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Aging Behaviour of a 2.25Cr-1Mo Steel
Roberto R. de Avillez
Fernando Cosme Rizzo Assunção
Bojan Marinkovic
Departamento de Ciência dos Materiais e Metalurgia,
Pontifícia Universidade Católica do Rio de Janeiro, Brasil
Introduction
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⇒ The 2.25Cr–1.0Mo steel is widely used in petroleum refining
units, for example, in pressure vessels, working under severe
operational conditions.
⇒ Due to the high temperatures of some processes, many
phenomena such as precipitation of carbides and their coarsening
occur during service, resulting in changes of material
microstructure.
⇒ Such changes may modify the mechanical properties, which are
responsible for the performance of the pressure vessels.
Theoretical Approach
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⇒ Using a software for thermodynamic calculations
(ThermoCalc), the thermodynamically stable carbides and the
theoretical carbide nucleation sequence were established over
the temperature range of interest (700-1040 K), supposing
ferrite as the original microstructure.
⇒ The evolution of the carbide precipitation along a period up
to 20 years was calculated using the software DICTRA. The
M2C, the carbide with the highest nucleation energy, was used
as the starting condition in a system composed of several
diffusion cells, one for each considered carbide.
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Calculation of Stable Carbides
(ThermoCalc)
Carbides stable in Ferrite
matrix (bcc):
M23C6 (700-1050oC) and
M6C (700-925oC)
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Mass fraction of Fe, Cr and Mo at metal site in the
stable carbide M23C6
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Calculation of the Driving Force for
Nucleation: Matrix γ, composition a.
T, P
γ
Gibbs Free Energy
β
α
a
1
Molar Fraction
Mats Hillert, 1951
∆Gα
∆Gβ
Theoretical Nucleation Sequence
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M2C → M3C2 → M7C3 → M23C6 → M3C → M6C → M5C2
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Precipitation Kinetic: Dictra
- Difusional (one-dimensional), spherical or planar interfaces
-2 or 3 diffusion cells connected by zero total flux and
identical potential on outside border.
- Cell 1 consists of a carbide precipitation (M2C) in metastable
equilibrium with a ferrite matrix.
- The radius of this carbide is calculated from the metastable equilibrium
data.
- The other cells consist initially of only a ferrite matrix with the same
composition of the matrix from the cell 1.
- Only one carbide type can precipitate in each of the cells.
Diffusion Geometry
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carb
carb
bcc
bcc
bcc
bcc
carb
bcc
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Theoretical Calculations (2 cells)
M2C mass fraction precipated in the Cell 1
(T= 1000K)
8
7
1 ano =34.536E6 sec
6
IWW(M2C)
5
4
3
2
1
-3
0
10
-1
0
6
10
2
4
6
TIME(s)
8
10
12
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M23C6 mass fraction in the cell 2
(T= 1000K)
1200
1195
1190
1185
IWW(M23C6)
14
12
IWW(M23C6)
10
1180
1175
1170
1165
8
1160
6
1155
10-5
1150
4
8
10
2
-3
10
0
0
0
6
10
2
4
6
TIME (s)
8
10
12
1
2
3
4
TIME (s)
5
6
7
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Fe metal sublattice fraction
in M23C6 (cell 2, T= 1000K), x linear scale
Cr
Fe
0.40
0.59
0.39
0.58
0.38
0.57
0.36
Fe Fraction
Cr Fraction
0.37
0.35
0.34
0.33
0.56
0.55
0.54
0.32
0.53
0.31
0.30
0
6
10
2
4
6
TIME(s)
8
10
12
0.52
0
6
10
2
4
6
TIME (s)
8
10
12
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Comparison with Experimental Data
Parvathavarthini et al., J. Nucl. Mat., 288 (2001) pp. 187-196:
- Corroborate the theoretical carbide nucleation sequence.
Yang et al., Mater. Charact., 30 (1993) pp.75-88:
-M23C6 presented in 2.25Mo-1.0Cr steel steam pipes after 5 years use,
were rich in Fe.
-M23C6 presented in 2.25Mo-1.0Cr steel steam pipes after 18 years use,
were rich in Cr.
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Cr and Fe metal sublattice fraction
in M23C6 (cell 2, T= 1000K), x log scale
Fe
Cr
0.40
0.59
0.39
0.58
0.38
0.57
0.36
Fe Fraction
Cr Fraction
0.37
0.35
0.34
0.33
0.55
0.54
0.32
0.53
0.31
0.30
0.56
1
10
100 100010000 105 106 107 108 109
TIME(s)
0.52
1
10
100 100010000 105 106 107 108 109
TIME (s)
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Effect of M2C radius on
M23C6 precipitation
14
12
IWW(M23C6)
10
0.5µm
8
2.0µm
6
4
2
10-3
0
0
6
10
2
4
6
TIME (s)
8
10
12
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Formation of M7C3 and M23C6
3 Cell-planar geometry
M7C3
M23C6
C
36
7
32
6
28
5
IWW(M23C6)
IWW(M7C3)
24
20
16
12
8
4
3
2
4
10-5
1
0
-4
-3
10
0
107
4
8
12
16
TIME
20
24
28
32
0
0
7
10
4
8
12
16
TIME
20
24
28
32
Conclusions
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- It is possible to use a simple system to have some insights
on the very long term microstructural changes in the steel.
- The composition of M23C6 could be used to follow service
degradation of a 2.25Mo-1.0Cr steel.
- Also, mass fraction of M23C6 could be used for the purpose
of evaluation of service degradation of a 2.25Mo-1.0Cr
steel.
Aknowledgements
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• CNPq-Brazilian Research Funding Agency
• FAPERJ-Rio de Janeiro State Reeaserch Funding Agency
Thank you for your attention
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Conditions of Thermodynamic Equilibrium
T, P
γ
Gibbs Free Energy
β
α
2
b
d
Gβ
Gα
Molar Fraction
Driving force is zero for the phses in equilibrium and it will be
negative for the phases out of thermodynamic equilibrium.