MEMS 0040 Materials and Manufacturing
Refining of Metals
Topic Coverage (not included in textbook)
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Reduction of iron ore in the blast furnace
Pig iron vs. steel
Basic Oxygen Furnace (BOF) processing of steel
Electric arc furnace (EAF) processing of steel
Continuous casting of steel
Types of steels
Sankey Diagram for manufacturing of iron and steel
Bayer process to produce aluminum oxide
Aluminum refining
Types of aluminum alloys
Sankey diagram for manufacturing of aluminum
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Iron and Steel Production
• In 2022, 86 million metric tons in U.S. Used about 1,000 trillion BTU of
energy, but only perhaps 200 trillion BTU electricity.
• Iron, and particularly steel, production is very important to structural
engineering. We must discuss how Iron is processed from ore and how
steel is manufactured from iron and scrap.
• Metallic iron is generally not present in nature, but must be processed from
ore, which is mostly hematite (Fe2O3) and other oxides such as magnetite
(Fe3O4).
• Obviously, these oxides must be reduced to get metal.
• Limestone, mainly made up of CaCO3, is added to flux the removal of
impurities such as SiO2, sulfur, and Al2O3, which together form a layer on top
of the molten metal called slag (lower density).
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MEMS 0040 Materials and Manufacturing
Total of 2.8 Billion Tonnes of Ores Mined in 2022
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MEMS 0040 Materials and Manufacturing
Quantities of Ores Mined in 2022
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MEMS 0040 Materials and Manufacturing
Quantities of Nonferrous Ores Mined in 2022
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Pig Iron Production using the Blast Furnace
• Using hematite as the starting ore:
Fe2O3 + CO  2 FeO + CO2
• This accomplishes final reduction of FeO to iron:
FeO + CO  Fe + CO2
• CO2 reacts with coke to form more CO: Why coke, not coal?
CO2 + C (coke)  2 CO
Why does liquid Fe collect in bottom of 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).
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Pig Iron Production using the Blast Furnace
• Iron ore, coke, and limestone
are added at the top.
• Hot air, gases, and fuel are
injected into the bottom to
burn the coke to form CO (base
of furnace reaches 1650°C).
• The coke is added to reduce the
iron oxide and provide heat
through exothermic oxidation of
carbon.
• Heat is controlled by blasting air
into the bottom of the furnace;
this accelerates the oxidation of
carbon, which generates heat
(like bellows in blacksmith’s
forge).
Slag is a liquid mixture of oxides that also absorbs
impurities like sulfur so that they can be removed.
• What is the function of slag?
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Pig Iron Production using the Blast Furnace
• 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 is recycled.
• 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 in STEELMAKING.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steels
The major objective
of steelmaking is?
To remove carbon
from liquid pig iron
from the blast
furnace.
What characterizes a
steel vs. pig iron?
Steel has a lower
amount of carbon
(<2%, usually <1%).
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
• Today, the two most important processes are:
Basic Oxygen Furnace (BOF)
Electric Arc Furnace (EAF)
• Both are used to produce plain carbon and alloy steels.
Basic Oxygen Furnace (BOF)
• Accounts for 30% of steel production in U.S., adaptation of the
Bessemer converter.
• Bessemer process used air blown upward through molten pig iron to
burn off impurities.
• BOF uses pure oxygen blown downward into the liquid metal.
• Typical BOF vessel is 5 m (16 ft) inside diameter, and can process 150 to
300 tons per heat.
• Cycle time (tap-to-tap time) takes 45 minutes.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
• Need to reduce impurities in iron by oxidation:
2 C + O2  2 CO then 2 CO + O2  2 CO2
CO and CO2 are removed as gases.
Fe is not oxidized (thermodynamic reasons) until late in the process.
Si + O2  SiO2
2 Mn + O2  2 MnO
4 P + 5 O2  2 P2O5
• How do these impurities become
incorporated into the slag?
The oxides come into contact with the
liquid slag and are dissolved into it.
• The CO/CO2 mixture rises out of the BOF
(furnace) and is captured by a hood.
• Tap-to-tap processing time is 45 minutes.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
• Steels contain less than
2%C, so steelmaking
mostly involves removal
of C by oxidation.
• Most alloying elements,
such as Mn, Si, Cr, V,
and Ti need to be
added to the melt after
this blow stage. Why?
These elements are
more reactive than iron
and will oxidize and be
lost in the slag.
Figure 6.7. Basic oxygen furnace, showing
BOF vessel during processing of a heat.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
Figure 6.8. BOF sequence: (1) charging of scrap; (2) pig iron; (3) blowing; (4) tapping
of molten steel; (5) pouring off slag. Draw the BOF vessel cycle as a flow diagram.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
• In the BOF, pure O2 is blown downward
(above Mach 2) onto the surface of
molten pig iron and scrap (20-30%)
using a lance above the surface of the
molten metal and slag.
• This causes combustion and heating of
the surface of the molten metal pool.
• It also produces a gas-slag emulsion
that fills much of the furnace volume.
• 5 m ID.
• Processes 150 to 300 tons in a batch
called a heat.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking using Electric Arc Furnace
• Accounts for 70% of steel production in U.S.
• Scrap iron and scrap steel are primary raw materials. Can reduce energy
requirement by 74% compared to reduction of iron ore.
• Capacities commonly range between 25 and 200 tons per heat.
• Tap-to-tap time can be as short as 1 hour or as long as 4 hours.
• Quality of steel depends significantly on quality of scrap used.
• Can produce both commodity products (rebar, low carbon sheet, etc.)
and high quality alloy steels, tool steels, and stainless steels.
• Cost per ton is sensitive to price of scrap and other iron sources.
MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking using Electric Arc Furnace
Figure 6.9. Electric arc furnace for steelmaking.
Draw the EAF cycle as a flow diagram.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking using Electric Arc Furnace
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking
• Frequently, the liquid metal must be further refined after the BOF or EAF.
This can be done in the ladle. Often, Al and/or Si are added to remove
oxygen by oxide formation. The oxides are less dense than the liquid metal
and may float to the slag on top. Some oxides remain in the steel.
• The refining continues until the oxygen content reaches the correct level
for casting. Degassing can be done to lower hydrogen (and carbon).
• Once the steel is tapped, it must be cast. It can be cast into ingot molds
from 1 to 300 tons, which are then thermomechanically formed. Today,
continuous casting is widely used.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking:
Continuous Casting
Figure 6.11. Schematic
of continuous casting
machine with straight
mold, also called a
vertical bending caster.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking: Continuous Casting
• In this process, the molten metal is poured into a tundish and then into a
water-cooled mold. A continuous strand is formed, which is pulled downward
through many pairs of rolls. The strand is cooled by water sprays and solidifies
from the outside inward. If the mold is straight, the strand is bent to form a
curve. The strand is then unbent or straightened into a horizontal position
while still hot and plastic.
• This process is continuous, and is therefore more efficient and easier to control
compared with casting ingots, which is a batch process. It is not necessary to
wait several hours or days for ingots to solidify.
• Intermediate products from continuous casting include billets (square or
round sections up to 150 mm), blooms (square or round sections >150 mm),
slabs (>600 mm by >40 mm), and other shapes (e.g., I-beam blanks). Can
vertically integrate to end with standard product, e.g., rolled sheet, etc.
• It is responsible for 90% of U.S. steel production, compared to 10% in 1970.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking: Continuous Casting
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Steelmaking: Ingot Casting
• Cast into ingots 1 to 300 tons, which are then thermomechanically formed. Why
does most of steelmaking use continuous casting?
Figure 6.10. A big-end-down ingot mold
typical of type used in steelmaking.
e.g., Ellwood Quality Steel uses EAF
and casts into 50-ton ingots for later
thermomechanical forming, such as
forging and extrusion.
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MEMS 0040 Materials and Manufacturing
Sustainability and Ecoselection
• Energies for materials production stage
From “Materials: Engineering, Science, Processing, and Design,”
2nd Edition, by Ashby, Shercliff, and Cebon, Elsevier (2009).
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
Source: https://crsreports.congress.gov/product/pdf/R/R47294
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
• In 2021, produced 908,000 metric tons of primary aluminum (from ore) in
the U.S.A., and 3.2 million tons of secondary aluminum (from scrap).
• U.S.A. imported 44% of aluminum used (more than half from Canada).
• The weight of aluminum recovered from purchased scrap in the U.S.A.
represented about 78% of total production tonnage. Much of this scrap is
new scrap from production facilities. The percentage of old scrap
recycled, such as from aluminum cans, is much lower. The U.S.A. is a
major exporter of aluminum scrap.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
• Principal ore of aluminum is bauxite, Al(OH)3. Why is bauxite not used
directly for smelting of aluminum? It is far too impure.
• First, the ore is washed and crushed.
• Bauxite is purified using the Bayer Process, in which bauxite is digested in
caustic soda (NaOH) solution at 135-150°C.
• The main contaminant is iron hydroxide (similar to rust). Al(OH)3 remains
in solution, but Fe hydroxide is insoluble and is removed by filtration.
• Then, the solution is pumped to precipitation tanks, the pressure and
temperature are reduced, and pure aluminum hydroxide precipitates.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
Reaction is:
• 2 Al(OH)3 + 2 NaOH  2 NaAlO2 (soluble) + 4 H2O (digestion)
• The red mud precipitate is then filtered (clarification).
• The clear liquid is pumped into precipitation tanks, in which it cools and
Al(OH)3 reprecipitates (precipitation).
• It is then heated to temperatures up to the range 1010 to 1260°C in
rotary kilns (calcination).
• Draw the Bayer process as a flow diagram.
• The by-product of clarification, called red mud, can be 50% of the ore. It
is far too high in pH to be released, and is held in settling ponds.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
• Red mud is caustic and is commonly kept in containment ponds.
• Accident in Ajka, Hungary in October 2010. Dam on reservoir No. 10
collapsed, releasing 1 million cubic meters of red mud sludge. A 1-2 m
high wave flooded nearby villages and towns. Killed 10 people, injured
150, killed all life in nearby Marcal River, and reached Danube.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
Hall-Heroult Process
• Aluminum oxide is dissolved in a bath of molten cryolite (Na3AlF6) at about
955-965°C; then, a large electric current is imposed between the graphitelined floor of the cell (cathode) and graphite electrodes (anodes).
• In the electrochemical reaction, the aluminum ions are attracted to the
cathode, where they are reduced to aluminum metal, which collects on the
bottom of the tank. The oxygen ions form O2 gas at the anodes. The oxygen
reacts with the graphite electrodes to form CO2.
• 2 tons of Al2O3 gives 1 ton Al metal.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
Reaction at cathode: Al3+ + 3 e-  Al (need 3 to 5 volts).
Reactions at anode: 2 O2-  O2 + 4 eC + O2  CO2
Which reaction is oxidation? Anode reaction is oxidation (electrons removed).
Power requirement is very high, but voltage is only 3-5 V, so current is high.
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MEMS 0040 Materials and Manufacturing
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MEMS 0040 Materials and Manufacturing
MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
1200 Grade Aluminum
Production energy
Minimum energy of melt (30% efficient)
Minimum energy to 90% deformation (30% efficient)
CO2 production
Commonly use hydroelectric
power.
Why Iceland?
Can reduce energy requirement
by 90% through recycling.
190-210 MJ/kg
3.5-3.8 MJ/kg
0.04-0.044 MJ/kg
12-13 kg/kg
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Aluminum refining
Why Iceland?
Electricity is 100% renewable!
70% hydro, 30% geothermal
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
0 to 2.11 wt% carbon
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
Plain carbon steels
• Carbon is principal alloying element, with only small amounts of other
elements.
• AISI-SAE designations have 4 digit numbers, with first two as 10XX,
where XX represents carbon content; e.g., 0.40% C is 1040.
• Low carbon (less than 0.2% C) used in plate steel, rails, and sheet metal
for cars.
• Medium carbon (0.2 to 0.5% C) have higher strength, so can be used for
crankshafts, etc.
• High carbon (>0.5 %C) have high strength and are very hard, but have
lower ductility. Used for springs, cutting tools, and for wear resistance.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
Proeutectoid ferrite and pearlite
What is pearlite? It is a two-phase
lamellar (parallel layers) mixture of
ferrite and iron carbide (Fe3C).
Figure GLU 2.39. Microstructure of
hypoeutectoid steel; proeutectoid
ferrite + pearlite.
Why is the ferrite called proeutectoid
ferrite? It forms before the eutectoid
temperature during cooling.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
Low Alloy Steels
• Additional alloy elements total less than 5wt%. Better properties, often
require heat treatment (quenching and tempering) to be realized.
Alloying elements include:
• Chromium (Cr): In solid solution, improves strength, hardness, etc., and very
good for hardenability. Used in stainless steels for corrosion resistance.
• Manganese (Mn): Improves strength, hardness, hardenability. Widely used.
• Molybdenum (Mo): Improves strength and toughness, formation of carbide
improves wear resistance.
• Nickel (Ni): Improves strength and toughness. Used in stainless steel to
stabilize austenite and for corrosion resistance.
• Vanadium (V): Inhibits grain growth at high temperatures and improves
strength and toughness.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
Low Alloy Steels
• Low alloy steels are not easily welded at medium and high carbon contents
because of martensite formation.
• Now we have HSLA (High Strength Low Alloy) steels, which have low C
content (<0.3%C) with low alloying elements (<3% total). These give good
welding and formability, while having higher strength than plain carbon
steels.
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MEMS 0040 Materials and Manufacturing
Figure GLU 2.45. TTT diagram of hypoeutectoid steel with martensite shown;
evolution of yield stress during tempering.
Isothermal heat treatment in a + Fe3C two-phase region.
M  a + Fe3C (tempered martensite)
Tempering gives precipitates of Fe3C in a; lowers strength, but improves toughness.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Classification of steels
Stainless Steels
• Cr is usually above 12%. Why? The chromium is what prevents rusting.
• Austenitic Stainless steels: Usually 18% Cr and 8% Ni (18-8). Nickel
stabilizes austenite at room temperature. Low carbon, very ductile.
Used in chemical and food industries, etc.
• Ferritic stainless steels: 15 to 20% Cr, no nickel, low carbon. Used for
many applications, from kitchen utensils to jet engines.
Tool Steels >0.5 wt%C
• Contain more than 0.5%C, especially hypereutectoid. Often large
amounts of alloying elements also. Produced using quenching and
tempering processes. Hard, but low ductility. Used for machine tools.
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MEMS 0040 Materials and Manufacturing
Strength and Ductility of Steels
Typical of metals,
steels show an inverse
relationship between
yield strength and
ductility when
strengthened.
The high ductility-low
strength alloys are not
usually heat treatable
into a high strength
condition.
Figure 3.12. Typical properties for groups of steels. From
“Sustainable Materials: With Both Eyes Open,” by J. M.
Allwood and J. M. Cullen, UIT, Cambridge, UK (2012).
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MEMS 0040 Materials and Manufacturing
Summary of Steel Classification
Table 3.1. The world of steel. From “Sustainable Materials: With Both Eyes Open,”
by J. M. Allwood and J. M. Cullen, UIT, Cambridge, UK (2012).
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MEMS 0040 Materials and Manufacturing
MATERIAL FLOWS FOR STEEL PRODUCTION
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MEMS 0040 Materials and Manufacturing
Some takeaways from the Sankey diagram for steels:
It is important to know the details.
2/3 of worldwide steel production comes from ore, and 1/3 from scrap.
Scrap sources: 1/5 from within steelmaking, 2/5 from manufacturing, 2/5 from
end-of-life products and buildings.
Dominant process for ore is BOF, but dominant process for scrap is EAF.
>90% of steel is continuously cast and rolled. 1/10 plate, 4/10 thin sheet as
strip, 4/10 is rod and bar, 1/10 constant cross-section profiles.
50% of steel is used in construction.
Largest proportion of steel used in vehicles is cold-rolled coiled steel.
MEMS 0040 Materials and Manufacturing
Refining of Metals
Production of Cars and Planes
• Worldwide car and commercial vehicle production in 2019:
67,149,196 cars and 24,637,665 commercial vehicles.
• 28% in China, 11% in Japan, 5% in Germany, 12% (10.8 million) in USA.
Source: International Organization of Motor Vehicle Manufacturers
• Commercial aircraft deliveries worldwide in 2017: 1740, with Boeing and
Airbus having more than 700 each.
• 31,600 aircraft in service in 2017.
Source: Centre for Aviation, CAPA
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MEMS 0040 Materials and Manufacturing
Body-In-White, BMW Mini Cooper
Note: Curvature (Formability), Spot Welds (Low Carbon),
Lightweighting (High Strength), Corrosion Protection (Zn)
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MEMS 0040 Materials and Manufacturing
Steels Application Guidelines
Area of Opportunity for Third
Generation (Gen3) Steels
(1)
Generation III
(2)
(3)
(1) Steels for safety-critical parts, especially for passenger
survival in crash events;
(2) High-strength steels with a good balance of strength,
formability, energy absorption, and durability;
(3) Steels with excellent formability (e.g., for deep drawing).
C. M. Tamarelli, "AHSS 101: Evolving
Use Of Advanced High-Strength
Steels For Automotive Applications,"
Energy Business Journal, p. 58, 2012.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
Metal Prices (December 2023)
U.S.A. Carbon Steel Hot Rolled Coil Price $1162/MT
Global Ex-China Stainless Steel Coil Price $2300/MT
North American Scrap Prices
Bundle #1 (from factory, no galvanized)
Heavy melting (no galvanized product)
Stainless steel, 304
Stainless steel, 316
$503/MT
$373/MT
$725/MT
$1410/MT
6063 Aluminum
Copper tubing
$1275/MT
$6200/MT
Why does stainless steel scrap command a high price?
Stainless steel contains valuable alloying elements.
Who would use it? Stainless and alloy steel producers.
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MEMS 0040 Materials and Manufacturing
Refining of Metals
ATI (Specialty Metals)
• $3.84 billion dollars in sales
(2022).
• Facilities in U.S., Europe, and
China for melting and
manufacturing.
• Sales offices in Europe, Middle
East, Asia, the Americas.
• Materials: stainless steel,
titanium, nickel, cobalt,
zirconium, niobium, etc.
• Forms: Flat-rolled, long,
forgings, castings, etc.
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MEMS 0040 Materials and Manufacturing
Summary of Types of Aluminum
Table 3.2. The world of aluminium. From “Sustainable Materials: With Both Eyes
Open,” by J. M. Allwood and J. M. Cullen, UIT, Cambridge, UK (2012).
MEMS 0040 Materials and Manufacturing
Typical of metals,
aluminum alloys show
an inverse relationship
between yield strength
and ductility when
strengthened.
Strength and Ductility of Aluminum Alloys
Highly strengthened
aluminum alloys still
have lower strength
than many steels.
Notice that castings
have properties that
are inferior to wrought
products.
Figure 3.13. Typical properties for groups of aluminium
alloys (‘wrought’ alloys can be deformed). From
“Sustainable Materials: With Both Eyes Open,” by J. M.
Allwood and J. M. Cullen, UIT, Cambridge, UK (2012).
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MEMS 0040 Materials and Manufacturing
Aluminum Alloys: natural and artificial aging
Why is there a peak?
• Before peak aging time, nucleation of precipitates
decreases precipitate spacing.
• Overaging occurs due to precipitate growth.
From M. Ashby, H. Shercliff, and D. Cebon, “Materials: Engineering,
Science, Processing and Design,” 2nd edition, Elsevier (2010).
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MEMS 0040 Materials and Manufacturing
MATERIAL FLOWS FOR ALUMINUM PRODUCTION
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MEMS 0040 Materials and Manufacturing
Some takeaways from the Sankey diagram for aluminum:
50% of aluminum is made from ore and 50% from scrap (remelted or added to
refiner)
Aluminum made from ore and some remelted scrap is low silicon and can be
used for wrought product (high quality products).
Aluminum from post-consumer scrap is refined and tends to be used in castings
(lower quality products).
2/3 wrought is rolled into sheet or plate, 1/4 is extruded, and remainder is cable
and wire. A third of liquid aluminum is cast (greater proportion than for steel).
So [(17.3/(29.8+17.2)) = 0.37] about 37% of new liquid aluminum ends up as
scrap in production or manufacturing of wrought product !! This figure is
about 25% for steel.
MEMS 0040 Materials and Manufacturing
Refining of Metals
Concept inventory
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•
•
Reduction of iron ore in the blast furnace (you must know equations).
Difference between pig iron and steel.
Description of Basic Oxygen Furnace (BOF) processing of steel.
Description of electric arc furnace (EAF) processing of steel.
Continuous casting of steel.
Definitions of plain carbon steels: low carbon, medium carbon, and high
carbon.
Definitions of low alloy steel, stainless steel, and cast iron.
Interpretation of Sankey Diagram for mass in manufacturing of iron and
steel.
Bayer process for removing iron contamination for bauxite to produce
aluminum oxide.
Description of aluminum refining and casting.
Heat treatable and non-heat treatable aluminum alloys.
Interpretation of Sankey diagram for mass in manufacturing of aluminum.
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