Rolling

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Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Rolling
It is a bulk deformation process involving thickness reduction and shaping cross sections of long products by the use of rollers

1- Flat rolling
Produces plates and sheets Examples:  Sheets used in automobile industry  Sheets used in fabrication of cans (0.15 mm thick)  Aluminum foils (0.008 mm thick) Initial material used in rolling is usually an ingot or a previously continues cast

Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Changes in the grain structure of cast or large-grain wrought metals during hot rolling. Hot rolling is an effective way to reduce grain size in metals for improved strength and ductility. Cast structures of ingots or continuous castings are converted to a wrought structure by hot working.

Mechanics of flat rolling

W/p enters the roll gap with a thickness ho, a width wo and a velocity Vo. As the volume flow rate through the roll gab must remain constant, the velocity must increase as the roll gap narrows. Since Vr ( roller velocity) is constant, sliding motion takes place at certain places between the roll and the rolled
Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

strip. In the entry the velocity of the roll is higher, whereas, towards the exit the velocity of the strip is higher. This implies that there exists a point where the speeds of the roll and the strip are equal. This point is known as the neutral point or the no-slip point

The figure to the right shows relative velocity distribution between roll and strip surfaces. Note the difference in the direction of frictional forces. The arrows represent the frictional forces acting on the strip As the strip must move towards the exit, the frictional force acting on the strip towards the exit must be greater. And the forward slip can be calculated by:
slip = V f − Vr Vr

Rolling forces, stresses ands pressure

By taking two material elements, at the entry and the exit (both sides of the neural point) and apply the force equilibrium we get:

(σ x + dσ x )( h + dh )( w) − (σ x )(h)( w) − 2 p (sin φ)( Rdφ)( w) ± 2 µp ( Rdφ )( cos φ )( w ) = 0
Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Simplifying, ignoring second order terms and assuming φ is very small such that sin φ =0 and cos φ = 1 p and σ x are two unknowns in the equation so another equation is needed. This can be obtained from the failure theory:
σ y −σ x = Y
_ f

Taking σy=p and give h= hfinal+Rφ
p entry = Y f
_

h µ( H o −H ) e ho

p exit = Y f

_

h −µ( H ) e ho

Where H is given by
H =2  R  R tan −1  .φ   hf  hf  

At the entry φ=α, so,
p entry = Y f
_

Η=Η ο

Whereas, at the exit, φ=0 and H =0 So
p exit = Y f
_

The analysis shows that pressures at the entry and exit are equal. This is illustrated in the figure to the right which shows pressure distribution in the roll gap as a function of the coefficient of friction. Note that as friction increases, the neutral point shifts toward the entry. Without friction, the rolls slip, and the neutral point shifts completely to the exit.

It back or forward tension is applied to the strip (i.e. tension at the exit or entry) the tensile stress is deducted from p in the previous analysis
Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

The figure to the right shows the pressure distribution in the roll gap as a function of reduction in thickness. Note the increase in the area under the curves with increasing reduction in thickness, thus increasing the roll-separating force.

Roll Force (F): F= Rolled area X Average flow stress in plain strain Maximum reduction in thickness ∆h (also known as draft) is a function of coefficient of friction µ and roll radius R. These are related by: ∆ h max= µ 2R The highest acceptance angle α can be be obtained from the simple theory of friction on inclined planes α max= tan-1µ so, if αmax is larger than this value the roll gegins to slip and the slap will not be drawn to the roll gap.

Rolling Defects
a- Wavy edges: caused by bending of rolls as shown. In this case thinning is higher at the edges. So, the edges elongate in the rolling direction more than the centre leading to buckling. If ductility is low, edges may crack to cause: b- Edge cracking: c- Zipper crack is caused by low ductility. d- Alligatoring: it can be a result of  piping defect in the original material  Non-homogeneous deformation. i.e. high friction and low pressure results in surface rather than through- section deformation

Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Flat Rolling Practice - Prior to hot rolling: Blooms, biollets and slabs are conditioned before
rolling. Conditioning can be torch scarfing or rough grinding to remove heavy scales. - Prior to cold rolling: scales are removined by pickling whch is a process involving sooaking in an aciod solution . Water blasting or grinding can also be used - Patch rolling: a process involving rolling of two layer in the same roll pass - Improving flatness of the product: the final sheet is passed through a set of leveling rolls as shown

Roll arrangement
Schematic illustration of various roll arrangements is shown below: (a) two

high; (b) three high; (c) four high; (d) cluster; (e) tandem rolling with three stands; (f) planetary.

Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Shape rolling
Section rolling Stages in shape rolling of an H-section part. Various other structural sections, such as channels and I-beams, are also rolled by this process.

Mutah University Dept. of Mechanical Engineering [email protected]

Dr W. R. Tyfour B.Sc. Productions and Metallurgical Engineering Ph.D. Mechanical Engineering

Ring-rolling
Reducing the thickness results in an increase in the part’s diameter. (b) Examples of cross-sections that can be formed by ring rolling.

Mutah University Dept. of Mechanical Engineering [email protected]

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