The cast products (such as steel ingots or continuous steel castings) are converted (or manipulated) in the solid state to wrought products (or mechanically worked products) by various deformation processes. The choice of deformation or manipulation process (hot-working or cold-working) depends on the metal being handled, desired shape of the end product, accuracy and finish and the quantity required. The cast steel ingots or continuous steel castings are always first hot worked to reduce them to the products of various shapes and sizes primarily for the refinement of grain structure of the cast product, improved directional properties or directional control of flow lines’, elimination of porosity in cast ingots, and breaking up and distribution of inclusions and impurities in the metal, thus avoiding weak points or locations in the product. Wrought products may be in the form of long-length forms such as sheets, plates, bars (black, hot rolled) or bright-drawn bars (cold drawn), rolled sections as I-beam, channels, angles, tubes, wire and other extruded sections. The other types of products coming under the category of wrought products are individual components like large forgings and forged small components such as connecting rods, crank shafts, etc.
Showing hot rolling and cold rolling of metals. Hot rolling refines the grain structure whereas cold rolling distorts it. Hot rolling is an effective way to reduce grain size in metals, for improved strength, ductility and impact resistance.
If a metal is subjected to some stress (loading), forming will take place when the deformation stress moves from the elastic to plastic range, i.e. beyond the yield point. For every material there is a limiting value of load (or stress) up to and within which the resulting strain or deformation disappears entirely on removal of load. The value of stress corresponding to this limiting load up to and within which metal shows elastic behaviour (i.e. coming to original shape on removal of load) is known as the elastic limit of metal. Whenever a metal is stressed beyond its elastic limit, the applied stress causes plastic deformation which is permanent in nature. It will be further noted that in loading beyond elastic limit, increase in strain is far more larger and rapid than the corresponding increase in stress level (i.e. metal does not obey Hooke’s Law). This situation continues till the yield point is reached at which the strain increases even without any further increase in stress and the stress at which this sudden. stretching occurs is called yield point of the metal. Plasticity is the property of metal because of which it can undergo permanent deformation under load without rupture or failure and is an important property for forming purposes. Plastic deformation (or permanent deformation) occurs only when metal is stretched beyond the yield point, i.e. from the elastic to the plastic range.
Classification of Metal Forming Processes
Metals can be plastically deformed or (worked) at room, warm or higher temperatures. Their behaviour and workability depend largely on whether the deformation takes place below or between 0.37 and 0.5T, where T is the absolute melting temperature of the metal. above the recrystallization temperature of the metal. Recrystallization temperature ranges Recrystallization is the process in which, at the said temperature range, new equiaxed (having equal dimensions in all directions) and strain-free grains are formed. Metal forming processes are traditionally classified as:
(a) Hot forming or hot-working processes
(b) Cold forming or cold-working processes
(c) Warm forming processes
The categorization of metal forming processes may be done in the following ways where T stands for the absolute working (or forming) temperature and T is the absolute melting point of the metal
For cold-working, ratio of T/Tm is less than 0.3.
For warm-working, ratio of T/Tm is 0.3 to 0.5.
For hot-working, ratio of T/Tm is greater than 0.6.
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