 regarding hot rolling is not entirely correct either. If you cut a piece of hot rolled material (sawing perhaps), there is a considerable possibility of warping due to the release of internal stresses.

    Item #10 does not make sense for either hot or cold rolling. More/less stress required for what?

    I have doubts regarding item #12 for hot rolling. It is my understanding that aluminium (which is moderately reactive) is hot rolled in air.

    But then, these are just details; who really cares? Right?

    The term "cold rolling" can  be misleading. The material may start out quite cold, even as low as 32 F (0 C), but when it comes out of the cold mill, it will be far too hot to touch. This is not because heat has been applied by flame or conduction, but simply because of the massive amount of working done in the rolling process.

    Some hot rolling operations involve only a single mill stand, such as a "break down" mill that initially breaks down the ingot, or a slab mill that rolls thick slabs. A rolled section mill (for I-beams, angles, channels, etc) or a rod mill (for rebar, other rod stock) are usually hot operations. The material is cherry red as it passes through the mill rolls.

    In a cold sheet mill, the stock is received from the pickling line, after having previously passed through the hot mill. Often multiple pieces are butt welded together to make a longer piece of incoming material. This is important because once the mill is up to speed, you really do not want to have to re-start until it is absolutely essential. Material rolled at low speed, before the mill is brought fully up to speed is not the same guage or other properties as that rolled at high speed. This is because of the difference in dynamic properties of the rolling mill at different speeds.

    When we look at a picture of a rolling mill (either hot or cold), we usually see the mill stand and the ends of the rolls. The typical rolling mill is a "4-high mill" which means that there are four rolls. From the top they are (1) top backup roll, (2) top work roll, (3) bottom work roll, (4) bottom backup roll. On the cold mill where I worked many years ago, the work rolls were about 11 inches in diameter while the backup rolls were about 60 inches in diameter. This was on a mill with a width of 120 inches (10 feet). The vertical squeezing is applied to the bearings of the backup rolls, and they in turn press against the work rolls. What most do not realize is that the vertical forces is primarily there to avoid material sllp; it does not reduce the guage significantly.

    Guage reduction, that is, thickness reduction, in the rolling mill is primarily the result of stretching. Thus the first mill stand, where the stock enters initially, acts primarily as a brake, holding back the stock. Each later mill stand runs faster than the incoming stock will allow, thus stretching the material over and over as it passes through each successive stand. The interstand tension is typically about 1000 lb/in of width, so for an 80 inch wide sheet, this comes to about 80,000 lb tension between the stands. At times, the interstand tension causes the strip to break, and then all havoc breaks loose.


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