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Material Grades for Strip Mill Backup Rolls
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Other Options
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Di-Hard Backup Rolls, in Alloy Cast Steel and new V-Chrome Alloy Cast Steel, have been developed to overcome the compromise between strength and wear resistance, which is so often a problem when manufacturing conventional rolls. Our production and complex heat treatment processes result in rolls with substantially extended campaign times and excellent wear profiles.
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The mould for the production of a differentially hardened monobloc static cast roll is made up in multiple sections, assembled at the bottom of a casting pit.
The bottom, or DRAG section of the mould, is constructed to contain the in-gate, which allows entry of the metal into the mould. The in-gate is moulded at a tangent so that the metal steam will enter the mould off-centre.
The bottom journal is moulded in cast iron, sand lined chills.
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The roll barrel is moulded in heavy section cast iron chills, and the sand lining here is carefully regulated to a narrow width to promote rapid solidification of the steel, and produce a fine-grained, porosity-free working layer.
The top journal is contained within the COPE section, which again, is made from sand-lined cast iron chills.
The top-most section of the chill is the HEAD-BOX. This split-flask mould is heavily lined with sand and contains insulating tiles over its entire inner surface.
The mould is then completed by the attachment of the down-gate and runner system, which carry the metal from the ladle into the bottom of the mould.
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The completed mould is shown to the left in cross section, and the rough shape of the cast roll can be seen.
The mould is filled from the bottom as rapidly as possible, and the tangential entrance of the metal stream creates a rapidly swirling motion.
This vortex purges any refractory debris from the mould, and carries it up into the head section, thus ensuring a very clean casting.
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Computer modelling has been employed in mould design. This dictates appropriate chilling or insulation in the various mould section, so that directional solidification is promoted. The last area to solidify, being the most segregated, is therefore deliberately contained within the sacrificial head section, and so forms no part of the finished roll.
After casting, the roll is left to cool for a specified time, after which the head box and cope sections are removed and the roll is lifted from the rest of the mould.
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The roll is transferred directly to a heat treatment stove for the first heat treatment. This consists of a high temperature normalising cycle, which produces a homogenised casting, and leaves the material in its strongest and most ductile condition.
Following the homogenising treatment, the rough cast is fettled and then machined close to finished size, in preparation for the barrel handening process.
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The hardening treatment is carried out in a specially designed SELAS furnace. This comprises 2 half shells, lined throughout with high velocity gas burners.
The rolls, having been previously pre-heated in a conventional stove, is lifted into the SELAS furnace. The shells are closed and the roll is rotated as the surface of the roll barrel is very rapidly heated.
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The surface is quickly taken to a specified temperature of more than 900 degrees C, and thereafter, the heating if carefully monitored so that a pre-determined depth is heated to hardening temperature. This depth having been determined using heat flow calculations to ensure heating extends beyond the final roll discard diameter. Heating is so rapid that the journal and axis materials remain at the pre-heated sub-critical temperature only.
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The next operation is to water quench the roll barrel. This is carried out on a quench rig, where the roll is rotated between two banks of spray nozzles previously adjusted to match the barrel dimensions.
The fine water sprays rapidly quench the previously heated surface to transform the micro-structure and harden the entire working layer.
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In the later stages of the quench, fans are brought in to use instead of water, when a less rapid colling rate is required.
At the specified time and surface temperature, the rolls is removed from the quench rig and transferred to a heat treatment stove to complete the process. This included tempering that both stress relieves and prodices a barrel hardness at the level required by the customer.
The roll is now completed by being machined to size.
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Micro-examination shows that the material within the quenched and tempered working layer of the barrel consists of tempered bainites at a hardness of 50 to 70 degrees Shore C - as dictated by the particular customer specifications. While the journals and axis remain in the normalised condition, consisting of fine pearlite at 30 to 40 degrees Shore C, and having very good strength and toughness. Hardness penetration measurements are made on all rolls, and show that specified levels of hardness are maintained at a constant level to beyond the discard diameter.
In conclusion, the methods used in the proudction of Di-Hard backup rolls guarantee high strength and spall resitance with excellent hardness penetration and wear characteristics.
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The Davy Roll Company Limited
Registered in England, Registration No. 162966, VAT registration 172 3848 50 501.
Registered Office: PO Box 21, Coulthards Lane, Close Works, Gateshead, Tyne and Wear, NE8 3DX.
The Davy Roll Company Limited is a member of the Union Electric Steel Corporation group of companies.
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