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Forming and rolling class 25 mar ppt
Metal forming processes use plastic deformation to change the shape of metal workpieces. Rolling is one of the most common metal forming processes, accounting for around 90% of metal shaping. In rolling, the metal workpiece is passed through one or more sets of rolls, reducing the thickness and changing the cross-sectional area under compressive forces applied by the rolls. The geometry of the final product is determined by the shape and contour of the roll gap. Rolling can be performed hot or cold, and is used to produce a wide variety of parts for structural applications and transportation.
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Forming and rolling class 25 mar ppt
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- 2. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Metal Forming Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces The tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the die
- 3. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Stresses in Metal Forming Stresses to plastically deform the metal are usually compressive Examples: rolling, forging, extrusion However, some forming processes Stretch the metal (tensile stresses) Others bend the metal (tensile and compressive) Still others apply shear stresses
- 4. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Essential Material Properties Desirable material properties: Low yield strength High ductility These properties are affected by temperature: Ductility increases and yield strength decreases when work temperature is raised Other factors: Strain rate and friction
- 5. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Basic Types of Metal Forming Processes 1. Bulk deformation Rolling processes Forging processes Extrusion processes Wire and bar drawing 1. Sheet metalworking Bending operations Deep or cup drawing Shearing processes
- 6. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Bulk Deformation Processes Characterized by significant deformations and massive shape changes "Bulk" refers to workparts with relatively low surface area to volume ratios‑ ‑ Starting work shapes are usually simple geometries Examples: Cylindrical billets Rectangular bars
- 7. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Bulk Deformation Processes (a) Rolling and (b) forging
- 8. ©2010 John Wiley& Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Considerations in Choosing a Lubricant Type of forming process (rolling, forging, sheet metal drawing, etc.) Hot working or cold working Work material Chemical reactivity with tool and work metals Ease of application Cost
- 9. Metals areworked within the yield point and breaking stress producing the plastic deformation. Hot working reduces the stress required to produce the yielding. Working Range A typical stress- strain curve for mild steel
- 10. CLASSIFICATION ON TEMPERATURE BASIS HOTWORKING COLD WORKING
- 11. Forming of metalsat a temperature above the re- crystallization temperature. Shape is obtained by virtue of their ability to flow plastically in the solid-state without accompanying deterioration of properties. Advantages and Limitations Porosity in metal is largely eliminated. Impurities in the form of inclusions are broken up and distributed thought the metal. Coarse grains are refined; Physical properties improved. HOT WORKING
- 12. HOT WORKING Energy requiredto change shape is less Good Machinability and weldability No residual stresses developed Ready to use products Rapid oxidation and scaling of the surface lead to poor surface finish and loss of metal Loss of carbon from steel surface- decarburization Weakening of surface lead to fatigue cracks Close Tolerance cannot be obtained
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- 14. Hot working Temperatures Hotworking is done above the recrystallization and below the melting temperature. Depends upon the material properly and sometimes may be at room temperature (Lead, Zinc etc.) Rolling, Forging, Extruding and Drawing are some major hot working in metals.
- 15. Working on metalsat a temperature below the re- crystallization temperature, usually at room temp. Deformation is brought by distortion of lattice about slip planes through slip and twinning mechanism Advantages and limitations: Improves strength, machinability, dimensional accuracy and surface finish of metals. Lesser oxidation and scaling cold working allows thinner sheets to be worked accurately. Many products are cold finished after hot working to make them commercially acceptable, close to tolerance and remove the scales and oxides. COLD WORKING
- 16. Strength and hardnessincreases, loss in ductility. Ideal for increasing hardness of metals which are not responding to heat treatment. Surface finish is better and no oxidation Close dimensional control is possible Much more Pressure is needed than hot working and there is no recovery of the distortion (Residual Stress). COLD WORKING
- 17. COLD WORKING Distortion andfragmentation of grain structure is created. Only applicable to ductile material Stresses are setup in metals that remains unless removed by subsequent heat treatment It results in loss of uniformity of metal composition and effect metal properties Impact strength and elongation are reduced Bending, Drawing, Spinning, Forming, Embossing, cold extrusion ,cold rolling and seaming are major cold working operations
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- 22. Effects of HotRolling
- 24. It is theprocess of reducing the thickess or changing the cross-section of a long workpiece by compressive forces applied through a set of rolls 90 % of Metals used are rolled sheets The metal moves through the rolling mill by a friction process. The metal is pushed or drawn through the rolling mill ROLLING PROCESS
- 25. Rolling Process :Introduction Rolling is the most extensively used metal forming process and its share is roughly 90% The material to be rolled is drawn by means of friction into the two revolving roll gap The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area
- 26. Rolling Process :Introduction The geometry of the product depend on the contour of the roll gap Roll materials are cast iron, cast steel and forged steel because of high strength and wear resistance requirements Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish
- 27. Rolling Process :Introduction In rolling the crystals get elongated in the rolling direction. In cold rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough. In the deformation zone the thickness of the strip gets reduced and it elongates. This increases the linear speed of the at the exit. Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverses.
- 28. Rolling Process :Introduction When the angle of contact α exceeds the friction angle λ the rolls cannot draw fresh strip Roll torque, power etc. increase with increase in roll work contact length or roll radius
- 29. Hot rolling The coarse–grained, brittle and porous structure of the ingot (or continuous cast metal) is broken in wrought structure Finer grain size, enhanced properties, strength and hardness Cold rolling Room temperature, hardness and better surface finish Anisotropic properties-preferential orientation This process requires higher energy Rolling processes
- 31. Pressure variation duringRolling Typical pressure variation along the contact length in flat rolling. The peak pressure is located at the neutral point. The area beneath the curve, represents roll force.
- 32. Friction in Rolling Frictionin rolling: It depends on lubrication, work material and also on the temperature. In cold rolling the value of coefficient of friction is around 0.1 and in warm working it is around 0.2. In hot rolling it is around 0.4. In hot rolling sticking friction condition is also seen and then friction coefficient is observed up to 0.7.
- 33. Rolling force andpower F=LwYavg F rolling force Yavg is the average true stress w is the width of the strip L length of the contact Add 20 % for friction Torque on the roll is the product of a and F equals in average kW FLN Power 000.60 2π =
- 34. W width ofthe strip Diameter of the contact roll Metal structure Temperature of the metal Rolling speed µ friction coefficient Reduction in thickness Thickness incoming sheet Pull or push at the beginning or at the end Parameters that affect Rolling Force
- 35. Spreading in FlatRolling
- 36. ROLLING TERMINOLOGY Absolute Draught: Difference between initial and final thicknesses of the metal being rolled (T1 – T2) Absolute Elongation : Difference between the final and initial length = ( L2 – L1) Absolute Spread : Difference between the final and initial width = ( W2 – W1) Relative Draught : Ratio of absolute drawght to initial thickness = (T1 – T2)/T1 Elongation Coefficient : Ratio of final length to initial length : L2/L1
- 37. ROLLING TERMINOLOGY Angle ofContact, No slip point, lagging zone, forward slip zone Forward slip : (V2 – V)/ V, V= roll velocity, V2 = velocity at delivery point. It vary from 3 to 10% and increases with the increase in roll diameter and coefficient of friction and also with reduction in thickness of the strip being rolled.
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- 39. Shape Rolling ofan H-section part
- 43. Two highrolling mills Reversing Mills Reversed direction Four High Mills and Clusters Mills (Sendzimir or Z mills) : Use for subsequent rolling of slabs Cluster roll mill : used for cold rolling Continuous rolling mill (Tandem Mill) : Material pass from initial to final stage . Speed of each roll is adjusted properly. Used for mass production TYPES OF ROLLING MILLS
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- 45. To control thefriction process lubricants have to be used Low friction has to be used to avoid damage of the equipment But friction is necessary Lubrication during rolling
- 48. Defects Wavy Edges :Due to roll bending, edge elongate more than the centre Spread : Due to high width to thickness ratio, width of material remain constant. With smaller ratio, width increases considerable in the roll gap. Crocodile crack : Due to weakness at the center, sheet bifurcate into two parts.
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- 50. Applications Straight and longstructural shapes Solid bars with various cross sections Channels I-beams Railroad rails Typical applications : 300 mm reactors vessels 100mm thick tanks 1, 8 mm Boeing aircrafts 0,7 mm car body panels 6 µm Al foil for packaging 0,28 mm Al beverage cans