Phase Transformation: Lecture Review of Phase Diagrams

MM-501 Phase Transformation in
Solids
Fall Semester-2015
Engr. Muhammad Ali Siddiqui
Lecturer,
Metallurgical Engineering Department,
NED UET
BE: Mehran UET, 2007
ME: NED UET, 2011

Important Instruction
• Class as per schedule Inshallah;
should maintain 75% attendance.
• LECTURES: soft copy can be taken after the
class or from my Office.
• Make your OWN NOTES and read the books
for in-depth understanding.
2

Books for Understanding the subject &
for Examination Preparation
1) Phase Transformations in Metals and Alloys,
Porter and Easterling, 2nd ed
(Library call No 669 POR)
Chapters, 1,2,3,5 and 6
2) Steels Microstructure and Properties,
HKDH Bhadeshia
(Library call No 669.142 BHA)
chapters 5, 6 & 10
3

Sessional Marks Distribution…..40Nos
• 02 Test …………………………………………………………..20 Nos
• 01 Assignment……………………………………………….. 10 Nos
• Assignment Presentation/Viva……………………......05 Nos
• Attendance………………………………………………………05 Nos
Important Dates
1. Test No:01  Sep 17,2015
2. Test No:02  Nov 5 or 12, 2015
3. Session Assignment  Oct 1, 2015 (Last Date)
4. Viva /presentation  on Last Lecture day. (Expected
Nov 19,2015)…………..Inshallah
Type of Test:
• MCQs/BCQs/
Descriptive/Problems
4

Assignment Criteria
• In-Time Submission
• Late Submission  will cost -1 marks/day and there will be
seven days
Style and Format:
• No: of pages > 30 but < 45, at least 25 pages contain full text.
• A-4 size paper, Typed in a single space,
• 1” Margin all around the text,
• Font  Time New Roman #12 for text and 12Bold sub heading,
14 Main heading,
• Reference  Harvard University Style.
• All figures and Tables must be captioned and discussed in the
text. 5

Topics for Session Assignment and
Presentation
Topic 1: Diffusionless Phase Transformation-I
Diffusionless Character, Nucleation and Growth of
Martensite, The Habit Plane, Orientation Relationships.,
Athermal Nature of Transformation.
Topic 2: Diffusionless Phase Transformation-II
Structure of the Interface b/w γ & α’, The Shape Deformation,
Bain Distortion Model, Phenomenological Theory of
Martensite, Morphology and crystallography of α’ (bcc or bct).
6

• Topic 3: Reconstructive Phase Transformation
Austenite to Allotriomorphic ferrite, Idomorphic ferrite,
Massive ferrite and Pearlite transformation.
• Topic 4: Displacive Phase Transformation
Bainite (upper & lower), Acicular Ferrite and
widmanstatten ferrite transformation.
• Topic 5: Precipitation and Age Hardening
Phenomena
Strengthening of Non Ferrous Alloy. E.g Al, Cu and Mg
• Topic 6: Solid Solution and Phase Diagram
Solid solution, Binary and Ternary phase diagram 7

Introduction Lecture
1. Iron - Phase Diagram.
2. Effect of Alloying Elements on Fe-Fe3C Diagram.
3. TTT Diagrams. (Construction )
8

Phases Observed in Fe-C Diagram
1. Ferrite
Ferrite is the interstitial solid solution of carbon in alpha iron. It has B.C.C.
Structure. It has very limited solubility for carbon (maximum 0.022%/0.025 at
727°C and 0.008% at room temperature). Ferrite is soft and ductile.
2. Austenite
Austenite is the interstitial solid solution of carbon in gamma (γ) iron. It has
FCC structure. Austenite can have maximum 2.14% carbon at 1143°C.
Austenite is normally not stable at room temperature. Austenite is non-
magnetic and soft.
3. Cementite
Cementite or iron carbide (Fe3C) is an intermetallic compound of iron and
carbon. It contains 6.67% carbon. It is very hard and brittle. This intermetallic
compound is a metastable phase and it remains as a compound indefinitely at
room temperature.
4. δ-ferrite
It is a solid solution of carbon in δ-iron. It is stable at high temperatures. It has
BCC structure.
10

Phase Mixtures Observed in Fe-C
Diagram
• 1. Pearlite
The pearlite consists of alternate layers of ferrite
and cementite. It has properties somewhere
between ferrite and cementite. The average
carbon content in pearlite is 0.76%
• 2. Ledeburite
Ledeburite is an eutetcic mixture of austenite and
cementite in the form of alternate layers. The
average carbon content in ledeburite is 4.3%.
11

• If alloying elements are added to the iron-carbon
alloy (steel), the position of the A1, A3, and Acm
boundaries and the eutectoid composition are
changed.
1) all important alloying elements decrease the
eutectoid carbon content,
2) the austenite-stabilizing elements manganese
and nickel decrease A1, and
3) the ferrite-stabilizing elements chromium,
silicon, molybdenum, and tungsten increase A1.
The Effects of Alloying Elements on
Iron-Carbon Alloys
12

Effect on the Eutectoid Point
13
Changes eutectoid Temperature Changes eutectoid Composition

Effect of alloying additions on the γ-phase field:
(a) Mn;
14

(b) Mo;
15

(c) Cr;
16

(d) Ti
17

The Effects of Alloying Elements on
Iron-Carbon Alloys
• For analyzes the effects of alloying elements on iron-carbon
alloys would require analysis of a large number of ternary
alloy diagrams over a wide temperature range.
• “Wever” pointed out that iron binary equilibrium systems fall
into four main categories: open and closed γ-field systems,
and expanded and contracted γ-field systems.
• This approach indicates that alloying elements can influence
the equilibrium diagram in two ways:
 by expanding the γ-field, and encouraging the formation of
austenite over wider compositional limits. These elements
are called γ-stabilizers.
 by contracting the γ-field, and encouraging the formation of
ferrite over wider compositional limits. These elements are
called α-stabilizers. 18

Fig. 1 Classification of
iron alloy phase
diagrams:
(a)open –field;
(b)expanded –field;
(c)closed –field;
(d)contracted –field
19

Open γ-field
• This group belong the important steel alloying elements nickel
and manganese, as well as cobalt and the inert metals
ruthenium, rhodium, palladium, osmium, iridium and
platinum.
• Both nickel and manganese, if added in sufficiently high
concentration, completely eliminate the bcc α-iron phase
and replace it, down to room temperature, with the γ-phase.
• So nickel and manganese depress the phase transformation
from γ to α to lower temperatures , i.e. both Ac1 and Ac3 are
lowered.
• It is also easier to obtain metastable austenite by quenching
from the γ-region to room temperature, consequently nickel
and manganese are useful elements in the formulation of
austenitic steels. 20

Closed γ-field
• Many elements restrict the formation of γ-iron, causing the
γ-area of the diagram to contract to a small area referred to
as the gamma loop.
• This means that the relevant elements are encouraging the
formation of bcc iron (ferrite), and one result is that the δ-
and γ-phase fields become continuous.
• Alloys in which this has taken place are, therefore, not
agreeable to the normal heat treatments involving cooling
through the γ/α-phase transformation.
• Silicon, aluminium, beryllium and phosphorus fall into this
category, together with the strong carbide forming
elements, titanium, vanadium, molybdenum and
chromium.
23

Expanded γ-field
• Carbon and nitrogen are the most important
elements in this group.
• The γ-phase field is expanded, but its range of
existence is cut short by compound formation.
• Copper, zinc and gold have a similar influence.
28

contracted y-field
• Boron is the most significant element of this
group, together with the carbide forming
elements tantalum, niobium and zirconium.
• The γ-loop is strongly contracted, but is
accompanied by compound formation.
31

The Effect on the Formation and
Stability of Carbides
Some
alloying
elements
form very
stable
carbides
when
added to
steel (Fig.
13.2)
33

Alloy Carbides
The periodic table showing the positions of strong
carbide-forming elements
34

• In steel six kinds of carbides can be formed as shown in Table 1, where M
denotes a sum of carbide-forming (metal) elements.
• The carbides placed in group I posses a complicated crystal structure; an
examples is cementite (Fe3C), or Cr23C6.
• A specific structural feature of the carbides of group II as interstitial
phases is a simple crystal lattice (e.g., TiC, WC, NbC and Mo2C).
35

Isothermal Transformation (I.T) /
Time Temperature Transformation (TTT)
Diagram
Today we will discuss about its Construction
36
What is TTT?
 T (Time) T (Temperature) T (Transformation) diagram is a plot
of temperature versus the logarithm of time for a steel of
definite composition.
 It is used to determine when transformations begin and end for
an isothermal (constant temperature) heat treatment of a
previously austenitized alloy.

Step: 4 After Cooling, check sample hardness
and microstructure.
38

39
• Step 2,3 and 4 are shown in the following Figure
• If we draw a curve b/w
%age of Pearlite Transformed and Time

41
Step: 5 all steps are repeated at different sub-critical
temperature below Ac1 in order to get sufficient points for
plotting the curve.

44
Composition of
AISI 1080 Steel
C= 0.75-0.88,
Mn= 0.60-0.90 ,
P= 0.040,
S= 0.050
Fe= balance

47
Example: Effect of Alloying Elements

48
 A cooling curve is
determined experimentally
by placing a thermocouple
at a definite location in a
steel sample and then
measuring the variation of
temperature with time.
 Since the coordinates of
the I-T diagram are the
same as those for a
cooling curve,
 it is possible to
superimpose various
cooling curves on the I-T
diagram. This is shown in
the figure given below.
Cooling Curves and the I-T Diagram

Re: Why is austenitic iron non-magnetic, but ferritic iron is?
• It's not actually the Austenite phase that causes the loss of magnetic properties of Iron, it's
the temperature of the iron itself.
• Magnetism in iron is believed to be caused by the alignment of the spins of the electrons
located in the third d shell of each atom. Each atom has a magnetic dipole moment, and in
ferromagnetic materials, zones of atoms with similarly aligned moments tend to form
throughout an object. Normally, there are enough different zones with differently oriented
magnetic directions to cancel out any net magnetic field. But when you apply an external
magnetic field to a piece of iron, the zones that are aligned with the field grow and the ones
that are not shrink. This causes a net magnetic field in the iron, and the iron piece is attracted
to the magnet. However, under certain parameters such as rubbing the iron on a magnet,
applying a strong enough magnetic field, or giving a few really hard whacks (highly scientific
term) on another iron object, the magnetic zones will align and cause a net magnetic field,
without needing to apply an external field.
• Ok, that's cool, but how does temperature affect magnetism? All ferromagnetic materials
have a Curie Temperature, above which, the material shows negligible magnetic properties.
This occurs because above the Curie Temperature, the thermal energy in the material is so
high that the atoms wiggle around quite a bit more, and it's harder for the zones to form. The
Curie temperature for iron is 1043 K. This is close to the temperature at which the Austenite
(gamma) phase of iron is stable (1183 to 1673 K) but the two properties are not related.
• In order to demonstrate this, you could heat a sample of ferritic (alpha) phase iron to about
1100 K, and check and see if it's magnetic. You could also heat a sample of iron to 1200-1600
K, quench it really quickly down to room temperaturre to keep it in the Austenitic phase, and
then check to see if it's magnetic as well. 51

Phase Transformation: Lecture Review of Phase Diagrams

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Phase Transformation: Lecture Review of Phase Diagrams