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Short Communication on “Self-welding susceptibility of NiCr-B hardfaced coating with and without NiCr-B coating on 316LN stainless steel in flowing sodium at elevated temperature”
Journal of Nuclear Materials 484 (2017) 141e147
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Self-welding susceptibility of NiCr-B hardfaced coating with and without NiCr-B coating on 316LN stainless steel in flowing sodium at elevated temperature Hemant Kumar a, b, *, V. Ramakrishnan b, S.K. Albert b, A.K. Bhaduri b, K.K. Ray a a b
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721 302, India Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2016 Received in revised form 23 November 2016 Accepted 30 November 2016 Available online 5 December 2016
The self-welding susceptibility between NiCr-B coated 316LN stainless steel and the base metal, and that between NiCr-B hardfaced coatings has been evaluated in flowing sodium at 823 K for 90 and 135 days under contact stress of 8.0 and 11.0 MPa using a fabricated set-up. Neither any self-welding could be observed nor could any damage be detected on the specimen surfaces of the selected materials under the imposed experimental conditions, which indicate their satisfactory potential for applications in Fast Breeder Reactors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Self-welding Flowing sodium NiCr-B hardfaced coating 316LN stainless steel
1. Introduction Austenitic stainless steel of 316LN grade is commonly used as one of the major structural materials in the Fast Breeder Reactors (FBRs). The extensive use of this material in FBRs is due to its excellent combination of mechanical properties and compatibility with liquid sodium. However, various components made of this material are exposed to liquid sodium during service; the latter is typically used as a coolant in Indian Prototype Fast Breeder Reactor (PFBR). Liquid sodium reacts readily with oxide films present on the surfaces of these materials. The oxygen content in liquid sodium is usually restricted to maximum 3 ppm to reduce corrosion rates of the structural materials; as a consequence, the oxide film barrier for diffusion through contact surfaces disappears. This phenomenon permits metal to metal contact at the mating surfaces. Since the contact stress and the temperature at the mating point are relatively high, there exists the possibility for self-welding of these mating surfaces. The phenomenon of self-welding between mating surfaces occurs at the contact points of the asperities [1] and is governed by
* Corresponding author. Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721 302, India. E-mail address: [email protected] (H. Kumar). http://dx.doi.org/10.1016/j.jnucmat.2016.11.032 0022-3115/© 2016 Elsevier B.V. All rights reserved.
several factors like temperature, contact stress, dwell time and surface roughness. The characteristics of self-welding for different material combinations have been investigated by several authors [2e6] in liquid sodium but with an insufficient focus to study the self-welding characteristics of different types of hardfaced coatings. The self-welding susceptibility of hardfaced coatings has been examined by Yoshida et al. [7] for different hardfacing materials and by Huber [5] for Stellite-6 and Colmonoy-6 at a few temperatures and contact pressures by assessing the self-welding ratio and the breakaway force. However, current developments and design requirements for Indian PFBR need understanding about the selfwelding susceptibility of NiCr-B (equivalent to Colmonoy-5) hardfaced coating on 316 LN stainless steel in flowing sodium for contact pressures up to 11 MPa and service temperatures of 673 K803 K specifically for grid plate for a service life of 40 years. The motivation in this investigation is to generate understanding to fulfill a part of these requirements.
2. Experimental details The combinations of materials selected for self-welding susceptibility tests in liquid sodium in this investigation are (a) NiCr-B hardface coating and 316 LN stainless steel, and (b) two identical NiCr-B hardface coatings. The 316LN stainless steel was available in
142
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
the form of commercial rods of about 30 mm diameter. Hardface coating of NiCr-B of approximately 2 mm thickness was made on the cross-section of 316LN stainless steel rods by Gas Tungsten Arc Welding (GTAW) process using ER NiCr-B filler wire. The welding parameters were: Current ¼ 120e140 A, Voltage ¼ 22 V, Shielding gas ¼ Argon, Flow rate ¼ 9e12 lpm etc. The chemical compositions of 316LN stainless steel and NiCr-B hardface coating were analysed by optical emission spectroscopy (Model: ARL 3460 Metal Analyzer, Switzerland). Specimens for microstructural examinations were suitably ground and polished up to 0.25 mm finish following standard metallographic practices. Samples of 316LN stainless steel were electrolytically etched using 10% oxalic acid whereas the coated surfaces were etched using a solution of HCl þ HNO3þH2O (0.4:1:1) to reveal the microstructures. The microstructures of the specimens were examined by an optical microscope (Olympus, GX51-F, Japan). The grain size of 316LN stainless steel was determined by linear intercept method following ASTM standard E112. The hardness of 316LN stainless steel and the hardfaced coatings were determined using cylindrical specimens (~15 mm diameter and ~10 mm height) by a Vickers Hardness Tester using a load of 5 kgf for the dwell time of 15 s. The chemical compositions of the 316LN stainless steel and the NiCr-B hardfaced coating are given in Table 1. Typical microstructures of these materials are shown in Fig. 1. The microstructure of 316LN stainless steel reveals polygonal grains of austenite with annealing twins whereas the microstructure of NiCr-B hardfaced coating reveals precipitates of carbides and borides on the matrix of g-Nickel phase. The carbides are commonly found in the shape of needles whereas the borides are found in the blocky form. The microstructure of the cross-section of NiCr-B coated specimen is also shown in Fig. 1c. The figure indicates a continuous interface between the coating and the 316LN substrate showing mixed zone with predominantly cellular mode of solidification. The average grain size of austenite in 316LN stainless steel was estimated as 85 ± 8 mm. The average macro-hardness values (HV5) of 316LN stainless steel and NiCr-B hardfaced coating, estimated from at least five readings, are found to be 178 and 542 respectively. Specimens for self-welding susceptibility tests were hollow cylinders of 21.4 mm outer diameter, 15.8 mm inner diameter and 15 mm height as shown in Fig. 2. The top and bottom surfaces of the cylindrical specimens form the mating surfaces for the self-welding tests. Specimens of 316LN stainless steel were machined from the available cylindrical rods in the solution annealed condition whereas hardfaced specimens were prepared by EDM wire cutting. The mating surfaces of both 316LN stainless steel and the hardfaced specimens were subsequently ground to achieve the surface finish of 0.8 mm as estimated by the profilometer (Talysurf, CLI-1000, UK). The self-welding susceptibility tests were carried out by using a fabricated set-up as reported earlier [8,9]. In brief, the set-up consists of an inner shaft, outer shell, load cell and provision for holding specimens at the bottom (Fig. 1 of Ref [8]). The contact pressure was applied by tightening the disc springs at the top of the fabricated set-up. The whole test set-up was immersed in a cylindrical vessel in which liquid sodium flows at a temperature of 823 K. The purity of the sodium in the vessel was maintained by passing liquid sodium through a purification system consisting of prefilter and cold trap; the cold trap was maintained in the
temperature range of 396e406 K. The self-welding susceptibility tests between “NiCr-B hardface coating and 316LN stainless steel” and between “two identical NiCrB hardface coatings” in reactor grade flowing sodium with oxygen content of less than 3 ppm were carried out using the procedure described above. The parameters used for these tests are summarized in Table 2. 3. Results and discussion The results of the self-welding susceptibility studies between “NiCr-B hardface coating and 316 LN stainless steel” and between “two identical NiCr-B hardface coatings” in reactor grade flowing sodium are summarized in Table 3. Self-welding could not be noticed in the ‘NiCr-B vs. 316LN stainless steel’ couple under 8.0 MPa contact stress tested for 90 days and ‘NiCr-B vs. NiCr-B’ couple under 8.0 and 11.0 MPa contact stresses for 90 and 135 days respectively. A typical observation for lack of self-welding between NiCr-B hardfaced specimens tested for 90 days durations is illustrated in Fig. 3. Specimens subjected to self-welding susceptibility tests for 90 days were taken out from the set-up and were first cleaned using ethyl alcohol and then using a mixture of alcohol þ water from the traces of sodium. These were subsequently examined under Scanning Electron Microscope (SEM) with Energy-Dispersive Analysis of X-rays (EDAX). These examinations indicated no significant surface damage on either 316LN stainless steel specimens or on NiCr-B hardfaced coatings as shown in Figs. 4 and 5 respectively. However, a few spherical shaped particles were observed on the mating surface of NiCr-B specimens (Fig. 5); the amount of these particles is higher on the surfaces of the specimens taken from ‘NiCr-B hardfaced coating - NiCr-B hardfaced coating’ couple than that on the specimens from‘NiCr-B hardfaced coating e 316LN stainless steel’ couple. These particles were traced as corrosion products (occurring due to the formation of different complex metal oxides like NaCrO2) resulting from the interaction between sodium and NiCr-B hardfaced coating. Subsequently, NiCr-B coated specimens after 90 days of test duration were also cut perpendicular to the coated surface and were examined under SEM. A typical microstructure beneath the mated surface region (after the self-welding susceptibility test) is shown in Fig. 6. Comparison of the microstructure on the surface and that at the cross-section revealed no significant changes in the size, morphology or the nature of the constituent phases in the coated layer. Typical results of EDAX analyses on NiCr-B coating and 316LN stainless steel specimens are shown in Fig. 7. The results in Fig. 7 indicate low sodium count rate, which is the signature of corrosion products resulted from the interaction of the specimen with sodium. Interestingly both EDAX plots show marginal sodium count, but it is lower in the case of 316LN stainless steel specimen. This may be due to higher interaction of NiCr-B surfaces with flowing sodium compared to that with 316LN stainless steel. Further, there was no change in the integrity of the interface between NiCr-B coating and the substrate after the tests. The corrosion traces observed on NiCr-B hardfaced coating is considered to be caused primarily due to the dissolution of nickel into sodium, since solubility of nickel is higher than that of the other metal elements. Accordingly, nickel concentration on
Table 1 Chemical composition of 316LN stainless steel and NiCr-B hardface coating. Material
Ni
Cr
Mo
Mn
Si
B
C
P
S
N
Fe
SS316LN NiCr-B coating
10.61 Bal
17.67 11.56
2.65 e
1.78 e
0.35 3.87
e 2.03
0.02 0.53
0.043 e
0.0021 e
0.06 e
Bal 3.97
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
143
Fig. 1. Microstructures of (a) 316LN stainless steel (b) NiCr-B hardfaced coating and (c) cross section of NiCr-B coated specimen showing interface between coating and the substrate.
exposed surfaces of NiCr-B coated specimen perpendicular to the mating surfaces was also determined using SEM-EDAX. SEM micrographs along with EDAX elemental counts are shown in Fig. 8. EDAX count rates revealed on average 78.9 wt% nickel near the mating surface which is exposed to liquid sodium for 90 days at
823 K compared to 85.7 wt% nickel approximately 20 mm away from the mating surface, which is supposed to be unexposed. These results indicate that there is some amount of dissolution of nickel from the NiCr-B surface exposed to liquid sodium. The parameters for self-welding susceptibility tests have been selected such that it fulfils the conditions prevailing in the one of the major assemblies between grid plate and core support structure of the PFBR. Since grid plate holds the active core of the reactor, it is considered to be one of the major components of the reactor. As per design, the bottom of the grid plate rests on the top flange of the core support structure [10]. In the event of self-welding at the
Table 2 Parameters used for self-welding susceptibility tests.
Fig. 2. Specimen for in-sodium self-welding susceptibility tests.
Parameters
Magnitude
Temperature Contact stress Duration of testing Operating medium Surface finish of mating surfaces
823 K 8 and 11 MPa 90 and 135 days Flowing Sodium 0.8 mm
Table 3 Results of self-welding susceptibility tests in reactor grade flowing sodium. Material combinations
Surface roughness (mm)
Test temp. (K)
Contact stress (MPa)
Contact load (kg)
Dwell time (days)
No. of specimens tested
No. of specimens self welded
Break away shear force (kg)
Self welding behaviour
NiCr-B vs.316LN SS NiCr-B vs. NiCr-B NiCr-B vs. NiCr-B
0.8 0.8 0.8
823 823 823
8.0 8.0 11.0
133.4 133.4 183.4
90 90 135
4 5 5
Nil Nil Nil
e e e
No self-welding No self-welding No self-welding
Fig. 3. NiCr-B coated specimens after self-welding test for 90 days (showing no selfwelding between two adjacent specimens).
mating surfaces of these two components, the relative radial movement (occurring due to thermal expansion or contraction) between these two components get arrested, and such a condition is not acceptable during operation of the reactor. The mating surfaces would be experiencing a temperature of 673 K during normal operation of the reactor for its designed service life of 40 years. However, during transient conditions such as reactor power ramping, this temperature (673 K) could shoot up to approximately 803 K. The total duration of all the transient conditions is 150 h during the reactor design life of 40 years. Since the total duration of contact between the above-mentioned mating components is high and the operating temperature varies, it is difficult to carry out
Fig. 4. SEM micrograph of mating surface showing no surface damage in 316LN stainless steel specimen after 90 days test in flowing sodium at 823 K (a) at lower magnification showing cross-section of mating surface (b) at higher magnification, region marked by arrow in (a).
Fig. 5. SEM micrograph of mating surface showing no surface damage in NiCr-B specimen after 90 days test in flowing sodium at 823 K (a) at lower magnification showing crosssection of mating surface (b) at higher magnification, region marked by arrow in (a).
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
145
Fig. 6. Microstructure taken perpendicular to the NiCr-B coating after testing for 90 days at (a) top region near to mated surface of the specimen and (b) 0.5 mm away from the mated surface region.
laboratory simulated tests particularly for such long duration. To overcome this difficulty, accelerated self-welding tests were carried out at 8 MPa contact stress for 90 days; estimated from theoretical calculations, in flowing sodium at 823 K. If there is no self-welding observed in the above material combinations after the accelerated tests, then it is unlikely that there will be self-welding between these two components under the prevailing operating conditions of the reactor. Limited information on self-welding of Ni-based hardfacing alloys (Colmonoy with some particular chemistry) in flowing sodium is available in the literature. However, some information explaining the correlation between susceptibility to self-welding and contact stress as well as the duration of sodium exposure is available only for austenitic stainless steel mating surfaces. Since no information is available for the investigated material combinations, data on selfwelding of austenitic stainless steel have been considered for
estimating the contact stress required for accelerated tests that would simulate the operating conditions in PFBR. Self-welding studies in flowing sodium at 823 K, carried out for austenitic stainless steel combinations, have shown that self-welding coefficient W, defined as the ratio of breakaway stress by shear to contact stress, varies with duration of test t as:
W2 ¼ K t
(1)
where K is the rate constant for self-welding, and t is contact period (in s). K is expressed as:
K ¼ K0 expðQ =RTÞ
(2)
where K0 is a constant and Q is the activation energy, and R is the gas constant. The values of K for temperatures of 803 K, 673 K and
Fig. 7. Elemental count rate by EDAX in (a) NiCr-B and (b) 316LN stainless steel specimens after 90 days test in flowing sodium at 823 K.
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H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
823 K are calculated using K0 (1.7 1010 s1) and Q (218 kJ/mol) values from the report of Yokota et al. [11]; this naturally incorporates the assumption that the material combinations under investigation behaves as if like stainless steel and has the same probability for self-welding in liquid sodium environment. Initially, K values corresponding to the operating temperatures and the temperature for the self-welding tests are estimated using equation (1). The estimated K values for different temperatures are found to be: K803 ¼ 1.1198 104 s1 K673 ¼ 2.0434 107 s1 K823 ¼ 2.4762 104 s1 Subsequently considering the service conditions prevailing in PFBR, the following calculations were made to estimate the contact stresses at which self-welding is likely to occur in the selected material combinations under the normal operating condition and transient condition. These are described below: Normal Operating Condition (T ¼ 673 K, contact stress ¼ 11 MPa and duration ¼ 40 yrs). For normal conditions, For normal conditions, equation (1) can be written as:
For the total reactor life : For the test duration :
W673 2 ¼ K673 t673
W823 2 ¼ K823 t823
(3) (4)
where t673 is 40 years and t823 is 90 days. Using equations (3) and (4), the ratio W673:W823 is 0.37. The ratio W673:W823 is considered equivalent to the ratio of contact stress at 823 K to the contact stress at 673 K; assuming that if selfwelding occurs, the breakaway shear stress would be the same both in service and in 90 days accelerated test. Since the contact stress at 673 K is 11 MPa (known), the contact stress for the accelerated test is estimated to be 4.07 MPa. Transient Condition (T ¼ 803 K, contact stress ¼ 11 MPa and duration ¼ 150 h)
For the total reactor life : For the test duration :
Fig. 8. NiCr-B coated specimen after 90 days test in flowing sodium at 823 K (a) SEM of coating cross section (b) elemental count rate by EDAX near the mating surface at location marked spectrum 5 and (c) elemental count rate by EDAX approximately 20 mm away from the mating surface at location marked as spectrum 4.
W803 2 ¼ K803 t803
W823 2 ¼ K823 t823
(5) (6)
Since the total duration to be spent in the reactor at 803 K is only 150 h, which is less than the normal test duration of 90 days, both t803 and t823 are taken as 150 h. Accordingly, the ratio W803:W823 is found to be 0.67. Assuming that if self-welding occurs, the breakaway shear stress required is same for both in service and in 150 h test, from the definition of W, the contact stress required for the accelerated test is estimated to be 7.4 MPa. The values of stress levels which initiate self-welding corresponding to 90 days at 823 K for the normal reactor operation and that for 150 h at 823 K for the transients were calculated as 4.07 and 7.4 MPa respectively. Since the stress level to simulate the transient condition is higher, the self-welding susceptibility tests were conducted at 8 MPa for the duration of 90 days (instead of 150 h, the maximum expected cumulative duration of transient condition) to get a conservative estimate of the combined effect of both the normal and transient conditions that exist in the reactor operation. Hence, if accelerated self-welding tests at stress levels estimated from the design stress do not result in self-welding, it can be safely concluded that for reactor operation of 40 years self-welding is
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
unlikely to occur in these mating surfaces if one of the mating surfaces is NiCr-B coating. 4. Conclusions (i) Self-welding could not be perceived between the mating surfaces of ‘NiCr-B vs. 316LN stainless steel’ under 8.0 MPa contact stress for the duration of 90 days and ‘NiCr-B vs. NiCrB’ under 8.0 and 11.0 MPa contact stresses for the duration of 90 and 135 days in high purity flowing sodium at 823 K. (ii) These critical experimental observations coupled with pertinent theoretical analyses infer that self-welding would not occur between the mating surfaces of grid plate hardfaced with NiCr-B coating and core support structure made of 316LN stainless steel of Prototype Fast Breeder Reactor (PFBR) under the prevailing operating conditions. Acknowledgement The authors acknowledge the support and advice received from Dr. K.K. Rajan, Former Director of Fast Reactor Technology Group, Dr. C. Meikandamurthy of Fast Reactor Technology Group and members of Material Technology Division of Indira Gandhi Centre for Atomic Research.
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References [1] S. Chander, C. Meikandamurthy, R.D. Kale, R. Krishnamurthy, Wear 162e164 (1993) 458e465. [2] W.H. Roberts, Tribol. Int. 17 (1981) 17e28. [3] K. Bendorf, Nucl. Eng. Des. 14 (1970) 83e98. [4] J.Y. Chang, S.L. Schrock, Self-welding evaluation of Co-Cr-W (Stellite) alloy in flowing sodium, in: Int. Conf. Liq. Met. Technol. in Eng. Prod., Richland, Washington, USA, June 21, 1980. [5] F. Huber, K. Mattes, Investigations into the self-welding behaviour of metallic materials exposed to sodium, in: 1st Int. Conf. Liquid Met. Technol. in Eng. Prod., Champion, Pennsylvania, USA, May 3-6, 1976. [6] P. Marshall, J.R. Gwyther, A.J. Hooper, The influence of high temperature sodium, vacuum and argontel bolted assemblies, in: H.U. Borgstedt (Ed.), Material Behaviour and Physical Chemistry in Liquid Metal Systems, Plenum Press, New York, 1982, pp. 467e474. [7] E. Yoshida, Y. Hirakawa, S. Kano, I. Nihei, In-sodium tribological study on cobalt-free hard facing materials for contact and sliding parts of FBR components, Proc. Int. Conf. on Liq. Met. Technol., Socite Francaise d’ Energie Atomque, Paris 502 (1988) 1e10. [8] Hemant Kumar, S.K. Albert, V. Ramakrishnan, C. Meikandamurthy, G. Amarendra, A.K. Bhaduri, J. Nucl. Mat. 374 (2008) 1e8. [9] C. Meikandamurthy, Hemant Kumar, G. Chakraborty, S.K. Albert, V. Ramakrishnan, K.K. Rajan, A.K. Bhaduri, J. Nucl. Mat. 407 (2010) 165e170. [10] Hemant Kumar, V. Ramakrishnan, S.K. Albert, C. Meikandamoorthy, A.K. Bhaduri, R. Sridharan, V. Balasubramaniyan, R. Vijayashree, V.R. Babu, Insodium Tribology Studies on Hardfaced Material Combinations of Core and Reactor Assembly Components e Experimental Plan and Current Status, October 2005. PFBR/31000/EX/1058/R-B. [11] N. Yokota, S. Shimoyashiki, Nucl. Technol. 81 (1988) 407e414.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Self-welding susceptibility of NiCr-B hardfaced coating with and without NiCr-B coating on 316LN stainless steel in flowing sodium at elevated temperature Hemant Kumar a, b, *, V. Ramakrishnan b, S.K. Albert b, A.K. Bhaduri b, K.K. Ray a a b
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721 302, India Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2016 Received in revised form 23 November 2016 Accepted 30 November 2016 Available online 5 December 2016
The self-welding susceptibility between NiCr-B coated 316LN stainless steel and the base metal, and that between NiCr-B hardfaced coatings has been evaluated in flowing sodium at 823 K for 90 and 135 days under contact stress of 8.0 and 11.0 MPa using a fabricated set-up. Neither any self-welding could be observed nor could any damage be detected on the specimen surfaces of the selected materials under the imposed experimental conditions, which indicate their satisfactory potential for applications in Fast Breeder Reactors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Self-welding Flowing sodium NiCr-B hardfaced coating 316LN stainless steel
1. Introduction Austenitic stainless steel of 316LN grade is commonly used as one of the major structural materials in the Fast Breeder Reactors (FBRs). The extensive use of this material in FBRs is due to its excellent combination of mechanical properties and compatibility with liquid sodium. However, various components made of this material are exposed to liquid sodium during service; the latter is typically used as a coolant in Indian Prototype Fast Breeder Reactor (PFBR). Liquid sodium reacts readily with oxide films present on the surfaces of these materials. The oxygen content in liquid sodium is usually restricted to maximum 3 ppm to reduce corrosion rates of the structural materials; as a consequence, the oxide film barrier for diffusion through contact surfaces disappears. This phenomenon permits metal to metal contact at the mating surfaces. Since the contact stress and the temperature at the mating point are relatively high, there exists the possibility for self-welding of these mating surfaces. The phenomenon of self-welding between mating surfaces occurs at the contact points of the asperities [1] and is governed by
* Corresponding author. Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721 302, India. E-mail address: [email protected] (H. Kumar). http://dx.doi.org/10.1016/j.jnucmat.2016.11.032 0022-3115/© 2016 Elsevier B.V. All rights reserved.
several factors like temperature, contact stress, dwell time and surface roughness. The characteristics of self-welding for different material combinations have been investigated by several authors [2e6] in liquid sodium but with an insufficient focus to study the self-welding characteristics of different types of hardfaced coatings. The self-welding susceptibility of hardfaced coatings has been examined by Yoshida et al. [7] for different hardfacing materials and by Huber [5] for Stellite-6 and Colmonoy-6 at a few temperatures and contact pressures by assessing the self-welding ratio and the breakaway force. However, current developments and design requirements for Indian PFBR need understanding about the selfwelding susceptibility of NiCr-B (equivalent to Colmonoy-5) hardfaced coating on 316 LN stainless steel in flowing sodium for contact pressures up to 11 MPa and service temperatures of 673 K803 K specifically for grid plate for a service life of 40 years. The motivation in this investigation is to generate understanding to fulfill a part of these requirements.
2. Experimental details The combinations of materials selected for self-welding susceptibility tests in liquid sodium in this investigation are (a) NiCr-B hardface coating and 316 LN stainless steel, and (b) two identical NiCr-B hardface coatings. The 316LN stainless steel was available in
142
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
the form of commercial rods of about 30 mm diameter. Hardface coating of NiCr-B of approximately 2 mm thickness was made on the cross-section of 316LN stainless steel rods by Gas Tungsten Arc Welding (GTAW) process using ER NiCr-B filler wire. The welding parameters were: Current ¼ 120e140 A, Voltage ¼ 22 V, Shielding gas ¼ Argon, Flow rate ¼ 9e12 lpm etc. The chemical compositions of 316LN stainless steel and NiCr-B hardface coating were analysed by optical emission spectroscopy (Model: ARL 3460 Metal Analyzer, Switzerland). Specimens for microstructural examinations were suitably ground and polished up to 0.25 mm finish following standard metallographic practices. Samples of 316LN stainless steel were electrolytically etched using 10% oxalic acid whereas the coated surfaces were etched using a solution of HCl þ HNO3þH2O (0.4:1:1) to reveal the microstructures. The microstructures of the specimens were examined by an optical microscope (Olympus, GX51-F, Japan). The grain size of 316LN stainless steel was determined by linear intercept method following ASTM standard E112. The hardness of 316LN stainless steel and the hardfaced coatings were determined using cylindrical specimens (~15 mm diameter and ~10 mm height) by a Vickers Hardness Tester using a load of 5 kgf for the dwell time of 15 s. The chemical compositions of the 316LN stainless steel and the NiCr-B hardfaced coating are given in Table 1. Typical microstructures of these materials are shown in Fig. 1. The microstructure of 316LN stainless steel reveals polygonal grains of austenite with annealing twins whereas the microstructure of NiCr-B hardfaced coating reveals precipitates of carbides and borides on the matrix of g-Nickel phase. The carbides are commonly found in the shape of needles whereas the borides are found in the blocky form. The microstructure of the cross-section of NiCr-B coated specimen is also shown in Fig. 1c. The figure indicates a continuous interface between the coating and the 316LN substrate showing mixed zone with predominantly cellular mode of solidification. The average grain size of austenite in 316LN stainless steel was estimated as 85 ± 8 mm. The average macro-hardness values (HV5) of 316LN stainless steel and NiCr-B hardfaced coating, estimated from at least five readings, are found to be 178 and 542 respectively. Specimens for self-welding susceptibility tests were hollow cylinders of 21.4 mm outer diameter, 15.8 mm inner diameter and 15 mm height as shown in Fig. 2. The top and bottom surfaces of the cylindrical specimens form the mating surfaces for the self-welding tests. Specimens of 316LN stainless steel were machined from the available cylindrical rods in the solution annealed condition whereas hardfaced specimens were prepared by EDM wire cutting. The mating surfaces of both 316LN stainless steel and the hardfaced specimens were subsequently ground to achieve the surface finish of 0.8 mm as estimated by the profilometer (Talysurf, CLI-1000, UK). The self-welding susceptibility tests were carried out by using a fabricated set-up as reported earlier [8,9]. In brief, the set-up consists of an inner shaft, outer shell, load cell and provision for holding specimens at the bottom (Fig. 1 of Ref [8]). The contact pressure was applied by tightening the disc springs at the top of the fabricated set-up. The whole test set-up was immersed in a cylindrical vessel in which liquid sodium flows at a temperature of 823 K. The purity of the sodium in the vessel was maintained by passing liquid sodium through a purification system consisting of prefilter and cold trap; the cold trap was maintained in the
temperature range of 396e406 K. The self-welding susceptibility tests between “NiCr-B hardface coating and 316LN stainless steel” and between “two identical NiCrB hardface coatings” in reactor grade flowing sodium with oxygen content of less than 3 ppm were carried out using the procedure described above. The parameters used for these tests are summarized in Table 2. 3. Results and discussion The results of the self-welding susceptibility studies between “NiCr-B hardface coating and 316 LN stainless steel” and between “two identical NiCr-B hardface coatings” in reactor grade flowing sodium are summarized in Table 3. Self-welding could not be noticed in the ‘NiCr-B vs. 316LN stainless steel’ couple under 8.0 MPa contact stress tested for 90 days and ‘NiCr-B vs. NiCr-B’ couple under 8.0 and 11.0 MPa contact stresses for 90 and 135 days respectively. A typical observation for lack of self-welding between NiCr-B hardfaced specimens tested for 90 days durations is illustrated in Fig. 3. Specimens subjected to self-welding susceptibility tests for 90 days were taken out from the set-up and were first cleaned using ethyl alcohol and then using a mixture of alcohol þ water from the traces of sodium. These were subsequently examined under Scanning Electron Microscope (SEM) with Energy-Dispersive Analysis of X-rays (EDAX). These examinations indicated no significant surface damage on either 316LN stainless steel specimens or on NiCr-B hardfaced coatings as shown in Figs. 4 and 5 respectively. However, a few spherical shaped particles were observed on the mating surface of NiCr-B specimens (Fig. 5); the amount of these particles is higher on the surfaces of the specimens taken from ‘NiCr-B hardfaced coating - NiCr-B hardfaced coating’ couple than that on the specimens from‘NiCr-B hardfaced coating e 316LN stainless steel’ couple. These particles were traced as corrosion products (occurring due to the formation of different complex metal oxides like NaCrO2) resulting from the interaction between sodium and NiCr-B hardfaced coating. Subsequently, NiCr-B coated specimens after 90 days of test duration were also cut perpendicular to the coated surface and were examined under SEM. A typical microstructure beneath the mated surface region (after the self-welding susceptibility test) is shown in Fig. 6. Comparison of the microstructure on the surface and that at the cross-section revealed no significant changes in the size, morphology or the nature of the constituent phases in the coated layer. Typical results of EDAX analyses on NiCr-B coating and 316LN stainless steel specimens are shown in Fig. 7. The results in Fig. 7 indicate low sodium count rate, which is the signature of corrosion products resulted from the interaction of the specimen with sodium. Interestingly both EDAX plots show marginal sodium count, but it is lower in the case of 316LN stainless steel specimen. This may be due to higher interaction of NiCr-B surfaces with flowing sodium compared to that with 316LN stainless steel. Further, there was no change in the integrity of the interface between NiCr-B coating and the substrate after the tests. The corrosion traces observed on NiCr-B hardfaced coating is considered to be caused primarily due to the dissolution of nickel into sodium, since solubility of nickel is higher than that of the other metal elements. Accordingly, nickel concentration on
Table 1 Chemical composition of 316LN stainless steel and NiCr-B hardface coating. Material
Ni
Cr
Mo
Mn
Si
B
C
P
S
N
Fe
SS316LN NiCr-B coating
10.61 Bal
17.67 11.56
2.65 e
1.78 e
0.35 3.87
e 2.03
0.02 0.53
0.043 e
0.0021 e
0.06 e
Bal 3.97
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
143
Fig. 1. Microstructures of (a) 316LN stainless steel (b) NiCr-B hardfaced coating and (c) cross section of NiCr-B coated specimen showing interface between coating and the substrate.
exposed surfaces of NiCr-B coated specimen perpendicular to the mating surfaces was also determined using SEM-EDAX. SEM micrographs along with EDAX elemental counts are shown in Fig. 8. EDAX count rates revealed on average 78.9 wt% nickel near the mating surface which is exposed to liquid sodium for 90 days at
823 K compared to 85.7 wt% nickel approximately 20 mm away from the mating surface, which is supposed to be unexposed. These results indicate that there is some amount of dissolution of nickel from the NiCr-B surface exposed to liquid sodium. The parameters for self-welding susceptibility tests have been selected such that it fulfils the conditions prevailing in the one of the major assemblies between grid plate and core support structure of the PFBR. Since grid plate holds the active core of the reactor, it is considered to be one of the major components of the reactor. As per design, the bottom of the grid plate rests on the top flange of the core support structure [10]. In the event of self-welding at the
Table 2 Parameters used for self-welding susceptibility tests.
Fig. 2. Specimen for in-sodium self-welding susceptibility tests.
Parameters
Magnitude
Temperature Contact stress Duration of testing Operating medium Surface finish of mating surfaces
823 K 8 and 11 MPa 90 and 135 days Flowing Sodium 0.8 mm
Table 3 Results of self-welding susceptibility tests in reactor grade flowing sodium. Material combinations
Surface roughness (mm)
Test temp. (K)
Contact stress (MPa)
Contact load (kg)
Dwell time (days)
No. of specimens tested
No. of specimens self welded
Break away shear force (kg)
Self welding behaviour
NiCr-B vs.316LN SS NiCr-B vs. NiCr-B NiCr-B vs. NiCr-B
0.8 0.8 0.8
823 823 823
8.0 8.0 11.0
133.4 133.4 183.4
90 90 135
4 5 5
Nil Nil Nil
e e e
No self-welding No self-welding No self-welding
Fig. 3. NiCr-B coated specimens after self-welding test for 90 days (showing no selfwelding between two adjacent specimens).
mating surfaces of these two components, the relative radial movement (occurring due to thermal expansion or contraction) between these two components get arrested, and such a condition is not acceptable during operation of the reactor. The mating surfaces would be experiencing a temperature of 673 K during normal operation of the reactor for its designed service life of 40 years. However, during transient conditions such as reactor power ramping, this temperature (673 K) could shoot up to approximately 803 K. The total duration of all the transient conditions is 150 h during the reactor design life of 40 years. Since the total duration of contact between the above-mentioned mating components is high and the operating temperature varies, it is difficult to carry out
Fig. 4. SEM micrograph of mating surface showing no surface damage in 316LN stainless steel specimen after 90 days test in flowing sodium at 823 K (a) at lower magnification showing cross-section of mating surface (b) at higher magnification, region marked by arrow in (a).
Fig. 5. SEM micrograph of mating surface showing no surface damage in NiCr-B specimen after 90 days test in flowing sodium at 823 K (a) at lower magnification showing crosssection of mating surface (b) at higher magnification, region marked by arrow in (a).
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
145
Fig. 6. Microstructure taken perpendicular to the NiCr-B coating after testing for 90 days at (a) top region near to mated surface of the specimen and (b) 0.5 mm away from the mated surface region.
laboratory simulated tests particularly for such long duration. To overcome this difficulty, accelerated self-welding tests were carried out at 8 MPa contact stress for 90 days; estimated from theoretical calculations, in flowing sodium at 823 K. If there is no self-welding observed in the above material combinations after the accelerated tests, then it is unlikely that there will be self-welding between these two components under the prevailing operating conditions of the reactor. Limited information on self-welding of Ni-based hardfacing alloys (Colmonoy with some particular chemistry) in flowing sodium is available in the literature. However, some information explaining the correlation between susceptibility to self-welding and contact stress as well as the duration of sodium exposure is available only for austenitic stainless steel mating surfaces. Since no information is available for the investigated material combinations, data on selfwelding of austenitic stainless steel have been considered for
estimating the contact stress required for accelerated tests that would simulate the operating conditions in PFBR. Self-welding studies in flowing sodium at 823 K, carried out for austenitic stainless steel combinations, have shown that self-welding coefficient W, defined as the ratio of breakaway stress by shear to contact stress, varies with duration of test t as:
W2 ¼ K t
(1)
where K is the rate constant for self-welding, and t is contact period (in s). K is expressed as:
K ¼ K0 expðQ =RTÞ
(2)
where K0 is a constant and Q is the activation energy, and R is the gas constant. The values of K for temperatures of 803 K, 673 K and
Fig. 7. Elemental count rate by EDAX in (a) NiCr-B and (b) 316LN stainless steel specimens after 90 days test in flowing sodium at 823 K.
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H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
823 K are calculated using K0 (1.7 1010 s1) and Q (218 kJ/mol) values from the report of Yokota et al. [11]; this naturally incorporates the assumption that the material combinations under investigation behaves as if like stainless steel and has the same probability for self-welding in liquid sodium environment. Initially, K values corresponding to the operating temperatures and the temperature for the self-welding tests are estimated using equation (1). The estimated K values for different temperatures are found to be: K803 ¼ 1.1198 104 s1 K673 ¼ 2.0434 107 s1 K823 ¼ 2.4762 104 s1 Subsequently considering the service conditions prevailing in PFBR, the following calculations were made to estimate the contact stresses at which self-welding is likely to occur in the selected material combinations under the normal operating condition and transient condition. These are described below: Normal Operating Condition (T ¼ 673 K, contact stress ¼ 11 MPa and duration ¼ 40 yrs). For normal conditions, For normal conditions, equation (1) can be written as:
For the total reactor life : For the test duration :
W673 2 ¼ K673 t673
W823 2 ¼ K823 t823
(3) (4)
where t673 is 40 years and t823 is 90 days. Using equations (3) and (4), the ratio W673:W823 is 0.37. The ratio W673:W823 is considered equivalent to the ratio of contact stress at 823 K to the contact stress at 673 K; assuming that if selfwelding occurs, the breakaway shear stress would be the same both in service and in 90 days accelerated test. Since the contact stress at 673 K is 11 MPa (known), the contact stress for the accelerated test is estimated to be 4.07 MPa. Transient Condition (T ¼ 803 K, contact stress ¼ 11 MPa and duration ¼ 150 h)
For the total reactor life : For the test duration :
Fig. 8. NiCr-B coated specimen after 90 days test in flowing sodium at 823 K (a) SEM of coating cross section (b) elemental count rate by EDAX near the mating surface at location marked spectrum 5 and (c) elemental count rate by EDAX approximately 20 mm away from the mating surface at location marked as spectrum 4.
W803 2 ¼ K803 t803
W823 2 ¼ K823 t823
(5) (6)
Since the total duration to be spent in the reactor at 803 K is only 150 h, which is less than the normal test duration of 90 days, both t803 and t823 are taken as 150 h. Accordingly, the ratio W803:W823 is found to be 0.67. Assuming that if self-welding occurs, the breakaway shear stress required is same for both in service and in 150 h test, from the definition of W, the contact stress required for the accelerated test is estimated to be 7.4 MPa. The values of stress levels which initiate self-welding corresponding to 90 days at 823 K for the normal reactor operation and that for 150 h at 823 K for the transients were calculated as 4.07 and 7.4 MPa respectively. Since the stress level to simulate the transient condition is higher, the self-welding susceptibility tests were conducted at 8 MPa for the duration of 90 days (instead of 150 h, the maximum expected cumulative duration of transient condition) to get a conservative estimate of the combined effect of both the normal and transient conditions that exist in the reactor operation. Hence, if accelerated self-welding tests at stress levels estimated from the design stress do not result in self-welding, it can be safely concluded that for reactor operation of 40 years self-welding is
H. Kumar et al. / Journal of Nuclear Materials 484 (2017) 141e147
unlikely to occur in these mating surfaces if one of the mating surfaces is NiCr-B coating. 4. Conclusions (i) Self-welding could not be perceived between the mating surfaces of ‘NiCr-B vs. 316LN stainless steel’ under 8.0 MPa contact stress for the duration of 90 days and ‘NiCr-B vs. NiCrB’ under 8.0 and 11.0 MPa contact stresses for the duration of 90 and 135 days in high purity flowing sodium at 823 K. (ii) These critical experimental observations coupled with pertinent theoretical analyses infer that self-welding would not occur between the mating surfaces of grid plate hardfaced with NiCr-B coating and core support structure made of 316LN stainless steel of Prototype Fast Breeder Reactor (PFBR) under the prevailing operating conditions. Acknowledgement The authors acknowledge the support and advice received from Dr. K.K. Rajan, Former Director of Fast Reactor Technology Group, Dr. C. Meikandamurthy of Fast Reactor Technology Group and members of Material Technology Division of Indira Gandhi Centre for Atomic Research.
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