IE 337: Materials & Manufacturing Processes

1. IE 337: Materials & Manufacturing Processes Lecture 11: Introduction to Metal Forming Operations Chapters 6 & 18

2. This Time • Iron/Steel Production • Overview of Metal Forming • Bulk Deformation • Sheet Metal Forming • Process Classifications • Hot Working • Warm Working • Cold Working Processes • Formability Properties • Yield Strength • Ductility

3. Iron and Steel Production • Iron making - iron is reduced from its ores • Steel making – iron is then refined to obtain desired purity and composition (alloying)

4. Iron Ores Required in Iron-making • The principal ore used in the production of iron and steel is hematite (Fe2O3) • Other iron ores include magnetite (Fe3O4), siderite (FeCO3), and limonite (Fe2O3‑xH2O, where x is typically around 1.5) • Iron ores contain from 50% to around 70% iron, depending on grade (hematite is almost 70% iron) • Scrap iron and steel are also widely used today as raw materials in iron‑ and steel making

5. Other Raw Materials in Iron-making • Coke (C) • Supplies heat for chemical reactions and produces carbon monoxide (CO) to reduce iron ore • Limestone (CaCO3) • Used as a flux to react with and remove impurities in molten iron as slag • Hot gases (CO, H2, CO2, H2O, N2, O2, and fuels) • Used to burn coke

6. Iron‑making in a Blast Furnace Blast furnace - a refractory‑lined chamber with a diameter of about 9 to 11 m (30 to 35 ft) at its widest and a height of 40 m (125 ft) • To produce iron, a charge of ore, coke, and limestone are dropped into the top of a blast furnace • Hot gases are forced into the lower part of the chamber at high rates to accomplish combustion and reduction of the iron

7. Figure 6.5 Cross‑section of iron-making blast furnace showing major components

8. Chemical Reactions in Iron-Making • Using hematite as the starting ore: Fe2O3 + CO  2FeO + CO2 • CO2 reacts with coke to form more CO: CO2 + C (coke)  2CO • This accomplishes final reduction of FeO to iron: FeO + CO  Fe + CO2

9. Proportions of Raw Materials In Iron-Making • Approximately seven tons of raw materials are required to produce one ton of iron: • 2.0 tons of iron ore • 1.0 ton of coke • 0.5 ton of limestone • 3.5 tons of gases • A significant proportion of the byproducts are recycled

10. Iron from the Blast Furnace • Iron tapped from the blast furnace (called pig iron) contains over 4% C, plus other impurities: 0.3‑1.3% Si, 0.5‑2.0% Mn, 0.1‑1.0% P, and 0.02‑0.08% S • Further refinement is required for cast iron and steel • A furnace called a cupola is commonly used for converting pig iron into gray cast iron • For steel, compositions must be more closely controlled and impurities brought to much lower levels

11. Steel-making • Since the mid‑1800s, a number of processes have been developed for refining pig iron into steel • Today, the two most important processes are • Basic oxygen furnace (BOF) • Electric furnace • Both are used to produce carbon and alloy steels

12. Basic Oxygen Furnace (BOF) • Accounts for  70% of steel production in U.S • Adaptation of the Bessemer converter • Bessemer process used air blown up through the molten pig iron to burn off impurities • BOF uses pure oxygen • Typical BOF vessel is  5 m (16 ft) inside diameter and can process 150 to 200 tons per heat • Cycle time (tap‑to‑tap time) takes  45 min

13. Basic Oxygen Furnace Figure 6.7 Basic oxygen furnace showing BOF vessel during processing of a heat.

14. Figure 6.8 BOF sequence : (1) charging of scrap and (2) pig iron, (3) blowing, (4) tapping the molten steel, (5) pouring off the slag.

15. Electric Arc Furnace • Accounts for  30% of steel production in U.S. • Scrap iron and scrap steel are primary raw materials • Capacities commonly range between 25 and 100 tons per heat • Complete melting requires about 2 hr; tap‑to‑tap time is 4 hr • Usually associated with production of alloy steels, tool steels, and stainless steels • Noted for better quality steel but higher cost per ton, compared to BOF

16. Figure 6.9 Electric arc furnace for steelmaking.

17. Casting Processes in Steel-making • Steels produced by BOF or electric furnace are solidified for subsequent processing either as cast ingots or by continuous casting • Casting of ingots – a discrete production process • Continuous casting – a semi-continuous process

18. Casting of Ingots Steel ingots = discrete castings weighing from less than one ton up to  300 tons (entire heat) • Molds made of high carbon iron, tapered at top or bottom for removal of solid casting • The mold is placed on a platform called a stool • After solidification the mold is lifted, leaving the casting on the stool

19. Ingot Mold Figure 6.10 A big‑end‑down ingot mold typical of type used in steelmaking.

20. Continuous Casting • Continuous casting is widely applied in aluminum and copper production, but its most noteworthy application is steel-making • Dramatic productivity increases over ingot casting, which is a discrete process • For ingot casting, 10‑12 hr may be required for casting to solidify • Continuous casting reduces solidification time by an order of magnitude

21. Figure 6.11 Continuous casting. Steel is poured into tundish and flows into a water‑cooled continuous mold; it solidifies as it travels down in mold. Slab thickness is exaggerated for clarity.

22. 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 yield strength of metal • The metal takes a shape determined by the geometry of the die

23. Metal Forming

24. 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 include cylindrical billets and rectangular bars

25. Rolling Basic bulk deformation processes: (a) rolling

26. Forging Basic bulk deformation processes: (b) forging

27. Extrusion Basic bulk deformation processes: (c) extrusion

28. Wire/Rod Drawing Basic bulk deformation processes: (d) wire/rod drawing

29. Sheet Metalworking • Forming and related operations performed on metal sheets, strips, and coils • High surface area‑to‑volume ratio of starting metal, which distinguishes these from bulk deformation • Often called pressworking because presses perform these operations • Parts are called stampings • Usual tooling: punch and die

30. Bending Basic sheet metalworking operations: (a) bending

31. Shearing Basic sheet metalworking operations: (c) shearing

32. Drawing Basic sheet metalworking operations: (b) drawing

33. Microchannel Process Technology 200 µm wide channels • Patterning: • machining (e.g. laser …) • forming (e.g. stamping …) • micromolding channel header • Channels • 200 µm wide; 100 µm deep • 300 µm pitch • Lamina (24” long x 12” wide) • ~1000 µchannels/lamina • 300 µm thickness channels Single Lamina

34. Microchannel Process Technology • Laminae (24” long x 12” wide) • ~1000 µchannels/lamina • 300 µm thickness • Patterning: • machining (e.g. laser …) • forming (e.g. stamping …) • micromolding 12” Cross-section of Microchannel Array 24” 24” 12” 12” • Device (12” stack) • ~ 1000 laminae • = 1 x 106 reactor µchannels • Bonding: • diffusion bonding • solder paste reflow • laser welding …

35. Microchannel Process Technology 12” 24” 12” Microchannel Reactor • Laminae (24” long x 12” wide) • ~1000 µchannels/lamina • 300 µm thickness • Device (12” stack) • ~ 1000 laminae • = 1 x 106 reactor µchannels • Interconnect • welding • brazing • tapping • Bonding: • diffusion bonding • solder paste reflow • laser welding … Bank of Microchannel Reactors (9 x 106 microchannels)

36. Microlamination [Paul et al. 1999, Ehrfeld et al. 2000*] 12” 24” 12” Microchannel Reactor Microlamination of Reactor *W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH, 2000.

37. Formability Variables • Material Properties • Recrystallization Temperature • Ductility • Fracture Resistance • Strain Hardening • Yield Strength / Elasticity • Process Variables • Temperature • Friction • Lubrication • Deformation Rates

38. Material Properties in Forming • Desirable material properties: • Low yield strength and 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

39. Material Properties Typical engineering stress‑strain plot in a tensile test of a metal

40. Material Behavior in Metal Forming • Plastic region of stress-strain curve is primary interest. In plastic region, metal's behavior is expressed by the flow curve: • where, K = strength coefficient; and • n = strain hardening exponent • Stress and strain in flow curve are true stress and • true strain

41. 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 shearstresses

42. Flow Stress • For most metals at room temperature, strength increases when deformed due to strain hardening • Flow stress = instantaneous value of stress required to continue deforming the material • where Yf = flow stress, that is, the yield strength as a function of strain

43. Average Flow Stress Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest where, = average flow stress; and  = maximum strain during deformation process

44. Effect of Temperature on Properties

45. Temperature in Metal Forming • For any metal, K and n in the flow curve depend on temperature • Both strength and strain hardening are reduced at higher temperatures • In addition, ductility is increased at higher temperatures

46. Temperature in Metal Forming • Any deformation operation can be accomplished with lower forces and power at elevated temperature • Three temperature ranges in metal forming: • Cold working • Warm working • Hot working

47. Hot Working: Deformation at temperatures above recrystallization temperature = 0.5 Tm on absolute scale Less powerful equipment More isotropic properties Less residual stress Less strain-hardening Ductility for deformation Easier secondary ops Shorter tool life Cold Working: Deformation performed at or slightly above room ambient temperature - no heating required Less reactive environment Better surface finish Better dimensional control More anisotropic properties More strain-hardening Strength for end-use Fatigue resistance Hot Working vs. Cold Working • Warm Working: Performed at 0.3 - 0.5 Tm, - intermediate effects

48. Recrystallization and Grain Growth Scanning electron micrograph taken using backscattered electrons, of a partly recrystallized Al-Zr alloy. The large defect-free recrystallized grains can be seen consuming the deformed cellular microstructure. --------50µm-------

49. Cold Working Processes • Primarily Sheet Metal Working • Primary Operations: • Shearing • Bending • Drawing • Primary Processes: • Punching / Blanking • Roll Bending • Roll Forming • Spinning

50. Strain Rate Sensitivity • Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0 • The metal should continue to flow at the same flow stress, once that stress is reached • However, an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivity