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Defects on Rolls for Cold Rolling - Article |
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Defects on Rolls for Cold Rolling
Cold rolling of sheet metal and strips of ferrous and non-ferrous metals is a substantial part of production and processing of metals. The use of cold rolled metal sheet and strips has spread rapidly, they are made from ordinary and alloyed steel and special alloys. Consequently, requirements regarding characteristics of rolls and the development of the rolling technology have also increased. Rolls are of course closely connected to the rolling technology. Rolls are among the most important tools used for cold rolling of metals and alloys. Of all tools made from hardened steel, rolls used in cold rolling are subjected to the biggest strains. They are the tools used for plastic deformation of metals or alloys under recrystallisation temperature, generally at the room temperature. At this temperature, metals are very resistant to deformation and are hardened in the process. The material's resistance against deformation thus increases with every pass and quickly increases to the rate, which subjects rolls to great strains. Various types of rolls are used for rolling, namely working and support rolls. Working rolls come into direct contact with the rolled material, while support rolls support the working rolls and at the same time prevent their bending under large pressure. Support rolls have bigger diameter than working rolls. Given their function, their production is of course also different. Working rolls must have very big surface hardness, substantial depth of the hardened layer and strong core. As regards support rolls, the most important is medium hardness with very good hardness of the core. The basic characteristic of all rolls is their resistance against wearing, spalling, heat shocks and large deformations. Various defects often appear on rolls during use, which may cause them to be damaged. Main defects appearing on rolls are: soft structure, welding on of the roll surface, cracks on rolls, spalling of surface, hair cracks, crushing of surface, cracks on the front side, broken edges, imprinted particles, fracture of the rolls, failure of the roll neck and corrosion pits on the rolls surface. The major part of defects on rolls occurs because of damage in the rolling plant or as a result of defective thermal treatment. Defects in the rolling plant usually result from overloading of rolls, inappropriate handling, incorrect grinding and cooling of rolls. This text presents basic defects appearing in rolls for cold rolling.
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