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Engineering
Procedía Engineering 55 (2013) 17 - 26 -
www.elsevier.com/locate/procedia
6th International Conference on Creep, Fatigue and Creep-Fatigue Interaction [CF-6]
Creep and Low Cycle Fatigue Behaviour of Fast Reactor
Structural Materials
M.D.Mathew*, K.Laha, R.Sandhya
Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, India
Abstract
Low cycle fatigue and long-term creep properties, as well as compatibility with liquid sodium coolant, govern the choice of materials for out-of-core structural and steam generator components of sodium cooled fast reactors (SFRs). 316L(N) SS is preferred for all high temperature structural components while modified 9Cr-1Mo steel is preferred for the steam generator components for which resistance to stress corrosion cracking in steam is an additional requirement. A thorough understanding of the deformation, damage and fracture behavior of these materials helps to improve confidence in the design of the components and performance in service, and also to develop improved materials. The cyclic and creep deformation behavior have to be understood under complex stress conditions and in the liquid sodium, operating environment. This paper highlights the creep, low cycle fatigue and creep-fatigue properties of current SFR materials in air and sodium environments, and discusses the studies underway to develop improved materials for future SFRs for longer design life.
© 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review ander responsibility of the Indira Gandhi Centre for Atomic Research. Keywords: Creep; low cycle fatigue; sodium; 316L(N) SS; fast reactor
1. Introduction
Structural materials for out-of-core nuclear steam supply system (NSSS) components of sodium cooled fast reactors (SFRs) are chosen mainly on the basis of their high temperature low cycle fatigue (LCF) and long-term creep properties as well as compatibility with the liquid sodium coolant. Austenitic stainless steels are used for all the NSSS components, except the steam generator for which modified 9Cr-1Mo steel is chosen because of its resistance to stress corrosion cracking. During service, the components are subjected to repeated thermal stresses, as a result of temperature gradients, which arise due to thermal cycles during start-ups, shut-downs and power transients. Intervening long periods of steady state loading introduce creep deformation. Therefore, there is a need for developing a comprehensive understanding of the creep and cyclic deformation and damage behavior of the structural materials, which in turn will help to improve their fatigue and creep resistance. The
* Corresponding Author:
E-mail address: mathew@igcar.gov.in
ELSEVIER
1877-7058 © 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi: 10.1016/j.proeng.2013.03.213
components operate in liquid sodium environment and therefore the influence of flowing sodium on the creep, LCF, and creep-fatigue interaction (CFI) properties also needs to be understood. Currently, 316L(N) stainless steel (SS) containing 0.02-0.03 Wt.% carbon and 0.06-0.08 Wt. % nitrogen, is the preferred material for the out-of-core structural components, and modified 9Cr1Mo steel is the preferred material for steam generator components. In order to increase the economic competitiveness of SFRs, the design life of future reactors will be increased to at least 60 years from the present 40 years. Increasing the high temperature mechanical properties of 316L(N) SS by increasing the amount of nitrogen is one of the options under investigation. Whereas grade 91 steel, is a rather well understood material, improving the long-term properties of the weld points by alleviating the issues of type IV cracking is a major challenge. This paper highlights the creep, low cycle fatigue and creep-fatigue properties of these materials in air and sodium environments, and discusses the studies underway to develop improved materials for longer design life for future SFRs. The extensive research work that has been carried out at Indira Gandhi Centre for Atomic Research in the past few years forms the background for this paper.
2. 316LN SS
2.1. Creep properties
Extensive studies have been carried out to understand the creep and low cycle fatigue (LCF) properties of nitrogen-alloyed (0.06-0.08 Wt.%), low carbon grade (0.02-0.03 Wt.%) of 316(L) N stainless steel (SS) base metal, weld metal and weld joints. These studies have shown that the creep rupture strength of the weld joint is higher than that of the weld metal but less than that of the base metal. The material showed very good microstructural stability after long term creep exposure [1]. The effect of nitrogen content on the tensile, creep and low cycle fatigue properties of four heats of 316LN SS, containing 0.07, 0.11, 0.14 and 0.22 Wt. % nitrogen have been studied extensively [2,3]. All the heats had a nominal chemical composition (in Wt.%) 0.02- 0.03 carbon, 17-18chromium, 12.0-12.5 nickel, 1.6-2.0 manganese and 2.3-2.5 molybdenum. The grain size was in the range of 80 to 90 microns. Creep properties were studied at 823, 873 and 923 K up to 20,000 hours. Figure 1 shows significant improvement in rupture life at 923 K with increasing amount of nitrogen for various applied stress levels. Rupture life was higher by as much as ten times by increasing the nitrogen content from 0.07 to 0.22 Wt. %. In contrast, rupture ductility decreased with increase in nitrogen content (Fig. 2). A minimum in ductility-life curve was observed after short duration tests of the order of a few hundreds of hours. At longer rupture times, ductility increased continuously, a trend exhibited generally by austenitic stainless steels, because of grain boundary strengthening by the M23C6 type of inter granular carbides. These inter granular carbides retard grain boundary sliding and thus allow matrix deformation. The time spent in all the three creep stages increased with increase in nitrogen content. Detailed studies were carried out on 316LN SS containing 0.14Wt.% nitrogen. Power law creep with stress exponent in the range of 6-8 indicated dislocation creep as the rate controlling deformation mechanism. The stress-rupture life curves at 873 and 923 K (Fig.3) showed a linear variation (on log-log scale which was indicative of the strong microstructural stability of the material at these temperatures.
The improvement in the creep strength of 316LN SS due to nitrogen results from several contributions. Nitrogen substantially increases the concentration of free electrons in the steel and this promotes metallic interatomic bonding, since it makes electron exchange between atoms less directional and the distribution of electrons in the crystal structure more homogenous. As a result, dislocation glide takes place without weakening or breaking of the interatomic bonds and the high temperature strength is improved. Nitrogen causes higher dilation of the crystal structure than that by carbon and causes strong pinning of dislocations. The Young's modulus of 316LN SS increased from 201 GPa to 221 GPa on increasing nitrogen content from 0.07 Wt.% to 0.22 Wt.%. The dislocation cores are characterized by a shortage of free electrons; i.e., they possess a positive electric charge. Nitrogen atoms in austenitic steels are negatively charged as a result of localization of free electrons in the vicinity of nitrogen atoms. This results in an electrostatic interaction between dislocations and nitrogen atoms. Experimental measurements of "effective grain boundary energy" showed that nitrogen lowers grain boundary misorientation resulting in more stable grain boundaries. Stacking fault energy (SFE),
which is an important parameter that influences creep deformation behavior of materials decreased appreciably from 26.4 mJ/m2 to 15 mJ/m2 on increasing the nitrogen content from at 0.07 Wt. % to 0.22 Wt. % N. Nitrogen delays the rate of coarsening of M23C6. 316LN SS containing 0.07 Wt.% nitrogen, did not show the presence of nitrides or carbonitrides in the matrix even after creep testing for 9200 h (Fig. 4a). Fine carbonitride precipitates were observed in the matrix in the steel containing 0.014 and 0.22 Wt.% nitrogen after creep testing for 9700 h and 16,000 h respectively (Fig.4b and 4c). Increase in Young's modulus, decrease in stacking fault energy and precipitation of fine matrix carbonitrides contributed predominantly to the increase in creep strength with increasing nitrogen content.
—i—
0.12 0.16 Nitrogen, Wt%
225 MPa 200 MPa 175 MPa 140 MPa
—I—
90 8070
£ 60-_o
n 50 m
"S. £ 20-|
• 0.07 N ■ 0.11 N i 0.14 N
* 0.22 N
100 1000 Rupture life, h
Fig. 1. Influence of nitrogen content on creep rupture life of 316LN SS at 923 K.
Fig. 2. Influence of nitrogen content on creep rupture ductility of 316LN SS at 923 K.
316 LN SS
• 923 K ■ 873 K
316 LN SS (0.14 Wt. % N)
1000 Rupture life,h
Fig. 3. Influence of test temperature on rupture ductility of 316LN SS containing 0.14 Wt.% nitrogen.
Fig. 4. Microstructure of 316LN SS after creep test. (a) nitrogen content=0.07 Wt.%, rupture life= 9200 h, (b) nitrogen content =0.14 Wt.%, rupture life= 9700 h and (c) nitrogen content =0.22 Wt.%, rupture life= 16,000 h.
2.2. Fatigue properties
The influence of nitrogen content on LCF properties was studied at various strain amplitudes between room temperature and 873 K [3]. The cyclic stress response was generally characterized by initial hardening, followed by a saturation stage and a final cyclic softening stage. LCF life was found to increase with increase in the nitrogen content, up to 0.14 Wt.%. Above 0.14 Wt.% nitrogen content, fatigue life decreased with further increase in nitrogen content, up to 0.22 Wt.%, as shown in Fig. 5. Increase in fatigue life with increase in the nitrogen content has been attributed to increasing planar glide of dislocations and slip reversibility (i.e., less slip localization) whereas the reduction in fatigue life at nitrogen contents above 0.14 Wt.% is attributed to high matrix hardening, and consequent decrease in residual ductility. Crack initiation and propagation were found to be transgranular. Fatigue crack initiation took place along slip bands on the specimen surface. Initial propagation occurred along slip planes oriented at 45° to the applied stress axis, designated as stage I. This type of cracking continued to only one or two grain diameters and was followed by transition to stage II cracking. Stage II cracking was evidenced by striations on the fracture surface. Based on the creep and LCF properties, the nitrogen content in 316LN SS, has been optimized with 0.14 Wt.% for improved properties.
Fig. 5. Influence of nitrogen on fatigue life of 316L(N) SS at various temperatures.
2.3. Creep design curves
High temperature design of SFR components is made according to RCC-MR fast reactor design code. This code contains design rules for 316L(N) SS containing 0.02-0.Wt % carbon and 0.06-0.08 wt.% nitrogen. RCC-MR code defines time dependent allowable stress (St) as the least value among (i) 2/3 of the minimum stress inducing fracture, (ii) 80% of the minimum stress leading to the onset of tertiary creep and (iii) 100% of the minimum stress inducing total strain (elastic+ plastic + creep) of 1%. Creep data generated on high nitrogen grade 316LN SS have been analyzed according to RCC-MR code procedures for generating the design curves [4]. Figure 6 shows the isochronous stress-strain curves, derived for various times up to 3x105 h, for the heat containing, 0.14 Wt.% nitrogen. Increase in the nitrogen content, increased the allowable stress significantly, as shown in Fig.7. The analysis showed that the long term St values were controlled by the stress to produce 1% total strain.
0.6 0.8 1.0 1.2 1.4
Total Strain, %
Fig. 6. Isochronous stress-strain curves at 923 K for 316LN SS containing 0.14 Wt.% nitrogen.
Fig. 7. Variation of St at 923 K with increase in nitrogen content.
100000
Time, h
2.4. Notch sensitivity in creep
Although creep rupture ductility is not explicitly considered in the design codes, the substantial decrease in creep ductility with increase in nitrogen content deserves careful consideration and is a cause of concern. The complex geometries and the potential for weld defects in fabricated components necessitate the evaluation of the creep behavior of 316LN SS in the presence of notches. It is known that materials with low ductility show notch weakening. Creep tests have been carried out on notched specimens containing a V-notch at an angle of 60 0, with root radius of 0.19 mm, which provided a theoretical stress concentration factor Kt of 4.2. The notch increased the rupture life significantly for all the four heats. The trend curve showed a peak value, at 0.14 Wt.%, nitrogen content (Fig.8)[5]. The ratio of rupture life of smooth to notched specimen decreased with increase in the nitrogen content from as high as 18 for the material containing 0.07 Wt. % nitrogen to a low value of 3 for the material containing 0.22 Wt. % nitrogen, thereby implying that the notch strengthening effect decreased drastically at high nitrogen levels, although still showing some amount of notch strengthening.
0.05 0.10 0.15 0.20 0.25
Nitrogen, Wt. %
Fig.8. Variation of rupture life with nitrogen content for smooth and notched specimens tested at 923 K at a stress level of 200 MPa.
In order to understand the role of nitrogen on notch strengthening, finite element (FE) analysis of stress distribution was carried out in terms of (i) Von Mises equivalent stress (ii) hydrostatic stress (iii) maximum principal stress and (iv) triaxiality factor. The variation of normalised maximum principal stress with distance, from the notch root, is shown in Fig.9(a) for the four heats of 316LN SS. The FE contours of maximum principal stress for the heat with 0.22 Wt.% N are shown in Fig. 9(b). The peak stress was observed, closer to the notch root, it increased with increase in nitrogen content. The average stress across the notch section decreased with increase in nitrogen content. Against the nominal applied stress of 200 MPa, the average value
of the maximum principal stress saturated between 150 MPa and 175 MPa, while the peak value was more than 250 MPa. The nitrogen content vs rupture life curve for the notched specimen was found to be very close to the curve for the smooth specimen corresponding to 150 MPa. It was therefore concluded that the notch lowered the average value of the maximum principal stress and this led to a higher rupture life. Creep failure under the presence of a geometrical notch is controlled by maximum principal stress.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Normalised radial distance from the notch root
Fig. 9(a). Variation of maximum principal stress with distance from notch root
Fig. 9(b): Hydrostatic stress distribution around the notch for316LN SS containing 0.07 Wt. % nitrogen after 500 hours of creep analysis.
2.5. Creep strength of 316LN SS weld joints
Creep properties of weld joints made of 316LN SS plate, containing 0.14 Wt.% nitrogen, welded with 316N SS electrode containing 0.10 Wt.% nitrogen and 0.05 Wt.% carbon, showed that rupture life of the weld joint was lower than that of the base metal. The difference between the rupture life of the base metal and the weld joint increased with decrease in applied stress (Fig.10). It was interesting to note that the creep strength of weld joints, made of base metal containing 0.14 Wt.%, was higher as compared with the strength of weld joints made of base metal containing 0.07 Wt.% nitrogen. Though the same electrode was used for preparing both kinds of the weld joints. Creep failure of the weld joint specimens always occurred in the weld metal; however the rupture life was more than that of the weld metal. The gradient in the microstructure and hence the creep strength across the base metal, heat affected zone, and the weld metal, which constitute the weld joint, is suggested to create a metallurgical notch in the weld joint and a notch strengthening effect. This proposition has been modeled using FE analysis. Figure 11 clearly shows that a stress concentration develops at the weld interface akin to stress concentration at a geometric notch at the interface. The stress in the weld metal is lower than that at the interface.
base metal - weld joint weld metal
1000 Rupture Life, h
Fig.10. Comparison of isothermal stress rupture curves of base metal, weld joint and weld metal.
Fig.11. Stress distribution in 316L(N) SS weld joint smooth specimen indicating stress concentration at the weld-heat affected zone interface.
2.6. Thermomechanical fatigue behaviour of 316L(N) SS
Thermomechanical fatigue (TMF) studies attempt at simulation of the thermally induced fatigue loadings in a component by cycling simultaneously under mechanical strain and thermal strain. Thermal strain is induced by cycling the specimen, between a maximum and a minimum temperature. The two basic TMF cycle types that are most often used to assess lives under TMF conditions are in-phase (IP) TMF (peak tensile strain and peak temperature coinciding) and out-of-phase (OP) TMF (peak tensile strain and the minimum temperature coinciding). Studies carried out on 316L(N) SS showed that TMF cycling resulted in a mean stress, which was compressive under IP and tensile under OP cycling conditions, as shown in Fig. 12. In contrast, the mean stress was zero under LCF tests. The development of mean stress under TMF cycling is related to the change in the elastic modulus of the material with change in the temperature; a decrease in temperature causes an increase in the modulus. Therefore, under IP cycling conditions, a higher stress is required in the compression cycle (which is also the cooling cycle) to achieve the same imposed mechanical strain. Hence, the mean stress is compressive and life is longer. The reverse holds true under OP cycling, therefore, a tensile mean stress is developed and life is shorter. However, in the domain of creep temperature, life under IP cycling was lower (Fig. 13) due to creep effects [6]. The progressive reduction, in the difference, between the TMF and peak temperature LCF life, seen in Fig. 13, appears to be a consequence of the damaging influence of DSA. It operates when the temperature range of TMF cycling encompasses the DSA domain.
a 1500
□ □ In-Phase
♦ Out-of-Phase
o Isothermal LCF
-», B \
* » —. _ \
* * » * , >
o * * » „ • V
Number of Cycles
Fig.12. Comparison of cyclic stress response behaviour under IP and OP TMF cycling.
Fig.13. Comparison of cyclic lives in IP TMF and isothermal LCF cycling at Tmax. A constant temperature interval of250°C in TMF has been used for the comparison.
3. Modified 9Cr-1Mo Steel
3.1. Type IV cracking in modified 9Cr-1Mo steel
Modified 9Cr-1Mo, ferritic-martensitic steel is selected for steam generator components in view of its virtual immunity to stress corrosion cracking in steam environment, and its good high temperature mechanical properties. The steel derives its creep strength from its complex microstructure consisting of (i) tempered martensitic matrix with a high density of dislocations, (ii) large number of lath-boundaries, decorated with M23C6 carbides, and (iii) distribution of fine MX type of V,Nb-carbonitride in the matrix. The weld joint of the steel, possesses lower creep rupture strength than the base metal (Fig.14). Failure of the weld joint occurs in the creep-soft microstructural region of the heat affected zone (HAZ), also known as the intercritical region. This failure is often termed as type IV cracking. Differences in creep strength of the various microstructural constituents of the weld joint (viz. base metal, weld metal, coarse grain HAZ, fine grain HAZ and inter critical HAZ) produce a triaxial state of stress across the joint, during creep exposure. Finite element analysis of stress
behavior showed that the maximum principal stress and Von-Mises stress components developed their peak values in the soft inter critical HAZ (Fig.15) and thus promoted nucleation and growth of creep cavities in this region as shown in Fig.16. This leads to the premature type IV failure of the weld joint [7].
S 1 SO
W <0 100-
M o d ffied BCr-1 Mo
" ^ ■ 5 5 0 °C
^^^ EDO °C
Esse Joint
■ 323 K □ 8 2 3 K S50 °C
• 873 K < 873 K
923 K 923 K
1 00 1000 1 001 Rupture life, hour
Fig.14. Comparison of creep rupture life of modified9Cr-1Mo base and weld joint.
Fig.15. Stress triaxiality in the intercritical region of HAZ, leading to strain concentration and creep cavitation in HAZ intercritical region of (150 MPa/823 K).
3.2. LCF and creep-fatigue interaction behavior of ferritic steel
LCF studies carried out on modified 9Cr-1Mo steel base metal and weld joints showed that the stress response behavior was characterized by cyclic softening, right from first cycle onwards, unlike that of the 316L(N) SS, which exhibited initial cyclic hardening. This behavior is attributed to the microstructural degradation occurring during cycling, both in the base metal as well as in the weld joint. In case of the weld joint, the response of each microstructural constituent, to the applied external strain cycling, is different, depending upon its fatigue properties. The degree of softening, which is directly related to the cumulative plastic strain, is not uniform in each microstructural constituent of the weld joint. This strain localization, due to heterogeneity in microstructure, and cavitation around coarsened carbides, due to multi axial stress occurring in the weld joint, lead to lower fatigue life of the weld joint. Creep-fatigue studies of base metal and weld joint showed that tensile strain hold and compressive strain hold lowered the fatigue life (Fig.17). Compressive hold had a more damaging effect than a tensile hold, due to the effect of oxidation [8]. The oxide layer which forms on surface of the sample, during the hold time, being brittle breaks down upon unloading, and further cycling in tensile direction. Oxygen impregnates through the freshly exposed metal layer causing more damage in the material.
0) -u D
E ro c TO
■ ■ i • Weld jnint (873 K, ±0.6%)
C Continuous cycling
B 1 min TH
H 1 min CH
e 10 min CH
© 10 mill TH
© 30 min TH
B as© Hs,
Fig.16. Preferential creep cavitation in the intercritical HAZ of modified 9Cr-1Mo weld joint (T = 923 K, s = 40 MPa, tr = 15,700 h).
Number of cycles to failure
Fig.17. Comparison of creep-fatigue life under tensile and compressive hold conditions.
4. Effect of Liquid Sodium Environment on Creep and Fatigue Properties
The use of liquid sodium, as the heat transfer medium in SFRs, necessitates the assessment of creep, LCF and CFI properties of structural and stream generator materials, in controlled reactor grade sodium. Creep, LCF and CFI properties of 316L(N) SS and modified 9Cr-1Mo steel have been studied in flowing sodium (velocity of 2.5 m/sec)[9]. The purity of sodium was controlled to very low levels of oxygen (less than 2 ppm) and carbon (less than 10 ppm). The creep rupture life of 316L(N) SS at 873 K in sodium and air environments are compared in Fig. 18. It shows that creep rupture life, in flowing sodium environment, was significantly higher than that in air. The steady state creep rates in liquid sodium was about half of that in air. The increase in rupture life in sodium environment has been attributed to delayed occurrence of the tertiary stage as well as the extended tertiary creep stage. As a result, creep rupture ductility was higher in sodium environment than that in air.
Sodium environment
o Air environment
316L(N) SS
100 1000 10000 Rupture life, hours
Fig.18. Enhanced creep rupture life of 316L(N) SS in sodium environment.
i 0.01 ■ a
£ 0 008
indigenous Mod. 9Cr-1Mo Steel
Temperature B73 K
Strain Rate 3x10 ~V
■ Sodium
ionn mono
Number of Reversals To Failure, 2Nf
Fig.19. Comparison of LCF life in air and sodium environments at 873 K for Mod.9Cr-1Mo steel.
Mod.9Cr-1 Mo Steel 873 K
Strain Amplitude 0.6% Strain Rate 3 x 10-3s-1
Sodium Air
0 2 4 6 8 10
Hold Time (Minutes)
Fig. 20. Influence of hold time on the creep-fatigue interaction lives of Mod. 9Cr-1Mo steel at 873 K in air and flowing sodium environments.
Figure 19 shows the influence of sodium environment on the fatigue life of 316 L(N) SS base metal and weld joints. It can be seen that fatigue life in liquid sodium is substantially higher. This beneficial effect is attributed to the absence of oxidation in high purity sodium environment. Fatigue studies conducted on modified 9Cr-1Mo steel also showed higher fatigue life in liquid sodium environment by as high as a factor of
20. As modified 9Cr-1Mo steel is more prone to oxidation-induced crack initiation as compared to 316L(N) SS, and crack initiation being the dominant factor at lower strain ranges, this improvement in life was found to be more pronounced at lower strain ranges. Figure 20 depicts the influence of tensile hold time on the creep-fatigue life for Mod. 9Cr-1Mo steel base material at 873 K. Introduction of long hold periods did not yield any beneficial effect in sodium due to the controlling role of creep damage. An interesting observation was that it in air environment compressive hold was much more deleterious than a tensile hold. However, in sodium environment, a tensile hold was found to be more deleterious than a compressive hold, indicating that oxidation has a major role to play in the compressive dwell sensitivity of this material.
5. Summary
Extensive studies have been carried out on the effect of nitrogen content in the range of 0.07-0.22 Wt.% on creep and LCF properties of 316LN SS. Creep strength increased with increase in nitrogen content whereas LCF life showed a peak at 0.14 Wt.% nitrogen content. The optimum content of nitrogen was thus specified as 0.14 Wt.%. For a given welding electrode and welding process, creep rupture life of the weld joint was found to increase with increase in the strength of the base metal. Finite element analysis showed that the creep notch sensitivity is controlled by the maximum principal stress component. Thermomechanical fatigue life of 316L(N) SS was lower than the LCF life at the peak temperature of cycling due to creep effects. In modified 9Cr-1Mo steel, creep-fatigue interaction was more severe under compression hold than under tension hold. Liquid sodium environment was beneficial to creep, LCF and creep-fatigue life of 316L(N) SS and modified 9Cr-1Mo steel.
References
[1] M.D.Mathew, Trans. Ind. Inst. Metals, 63(2010)151-158.
[2] M.D.Mathew, K.Laha and V.Ganesan, "Improving creep strength of 316LN stainless steel by alloying with nitrogen", Mater. Sci. Engg. (in print).
[3] G.V.Prasad Reddy, R.Sandhya, K.Bhanu Sankara Rao and S.Sankaran - Procedia Engineering, 2(2010)2181.
[4] J.Ganesh Kumar, M.Chowdary, V.Ganesan, M.D.Mathew, R.K.Paretkar and K.Bhanu Sankara Rao, Nucl. Engg. Design, 240(2010)1363-1370.
[5] M.D.Mathew, V.Ganesan, J.Ganeshkumar and K.Laha, Creep Notch Sensitivity of 316 L(N) SS and the Effect of Nitrogen , 21st Intl Conf. On Structural Mechanics in Reactor Technology, SMIRT 21, Nov. 6-11, 2011, Delhi, CD Proc. Paper 191 (2011).
[6] A.Nagesha, R.Kannan, P.Parameswaran, R.Sandhya, K.Bhanu Sankara Rao and Vakil Singh, Materials Science and Engineering A, 527(2010)5969.
[7] K.Laha, K.S.Chandravathi, P.Parameswaran and K.Bhanu SAnkara Rao, Met. Trans, A, 40A(2009)386.
[8] Vani Shankar, M.Valsan, K.Bhanu Sankara Rao, R.Kannan, S.L.Mannan and S.D.Pathak, Materials Science and Engineering A, 437(2006)413.
[9] R.Kannan, R.Sandhya, V.Ganesan, M.Valsan, and K.Bhanu Sankara Rao, J. Nucl. Mater., 384(2009)286-291.
