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Received 16 May 2008; revised 7 July 2008; accepted 7 July 2008Available online 22 July 2008

magnetic test

process, together with the characteristics of the uidspresent, lead to carbon diusion into the tube walls fromthe internal surface towards the external one, resulting

matrix [5,7], reducing tensile strength and creep resis-tance [4].

Therefore, there is an interest in developing nonde-structive methods in order to evaluate quantitativelythe degree of carburization in pyrolysis furnace tubes.Traditionally, magnets are used to qualitatively evaluatethe carburization of the tubes. Indeed, the as-cast HP

*Corresponding author. Tel.: +32 2 650 27 23; fax: +32 2 650 27 86;e-mail: jdille@ulb.ac.be

Available online at www.sciencedirect.com

Scripta Materialia 59 (2008) 101010The classical industrial process formanufacturingethylene is based on the thermal cracking of a mixture ofhydrocarbons in the presence of steam in a pyrolysis fur-nace. Nowadays, the alloys used to manufacture the ser-pentine tubes of a pyrolysis furnace are of the high-pressure (HP) type. These materials guarantee corrosionand creep resistance at the working temperatures of>1000 C that are encountered in specic regions of thetubes [1,2]. Based on the ASTM composition [3], manu-facturers of HP alloys have developed alloys via additionof other elements to achieve improved mechanical prop-erties at high temperatures. For example, the addition ofNb and Ti leads to the formation of more stable carbides,improving the creep resistance [4].

The high-temperature conditions during the cracking

in carburization. This detrimental eect leads to metal-lurgical alterations and brittleness, aecting the reliabil-ity of the tubes and reducing their lifetime.

The carburization process has been studied by vari-ous authors [1,2,46]. At the beginning of the operation,with temperatures between 850 and 1000 C, a protec-tive oxide layer (Cr2O3) begins to form. This layer actsas a barrier to carbon diusion. At temperatures above1000 C the oxide layer becomes thermodynamicallyunstable, allowing the diusion of carbon. Initially, thecarbon deposited on the internal wall of the coils is ab-sorbed by the internal surface. In a second stage, the car-bon diuses into the austenitic matrix, precipitating asM7C3 and M23C6 carbides [2]. Carbide precipitationbrings about a decrease in the Cr concentration in theA magnetic ux measurement technique was used to correlate the magnetic response of carburized cast austenitic steel tubes withthe volumetric fraction of chromium carbides in the tube wall. Two dierent types of chromium carbides were identied by back-scattered electron diraction: Cr23C6 in the outer part and Cr7C3 in the inner part of the tubes. Magnetic force microscopy andenergy-dispersive X-ray analysis reveal a correlation between the ferromagnetic behavior of the tubes and chromium depletion inthe matrix. 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Austenitic steels; Magnetic force microscopy (MFM); Electron backscattering diraction (EBSD); Carburization; Non-destructiveStructural and magof a carburized c

I.C. Silva,a J.M.A. Rebello,a A.C. BrunoaFederal University of Rio de Janeiro, Cidade U

bDepartment of Physics, Pontical Catholic Univers

Rio de Janeiro, RcUniversite Catholique de Louvain, Departement

Place Sainte Barbe 2, 134dUnite de Chimie et de Physique des Hauts Polyme`res

Materials and Electronic Devices (CeRMiN), Universite catholieChemicals and Materials Department, Universite Libre d1359-6462/$ - see front matter 2008 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2008.07.015ic characterizationt austenitic steel

P.J. Jacques,c B. Nystend and J. Dillee,*

rsitaria, PO Box 68505, Rio de Janeiro, Brazil

Rio de Janeiro, Rua Marques de Sao Vicente 225,

451-900, Brazil

ciences des Materiaux et des Procedes, IMAP,

uvain-la-Neuve, Belgium

LY) and Research Center for Micro- and Nanoscopic

de Louvain Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium

uxelles, Avenue F. Roosevelt 50, 1050 Brussels, Belgium

13

www.elsevier.com/locate/scriptamatsevier Ltd. All rights reserved.

alloy is paramagnetic and gradually becomes ferromag-netic as carburization progresses. In a previous paper [8]the carburization level in samples extracted from HP al-loy tubes was characterized through measurements of

each of the four samples. Figure 1 shows a macroscopicview of sample S2. Going from the internal wall to theexternal wall, two dierent regions can clearly be distin-guished. These two regions can be found on all threecarburized samples although the transition zone posi-tion varies with the operating time. The longer the oper-ating time, the farther away from the internal wall is thetransition zone position.

Back-scattered electron imaging by SEM allows thechromium carbides to be distinguished from other pres-ent phases. In Figure 2A, the chromium carbides appearin black. The corresponding energy-dispersive X-rayanalysis (EDX) spectrum is given in Figure 2C. Nio-bium-rich compounds are colored in white (niobiumcarbides) or pale gray (nickelniobium silicide Ni16Nb6-Si7, named the G phase). The G phase precipitates in HPalloys above 900 C [10]. Figure 2B shows an EDX spec-trum of an NbC particle, and the EDX analysis of thematrix is presented in Figure 2D.

From the electron backscattered images, it waspossible to calculate the volume fraction of chromium

Mn Nb Ti Mo Fe

Table 2. Sample operating time

Sample Operating time (h)

S0 0S1 6800

I. C. Silva et al. / Scripta Materialia 59 (2008) 10101013 1011the magnetic eld over their external surface. The mag-netic measurements were then correlated with the vol-ume fraction of chromium carbides in each sample. Inthis paper, the objective is to characterize the structuralevolution in the tubes during carburization and to corre-late this with their ferromagnetic behavior. The para-magnetic as-cast microstructure consists of anaustenitic matrix with M23C6 carbides, and the purposeof this work is to investigate which structural change in-duces the ferromagnetic evolution of the alloy.

The composition of the HP alloy investigated is pre-sented in Table 1. Three HP alloy samples (S1, S2, S3)with dierent operating times were extracted from thehighest-temperature regions of pyrolysis furnace tubes.A fourth sample (S0) corresponding to the as-cast con-ditions was also analyzed. This sample treatments aresummarized in Table 2.

The level of carburization of the samples was deter-mined by nondestructive magnetic ux density measure-ments before they were removed from the tubes [9]. Thistechnique measures the magnetic ux density near theexternal surface of the tubes by means of a magnetore-sistive sensor biased by a small ferrite magnet. Chro-mium carbide volumetric fraction measurements werecarried out by scanning electron microscopy (SEM) atselected points of the samples in accordance with theASTM E 562 standard. Electron back-scattered dirac-tion (EBSD) analyses were performed in order to iden-tify the phases present in the samples at dierentcarburization levels.

After polishing with diamond paste down to 1 lm,the specimens were further polished with a 0.03 lm col-loidal silica suspension for 1 h. The EBSD measure-ments were carried out with a eld emission gunscanning electron microscope operating at 20 kV and aworking distance of 15 mm. Step sizes ranging from0.1 to 5 lm were used.

Magnetic force microscopy (MFM) imaging allowedthe identication of the ferromagnetic regions in thecross-section of the tubes after carburization. MFManalyses were performed on cross-sectioned sample S2under ambient conditions with a PicoPlus microscope(Agilent Technologies) using PointProbe MFM probes(NanoSensorsTM) with a resonance frequency around75 kHz and a typical spring constant value of3 N m1. The magnetic images were acquired in theso-called Lift Mode. During a rst pass, the topogra-phy of a scan line was acquired in the standard intermit-tent-contact mode (Tapping mode). Then the tip waslifted to a specic height above the surface (75 nm inthe present study) and the magnetic proles (phase-shiftand cantilever vertical deection) were acquired while

Table 1. Composition of the alloy investigated (wt.%)

C Cr Ni Si0.45 26.0 34.6 2.4 1.0the tip scanned the same line at constant tipsurface dis-tance, following the previously recorded topography.This method yields images free of topographic artifacts.The cantilever vertical deection is proportional to themagnetic force acting on the magnetic tip. If the oscillat-ing cantilever is considered as a linear harmonic oscilla-tor, the phase-shift is roughly proportional to the secondderivative of the vertical component of the local mag-netic induction of the sample.

After extraction, a cross-section was prepared for

S2 25,600S3 >90,000

Figure 1. Macroscopic view of a cross-section of sample S2.1.0 0.15 0.2 Bal.

carbides through the tubes cross-section by image anal-ysis. These results are shown in Figure 3. As expected,the chromium carbide volume fraction increases withthe exposure time at high temperature. In each cross-sec-tion, there is also an increase in the chromium carbidevolume fraction from the outer wall to the inner wall.InFigure 3, the magnetic ux density values are also dis-

Figure 2. (A) Back-scattered electron image; (B) EDX analysis of point 1; (

1012 I. C. Silva et al. / Scripta Materialia 59 (2008) 10101013played for each sample.Taking into account the chromium carbide volume

fraction curves (Fig. 3), it is possible to establish a linearrelationship between the magnetic ux density and theareas under these curves. This is shown in Figure 4.Figure 4. Area under the chromium carbide volume fraction curves(arbitrary units) versus the measured magnetic ux density.

Figure 3. Chromium carbide volume fraction through the tubethickness. The depth is measured from the external wall.The measurement of the magnetic ux can thereforebe considered as a valid non-destructive method for aquantitative determination of the degree of carburiza-tion in pyrolysis furnace tubes and for the predictionof their remaining life.

EBSD analyses were performed in order to dierenti-ate the various phases based on their crystal structures.Two dierent zones were observed. For each carburizedsample, they are identical to the two macroscopic re-gions shown inFigure 1. In the outer region of the tubes,the microstructure is similar to the as-cast microstruc-ture. It consists of a face-centered cubic (fcc) austeniticmatrix (a = 0.36 nm) containing Cr23C6 carbides havinga fcc structure (a = 1.064 nm). In the inner part of thetubes, the Cr23C6 carbides are replaced by carbon-richCr7C3 carbides in the austenitic matrix. These Cr7C3carbides exhibit a hexagonal structure (a = 1.398 nm;c = 0.452 nm). These results are shown in Figure 5. Thisclear dierentiation between Cr23C6 and Cr7C3 carbidesby EBSD was not possible by traditional back-scattered

C) EDX analysis of point 2; (D) EDX analysis of point 3.electron imaging in the SEM. Indeed, with this method,both types of carbide appeared in black in the matrix(see Fig. 2).

Figure 5. EBSD phase identication: red, austenite; blue, Cr23C6;yellow, Cr7C3.

Table 3. Chromium content (wt.%) measured by EDX analysis ofdierent regions shown in Figure 6

Region on sampleS2 cross-section

Matrix Crcontent

Global Crcontent

I. C. Silva et al. / Scripta Materialia 59 (2008) 10101013 1013Finally, a MFM study was carried out in order todetermine which structural component is responsiblefor the ferromagnetic behavior of the carburizedtubes. Figure 6 shows six MFM images obtained fromdierent areas on a cross-section of sample S2, fromthe external wall (region a) to the internal wall (re-gion f). On these images, a negative phase-shift, i.e.dark color level, corresponds to ferromagnetic regions,while a zero phase-shift, i.e. bright color level, corre-sponds to paramagnetic regions. These measurementsshow that M23C6 and M7C3 carbides are both para-magnetic. On the other hand, it can be seen that thematrix becomes more and more ferromagnetic whenmoving from the external surface towards the internalone. It is clear from these results that the austeniticmatrix, close to the external wall, is paramagnetic withonly thin ferromagnetic domains in the areas adjacentto the chromium carbides. The ferromagnetic areas

Figure 6. MFM phase-shift images acquired on a cross-section ofsample S2. The images (a)(f) were successively acquired at dierentlocations from the external wall (a) to the internal wall (f). In (a) and(f), the insets are the magnetic force images given for comparison.progressively widen and, nally, the whole matrix be-comes ferromagnetic. On image f of Figure 6, themagnetic domains are clearly visible in the entire ma-trix. EDX analyses were also performed in dierentregions of Figure 6 in order to measure the chromiumcontent. The results are given in Table 3. Whereas theglobal chromium content is constant between each re-gion, the chromium content of the matrix decreasesappreciably when moving from the external wall tothe internal one. These results conrm that the magne-tization of the carburized tubes is due to chromiumdepletion in the matrix, as proposed by Stevens etal. [7,11]. On a ternary phase diagram, the positioncorresponding to HP alloy composition is located nearthe boundary between the paramagnetic domain andthe ferromagnetic domain [12]. This fact can explainwhy even slight chromium depletion makes the austen-itic matrix ferromagnetic instead of paramagnetic.

In conclusion, a linear relationship between the de-tected magnetic ux density and the total area underthe chromium carbide volume fraction curve throughthe thickness of the tube was found. This result conrmsthe adequacy of magnetic non-destructive evaluation asa tool to characterize the carburization and the subse-quent mechanical degradation of the tubes. The EBSDanalysis shows a transition from Cr23C6 carbides inthe external part of the tubes to Cr7C3 carbides in theinternal part. The position of the transition zone de-pends on the carburization level. MFM analysis con-rms that the magnetization of the HP steel is due tochromium depletion in the matrix, resulting from car-bide precipitation.

The authors thank CNPq, CAPES, FAPERJ for theGrants given to support this work and TSEC LTDA forproviding the HP carburized samples. B.N. is Senior Re-search Associate of the Belgian Funds for Scientic Re-search (FRS-FNRS). P.J.J. acknowledges the FNRSand the FRFC.

[1] T.F. da Silveira, M.Sc. Thesis, COPPE/Federal Universityof Rio de Janeiro, 2002.

[2] H.J. Grabke, Materials at High Temperature 17 (2000)483.

[3] ASTM, Standard Specication ASTM A 297/A 297M 97(Re-approved 2003).

[4] H.J. Grabke, I. Wolf, Materials Science and Engineering87 (1987) 23.

[5] S.B. Parks, C.M. Schillmoller, Hydrocarbon ProcessingInternational Edition 75 (1996) 53.

[6] F. Liu, F. Chen, Materials Chemistry and Physics 82(2003) 288.

[7] K.J. Stevens, A. Parbhu, J. Soltis, D. Stewart, Journal ofPhysics D: Applied Physics 36 (2003) 164.

[8] I.C. Silva, A.C. Bruno, J.M.A. Rebello, L.L. Silva,Scripta Materialia 56 (2007) 317.

[9] I.C. Silva, A.C. Bruno, J.M.A. Rebello, R.S. da Silva,T.F. Silveira, NDT& E International 39 (2006) 569.

[10] G.D.A. Soares, L.H. de Almeida, T.L. da Silveira, I. LeMay, Materials Characterization 29 (1992) 387.

Region a 17.1 0.1 25.6 0.1Region d 9.8 0.4 26.0 0.1Region f 5.2 0.3 26.2 0.1[11] K.J. Stevens, A. Parbhu, J. Soltis, Current AppliedPhysics 4 (2004) 304.

[12] A.K. Majumbar, P. van Blanckenhagen, Physical ReviewB 29 (1984) 4079.

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