enmat101a engineering materials and processes · converting pig iron to steel is done by oxidation...

TAFE NSW -Technical and Further Education Commission

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ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies)

Lecture 11 – Iron and steel

wikipedia


Iron and steel

EMMAT101A Engineering Materials and Processes

Reference Text Section

Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann

Ch 11

Additional Readings Section


Iron and steel


Note: This lecture closely follows text (Higgins Ch11)


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


Iron and steel: Intro (Higgins 11.1)


China now

dominates steel

production – almost

half the world’s

production!


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

http://www.youtube.com/watch?v=9l7JqonyoKA

http://www.youtube.com/watch?v=9l7JqonyoKA

videos/STEEL_From-Start-to-Finish.mp4

http://www.youtube.com/watch?v=Ea_7Rnd8BTM



videos/Steelmaking.mp4


videos/Steelmaking_Continuous-Casting.mp4







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 blast-

furnace releases about 6600

tonnes of carbon dioxide

every day.

Hebei province accounts for a quarter of the China's

total steel production capacity


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.

Corus Steel (UK)

Description of steel

making processes

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.




Basic Oxygen Process.

Higgins




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.


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

http://www.sciencedirect.com/science/journal/13811169


Carbon in Steel




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

images/FC_Equilibrium_Diagram_2000.gif



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.


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

watlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures.html


Eutectoid




Iron Carbon Equilibrium Diagram

Follow Higgins notes 11.5.1

Handout Teach yourself phase diagrams

http://www-g.eng.cam.ac.uk/mmg/teaching/phasediagrams/i2a.html


docs/cambridge-handout.pdf






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.


Hyper Eutectic



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.


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



Larger version

Normalising (Higgins 11.6.1)

images/FC_Equilibrium_Diagram_2000.gif


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.




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 cold-

drawing.




Type 3: Annealing of cold-worked steel

Recrystallisation of distorted ferrite grains to restore ductility (e.g. to allow

further cold working processes).




Summary of ranges on the Fe-C diagram.


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.



Handout

Wikipedia: Steel Production

Online Resources.

Scale of material structure

Teach yourself phase diagrams



http://en.wikipedia.org/wiki/Steel

http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=1.15

docs/cambridge-handout.pdf






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


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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