Effect of thermal ageing on impact ductility of the nuclear reactor pressure vessel steel SA533B and its weld metal

Theoretical and Applied Fracture Mechanics 8 (1987) 25-31 North-Holland

25

EFFECT OF THERMAL AGEING ON IMPACT DUCTILITY OF THE NUCLEAR REACTOR PRESSURE VESSEL STEEL SA533B AND ITS WELD METAL R. PELLI and J. FORSTI~N Technical Research Centre of Finland, Metals Laboratory, SF-02150 Espoo, Finland

The effects of long term thermal ageing on the impact toughness of a reactor pressure vessel (RPV) steel SA533B and its submerged arc weld metal have been studied. The annealings were carried out at 350 450 ° C with a duration of 1-5 years. An estimate of the degree of embrittlement at the expected end-of-life of a RPV was made. The studied base metal did not exhibit any noticeable embrinlement during the anticipated reactor life. The studied weld metal proved to be susceptible to the thermal ageing resulting in a small rise in the ductile-brittle transition temperature at the expected end-of-life of the reactor pressure vessel.

1. Introduction

Pressure vessel steels may have a tendency to the toughness degradation during long term thermal ageing. The phenomenon of thermal ageing is similar to temper embrittlement but longer annealing times are required due to the lower temperature. The main reason for thermal ageing is the segregation of impurities into grain boundaries. The degree of thermal ageing is assumed to depend on the total concentration as well as on the local segregation of certain impurities. The long service life of RPV's makes the estimation of the final degree of embrittlement difficult. The only way to do the estimation is to accelerate the degradation by using higher annealing temperatures. The estimation of the degradation of the RPV is then extrapolated from the measured degradation of raised temperature but shorter annealing time. The purpose of this investigation was to measure the shift in transition temperature caused by thermal ageing and to evaluate the significance of thermal ageing in assessing the structural integrity of reactor pressure vessels.

2. Test methods

The steel used in this study was ASME SA533B CI.1 and its submerged arc weldment. The thickness of the test plate was 165 ram. The material was heat treated according to the common prac-

tice in reactor pressure vessel manufacture. An additional test material of the weld metal (submerged arc welded) was only post weld heat treated (PWHT) at 575°C for 0.5 hours. The chemical analyses of the test materials are presented in Table 1. The simulation of the degradation of toughness due to long term thermal ageing in operation was done by exposing the test material to elevated annealing temperatures. They were chosen to be 350°C, 400 °C, and 450 °C for shortening the

Table 1 Chemical analyses of the test material SA533B CI.1 (wt-~,) SA533B C1.1

C Si Mn S P Cr Ni Mo Cu AI V Sn As Ti Co W Sb Bi

0167-8442/87/$3.50 ~i5 1987, Elsevier Science Publishers B.V. (North-Holland)

Plate

Weld metal

0.18 0,22 1,42 0.006 0.005 0.18 0.63 0.50 0.04 0.025 0.01 0.006 0.005 0.00 0.01 0.00 0.003 < 0.001

0.07 0,16 1,52 0.011 0.008 0.04 1.56 0.43 0.07 0.02 0.01 0.005 0.01 0.00 0.02 0.005

26

R. Pelli, J. Forstdn / Effect of thermal ageing on impact ductility,

required ageing times to 1-5 years. The degradation of the toughness was studied using transverse Charpy V impact tests. The base metal specimens were cut from the centre position (½T) of the plate. They had the transverse (TL) orientation. The energy absorption was measured as a function of the test temperature. Also the lateral expansion was measured. Each transition curve is based on about 15 test specimens with exception of HAZ when the number of specimens was 5 only. The grain boundary segregation of impurities was studied with Auger-electron spectroscopy (AES) and scanning electron microscopy (SEM). The fracture surfaces were studied from specimens broken at - 196 ° C.

ment due to thermal ageing in operating condition during the reactor life time. Contrary to the base material, the weld metal of SA533B steel exhibited a definite susceptibility to thermal ageing. After one year exposure at 400 ° C the increase in the transition temperature was about 43 °C irrespective of the used transition criterion. The thermal ageing for one year at 3 5 0 ° C had a considerably smaller effect on the impact ductility. The corresponding shift of the transition temperature is in this case about 10 ° C, The thermal ageing of the heat-affected zone (HAZ) was studied after annealing one year at 350 ° C. An attempt was made to characterize the ductile-brittle-transition with a limited number of test specimens (5 per each seriesl. No embrittlement of the HAZ could be observed, on the contrary, the results show a slight ~mprovement of the toughness. However. one must remember that the scatter of results is large due to difficulties in positioning the notch correctly in the HAZ and the small number of specimens used in each series. The weld metal subjected o n b to intermediate stress relief heat treatment at 575 °C for 0.5 hours differs from an ordinary reactor pressure vessel weld metal and represents an inadequately stressrelieved material. This material can be considered to resemble repair weldings which are welded utilizing the temper-bead or the half-bead method but not post weld heat treated. The impact test

3. R e s u l t s

The results of the impact tests given as ductilebrittle-transition curves are seen in Figs. 1-3 and in Table 2. For the base metal of SA533B CI.1 the effect of thermal ageing is negligible. Even the toughness after ageing at 450 ° C for one year as well as at 350 ° C for 5 years are within the scatter of impact test results of the virgin material. Thus at least with the composition used in this study (e.g. low impurity content such as P = 0.005%) the steel SA533B CI.1 is not susceptible to embrittlei

~

t

~

I ~

T

~ - ~

1 ~ 1 ~

E ! 200

v

SA533B c L 1 Therma I agei ng :

.~// HJ

///

1o0

// / .S /

....

Parent metal

....

450°ci8800 h

.....

400°C/8500 h

....... 350°C/17500 h ....

,

t

I

-100

I

1

,

~

I

L

L

,

0

~

J

+100 TEMPERATURE

l

350°C/43500 h i

t

i

l

,

+200

(°C)

Fig. l. The effect of ageing on impact ductility of the SA533BC1.1 base material.

R. Pelli, J. ForstOn / Effect of thermal ageing on #npact ductility I

I

I

27

i

200

3

-- --

,/

/

m

Of/

"--'~

o

.iZ. /o/ o'

/#//0

SA 533

;O ./

...~',f...

--

initial

0

400 oc18600 h

•

900

i

1

i

-100

ELD METAL

THERMALAGEING:

O/

O//~# ~ / O

o

--

/

A/

~100

-- -- --2-:

. . . .

I

state

350 °c/aT00 h I

I

0 TEST TEMPERATURE(Oc)

+100

Fig. 2. The effect of ageing on the impact ductility of submerged arc weld metal in a SA533B plate. The weld metal has received a normal post weld heat treatment.

results indicate lower susceptibility to thermal ageing than after normal post weld heat treatment. The isothermal annealing of one year at 4 5 0 ° C I

I

I

r

r

,

,

clearly increased the transition temperature. The shift in transition temperature was 1 6 - 3 4 ° C depending on the used criterion. The effect of the

I

'

r

i

[

r

r

,

t

,

]

,

r

200

-)

.-"

'

_._.;°..

•

(2c w

",,/~/

/ // / i ,/" / / / ,//~'" / / / ," ~

~- 100 lg

0

,

,

I

-100

,

I

J

J

I

0

SA533B WELD METAL

~

LOwPWHT Thermalageing: -Parentmetal ----- 450°C/8800h ---- 400°C/8500h ........ 350°C/17500h i

i

i

I

I

+I00

i

I

i

i

I

i

,

+200

TEMPERATURE (°C)

Fig. 3. Theeffectofageing on theimpactductility ofsubmerged arc weld metalin a SA533B plate. The weld metalhasreceived only alow post weld heattreatment(575 ° C/0,5 h).

28

R. Pelli, J. Forst~n / Effect of thermal ageing on impact ductility

Table 2 T h e effect of t h e r m a l a g e i n g o n C h a r p y - V e n e r g y a b s o r p t i o n ( E A ) a n d lateral e x p a n s i o n (LE). T h e b a s e met',d s p e c i m e n s a r e t a k e n f r o m the ~ T p o s i t i o n in the t r a n s v e r s e ( T L ) o r i e n t a t i o n Material

Ageing temperature/

T r a n s i t i o n t e m p e r a t u r e (TT) c r i t e r i o n E A 41 J

E A 68 J

L E 0.89 m m

holding time TT S A 5 3 3 B C1.1 Base material

Initial state 450°C/8800h 400°C/8500 h 350 o C / 4 3 500 h 350 ° C / 1 7 500 h

SA533B W e l d metal, ordinary PWHT (620 o C / 3 0 h)

Initial state 400 o C / 8 6 0 0 h 350°C/8700 h

SA533B W e l d metal, low PWHT (575 o C / 0 , 5 h)

Initial s t a t e 450 o C / 8 8 0 0 h 400 o C / 8 5 0 0 h 350 ° C / 1 7 5 0 0 h

SA533B HAZ ( 6 2 0 o C / 3 0 h)

Initial s t a t e 350 o C / 8 7 0 0 h

ATT

TT

ATT

T'I

ATT

- 23 -19 -24 - 19 - 27

+4 1 ~-4 - 4

9 -7 12 ~ 10 -9

62 3 1 0

- 11 ~ I{} i7 lt - 15

- 105 - 63 -94

+ 42 + 11

- 88 - 45 78

+ 43 4 10

t}3 50 ~i}

+ 43 ~ t3

- 76 - 60 - 81 -82

+ 16 - 5 -6

- 64 --- 30 - 66 -58

+ 34 - 2 ~6

- 96 - 103

7

- 87 - 96

-9

q6

10

less severe annealings cannot be distinguished from the normal scatter of the test results. A SEM fractograph of a SA533B specimen (450 o C/8800 h) of the base material is given in Fig. 4. The fracture surface is partly covered by intergranular fracture indicating some susceptibility to thermal ageing. Intergranular fracture was not observed on base material specimens annealed at lower temperatures (350 and 400 ° C).

Fig, & S E M f r a c t o g r a p h o f a S A 5 3 3 B C i . 1 b a s e m a t e r i a l aged a t 4 5 0 0 C for 8800 hours and broken a t - 196 a C.

~ 1 - 6 4~ 4

The fractographs of the weld metal and the HAZ were also taken by SEM. Even in the case of undoubtedly embrittled specimens no clear intergranular fracture could be observed. A typical fracture surface is seen in Fig. 5. The SA533B base material specimen annealed at 450°C for 8800 hours resulted in an Auger electron spectroscopy (AES) spectrum seen in Fig.

Fig. 5. S E M f r a e t o g r a p h of a S A 5 3 3 B Weld m e t a l ( P W H t 575 o C / 0 . 5 h) a g e d a t 450 0 C f o r 8 8 0 0 hours and broken at -196°C.

R. Pelli, J. Forst~n / Effect of thermal ageing on impact ductihO'

SA 533 B 450 OC/ 8800 h

Fe

Fe

KINETIC ENERGY (eV)

Fig, 6. Auger electron spectrograph of a SA533B CI.I base metal steel aged at 450 ° C for 8800 hours.

6. The AES spectrum indicates small peaks of phosphorus and carbon, They can be the reason for the observed embrittlement.

4. Discussion When comparing the results from this study with literature data one can notice that Druce [3] has observed a higher thermal ageing effect on a SA533B C].I base material. The increases of the transition temperatures (68 J) due to ageing for 10000 hours at 450°C, 4 0 0 ° C and 300°C were in his study 30°C, 14°C and 19°C, respectively. The amount of impurities in both test materials was almost similar, only the contents of arsenic (0.02%) and tin (0.03%) were slightly higher in the material tested by Druce. In the case of the weld metal the effect of the thermal ageing was evident in this study. The reason for the extensive thermal embrittlement is probably due to the high nickel content (1.56% Ni) as the amount of alloying elements and impurities in the weld material otherwise does not significantly differ from the base material. The powerful effect of nickel on thermal ageing of a pressure vessel steel is verified for example by Balandin et al. [2]. The content of phosphorus is

29

slightly higher in the weld metal (0.008% instead of 0.005%) but the contents of tin and arsenic are about equal. Further, the amount of copper is also slightly higher in the weld metal (0.07%) as compared to that of the base metal (0.04%). In irradiation tests it has been shown that the combined effect of phosphorus and copper determines the irradiation embrittlement. One cannot rule out the possibility that they together also determine the thermal embrittlement. Druce et al. [4] have studied thermal ageing effects in the weldments of the steel A508 C1.3. Their results indicated no clear effect on property degradation after ageing at 4 5 0 ° C for 10000 hours. The weld metal did not contain nickel. The estimation of the thermal embrittlement at the end-of-life of a reactor pressure vessel is difficult. The effect of thermal ageing on clean reactor pressure vessel steels seems to be minor. Therefore are trends and influencing parameters often hidden under the normal scatter of the results. There is also uncertainty in converting the results from relatively short time annealings into the situation after 40 years of reactor operation. However, a proposal for carrying out the conversion is made here. The proposal is based on a pure thermal ageing effect with no combined effect of radiation a n d / o r stress. Based on literature data [6,8,9], the combined effect of temperature and time can be characterized by the following modified Hollomon- Jaffe temper parameter ( P ):

P = 7"(10 + log t),

(1)

where T is the temperature in Kelvins and t the time in hours. The same parameter P in eq. (1) as that corresponding to the full reactor lifetime (290 ° C / 3 0 years) can be achieved in an isothermal heat treatment at 350°C for one year. The corresponding annealing time at 400 °C would be about 800 hours. The conversion is possible only if the degradation mechanisms do not change as a function of the annealing temperature. In the steels SA533 and SA508 there are no indications of mechanism changes but recent studies [7] on low temperature sensitation of stainless steels have shown that the degradation mechanism might change in the low temperature range due to changes in chromium diffusion from bulk to grain boundary diffusion. One has to consider the possibility of a mechanism change also in the ferritic

30

R. Pelli, J. Forstdn / EffeCt of thermal ageing on impact ductility SA 533 B 80 base material D weld metal 60 c~

•

(PWHT 620 °C/SO h)

l we

i

0

v

40

20 Za

[

I i ii

I

I

350/Ia 350/2a 350/5a 400/Ia ANNEALING TEMPERATURE / DURATION (°C/years) I I iI 8500 9000 9500 THERMAL AGEING PARAMETER T (10 + logt) III 30 40 60 REACTOR OPERATION TIME (years) at 290 °C

I, 45011a I

10000

Fig. 7. The transition temperature shift ATI" as a function of the thermal ageing parameter for prediction of the thermal embrittlement during the service life of a reactor.

steels at low temperatures. Some authors have proposed other constants than 10 in eq. (1). Astafjev et al. [1] gave a value between 8 and 9, Engl and Fuehs [5] arrived at a value 12.6. The value of the constant is probably dependent on the content of alloy elements. Especially molybdenum seems to increase the activation energy resulting in a higher constant, The choice of the value 10 is in conformity with the results of Druce et al. [4]. The conversion of the measured embrittlement to end-of-life conditions shows (Fig. 7), that the thermal ageing of the base material SA533B is negligible for materials having compositions resembling that of the studied material. The weld metal corresponding to that used in this study is expected to exhibit a shift of about 1 5 ° C in the transition temperature at the end-of-life. However, its original transition temperature is very low. The results obtained here cannot directly be claimed to represent all compositions of SA533B C1.1, However, it is reasonable to believe that the results are representative for materials having a low impurity content. In a real reactor the interplay between thermal ageing, radiation and stresses goes on for the whole life span. It is not clear that the different degradation effects can be separated but there are no indication of important enhancements due to combined effects of the degradation mechanisms.

One has to remember that irradiations of surveillance tests are carried out at operating temperatures and that some specimens have been irradiated under stress. Long term thermal ageing on SA533B C1,1 is therefore assumed to contribute no more than 2 0 ° C to the transition temperature increase of the weldments. It is believed that careful composition control of weldments and base material can further reduce the thermal embrittlement.

5. Summary The base material and its heat affected zone of the reactor pressure vessel steel SA533B C1.1 h a s in long term thermal ageing proved to be immune to thermally induced embrittlement during a reactor lifetime. The corresponding weld metal exhibits a moderate susceptibility to thermal embrittlement. The shift of the transition temperature caused by thermal ageing seems to have no major influence on the integrity of a reactor pressure vessel.

References [1} A:A.Astafjev;V.A. Yukhanov, A.D. Shut and V.V. Bobkov, Method of determining the shift of critical brittleness temperature of vessel steels as a result of long-term thermal effects. Industrial Laboratory 50 (It)); 1014-1017 (t984),

R. Pelli, J. ForstOn / Effect of thermal ageing on impact ductili(v [2] J.F. Balandin, I,V. Gorynin, Ju.I. Zvezdin, B,A. Maslenok, V.A, Nikolajev and Ju.V. Sobolev, Perspektivy sover~enstvovanija perlimyh stalej dlja korpusov reaktorov i drugogo oborudovanija pervogo kontura AES. Energoma~inostroenie 10, 25-28 (1976). {3] S.G. Druce, A study of long term ageing effects in A533B Class 1 and A508 Class 3 steels, UKAEA Harwell, Report AERE-R 10252. 35 p. (1981). [4] S,G. Druce, G. Gage, G.R. Jordan and J.A. Hudson, Effect of thermal ageing on mechanical properties of PWR PV steels and weldments. Trans. 8th Internat. Conf. on Structural Mechanics in Reactor Technology. Brussels, 19-23 August, Vol. F2, pp. 401-408 (1985). [5] B. Engl and A. Fuchs, M/3glichkeiten einer zus~itzlichen Zfihigkeitssteigerung an thermomechanisch gewalztem Warmbreitband ffir Grossrohre,'Smhl und Eisen 10l (18), 25 30(1981).

31

[6] E.D. Hondros and M.P. Seah, Segregation to interfaces, Internat. Metals Rev. 22, 262-301 (1977). [7] T. Kekkonen, P. Aaltonen and H. H~inninem Metallurgical effects on the corrosion resistance of a low temperature sensitized welded AISI 304 stainless steel, Corrosion Sci. 25 (8, 9), 821-836 (1985). [8] J. Lagrange, Ph. Maynier, M. Palmier, A. Ponsot and J. Dollet, l~tude de la cin6tique de la fragilisation de revenu r6versible des aciers au carbone et faiblement allies, M6moires Scientifiques, Re~,. M~tallurgie 74 (12), 757-763 (1977). [9] M.P. Seah, Grain boundary segregation and the T - t dependence of temper brittleness, Acta Metallurgica 25 (3), 345-357 (1977).