Introduction to Engineering Materials

ENGR2000

Chapter 9: Phase Diagrams II

Dr. Coates

The Iron-Carbon System

• Ferrous alloys (iron + C & other elements)

– Iron: less than 0.008 wt. % C

– Steel: between 0.008 and 2.14 wt. % C

– Cast iron: between 2.14 and 6.70 wt. % C

Terminology for microstructures in

Fe-C alloys

9.l7 The Iron-Iron Carbide Phase Diagram

Eutectoid

reaction

Peritectic

reaction Eutectic

reaction

Properties of the different phases

• α-ferrite is relatively soft

• δ-ferrite is very similar but is stable over a higher

range of temperatures.

• γ-austenite is non-magnetic

• Cementite is hard & brittle

Eutectoid

reaction

( ) ( ) ( )CwtCFeCwtCwt .%7.6.%022.0.%76.0

:reaction Eutectoid

3+⇔ αγ

CwtC %76.0

:ncompositio Eutectoid

0 =

pearlite

Eutectoid alloys

CwtC %76.0

:ncompositio eutectoid-Hypo

0 <

Hypo-eutectoid alloys

Eutectoid ferrite and Pro-eutectoid ferrite

• α-ferrite is present in the pearlite (eutectoid

ferrite) and

• Is formed above the eutectoid temperature as well

(pro-eutectoid ferrite)

022.076.0

76.0

: idproeutecto of fraction) (massamount Phase

022.076.0

022.0

:pearlite of fraction) (massamount Phase

0

0

−

−=

−

−=

CW

CW

idproeutecto

pearlite

α

α

CwtCCwt %14.2%76.0

:ncompositio eutectoid-Hyper

1 <<

Hyper-eutectoid alloys 10-18-10

76.070.6

76.0

:C idproeutecto of fraction) (massamount Phase

76.070.6

70.6

:pearlite of fraction) (massamount Phase

1

3

1

3 −

−=

−

−=

CW

Fe

CW

CFeidproeutecto

pearlite

Example 9.4

• For a 99.65 wt. % Fe-0.35 wt. % C alloy at a

temperature just below the eutectoid, determine

the following:

(a) Fractions of total ferrite and cementite phases

(b) Fractions of the proeutectoid ferrite and pearlite

(c) Fraction of eutectoid ferrite

Fractions of total ferrite and cementite phases

Cwt.%35.0C

:ncompositioalloy Given

0 =

05.0022.070.6

022.035.0

:cementite totalofFraction

95.0022.070.6

35.070.6

:ferrite totalofFraction

3=

−

−=

=−

−=

CFeW

Wα

Fractions of the proeutectoid ferrite & pearlite

44.0022.076.0

022.035.0

:pearlite ofFraction

56.0022.076.0

35.076.0

:ferrite idproeutecto ofFraction

=−

−=

=−

−=

pearlite

idproeutecto

W

W α Remember the γ becomes

eutectoid pearlite

Fraction of eutectoid ferrite

39.056.095.0

:ferrite eutectoid ofFraction

=−=

=+

α

ααα

eutectoid

totaleutectoididproeutecto

W

WWW

The eutectoid α will be 89% of the pearlite that is present at ANY

composition

89.0022.070.6

76.070.6=

−

−=αeutectoidW

:reaction eutectoid the from ferrite eutectoid of Fraction

39.044.089.089.0 =×=×=

=

pearliteeutectoid WW

ferrite eutectoid of fraction The

α

At a composition of 0.35%, the amount of pearlite present is 44%,

therefore:

Fraction of eutectoid ferrite

(another method)

Problems

9.12 A 50 wt% Pb-50 wt% Mg alloy is slowly cooled from 700°C (1290°F) to 400°C (750°F).

(a) At what temperature does the first solid phase form?

(b) What is the composition of this solid phase?

(c) At what temperature does the liquid solidify?

(d) What is the composition of this last remaining liquid phase?

Solution in

class!

9.33 The microstructure of a lead-tin alloy at 180°C (355°F) consists of primary β

and eutectic structures. If the mass fractions of these two microconstituents are

0.57 and 0.43, respectively, determine the composition of the alloy.

Solution

in class!

9.51 Consider 2.5 kg of austenite containing 0.65 wt% C, cooled to below 727°C (1341°F).

(a) What is the proeutectoid phase?

(b) How many kilograms each of total ferrite and cementite form?

(c) How many kilograms each of pearlite and the proeutectoid phase form?

(d) Schematically sketch and label the resulting microstructure.

The influence of other alloying elements