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Process is similar to rolling dough with rolling pin to reduce thicknessRolling accounts for about 90% of all metals produced by metalworking processes

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Rolling first is carried out at elevated temperatures; known as “Hot Rolling”From Ingot to slab, billet, bloomChanges grain structure: from ingot’s coarse grained, brittle & porous structure to wrought structure with finer grain structure and enhanced properties, e.g. strength & hardnessCold rolling (in later stages) increases strength & hardness and provides a better surface finishbetter surface finishCold rolling requires more energy than hot rollingNote slab, billet & bloom are intermediate stage for different family of final products

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A metal strip is reduced in thickness by a pair of rotating rolls, each powered individually by electric motors.The velocity of the strip increases as it passes through the roll gap.The velocity is highest at exit from rollsThe surface speed of each roll is constant.Therefore there is relative sliding between the roll and the strip and this varies.At one point along the contact length the velocity of roll and strip are the same. This is no slip point. (see (b)). Some friction is necessary in process (similar to friction keeping car tires on a road surface) but requires energy. In practice lubricants are used to reduce friction and energy requirements

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Roll forces can cause significant deflection.(b) Two high rolling mills are used for HR in initial passes(c) Three high or Reversing Mill: Used for repeated, successive passes of material (plate) in opposite directions(d) 4-high and cluster mills are based on the principle that small diameter rolls lower the roll forces (because of smaller roll-strip contact area); also lower power requirements and reduce spreadingpower requirements and reduce spreading. Small rolls are cheaper to replace; but deflect more, therefore they are supported by other large-diameter rolls.Common roll materials are C.I., cast steel, forged steel, Tungsten carbide is used for small diameter rollsForged steel rolls are more costly than C I rolls but have higher strengthForged steel rolls are more costly than C.I. rolls but have higher strength, stiffness, and toughness.

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Roll forces tend to bend the rolls elastically along their lengthUsual method to prevent roll deflection is to grind the rolls with a camber (thicker at centre than edges)Amount of camber varies with product.For rolling sheet metals, the radius of maximum camber point is generally 0.25 mm (0.01 in.) greater than at edges of roll

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Spreading: increase in width by rollingA phenomenon observed when rolling cookie doughIn equations calling for width, use average width

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Hot rolling converts the cast grain structure to a wrought grain structureProduct has finer grains and enhanced ductility

- Both are a result of the breaking up of brittle grain boundaries and the closing up of internal defects (especially porosity)Recall the slabs, billets and blooms (Fig. 13.1) These are the products of the first hot rolling operation.

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Use series of leveling rolls to improve flatnessWorkpiece is flexed in opposite directions as it passes through a set of rollersEach roll driven separately by individual electric motor

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(a) Wavy edge – caused by roll bending; strip is thinner at edges than at centre, thus edges elongate more than the centre. Then edges buckle because they cannot expand freely in longitudinal direction

(b) Zipper cracks & (c) Edge cracks are result of poor material ductilityRemove edge cracks by slitting (a process covered later) to maintain integrity of

material used(d) Alligatoring complex phenomenon; typically caused by nonuniform bulk(d) Alligatoring – complex phenomenon; typically caused by nonuniform bulk

deformation of the billet during rolling or by presence of defects in the original cast material

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Residual StressesBecause of non uniform deformation of the material in the roll gap,Can develop in rolled plates and sheets, especially during cold rollingSmall diameter rolls or small thickness reductions per pass ( see (a))

- tend to plastically deform the metal more at its surfaces than in the bulk- results in Compressive residual forces at the surfaces and Tensile stresses in p

the bulkConversely, large-diameter rolls or high reductions per pass (see (b))

- tend to deform the bulk more than the surfaces- Due to higher frictional constraint at the surfaces along the arc of contact- Result is opposite: C in bulk and T at surface

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Illustrates scale of facility; significant investment

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Tandem rollingThis is what is in the mill stands in the previous pictureStrip is rolled continuously through a number of stands to thinner gauges with each passEach stand consists of a set of rolls with its own housing and controlsA group of stands is called a trainNote increasing speeds as material becomes thinnerThe control of the strip thickness and the speed at which the strip travels through each roll gap is critical.Extensive electronic and computer controls are used, particularly in precision rolling.

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Shape RollingUsed for straight and long structural shapes (RR rails, I-beams, channels, solid bars, etc.)Performed at elevated temperaturesThe stock goes through a set of specially designed rolls

Cold Shape FormingStart with material in the shape of wire with various cross sectionsMaterial’s cross section is reduced nonuniformly.Therefore, design of roll series requires considerable experience to avoid external and internal defects, hold dimensional tolerances, reduce roll wear.

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Roll-forging (aka cross rolling)Cross section of a round bar is passed through a pair of rolls with profiled grooves. Typically used to produce tapered shafts and leaf springs, table knives, hand tools

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Making Ball Bearings(a) Skew Rolling

feed round wire into roll gap between two rotating rolls; cut rough ball,

(b) Shear round cylindrical blanks from round bar; upset the pieces in headers between two dies with hemispherical cavities;

Both methods:Later ground and polished

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Ring RollingThick ring is expanded into large diameter, thinner one.Thickness is reduced by bringing rolls closer together as they rotate.

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Typically made of hardened steel, rolling dies are expensive because of complex shape.

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Note continuous grain structure in rolled thread, resulting in higher strength

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Rotary Tube PiercingHot working operationFor making long, thick walled, seamless, tube or pipePrinciple:

When a round bar is subjected to radial compressive forces, tensile stresses develop at the centre of the bar.

When it is subjected continuously to these cyclic compressive stresses, the bar begins to develop a small cavity at its centre, which then begins to grow.

Try it with around eraser.

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Tube Rolling

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