Material Properties: STRENGTH, HARDNESS, DUCTILITY, TOUGHNESS
Polymorphic
CAST STEELS
(Hard)
Soft
L - Liquid solution of carbon in iron;
1040 = .4% Carbon
δ-ferrite Crystal structure is BCC (cubic body centered). Exists at room temperature. Soft and
Ductile.
Austenite – Crystall structure is FCC (cubic face centered). Austenite does not exist below
1333 ºF (723ºC) and maximum carbon concentration at this temperature is 0.83%.
Cementite – iron carbide, hard and brittle intermetallic compound, having fixed composition
Fe3C.
Critical temperatures
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Upper critical temperature (point) A3 is the temperature, below which ferrite
starts to form as a result of ejection from austenite in the hypoeutectoid alloys.
Upper critical temperature (point) ACM is the temperature, below which cementite
starts to form as a result of ejection from austenite in the hypereutectoid alloys.
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
Lower critical temperature (point) A1 is the temperature of the austenite-topearlite eutectoid transformation. Below this temperature austenite does not exist.

Magnetic transformation temperature A2 is the temperature below which α-ferrite is
ferromagnetic.
Eutectoid steel (carbon content 0.83%) entirely consists of pearlite. Eutectic system is a

mixture of elements that have a single chemical composition that solidifies at a lower temperature than any other
composition made up of the same ingredients. This composition is known as the eutectic composition and the
temperature at which it solidifies is known as the eutectic temperature.
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TTT Diagram
1. Slow Cooling > Carbon atoms diffusing. Results: soft but not ductile.
2. Faster Cooling > Higher strength (more pearlite).
3. Rapid Cooling > Hold at 300OC (Nose missed) Fine pearlite.
4. Cooling misses nose entirely > Room temperature (Hard & Brittle).
5. Ductility Restored >> Strength & Hardness decreased with increased ductility & toughness.
6. Nose crossed >> Hard
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Dendrite Crystal
Crystals As molten steel begins to cool, the lattice structures
begin to attach to one another and form crystal skeletons
called dendrites, which resemble the look of a pine tree.
Dendrites start to form everywhere, and each one continues to
grow independently. This is the beginning of the transformation
to a solid state.
Grains Dendrites grow to a point
where they begin to bump into one
another, and eventually, they can
not grow any larger because of
being restricted by other dendrites
around them. At that point, the
dendrite has reached its maximum
size and is referred to as a grain.
Grain boundaries are interfaces where crystals of different orientations meet.
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Grain size greatly affects the toughness of
steel. The smaller the grain size, the more
ductile the steel. See Picture 3.
Cooling rate If molten steel is cooled slowly,
dendrites have a longer time to grow before
they begin to bump into neighboring
dendrites. Thus, a large grain size is formed.
When steel is cooled quickly, the number of
dendrites that begin growing increases, and
the dendrites form quickly and begin to be
restricted by other dendrites. Smaller grain
size is the result.
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