trip steels.ppt

Fundamentals of Advanced Materials
17th of Februaryr 2004 1
LECTURE 4
Exercise: How to make your own alloy?
Lecturers: Dr. ir. Pedro Rivera
Prof. dr. ir. Sybrand van der Zwaag
New Building, office 1.36
p.rivera@lr.tudelft.nl, phone 00 3115 2784559

Objective of this lecture:
Provide critical examples of alloy design.
TRIP steel and TRIP-Ti alloys.

LECTURE OUTLINE
1. The design of P TRIP steel.
2. The design of Ti alloys with TRIP effect.
3. Review of student projects and setting timetable.

LECTURE OUTLINE

P Influences
Reviewed in Lecture 2
1. For avoiding phosphide precipitation, P content should be
lower than 0.3%.
2. For casting limitations and segregation, P content should
be lower than 0.3%.
3. From the peritectic reaction, segregation is minimised if P
content is APPROXIMATELY larger than 0.25%.
4. P content should be as large

Principles of P Alloy Optimisation
1. The amount of plasticity depends on the volume of
martensite induced and the magnitude of the martensite
transformation strain.
2. Residual austenite is no more than 4 vol%.
3. The formation of ferrite and bainite enriches or depletes
alloying elements in the remaining austenite, improving its
stability (chemical stabilization).
4. The formation also decreases austenite grain size (grain
size stabilization).

Non-equilibrium regimes
1. LE: Local equilibrium or orthoequilibrium. All alloying
elements redistribute.
2. PE: Paraequilibrium. Only interstitial atoms redistribute.
3. NE: Non-equilibrium. No atom redistribution between
matrix and product (same chemical composition).
PE
LE
NE
NPLE
XE
NPLE=non partition
local equilibrium

Phase Transformations in TRIP steels
1. It is required to know if the ferrite decomposition is under
LE or NPLE.
2. Bainitic decomposition is assumed to be NPLE.
3. Martensitic decomposition is under NE.

Ferrite decomposition (1/3)
1. Ferrite increases carbon
concentration in austenite.
Why we want this?
2. It decreases grain size of
residual austenite.
800
1000
1200
1400
1600
1800
0.0 0.5 1.0 1.5 2.0
C, mass %
T,
K
a + q
g
g + q
g + a
liquid
d + g
d
liquid + g
A
B
E
J
H
N
G
S K
P
Q
C
(a)
Fe-C-1.5Mn-0.4Si

1. Ferrite increases
carbon
concentration in
austenite. Why we
want this?
2. It decreases grain
size of residual
austenite.
0
20
40
60
80
950 1000 1050 1100 1150
T, K
Ferrite,
mass
%
0.25P
0.05P
0.15P
(a)

0
20
40
60
80
100
0 0.2 0.4 0.6 0.8
P, mass %
Ferrite,
mass
%
(b)
Due to phosphide
Due to cementite
Max. ferrite amount
If the combined effects
of phosphide and
cementite formation are
accounted, it is required
that P0.25

C in austenite
0.2
0.3
0.4
0.5
0.6
0.7
0.8
950 1000 1050 1100 1150
T, K
C,
mass
%
0.25P
0.05P
0.15P
(a)
Intercritical annealing temperature -------------------------
C enrichment chemically
stabilises metastable
austenite as it decreases
Ms temperature.
P increases carbon
solubility in austenite

Si in Austenite and Ferrite
0.4
0.42
0.44
0.46
0.48
0.5
920 970 1020 1070 1120
T, K
Si
in
ferrite,
mass
%
0.3
0.32
0.34
0.36
0.38
0.4
Si
in
austenite,
mass
%
in austenite
in ferrite
0.25P
0.15P
0.05P
(b)
P redistributes Si. This slows
down the kinetics of
cementite formation.
The intercritical annealing
temperature should be
lower than 1030 K.

Mn in Austenite and Ferrite
0.72
0.76
0.8
0.84
920 970 1020 1070 1120
T, K
Mn
in
ferrite,
mass
%
1.5
2.5
3.5
4.5
Mn
in
austenite,
mass
%
austenite
ferrite
0.25P
0.15P
0.05P
(c)
The intercritical temperature
has to be in the range
970<T<1030 for maximising
the partitioning of Mn, and
thus improve the
metastability of austenite by
lowering the Ms.

P in Austenite and Ferrite
0.00
0.10
0.20
0.30
0.40
0.50
0.60
950 1000 1050 1100 1150
T, K
P
in
ferrite,
mass
%
0.00
0.05
0.10
0.15
0.20
0.25
P
in
austenite,
mass
%
0.25P
0.15P
0.05P
ferrite
austenite
(d)
P concentration increases P
content in austenite and
ferrite.
Intercritical T increases
austenite P content,
stabilising metastable
austenite.
T must be maximised.

Influence of Metastability
20
40
60
80
100
950 975 1000 1025 1050
T, K
Ferrite,
mass
%
0.2
0.4
0.6
0.8
1
C
in
austenite,
mass
%
Fe-0.2C-0.15P-1.5Mn-0.4Si
full equilibrium
paraequilibrium
The choice of PE or NPLE
makes no significant
difference in the results.
At T>1000 K C
concentration and amount
of ferrite are slightly
overestimated.

CONCLUSIONS REACHED SO FAR:
• Annealing temperature between 1030 and 1040 K
• P content between 0.15 and 0.25 wt%

Bainitic decomposition (1/4)
• Austenite grain size reduction
• Possible cementite precipitation (retained austenite C decrease)
• C composition of the Retained Austenite

• Austenite grain size
reduction
Can be determined from
previous experimental work.
Is essential for
micromechanical behaviour.
Grains should be of a lower
size.

• Possible cementite precipitation (retained austenite C
decrease)
No prior experience on this regard
It is undesirable
Hot deformation at high temperature recommended (e.g.
in Si-Mn TRIP steels)

Bainite and austenite
compositions can be
determined by
extrapolation in the
appropriate metastable
phase diagram.
From this composition
of martensite may be
determined.
Carbon concentration
Temperature
austenite
austenite
+
ferrite
G
S
P
Q P1 P2
ferrite
/
a
a
x q
/
a
a
x g
/ a
xg
g
T

Martensitic Decomposition of Austenite
• Ultimately,
austenitemartensite
transformation is wanted.
• Ms can be related to the critical
temperature T0 at which ferrite
and martensite have the same
Gibbs energy.
• Using this concepts, for the
calculated austenite
composition, Fe-1.29C-0.12P-
0.6Si-2.29Mn, Ms ranges
between 252 to 292 K
depending on grain size.
840
844
848
852
856
0 0.1 0.2 0.3 0.4 0.5
P, mass %
T
0
,
K

LECTURE OUTLINE

Ti-Systems with TRIP Effect
How can the TRIP effect be achieved in Ti-alloys?
1. Take the alloy to the +a phase.
2. Quench to room temperature retaining  phase.
3. Stress induce martensitic transformation at room
temperature.

Motivation

What is required for TRIP in Ti-
Alloys?
1. Have a system with a+ high temperature phase.
2. Have the ability to retain  phase at room temperature.
3. Control the Ms temperature around room temperature.
4. Avoid (control) undesired precipitate phases.
5. Control retained phases hardness at room temperature.

Some Previous Work (1/5)
1. TRIP effect has been reported in some Ti-alloys:
Ti-Al-Sn, Ti-V-Fe-Al, Ti-Ta, Ti-Mo
2. The conditions for the presence of martensite have been
recorded.

3. Some phase diagrams and heat treatment schemes have
been determined.

4. Some mechanical properties have already been recorded.
Ti-40wt% Ta

Ti-40wt% Ta

LECTURE OUTLINE

Evaluation Method and Project
Weighted evaluation measuring three aspects:
- Understanding of the lectures
and reading material
- Knowledge of the theory
- Skills for alloy design  Aid for using software and
critical decision process
Oral examination

Suggested projects
1. Controlling the microstructure in TRIP steels.
A thermodynamical analysis will be performed in order to
link the phases present in a commercial TRIP steel.
• Retained austenite and its grain size
• Martensite
• Ferrite
• Bainite
• Cementite
• Precipitate phases.
OBJECTIVE: Excel strength-ductility properties by
controlling the alloying elements in each phase.
Determine optimum rolling schemes and composition.

Suggested projects
2. Design of Ti alloys with TRIP effects.
Investigate the ranges of binary and ternary systems that
have the required a+ fields and their sizes. Tailor
volume fractions and electron to atom ratios for
inducing martensite transformation.
Determine also optimum heat treatment schemes.
OBJECTIVE: Quantify the most feasible options for Ti-
alloys with TRIP effects.

Suggested projects
3. Precipitation sequences in Al 2024 T3 and minimisation of
its softening.
Taking the commercial alloy composition and heat
treatment, reproduce its expected precipitation sequences
and suggest modifications in its composition and heat
treatment scheme to avoid undesirable softening after
aircraft long term use.
OBJECTIVE: Optimise strength-ageing behaviour in
commercial Al 2024 T3.

BIBLIOGRAPHY
J.J. Wang and S. van der Zwaag; Theoretical study of P containing TRIP
steels. Z. Metallkd 92 (2001) 1299-1311
Q. Y. Sun, S. J. Song, R. H. Zhu and H. C. Gu, Journal of Materials Science
37, 2543 (2002) “Toughening of Titanium Alloys by Twinning and Martensite
Transformation”
J. D. Cotton, J. F. Bingert, P. S. Dunn and R. A. Patterson, Metallurgical and
Materials Transactions A 25A, 461 (1994) “Microstructure and Mechanical
Properties of Ti-40wt Pct Ta
O. M. Ivasishin and R. V. Teliovich, J. Phys. IV 11 (Pr4), 165 (2001)
“Transformation Plasticity in Titatnium Alpha Double Prime Martensite”

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