ENMAT101A Engineering Materials and Processes
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Lecture 11 – Iron and steel
wikipedia
www.highered.tafensw.edu.au
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Iron and steel
Reference Text
Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians,
5th ed, Butterworth Heinemann
Ch 11
Additional Readings
Section
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Iron and steel
Note: This lecture closely follows text (Higgins Ch11)
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Iron and steel: Intro (Higgins 11.1)
Since the onset of the Industrial Revolution, the material wealth and
power of a nation has depended largely upon its ability to make steel.
Every new country ramping up into industrialisation begins by focussing
on steel production – Britain and Europe, then US then USSR then
Japan and Korea, and the lastest example China…
The last few decades of Asian
development have been good for
Australia’s mining industry.
http://www.independentaustralia.net
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Iron and steel: Intro (Higgins 11.1)
China now
dominates steel
production – almost
half the world’s
production!
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Smelting (Higgins 11.2)
Blast Furnace turns iron ore to pig iron, which has too much carbon. This
is removed in later process such as oxygen process, to make steel.
Steel from Start to Finish (Promo. US)
http://www.youtube.com/watch?v=9l7JqonyoKA
Steelmaking (UK)
http://www.youtube.com/watch?v=Ea_7Rnd8BTM
Continuous Casting (More modern system that suits
electric arc and recycled steel, but not really suited to blast
furnace which is a batch process)
http://www.youtube.com/watch?v=d-72gc6I-_E
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Smelting (Higgins 11.2)
Despite research on 'direct reduction' of iron ore, the blast furnace still
dominates iron production. The thermal efficiency of the blast-furnace is
very high, also helped by injection of oil or pulverised low-cost coal to
reduce the amount of expensive metallurgical coke consumed.
A blast furnace runs non-stop
for several years (life of the
lining) since it is quite a
procedure to stop and start it.
However, a typical blastfurnace releases about 6600
tonnes of carbon dioxide
every day.
Hebei province accounts for a quarter of the China's
total steel production capacity
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Steel-making (Higgins 11.3)
Converting pig iron to steel is done by oxidation of impurities, so that
they form a slag which floats on the surface of the molten steel or are
lost as fume.
The Bessemer process 1856
brought steel to the masses. That
process is now obsolete. The
open-hearth process followed but
modern processes are basic
oxygen processes (1952) or in the
electric-arc furnace.
Corus Steel (UK)
Description of steel
making processes
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Steel-making (Higgins 11.3)
Basic Oxygen Process.
Higgins
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Steel-making (Higgins 11.3)
Plain-carbon steels: less than 1.7 % C.
Ordinary steels: up to 1.0 % Mn (left over from a deoxidisation process
that slightly increases strength and hardness, and reduces sulphur
content of the steel.
Both sulphur and phosphorus are extremely harmful impurities which
give rise to brittleness in steels. Usually specify max 0.05% S and Ph,
and high quality steels no more than 0.04%. (or as low as 0.002% in
modern steel for pipelines).
The majority of steel is mild steel and low-carbon steel for
structural work, none of which is heat-treated except for stress relief.
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Cementite (Higgins 11.4.1)
Ordinarily carbon in steel exists as iron carbide (cementite).
Cementite is very hard. So increasing carbon content increases
the hardness of the steel.
Cementite is actually an intermetallic compound
in steel alloys with the chemical formula Fe3C.
This phase has a specific chemical formula,
unlike most phases which have ranges of
chemical composition. Cementite is hard and
brittle.
IMAGE: Journal of Molecular Catalysis A: Chemical
Volume 269, Issues 1–2, 18 May 2007, Pages 169–178
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Carbon in Steel
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The Iron-Carbon equilibrium
diagram over a very small
range of Carbon (0 to 2% by
weight, or 0 to 7% by atoms)
This is as much carbon as
steel can handle before it
turns into cast iron, and then
into useless rock.
This diagram will meet you
again soon (not today).
Larger version
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Figure 11.4 The
iron-carbon
equilibrium
diagram.
The small dots in the
diagrams depicting
structures
containing austenite do
not represent visible
particles of cementite
— they are meant to
indicate the
concentration of carbon
atoms dissolved in the
austenite and in the
real microstructures
would of course be
invisible. The inset
shows the 'peritectic
part' of the diagram in
greater detail.
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Steel grain structures
Equilibrium grain
structures
Identify:
• Ferrite
• Cementite
• Pearlite
Austenite is not
visible in any of
these – why not?
watlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures.html
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Eutectoid
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Iron Carbon Equilibrium Diagram
Follow Higgins notes 11.5.1
Teach yourself phase diagrams
Handout
http://www-g.eng.cam.ac.uk/mmg/teaching/phasediagrams/i2a.html
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0.4 % C
These then are the main stages in the foregoing process of solidification
and cooling of the 0.4 per cent carbon steel:
1 Solidification is complete at Si and the structure consists of uniform
austenite.
2 This austenite begins to transform to ferrite at Ui, the upper critical
temperature of this steel (about 825°C).
3 At 723°C (the lower critical temperature of all steels), formation of
primary ferrite ceases, and, as the austenite is now saturated with carbon,
the eutectoid pearlite is produced as alternate layers of ferrite and
cementite.
4 Below 723°C, there is no further significant change in the structure.
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Hyper Eutectic
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Carbon vs Properties
Figure 11.8 A diagram
showing the relationship
between carbon
content, mechanical
properties, and uses of
plain-carbon steels which
have been slowly cooled
from above their upper
critical temperatures.
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Normalising (Higgins 11.6.1)
The main purpose in normalising is to obtain a structure which is
uniform throughout the work-piece, and which is free of any 'locked-up'
stresses.
Read Higgins 11.6.1
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Normalising
(Higgins 11.6.1)
Larger version
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Annealing (Higgins
11.6.2)
Three types of annealing:
Type 1: Annealing of
castings
Same as normalising but
slower cooling (controlled
in furnace) to prevent
cracking.
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Annealing (Higgins 11.6.2)
Type 2: Spheroidisation annealing
An annealing process which is applied to high carbon steels in order to
improve their machinability and, in some cases, to help with colddrawing.
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Annealing (Higgins 11.6.2)
Type 3: Annealing of cold-worked steel
Recrystallisation of distorted ferrite grains to restore ductility (e.g. to allow
further cold working processes).
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Annealing (Higgins 11.6.2)
Summary of ranges on the Fe-C diagram.
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Brittle Fracture in Steels (Higgins 11.7)
Ferrite is very susceptible to brittle fracture at low temperatures,
especially below the transition temperature.
This transition temperature can be depressed to a safe limit by
increasing the manganese content to about 1.3%.
For use at even lower temperatures, it is better to use a low-nickel
steel.
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Online Resources.
Teach yourself phase diagrams
Handout
http://www-g.eng.cam.ac.uk/mmg/teaching/phasediagrams/i2a.html
Scale of material structure
Wikipedia: Steel Production
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GLOSSARY
Smelting
Pig Iron
Basic Oxygen Process
Blast Furnace
Electric arc furnace
Ferrite
Cementite
Austenite
Pearlite
Eutectic
Eutectoid
UCT
LCT
Hypo eutectoid
Hyper eutectoid
Normalising
Annealing
Spheroidisation annealing
Work-hardened annealing
Brittle fracture transition temperature
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QUESTIONS
Moodle XML: Some questions in 10105 Steel
1. Define all the glossary terms.
2. Give at least 4 reasons why iron is by far the most important metal to man.
3. Explain how carbon atoms join the iron structure in equilibrium conditions of
solidification. Give the chemical name and the metallurgical name for this
structure. Is this structure substitutional, interstitial or intermetallic? Which is the
solute and solvent element? Is this non, complete or partial solubility?
4. Describe the cooling of a hypo-eutectoid iron-carbon mixture under equilibrium
conditions. What differences are there with a hyper-eutectoid steel?
5. In the Fe-C thermal equilibrium diagram, identify the a b g and d phases. Which
phases exist at room temperature. At what temperatures do the others exist?
Explain why the d phase gets very little mention.
6. What is the main difference in the process of normalising of a forging vs
annealing of a casting?
7. What is the main difference in the process of annealing rolled sheet vs
annealing of a casting?
8. Identify Ferrite, Cementite and Pearlite in photomicrographs.
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