REVISTA MEXICANA DE FISICA 50 SUPLEMENTO 1, 49–53 JUNIO 2004

Mechanical and microstructural effects on the stress corrosion cracking of weldbeads of X-52 and X-70 pipeline steels

A. Contreras, A. Albiter, C. Angeles-Chavez, and R. PerezInstituto Mexicano del Petroleo, Programa de Investigacion y Desarrollo de DuctosEje central Lazaro Cardenas # 152, San Bartolo Atepehuacan, 07730, Mexico D.F.,

Tel. 01(55)9175-6414, Fax: (55)9175-6429,e-mail: acontrer@imp.mx

Recibido el 27 de marzo de 2003; aceptado el 22 de septiembre de 2003

One of the greatest challenges that the petroleum industry faces today is the corrosion problem caused by aggressive fluids such as CO2 andH2S. In this work the susceptibility to stress corrosion cracking (SCC) of the X-52 and X-70 steels, immersed in a NACE solution saturatedwith H2S, was studied using slow strain rate tests (SSRT) at a strain rate of1×10−6 s−1. The samples used for the SCC tests were taken fromthe weld bead zone of the pipeline steels and the experiments were carried out at room temperature and 50◦C. SSC experiments performed inair at room temperature where taken as a reference. After failure, the fracture surfaces were observed using a scanning electron microscope(SEM) and the identification, distribution and types of phases were determined by energy dispersive spectroscopy (EDS). It was found thatthe mechanisms responsible of fracture propagation are likely to be a combination of dissolution material, interfacial decohesion, primaryand secondary cracks as well as void formation around the inclusions. The specimens tested in air at room temperature exhibited ductilefracture while the specimens tested under corrosive environment showed brittle fracture.

Keywords: Weld bead; stress corrosion cracking (SCC); slow strain rate tests (SSRT); hydrogen sulfide (H2S); pipeline steel.

Uno de los grandes retos que enfrenta la industria petrolera son los problemas producidos por la corrosion causada por fluidos agresivos talescomo son CO2 y H2S. En este trabajo se estudio la susceptibilidad a la corrosion bajo esfuerzos de tension de la soldadura longitudinal dedos ductos de aceros X-52 y X-70 inmersos en una solucion NACE saturada con H2S usando pruebas de tension a deformacion lenta (SSRT)a una rapidez de deformacion de1 × 10−6 s−1. Los ensayos se llevaron acabo a temperatura ambiente y 50◦C. Los ensayos realizados enaire fueron tomados como referencia. Las caracterısticas morfologicas de las superficies fracturadas, distribucion y productos de corrosionfueron observadas en el microscopio electronico de barrido (MEB). La identificacion, composicion y tipos de fases fueron determinadaspor espectroscopia de energıa dispersiva (EDS). Se encontro que el mecanismo responsable de la propagacion de la grieta puede ser unacombinacion de la disolucion del material, decohesiones interfaciales, grietas primarias y secundarias ası como la formacion de huecosalrededor de las inclusiones. Los especimenes ensayados en aire exhiben un tipo de fractura ductil, mientras que los especimenes ensayadosbajo el ambiente corrosivo mostraron fractura fragil.

Descriptores: Cordon de soldadura; corrosion bajo esfuerzo (SCC); pruebas a velocidad de deformacion lenta (SSRT); sulfuro de hidro-geno (H2S); tuberıa de acero.

PACS: 62.20.Mk

1. Introduction

Stress corrosion cracking (SCC) is a local corrosion processwhich is characterized by the initiation and propagation ofcracks. It takes place under the simultaneous action of sus-tained tensile stresses and a specific corrosive environment.Generally, the SCC susceptibility of high-strength steels hasbeen estimated using several tests, including the National As-sociation of Corrosion Engineers (NACE) TM-0177 tensiontest [1,2]. This includes the slow strain rate tests (SSRT)which was used to produce a result in a reasonably shorttime [3]. The obtained SSRT tests results include: time tofailure, plastic elongation to failure, reduction in area, ulti-mate tensile strength, and the load at fracture.

For underground pipeline steels, the susceptibility tostress corrosion cracking (SCC) is of great interest [4]. TheH2S environment is probably the most significant concern incurrent day corrosion and cracking evaluation. However, H2Srelated corrosion and cracking has remained as one of thebiggest problems involved in processing and transportation of

hydrocarbides containing these sulfides. Laboratory data andfield experience have demonstrate that extremely low con-centrations of H2S may be sufficient to lead to SSC failure insusceptible materials.

The purpose of this investigation was to characterize thebehavior of the longitudinal weld bead in X-52 and X-70 steelpipelines. In order to assess this behavior a study of the ten-sile properties and the fracture characteristics of these steels,experiments in air, and NACE solution saturated with H2S atroom temperature and at 50◦C using SSRT were carried out.

2. ExperimentalThe API X-52 and X-70 pipeline steels with longitudinalwelding acquired in PMT (Productora Mexicana de Tuberıa)were used in this study. These had 36” of diameter and 0.375”and 0.902” thickness, respectively. The longitudinal weldingwas carried out by the technique of submerged arc welding(SAW). The chemical composition of the pipeline steels isshown in the Table I.

50 A. CONTRERAS, A. ALBITER, C. ANGELES-CHAVEZ AND R. PEREZ

TABLE I. Chemical composition of the API X-52 and X-70 steels (wt%).

Steel C Mn Si P S Al Nb Cu Cr Ni Mo V Ti Ca B

X-52 0.08 1.06 0.26 0.019 0.003 0.039 0.041 0.018 0.02 0.019 0 0.054 0.003 0.0002 0.0003

X-70 0.031 1.48 0.13 0.012 0.002 0.033 0.1 0.29 0.27 0.16 0 0.004 0.012 0.0011 0.0003

TABLE II. Summary of the SSRT results.

Steel Environment UTS (MPa) EL (mm) % RA RRA

X-52Air 391.3 2.03 55.69 N/A

NACE+H2S 249.0 1.42 7.25 0.130

NACE+H2S+50◦C 233.9 1.88 13.83 0.248

X-70Air 462.5 2.64 50.98 N/A

NACE+H2S 213.4 1.21 6.91 0.135

NACE+H2S+50◦C 355.8 2.03 4.38 0.085

FIGURE 1. Schematic drawing of a slow strain rate specimen.

FIGURE 2. Schematic representation of the slow strain rate testingmachine used.

Prior to the SSRT tests, samples around the weld beadwere taken and prepared to perform the metallographic char-acterization and observe the different zones of the weld.

Cylindrical tensile specimens having a gauge diameterof 2.5 and∼25mm in length were selectively machined fromthe tube as is shown in Fig. 1.

The SSRT were performed at room temperature in air andin a NACE solution saturated with H2S at room temperatureand at 50◦C with 1 × 10−6 s−1 strain rates. The corrosivesolution was prepared according to the NACE TM-0177 stan-dard [2]. To perform the SSRT tests in the NACE solution,a 300mL AISI 316 stainless steel autoclave was used. Theschematic representation is shown in Fig. 2.

The susceptibility to SCC was expressed in terms of thepercentage reduction in area (%R.A.) calculated by the fol-lowing equation [5]:

RA(%) =(D2

i −D2f )x100

D2i

(1)

where Df and Di are the final and the initial diameters ofthe tensile specimen, respectively. The ratio of reduction inarea after fracture for the specimen in the tests environment(Rae) to the corresponding value determined in the controlenvironment (RAc) was calculated according to the follow-ing expression:

RRA = Rae/RAc (2)

The morphological characteristics of the fracture surfacesand corrosion product were analyzed in a XL-30 ESEMPhilips Environment Scanning Electron Microscope, whichwas attached to an energy dispersive spectroscope.

3. Results and discussion

The Microstructure of the X-52 and X-70 weld bead samplesis shown in Fig. 3. The macrostructure (Figs. 3a and c) showthe heat affected zone (HAZ), the base metal (BM), and theweld beads (WB). In Figures 3(b) and (d) the microstructure

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MECHANICAL AND MICROSTRUCTURAL EFFECTS ON THE STRESS CORROSION CRACKING OF WELD BEADS OF. . . 51

of the base metal is shown. The microstructure shows thetypical pearlite colonies (dark areas) in the ferrite matrix.

The types of predominant inclusions were globular withdifferent sizes ranging 1 to 5µm. The composition of theseinclusions consists mainly of Mn, S, Ca, O, and Fe. Beingpossible the formation of FeO, MnS, CaO and FeS.

3.1. Stress corrosion cracking (SSRT tests)

The SSRT are widely used to evaluate the susceptibility tostress corrosion cracking of various materials [6-8]. Figure 4shows the stress-elongation behavior obtained from the slowstress rate tests performed in air and in a NACE solution sat-urated with H2S at room temperature and 50◦C for the X-52and X-70 pipeline steels.

Table II shows the ultimate tensile strength, elongation, %reduction in area, and ratio of reduction in area. These val-ues were obtained from the curves of Fig. 4 for the fracturedspecimens.

The ratio of reduction in area after fracture was calculatedusing Eq. (2). Ratios in the range of 0.8-1.0 normally de-note high resistance to environment assisted cracking (EAC),whereas low values (i.e.<0.5) mean high susceptibility.

From Table II it can be observed that both specimenstested in air showed the maximum %RA, which indicates ahigh ductility compared with those tested in aggressive me-dia. The X-52 specimens tested in the NACE solution satu-rated with H2S at room temperature presented the maximalsusceptibility to SCC. This is in agreement with results re-ported in the literature [1,2,9]. Meanwhile, the maximal sus-ceptibility to SCC in X-70 specimens tested in the NACEsolution saturated with H2S was at 50◦C.

3.2. Fracture behavior

The mechanical fracture has been used to quantify the stresseffects and sour environment in the cracking. SEM frac-tographs of tensile fractured specimens are shown in Figs. 5and 6.

Figure 5 shows the cross section of the X-52 sample il-lustrating the different types of fracture (5a as ductile, and 5band 5c as brittle). The chemical analysis carried out in thesample tested in air does not show the presence of O, S

and Cl (Fig. 5d), which were found in the sample tested inthe NACE solution saturated with H2S. Great amounts of Cland S were found in some areas, others were rich in S, and Clwas not detected. As illustrated in Fig. 5(e-f). Similar re-sults are observed in Fig. 6, which correspond to the X-70specimen.

A chemical analysis of the typical inclusions found in thesample tested in air is illustrated in Figure 6(d). The spectrumshows clearly the presence of Ca, Ti and Mn. These chemicalelements are typical of the fluxes used in the SAW process.The flux is specially formulated to be compatible with a givenelectrode wire type so that the combination of flux and wiregives rise to the desired mechanical properties.

It is evident that in air, both specimens exhibited a duc-tile type of failure, since they showed micro plastic deforma-tion, nucleation and coalescence of microvoids around someinclusions as shown in Figs. 7(a) and (b). In the corrosivesolution, the specimens showed a brittle fracture with trans-granular appearance type of cracking as shown in Figs. 7(c)and (d).

The failure in air occurred in some cases in the weld jointfor the X-70 steel, while for the X-52 the fracture occurredin the base metal and in the HAZ. SCC of low strength steelsis characterized by transgranular fracture, in contrast to inter-granular fracture of high strength steels [10-12].

FIGURE 3. Optical micrographs of the two-pass weld bead, (a) X-52, (c) X-70 steel, (b) and (d) typical base metal microstructure forthe X-52 and X-70 steel, respectively.

FIGURE 4. Stress vs elongation behavior under different conditions.

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52 A. CONTRERAS, A. ALBITER, C. ANGELES-CHAVEZ AND R. PEREZ

FIGURE 5. SEM fractographs of tensile fractured samples for the X-52 steel, showing the appearance of the fracture surface, (a) ductilefracture (tested in air), (b) and (c) brittle fracture (tested in a NACE solution at room temperature and 50◦C respectively), (d-f) Chemicalanalysis of different zone of each sample.

FIGURE 6. SEM fractographs of tensile fractured samples for the X-70 steel, showing the appearance of the fracture surface, (a) ductilefracture (tested in air), (b) and (c) brittle fracture (tested in a NACE solution at room temperature and 50◦C respectively), and (d-f) Chemicalmicroanalysis of different zone of each sample.

Figure 8 shows SEM fractographs of the near surfacecross-section micrographs of the failed SSRT specimens forNACE solution saturated with H2S at 25◦C. The path of thecrack was very irregular with a brittle appearance. In ad-dition, it is observed that X-70 steel is more susceptible tothe corrosion attack as shown in Fig. 8(b). The X-52 steelpresented minor degrees of corrosion compared to the X-70steel due to the dissolution of material produced by the corro-sive attack. This explains the behavior of the curve shown inFig. 4(a) where the failure of the X-52 specimen occurred inbase metal, while in the X-70 specimen the failure occurredin weld bead [13]. The EDS microanalysis revealed the pres-ence of Na, Cl, S, Mn, C, O and Fe mainly. Therefore, it

is possible that corrosion products like FeO, MnS, FeS andNaCl could be present. Corrosion was clearly a factor in theinitiation of some of the cracks.

In the case of X-70, it is clearly observed that at 25◦Cthe attack in the pitting form (Figure 8b), and was less severethan at 50◦C, where the attack was in the form of microc-racks and more homogeneous along the lateral surface of thespecimen.

4. Conclusions

The susceptibility to SCC of the API X-52 steel was greaterin a NACE solution saturated with H2S at room temperature.

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MECHANICAL AND MICROSTRUCTURAL EFFECTS ON THE STRESS CORROSION CRACKING OF WELD BEADS OF. . . 53

FIGURE 7. SEM fractographs of tensile-fractured specimens (a)and (b) X-52 and X-70 steel tested in air at room temperature, (c)and (d) Transgranular stress corrosion cracking (TGSCC) on thefracture surface of X-52 and X-70 steel after failure in a NACEsolution saturated with H2S tested at 50 and 25◦C, respectively.

FIGURE 8. SEM fractographs of tensile fractured samples (a) X-52and (b) X-70 steel, showing the appearance of the fracture surface,both specimens were tested in a NACE solution saturated with H2Sat 25◦C.

Meanwhile, for the X-70 steel the susceptibility was greaterin a NACE solution saturated with H2S at 50◦C. In summary,according to the values obtained for RRA, it is clear that bothspecimens tested in the NACE solution saturated with H2Spresented high susceptibility to SCC.

From the SSRT tests it could be observed that the spec-imens tested in air exhibited a ductile type of failure, sincethey showed micro plastic deformation, nucleation, and coa-lescence of microvoids around some inclusions. Whereas, inthe corrosive solution both specimens showed a brittle frac-ture.

Among all testing environments, the saturated H2SNACE solution had a strong influence on SSRT results, re-flected in the degradation of tensile properties. Both steelspresented a corrosive attack in the form of dissolution of thematerial. The X-52 steel showed the best resistance to thecorrosive attack. All the cracks, primary and secondary, wereperpendicular to the applied tension axis, being indicative ofSSC.

Cracking susceptibility was indicated by a decrease in themechanical properties (e.g., strain to failure, ultimate tensilestrength, reduction in area), and also due to, the presence ofsecondary cracking along the gauge length of the specimen.

Acknowledgements

The authors wish to thank to Centro de CienciasFısicas (CCF) of Cuernavaca Mor., Mexico and the Institutode Investigaciones Metalurgicas, U.M.S.N.H. for the techni-cal support during the experimental tests.

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