Development of Structural and Steam Generator Materials for Sodium Cooled Fast Reactors

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Energy Energy Procedia Procedia 7 (2011) 6 (2011) 250–256 1–5

Asian Nuclear Prospects 2010

Development of Structural and Steam Generator Materials for Sodium Cooled Fast Reactors M.D. Mathew, R. Sandhya , K. Laha Mechanical Metallurgy Division Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

Abstract Nitrogen-alloyed low-carbon grade type 316L(N) SS is the principal material for high temperature structural components of the Prototype Fast Breeder Reactor (PFBR). Creep, and low cycle fatigue (LCF) properties of 316L(N) stainless steel base metal, weld metal and weld joints have been evaluated and found to fully meet the properties assumed in the design of PFBR components. In-sodium experiments have shown that flowing sodium has no detrimental effect on the creep and LCF properties of 316L(N) SS. Modified 9Cr-1Mo is the steam generator material of PFBR. Creep behaviour of indigenously produced modified 9Cr-1Mo steel and its weld joint have been characterized. The mechanism of degradation of creep rupture strength of modified 9Cr-1Mo steel weld joint due to type IV cracking has been established.

© by by Elsevier Ltd. Ltd Selection and/or peer-review under responsibility of Indra Gandhi © 2011 2010Published Published Elsevier Centre of Atomic Research Key words: Fast reactors, PFBR, LCF, 316L(N) SS

1. Introduction Economic success of Sodium Cooled Fast Reactors (SFRs) is largely dependent on the high temperature mechanical properties of structural components so that they can be designed for very long life - as long as 60 years and more. Materials for high temperature structural components should have good creep, low cycle fatigue (LCF) and creep-fatigue interaction properties, compatibility with liquid sodium coolant, weldability, and the design data shall be available in international codes. Efforts are underway to design future SFRs with a design life of 60 years in order to improve the economic competitiveness. Studies are therefore being carried out to increase the high temperature creep and LCF strength of 316L(N) stainless steel (SS) by increasing the nitrogen content from the present 0.06-0.08 wt.% to 0.12-0.14%. Steam generator plays an important role in economic operation of SFRs. Modified 9Cr-1Mo steel is selected for steam generator of PFBR. It is therefore important to characterize the

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1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Indra Gandhi Centre of Atomic Research doi:10.1016/j.egypro.2011.06.032

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high temperature mechanical properties such as creep and fatigue properties of higher nitrogen alloyed 316 LN SS, modified 9Cr-1Mo ferritic steel and their weld joints. The use of liquid sodium as a coolant in fast reactors, necessitates the assessment of the compatibility of structural materials, and the influence of long–term exposure to sodium on the mechanical properties of reactor structural materials. Currently the design of core and structural components of SFRs is based on test data generated in air while the actual environment seen by these components during service involves sodium. Moreover, the current trend being life extension of power plants, it is important to accurately determine the life of components in the actual environment, so as exploit potential extension of life beyond the envisaged design life. With this view, an elaborate programme has been initiated at Mechanical Metallurgy Division to evaluate the effects of dynamic sodium on the low cycle fatigue, creep and creep-fatigue interaction behaviour of indigenously manufactured reactor materials such as 316L(N) stainless steel and Modified 9Cr-1Mo ferritic steel and their weld joints. This paper presents the results of the creep and LCF experiments conducted on 316L(N) stainless steel and modified 9Cr-1Mo ferritic steel and their weld joints in both air and sodium environments. The effect of nitrogen alloying in 316 LN SS has also been evaluated. 2. Experimental Creep tests have been conducted on 316 SS having nitrogen in the range 0.07 wt. % to 0.22 wt. % and modified 9Cr-1Mo base metal, weld metal and weld joints over a wide stress and temperature ranges. Creep tests have also been conducted on base and weld metal of 316 SS. LCF tests have been conducted in the temperature range 300 to 873 K on 316 LN SS with nitrogen content varying from 0.07 to 0.22 wt.%. Weld joints of 316L(N) SS have also been evaluated for their fatigue resistance. Chambers, designed in-house has been used to study the effect of sodium environment on fatigue properties of 316L(N) SS and modified 9Cr-1Mo steel and their weld joints at 823 and 873 K and creep properties of 316 LN SS. 3. Results and Discussion 3.1. Creep Properties Figure 1a shows the variation of steady state creep rate of type 316L(N) SS with applied stress. The creep rates of 316 SS are also superimposed in Fig.1(a) for comparison. An order of magnitude decrease in steady state creep rate was observed in type 316L(N) SS due to addition of about 0.07 wt.% nitrogen. Significant increase in creep rupture life was also observed (Fig.1b) [1]. The beneficial effects of nitrogen arise due to higher solubility of nitrogen in the matrix compared to carbon, reduction in stacking fault energy of the matrix and introduction of strong elastic distortions into the crystal lattice, giving rise to strong solid solution hardening [2]. Nitrogen slows down the diffusivity of chromium in austenitic stainless steels leading to retardation in coarsening of M23C6 thereby retaining the beneficial effects of fine carbide precipitation to longer times [3]. Solubility of nitrogen in austenitic SS being high (typically 0.15 wt% at 923 K), nitrides are not formed during creep at these temperatures.

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-5

Steady state creep rate, s-1

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923

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316L(N) SS 316 SS

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(a)

-11

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150

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Stress, MPa 400

873 K base metal

Applied stress, MPa

350 300

22% increase in strength

250

200

150 316L(N) SS 316 SS

(b)

100 10

100

1000

10000

Rupture Life,h

Fig.1. Influence of nitrogen on creep properties of 316L(N) SS base metal. (a) on steady state creep rate and (b) on rupture life.

The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt.% by keeping the rest of the composition unaltered [4,5]. The carbon content in these heats was 0.03 wt.%. Creep rupture strength increased substantially with increase in nitrogen content as shown in Fig. 2. Rupture life increased almost 10 times by increasing the nitrogen content from 0.07 wt% to 0.22 wt.%. The increase in creep rupture life with increasing nitrogen has been attributed to the difference in substructure formed during creep deformation and the decreased crack density with increasing nitrogen content . The creep properties of modified grade of type 316 SS and its welds (prepared using E316-15 basic coated electrodes with a ferrite number of 3 to 6) at 823, 873 and 923 K over a wide range of stress levels have been studied [6]. 316 SS weld metal crept at a faster rate than the base metal by a factor of ten at 823 K (Fig. 3) and by a factor of two at 873 K. At 923 K, the minimum creep rates of the base and weld metals were found to be similar at all stress levels. Higher creep rate of the weld metal at 823 K has been attributed mainly to the lack of precipitation of M23C6 in the weld metal matrix where precipitation of carbides was mainly confined to delta/austenite interphase unlike in the case of base metal where matrix precipitation of carbides helped in strengthening the material.

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316 LN SS

Rupture Life, h

10000

1000

100

140 MPa 175 MPa 200 MPa 225 MPa

0.04

0.08

0.12

0.16

0.20

0.24

0.28

Nitrogen content, wt%

Fig. 2. Influence of nitrogen on the creep rupture life of 316 SS.

Fig. 3. Comparitive evaluation of creep rates of base and weld metal of 316 SS

The creep-rupture strength of indigenously developed modified 9Cr-1Mo steel in rolled, forged and tube product forms were found to be higher than the average strength values reported in RCC-MR design code (Fig.4). Creep rupture strength of modified 9Cr-1Mo steel and its fusion welded joint is compared in Fig.5. The weld joint of the steel possessed lower creep rupture strength than base metal. The strength reduction was more at higher temperature and longer creep exposure (Fig.5) [7]. Failure in the weld joint occurred in the outer edge of HAZ, commonly known as Type IV failure. Creep strength gradient across the joint led to the preferential elongation coupled with cavitation in the soft intercritical region of heat affected zone (HAZ) causing premature failure in the steel weld joint. 500

Applied Stress, MPa

Mod. 9Cr-1Mo Steel

RCC-MR Average RCC-MR Minimum

400 300 200

823 K

100 90 80 70

Forging (70 mm diameter) 873 K Plate (12 mm thickness) Tube (17.2 mm diameter and 2.3 mm thickness) 1

10

2

10

3

10

4

10

5

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Rupture Life, h

Fig. 4. Comparison of the creep rupture strength of modified 9Cr-1Mo steel in different product forms

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APPLIED STRESS, MPa

Modified 9Cr-1Mo 200

100 90 80 70 60 50 40

BASE 823 K BASE 873 K BASE 923 K JOINT823 K JOINT873 K JOINT923 K

10

100

1000

10000

RUPTURE LIFE, HOURS

Fig. 5.Comparison of the creep ruptures strength of modified 9Cr-1Mo base metal and weld joint.

3.2. Fatigue Properties In nitrogen alloyed 316LN SS, LCF life has been found to increase with increase in nitrogen under continuous cycling conditions, up to 0.14 wt% nitrogen. Above this nitrogen content saturation/decrease in fatigue life has been observed in the investigated nitrogen content range from around 0.07 to 0.22 wt% [8]. Increase in fatigue life has been attributed to the increased planar glide of dislocations and slip reversibility (i.e., less slip localization) as compared to low nitrogen steels [9]. It is suggested that nitrogen retards dynamic strain ageing (DSA) by decreasing Cr-diffusivity and promotes tendency towards formation of short range order (SRO) of Cr and N. Based on the continuous cycling LCF tests in the present study, conducted in the temperature range 300-873 K on 316LN SS with 0.11, 0.14 and 0.22 wt% nitrogen, it is found that the beneficial effect of nitrogen on fatigue life is observed to be increasing/saturating with increase in nitrogen content for temperatures < 673 K while it is found to be maximum at 0.14 wt% nitrogen at temperature > 673 K (Fig.6). The reduction in fatigue life beyond 0.14 wt. % could be attributed to high matrix hardening (and hence decrease in residual ductility), with a consequent decrease in LCF life.

Fig. 6.Influence of nitrogen on fatigue lives of 316L(N) SS at various temperatures

Ductility plays a governing role in fatigue life of base and weld joints of 316LN SS also. LCF life is generally governed by the ductility of the material at high strain amplitudes and by strength of the material at low strain amplitudes. The lower ductility of weld metal causes reduction in its LCF life compared to the base-material. Delta-ferrite in welds and weldments with a vermicular morphology undergoes transformation to M23C6 and V-phase during LCF testing. The transformed amount of Gferrite increases with increasing number of cycles to failure and increasing temperature. The fine duplex austenite-ferrite microstructure in the weld metal, with its transformed G-V phase boundaries, offers greater resistance to the extension of fatigue cracks by causing the deflection of crack path. Crack deflection could cause reduced stress intensity at the crack tip with an associated reduction in the

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crack propagation rate. The beneficial effects of G-V transformation could be clearly seen in the comparative evaluation of fatigue lives of 316L(N) base and 316 SS weld metals [10]. At 773 K, the weld metal showed a lower life than the base metal, as the transformation of the G-ferrite to V is less and the beneficial effects of crack deflection could not be fully realized. At 873 K, (Fig.7), the LCF life of 316 SS weld metal is superior to 316L(N) base metal.

PLASTIC STRAIN AMPLITUDE

TEM PER ATU R E : 873 K -3 S T R A IN R A T E : 3 x 10 s

10

-2

10

-3

10

-4

-1

3 1 6 L (N ) B a s e M e ta l 3 1 6 W e ld M e t a l 3 1 6 L ( N ) / 3 1 6 W e ld J o i n t 10

2

10

3

10

4

N U M B E R O F R E V E R S A L S T O F A IL U R E , 2 N

f

Fig. 7. Influence of G-V transformation on LCF life of 316 LN SS.

3.3. Creep and LCF Properties in Flowing Sodium Figure 8 shows the influence of sodium environment on the fatigue life of 316 LN SS base and weld joints. It is seen that compared to fatigue life in air, the fatigue life is substantially improved in sodium environment. Sodium has been found to have a beneficial effect on the fatigue properties of materials due to the lack of oxidation effects in a high purity sodium environment [11]. Fatigue tests conducted on modified 9Cr-1Mo steel show an enhanced difference in fatigue life between air and sodium environments compared to 316 L(N) steel. As modified 9Cr-1Mo is prone to oxidation induced crack initiation compared to stainless steel, and crack initiation being the dominant factor at lower strain ranges, this improvement in life is more pronounced at lower strain ranges in sodium environment in this material. Fatigue life at lower strain ranges in modified 9Cr-1Mo steel is enhanced by a factor of 20 as compared to that in air [12]. The results of the creep tests conducted at 873 K in dynamic sodium environment are shown in Fig. 9. It can be seen from the figure that there is only a marginal beneficial effect of sodium on the creep rupture life; the rupture life a lower stress levels are increased by a factor of 2, while at higher stress levels the life in sodium and air are comparable. Creep being a bulk damage phenomenon, the effect of environment is only marginal.

TOTAL STRAIN AMPLITUDE

Air - Base Metal Air - Weld Joints

316L(N) SS -3 -1 3 x 10 s , 823 K

0.01

Flowing Sodium - Base Metal Flowing Sodium - Weld Joints 100

1000

10000

100000

NUMBER OF REVERSALS TO FAILURE, 2N f

Fig. 8. Influence of flowing sodium environment on the fatigue life of 316L(N) SS base and weld joints at 823 K.

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320

Air test Sodium test

Stress,MPa

300 280 260

240

316L(N) SS 873 K

50

100

500

1000

5000

Rupture life, h Fig. 9.Influence of flowing sodium environment on the creep rupture life of 316L(N) SS at 873 K.

4. Summary Creep life of 316LN SS tested with varying amounts of nitrogen exhibits an increase in life with increasing nitrogen content, while fatigue life shows a saturation at a nitrogen content of 0.14 wt.%. Creep studies show that the base metal has the highest creep strength followed by the weld joint and the weld metal, while joints exhibited the lowest fatigue lives, and the base material the highest. Sodium environment has been found to be beneficial for fatigue properties. Creep rupture strength of modified 9Cr-1Mo steel possess lower creep rupture strength than the base metal due to type IV failure in the intercritical HAZ. Acknowledgements The authors thank Dr.Baldev Raj, Director, IGCAR and Dr.T.Jayakumar, Director, Metallurgy and Materials Group for their keen interest in this work. References [1] M.D.Mathew and V.S.Srinivasan, in Proc. Monograph on High Nitrogen Austenitic Steels and Stainless Steels, U. Kamachi Mudali and Baldev Raj (Eds.), Narosa Publications, New Delhi, (2004) pp.182-204. [2] Girish Shastry, M D Mathew, K Bhanu Sankara Rao and S L. Mannan, Trans. IIM, 58, 275-279 (2005) [3] G Sasikala, S K Ray, S L Mannan and M D Mathew, Trans. IIM, 53, 223-229, (2000) [4] V Ganesan, M D Mathew, K Bhanu Sankara Rao and Baldev Raj in 6th European stainless steel conference science and market, Helsinki: June 10–13, 2008, (Eds.) Pentti Karjalainen and Staffan Hertzman, 445-451, 2008. [5] J. Ganesh Kumar, M. Chowdary, V. Ganesan, M.D. Mathew, R.K. Paretkar and K. Bhanu Sankara Rao, Nucl. Engg. Design, 240, 1363-1370 (2010). [6] M D Mathew, G Sasikala, S L Mannan, and P Rodriguez , Mater Sci Tech 7, 533 (1991). [7] K. Laha K..S. Chandravathi, P. Parameswaran, K Bhanu Sankara Rao and SL Mannan Met. and Mats Trans A 2007; 38A: 58-68. [8] Dae Whan Kim, Woo-Seog Ryu, Jun Hwa Hong, Si-kyung Choi, J. Nucl. Mater. 254 226 -233 (1998). [9] J.O.Nilsson., Scripta Metall. 17 593 (1983). [10] S.L.Mannan,“G.D.Birla Medal Lecture” delivered during the National Metallurgists’ day (1999). [11] M.D.Mathew, K. Laha, M. Valsan, R.Sandhya, S.Latha, and K. Bhanu Sankara Rao, in Proc. International Conference on Peaceful Uses of Atomic Energy-2009, New Delhi, India. [12] R.Kannan, R.Sandhya, V.Ganesan, M.Valsan and K.Bhanu Sankara Rao, Journal of Nuclear Materials 2009, Vol. 384 286-291.