vanadium and niobium.pdf

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ECCC Creep Conference, 1214 September 2005, London. [email protected]

153

Influence of Normalizing Heat Treatment onPrecipitation Behavior in Modified 9Cr-1Mo Steel

Masataka Yoshino1), Yoshinao Mishima1), Yoshiaki Toda2), Hideaki Kushima2), Kota

Sawada

2)

and Kazuhiro Kimura

2)

1) Tokyo Institute of Technology, Japan.

2) National Institute for Materials Science, Japan.

Abstract

Precipitation behavior during normalizing heat treatment has been investigated on modified

9Cr-1Mo steel. Heterogeneously distributed spherical MX particles and platelet M 3C were observed in

the as normalized condition. Number of the precipitates decreased with increasing normalizing

temperature and no precipitates was observed after normalizing at 1250oC. The size of MX increased

with increase in normalizing temperature up to 1200oC. For MX, not only size, but also composition of

metallic elements was influenced by normalizing temperature. Since equilibrium composition of MX

depends on temperature, MX particle with non-equilibrium composition dissolves and precipitation of

it takes place with its equilibrium composition at the normalizing temperature. A phase field diagram

of NbX-VX quasi binary system in modified 9Cr-1Mo steel was experimentally determined. It has

been supposed that precipitation of M3C takes place during cooling from normalizing temperature in

the surrounding area of MX particles where the concentration of niobium and vanadium in matrix is

poor.

Keywords: ferritic creep resistant steel; modified 9Cr-1Mo steel; MX; M3C; phase equilibrium;

precipitation behavior; two-phase separation; normalizing

1. Introduction

High Cr ferritic creep resistant steels such as a modified 9Cr-1Mo steel (ASME P91/T91) have been

widely used as materials for high temperature structural components such as header and main steam

pipe in power generation plant. High strength ferritic creep resistant steels are usually subjected to

normalizing and tempering heat treatment prior to service and, therefore, microstructure of those is a

tempered martensite. Creep strength of these steels are improved by its martensitic lath structure,

precipitation strengthening effects of M23C6 carbide and MX carbonitride (M=Nb, V, Cr and X=C, N)

and solid solution strengthening effects of Mo and W atoms in the matrix [1]. Especially, precipitation

strengthening effect of MX is important because its coarsening rate is small and fine particle size is

maintained for long-term [2-4]. Many researches have been conducted on features and role of MX [2,

5-10]. A possibility of further improvement in creep strength by controlling MX has been also

investigated [11].

Recently, Inoue et al. reported that two-phase separation behavior of primary MX into Nb-rich MX


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M.Yoshino, Y.Mishima, Y.Toda, H.Kushima, K.Sawada and K.Kimura

154

and V-rich MX was caused by the miscibility gap between primary MX and another two phases at the

temperature [9]. Such two-phase separation behavior has been clearly demonstrated by experimental

results during tempering heat treatment at 765oC in a 9Cr-1Mo-V-Nb steel by Suzuki et al. [10]. Since

MX particles are major strengthener, precipitation behavior in cooperate with two-phase separation of

those during normalizing and tempering heat treatment strongly influences on creep strength. However,

it has not yet been clearly understood, and presence of M3C in the as normalized condition in 9-12Cr

ferritic creep resistant steels [12-17] has not yet been authorized. Aim of the present study is to

understand phase equilibrium between austenite and MX at the elevated temperature in modified

9Cr-1Mo steel and influence of normalizing temperature on the precipitates in the as normalized

condition has been investigated.

2. Experimental procedure

The steel used in this study was a modified 9Cr-1Mo steel, chemical composition was shown in

Table 1. An ingot of the steel with weight of 10kg was prepared by high frequency vacuum induction

furnace. The ingot was heated to 1150oC for 1.5h and hot rolled in a range of temperatures from

1150oC to 900oC into bar with a diameter of 16mm. Normalizing heat treatment conditions are shown

in Table 2. Normalizing heat treatments were performed for 600 sec in a range of temperatures from

1050oC to 1250oC and for 3600 sec at 1100 and 1200oC, followed by air cooling.

Table 1. Chemical composition of the steel used in this study.

Fe C Si Mn P S Ni

Bal. 0.090 0.26 0.41 0.001


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Normalizing Heat Treatment & Precipitation in Modified 9Cr-1Mo Steel

155

Transmission Electron Microscope (FE-TEM). Each precipitates were analyzed by Energy Dispersion

X-ray Spectroscopy attached to FE-TEM (TEM-EDS). Mechanically polished specimen surface was

etched in a saturated solution of picric acid in ethyl alcohol with 1% hydrochloric acid (Villelas

reagent). The carbon was deposited on the etched surface, and the carbon film was detached from the

specimen surface in a Villelas reagent. The carbon extracted replica was cleaned in ethyl alcohol and

collected on copper grids for TEM examination. Precipitates were identified by means of X-ray

diffraction analysis on an electrolytically extracted residue.

3. Results and discussion

3.1. Macrostructure

Figure 1 shows optical micrograph of the steels in the as normalized condition at (a) 1050 oC, (b)

1100oC, (c) 1150oC, (d) 1200oC and (e) 1250oC. Microstructure in the as normalized condition was

martensite and no delta ferrite was observed for the range of normalizing temperatures from 1050 to

1250oC. The material was austenite single phase at the normalizing temperatures investigated.

Figure 2 shows Vickers hardness and prior

austenite grain size number in the as normalized

condition. Prior austenite grain size number was

measured according to JIS G 0552 [18]. Prior

austenite grain size of the steel increased with

increase in normalizing temperature, but

hardness of about HV400 was almost the same

independent of normalizing temperature.

3.2. Precipitates in the as received condition

Figure 3 shows a TEM micrograph of the carbon extracted replica prepared from the steel in the as

received condition. Fig. 3(b) is a higher magnification image of the area indicated by square in Fig.

Fig. 1.Optical micrograph of the steels normalized for 10min. at (a) 1050oC, (b) 1100oC, (c)

1150oC, (d) 1200oC and (e) 1250oC.

Fig. 2.Normalizing temperature dependence of a

grain size number and hardness of the steel.

0

2

4

6

8

10

12

0

100

200

300

400

500

1000 1050 1100 1150 1200 1250 1300

Grainsizenumber

Hardness/HV

Normalizing temperature /

: Grain size number

: Hardness

as received

Load : 98N, 30s

//

//


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3(a). Precipitates distributed within grain were observed. There were two types precipitate which is

spherical shape with a diameter of about 20~30 nm and coarse platelet one with a length of about 100

nm. According to TEM-EDS and XRD analysis, spherical precipitate and platelet one was identified as

Nb-rich MX and M3C, respectively. A detail of phase identification and the morphology of precipitates

were described in a previous work [19].

3.3. Precipitates in the as normalized condition

Figure 4 shows the TEM micrographs of carbon extracted replica prepared from the steels

normalized for 600 sec in a range of temperatures from 1050 to 1250 oC. In the steel normalized at

1050oC (Fig. 4(a)), a lot of precipitates were observed within prior austenite grain. Many precipitates

were also observed in the steels normalized at 1100oC (Fig. 4(b)) , 1150oC (Fig. 4(c)) and 1200oC (Fig.

4(d)), however, number of those decreased with increasing normalizing temperature and distribution of

those was heterogeneous, in comparison with that in the steel normalized at 1050 oC (Fig. 4(a)). In the

steel normalized at 1250oC (Fig. 4(e)), no precipitate was observed. Consequently, it has been

supposed that amount of undissolved precipitate decreased with increasing normalizing temperature

and it does not exist at 1250oC. Moreover, any precipitation did not take place during cooling from

normalizing temperature of 1250oC. Precipitates observed in the steels normalized in a range of

temperatures from 1050 to 1200oC were also identified by X-ray diffraction and TEM-EDS analysis to

be MX and M3C similar to those in the as received condition. It has been also observed that M3C

cementite tends to precipitates around MX.

Fig. 3. TEM micrographs of carbon extracted replica prepared from the steel in the as

received condition: (b) is an enlarged image of the area indicated by square in (a).

Fig. 4. TEM micrographs of carbon extracted replica prepared from the steel normalized for 600 sec

at (a) 1050oC, (b) 1100oC, (c) 1150oC, (d) 1200oC and (e) 1250oC.


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Fig. 5. Changes in particle diameter of MX

and M3C with increase in normalizing

0

50

100

150

200

250

300

350

1000 1050 1100 1150 1200 1250

Particlediameter/nm


as received

//

//

: MX

: M3C

Figure 5 shows temperature dependence of

average size of MX and M3C particles. Average

size of M3C was in a range of 50 to 60 nm

independent of normalizing temperature. On the

other hand, that of MX increased with increasing

normalizing temperature from 35 nm after

normalizing at 1050oC to 315 nm after normalizing

at 1200oC. It should be noted that average size of

MX particles after normalizing was larger than that

of 24 nm observed before normalizing heat

treatment, and no MX particle was observed after

normalizing at 1250oC. Above observation

indicates that no precipitation of MX takes place during cooling from the normalizing temperature of

1250oC, therefore, observed MX particles after normalizing heat treatment are undissolved particles.

Increase in average size of MX particles with increasing normalizing temperature could be caused by

following two mechanisms, (1) finer MX particles tends to dissolve at the lower normalizing

temperature and coarser one remains after normalizing heat treatment at higher temperature and/or (2)

coarsening of MX particles takes place at the normalizing temperature. On the other hand,

precipitation of M3C should take place during cooling from the normalizing temperature, since

solubility of M3C in austenite phase at the normalizing temperature is high enough to dissolve it [20].

As a result of that, average size of M3C was almost the same independent of normalizing temperature.

However, M3C was not observed at 1250oC. Thus, it is considered that precipitation of M3C was

closely related to presence of MX particle.

3.4. Influence of normalizing temperatures on MX

According to TEM-EDS analysis, major composition of MX was niobium, vanadium and chromium,

and sum of concentrations of those three elements was higher than 93 mass% of metallic elements,

moreover, sum of niobium and vanadium was about 90 mass%. In this section, influence of

normalizing temperature on MX particle is investigated from a viewpoint of composition of MX, with

a special attention to niobium.

A sum of niobium, vanadium and chromium concentrations of MX particles was standardized to be

100 mass% and plotted in a Cr-Nb-V ternary triangle as shown in Figure 6. In the as received

condition (Fig.6(a)), distribution of composition in a range of niobium from 50 to 90 mass% was

observed along a line between 100 mass% Nb and 80 mass% V - 20 mass% Cr. This result was similar

to that reported by Suzuki et al. [10] on a 9Cr-1Mo-V-Nb steel in the as normalized condition,

although low niobium particles less than 50 mass% were not observed in this study.

On the other hand, composition of MX particle was clearly divided into two groups by its niobium

concentration after normalizing at 1050oC (Fig.6(b)). In addition to high niobium particles, those with

low niobium concentrations of 20 to 30 mass% were observed. Those compositions distributed along


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the same line as that observed in the as received condition. The number of particles with niobium

concentration of 50 to 80 mass% observed in the as received condition decreased after normalizing at

1050oC. After normalizing at 1100oC (Fig.6(c)), distribution of compositions along the same line as

those observed in the as received condition and after normalizing at 1050 oC (Figs.6(a),(b)) was

observed. However, many precipitates with niobium concentration of 40 to 70 mass% were observed

instead of those with low niobium observed after normalizing at 1050oC. After normalizing at 1150

and 1200oC (Figs.6(d),(e)), small number of MX particles with high niobium concentration of 85 to 95

mass% were observed.

Fig. 6. Nb-V-Cr balances of MX in the steels in the as received condition (a) and normalized

for 600 sec at (b) 1050o

C, (c) 1100o

C, (d) 1150o

C and (e) 1200o

C.

Fig. 7. Relationship between particle diameter and Nb concentration of MX in the as received steel

(a) and steels normalized for 600 sec at (b) 1050oC, (c) 1100oC, (d) 1150oC and (e) 1200oC.

0

20

40

60

80

100

1 10 100 1000

Nbcontent/mass%

Particle diameter / nm

1050/10min

(b)

0

20

40

60

80

100

1 10 100 1000

Nbcontent/mass%


as received

(a)

0

20

40

60

80

100

1 10 100 1000

Nbcontent/mass%


1100/10min

(c)

0

20

40

60

80

100

1 10 100 1000

Nbcontent/

mass%


1150/10min

(d)

0

20

40

60

80

100

1 10 100 1000

Nbcontent/

mass%


1200/10min

(e)


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159

Relationship between diameter and niobium concentration of MX is shown in Figure 7. In the as

received condition (Fig.7(a)), diameter and niobium concentration of MX was widely distributed in a

range of 10 to 100 nm and 50 to 85 mass% of Nb, respectively. After normalizing at 1050oC (Fig.7(b)),

low niobium MX with 20 to 30 mass% of Nb which did not detected in the as received condition was

observed. Number of high niobium MX with 80 to 90 mass% of Nb increased and those with 50 to 80

mass% of niobium decreased. After normalizing at 1100oC (Fig.7(c)), diameter of MX distributed from

30 to 300 nm and was larger than that in the as received condition. Concentration of MX was roughly

divided into two groups by niobium concentration of 75 mass% and low Nb MX indicated larger

diameter than that of high Nb one. MX particles with niobium concentration of 40 to 70 mass% were

not detected after normalizing heat treatment, except for normalizing temperature of 1100oC. After

normalizing at 1150 and 1200oC (Figs.7(d),(e)), all MX particles contained high niobium of 80 to 90

mass% and those were larger than 100 nm.

From the above results, it has been found that change in chemical composition of MX takes place

during normalizing heat treatment and normalizing temperature influences on chemical composition of

MX. Since low niobium MX observed after normalizing at 1050oC does not exist in the as received

condition and MX particles are considered as undissolved precipitate that exist at the normalizing

temperature as mentioned above, not only dissolving of MX, but also precipitation of it should take

place during normalizing.

Mean diameter of high niobium MX particle

and low niobium one was plotted against a

normalizing temperature and shown in Figure 8.

Average diameter of MX particles increased with

increasing normalizing temperature, as mentioned

in Fig.5. Low niobium MX particle was larger

than high niobium MX for the same normalizing

temperature.

Amount of undissolved MX precipitates

decreased with increasing normalizing

temperature (Fig.4), however, low niobium MX

particle which did not exist in the as received

condition was observed after normalizing at 1050 and 1100oC. It has been supposed that, consequently,

not only dissolving of MX particle, but also precipitation of it with a chemical composition depends on

normalizing temperature should take place during normalizing heat treatment. Inoue et al. has

investigated phase equilibrium between austenite and (Nb,V)(C,N) complex carbonitride in

Fe-Nb-V-C-N alloys, and binodal curve and phase separation behavior in quasi binary system of

NbC-VC have been reported [9]. Suzuki et al. [10] investigated on precipitation behavior of MX

during normalizing and tempering heat treatment in 9Cr-1Mo-V-Nb steel and observed a distribution

of chemical composition of MX carbonitride along the line between 100 mass% Nb and 85 mass% V -

15 mass% Cr in a Cr-Nb-V ternary triangle, similar to the results in the present study (Fig.6). It has

0

50

100

150

200

250

300

350

1000 1050 1100 1150 1200 1250

Particlediameter/nm


as received

: High Nb MX

: Low Nb MX

//

//

Fig. 8. Changes in diameter of high Nb and

low Nb MX particles with increase in

normalizing temperature.


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160

been reported by Suzuki et al. [10] that chemical composition of MX varies along the tie line between

Nb-rich MX and V-rich MX and phase separation of undissolved primary MX into Nb-rich MX and

V-rich MX takes place during tempering. Temperature dependence of chemical composition of MX

observed in the present study should be influenced by phase equilibrium between austenite and MX.

Phase field diagram of NbX-VX quasi binary system obtained from the experimental result is shown in

Figure 9. Solid circle indicates average niobium concentration in MX precipitates and error bar means

range of the experimental data.

Additionally, the data examined by

Suzuki [21] which was measured on

9Cr-1Mo-V-Nb steel in the as

tempered condition after normalizing

for 600 sec at 1050oC, was also

indicated by square symbol.

Solubility limit temperature of MX

indicated by dashed line was

determined by Thermo-calc

calculation for NbX that was 1216oC

and experimental results on NbX and

VX reported by Iseda et al. [20]. Ac1

and Ac3 temperatures determined by

dilatometer were also indicated. Peak

temperature of binodal curve between

1100 and 1150oC is significantly

lower than the result reported by Inoue et al. [9] in which that is about 1530 oC. The difference in peak

temperature of binodal curve should be derived from difference in niobium, vanadium, carbon and

nitrogen concentrations. Concentrations of 0.61 - 2.08 mass% Nb, 0.45 - 2.29 mass% V, 0.18 - 0.42

mass% C and 0.057 - 0.119 mass% N for the steels used in the study of Inoue et al. [9] are

significantly higher than those of the steel used in the present study. During normalizing heat treatment,

dissolving of MX with non-equilibrium composition and precipitation of it with equilibrium

composition takes place corresponding to binodal curve. It has been considered that temperature

dependence of equilibrium composition of low niobium MX is larger than that of high niobium MX,

since solubility limit temperature of VX is about 100oC lower than that of NbX. Moreover, larger size

of low niobium MX than that of high niobium MX (Fig.8) should be also caused by lower solubility

limit temperature of VX, because higher concentration of vanadium in solid solution of austenite

matrix may result in higher coarsening rate of low Nb MX.

3.5. Re-precipitation of MX

Amount of undissolved MX decreased with increasing normalizing temperature, however, not only

dissolving, but also precipitation of it took place, corresponding to equilibrium composition along a

1216

1100

1300

1200

Temperature/oC

1000

0Nb content / mass%NbX VX

Austenite

Austenite + MX

Ferrite + NbX + VX

100

900

800

700

600

Austenite + NbX + VX

: Suzuki [20]

Ac3: 835oC

Ac1: 815oC

1216

1100

1300

1200

Temperature/oC

1000

0Nb content / mass%NbX VX

Austenite

Austenite + MX

Ferrite + NbX + VX

100

900

800

700

600

Austenite + NbX + VX

: Suzuki [20]

Ac3: 835oC

Ac1: 815oC

Fig. 9. Phase field diagram of NbX-VX quasi binary


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binodal curve. It should require enough time for normalizing in order to attain phase equilibrium at the

temperature. In order to investigate an influence of normalizing time on phase equilibrium, the steel

was normalized for 3600 sec that is six times longer than a condition for the steels discussed in the

above.

Figure 10 shows TEM micrographs of carbon

extracted replica prepared from the steel

normalized for 3600 sec at 1100 and 1200oC.

Although heterogeneously distributed precipitates

were observed in both conditions similar to those

normalized for 600 sec (Fig.4), amount of particles

after normalized for 3600 sec was smaller than that

after normalized for 600 sec, especially in the steel

normalized at 1200oC. Precipitation of M3C takes

place during cooling from the normalizing

temperature in cooperation with a presence of MX,

since M3C tends to precipitate around MX particles

and no M3C precipitation takes place during

cooling from the normalizing temperature of

1250oC where no undissolved MX particle exist.

Since no M3C was observed in the steel

normalized at 1250oC where no undissolved MX

particle exist, it is considered that precipitation of

MX particle during normalizing heat treatment

should influence on precipitation of M3C during

cooling from the normalizing temperature. Area

fraction where the M3C precipitated was

measured from TEM micrographs (Figs.4 and 10)

by image analysis and shown in Figure 11. Area

fraction where M3C was precipitated gradually decreased with increase in normalizing temperature

and time. Therefore, it has been supposed that decrease in amount of M3C precipitate with increase in

normalizing time should be influenced by time dependent phenomenon such as diffusion related to

precipitation of MX particle.

Schematic illustrations on changes in MX particle during normalizing heat treatment are shown in

Figure 12. Only dissolving phenomenon of MX particle corresponding to solubility limit of it is

described in Fig.12(a) and multiple behavior of dissolving of MX particle with non-equilibrium

composition and precipitation of it with equilibrium composition is demonstrated in Fig.12(b). Vertical

axis indicates concentration of niobium and/or vanadium. If only dissolving of MX particles takes

place during normalizing (Fig.12(a)), diffusion of niobium and vanadium atoms in austenite matrix

occurs from dissolved MX. In this case, coarser MX particles tend to remain after normalizing, since

Fig. 10. TEM micrographs of carbon extractedreplica prepared from the steel normalized for

3600 sec at (a) 1100oC and (b) 1200oC.

0.0

0.2

0.4

0.6

0.8

1.0

1050 1100 1150 1200 1250 1300

AreaFraction


: 10min.

: 60min.

0

Fig. 11. Normalizing temperature dependence

of area fraction where the M3C was


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finer particles are easy to dissolve. However, experimental results on the change in composition of MX

particles that strongly depends on normalizing temperature and coarsening of undissolved MX

particles with increasing normalizing temperature can not be explained by such simple dissolving

model.

On the other hand, both dissolving and precipitation of MX particles should take place during

normalizing heat treatment (Fig.7(b)). Since equilibrium composition of MX strongly depend on

temperature, especially at the temperatures higher than 1000oC (Fig.9), particles with non equilibrium

composition dissolve and MX precipitates with its equilibrium composition during normalizing. By

considering these precipitation behavior, temperature dependence of composition of MX and

coarsening of it with increase in normalizing temperature can be understood. In contrast to simple

dissolving model, concentration of niobium and vanadium in solid solution in the surrounding area of

precipitated MX particle should be lower than the other region far from the particle, until the

equilibrium is attained. With increase in normalizing time, niobium and vanadium poor area is reduced

with progress in diffusion in austenite matrix. Although a lot of M3C particles have been observed in

the as normalized condition, M3C is easily replaced by more stable carbide and/or carbonitride such as

M23C6 and MX [22]. Affinity of niobium and vanadium with carbon and nitrogen is much stronger

than that between iron and carbon. It has been considered that precipitation of M 3C takes place in a

surrounding area of MX particles precipitated during normalizing heat treatment, since concentration

of niobium and vanadium in solid solution should be poor. Consequently, amount of M3C particles

decreased with increase in normalizing time since niobium and vanadium poor region was reduced by

diffusion and no precipitation of M3C was observed after normalizing at 1250oC where precipitation of

MX did not take place.

4. Conclusions

The precipitation behavior of MX carbonitride during normalizing heat treatment has been

investigated in a modified 9Cr-1Mo steel. The following results were obtained.

(1) In the as received condition and after normalizing heat treatment, heterogeneously distributed

fine spherical MX and coarse platelet M3C cementite were observed, except for the steel normalized at

1250oC. MX and M3C were clearly distinguished by those chemical compositions. Amount of

Nb, V

Distance

MX

(a)Nb, V

Distance

MX

Nb, V

Distance

MX

(a) Nb, V

Distance

MX

M3C

(b) Nb, V

Distance

MX

M3C

(b)

Fig. 12. Schematic illustrations of (a) dissolution model and (b) dissolution and precipitation

model of MX particles at the normalizing temperature.


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precipitates decreased with increase in normalizing temperature.

(2) Although a size of M3C was almost the constant independent of normalizing temperature,

average diameter of MX particle increased with increase in normalizing temperature from 35 nm after

normalizing at 1050oC to 315 nm after normalizing at 1200oC.

(3) Chemical composition of metallic elements in MX consisted mainly niobium, vanadium and

chromium, and sum of those three elements was higher than 93 mass% of the metallic elements.

Chemical composition of metallic elements in MX was influenced by normalizing temperature. Phase

field diagram of NbX-VX quasi binary system with binodal curve based on experimental results was

proposed.

(4) Not only dissolving of MX with non-equilibrium composition, but also precipitation of it with

equilibrium composition takes place corresponding to binodal curve during normalizing heat

treatment.

(5) It has been considered that precipitation of M3C occurs in a surrounding area of MX precipitated

during normalizing heat treatment, since concentration of niobium and vanadium in solid solution is

lower than the other region.

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