1. Manufacturing Engineering Technology in SI Units, 6th Edition PART III: Forming and Shaping Processes and Equipment
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2. PART III: Forming and Shaping Processes and Equipment
“Forming” indicates changing the shape of an existing solid body
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3. PART III: Forming and Shaping Processes and Equipment
For forming processes, the starting material may be in the shape of a plate, sheet, bar, rod, wire, or tubing of various cross sections
Shaping processes involve the molding and casting of molten materials and the finished product is near the final desired shape
Molten metalis cast into individual ingots or continuously cast into slabs, rods, or pipes
Cast structures are converted to wrought structures by plastic-deformation processes
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4. PART III: Forming and Shaping Processes and Equipment
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5. Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 13: Metal-Rolling Processes and Equipment
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6. Chapter Outline Introduction The Flat-rolling Process
Flat-rolling Practice
Rolling Mills
Various Rolling Processes and Mills
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7. Introduction
Rolling is the process of reducing the cross section of a long workpiece by compressive forces applied through a set of rolls
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8. The Flat-rolling Process
Flat-rolling process is shown
Friction forces act on strip surfaces
Roll force, F, and torque, T, acts on the rolls
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9. The Flat-rolling Process
As the surface speed of the rigid roll is constant, there is relative sliding between the roll and the strip along the arc of contact in the roll gap, L
At neutral point or no-slip point, the velocity of the strip is the same as that of the roll
The maximum possible draft is defined as the difference between the initial and final strip thicknesses
From the relationship, higher the friction and the larger the roll radius, the greater the maximum possible draft becomes
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10. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
Rolls apply pressure on the flat strip to reduce its thickness, resulting in a roll force, F
Roll force in flat rolling can be estimated from
Total power (for two rolls) is
L = roll-strip contact length
w = width of the strip Yavg = average true stress of the strip
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11. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
EXAMPLE 13.1
Calculation of Roll Force and Torque in Flat-rolling
An annealed copper strip 228 mm wide and 25 mm thick is rolled to a thickness of 20 mm in one pass. The roll radius is 300 mm, and the rolls rotate at 100 rpm. Calculate the roll force and the power required in this operation.
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12. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
Solution
Roll-strip contact length is calculated through geometry,
Absolute true strain of the strip is
Average true stress is
The roll force is
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13. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
Solution
With 100 rpm, the total power is calculated from
Force and the power requirements is difficult to obtained due to (a) the exact geometry between the roll and the strip and (b) coefficient of friction and the strength of the material in the roll gap
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14. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
Reducing Roll Force
Roll forces can cause deflection and flattening of the rolls
The columns of the roll stand may deflect under high roll forces
Roll forces can be reduced by:
Reducing friction at the roll–workpiece interface
Using smaller diameter rolls
Reduce the contact area
Rolling at elevated temperatures
Applying front and/or back tensions to the strip
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15. The Flat-rolling Process: Roll Force, Torque, and Power Requirements
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16. The Flat-rolling Process: Geometric Considerations
Roll forces will bend the rolls elastically during rolling
When the roll bends, the strip has a constant thickness along its width
The heat generated by plastic deformation cause the rolls to be slightly barrel shaped (thermal camber)
Roll forces also tend to flatten the rolls elastically
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17. The Flat-rolling Process: Geometric Considerations
Spreading
Increase in width is called spreading
Spreading increases with:
Decreasing width-to-thickness ratio of the entering strip
Increasing friction
Decreasing ratio of the roll radius to the strip thickness
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18. The Flat-rolling Process: Vibration and Chatter
Vibration and chatter have effects on product quality and the productivity of metalworking operations
Chatter defined as self-excited vibration
Occur in rolling and in extrusion, drawing, machining, and grinding operations
Chatter results from interactions between the structural dynamics of the mill stand and the dynamics of the rolling operation
Chatter can be reduced by increasing the roll radius, strip-roll friction and incorporating dampers in the roll supports
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19. Flat-rolling Practice
Initial rolling steps (breaking down) of the material is done by hot rolling
A cast structure is dendritic and is brittle and porous
Hot rolling converts the cast structure to a wrought structure with finer grains and enhanced ductility
Product of the first hot-rolling operation is called a bloom, a slab, or a billet
To improve flatness, the rolled strip goes through a series of leveling rolls
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20. Flat-rolling Practice
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21. Flat-rolling Practice: Defects in Rolled Plates and Sheets
Defects may be present on the surfaces or there may be internal structural defects
They are undesirable as they compromise surface appearance and adversely affect strength, formability, and other manufacturing characteristics
Surface defects may be caused by inclusions and impurities in the original cast material
Wavy edges on sheets are the result of roll bending
Cracks are due to poor material ductility at the rolling temperature
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22. Flat-rolling Practice: Other Characteristics of Rolled Metals
Residual Stresses
Residual stresses develop in rolled plates and sheets due to nonuniform deformation of materials in roll gap
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23. Flat-rolling Practice: Other Characteristics of Rolled Metals
Dimensional Tolerances
Thickness tolerances for cold-rolled sheets range from ±0.1~0.35 mm
Flatness tolerances are within ±15 mm/m for cold rolling and ±55 mm/m for hot rolling
Surface Roughness
Cold rolling can produce a very fine surface finish
Cold-rolled sheets products may not require additional finishing operations
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24. Rolling Mills
Automated mills produce close-tolerance, low cost and high quality plates and sheets at high production rates
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25. Rolling Mills
Two-high rolling mills are used for hot rolling in initial breakdown passes (cogging mills) on cast ingots or in continuous casting
In tandem rolling, the strip is rolled continuously through a number of stands to thinner gages with each pass
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26. Rolling Mills Roll Materials
Basic requirements for roll materials are strength and resistance to wear
Forged-steel rolls have higher strength, stiffness, and toughness than cast-iron rolls
Rolls made for cold rolling should not be used for hot rolling as they may crack from thermal cycling (and spalling
Lubricants
Hot rolling of ferrous alloys do not need lubricants
Water-based solutions are used to cool the rolls
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27. Various Rolling Processes and Mills
Shape Rolling
Straight and long structural shapes are formed at elevated temperatures by shape rolling
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28. Various Rolling Processes and Mills
Roll Forging
Cross section of a round bar is shaped by passing it through a pair of rolls with profiled grooves
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29. Various Rolling Processes and Mills
Skew Rolling
Similar to roll forging and used for making ball bearings
Another method is to shear pieces from a round bar and then upset them in headers between two dies with hemispherical cavities
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30. Various Rolling Processes and Mills
Ring Rolling
A thick ring is expanded into a large-diameter thinner one
Thickness is reduced by bringing the rolls closer together as they rotate
Short production times, material savings and close dimensional tolerances
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31. Various Rolling Processes and Mills
Thread Rolling
Thread rolling is a cold-forming process by which straight or tapered threads are formed on round rods or wire
Threads are formed with rotary dies at high production rates
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32. Various Rolling Processes and Mills
Thread Rolling
Thread-rolling process has the advantages of generating threads with good strength without any loss of material
Internal thread rolling can be carried out with a fluteless forming tap, produces accurate internal threads with good strength
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33. Various Rolling Processes and Mills
Rotary Tube Piercing
Also known as the Mannesmann process
It is a hot-working operation for making long, thick- walled seamless pipe and tubing
The round bar is subjected to radial compressive forces while tensile stresses develop at the center of the bar
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34. Various Rolling Processes and Mills
Tube Rolling
Diameter and thickness of pipes and tubing can be reduced by tube rolling, which utilizes shaped rolls
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35. Various Rolling Processes and Mills: Various Mills
Integrated Mills
Integration process starts from production of hot metal to the casting and rolling of finished products to shipping
Minimills
Scrap metal is:
Melted in electric-arc furnaces
Cast continuously
Rolled directly into specific lines of products
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