Hydrogen and radiation embrittlement of CrMoV and CrNiMoV ferritic RPV steels

Int. J. Pres. Ves. & Piping 24 (1986) 13-26

Hydrogen and Radiation Embrittlement of CrMoV and CrNiMoV Ferritic RPV Steels J. K o u t s k ~ a n d K. ~plichal Nuclear Research Institute, i~e~,Czechoslovakia (Received: 20 November, 1985)

ABSTRACT Hydrogen embrittlement of low-alloy steels has been investigated in relation to its dependence on hydrogen trapping and release, on the electrolytic hydrogen charging parameters, and on irradiation. The interaction of hydrogen with the structure of irradiated and unirradiated steels at higher charging current densities causes structural defects which can lead to a loss of ductility of the steel even after hydrogen release. The presence and the character of grain boundaries, secondary phases and other defects in the steel structure are of great importance from the viewpoint of the hydrogen embrittlement effect.

INTRODUCTION In recent years, increasing attention has concentrated on the problem of the through-wall direction failure possibility in corrosion-resistant welded cladding and the development of underclad cracks and other types of cracks in the base material of PWR pressure vessels. Crack growth in the presence or absence of a corroding environment can be affected by hydrogen and radiation embrittlement. In the present paper the conditions of hydrogen trapping and release have been studied in relation to their dependence on the presence of structural defects in both irradiated and unirradiated steel. The influence of these parameters has been determined from the decrease in ductile and strength properties of the steel. 13 Int. J. Pres. Ves. & Piping 0308-0161/86/$03.50 © ElsevierApplied SciencePublishers Ltd, England, 1986. Printed in Great Britain

14

J. Koutskf', K. ~pllchal

EXPERIMENTAL The experiments were done on the low-alloy CrMoV and CrNiMoV steels which are currently used for light water reactor pressure vessels. The material composition of the steels is given in Table 1. Tensile tests were conducted at a strain rate of 8.3 x 10-4s -t on cylindrical tensile specimens of diameter d = 4 m m and working section length l = 20 mm. For constant load testing the method of delayed fracture was used with cylindrical 60 ° V-notched specimens of dimensions d~ = 3.5 ram, d2 = 5 m m and l = 17.5 m m and notch radius 0.05 ram. Both series of tests were performed at room temperature. The specimens were irradiated in an irradiation rig using a helium environment at temperatures of 130, 180 and 290°C for fast neutron fluences of 2.6-4.9 x 1023n m -2 (E > 0.5 MeV). Unirradiated specimens prior to tensile test were hydrogen-charged for 1 h at room temperature in 1N H2SO4 solution (with addition of 3 0 p p m As203). The delayed fracture test was carried out only on unirradiated notched specimens which had been hydrogen-charged under constant load in 1N H2SO4 with current densities of 10, 100, 300 and 3 0 0 0 A m -2. The hydrogen content was determined by vacuum extraction of melted specimens in an adapted BALZERS EAH-220 analyzer. The specimens had been stored in liquid nitrogen before tensile testing or hydrogen content determination.

RESULTS

Hydrogen and radiation embrittlement The mechanical properties of the CrMoV steel depend on the extent of irradiation-induced defects, on the hydrogen content and on hydrogen charging parameters. The influence of irradiation becomes evident as radiation strengthening of the steel which increases with decreasing irradiation temperature in the range 290-120°C. F r o m the data given in Table 2 this is obvious; for example, for the samples irradiated at 130°C to fluence 9 x 102anm -2, the yield strength (Rp)o. 2 increases by 59% and the tensile strength Rm by 41%. The influence of hydrogen was not observed at concentrations up to

Initial state H charged 5 - 2 0 A m -2 H charged 100-200A m -2

CrMoV CrNiMoV

Steel

0-15 0"13

C

Si

0.27 0.20

Rm (MPa)

628 625 635

(Rp)o.2

(MPa)

560

560

563

Unirradiated

0.44 0.45

Mn

0-01 0-003

As

TABLE 2

0-012 0.010

S

2.67 2.20

Cr

639

619

600

(MPa)

(Rp)o.2

689

693

668

(MPa)

Rm

tit = 290"C 2.4 x 1023nm -2

Rm (MPa)

734 728 657

(Rp)o.2 (MPa)

698 689 656

tir = 180"C 2.6 x 1023nm -2

Strength Properties of CrMoV Steel

0.01 0.007

P

TABLE 1 Chemical Composition of Steels (Mass %)

0"19 1"26

Ni

0.24 0.12

V

796

863

860

(MPa)

(gp)o.2

795

842

865

(MPa)

Rm

tit = 130"C 4"6 x 1023nm -2

0.61 0"57

Mo

130"C

0.018 0.001

Sb

725

882

888

(MPa)

(Rp)o2

725

883

888

(MPa)

1~

9"0 x 1023nm -2

/it =

0-21 0-05

Cu

e,

J. Koutsk~, K. ~plichal

16

8 - 1 0 p p m after charging at current densities up to 5 0 A m -2. On the other hand, strength properties of the steel noticeably decreased with higher hydrogen contents after charging at current densities over 100A m -2. For the above-mentioned specimens irradiated at 130°C to fluence 9 x 1023nm -2 both yield and tensile strengths decreased by ~ 30% compared with hydrogen-free specimens. Hydrogen and radiation embrittlement may impair the ductility of the steel. A substantial decrease in total elongation and reduction of area of both irradiated and unirradiated specimens was observed with hydrogen contents over 2.5 ppm. Up to approximately 2 p p m hydrogen charging at current densities of 1-10 A m -2, no changes in ductility were observed. Above 2.5ppm hydrogen and at current densities of 105 0 A m -2 there was a marked decrease of elongation (Figs 1 and 2), reduction in area and fracture stress. The effect of radiation embrittlement on hydrogen embrittlement was inversely correlated with irradiation temperature (Fig. 2). With decreasing irradiation temperature the current density showing the greatest rate of decrease of elongation and reduction in area moved towards the lower values of current densities and the downward slope of the elongation-density curve increased.

20 -'

--... ° N \

180"C '~3 ~"~='- '~' 2 , 6 " 1 0 "n l ~ - "="~o~,ik

290" 3'4"1023nm-2 ..,,

0,8 1 Fig. 1.

,

,

,

2

= ~ N , . , , . . - .

5

10 H2 [ ppm]

Elongation As of irradiated and unirradiated CrMoV steel as a function of hydrogen content.

Hydrogen and radiation embrittlement of RPV steels

17

20

~;

~o

~3o c

.........

\

L

x= 290 c ° N.

2

5

~o 20

\

0-"=-*

1180%

5o ~oo soo [Am-2 ]

Fig. 2. Elongation A5 of irradiated and unirradiated CrMoV steel as a function of current density.

The hydrogen charging of specimens irradiated at 130 and 180"C, and with higher current densities, led to the superposition of hydrogen and radiation embrittlement. This was shown by the decrease of elongation and reduction in area for hydrogen contents over 8-10 ppm and current densities over 100Am -2, at which full loss of ductile behaviour of steel occurred, as was the case reported by Koutsk~, e t al. 1'2 Comparison of the conventional engineering load-displacement curves in a tensile test demonstrated that specimens charged at current densities of 100 A m -2 fractured before the material tensile strength was reached (Fig. 3). Radiation embrittlement caused a decrease in the uniform elongation value which, for both hydrogen-charged and hydrogen-free specimens (at current densities up to 50A m-2), reached approximately the same value and decreased with decrease of irradiation temperature (Fig, 4). On the contrary, for the specimens irradiated at 290"C the uniform elongation value after hydrogen charging increased in comparison with the unirradiated specimens. The local necking elongation was affected by hydrogen content over 2.5 ppm (Fig. 5). A considerable decrease of the local necking elongation was observed above 10 A m -2 and the data plotted semilogarithmically suggested a linear relationship.

J. Koutsk~, K. ~plichal

18

10

Z 10An~2

"10

~ uncharged

.J

|

I

0

Fig. 3.

5

10 Engineering Strain

15

Load-engineering strain curves for tensile test of specimens irradiated at 1300C.

The effect o f the initial stress R on the time to fracture up to 167 h was investigated for both steels charged at the same time (Figs 6 and 7). F r o m these curves the lower threshold stress was obtained and the dependence R~ on the charging current density was determined (Fig. 8). The value o f the limit threshold stress R~t can be determined, below which the specimens show no failure even at higher current densities. 10 "%'

290"Cs3,4.1023nm-2

A

unir radiQted

A

-*

° n

J~ < 4

180"C;2,6.1023nm 2

a

130*C~4,g.1023nm2 • ~o~o~ I , i 5 10 20

q~""~o~' ~ ' ~

2

0

--o %' i 2

50

,

100 200

500

[ A m -2 ] F i g . 4.

Effect of charging current density on the uniform elongation steel.

Am

of CrMoV

Hydrogen and radiation embrittlement of RPV steels

19

20

15

\

-°-----\~°-- o,,_ II..~

2,6.I023nn~2 s ~

'~

O ~

- ~

•~ ,

0

~

,o~'3° c_

4.,

. . . unirraaiatea

X.

~

o

I

I

I

I

l

2

5

10

20

50

.

-12'1~,..~ D ~

I

100 200

500

[Am-2] Fig. 5.

Effect of charging current density on the local necking elongation CrMoV steel.

A5-

Am of

R

[MPa]

Cr-Mo-V

1000

500

.o~, " ~~ ~ o , IoA.T2 • ~ j • 0A4

° 100 A rfi 2

• 300 AnT2

$

+3000 AnT2

6.105

I

f

I

103

104

105

f

!

106

t [s] Fig. 6.

Effect of applied stress intensity on time to failure of unirradiated CrMoV steel.

J. Koutskp, K. ~plichal

20

R Cr-Ni -Mo -V

[MPo] 1000

~

500

•

o

-

-

0 A.~ 2

A 10 AI~ z

o 100 Arn 2 • 300 Arn 2 ~- 3000 Arn2 I

103

I

104

I

105

106 t is]

Fig. 7.

Effect of applied stress intensity on time to failure of unirradiated CrNiMoV steel.

For the CrMoV and CrNiMoV steels Rit equals 210 MPa and 410 MPa, respectively. A fractographic investigation proved that the controlling mechanism of fracture was intercrystalline separation, the relative area of which on the fracture face was inversely proportional to the initial stress, as found by Axamit e t al. 3 Interaction of hydrogen with steel structure

Hydrogen trapping is affected by the presence and activity of structural defects. Up to 2.0 ppm hydrogen, the hydrogen distribution on the crosssection can be assumed to be uniform. Further increase of hydrogen content is observed mostly on traps, i.e. the initial austenitic grain boundaries, inclusions, and interphase boundaries. Attainment of local critical hydrogen concentration can then lead to crack formation even in non-deformed specimens (Fig. 9). For example, electrolytic charging of unirradiated samples at current densities over 300-500 A m -2 and for periods longer than 1 h causes irreversible changes resulting in internal cracks, oriented along the grain boundaries and along the interphase of non-metallic inclusion and matrix. The r61e of the grain structure is particularly important after hydrogen charging of specimens irradiated at lower temperature and in delayed

1000

"-9

•

Cr- M o - V

Fig. 8. Lower threshold stress Ri as a function of charging current density for unirradiated notched specimens.

(a)

(b) Fig. 9. Hydrogen-induced cracks in the structure of unirradiated CrMoV steel: (a) on grain boundaries; (b) at non-metallic inclusion.

J. Koutsk~, K. Splichal

22

(a)

(b)

Fig. 10. Intergranular fracture of notched specimens, delayed fracture test: (a) crack initiation near the surface; (b) local region on the surface.

fracture; in these cases the influence of inclusions is suppressed. Failure is initiated by intercrystalline fracture on the surface which focuses either in the local area or in the coherent layer under the whole surface of the specimen (Fig. 10).

Hydrogen trapping and release The CrMoV steel in the as-delivered state contained 0.4-0.6ppm and after irradiation 1.2-2.0ppm of hydrogen. The dependence of the hydrogen content on charging current density was characterized by two stages. In the first, the hydrogen content in irradiated and unirradiated specimens reached 2-3 ppm. In the second, the course of the hydrogen charging curve is determined by the presence and activity of trapping sides, which after irradiation are dependent on the level of radiation damage in the steel. With decreasing irradiation temperature (i.e. increasing radiation damage of the steel) the hydrogen content increases after irradiation at 290°C by approximately up to 1.5 times and at 130°C by 3-4 times, as shown by Koutsk~, e t al. ~ The effect of traps for hydrogen embrittlement was evaluated from the course of hydrogen charging and hydrogen release in both the unirradiated specimens and those irradiated at 290°C to a fluence of 3"4 x 1023nm-2(Figs 11 and 12).

Hydrogen and radiation embrittlement of RPV steels

20 Am-2

6

300 Am-2

ir radiated ~

"-'"

23

uni rrodiated

i

,_.=___

H charged

--,,., o

•

~ G"";

2 "B . . . . .

"~H

re t e a s e d

H released

0

I

I

10

20

I

I

I

30 40 50 charging time [h~]

10

I

I

I

20 30 40 50 charging time [hr]

Fig. 11. Hydrogen content as a function of charging time for specimens in H charged condition and for specimens in H charged and H released (48 h, 25"C) condition.

20

----m ~ ..O ,=..O

u N H released

o O

20

20 An~2

300 Am-2 15

15

\ \\

,•

H charged

",%.___

\ 10

O

H rel~e°sed

• m

5

|

10

I

e,~

ir radiated

~'~

uni r r a d i a t e d

i

I

20 30 40 50 charging time [ hr]

5

~

~..=. ==....~.=,= ,..~..

-

"%-

/ H charged

0

I

10

i

|

|

20 30 40 50 charging time [hr']

Fig. 12. Total elongation .45as a function of charging time for specimens in H charged condition and for specimens in H charged and H released (48 h, 25°C) condition.

24

J. Koutskj:, K. ,~plichal

Significant increase of hydrogen content was achieved within l h charging time. Further charging time up to 48 h had little effect. The hydrogen content was higher for the irradiated specimens compared with the unirradiated ones (Fig. 11). The decisive factor for interaction of hydrogen with traps is the current density of hydrogen charging. By increasing current density from 20 to 300 A m-2 the total elongation decreased and was lower for irradiated specimens practically throughout the whole range of hydrogen charging times (Fig. 12). The hydrogen release during 48 h degassing time at room temperature resulted in recovery of ductility. For the specimens charged at 20 A m -2 and those unirradiated and charged at 300 A m -2 (for 1-24 h) the total elongation and the total hydrogen content reverted nearly to their initial values. For the irradiated specimen and an unirradiated specimen hydrogen-charged for 48h we can assume that the interaction of hydrogen with structural defects (charging at 3 0 0 A m -2) results in stable defects which prevent recovery of ductility even when the hydrogen content falls to the same values as those of unirradiated specimens, as was found by Hrub~ e t al. 4 DISCUSSION A N D CONCLUSIONS The CrMoV steel in both unirradiated and irradiated states was sensitive to hydrogen embrittlement at hydrogen contents above 2-5 ppm after charging at current densities of 10-50Am -2. The hydrogen content apparently increased with decrease of irradiation temperature from 290°C to 130°C. This means that the effect of radiation-induced defects on trapping and maintaining hydrogen appears to be substantially higher in comparison with that of structural defects of unirradiated steel. A substantial decrease of elongation observed above those values is controlled by the hydrogen content or by the current density of charging. Irradiation at 130 and 180"C for a neutron fluence >2.6 x 1 0 2 3 n m -2 (E > 0-5 MeV) followed by hydrogen charging suggested mutual superposition of hydrogen and radiation embrittlement, which was proved by the decrease of elongation with hydrogen content over 8-10ppm, when full loss of ductility may appear. Radiation embrittlement decreases the uniform elongation; hydrogen embrittlement affects the decrease of local necking elongation.

Hydrogen and radiation embrittlement of RPV steels

25

A distinct reduction of ductility at comparable conditions of hydrogen charging has been ascertained for irradiated and unirradiated A302B steel, 5 unirradiated steels A533B and A542, 6 and irradiated pure iron and A542 steel. 7 The effect of hydrogen on the time to fracture depends on the charging current density and appeared in the CrMoV steel with hydrogen content above 3 ppm. The lower threshold stress Rit of the CrNiMoV steel was found to be twice that of the CrMoV steel. The higher resistivity of the CrNiMoV steel can be explained by a higher micropurity, causing a higher cohesion strength of the grain boundaries and therefore a higher resistivity against intercrystaUine failure. The critical development of initial defects proved to be located on the grain boundaries. It is evident that the quantity of hydrogen trapped on the defects both inside grains and on the grain boundaries as well as the quantity of necessary additional hydrogen on the potential failures increased with increasing current density. Under these conditions the leading r61e can be considered to be played by the interaction of hydrogen with the steel structure. At lower charging current densities hydrogen trapping on structural defects does not lead to stable defects: after hydrogen release from the specimens ductility reverted to initial values. Increased current densities of hydrogen charging and radiation damage of the steel result in irreversible hydrogen embrittlement. When the critical hydrogen content is exceeded, trap sides are formed. The given defects remain in the steel structure even after hydrogen release and are capable of initiating and propagating specimen failure. Hydrogen also contributes to crack growth by means of diffusion in the direction of the stress gradient, by which a specific local critical concentration of hydrogen in a crack front region can be reached.8'9 From this point of view, radiation-induced defects could play a significant part as sources for hydrogen diffusion to regions of stress concentrators, thus affecting the behaviour of defects in the RPV wall. In PWR operating conditions with the corrosion effect of the coolant, the influence of hydrogen on the crack propagation process is determined mostly by hydrogen overpotential (in model conditions it is dependent on the current density of hydrogen charging) and by the degree of hydrogen trapping in stainless steel cladding and in the base material of the RPV. For cladding specimens irradiated for a long time in overheated water under PWR operating conditions, a hydrogen content

26

J. Koutsk~, K. ~plichal

of 3-4 p p m was ascertained. Assuming that a concentration equilibrium is established on the cladding-base material interface, the hydrogen content could reach values significant from the viewpoint of hydrogen embrittlement for low-alloy steels.

REFERENCES 1. Koutsk~, J., Otruba, J. and ~plichai, K., Trans. 7th S M I R T Conf., Paper G 2/9, Chicago, 1983. 2. Koutsk~, J. and ~plichal, K., Proc. IAEA Specialists' Meeting on Corrosion and Stress Corrosion of the Steel Pressure Boundary Components and Steam Turbines, J. Forsten (ed.), VTT Espoo, 1984, pp. 271-85. 3. Axamit, R., Novosad, P. and Burda, J., Rep. 6119-M, Nuclear Research Institute, l~e~, Czechoslovakia, 1982. 4. Hrub~,, J. et al., Proc. Material and Technological Problems of VVER Nuclear Reactors, ~elezmi Ruda, Czechoslovakia, 1978, pp. 149-66. 5. Brinkmann, C. R. and Beeston, J. M., The effect of 112 on the ductile properties of irradiated pressure vessel steels, ASTM STP 484, Philadelphia, PA, 1971. 6. Takaku, H. and Kayano, H., J. Nucl. Mater., 78 (1978), 299-308. 7. Takaku, H. and Kayano, H., J. Nucl. Mater., 110 (1982), 286-95. 8. Gerberich, W. W. and Chen, Y. T., Met. Trans., 6A (1975), 271-8. 9. Gerberich, W. W., Chen, Y. T. and St John, C., Met. Trans., 6A (1975), 1485-98.