MSE-226 Engineering

Materials

Lecture-2

‘’IRON-IRON CARBIDE PHASE DIAGRAM’’

2

Classification of Metal Alloys

Metal Alloys

Steels

Ferrous Nonferrous

Cast Irons

<1.4wt%C 3-4.5 wt%C

Steels <1.4 wt% C

Cast Irons 3-4.5 wt% C

Fe 3 C

cementite

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

austenite

g +L

g +Fe3C a

ferrite a +Fe3C

L+Fe3C

d

(Fe) Co , wt% C

Eutectic:

Eutectoid: 0.76

4.30

727ºC

1148ºC

T(ºC)

PURE IRON

Carbon content < 0.1 % PURE IRON, Tm=1535oC

Iron is an allotrophic metal, which means that it can exist in more than

one type of lattice structure depending on the temperature

• Pure iron (ingot) is ductile and malleable.

• Small additions of other elements (C, Mn, Mo, Cr, Ni, etc.) enhance the

properties significantly

STEEL

STEELS

Steels are iron-carbon alloys that may contain appreciable

concentrations of other alloying elements,i.e. Cr, Mo, Co, W.

Steels are classified according to carbon content as follows;

1- Low carbon steels (<0.25 wt.% carbon)

2-Medium carbon steels (0.25 < wt%C < 0.6)

3-High carbon steels (0.6 < wt%C < 1.4)

Subclasses also exist within each group according concentration of

other alloying elements;

1- Plain carbon steels: Contain only residual concentrations of impurities

other than carbon and a little Mn

2-Alloy steels: More alloying elements are intentionally added

Examples:

SAE 1020 steel: Plain carbon steel, low carbon steel

SAE 1080 steel: Plain carbon steel, high carbon steel

SAE 4340 steel: Alloy steel, medium carbon steel

Classification of STEELS

1- Low carbon steels (<0.25 wt.% carbon)

Properties: Relatively soft and weak but have outstanding ductility and

toughness.

Machinable and weldable

Typical applications: automobile body components, structural shapes

(I-beams), sheets used in pipelines, buildings,

2-Medium carbon steels (0.25 < wt%C < 0.6)

Properties: Can be heat treated to improve mechanical properties. Heat treated

alloys are stronger than low carbon steels but sacrifice of ductility and

toughness.

Typical applications: Railways wheels and tracks, gears, crankshafts and

structural applications required combination of high strength, wear resistance

and toughness.

3-High carbon steels (0.6 < wt%C < 1.4)

Properties: Hardest, strongest and yet least ductile class of steels. They are

always used in hardened and tempered condition.

Tool and die steels are high carbon steels which contain Cr, V, W and Mo as

alloying elements. Alloying elements form very hard and wear resistance

carbides (Cr23C6, V4C3 and WC)

Applications: Drills, saw, lathe and planer tools

Properties and Applications of STEELS

Iron-Iron Carbide Equilibrium Phase Diagram

Phases and phase mixtures present in iron alloys;

Ferrite (α); Cementite (Fe3C); Pearlite (ferrite + cementite); Austenite (γ); d-ferrite;

Ledeburite (austenite + cementite)

Fe 3 C

cementite

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

austenite

g +L

g +Fe3C

a

ferrite

a +Fe3C

L+Fe3C

d

(Fe) Co , wt% C

Eutectic:

Eutectoid:

0.76

4.30

727ºC

1148ºC

T(ºC)

Definition and Properties of Phases

1) Ferrite : a-iron, Solid Solution, max. Carbon solubility 0.022%wt. at 727oC

BCC structure, SOFT

2) Cementite : Iron carbide(Fe3C), contains 6.67% wt. C

Orthorhombic structure, HARD and BRITTLE

3) Pearlite : Phase mixture (ferrite+cementite), Lamellar structure, contains ~0.8% wt. C

Produced from austenite decomposition

4) Austenite : g-iron, Solid solution, stable at higher temperatures (>727oC)

Max. Carbon solubility is 2.14%wt. at 1147oC, FCC structure

HIGH TOUGHNESS

5) Ledeburite: Eutectic phase mixture(austenite+Fe3C), seen in cast irons

Contains 4.3 %wt. Carbon, forms at 1147oC

6) d-ferrite : Solid solution, max. carbon solubility is 0.1%wt. At 1493oC

Invariant reactions in Fe-Fe3C Phase diagram

1) At 1493oC, 0.18 %wt C (PERITECTIC REACTION)

Liquid(l, 0.5%C)+d-ferrite(d,0.1%C) Austenite(g, 0.18%C) cooling

heating

2) At 1147oC, 4.30 %wt C (EUTECTIC REACTION)

Liquid(l, 4.30 %C) Austenite(g, 2.14 %C) + Cementite(Fe3C,6.67%C) cooling

heating

3) At 727oC, 0.77 %wt C (EUTECTOID REACTION)

Austenite(g, 0.77 %C) Ferrite(a, 0.022 %C) + Cementite(Fe3C,6.67%C) cooling

heating

PEARLITE

LEDEBURITE

Carbon solubility in BCC-ferrite and FCC-Austenite

FCC

BCC

Octahedral sites Tetrahedral sites

• Teutectoid changes: • Ceutectoid changes:

Alloying steel with alloying elements

Eutectoid Steel

Transformation of Austenite to pearlite in Eutectoid Steel

under equilibrium conditions (very slow cooling or annealing)

Upon very slow cooling transformation of austenite to pearlite occurs by diffusion

of carbon atoms(time is required for carbon diffusion). So, this type of

transformation is called DIFFUSIONAL (Time Dependent) TRANSFORMATION.

Microstructure of eutectoid steel (~0.77wt.%C)

Pearlite

(ferrite and cementite)

Hypoeutectoid Steel

Microstructure of Hypoeutectic Steel (~0.3wt.%C)

X200

X500

Primary ferrite

Pearlite

(ferrite and cementite layers

cannot be resolved)

Hypereutectoid Steel

Microstructure of Hypereutectoid steel(1.4 wt%C)

Primary

cementite Pearlite

(cementite+ferrite)

! ! ! The microstructure of annealed hypereutectoid steel consists of coarse

lamellar pearlite areas surrounded by a network of proeutectoid cementite.

Because this proeutectic cementite network is brittle and tends to be a plane of

weakness, annealing should never be a final heat treatment for hypereutectoid

steels.

The mechanical properties of an alloy depend upon the properties of the

phases and the way in which these phases are arranged to make up the

structure.

In steel

ferrite is relatively soft with low tensile strength

While cementite is hard with very low tensile strength

Pearlite (ferrite + cementite) greater tensile strength than that of ferrite and

cementite.

The approximate tensile strength of annealed hypoeutectoid steels may

be determined by the proportion of ferrite and pearlite present:

Approximate tensile strength (psi) = [40,000 (wt.%ferrite) +120,000 (wt.% pearlite)] / 100

Microstructure-Property Relationship of Steels

Effect of Carbon on Mechanical Properties

HOMEWORK-I

(1) For an annealed (cooled in equilibrium conditions after austenitization) hypoeutectoid

steel;

(i) Determine the composition of the steel (C wt.%) if it contains 22% secondary ferrite

at room temperature.

(ii) Determine the amount of primary phase present in the steel.

(iii) Draw the room temperature microstructure.

(2) Why hypereutectoid steels shouldn’t be annealed?