Section 4. Steel
Did you know? Iron is abundant in the universe, found in the sun and many types of stars in considerable
quantity. The core of the earth is thought to be made up of nickel and iron, and is hotter than the Sun's
surface. This intense heat from the inner core causes material in the outer core and mantle to move around
(convection currents).
Carbon Steel
Steel is an alloy of iron (Fe) and carbon (C), with 0.2 to 2.04% carbon by weight. Carbon is the most
cost-effective alloying material for iron, but various other alloying elements are used such as manganese,
chromium, vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing
dislocations in the iron atom crystal lattice from sliding past one another.
Carbon Steel
ANSI def'n
General Def'n
Low carbon steel
0.05–0.15%
<0.1%
Mild Steel
0.16-0.29%
0.1-0.25%
Low tensile strength, but it is cheap and
malleable; surface hardness can be increased
through carburizing.
Medium carbon
steel
0.30–0.59%
0.25-0.45%
Balances ductility and strength and has good
wear resistance; used for large parts, forging and
automotive components.
High carbon steel
0.6–0.99%
0.45-1.0%
Very strong, used for springs and high-strength
wires.
1.0–2.0%
1.0-1.50%
(>1.5% rare)
Very hard - knives, punches. Usually anything
over 1.2% would be made with powder
metallurgy and is considered a high alloy carbon
steels.
–
2.5-4.0%
Ultra-high carbon
steel
Cast Iron
Applications and properties
Soft, ductile. Easy to form.
Lower melting point, easy casting, lower
toughness and strength than steel.
Varying the amount of alloying elements and the way they incorporated into the steel (solute elements,
precipitated phase) influences such properties as hardness, ductility and tensile strength of the resulting
steel. With increased carbon content steel becomes harder and stronger than iron, but also more brittle.
The maximum solubility of carbon in iron (in austenite region) is 2.14% by weight, occurring at 1149 °C;
higher concentrations of carbon or lower temperatures will produce cementite (very brittle). Add any
more carbon and you get cast iron, which has a lower melting point and is easier to cast.
Wrought iron containing only a very small amount of other elements, but contains 1–3% by weight of
slag in the form of particles elongated in one direction, giving the iron a characteristic grain. It is more
rust-resistant than steel and welds more easily. It is common today to talk about 'the iron and steel
industry' as if it were a single entity, but historically they were separate products.
Steel has been produced for thousands of years, but it became common after more efficient production
methods were devised in the 17th century. The Bessemer process in the mid 1800's made steel relatively
inexpensive for mass-produced goods. Further refinements in the process, such as basic oxygen
steelmaking, further lowered the cost of production while increasing the quality of the metal. Today, steel
is one of the most common materials in the world and is a major component in buildings, tools,
automobiles, and appliances.
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Hardening of Steel
Temper colours can be used as a guide to temperature. Alloys such as stainless steel form thinner films
than do carbon steels for a given temperature and hence produce a colour lower in the series. For
example, pale straw corresponds to 300°C for SS, instead of 230°C for CS.
Temper Colour
Temp °C
Examples applications
Pale straw
230
Planing and slotting tools
Dark straw
240
Milling cutters, drills
Brown
250
Taps, shear blades for metals
Brownish-purple
260
Punches, cups, snaps, twist drills, reamers
Purple
270
Press tools, axes
Dark purple
280
Cold chisels, setts for steel
Blue
300
Saws for wood, springs
Dark Blue
450-650
Toughening for constructional steels
VIDEO: Properties and Grain Structure. BBC 1973
(These are available on the network, but not on the website)
Part 1: What is a grain? (Video)
1. The patches seen on a galvanised object are crystals or grains of zinc.
2. All metals are made up of grains, but they are usually invisible (too small to see or same shine/colour).
3. Etching Process: Mirror finish, powerful acid, washed and sealed.
4. In a pure metal, the grains are different colours because of the way they reflect the light.
5. Tiny crystals grow outward until they meet. Each fully grown crystal is called a grain.
Part 2: Recrystallisation (Video)
1. Before cold working the grains are similar size and shape
2. Cold working elongates the grains, increases hardness and strength increases, reduces ductility.
4. At 350C, new grains form in the Al to replace old grains. Called recrystallisation
5. Recrystallisation softens, lowers the strength, ductility increased
6. Excessive recrystallisation temp gives poor mechanical properties
Part 3: Heat Treatment of Steel (Video)
1. Steel grains are too small to be visible - need a microscope approx 250 times magnification.
2. Ferrite: Light coloured. Made of iron. Ductility to the steel
3. Pearlite: darker coloured. Layers of Iron + Iron Carbide. Hardess and strength to the steel
4. 100% Pearlite is about 0.8%C. Pearlite, recrystallisation temperature 720C.
5. Normalising - cooled in air, reduced grain size and more uniform shape, toughness increased
6. Quenching - increases hardness. Not enough time for pearlite to form, so a needlelike strcture forms martensite. Very hard and brittle.
7. Tempering - (after quenching) restores toughness. Modifies the martensite needles with small flakes of
carbon. This gives hardness AND toughness.
8. 0.1%C steel (Mild Steel). Recrystalisation 900C. Not enough carbon to produce martensite.
Iron-Carbon Equilibrium Diagram
An Equilibrium diagram is a graph of the different structural arrangements that occur within a range of an
alloying element.
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This diagram shows how iron and carbon combines IF it is cooled slowly (in equilibrium). Under 2% is
steel, over 2% is cast iron. Cementite Fe3C has 6.67%C and it is basically a ceramic. The eutectoid
(pearlite) at E has 0.83% C, less carbon is a hypoeutectoid steel (A), and greater is hypereutectoid (B).
Alpha iron (ferrite), gamma iron (austenite, which only exists at high temperature), and delta iron
(another high temp structure).
Allotropic changes take place when there is a change in crystal lattice structure. From 1539º-1400ºC the
delta iron has a body-centered cubic lattice structure. At 1400ºC, the lattice changes from a body-centered
cubic to a face-centered cubic lattice type. At 210ºC, the curve shows a plateau but this does not signify
an allotropic change. It is called the Curie temperature, where the metal changes its magnetic properties.
Two very important phase changes take place at 0.83%C and at 4.3% C. At 0.83%C, the transformation is
eutectoid, called pearlite. These 2 phases separate out in layers.
gamma (austenite) --> alpha + Fe3C (cementite)
At 4.3% C and 2066ºF, the transformation is eutectic, called ledeburite.
L(liquid) --> gamma (austenite) + Fe3C (cementite)
This is way too much carbon for steel.
Alloy Steels
Effects of alloying elements on tool steel properties:
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Carbon: Raising carbon content increases hardness slightly and wear resistance considerably.
Dramatic increases to hardness & strength when heat treated.
Manganese: Small amounts of of Manganense reduce brittleness and improve forgeability. Larger
amounts of manganese improve hardenability, permit oil quenching, and reduce quenching
deformation.
Silicon: Improves strength, toughness, and shock resistance.
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
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Tungsten: Improves "hot hardness" - used in high-speed tool steel.
Vanadium: Refines carbide structure and improves forgeability, also improving hardness and wear
resistance.
Molybdenum: Improves deep hardening, toughness, and in larger amounts, "hot hardness". Used
in high speed tool steel because it's cheaper than tungsten.
Chromium: Improves hardenability, wear resistance and toughness.
Nickel: Improves toughness and wear resistance to a lesser degree.
Including these elements in varying combinations can act synergistically, increasing the effects of using
them alone.
Tool Steels
Tool steels are covered in Australian Standard AS1239 and is virtually the same as the American AISI
tool steel classification. (Similarly with British Standard 4659) For example: AS 1239 grade H13 hot
work tool steel containing 0.35% carbon, 5.0% chromium, 1.5% molybdenum and 1% vanadium would
be written as X40CrMoV51 in DIN (German).
High Speed Steels, for example: AS 1239 grade M2 Containing 0.85% carbon, 4.0% chromium, 5.0%
molybdenum, 6.0% tungsten, 2.0% vanadium would be written as S 6-5-2 in DIN.
Glossary
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Alloy: A metallic substance that is composed of two
or more elements.
Austenite: Face-centered cubic iron or an iron alloy
based on this structure.
Bainite: The product of the final transformation of
austenite decomposition.
Body-centered: A structure in which every atom is
surrounded by eight adjacent atoms, whether the atom
is located at a corner or at the center of a unit cell.
Cementite: The second phase formed when carbon is
in excess of the solubility limit.
Critical point: Point where the densities of liquid and
vapor become equal and the interface between the two
vanishes. Above this point, only one phase can exist.
Delta iron: The body-centered cubic phase which
results when austenite is no longer the most stable
form of iron. Exists between 2802 and 2552 degrees
F, has BCC lattice structure and is magnetic.
Eutectic: A eutectic system occurs when a liquid
phase tramsforms directly to a two-phase solid.
Eutectoid: A eutectoid system occurs when a singlephase solid transforms directly to a two-phase solid.
Face-centered: A structure in which there is an atom at
the corner of each unit cell and one in the center of
each face, but no atom in the center of the cube.
Ferrite: Body-centered cubic iron or an iron alloy
based on this structure.
Fine pearlite:Results from thin lamellae when cooling
rates are accelerated and diffusion is limited to shorter
distances.
Hypereutectoid: Hypereutectoid systems exist below
the eutectoid temperature.
Hypoeutectoid: Hypoeutectoid systems exist above
the eutectoid temperature.
15. Ledeburite: Eutectic of cast iron. It exists when the
carbon content is greater than 2 percent. It contains
4.3 percent carbon in combination with iron.
16. Liquidus Line: On a binary phase diagram, that line or
boundary separating liquid and liquid + solid phase
regions. For an alloy, the liquidus temperature is that
temperature at which a solid phase first forms under
conditions of equilibrium cooling.
17. Martensite: An unstable polymorphic phase of iron
which forms at temperatures below the eutectoid
because the face-centered cubic structure of austenite
becomes unstable. It changes spontaneously to a bodycentered structure by shearing action, not diffusion.
18. Microstructure: Structure of the phases in a material.
Can only be seen with an optical or electron
mircoscope.
19. Pearlite: A lamellar mixture of ferrite and carbide
formed by decomposing austenite of eutectoid
composition.
20. Phase: A homogeneous portion of a system that has
uniform physical and chemical characteristics.
21. Phase diagram: A graphical representation of the
relationships between environmental constraints,
composition, and regions of phase stability, ordinarily
under conditions of equilibrium.
22. Polymorphic: The ability of a solid material to exist in
more than one form or crystal structure.
23. Quench: To rapidly cool
24. Solidus Line: On a phase diagram, the locus of points
at which solidification is complete upon equilibrium
cooling, or at which melting begins upon equilibrium
heating.
25. Solubility: The amount of substance that will dissolve
in a given amount of another substance.
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