1.Cold Work Process

2. Manufaturing processes Common manufaturing process categories : Cold working processes Hot working processes Casting processes 3. Cold-Working Processes : Cold rolling Reduce a plate or sheet of metal in rolling mills. Done at room temperatures. From decoiler, strip is unwound, straightened and flattened in the pinch rolls reduces thickness. After thickness is reduced, it is recoiled on the right- hand coiler. Then the mill rolls are reversed for further reduction. Finally, recoiled on the left-hand, the middle coiler. 4. Cold-Working Processes : Drawing wire and tube Wire and tubes are cold drawn from stock. Die used are the hardened tool steel or tungsten carbide. The drawing force in wire manufacture is provided by winding the wire on to a rotating drum or block. Wire are work-hardened during manufacture; work hardened brings brittleness thus need soften by annealing process. 5. Cold-Working Processes : Drawing wire and tube Wire Drawing Process 6. Cold-Working Processes : Drawing wire and tube Production of tubes need some form of draw bench. The internal bore of the tube needs support. Its done by drawing the tube on to a steel rod or mandrel which passed through the die along with the tube from which it is subsequently extracted. 7. Cold-Working Processes : Drawing wire and tube 8. Cold-Working Processes : Advantages & Disadvantages Advantages Disadvantages Good surface finish. Higher cost than hot-worked materials. A process for material previously hot worked. Therefore the processing cost is added to the hot-worked cost. Relatively high dimensional accuracy. Materials lack ductility due to work hardening, less suitable for bending etc. Relatively high geometrical accuracy. Clean surface is easily corroded. Work hardening during the processes: Increases strength and rigidity. Improves the machining characteristics of the metal, good finish is more easily achieved Availability limited to rods and bars also sheets and strips and solid drawn tubes. 9. Hot Work Process 10. Hot Working Processes : Intro Metals weaken and become more ductile at high temperatures. Forming can take place without exhausting material plasticity in steels at high temperatures. Deformation of weak austenite structure, then cools to the stronger roomtemperature, ferrite, or much stronger structures. Some of the hot working processes : Rolling. Forging. Extrusion. 11. Hot Working Processes : Recrystallization Above the recovery range, recrystallization takes place. In this temperature range, the formation of new stress- free and equiaxed grains leads to lower strength and higher ductility. With less cold work there are fewer nucleuses for the new grains, and the resulting grain size is larger. 12. Hot Working Processes : Drop Forging Localized compressive forces. Done manually anual or using power hammers, presses or special forging machines. Done above the recrystallization temperature. The metal may be (1) drawn out to increase its length and decrease its cross section (2) upset to decrease the length and increase the cross section (3) squeezed in closed impression dies to produce multidirectional flow. 13. Hot Working Processes : Drop Forging (Closed Imoression Die) Use of a closed die, one half of which is fixed to an anvil, whilst the other half is attached to a guided hammer. Heated work piece is interposed between the die. Excess of metal (flash) must be available to ensure that the die cavity is filled. 14. Hot Working Processes : Hot Rolling Usually is the first process to convert material into a finished wrought product. Recrystallization takes place during process. 15. Products of rolling process: A bloom - square or rectangular cross section, thickness more than 6 inches and a width no more than twice the thickness. A billet is usually smaller than a bloom and has a square or circular cross section. A slab is a rectangular solid where the width is more than twice the thickness. Slabs can be further rolled to produce plate, sheet and strip. 16. Hot Working Processes : Forward and backward extrusion By extrusion, metal is compressed and forced to flow through a shaped die. Hot extrusion reduces the forces required, eliminates cold- work effect and reduces directional properties. Extrusion process is like squeezing toothpaste out of a tube. A heated billet placed inside a confining chamber. A ram advances from one end, causing the billet to upset and enter the confining chamber. As the ram continues to advance, pressure builds until the material flows through the die. 17. Hot Working Processes : Forward and backward extrusion In direct extrusion, a solid ram drives the entire billet to and through a stationary die. In indirect extrusion a hollow drives the die back through a stationary, confined billet. 18. Hot Working Processes : Advantages & Disadvantages Advantages Disadvantages Low cost. Poor surface finish rough and scaly. Grain refinement from cast structure. Dimensional accuracy of a low order due to shrinkage during cooling. Materials are left in a fully annealed condition and are suitable for cold working. Due to distortion on cooling; leads to geometrical inaccuracy. Scale gives some protection against corrosion during storage. Poor finish when machined. Availability as sections (girders) and forgings as well as the more usual bars, rods, sheets and strip and butt-welded tube. Low strength and rigidity for metal considered. 19. Casting process 20. Casting Process Pouring molten metal into a mould patterned after the part to be manufactured. The important factors in casting operation are : Flow of the molten metal into the mould cavity Heat transfer during solidification and cooling of metal Type of mould material. Solidification of the metal from its molten state. There are a few ways of casting prosess which are : Sand casting Investment casting Pressure die casting 21. Casting Process : Sand Casting Sand casting consists of: placing a pattern in sand to make an imprint, filling the resulting cavity with molten metal, allowing the metal to cool until it solidifies, breaking away the sand mould and removing the casting. 22. Casting Process : Sand Casting Typical parts - machine-tool bases, engine blocks, cylinder heads and pump housings Sand moulds are characterized by the types of sand that comprise them : Green-sand - the sand in the mould is moist or damp while the metal is being poured into it Cold-box mould - various organic and inorganic binders are blended to bond the grains for greater strength No-bake mould - a synthetic liquid resin is mixed with the sand 23. Casting Process : Investment Casting Wax patterns produced in precision metal moulds and then assembled to a tree. The wax assembly is then invested with a mixture of powdered sillimanite and ethylsilicate. By heating it form a strong silica bond between the particles. The heating process also melts out the wax pattern leaving the mould cavity. Extremely complex shapes can be cast since the pattern is not withdrawn. Extreme precision in dimensions is possible. Importance for making small components from very hard, strong materials. Blades for gas-turbines and jet-engines can be cast by this process. High operational cost. 24. Casting Process : Investment Casting 25. Casting Process : Pressure Die Casting Molten metal is injected into the mould cavity under pressure. Much more accurate impression of the mould cavity is obtained. Pressure die casting is the more common process and cycling is rapid. As soon as the casting is solid the die is parted. Detached by a system or ejector pins. 26. Casting Process : Pressure Die Casting 27. Casting Process : Comparison Item Advantages Limitations Sand Casting Low cost. Poor surface finish Accurate dimension Weak brittle structure Investment Casting Complex shape can be cast. High operational cost. Precession in dimensioning Difficult to build the patterns Pressure Die Casting High productivity High operational cost. Good surface finish Need high pressure. 28. Heat Treatment Procecesses 29. Heat Treatment Heat treatment - controlled heating and cooling of metals Change properties to improve performance or for further processing. Consist of heating, soaking cooling Vary the heating, soaking & cooling of p.c.s, different combinations of mechanical properties can be obtained. 30. Heat Treatment : Annealing Rendering soft and ductile steel for further cold work and machining. Three annealing processes : stress-relief annealing spheroidising Full-annealing Chose process depends on carbon content pretreatment processing subsequent process and use 31. Fig : Temperature bands of annealing processes on iron carbon phase equilibrium diagram. Cooling rate is as slow as possible. 32. Annealing : Full Annealing P.c.steels solidify above the heat treatment temperatures. Large castings, insulated by the sand mould, take a very long time to cool down. Large forgings processed at temperatures above their upper critical temperatures for long period. 33. Annealing : Stress-Relief Annealing Steels below 0.4% carbon content. Such steels will not fully quench harden, but, they are frequently cold worked and become work hardened. Grain structures are distorted during working, recrystallisation can initiate at 5000C. In practice, annealing is done at 630 to 7000C to speed up the process and limit grain growth. 34. Annealing : Speroidising Annealing Crystals of pearlite have a laminated structure consisting of alternate layers of cementite and ferrite. If steels (>0.4 %C) are heated below the critical temperature (650-700 C), the cementite tends to 'ball up'. Aspheroidisation of pearlitic cementite takes place. No phase changes happen at sub critical temp. Spheroidisation of the cementite is a surface tension effect. 35. Normalising Resembles full annealing but cooling rate is accelerated. Work is taken out from the furnace for cooling in free air. Process temperature is the same as full annealing. After 'soaking' the steel at the process temperature, more rapid cooling is performed. Grain transform from fine grain austenite into fine grain ferrite + pearlite. Rapid cooling avoids grain growth associated with annealing. 36. Normalising Frequently used for stress relieve between rough machining and the finish machining of large castings to avoid 'movement' due to slow release of internal stresses and loss of accuracy. 37. Quenching Heat steel to its hardening temperature it becomes austenitic. Cooled it quickly, not enough time for transforming austenite into pearlite and ferrite or pearlite and cementite. Instead, a structure called martensite is formed. It is the hardest structure to produce in a P.C.S Under microscope, it appears as a needle-shaped (acicular) crystal. 38. Quenching : Quenching Media In order of severity: Compressed air blast - high-speed steel tools and components of small section Oil - high-carbon steels (1.2-1.4 %) and alloy steels Water P.C.S & alloy steels Brine (10 per cent solution) max hardness Care must be taken to ensure that distortion is kept to a minimum. Selection of media depends on the type of steel and the required properties. 39. Tempering Process Quenched P.C.S is hard but brittle and hardening stresses are present - Little practical use. Reheating, or tempering, to relieve the stresses and reduce the brittleness. Tempering transforms martensite into less brittle structures. Increase in toughness is accompanied by decrease in hardness, unfortunately. Transform unstable martensite into more stable pearlite. Tempering temperatures Below 200oC only relieve the hardening stresses. Above 220oC, martensite transforms into troostite. Tempering above 400oC giving a structure called sorbite. Troostite (for most cutting tools) is much tougher but less hard. Sorbite is tougher and more ductile then troostite; used in shock loadcomponents e.g springs. 40. Case Hardening Often components require a hard case to resist wear & tough core to resist shock loads. Hardness & toughness do not exist in a single steel since, Core should not exceed 0.3-0.4% carbon for toughness, while Surface should have 1.0%C for adequate hardness. Case hardening is the answer. Carburising - Add carbon to the surface layers of L.C.S to a depth. Heat-treatment - Harden the case and refine the core. Case harden by Carburising Nitrogen hardening (nitridization) 41. Case Hardening : Carburising L.C.S (approximately 0.1 per cent carbon) absorb carbon when heated to austenitic condition. Various carbon containing materials used: Solid media (pack carburising) - bone charcoal or charred leather, together with sodium and/or barium carbonate as energiser. Gaseous media (Gas carburising) Natural' gas (methane C2H6). 42. Case Hardening : Nitrogen Hardening Hard, wear-resistant coating on component made from special alloy steel, e.g drill bushes. Absorption of nitrogen gas into the surface of the metal to form very hard nitrides. Components are heated in ammonia gas at 500~600o C in 40 hours. Ammonia gas breaks down and the atomic nitrogen is absorbed into the surface of the steel. 43. Case Hardening Process Case Hardening process Advantages Disadvantages Solid media Lowest processing cost 6-8 hours to heat the crucible. Gaseous media Widely available because the using of natural gas (methane). Need more advance process plant Nitrogen Hardening Corrosion resistant of steel is improved. No subsequence grinding or finishing process possible.