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楊宏智（台大機械系教授） 2

PART III: Forming and Shaping Processes and Equipment  “Forming” indicates changing the shape of an existing solid body

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

Cast Structure and Wrought Structure  cast structure - solidification front moves thr the molten metal from the mold walls towards the center - at the mold walls the metal cools rapidly and produces solidified skin, or shell - brittle grain boundaries and internal defects are formed  wrought structure - converted from cast structure by hot rolling - gives finer grains and enhanced ductility, improved strength resulting from breaking up brittle grain boundaries and closing up internal defects 5

Chapter 13: Metal-Rolling Processes and Equipment

Chapter Outline  Introduction  The Flat-rolling Process  Flat-rolling Practice  Rolling Mills  Various Rolling Processes and Mills

Introduction  Rolling is the process of reducing the cross section of a long workpiece by compressive forces applied through a set of rolls ( 本圖表請參考 Manufacturing Engineering Technology in SI Units, 6th P.317 Figure 13.1 ) Slab: rectangular shape Billet: (< 6”) square or circular Bloom: (> 6”)

Plates and Sheets  Plates: 300mm>t>6mm reactor vessels(150mm) tanks (125mm) boiler supports (300mm)  Sheets: t<6mm B747 (1.8mm) sedan ( mm) can (0.1mm) Al foil (0.008mm)  Slab, Billet and Bloom

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 ( 本圖表請參考 Manufacturing Engineering Technology in SI Units, 6th P.319 Figure 13.2 )

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

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 Y avg = average true stress of the strip

Types of Rolling  Based on workpiece geometry  Flat rolling - used to reduce thickness of a rectangular cross section  Shape rolling - square cross section is formed into a shape such as an I-beam  Based on work temperature  Hot Rolling – can achieve significant deformation  Cold rolling – produces sheet and plate stock

Hot Rolling  process of reducing the thickness or changing the cross-section of a long workpiece by compressive forces thr a set of rolls  Performed above recrystallization temp  converted from cast structure to wrought structure  gives finer grains and enhanced ductility, improved strength resulting from breaking up brittle grain boundaries and closing up internal defects

Grain Structure During Hot Rolling ( 本圖表請參考 Manufacturing Engineering Technology in SI Units, 6th P.323 Figure 13.6 )

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: 1. Reducing friction at the roll–workpiece interface 2. Using smaller diameter rolls 3. Reduce the contact area 4. Rolling at elevated temperatures 5. Applying front and/or back tensions to the strip

The Flat-rolling Process: Roll Force, Torque, and Power Requirements ( 本圖表請參考 Manufacturing Engineering Technology in SI Units, 6th P.321 Figure 13.3 )

The Flat-rolling Process: Geometric Considerations  Roll forces will bend the rolls elastically during rolling crown: strip thicker at center - wavy edges or fractures in center  Use of a “crowned” roll for compensation, When the roll bends, the strip has a constant thickness along its width

The Flat-rolling Process: Geometric Considerations  Equipment is massive and expensive  Rolling mill configurations: Two-high – two opposing rolls Three-high – work passes through rolls in both directions Four-high – backing rolls support smaller rolls Cluster mill – multiple backing rolls on smaller rolls Tandem rolling mill – sequence of two-high mills

Rolling Mill Configurations  two-high rolling mill  two-high reversing - used for hot rolling in initial breakdown passes on cast ingots or in continuous casting, with the direction of material movement is reversed after each pass. - drawbacks are the energy waste in reversing the roll rotation and time consuming in adjusting the roll gap

Rolling Mill Configurations  four-high rolling mill - smaller rolls require lower force (fig 18-4) to reduce spread and roll deflection - the smaller cross-section however provides reduced stiffness - when worn or broken smaller rolls replaced at lower cost - large back-up rolls are used to provide the necessary support for the smaller work rolls (reduce crown problem)

Rolling Mill  Automated mills produce close-tolerance, low cost and high quality plates and sheets at high production rates  ( 本圖表請參考 Manufacturing Engineering Technology in SI Units, 6th P.326 Figure )

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

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

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

The Flat-rolling Process: Geometric Considerations Spreading  Increase in width is called spreading  Spreading increases with: 1. Decreasing width-to-thickness ratio of the entering strip 2. Increasing friction 3. Decreasing ratio of the roll radius to the strip thickness

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

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

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

Various Rolling Processes and Mills Shape Rolling  Straight and long structural shapes are formed at elevated temperatures by shape rolling

Various Rolling Processes and Mills Tube Rolling  Diameter and thickness of pipes and tubing can be reduced by tube rolling, which utilizes shaped rolls

Shape Rolling  Work is deformed into a contoured cross section rather than flat (rectangular) Accomplished by passing work through rolls that have the reverse of desired shape  Products Construction shapes such as I-beams, L-beams, and U-channels Rails for railroad tracks Round and square bars and rods

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

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

Thread Rolling Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies  Important process for mass producing bolts and screws  Performed by cold working in thread rolling machines  Advantages over thread cutting (machining):  Higher production rates (80 pieces/sec)  Better material utilization  Surface finish is very smooth  Stronger threads and better fatigue resistance

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

Ring Rolling Deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter  As thick-walled ring is compressed, deformed metal elongates, causing diameter of ring to enlarge  Hot working process for large rings and cold working process for smaller rings  Products: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery

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

Mannesmann Mills  principle: -when round bar subjected to radial compressive forces tensile stresses developed at the center of the bar - radial compressive force + bar roll, then gives cyclic compressive stresses cavity formed at the center of the bar

Mannesmann Mills  arrangement: - skew rolls pull the round bar by the axial component of the rotary motion - internal mandrel assists the expanding of the hole and sizing of the tube inside diameter

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

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