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Commercial metal forming processes may fundamentally include the following major operations.
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Metals to be extruded are normally applied with temperatures almost close to their melting point, and they are literally squeezed out from the die in their solid state form.
In this process, proper metal billets or alloy are subjected to high standard extruding temperatures, then put in the compartment of the extruding compressor and thrust out through the relevant shape of the die with the help of the force exerted by a ramming piston of the machine.
Metals like aluminum, copper, brass, lead, and their alloys can be converted into any extruded form such as round, square, hexagonal, angular, and any complex shape, including shapes which are difficult rolling out.
Shapes produced through extrusion are usually smooth, correctly shaped, and retain excellent levels of tensile strength.
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Hydraulic Metal Forming Press
Image Credit - http://www.forging-hydraulic-press.com/
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This process involves “cold” working of the metal whereby a ductile metal rod is squeezed out through a tapered nozzle of a die so as to force or make the metal to transform into a smaller cross-sectional area, resulting in an enhancement in its tensile strength.
Though normally the drawn cross sectional shape is round, any desired shape may be produced depending on the die being used.
The material used for making the dies for operating the above procedures may range from diamond, steel, or tungsten carbide to chilled cast iron.
Nowadays, tungsten carbide has become more popular and has replaced other materials due to its excellent performance with retaining its original shape even after a huge number of drawing operations.
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Steels appearing reddish blue in color with a non-polished finish typically indicate that they have been hot rolled.
The hot rolling process is normally initiated immediately after the solidification of the ingot mold is completed. The process involves shaping of the metal by rolling it past between a pair of revolving, chilled iron rollers that squeeze the metal and at the same time push it across the other side of the roller. The procedure is repeated in a to-and-fro manner until the thickness is significantly reduced with an increase in its length.
The process is repeated a number of times through different stages until the final desired result is reached.
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When drawing metals as wires, a hot-rolled rod having an uneven diameter is first cleaned in an acid pickling tub, which helps in removing scaly outer corrosion.
After the metal is further washed in a rinsing tub to get rid of the acid coating, next follows the water wash, making the metal absolutely free from all unwanted coatings.
After the water wash, the metal is treated with air so that a thin layer of rust gets deposited over it, which later on helps the drawing operations to be done more smoothly.
The next step involves dipping of the rod in lime water, followed by force drying of the metal at about 300 degree F. The dried rod is then forwarded to the drawing bench, where the rod end is made into a point like shape by hammering.
This end is then used to “feed” the die tapered opening, and the drawing process is initiated with the help of a power rod. This power rod starts drawing the metal until the entire metal passes through the die slot smoothly due to the actions of the lime and the rust layers. Other forms of external lubricants like grease, tallow or soap are also sometimes utilized.
Through some controlled number of drawing passes, the degree of metal reduction or shaping and through regular in between annealing of the metal, literally any possible level of strength and toughness can be induced into the metal.
With mild steel, the above procedure can be used for producing wire rods with significantly enhanced strengths.
This method is fundamentally used throughout the world to produce copper wires used as electrical conductors.
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This process is generally followed after the hot rolling, mainly to provide an improved finish or appearance to the processed metal.
The objective behind the process may be assumed to be:
- For establishing an enhanced surface finish
- For improving thickness uniformity which might have possibly not been accomplished during hot rolling
- For enhancing the positive physical attributes of the metal through a combined processes of cold rolling and further heat treatments
Normally, the cold rolling process may be considered quite similar to hot rolling, however here the rolling forces are increased greatly because the metal, being cold, offers much greater resistance to the rolling movements compared to hot rolling.
Caution is observed that the metal to be cold rolled is first cleaned and set free from scaly layers, as otherwise these impurities would get embedded in the metal while carrying out the clod rolling process.
A commercial metal forming process may involve a number of different further stages like cold pressing, spinning, casting, hollow casting, centrifugal casting, chilled casting, die casting, etc.
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The author's own experience
Capital Steel and Wire Inc. - http://www.capitalsteel.net/blog/hot-and-cold-rolling-explained