ferrous alloys chapter 12 – 4 th edition chapter 13 – 5 th edition
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Ferrous AlloysChapter 12 – 4th EditionChapter 13 – 5th Edition
Where Does Iron Come From? Naturally
occurring iron exists as iron-oxide (rust)
Iron ore mine in Labrador, Canada
http://upload.wikimedia.org/wikipedia/commons/f/f1/Iron_ore_mine-01_(xndr).jpg
Iron is also recycled
The only naturally occurring metallic iron on earth comes from meteorites
The largest meteorite discovered in Antarctica is about 2 feet by 2 feet by 1.5 feet. Due to its size it was not able to be thawed in the 100% nitrogen atmosphere and therefore the ice inside melted. The liquid water dissolved minerals inside the meteorite, and when it evaporated, white salts were left on the surface of the meteorite. NASA Lyndon B. Johnson Space Center, Houston, TX.
http://www2.ifa.hawaii.edu/newsletters/images/23largeMeteorite.jpg
Iron oxide is converted to metallic iron in a blast furnace
The main chemical reaction producing the molten iron is: Fe2O3 + 3CO → 2Fe + 3CO2[32]
Preheated blast air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat.
The carbon monoxide then reacts with the iron oxide to produce molten iron and carbon dioxide
1. Hot blast from Cowper stoves2. Melting zone3. Reduction zone of ferrous oxide4. Reduction zone of ferric oxide5. Pre-heating zone6. Feed of ore, limestone and coke7. Exhaust gases8. Column of ore, coke and limestone9. Removal of slag10. Tapping of molten pig iron11. Collection of
waste gases
Blast Furnace
http://en.wikipedia.org/wiki/File:VysokaPec.jpg
Pig Iron An intermediate product – the result
of smelting with iron ore and carbon Iron and typically about 4% carbon
Also includes sulphur phosphorus and other impurities
Brittle and not very useful
http://www.manufacturer.com/images/buyLeads/www.alibaba.com/1118/u/Pig_iron.jpg
Steel It wasn’t possible to make
steel until about 1850 We don’t call it steel
unless it is less than 2% carbon
An open hearth furnace was used to burn off the excess carbon up until the 1990’s
Carbon can also be burned off with Electric Furnace Oxygen Furnace
Sheet and Tube Open Hearth Furnace – Youngstown Ohio
Steel is a major structural component
Photo by Ian Britton http://www.freefoto.com/preview/42-12-6?ffid=42-12-6
The Palmer is named for a U.S. seal hunter who sailed along the west coast of Antarctica in 1820 looking for seal rookeries. Many believe he was the first to discover the continent.
Photograph courtesy Woods Hole Oceanographic Institution http://www.nationalgeographic.com/sealab/antarctica/ship.html
Carbon composition Steel generally has less than about
0.7% C, but can have up to 2.11% C.
Look at the iron phase diagram to remind yourself why
400 C
1400 C
1200 C
1000 C
800 C
600 C
1600 C
Fe 1% C 2% C 3% C 4% C 5% C 6% C 6.70% C
L
Steel Cast Iron
Steel and Iron The phase diagram only strictly
applies to an iron – carbon combination
Steel and iron often have other alloying elements in them, which modify the phase diagram
Stainless Steel Phase Diagram at 9000C
18-8 Stainless steel is the most common composition – The terminology refers to 18%Cr and 8% Ni – with the balance Fe (and other trace elements)
http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/experimental/ternary2.html
Stainless Steel Solidus Temperatures
http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/experimental/ternary2.html
Cast Iron Has quite a bit more
cementite in it than steel
That makes it hard and brittle
But cementite is a “metastable” compound, that can decompose into iron and graphite with the appropriate thermal treatment http://www.trademadesimple.co.uk/
companies/olymberyl-manufacturers/images/cast-iron-stove-hf332-1.jpg
http://www.georgesbasement.com/Microstructures/CastIronsHighAlloySteelsSuperalloys/Lesson-1/Introduction.htm
George Langford, Sc.D., Massachusetts Institute of Technology,
Wrought Iron This iron is ductile
and malleable and can be “wrought” into a final shape
Wrought iron was the primary high strength structural material until steel became available in the 19th century
http://upload.wikimedia.org/wikipedia/commons/b/bd/Eiffel_tower_from_below.jpg
The Eiffel Tower was made from Puddle Iron – a form of Wrought Iron
Heat Treatments Process Annealing
Heat the steel just below the eutectoid Removes the effect of cold work
Austenitizing Heat into the region to dissolve the
carbon
Microstructure We’ve already discussed the formation of
the eutectoid microstructure If you force the phase change to occur
just below the equilibrium transformation temperature you get spheroidite Large spheroidal particles Steel is easily machined Low strength and hardness
After machining it is heat treated again to improve the properties
Alloying Elements May… Modify the phase diagram Modify the TTT curve Strengthen the steel by precipitation
hardening Reduce Corrosion
Stainless Steel >12% Chromium May also contain large amounts of nickel In some stainless steels the austenite
structure survives at room temperature Makes the steel especially corrosion
resistant Non Magnetic
http://www.calphad.com/graphs/Fe-Ni%20Phase%20Diagram.gif
Iron Nickel Phase Diagram
Alloy Steel Alloying elements make it harder to
predict the effect of heat treatments The equilibrium structures are not
always known Even if they are – they aren’t always
achieved
Shopping? Moderate chromium steels can form
Martensite, which is hard and corrosion resistant
Austenitic steel is more corrosion resistant and more ductile (less brittle)
Ian O'Leary (c) Dorling Kindersley
Stainless steel has a broad range of properties
You want Martensite for your
knives and Austenite for your
bowls Remember –
Austenite is not magnetic
Surface Treatments Coatings
Tin, Zinc (galvanized), Aluminum Surface Hardening
Heating, followed by quenching Diffusion of carbon or nitrogen
Welding Problems with welds
do not usually occur in the weld itself
The area around the weld is heated, and changes the microstructure Excessive grain growth Formation of Martinsite
(makes it brittle)
Titanic
A detailed analysis of small pieces of the steel plating from the Titanic's wreck hull found that it was of a metallurgy that loses its elasticity and becomes brittle in cold or icy water, leaving it vulnerable to dent-induced ruptures. The pieces of steel were found to have very high content of phosphorus and sulphur (4x and 2x respectively, compared to modern steel), with manganese-sulphur ratio of 6.8:1 (compare with over 200:1 ratio for modern steels). High content of phosphorus initiates fractures, sulphur forms grains of iron sulphide that facilitate propagation of cracks, and lack of manganese makes the steel less ductile. The recovered samples were found to be undergoing ductile-brittle transition in temperatures of 32 °C (for longitudinal samples) and 56 °C (for transversal samples—compare with transition temperature of -27 °C common for modern steels—modern steel would became so brittle in between -60 and -
70 °C).
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