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Chapter 13

Rolling of Metals. Chapter 13. Introduction. This chapter describes Flat rolling Shape rolling Production of seamless tubing & pipe Rolling – process of reducing the thickness of a long work piece Plates – having thickness greater than 6mm Sheets – generally less than 6mm thick.

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Chapter 13

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  1. Rolling of Metals Chapter 13

  2. Introduction • This chapter describes • Flat rolling • Shape rolling • Production of seamless tubing & pipe • Rolling – process of reducing the thickness of a long work piece • Plates – having thickness greater than 6mm • Sheets – generally less than 6mm thick

  3. Flat Rolling Flat Rolling Process

  4. Flat Rolling Process • Metal strip enters the roll gap • The strip is reduced in size by the metal rolls • The velocity of the strip is increased the metal strip is reduced in size • Factors affecting Rolling Process • Frictional Forces • Roll Force and Power Requirement

  5. Frictional Forces Friction Forces acting on strip forces • Max Draft • h0-hf = µ2R Roll Force • F= W0.L.Yavg • L=̃ sqrt{R(ho-hf)}

  6. Flat-Rolling Practice • Hot rolling • The initial break down of an ingot • Continuously cast slab • Structure may be brittle • Converts the cast structure to a wrought structure • Finer grains • Enhanced ductility • Reduction in defects • Continuous Casting • Is replacing traditional methods • Faster & better Product of the first hot-rolling operation - Bloom or slab • Square cross section of 150mm (6in) on one side • Processed father by shape rolling • I-beams • Railroad rails

  7. Flat-Rolling Practice Con’t • Billets – smaller than blooms and rolled into bars and rods • Cold rolling • carried out at room temperature • Produces sheet and strip metal • Better surface finish – less scale • Pack rolling – when two or more layers of metal are rolled together Changes in grain structure during hot-rolling

  8. Defects in Rolled Plates & Sheets • Undesirable • Degrade surface appearance • Adversely affect the strength • Sheet metal defects include: • Scale, Rust, Scratches, Gouges, Pits, & Cracks • May be caused by impurities and inclusions • Wavy edges – result of roll bending • Alligatoring – complex phenomenon

  9. Other Characteristics • Residual stresses – produces: • Compressive residual stresses on the surfaces • Tensile stresses in the middle • Tolerances • Cold-rolled sheets: (+/- ) 0.1mm – 0.35mm • Tolerances much greater for hot-rolled plates • Surface roughness • Cold rolling can produce a very fine finish • Hot rolling & sand have the same range of surface finish • Gauge numbers – the thickness of a sheet is identified by a gauge number

  10. Schematic Illustration of Various Roll arrangements

  11. Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill

  12. Shape-Rolling Operations • Various shapes can be produced by shape rolling • Bars • Channels • I-beams • Railroad rails • Roll-pass design requires considerable experience in order to avoid external and internal defects

  13. Stages in Shape Rolling of an H-section part. Various other structural sections such as channels and I-beams, are rolled by this kind of process.

  14. Ring Rolling • A thick ring is expanded into a large diameter ring • The ring is placed between the two rolls • One of which is driven • The thickness is reduced by bringing the rolls together • The ring shaped blank my be produced by:\ • Cutting from plate • Piercing • Cutting from a thick walled pipe Various shapes can be produced by shaped rolls • Typical applications of ring rolling: • Large rings for rockets • Gearwheel rims • Ball-bearing and roller-bearing races • Can be carried out at room temperature • Has short production time • Close dimensional tolerances

  15. RING ROLLING (a) Schematic illustration of Ring-rolling operation. Thickness reduction results in an increase in the part diameter. (b) Examples of cross-sections that can be formed by ring-rolling

  16. Thread Rolling • Cold-forming process • Straight or tapered threads are formed on round rods by passing the pipe though dies • Typical products include • Screws • Bolts

  17. Thread Rolling Con’t • Threads are rolled in the soft condition • Threads may then be heat treated, and subjected to final machining or grinding • Uncommon or special-purpose threads are machined

  18. Production of Seamless Pipe & Tubing • Rotary tube piercing (Mannesmann process) • Hot-working process • Produces long thick-walled seamless pipe • Carried out by using an arrangement of rotating rolls • Tensile stresses develop at the center of the bar when it is subjected to compressive forces

  19. Continuous Casting & Integrated Mills & Minimills • Continuous casting • Advantages • Highly automated • Reduces product cost • Companies are converting over to this type of casting

  20. Continuous Casting & Integrated Mills & Minimills Con’t Integrated Mills utilize everything from the production of hot metal to the casting and rolling of the finished product Minimills • Scrap metal is melted • Cast continuously • Rolled directly into specific lines of products • Each minimill produces one kind of rolled product • Rod • Bar • Structural steel

  21. Spray Casting : In spray casting the molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe

  22. THE END

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