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Chapter-2 METAL WORKING PROCESSES

Chapter-2 METAL WORKING PROCESSES. -Vishal Sir. Definition. Metal Working/Mechanical Working/Metal Forming Processes. Definition. The process in which permanent change is obtained in a metal shape by plastic deformation under the action of external forces. Why Metal Forming is required?.

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Chapter-2 METAL WORKING PROCESSES

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  1. Chapter-2METAL WORKING PROCESSES -Vishal Sir

  2. Definition • Metal Working/Mechanical Working/Metal Forming Processes

  3. Definition • The process in which permanent change is obtained in a metal shape by plastic deformation under the action of external forces.

  4. Why Metal Forming is required? • Economical and time saving process to obtain desired shape from the given material. • To improve mechanical properties. • To minimize defects such as holes, cracks etc • To distribute impurities equally in metal.

  5. Classification of Metal Working • Hot Working of metals. • Cold Working of metals.

  6. Hot Working of metal

  7. Definition • It can be defined as the plastic deformation of metals and alloys at a temperature above recrystallisation and below melting point such that fine refined grain is obtained. • Maximum working temperature is 0.7 to 0.9 times the melting point.

  8. Advantages • Properties such as strength, ductility and toughness is improved. • Density increases by removing voids. • Desired shape can be easily obtained under plastic deformation. • Effect of impurities can be reduced. • Good grain structure. • Atoms in same direction leads to better strength.

  9. Disadvantages • Surface finish is not good. • Required accuracy is not obtained. • Process takes place at higher temperature, so tool life is minimum. • Thus handling cost is high.

  10. Hot working Methods • Hot Rolling

  11. Hot working Methods • Hot Forging

  12. Hot working Methods • Hot Drawing

  13. Hot working Methods • Hot Extrusion

  14. Hot working Methods • Hot Spinning

  15. Hot working Methods • Hot Piercing

  16. Cold Working of Metals • Hot Working of metal • Cold Working of metal • Normal Temperature Temp Rise Melting Point Recrystallization temperature

  17. Definition • The process in which plastic (permanent) deformation occurs when the process is carried out at a temperature lower than recrystallisation temperature. • The temperature is half of absolute melting temp. • Theoretically, it is room temperature. (25 to 30 “C)

  18. Advantages • Improves hardness and strength • Desired accuracy is possible. • Heating is not required. • Thus, cost and time is reduced.

  19. Disadvantages • Reduces ductility, thus difficult to process. • Force required is more. • Produce internal and residual stresses. • Atoms are deformed. • Low ductility -> Difficult to process. • Cost increases.

  20. Cold working Methods • Cold Rolling • Cold Drawing • Cold Extrusion • Cold Spinning • Squeezing • Bending • Peening • Coining • Hobbing

  21. Difference between Hot and Cold Working Hot Working Cold Working Below recrystallisation temperature. Process is done on metals at room temperature. More force is required. Residual stresses formed is more. • Process is done above recrystallisation temperature • Process is done on hot metal. • Less force is required. • No residual stress is formed in the metal.

  22. Difference between Hot and Cold Working Hot Working Cold Working Improves strength and hardness. Accuracy in surface finish is more. Maintenance cost is low. • Improves ductility, stiffness and toughness. • Accuracy in surface finish is less. • Maintenance cost is high.

  23. Rolling Process

  24. ROLLING PROCESS INGOT BLOOMS SLAB Sheets Plates

  25. BLOOMS Channels I-Beams Rails Rounds Billets Rods Wires Bars

  26. Definition • Rolling process is the process in which the metals and alloys are plastically deformed by passing them between circular or cylindrical rollers.

  27. What actually happens in Rolling Process???? • Rolls or rollers are arranged. • The metal piece which is to be deformed is drawn between the space of the rollers (by using frictional force between the rolls and metal surface) • Metal is squeezed (compressive force)

  28. Angle of Contact O R Angle of contact Feed direction B New grain A C V2 t1 V1 t2 Old Grain Normal Force Friction Force

  29. Angle of Contact • From the fig. we can say that, when the roller and work piece make contact, then we get arcAB. • Here arc AB is the contact portion. • Now, arc AB makes an angle “θ” with O. • This angle formed is known as angle of contact.

  30. Angle of Contact: Cosθ = 1- (t1-t2) 2R Where, t1 = thickness before rolling t2 = thickness after rolling R = radius of roll θ = Angle of contact or bite

  31. Coefficient of Friction • Now, coefficient of friction is given by, µ = Frictional Force Normal Force =tanβ Where, β = Friction angle

  32. Angle of Friction (β)

  33. Principle working of Rolling • Stage 1: At the beginning, the metal enters smoothly and it should not come back from roller. For that, θ‹β

  34. Stage 2: O R Angle of contact Feed direction B New grain A C V2 t1 V1 t2 Old Grain Normal Force Friction Force

  35. Stage 2: Between B and C, speed of work piece is less than roller speed • Stage 3: At point C, the speed of work piece and roller is same. This point is known as no slip point. • Stage 4: From point C to A, the speed of work piece is more than the roller • i.e. V2 > V > V1

  36. Forward Slip • Forward slip is the slip that is caused due to difference between, the strip velocity and roller velocity. • i.e if the strip velocity is greater than roller velocity, then a slip occurs, which is known as forward slip. • Forward Slip = V2 –V where, V2 = Strip Velocity V V = Roller peripheral velocity

  37. Types of Rolling Hot Rolling Cold Rolling

  38. Hot Rolling • Definition: Hot rolling is defined as the rolling process which occurs above recrystallisation temperature.

  39. Hot Rolling Process Video

  40. Hot Rolling Process Video

  41. Points to remember • The metals are recrystalized parallelly along with deformation. • Temperature is made constant during the rolling process. • The distance between the roller is less than the thickness of the metal (ingot), the thickness of metal reduce and length increases.

  42. Advantages • The grains become finer, thus mechanical properties improve. • Productivity increases. • Due to high temperature, high pressure is created. This pressure reduces the cavity and voids. • The grains are equally divided. Thus,shape is uniform.

  43. Advantages • Since the impurities are distributed equally, its effect is minimized. • Entire process is economic.

  44. Disadvantages • Surface finish is not good due to oxidation. • Since the process is carried out at high temperature, its handling and tooling cost is more. • Metal (output) of desired measurement is not obtained.

  45. COLD ROLLING • Definition: It is defined as the process that is carried out at room temperature or temperature below recrystallisation temperature.

  46. Cold Rolling Process Video

  47. Points to remember • Since in cold working, tremendous force is applied and when such force is removed, some residual forces remain inside the metal. • Before cold working is carried out, the scale (formed due to hot rolling) are removed by submerging the metal into sulphuric acid. Then, it is washed with water and cold working is done. • It is done for softer metals. Eg. Aluminum, copper, silver.

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