ie 337: materials & manufacturing processes
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IE 337: Materials & Manufacturing Processes. Lecture 11: Introduction to Metal Forming Operations. Chapters 6 & 18. This Time. Iron/Steel Production Overview of Metal Forming Bulk Deformation Sheet Metal Forming Process Classifications Hot Working Warm Working Cold Working Processes - PowerPoint PPT PresentationTRANSCRIPT
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
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)
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
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
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
Figure 6.5
Cross‑section of iron-making blast furnace showing major components
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
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
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
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
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
Basic Oxygen Furnace
Figure 6.7 Basic oxygen furnace showing BOF vessel during processing of a heat.
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.
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
Figure 6.9 Electric arc furnace for steelmaking.
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
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
Ingot Mold
Figure 6.10 A big‑end‑down ingot mold typical of type used in steelmaking.
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
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
Basic bulk deformation processes: (a) rolling
Rolling
26Basic bulk deformation processes: (b) forging
Forging
27Basic bulk deformation processes: (c) extrusion
Extrusion
28Basic bulk deformation processes: (d) wire/rod drawing
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
30Basic sheet metalworking operations: (a) bending
Bending
31Basic sheet metalworking operations: (c) shearing
Shearing
32Basic sheet metalworking operations: (b) drawing
Drawing
200 µm wide channels
Microchannel Process Technology
channel header
channels
Single Lamina
• Channels – 200 µm wide; 100 µm deep
– 300 µm pitch
• Lamina (24” long x 12” wide)– ~1000 µchannels/lamina
– 300 µm thickness
Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding
Microchannel Process Technology
• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels
• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina
– 300 µm thickness Bonding: • diffusion bonding• solder paste reflow• laser welding …
Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding
24”
12”
12”
12”
24”Cross-section of
Microchannel Array
Microchannel Process Technology
Bonding: • diffusion bonding• solder paste reflow• laser welding …
Interconnect• welding• brazing• tapping
24”12”
12”
Microchannel Reactor
Bank of Microchannel Reactors(9 x 106 microchannels)
• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels
• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina
– 300 µm thickness
Microlamination [Paul et al. 1999, Ehrfeld et al. 2000*]
*W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH,
2000.
24”12”
12”
Microchannel Reactor
Microlamination of Reactor
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
Typical engineering stress‑strain plot in a tensile test of a metal
Material Properties
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:
nK 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 shear stresses
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
nf KY
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
nK
Yn
f
1_
_
fY
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 vs. Cold Working
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
Warm Working: Performed at 0.3 - 0.5 Tm, - intermediate effects
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-------
48
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
51
What is Strain Rate?
Strain rate in forming is directly related to speed of deformation v
Deformation speed v = velocity of the ram or other movement of the equipment
Strain rate is defined:
where = true strain rate; and h = instantaneous height of workpiece being deformed
hv
.
.
52
Effect of Strain Rate on Flow Stress
Flow stress is a function of temperature At hot working temperatures, flow stress also
depends on strain rate As strain rate increases, resistance to deformation
increases This effect is known as strain‑rate sensitivity
53
(a) Effect of strain rate on flow stress at elevated work temperature. (b) Same relationship plotted on log‑log coordinates
54
Strain Rate Sensitivity Equation
where,
C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent
mf CY
55
The constant C, indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature
Effect of Temperature on Flow Stress
56
Strain Rate Sensitivity
Increasing temperature decreases C, increases m At room temperature, effect of strain rate is almost
negligible Flow curve is a good representation of material behavior
As temperature increases, strain rate becomes increasingly important in determining flow stress
57
Friction in Metal Forming
In most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increased Wears tooling faster
Friction and tool wear are more severe in hot working
58
Lubrication in Metal Forming
Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction
Benefits: Reduced sticking, forces, power, tool wear Better surface finish Removes heat from the tooling
59
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
60
Next Time
Sheet Metal Forming Analysis
Chapter 20