Optimized materials for the future breeder line

Nuclear Engineering and Design 130 (1991) 1-5 North-Holland

1

Optimized materials for the future breeder line E. O h r t a n d E. te H e e s e n

Interatom GmbH, Bergisch Gladbach, Germany Received 7 May 1989, revised version 21 May 1990

This paper presents a survey of developments which form part of ongoing activities for the construction of breeder plants. Following a brief introduction it describes the history of an internationally coordinated material for the major components of a European breeder. Some material properties which are of importance for the design are discussed. The task of finding a suitable filler metal for steel 316L(N) (1.4909) is considered in greater detail. In this case too, selection criteria are the mechanical properties of the weld metal, its chemical and thermal resistance and its behaviour during welding. Finally, processes which are absolutely necessary in the construction phase of a power plant are discussed in the outlook. These have not been optimized to date and will therefore be the subject of internationally distributed activities in the subsequent phase.

1. Introduction

In the course of development work on sodium-cooled fast nuclear reactors all over the world it has become apparent that unstabilized "austenitic" steels of type AISI 304/316 are most suitable. They are used as structural material for the main components of the primary and secondary sodium systems. Steel X 6 CrNi 18 11 (type 304; material number 1.4948) was employed as plant material for the prototype breeder power plant SNR-300 constructed in Kalkar, Federal Republic of Germany. Within the context of national studies and considerations, extensive experience with the design and construction of the Kalkar nuclear power plant and with planning work for a follow-up project has led to a material change. As a result, steel type AISI 316 (e.g. 1.4919 or X 6 CrNiMo 17 13) is to be used for future breeder plants.

2. Structural material for the E u r o p e a n F a s t R e a c t o r (EFR)

Since experience has shown that the time and expenditure required for the development, full-scale testing and qualification of a new structural material are too high to be borne by any single European nation within the scope of construction of a nuclear power plant, the decision was taken to terminate independent

national work on the breeder line in favour of a European version. In the course of international project harmonization, a modified low carbon steel type 316L was developed for the main plant components, e.g. reactor vessel, hot tank internals, steam generators, IHX-structures and sodium-carrying piping. The alloy is an optimisation of that previously used for construction of the French breeder "Superph6nix". As a consequence of this international agreement between France, England, Italy and Germany, this new steel is currently being introduced in the Federal Republic of Germany with the designation X 2 CrNiMoN 17 12 and the material number 1.4909. It will be tested for all requirements in an extensive material testing program funded by research laboratories of the countries listed above. Its chemical composition is compared to that of steels 304 (1.4948) and 316 (1.4919) in table 1. The E F R material has the following characteristic features: - The carbon content which in contrast to 304 and 316, is lowered to values < 0.03% leads to a reduction in strength which is compensated for by the nitrogen content in the range of 0.06 to 0.08%. - At the same time, limitation of the C content and controlled nitrogen alloying significantly improve resistance to intercrystalline corrosion and aging. - A special feature of the E F R material 316L(N) is the close tolerance of the alloying elements, one example of which is the nickel content of 12.0-12.5% compared with 10-12% for steel 304 and 12-14% for

0 0 2 9 - 5 4 9 3 / 9 1 / $ 0 3 . 5 0 © 1991 - Elsevier Science Publishers B.V. All rights reserved

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E. Ohrt, E. te Heesen / Optimized materials for the future breeder line

Table 1 Chemical composition of unstabilized high-temperature austenitic steels Chemical analysis (wt%)

Steel type Designation

Material-No.

C 0.04

X 6 CrNi 18 11

1.4948 (304)

X 6 CrNiMo 17 13

1.4919 (316)

X 2 CrNiMoN 17 12

1.4909(316L(N))

0.07 0.040 0.080 0.015 0.030

Si < 0.75

< 2.00

< 0.75

< 2.00

< 0.50

steel 316. These chemical limitations also have positive effects on the scatter bands of the mechanical properties which are within relatively narrow limits in this steel. The strength behaviour of steel 304 is illustrated in fig. 1 using the example of the extrapolated creep strength for 105 h. Apart from the high-temperature range > 650°C there are practically no differences between 316L(N) (1.4909) and 316 (1.4919). However, the newly developed E F R material 316L(N) has clear advantages over the S N R structural material 304. Steel 316L(N) has meanwhile been used in G e r m a n y for the first time for a sodium test facility for temperatures up to 650 ° C which is being constructed on the Interatom site in Bergisch Gladbach. The licensing procedure for the base material and various filler metals

200

E E ~" 100 o E n,50

20 500

~ . ~ ~

600

./--1.4909 --1,4919 \, ~/-1.4948

Temperatu~~C)

700

Mn

800

Fig. 1. 105 h creep strength of high-temperature austenitic steels.

Cr 17.0 19.0

Mo < 0.50

Ni

N

B

10.0 12.0

16.0

2.0

12.0

18.0

2.5

14.0

1.60

17.0

2.30

12.0

0.06

2.00

18.0

2.70

12.5

0.08

< 0.0020

has already been practically completed for this construction project.

3. Adaptedffilermetais With the change in structural material from 304 to 316 and 316L(N) respectively it became necessary to investigate apparently suitable filler metals. In order to limit as far as possible the reduction factor for the calculation of the strength of components with welds in the licensing procedure it was necessary to find a filler metal with short and long-term mechanical properties which were as close as possible to the middle of the scatter band of the base material properties. Consequently, an alloy of the same composition as the base was first considered as the most obvious solution for the filler metal. However, because of the need for welding without hot cracks it has a slightly higher Cr and lower Ni content so that a &ferdte fraction results on solidification. F r o m the base material analysis C r N i M o 17 13 2, a weld metal C r N i M o 19 12 2 is obtained as shown in line 1 of table 2. F r o m the results of American [1] and European research [2,3,4] it is known that although this material has high short-term strength and good resistance to intercrystalline corrosion it embritfles on aging at temperatures around 600-650 ° C and is still susceptible to hot cracking if the necessary ferrite content is not carefully controlled. For this reason, work has been continuing on consumables similar in composition to the base metal for some time now. The filler metal

E. Ohrt, E. te Heesen / Optimized materials for the future breeder line

3

Table 2 Survey of steps for the electrode development 16-8-2; list of compositions Series Designation

Chemical analysis (wt%) C

Si

P

1.2 1.8

18.0 0.025 0.020 19.0

S

Cr

Mo

1.9 2.2

Ni

N

11.0 12.0

for inf.

Cr-equiv.

Ni-cquiv.

Delta-Ferrite

[%]

[%]

[%1

0

19-12-2

1

16-8-2-Elect. 0.055 0.21 1.18 0.016 0.007 16.50 3% 8-Fe 16-8-2-Elect. 0.050 0.21 1.14 0.017 0.006 16.87 6% 8-Fe

1.67

9.30 0.036

18.49

11.54

3.0

1.65

8.70

0.040

18.84

10.77

6.0

2

0.2% N 4% Mn

0.022 0.15 1.22 0.009 0.007 16.0 0.047 0.21 4.0 0.010 0.008 16.07

1.59 1.57

7.64 0.190 9.26 0.042

17.81 17.95

14.61 13.93

-4.5 - 2.5

3

Nh N h + Mn

0.022 0.21 2.25 0.009 0.007 17.40 0.025 0.19 4.38 0.009 0.007 17.05

2.30 2.28

8.20 0.177 7.90 0.185

20.02 19.63

15.3 16.4

1.0 - 3.5

( N h ffi

0.045 0.4 0.055 0.7

Mn

Nitrogen _highlevel)

CrNiMo 16 8 2, which was originally developed in USA, was used by various institutions in Europe for further development of certain properties. Its advantage was weldability without hot cracks and a high structural stability with only slight aging embrittlement. However, in its original composition, this alloy was not resistant to intercrystalline corrosion and its strength attained only the lower bound of the scatter band for the base material. First optimization tests performed by Interatom in close cooperation with Bt~hler on coated electrodes concentrated on the variation of the Cr-Ni-Mo ratios and hence different 8-ferrite contents in the weld metal [5]. Series 1 of table 2 is an example of this (Calculated values using the De Long-Diagram). However, the results of this study clearly showed that no significant improvements in strength and corrosion resistance could be attained although the strength of melts with low 8-ferdte content exceeded that of melts with higher 8-ferrite fractions. The creep results obtained for 3% and 6% ferrite containing welded joints were also in the lower scatter band of the base material values. This is indicated by the curves marked with a triangle in fig. 3. In order to increase resistance to intercristalline corrosion and come closer to the base material's strength properties, two electrode types were produced as an intermediate step. The &ferrite contents of these approximated to those of the 1st series while the C-contents were lowered to between 0.020-0.030%. The anticipated loss of strength was compensated by increasing the nitrogen content to 0.13% N. Since a positive trend was observed in weldability

and short-term mechanical properties, a second investigation series was produced in which the effects of carbon and nitrogen in this alloy were compared in detail. In one electrode with a lower carbon content the amount of nitrogen was increased to approx. 0.2%. The other high carbon variant was alloyed with 4% Mn and with the random nitrogen content of about 0.04%. The analysis of these two electrodes are presented as series 2 in table 2. The addition of an increased Mn-content to 4% was chosen because of two reasons. In the welding area also the lowest quantities of sulfur - wherever they derive from - should combine to Mn-sulfide and the remainder of Mn in solid solution is to strengthen the material by substitutional hardening. In case of an N-enriched variant Mn increases the stability of solved nitrogen in the lattice and thus postpones the formation of nitrides. For the electrode containing nitrogen, the cross weld tensile tests confirmed the anticipated increase in strength up to high testing temperatures with t h e fracture occurring in the base material in each case (open circles fig. 2). In contrast the high carbon variant containing Mn was very ductile but fractured exclusively in the less strong weld metal. These results led to the third series of tests which checked reproducibility of the nitrogen variant and compared its values with those of a high N + Mn version. These are given in the bottom line of table 2. Since the variation of the nitrogen content does not affect other features than the tensile strength the designation of electrodes with only nitrogen enrichment was N h (_high N content).

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E. Ohrt, E. te Heesen / Optimized materials for the future breeder line

Stress N/mm2

T

:3~/~-~. }.1Series o+N • + Mn

600 ,~X

}

2.Series

a+N } • + N + Mn 3.Senes 500 ~

3OO

200

100

0

200

400

• 600 Temperature (°C)

Fig. 2. Results of tensile and yield strength of 16-8-2 welded joints in 316L(N) steel.

The contents of/~-ferrite given in table 2 were calculated using the de Long-diagram. - Although no negative 8 ferrite can exist, there are such figures shown in the table in order to indicate how far the calculated value is located from the equilibrium line in the fully austenitic field of the diagram. Measured data are not given as they vary in a wide range in dependence of the investigated area of the weld. The contents normally decreases from the top layer to the root, but also varies with the amount of base metal penetration. In general could be stated that the de Long-calculation gives too low results. The higher the amount of N and especially Mn was the more differed the measured from calculated figures. As there exists in fact no equilibrium after welding, each weld showed very little &ferrite up to 3%. The welding characteristics of these electrode types proved to be good in all cases even though the electrode containing Mn burns softer than the variant alloyed only with nitrogen. There was no other difference in the production of the two types of electrodes. The used core wire was the same, and the composi-

tion of the coating differed from each other only in the amount of maganese. Nevertheless, there was a tendency of the low Mn-containing electrode to give a hard sound during welding with 140 A and 28 V DC and a mean heat input of 15-16 kJ/cm. Besides the Mn-rich type of electrode showed minor spattering as the other type did, and the nearly self removing slag was not so brittle with 4% Mn. The comparison of the tensile strengths for series 3 (fig. 2, square symbols) clearly indicates that there is practically no difference in the mechanical properties of the two electrodes and that these compositions did not only reproduce the good results of the N-rich filler (series 2) but even surpassed them. The tensile strength corresponds to that of the base material, and there is no loss in ductility as compared to previous electrode types. The notch toughness at room temperature is high at more than 100 J / c m 2 and only changes negligibly when aged at temperatures of up to 650 o C. However, the greatest improvement is exhibited by the creep behaviour where the strength of the welded joints with high nitrogen weld metal are in the centre of the scatter band of the base material 316L(N). (Note: The values presented in fig. 3 were obtained using the first nitrogen-alloyed electrode from series 2. Data for series 3 are not yet available). The good behaviour of the mechanical properties of the welded joint can be attributed to the high structural stability of the weld metal. This statement can be verified by tensile tests with specimens of the second series after aging at 550, 600 and 650°C for up to 15000 hours. The results for aging at 6 5 0 ° C are shown in fig. 4. Tests were performed at room temperature and at 650 o C. A major advantage of this new filler metal is that it

600 500-

~' 200

""~ ~'~

100 90" 70. 60. 50-

80"

~Base,l.4909 ----joint -16-8-2 solidSymbolslStSeries open SymbolsNh2ndSeries 40: I I ] 100 101 102 t [h] 103 104 .

.

.

.

.

.

.

.

.

.

105

Fig. 3. Creep strength of welded joints 16-8-2 in 316L(N) stainless steel.

E, Ohrt, E. te Heesen / Optimized materials for the future breeder line i

StreSS6 N/mm2 700

t

i

llllJ:

i

i

i

L

I

llll

,

,

,

I

ll*,

Aged at 650°C Rm

600

~\

500

Roo,2 ~

T,=RT

Tt Testtemperature

4OO 300 200

100

. . . . . . . . . .

~.

~T1=650 /

°C

~

5

103

5

104

t [hi

5

105

Fig. 4. Tensile tests of weldedjoints 16-8-2 in 316L(N) SS after aging at 650 o C (tested at room temperature and at 650 o C). is extremely resistant to hot cracks due to a little ferrite content. Even at external loading in the Varestraint test up to 4% strain it was impossible to produce cracks either in the weld metal or in any other part of the specimen.

4. Outlook Considering the results obtained for these coated electrodes it can be assumed that with the use of a good, proven base material a developed filler metal with similar properties will be available for high-temperature anstenitic components in a new large-scale project. After establishing a suitable filler metal composition it is now important to optimize the necessary welding processes. A suitable wire must be fabricated for the inert-gas welding process and a wire/flux combination has to be found for submerged-arc welding. In view of the tendency to make greater use of mechanical welding

processes, the parameter sets must be optimized for the different welding techniques and qualified in examination programmes. A start has already been made on such activities. This paper presents an overview of the development of base and filler materials for breeder plants during the last few years. Its purpose was to describe how European cooperation led to the production of a closely toleranced austenitic steel, the properties of which meet the imposed requirements. Since the filler metal of the same composition as the base is not as stable as the base material itself, a coated electrode of type CrNiMo 16 8 2 similar in composition to the metal being welded was developed in several steps. The promising development of fillers follows with delay that of the base material. Therefore, it is still necessary to evaluate all the properties, particularly the long term behaviour. It can be welded without hot cracks, exhibits mechanical properties similar to those of the base material and is structurally stable up to an aging time of 15000 hours and 650 o C. The creep results of the final version are not available yet. The adaptation to other welding processes must still be performed on the basis of the materials described above.

References [1] Lundin, De Long, Spond, Welding Research 8 (1975) p. 241s. [2] King, Goodwin, Boiling, ORNL 5105 4 (1975). [3] R. Boudot, EdF, Unpublished paper HC/PV, D365 MAT/T. 43 (1976). [4] G. Zacharie, EdF, Unpublished paper HC/PV, D373 MAT/T. 41 (1976). [5] E. Ohrt, Unpublished interim report 55.06776.8 (1984).