Journal of Nuclear Materials 421 (2012) 172–175

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Journal of Nuclear Materials

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Letter to the Editor

Young’s modulus anisotropy in reactor pressure vessel cladding

W. Vandermeulen a,⇑, M. Mertens b, M. Scibetta a

a Studiecentrum voor Kernenergie – Centre d’Etudes de l’Energie Nucléaire (SCK�CEN), NMS Unit, Boeretang 200, B2400 Mol, Belgiumb Flemish Institute for Technological Research (VITO), Materials Department, Boeretang 200, B2400 Mol, Belgium

a r t i c l e i n f o

Article history:Received 8 September 2011Accepted 25 November 2011Available online 8 December 2011

0022-3115/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jnucmat.2011.11.066

⇑ Corresponding author. Tel.: +32 14333087; fax: +E-mail address: wvanderm@sckcen.be (W. Vander

a b s t r a c t

In a previous study it was shown that the anisotropy of Young’s modulus in the stainless steel cladding ofa reactor pressure vessel could be attributed to the solidification texture of the cladding. Further it wasfound that annealing the samples to remove the delta phase caused a modulus change but only in somedirections. Since the texture was only estimated from X-ray diffraction patterns the moduli, calculated forsome principal directions, differed considerably from the measured ones. In the present study, executedon a practically identical cladding, the texture was determined by actual texture measurements. It wasfound to be close to a fibre texture with h001i perpendicular to the cladding plane and the values calcu-lated from it agreed much better with the experimental ones. The annealing effect found in the previousstudy was shown to be due to surface recrystallization induced by milling damage.

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1. Introduction

It is common practice to clad the inner surface of nuclear reac-tor pressure vessels with stainless steel, mainly to protect the ves-sels against corrosion and to avoid changes in the water chemistry.This cladding is made by depositing two strip layers of stainlesssteel weld beads. During the solidification of the molten metal heatflows nearly perpendicular to the cold substrate. As a consequencea solidification texture develops similar to the one found in welds[1–4]. In a previous article it was shown that such a texture couldexplain the anisotropy of Young’s modulus of the cladding material[5]. In that case the texture was estimated from the relativeintensities of the peaks on X-ray diffraction patterns from sampleswhich were either parallel or perpendicular to the cladding plane.From these, it was deduced that the h001i direction was mainlyperpendicular to the cladding plane and that the h110i directionshowed a preference for the deposition direction. A simplified rep-resentation of this texture thus consists of a single crystal with theabove described orientation relative to the cladding. The modulicalculated for this ‘‘single-crystal’’ explained the ranking of theexperimental data but the absolute values diverged substantially.

The most common methods for modulus determination areultrasonic or resonance vibration methods. Unfortunately the firstmethod was not available and the second method requires a sam-ple length that cannot be obtained from the cladding in the per-pendicular direction. Therefore the modulus was determined bymeans of cantilever bending tests on small beams cut from the

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32 14321216.meulen).

cladding in different directions. To assess a possible effect of thedelta phase on Young’s modulus anisotropy the same specimenswere tested again after a solution treatment of 2 h/1050 �C. A mod-ulus change was found only in some directions. As a possibleexplanation for this phenomenon a nonlinear behaviour, depend-ing on the test direction, was supposed. Unfortunately specimenloading had been done in one single step and this hypothesis couldtherefore not be checked.

To get a better insight in these observations a new study, similarto the one mentioned above, was made on a cladding which hadbeen deposited on large fracture mechanics specimens used to as-sess the fracture behaviour of sub cladding cracks. In preparing thiscladding care was taken to replicate the previously used pressurevessel cladding as close as possible [6]. Cladding replication hasthe advantage to provide test material in large quantities and alsoallows repeating experiments in order to challenge previous find-ings. In order to check for the occurrence of nonlinear behaviourspecimen loading was done stepwise. Furthermore the textureassessment was based on actual texture measurements. The re-sults thus obtained allow presenting a more accurate picture ofthe modulus anisotropy and of the effect of annealing.

2. Experimental

The bottom layer of the cladding consisted of AISI 309L and thetop layer consisted of AISI 308L. The 0.2% yield strength wasaround 320 MPa. The fabrication and further characterisation havebeen described elsewhere [6].

The bend test specimens consisted of a cantilever of 0.8 � 2.0 �8.0 mm protruding from the centre of the 5 � 10 mm face of a

Fig. 1. (a)–(d) Results of modulus measurements from the present (diamonds, leftside) and a previous study (squares, right side) on two similar claddings. Thevertical scale gives the modulus in GPa. Results from the different specimens areplotted along the horizontal axis. Filled symbols represent the as welded conditionand open symbols the annealed condition. The results from the pulse excitationmeasurements in the L and T directions fall within the rectangular boxes. Themodulus values calculated from the texture are indicated by horizontal lines. Shortdashes: single crystal texture from previous study [2], long dashes: fibre texturefrom present work. Note that the bulk modulus is about 195 GPa.

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rectangular supporting beam [5] and were machined by spark ero-sion. Four specimen orientations were used. These were respec-tively: perpendicular to the cladding plane (S direction); in thecladding plane, parallel or perpendicular to the deposition direc-tion (L and T) and an ‘‘oblique’’ direction at an angle of 55� fromS, in the L–S plane (In the following the latter will be called OBL).Based on the previous texture estimation these directions were ex-pected to correspond respectively to the crystallographic h001i,h110i and h111i directions. In order to examine a possible differ-ence between top and bottom layers, specimens were machinedfrom both layers in the L and T directions. In the S and OBL direc-tions the specimens contain necessarily both layers. For the L and Tdirections the results obtained by the bending tests werecompared with results obtained by the impulse excitation method[7]. The latter was used on two plate specimens of 5 � 2 �25 mm for each direction.

In order to detect possible plastic deformation, loading wasdone in four steps by dead weights and the deflection was mea-sured with a displacement transducer (LVDT) mounted below theloading point. The maximum stress was 220 MPa which is well be-low the 0.2% yield stress (320 MPa). Within the sensitivity of themeasurements all load–deflection plots were found to be linear.All tests were performed at room temperature.

Since previous tests had indicated a modulus change in the L, Sand T directions after annealing at 1050 �C, the measurementswere repeated after the same annealing treatment.

The crystallographic texture was determined at three positionsin the cladding on planes parallel with the surface. A Philips X’Pertsystem using an X-ray lens was used. The spot on the sample wasabout 3 mm wide and 6 mm long which is close to the size of thebending specimen.

3. Results and discussion

3.1. Modulus measurements

The moduli obtained from the present study are plotted as dia-monds on the left side of Fig. 1 which each represent the results forone of the four orientations. The data for the as welded conditionare marked by filled symbols, those for the annealed conditionby open symbols. Each data pair relates to a single specimen. Theorder of the specimens along the horizontal axis is at random.Since the specimens from the top and bottom layers showed nosystematic difference they are not differentiated in the figures.The average moduli for each of the four directions in the as weldedcondition are respectively (in GPa): S (128), T (154), L (148) andOBL (206).

The results of the previous study [5] are shown for comparisonon the right hand side of the diagrams (squares). The correspond-ing averages are: S (144), T (167), L (160) and OBL (211). For a par-ticular orientation the scatter is of the order of 10 to 20 GPa. This ispartly experimental and partly due to the sample-to-sample varia-tion of the texture (See Section 3.2).

The ranges of the moduli measured in the annealed conditionby the impulse excitation method are shown by dotted rectangles.For the as-welded condition impulse excitation tests were onlymade in the previous study. These showed no effect of annealing,as opposed to the bending tests. Taking into account that the im-pulse specimens are much larger than the cantilever ones andtherefore give averages over a larger volume the agreement withthe bending tests is good.

From the above mentioned results it thus can be concluded thatthe simulated cladding, used in the present study, is a good repli-cation of the previously investigated one. However, as can be seenfrom the right-hand side of the figures, a modulus increase was

found in the previous study in the L, S and T directions after anneal-ing but not in the OBL direction. The impulse excitation measure-ments (only possible in the L and T directions) did not show suchan increase. In the present study none of the four orientationsyielded a significant modulus change due to annealing. Afterreconsideration of the experimental conditions it was found thatonly those specimens whose surface had been milled showed anincrease upon annealing while in the spark eroded ones the effectwas absent. To explain this correlation, specimens from both stud-ies were examined by metallography. It was found that afterannealing, in all milled specimens a recrystallized surface layerof about 50 lm was present. This layer was also found on surfaceswhich had not been loaded mechanically during testing. Thisshows that the cause of recrystallization is machining damagerather than plastic deformation during testing. In addition, in theS specimens, some of the surface grains had grown considerablyinto the specimen interior thereby increasing the recrystallizedvolume considerably. Supposing the recrystallized layer to havean isotropic grain orientation with a modulus of 195 GPa, it wascalculated that this layer would increase a specimen modulus of160 GPa (L and T) by about 10 GPa. This is close to the observed in-crease for these directions. The greater increase in the S directioncan be explained by the larger volume of the recrystallized zone.At the surface of spark eroded specimens no recrystallization wasobserved which explains why they show no modulus change.These considerations prove that no other factors contributed sig-nificantly to the modulus change. Finally, since the milled impulseexcitation specimens are 10 times thicker than the bending speci-mens it is obvious that those measurements did not detect thepresence of the recrystallized layer since this occupies only 0.5%of their thickness. In the bending case each layer accounts for 5%of the specimen thickness.

3.2. Texture determination

The lines with short dashes in Fig. 1 show the moduli calculatedfrom the elastic constants supposing a perfect texture as put for-

Fig. 2. Texture measurements on three samples (A)–(C) of the cladding used in the present study. Upper row: 111 poles; lower row: 200 poles. The orientations of the testspecimens (L, S, T and OBL) are shown in the upper left plot.

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ward in the previous study: h001i// S; h110i// L and T and h111i//OBL at 55� from S in the SL plane. Since this assumes the specimento be a single crystal there was no need for grain interaction cor-rection. The discrepancy with the experimental values is clear.

In order to get a more precise idea of the texture three samples(A)–(C) were taken from the cladding for actual X-ray texturedetermination. The h200i and h111i pole plots for the threesamples are shown in Fig. 2. It can be seen that the h200i polesof samples A and C lie mainly within an angle of about 20� fromthe S direction. The h111i poles mostly lie within a zone, borderedby the 30� and 60� parallels. It can be seen that the poles are notuniformly distributed around the S axis. This is ascribed to the factthat the small area covered by the X-ray beam does not contain allpossible grain orientations. It should be noted that this area waskept intentionally small in order to be representative for the spec-imen size. The picture shown by sample B is more complex. Afteretching it was found that it contained the boundary between twoweld beads. The intricate pole figure can be understood by consid-ering that the heat flow near the interface of beads is probably notperpendicular to the substrate and thus causes the solidificationdirection to be strongly inclined to the substrate plane.

Fig. 2 confirms that the h001i direction is predominantly paral-lel with the S direction as found previously. The h111i directionhowever, tends to be uniformly distributed around the S direction,as opposed to the fixed positions assumed previously. This corre-sponds to a so called fibre texture. The ‘‘perfect’’ fibre texture is de-rived from the single crystal one by considering a uniformdistribution of unit cell cubes which have been rotated aroundtheir [001] direction coinciding with the S direction. A large sam-ple will contain all rotational positions but small samples, as in thecase of the bending specimens, will only contain a limited numberof these cube orientations. In the perfect fibre texture all directionsmaking an angle of 55� with the [001] direction contribute to themodulus found for the OBL specimens. The measured modulusshould be the average of the moduli of all these directions. Like-wise, all directions making an angle of 90� with [001] contribute

to the modulus found for the L and T specimens. In the perfect fibretexture the S direction only samples the [001] direction. The mod-ulus calculated for the S specimens is therefore the same as calcu-lated previously. The averaged moduli, calculated for the fourtested directions were found to be: ES = 102 GPa, EL = ET = 141 GPaand EOBL = 225 GPa. By comparing these values with the experi-mental results in Fig. 1 it can be seen that the agreement is consid-erably better than found previously, except for the S direction.

A further improvement can be obtained by taking into accountthat the fibre texture is not perfect as can be seen from Fig. 2. Thisfigure shows that the [001] poles are in fact distributed in a zonearound the S direction. A new calculation was made in which it wasassumed that the zone of 20� around the S direction contained auniform distribution of [001] poles. This results in also the 55�and 90� parallels being replaced by broader bands. The averagemoduli for these zones were found to be: ES = 113 GPa, EL = ET =147 GPa and EOBL = 213 GPa. These values have been indicated bylong-dashed lines in Fig. 1. It can be seen that in all directions abetter estimation of the experimental values is obtained. Neverthe-less, differences between calculated and experimental values per-sist. They are very likely to be attributed to the omission of graininteractions in the calculations. In the above calculations it was in-deed assumed that each grain deforms elastically without beinghindered by the surrounding grains having a different orientation.Approximate formulae, valid for textureless materials, taking thiseffect into account have been derived by McGregor Tegart [8].For textured materials no such formulae are available. To get anidea of the error introduced by neglecting the grain interaction ef-fect the average modulus was calculated for a uniform distributionof grain orientations. A value of 177 GPa was obtained. This has tobe compared with the experimental modulus of 195 GPa. The def-icit of about 10% can be attributed to grain interactions. This is ofthe order of the differences between calculated (texture based)and experimental values. However, since the grain interaction cor-rections are texture dependent and the fact that material variabil-ity would need to be assessed through appropriate statistics any

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further refinement for improving the calculation of Young’s modu-lus would require a large effort with little additional benefit. There-fore no attempt was made to further improve the model.

4. Conclusions

An assessment of the anisotropy of Young’s modulus of a simu-lated stainless steel cladding, deposited on a pressure vessel steelsubstrate, was made by means of cantilever bending tests on spec-imens, cut in specific directions relative to the cladding. The resultswere found to be rather well comparable with those obtained in aprevious study on an actual pressure vessel cladding. Texture deter-minations showed that the solidification texture can be describedas a fibre texture with the symmetry axis mainly perpendicular tothe cladding plane. The modulus values calculated from this texturefor the four test directions agree within about 10% with the exper-imental values although material variability and the grain–graininteraction was not taken into account.

The modulus increase found previously after annealing at1050 �C in three of the four specimen directions could be shownto result from recrystallization induced by milling damage at thespecimen surface. Specimens with a spark eroded surface showedno recrystallization and consequently no modulus change.

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