Ultrasonic and structural characterization of anisotropic austenitic stainless steel welds

Ultrasonic and structural characterization of anisotropic austenitic stainless steel welds:Towards a higher reliability in ultrasonic non-destructive testingB.Chassignole a,R.El Guerjouma b,n,M.-A.Ploix c,T.Fouquet da EDF R&D,Materials and Component Mechanics Department,Moret sur Loing77818,Franceb Universite´du Maine and CNRS,LAUM UMR CNRS6613,Le Mans72000,Francec INSA de Lyon and CNRS,MATEIS UMR CNRS5510,Villeurbanne69621,Franced EDF R&D,Sinetics Department,Clamart92141,Francea r t i c l e i n f oArticle history:Received22September2006Received in revised form28November2009Accepted7December2009Available online29December2009Keywords:WeldUltrasonicNon-destructive testingNuclear engineeringAnisotropyModelingAttenuationa b s t r a c tThe non-destructive testing of austenitic stainless steel welds of the primary coolant piping system is asignificant problem for the nuclear industry.Ultrasonic techniques would be very helpful to detect,locate and size potential defects.Unfortunately,austenitic welds are coarse-grained,heterogeneous andanisotropic.This leads to aberration and scattering of the ultrasonic waves.In this paper,we presentseveral experimental results of ultrasonic testing of two austenitic welds exhibiting high anisotropy.In order to explain the observed display of wave propagation phenomena such as beam deviation,weusefinite element modeling.The modeling is associated with a complete characterization of theinspected welds.Two essential characteristics of the welds are determined:the average elasticconstants of the weld and the grain orientations.The capability of the model is illustrated in differenttesting configurations.This work associating structural characterization and modeling shows that abetter understanding of the phenomena of ultrasonic propagation should allow the interpretation andreliability of the industrial inspections of heterogeneous anisotropic welds to be improved.&2009Elsevier Ltd.All rights reserved.1.IntroductionTo assure the structural integrity of nuclear pressurized waterreactors,the non-destructive testing(NDT)techniques must allowthe detection of potential defects in the austenitic stainless steelpipes and welds of the primary coolant piping system.Inparticular,manufacturingflaws(inclusions,voids or lacks offusion)or in-serviceflaws(cracks)could be present in the weldedjoints.In French nuclear power plants,the in-service inspections ofaustenitic welds are carried out mainly on the basis of radio-graphic techniques.Ultrasonic inspections are rarely used becausethey are confronted with all the problems of testing coarse-grained,heterogeneous,anisotropic material[1–3].However,ultrasonic techniques would help to complement the informationprovided by radiography which efficiently detects defects but hassome limitations as to their location and size.A weld made of austenitic stainless steel in some casespresents strong anisotropy varying continuously from one areato another all over the weld giving rise to heterogeneousanisotropy[1–3].The effect of anisotropy on the propagation ofultrasound makes the velocity direction-dependent and makesthe group velocity different from phase velocity in both amplitudeand direction[4].Furthermore,deviation of the ultrasonic beam within the weldmaterial may cause difficulties in locating defects.Scattering ofthe ultrasonic energy results in some directions of propagationhaving a poor signal-to-noise ratio.To understand the anisotropyof the wave amplitudes recorded in ultrasonic inspectionexperiments,the complexity of wave propagation in these mediahas to be considered.To study these phenomena,modeling is very helpful.Indeed,modeling can reveal the quantitative features and thus help in theoptimization of conventional ultrasonic NDT techniques and inthe development of new ones.The simulation of ultrasonic testingusing appropriate models allows us to perform,for example,parametric studies and obtain quantitative simulated results.Thetheory of wave propagation into anisotropic and homogeneousmedia already allows the prediction of beam skewing anddivergence effects[4–7].When considering the more complicatedcase of heterogeneous anisotropic structures,modeling studiesrequire realistic descriptions from the mechanical and metallur-gical point of view of the various kinds of weld structures,especially the ones where anisotropy is strong.Over the last few years many studies have been undertaken toevaluate the effect of metallurgical structures of austeniticContents lists available at ScienceDirectjournal homepage:/locate/ndteintNDT&E International0963-8695/$-see front matter&2009Elsevier Ltd.All rights reserved.doi:10.1016/j.ndteint.2009.12.005n Corresponding author.E-mail address:rachid.elguerjouma@univ-lemans.fr(R.El Guerjouma).NDT&E International43(2010)273–282stainless steel welds on wave propagation for the application of ultrasonic non-destructive testing [8–10].However,to be efficient in comparing experimental results and modeling,the weld characterization should take into account both the metallurgical and mechanical aspects and this is rarely done.In this paper,we present several experimental ultrasonic testing results of two austenitic welds.In order to explain the wave propagation phenomena observed in the case of strong anisotropy,we use finite element modeling.This modeling is associated with a complete characterization of the inspected welds.In the first part,we describe the characteristics of the mock-ups inspected.In the second part,we present results of weld structural characterization by metallographic and crystallo-graphic analyses.In the third part,we determine the anisotropic elastic properties of the welds using an ultrasonic method.Then,in the last part,we compare experimental and modeling results in pulse-echo mode for different testing configurations (different probes and defect localization).2.Mock-up characteristics and material characterization The extensive columnar grain structure in multipass austenitic welds differs greatly from that in ferritic welds.In austenitic welds,the deposition of successive weld beads does not destroy the grain structures in the previous beads.Moreover,the graingrowth is parallel to the heat flow direction and is governed by an epitaxic process [11].The columnar grains then grow through the boundaries of the beads.Consequently,grains of substantial length and with specific crystallographic orientations are pro-duced.The long dendrite axis is almost vertical along the center of the weld and nearly perpendicular to the fusion lines (and the upper boundary of the weld).The present study used two flat position welding molds in AISI 316L steel (austenitic stainless steel),and the welds were manufactured with an automatic arc welding process that used coated electrodes.The difference between those two welds comes from the electrode scanning.For weld B,the weaving rate is equal to 130oscillations/min which lead to a run width of about 20mm.For weld A,the width of the string beads is equal to 10mm.For both welds the overlap is equal to 35%.The dimensions of the mock-ups are indicated on Fig.1.Metallographic observations in cross sections are presented in Figs.2and 3,respectively,for weld A and weld B.For each weld,the first observation was made in the plane transverse to the welding direction ((yz )plane)and the second one in a plane parallel to the welding direction ((xz )plane).Those pictures clearly reveal a dendritic type growth parallel to the heat flow direction.In the case of a sufficiently low welding energy,this leads to elongated and oriented grains which can grow by epitaxic process on several millimeters length.An X-ray diffraction analysis was performed on samples of 3mm thickness taken in a relatively homogeneous area of each weld (central area).The pole figure of {200}planes for weld A is given in Fig.4.It clearly reveals the strong anisotropy of the material.In the first approach,for the sake of simplicity,the material could be considered as transversely isotropic with a symmetry axis parallel to a [100]axis.Such material symmetry was chosen by several authors to describe an austenitic stainless steel weld [10,12].However,the analysis of the orientation distribution functions (ODF)shows that in a strict way,the material must be considered as orthotropic.The orientations of the symmetry axis,determined from the X-ray diffraction analysis and defined by the a and g angles,are given in Figs.2and 3.We confirm that the symmetry axis is very close to the elongation axis of the grain [11].Fig.1.Dimensions of themock-ups.Fig.2.Macrographic structure of weld A.(a)Plane transverse to the welding direction.(b)Plane parallel to the welding direction.B.Chassignole et al./NDT&E International 43(2010)273–282274For weld A,the columnar structure in the middle zone is rather homogeneous with a tilt of the elongation axis of the grain (a angle)of about 201in the transverse plane.Near the chamfers of the weld,the structure becomes more heterogeneous.In the (xz )plane,the grains are almost vertical (g =31).The tilt in this plane is directly linked to the welding velocity.For up vertical welds,which are manufactured with a lower welding velocity,a tilt of about 201was found [1].As far as weld B is concerned,the tilt is weaker in the transverse plane (about 31)because of the large electrode scanning which leads to flatter weld runs (weaving rate is equalto 130oscillations/min which lead to a run width of about 20mm).This scanning also leads to a more developed grain growth and a more homogeneous structure than for weld A.In the (xz )plane we clearly observe a layback of the structure (g =91).This layback is once again due to the flat shape of the runs.A multi-scale analysis (neutron diffraction,electron back-scattering diffraction and transmission electronic microscopy)on a weld metal deposit of 316L stainless steel is proposed by Bouche et al.[13].The authors also found a fiber-like /100S texture with reinforcements (orthotropic symmetry).Their weld is an intermediate case with both grain tilts in the (yz )plane (201)and in the (xz )plane (101).3.Determination of the elastic properties by ultrasonic methodsThe velocities and the polarizations of plane waves propagat-ing in a specific direction in a perfectly elastic anisotropic solid can be determined from the Christoffel equations [14]:G il Àr V 2d il ¼0ð1Þwith G il =C ijkl n j n k (i ,j ,k ,l =1,2,3)and where r is the density,n is the unit vector in the wave propagation direction,V is the phase velocity of ultrasonic waves in the medium,C ijkl are the elastic constants of the anisotropic medium and d il is the Kronecker symbol.With appropriate index changes,the elastic constants can be expressed with the more simple matrix form C mn .Christoffel equations admit three eigenvalues corresponding to the phase velocities V of the three wave modes which can propagate.The associated eigenvectors are the polarizations of each wave mode.Knowing the elastic constants of the material,it is possible to calculate any velocity for a given direction of propagation by solving Eq.(1).The inverse problem consists in recovering elastic constants from suitable experimental values of measured velocities.Such measurements are performed on a set-up in transmission mode [15]:a homogeneous sample is immersed in water andtheFig.3.Macrographic structure of weld B.(a)Plane transverse to the welding direction.(b)Plane parallel to the weldingdirection.Fig.4.{200}pole figure for weld AB.Chassignole et al./NDT&E International 43(2010)273–282275velocities are obtained in all the accessible planes of propagation by varying the angle of incidence.In practice,the unknown material properties are determined simultaneously by minimizing the sum of the squares of deviations between the experimental and calculated velocities [15].For an orthotropic medium,when the material symmetry coordinate system coincides with the geometrical axes of the sample,the unknowns are the nine independent elastic constants.In our study,the material symmetry axes are unknown.Then the grain disorientation with respect to the geometrical sample coordinate system results in three additional unknowns,the Euler angles (j 1,f ,j 2)which are also determined by the optimization process.The sample geometrical axes and the material symmetry axes are presented in Fig.5.We can also see in this figure some of the planes where ultrasonic velocities are measured.The values of the nine elastic constants determined by this method for our welds are presented in Table 1.These elastic constants are determined with a very good accuracy when the ultrasonic velocities are themselves measured with a good accuracy as it is the case for these materials [15].The 1,2and 3axes are linked to the material symmetry,the 3axis corre-sponding to the /100S fiber axis.If we compare the phase velocities calculated in the (23)plane (Fig.6),we can note that weld B presents a more pronounced anisotropy (higher variations of velocities).This is coherent with the conclusions of Section 1.Intermediate values were found for a sample taken in a weld representative of a butt weld of the surge line of nuclear power plants.In the table the values of a and g angles (orientation of the fiber axis)linked to the values of the Euler angles are also indicated.The results are close to those determined by the X-ray diffraction analysis.4.Ultrasonic inspections in pulse-echo mode 4.1.Configurations of inspectionTwo side-drilled holes with a diameter of 2mm were machined in the mock-ups (Fig.7).Their depths are 20and 40mm.The same defects were also machined in a reference mock-up in wrought austenitic stainless steel which presents an isotropic structure.The mock-ups were then inspected in pulse-echo mode with four contact testing probes whose characteristics are given in Table 2.They radiate by refraction through a Plexiglas wedge.Three of them produce longitudinal waves and the last one produces transverse waves with vertical polarization.The characteristics of these probes (probe size,center frequency,andbandwidth)are very close to those used for in-service inspections.In particular,the center frequency of the probes is close to 2MHz,which is the value recommended for the inspection of this type of weld.Moreover,the transmission pulse indication is highly damped to get good performances in terms of echo resolution.The transverse waves were only used to inspect weld B.Automatic and parallel immersion scans were performed with signal registrations every 0.5mm in the y direction (Fig.6).This allows perfect coupling conditions between the contact probes and the mock-up tobe.Fig.5.Sample geometrical axes and material symmetry axes.Table 1Calculated elastic constants (GPa)and a and g angles (1)from ultrasonic measurements.Weld C 11C 22C 33C 44C 55C 66C 23C 13C 12agA 2342402209911095146148118154B237247210122125701341328427Fig.6.Variations of velocity versus the angle of propagation in the (23)plane (a)longitudinal waves (b)transversewaves.Fig.7.Automatic testing of the mock-ups.B.Chassignole et al./NDT&E International 43(2010)273–282276In this paper,we are going to study more particularly the beam skewings and attenuations in the plane of incidence((yz)plane). We will compare the experimental results to the modeling ones to demonstrate the relevance of the modeling approach to reproduce the ultrasonic phenomena.Heterogeneity of most actual welds is known to be the origin of poor signal-to-noise ratios that greatly complicates the NDT. We have considered this crucial problem in a detailed study[17]. The two welds considered here for experiments are not representative of all real welds as their large volume leads to a more homogeneous anisotropic structure.For such welds the signal-to-noise ratio is rather high in several directions of propagation and the problems of ultrasonic testing are mostly related to the anisotropy of the material,resulting in beam splitting and skewing and which makes the attenuation direction-dependent.In this study we focus on such problems and we consider specific welds which exhibit strong anisotropy.Those phenomena are not clearly illustrated on welds that exhibit very strong heterogeneity with poor signal-to-noise ratios[18].4.2.Modeling codeFor this study,we used a two-dimension code called ATHENA developed by EDF R&D and the French National Institute for Research in Computer Science and Control.This code solves the equations of elastodynamics expressed with the stresses and the velocities of the displacements by afinite element method. The convergence of the code was demonstrated in previous work. The elements are squares with a size equal to afifteenth of the wave length which is known to be sufficient for an accurate solution.ATHENA simulates the propagation of the ultrasounds in an anisotropic and heterogeneous complex medium,with the assumption that such a medium can be described by afinite number of anisotropic and homogeneous areas.In this study,2D computations are helpful because we limit the analysis to the ultrasonic propagation in the plane transverse to the welding direction.X-ray diffraction analysis has shown that this plane can be assumed as a plane of symmetry of the orthotropic material.So the beam skewing out of this plane will be weak for the majority of the configurations.For a weak grain layback,this skewing in the welding direction can be significant only for normal incidence,as it was highlighted in a previous study in transmission mode[19].However,even in this case,it does not influence the skewing value in the transverse direction as we will demonstrate in Section4.5.1.ATHENA also takes into account the interactions of the beam with defects.These defects are simulated by a free boundary condition(s n=0)using thefictitious domain method that allows defects of complex geometry to be modelled[16].Moreover,it is possible to simulate contact or immersion probes.The transducer modelled as a piston source by the Stepanishen’s formula,to describe the propagation between the transducer and the block [20].The only difference between immersion and contact probes comes from differences on ultrasonic velocities in water and in a Plexiglas wedge.Lastly,work is in progress to take into account in ATHENA the ultrasonic scattering related to the coarse grain structure[18].The aim is to get a better modeling of the attenuation and back-scattering phenomena.4.3.Weld description for modelingWe have seen that an appropriate metallographic analysis can give some information about the crystallographic orientations. We therefore use an image processing analysis to obtain a cartography of the grain orientation in the(yz)transverse plane for each weld.The orientations are measured in square domains of5mm side.Considering the grain size,several grains are contained in each domain which justifies assumption of a macroscopic elastic symmetry(transversely isotropic or orthotropic).Finally,we obtain the description with afinite number of anisotropic and homogeneous domains presented in Fig.8(part of weld B).Note that the grain orientation can be slightly different between the two zones of localization of the defects.The elastic constant values determined in Section3are associated with the grain orientation in each domain.As modeling is performed with a two-dimensional code,only four elastic constants are necessary to describe the ultrasonic propagation(C22,C33,C44and C23), whether the material symmetry is orthotropic or transversely isotropic.Moreover,it is important to note that this modeling approach can be applied to industrial welds that exhibit a more hetero-geneous structure than those considered here.The reference mock-up in wrought austenitic stainless steel will be described by an isotropic domain whose ultrasonic velocities are5740and3080m/s,respectively,for longitudinal and transverse waves.4.4.ResultsWe compare the experimental results to the simulated ones for each weld.We analyze the following parameters for each probe:D y(mm):difference between the probe positions related tothe maximum amplitudes of the echo reflected by a defect in the weld and the echo reflected by the same defect in the reference mock-up.A positive value stands for a shift of the probe in the positive y direction for the propagation in the weld;D A(dB):defined as the difference between the maximumamplitudes of the echoes reflected by the defects at20and 40mm depth.A positive value stands for a decrease of the amplitude for the defect at40mm depth.Results for the differences of time offlight between propaga-tions in a weld and in the isotropic steel could also have been presented.In fact,it is demonstrated that the results for this parameter are globally the same as for the D y parameter.Indeed, variations of time offlight due to the shift of the probe are moreTable2Probes characteristics.Wave type Angle of refraction in austenitic steel(1)Transducer size(mm)Frequency(MHz)Longitudinal0|13 2.25 Longitudinal45|20 2.25 Longitudinal60|20 2.25 Transverse4515n152B.Chassignole et al./NDT&E International43(2010)273–282277influential than variations due to ultrasonic velocity variations (Fig.6).The results are presented in the form of histograms from Figs.9–12.Figs.9and 10give the D y values for longitudinal waves and for weld A and weld B,respectively.In Fig.11,the D y values of 451transverse waves are compared to the 451longitudinal waves ones.As far as the D A values are concerned,the results are presented in Fig.12.4.5.DiscussionIn this section,we are going to discuss separately the skewing and attenuation results.4.5.1.SkewingFirstly,we note that the modeling reproduces the experi-mental skewing very well.The results validate the model of weld description assumed.Moreover,the assumption of a propagation in a symmetry plane (2D case)does not lead to significant differences.We also observe strong skewings of the beam for the longitudinal waves,D y reaching a value of +15mm for certainconfigurations.This is due to the anisotropy of the material which leads to skewing between the direction of the wave energy and the direction of the wave vector.To discuss these skewing values,we are going to apply a simplified theoretical analysis consisting of plotting slowness surfaces of longitudinal waves with the assumption of a homo-geneous anisotropic medium (Fig.13).Even if the structures are rigorously heterogeneous (see Fig.8),this assumption is justified because the particular dimensions of the two welds lead to large globally homogeneous areas.We evaluate for each weld,each probe and each defect the average grain orientation in the area crossed by the ultrasonic waves.The application of Snell’s law allows the direction of the wave vector in the anisotropic medium to be determined and then the perpendicular to the slowness surface gives the direction of the group velocity (or direction of the energy).We can study the skewing of the energy of the beam compared to the isotropic case (D 1angle which is the difference between the values of c and y 1angles as defined in Fig.13),but usually skewing is expressed by the D 2angle which is defined as the angle between the wave normal and the wave’s energy flow direction (difference between the values of c and y 2angles as defined in Fig.13).Note that for L0wave,y 1and y 2and,as a consequence D 1and D 2,areequal.Fig.8.Cartography of the grain orientation in a part of weld B determined by imageprocessing.Fig.9.D y values for longitudinal waves and weld A.B.Chassignole et al./NDT&E International 43(2010)273–282278In Fig.14,D 2values are drawn versus the angle between the grain orientation and the wave normal (beam/grain angle).These values can be determined for each weld from the values of the elastic constants given in Table 1.D 2values are lower for weld A because of the difference of anisotropy highlighted in Section 3.For L0probe,it is interesting to note that although the grain orientations are different for the two welds,the skewings are similar.Indeed,we see in Fig.14that,for this probe,beam/grain values of 151and 51for weld A and weld B,respectively,give approximately the same D 2value (151).Moreover,the L0wave’s case illustrates well that it is important to take into account the correct grain orientation.Indeed,for weld B,a simplified description with vertical grains would lead in modeling to no skewing with L0wave.Other studies showed that descriptions that are too simplified lead to erroneous predictions [8,19].For L45wave,skewing in the case of defect at 40mm depth is higher for weld A than for weld B.Indeed,for weld B,as the grain is almost vertically oriented in the middle zone,the D 2angle is close to 0as shown in Fig.14.For L60wave,the skewings are weak with regard to the sound travel path distances.Thus,they correspond to angular deviations inferior to 51.However,the disturbances are different according to the defect or the weld studied.For instance,values of D y are higher for defect at 20mm depth for weld A and skewing directions are different between the two defects for weld B.To explain these observations,the values of D 1angle (previously defined)are indicated in Table 3.Those values fully agree with the measured values of D y and show slight differences between each configuration.Above all,this analysis confirms that even if those welding molds seem to be globally homogeneous,it is important to know with sufficient accuracy the right grain orientations in each zone crossed by the ultrasonic beam in order to obtain pertinent modeling results.As far as 451transverse waves are concerned,they are far more disturbed than longitudinal ones.With regard to the deviations,the experimental values are very significant and still in agreement with the theory.In fact,the energy of the beam is propagated with an angle close to 01compared to the vertical instead of the angle of 451expected in an isotropic medium.In consequence timesofFig.10.D y values for longitudinal waves and weldB.parison of D y values for 451transverse waves and 451longitudinal waves.B.Chassignole et al./NDT&E International 43(2010)273–282279flight are much shorter.Significant errors in the positioning of the defects are thus to be expected during an inspection of this type of weld with 451transverse waves.Moreover,when the ultrasonic propagation path through the weld is too great,transverse waves are proscribed [21].For example,in case of inspection of more heterogeneous welds,some defects were not detected [5].4.5.2.AttenuationWith regard to the attenuation,we note that modeling systematically underestimates its value.In fact,two phenomena contribute to attenuation.The first is relative to the divergence of the probe.This contribution is partially taken into account in ATHENA since the true characteristics of the probe are simulated but only in a two-dimension space.The second phenomenon is the grain scattering due to the coarse grain structure of the weld.This parameter was not simulated in our calculations.Work is in progress to introduce scattering attenuation coefficients in the models.Experimental evaluations of this coefficient versus the angle between the grain orientation and the direction of propagation are proposed in the literature for longitudinal waves [22,23].These studies highlight the anisotropic feature of the scattering attenuation in austenitic welds,with a regular increase of this parameter from 0to 901angle of propagation.This is confirmed by a theoretical analysis proposed by Ahmed and Thompson [24].In particular,the attenuation coefficient of longitudinal waves at 2MHz frequency is estimated at 0.1dB/mm for a beam/grain orientation of 601,whereas it is only around 0.01dB/mm for the isotropic base metal.In our study,for this angle of propagation of 601,the experimental variation of amplitude between the side-drilledholesFig.12.D Avalues.Fig.13.Determination of the direction of propagation of the energy from slowness surfaces of longitudinal waves with the assumption of an anisotropic homogeneous medium.B.Chassignole et al./NDT&E International 43(2010)273–282280。