Procedia Engineering 55 ( 2013 ) 823 – 829
1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi: 10.1016/j.proeng.2013.03.338
6
thInternational Conference on Cree
A Comparison of Creep De
316L(N) Austenitic Stainles
S. Ravi
a∗, K. Laha
a, M.D. Mathe
K.K. Ra
a Metallurgy & Materials Group, Indira Ga b Fast Reactor Technology Group, Indira G
Abstract
Type 316L(N) austenitic stainless steel is used for advanced stage of construction at Kalpakkam, India steel has been investigated and results are compare Creep test on the material both in flowing sodium and Sodium velocity across the creep specimen was ma change the rate of steady state creep deformation si found to change significantly by the testing environm later for testing in sodium environment than that in ai in liquid sodium environment that in air environment companied with higher creep rupture elongation. Op showed extensive intergranular creep cavitation both in sodium showed relatively less creep cavitation. Al in flowing sodium and also no evidence of surface d SEM fractrographs of the creep ruptured specimens t whereas predominantly intergranular creep failure wa © 2013 The Authors. Published by Elsevier Ltd Gandhi Centre for Atomic Research.
Keywords: Creep; sodium; 316L(N) SS; damage tolerance fa
1. Introduction
AISI 316L(N) austenitic stainless steel (SS) is th secondary circuits of Liquid Metal cooled Fast primarily based on a god combination of i
∗ Corresponding author:
E-mail address: sravi@igcar.gov.in
ep, Fatigue and Creep-Fatigue Interaction [CF-6
eformation and Rupture Behaviour
ss Steel in Flowing Sodium and in
ew
a, S. Vijayaraghavan
b, M. Shanmugavel
b,
ajan
b, T. Jayakumar
aandhi Centre for Atomic Research, Kalpakkam – 603102, India Gandhi Centre for Atomic Research, Kalpakkam – 603102, India
r the fabrication of Proto-type Fast Breeder Reactor (PFBR) a. The influence of flowing sodium on creep rupture behaviour d with those obtained on carrying out creep test in air enviro d in air were carried out at 873 K over a stress range of 225- 30 aintained around 2.5 m/s. The testing environment was found ignificantly. The tertiary stage of creep deformation of the ste ment. The tertiary stage of creep deformation in the steel starte
ir environment. The steel possessed higher creep rupture life for t. Higher creep rupture strength of the material in flowing sodiu ptical micrographic investigation of the creep ruptured specime
in interior as well as on the specimen surface, whereas specimen lmost no oxidation was observed on the specimen surface creep damage due to possible carburization and decarburization was n tested in flowing sodium showed predominantly ductile dimple as observed in the creep ruptured specimen tested in air.
. Selection and/or peer-review under responsibility of the
actor
he chosen material for reactor vessel and primary and hot Breeder Reactors (LMFBR). The choice of type 316L(N ts tensile and creep properties and enhanced resistan
6]
r of
Air
) under r of the onment. 05 MPa. d not to eel was d much r testing um was en in air n tested p tested noticed. e failure Indira t leg of ) SS is nce to© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research.
sensitization. International experience has show of other variations of type 316 stainless steels 316L(N) SS include existence of vast data base data, ease of availability and fabrication and a selected for PFBR design. Creep rupture behav have been reported previously [2-3]. The findin 10,000 h at 873 K and results of post-rupture results are compared with the base line data gene
2. Experimental details
Chemical composition of the 316L(N) SS em the sodium loop and liquid sodium are summari 4 mm and gauge length of 21 mm (Fig.1) were Tests were conducted in the stress range of 225 the ranges of 100 – 10,000 h. Table. 1. C Element (Plate) C Cr Ni Mo C 0.02 17.93 12.09 2.43 Fig. 1. A Table. 2. INSOT Loop us Facility
Materials of construction Inventory of sodium Flow of sodium in the tes Sodium velocity at the sp Temperatures Specimen Main loop Cold leg Sodium chemistry Oxygen Carbon
wn that creep rupture strength of 316L(N) SS is superior especially at longer creep exposures [1]. Major advanta e on the mechanical properties including very long term above all, the availability of design data in the RCC-MR
vior of 316L (N) materials as affected by sodium enviro ngs of creep rupture tests performed on flowing sodium metallographic examination are presented in this paper. erated on the same heat of base materials in both environm
mployed in this study is given in Table 1. The characteris ized in Table 2. Cylindrical specimens with a gauge diam e used to carry out creep tests in air and sodium environ 5 – 305 MPa and the corresponding creep rupture lives w
Chemical composition (Wt. %). Mn Cu Si N P S B (ppm) Gra siz (um 1.76 0.44 0.3 0.06 0.03 0.01 20
60-schematic of creep specimen.
sed for creep rupture tests in flowing sodium INSOT Facility n SS316LN/316L 500 litre st section 0.45 – 0.5 m3/h pecimen 2.5 m/sec 873 ± 2 K 673 ± 5 K 398 ±5 K < 3 ppm < 28 ppm to that ages of m creep R code onment m up to These ments. stics of meter of nments. were in ain ze m) -70
3. Results and discussion 3.1. Creep deformation
Creep tests were carried out on the 316L(N) SS at 873 K over a stress range of 225 – 305 MPa in air and in flowing sodium environments and summary of results are given in Table. 3. The velocity of sodium was maintained at around 2.5 m/sec. Typical creep curves of the steel at 225 MPa and 873 K obtained on performing creep tests in flowing sodium and air environments are compared in Fig. 2. The variations of steady state creep rate (ȑs) with applied stress (ı) for both the environments are shown in Fig. 3. The stress
exponent ‘n’ was found to be 13.7 for creep tests in sodium environment and 12.5 for creep tests in air. The values of ‘n’ indicate that creep deformation of the material was controlled by dislocation creep mechanism in both the testing environments. The onset of tertiary stage of creep deformation occurred much early for sample tested in air than that in sodium environment, especially for creep tests at lower stresses (Fig. 2).
Table. 3. Summary of results obtained from the creep tests conducted in sodium and in air.
Stress (MPa)
Sodium environment, 873 K Air environment, 873 K
Life ratio to air test Life (hour) Elongation,
(%) SS rate (h-1) Life, (hour) Elongation, (%) SS rate (h-1) 305 72 52 0.0021 60 38 0.0027 1.2 275 338 54 6.2 x 10-4 210 39 9.28 x 10-4 1.6 265 757 45 2.1 x 10-4 350 42 5.9 x 10-4 2.1 250 1345 42 1.07 x 10-4 650 32 2.52 x 10-4 2 235 2700 44 7.11 x 10-5 1300 26 1.04 x 10-4 2.1 225 7800 47 3.52 x 10-5 3500 48 6.30 x 10-4 2.2
Fig. 2. Typical creep curves of 316L (N) SS tested in air and sodium environments at 873K at an applied stress of 225MPa.
Fig. 3. Variation of steady state creep rate with applied stress of the steel, creep tested in air and sodium environments.
3.2. Creep rupture life and damage
The variations of creep rupture life (tr) of the steel with applied stress (ı) for both the environments are
shown in a double-logarithmic plot in Fig. 4. The variation obeyed a power law relation as tr = A' ın', where
‘A'’ and ‘n'’ are the stress coefficient and the stress exponent respectively. Creep rupture life of the steel was found to increase in the sodium environment over that in air environment, the extent of which was more at lower applied stresses. The variation of creep rupture ductility of the material (percentage elongation) as a function of rupture life in shown Fig. 5. The creep rupture ductility of the material in sodium environment was much higher than that in air especially at longer creep exposures. Scanning electron microscopic (SEM) examinations of the fracture surfaces of the creep ruptured specimens are shown in Fig. 6. The fractographs
revealed predominantly transgranular failure characterized by the appearances of dimples appearances resulting from coalescence of microvoids (Fig.6 (a)) for the steel tested in sodium environment, whereas predominantly intergranular creep failure was observed in the steel for testing in air environment (Fig.6(b)).
Fig. 4. Variation of creep rupture life with applied stress of the steel, creep tested in air and sodium environments.
Fig. 5. Variation of creep rupture ductility with rupture life of the steel, creep tested in air and sodium environments.
(a) (b) Fig. 6. SEM factrographs of 316L(N) steel creep tested at 235 MPa, 873 K for testing in (a) flowing sodium, showing predominantly
ductile dimple failure (b) air, showing predominantly creep brittle failure.
The material was found to follow the Monkman-Grant relationship in both the testing environments (Fig. 7). It followed a linear equation of the form tot = f. tr where “f” is a constant and was found to depend on the testing
environment as shown in Fig. 8. The constant “f” was 0.39 and 0.49 respectively for the creep tests in the air and sodium environments. Based on Continuum Creep Damage Mechanisms (CDM) approach, an indication of the damage process initiating tertiary creep is provided by the creep damage tolerance parameter defined as λ = εf / (ȑs.tf), where εf is strain to failure, ȑs is steady state creep rate and tf is rupture life. Each damage
micromechanism, when acting alone, results in a characteristic shape of the creep curve and a corresponding characteristic value of λ. The value of damage tolerance parameter λ offers an insight into the damage mechanisms responsible for tertiary creep and eventual fracture. It has been predicted that for values of λ between 1.5 to 2.5, the tertiary stage of creep deformation is due to the growth of creep cavities; whereas it can be as high as 4 or more when microstructural degradation causes the damage. Fig. 9 shows the variation of creep damage tolerance factor λ with rupture time of the steel creep tested in air and flowing sodium environments. The average value of λ for the steel was around 2.5 for testing in flowing sodium and around 2 for testing in air. Such relatively low values of λ for the steel indicates that the intergranular creep cavitation
was the main damage mechanism in the steel and the creep cavitation was expected to be more prevalence in testing in air environment than that in flowing sodium environment.
Fig. 7. Variation of steady state creep rate with rupture life of the steel, creep tested in air and sodium environments.
Fig. 8. Variation of time to onset of tertiary stage of creep deformation with rupture life of the steel, creep tested in air and
sodium environments
Fig. 9. Variation of creep damage tolerance parameter Ȝ with rupture life of the steel, creep tested in air and sodium environments.
Optical micrographs describing intergranular creep cavitation both close to specimen surface and interior are shown in Figs. 10 and 11 respectively for testing in air and flowing sodium environments. The enhanced creep cavitations in specimen tested in air might be also due to oxygen adsorption and further diffusion along the grain boundaries which reduces the energy required for grain boundary sliding which in turn leads to creep cavity nucleation [4 -5]. Almost no oxidation was observed on the specimen surface creep tested in flowing sodium (Fig. 12) and also no evidence of surface damage due to possible carburization and decarburization was noticed. SEM micrographs (Fig. 13) show the possibility of ferrite phase formation on the surface of creep specimen due to leaching of alloying elements due to exposure in liquid sodium. It might be possible that the formation of ferrite phase on specimen surface due to leaching of element and the associated creep embrittlement effects would reduce the enhancement of creep rupture life of the steel in flowing sodium than that in air.
Fig. 10. Optical micrographs of creep ruptured test spec specimen an
Fig. 11. Optical micrographs of creep ruptured test specim creep specime
Fig. 12. SEM micrograph of creep tested specimen, tested MPa, 873 K in flowing sodium, showing almost no eviden
oxidation on the specimen surface.
cimen ( 235 MPa, in air ) showing creep cavities in (a) interior of the cre nd (b) at the specimen surface.
men (235 MPa, in flowing sodium) showing creep cavities in (a) interior n and (b) at the specimen surface.
at 235 nce of
Fig. 13. SEM micrograph of creep tested specimen, tested a MPa, 873 K in flowing sodium, showing surface leaching
possible ferrite formation
eep
of the
at 235 g and
4. Conclusions
• Creep curves of the specimen tested in sodium environment were characterized by primary, secondary and tertiary at most of the stress levels.
• The creep rupture life of 316L(N) SS at 873 K was longer in flowing sodium environment than that in air the extent of which was more at lower applied stresses.
• Steady state creep rate of the steel was not significantly effected by the testing environments.
• The tertiary stage of creep deformation of the steel started much early in air environment than that in sodium environment.
• The steel possessed relatively higher rupture ductility in sodium environment than that in air environment.
Acknowledgments
The authors thank Shri S.C. Chetal, Director, Indira Gandhi Centre for Atomic Research, for his constant encouragement during this work. The authors gratefully acknowledge Dr.A.K.Bhaduri, Associate Director, Materials Development & Technology Group, IGCAR for his constant support and encouragements. The authors acknowledge Shri. David Vijayanand and Ms. S. Paneer Selvi for this support in carrying out optical and scanning electron microscopy.
References
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[2] M.P.Mishra, H.U.Borgstedt, G.Frees, B.Seith, S.L.Mannan, P.Rodriguez, Microstructural aspects of creep-rupture life of Type 316L(N) stainless steel in liquid sodium environment, J. Nucl. Mater. 200(1993)244-255
[3] S.Ukai, S.Mizuta, T.Kaito, H.Okada, In-reactor creep rupture properties of 20% modified 316 stainless steel, J. Nucl. Mater. 278(2000)320-327.
[4] C.Phaniraj, K.G.Samuel, S.L.Mannan, P.Rodriguez, Effect of environment on creep properties of AISI stainless steel, International conference on creep, 1986, 205-208.
