Microsoft Word - numero_46_art_28 A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 306 Fatigue crack propagation in welded joints X70 Adel Deliou, Benattou Bouchouicha Laboratory of Materials and Reactive Systems LMSR, University Djillali, Liabes, Sidi Bel-Abbes, Algeria . del032003@yahoo.fr, benattou_b@yahoo.fr ABSTRACT. Structural failure assessment approaches take into account local parameters, specimen geometry, loading and material. In the case of welded joints, in addition to these parameters, consideration must be given to the effect of the heterogeneity of properties due to welding. The objective of our work is to study the fatigue crack propagation of welded joint in API X70 pipeline steel. This experimental study focused on welded joints in the different parts, base metal, weld metal and heat affected zone. The concepts of fracture mechanics are used to analyze the harmfulness of defects in welded joints and the main part of fatigue life falls on the crack propagation. The results obtained show that the fatigue crack propagation rate of cracks in the heat affected zone is delayed compared to the other zones. The effect of the microstructure and the quality of submerged arc welding of the studied X70 steel are significant. Tensile tests, hardness and measurement of energetically parameters complemented this work. KEYWORDS. X70; Mechanical behaviors; Fatigue crack propagation; Energy. Citation: Deliou, A., Bouchouicha, B., Fatigue crack propagation in welded joints in X70, Frattura ed Integrità Strutturale, 46 (2018) 306-318. Received: 16.08.2018 Accepted: 03.09.2018 Published: 01.10.2018 Copyright: © 2018 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. INTRODUCTION elding is one of the important assembly methods for manufacturing in steel structures. In current industrial practice, welded joints are an integral section of these complex configurations [1]. In general, fatigue behavior of welded materials is complicated by large variation in nature and properties of materials, presence of geometric defects and many factors intrinsic (inclusions, lack of penetration, gas pore), etc. [2-8]. Many studies show that, from such defects, the stage of initiation of the fatigue crack can be reduced [9] and that consequently a large part of the life of the welded joints subjected to fatigue propagation occurs [10]. For the design of these assemblies, it is interesting to use the crack propagation laws given by the Linear Elastic Fracture Mechanics [LEFM]. In order to estimate the life of the joints, by calculating the number of cycles needed to propagate a crack from these defects until the rupture [11-21]. Welded structures present a gradient of microstructure and mechanical behaviors from the weld metal to base metal. Important studies regarding the microstructural change and the mechanical behaviors of API X70 steel have been executed [22- 24]. Nanninga [25] examined the fatigue crack propagation of API X42 and APIX70 pipelines at a load ratio R = 0.1 under H2 or N2 environment. The presence of hydrogen shows an important damage compared to Nitrogen. The W http://www.gruppofrattura.it/VA/46/28.mp4 A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 307 fatigue crack propagation FCP tested under the same type of environment (Hydrogen or Nitrogen) is higher than the API 5L X70 steel because of the microstructure and the chemical composition. The effect of toughness and microstructure on fatigue crack propagation rate in different steels X60 and X70 investigated by Y. Zhong et al. [26]. The results demonstrated that the toughness and strength have a significant influence to the fatigue behavior of pipeline steels. Maamache [27] inspected the influence of successive welding repairs on the microstructure and mechanical properties of the heat affected zone HAZ of an API 5LX70 steel. Microstructural analysis has shown that beyond the second repair, the HAZ microstructure undergoes a significant change in grain morphology and size that increases with the number of repairs. As a result, the yield strength and toughness of the repaired specimens were degraded. After the second repair, the properties of the welded joints do not meet the acceptability criteria defined by the standards applied by API. Fatoba [28] studied the oligocyclic fatigue behavior of API 5L X65 pipeline steel at room temperature. The author has shown that the increase in the amplitude of total deformation has the effect of decreasing the life of fatigue oligocyclic, and the amplitude of stress increases with the amplitude of plastic deformation. The investigation [29] showed that the cracking rate of X65 steel is influenced by the crack direction. The parameters of the propagation model have been determined admitting to the Paris law and show that the T-S direction has an excellent resistance to fatigue cracking with respect to the T-L direction (rolling direction). Kim [30] characterized the fatigue of the X65 steel in the three regions base metal, welded and heat affected zone, he noticed that the difference in the cracking rate decreases when the values of the stress intensity factor K increase. The finite element analysis performed by Hadjoui [30] for the X60 and X70 steels for a load ratio R = 0.2 shows that the X70 presents a better resistance to fatigue. Recently Maachou [31] analyzed the fatigue crack growth behavior under constant amplitude and variable amplitude loading using terms of energy parameters. The main objective of this work is to study the fatigue crack growth rate of low-carbon micro alloyed steel X70 welded joints used in pipeline manufacturing. This experimental study focused on the welded joints in the three different parts, namely in the unaffected base metal, the heat affected zone and the weld metal. MECHANICAL PROPERTIES Materials he studied material is a microalloyed steel, API 5L X70 grade, used in pipeline manufacturing. The pipe is spirally welded by Submerged Arc Welding SAW process. Chemical composition In order to evaluate if the chemical composition of our material conforms to the specification imposed by API 5L, an analysis by optical emission spectroscopy has been used. The chemical compositions of X70 pipeline steel and of SAW weld metal in weight percent are shown in the Tabs. 1 and 2 respectively. C Mn Si Cr Ni Mo V Al Ti Nb Cu P Sn S 0.064 1.640 0.290 0.051 0.009 0.021 0.050 0.038 0.020 0.056 0.023 0.011 0.036 0.004 Table 1: Chemical composition of X70 (base metal, BM; wt%). C Mn Si Cr Ni Mo V Al Ti Nb Cu P Sn S 0.070 1.610 0.250 0.035 0.030 0.133 0.060 0.017 0.017 0.040 0.046 0.014 0.003 0.004 Table 2: Chemical composition of filler metal (weld metal, WM; wt%). The equivalent carbon contents of the base metal and that of weld metal calculated to the expression (1) below proposed by the international institute of welding [32] are respectively 0.36% and 0.39%. T A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 308 6 5 15 eq Mn Cr Mo V Ni Cu C C        (1) Tensile test In order to evaluate the load strain curve and mechanical properties of X70, flat specimens according to API requirements were used .The tensile tests were carried out in 600 KN capacity, MFL UHP PRUFSYSTEME servo hydraulic testing machine, at room temperature with a low displacement rate(1mm/min). Figure 1: Schematic of the sampling direction of the specimens. The samples were taken in the rolling direction (Fig.1). The dimensions of weld metal specimens used according to ASTM A370 and API 5L standard are presented in Fig. 2. Tab. 3 and Fig. 3 present the results of conventional and true tensile of evolution of the stresses according to the deformation in base metal X70, where E the Young module, ν the Poisson's ratio, 𝜎 the yield strength, 𝜎 ultimate strength, A% the ductility and k and n being Hollomon's parameters. Tab. 4 shows the mechanical properties of the welded metal obtained. Figure 2: Diagram and dimensions of weld metal specimens used for tensile tests. Figure 3: Conventional and true tensile curve of X70 A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 309 E [GPa] ν   Y [MPa]   U [MPa] A (%) k n 221 0.3 500.3 573.14 33 840 0.25 Table 3: Tensile properties of X70. Material Yσ [MPa] Uσ [MPa] Elongation (mm) WM 603.44 642.80 6.33 Table 4: Tensile properties of weld metal Microstructure analysis In order to carry out the microstructural study of our material, we performed optical microscope observations on polished sections (1 μm) then etched with 3% Nital at the transverse surface of the welded joint (Fig. 4). Examinations performed on unaffected sections reveal the presence of numerous alignments of MnS inclusions in the base metal and globular oxides in the welded metal. Optical micrograph shows an increase in grain diameter from the base metal to the weld joint. According to Fig. (5.a), the microstructure of the base metal consists as expected essentially of polygonal ferrite grains (white) and pearlite (black) organized in strips. This structure is produced by the segregation of Mn and P during rolling [34-37]. At the approach of the weld we find a more homogeneous organization of the grains The microstructure of the region near base metal consisted by equiaxed very fine grains of ferrite and pearlite.. For the heat affected zone (HAZ) (Fig. 5b), they appear acicular ferrite grains in the form of needles with coarse polygonal ferrite grains. The microstructure of the weld metal (WM) consists of dominant polygonal ferrite, acicular ferrite around the inclusions, and islands of pearlite (Fig.5c). The microstructure obtained is agreed with a number of studies oriented towards the analysis of the structure [26], [38-41]. Figure 4: Transversal section for micrographic examinations and hardness measurement Figure 5: Welded joint microstructure as observed through optical microscopy: (a) base metal (M: X200) (b) heat affected zone (M: X200) and the weld metal (M: X500) (c) A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 310 Hardness measurement Vickers hardness filaments were carried out in the thickness on a cross section of the weld joint. The measurements are taken under 10 kgf of load using a ZWICK ZHV10 type Durometer, the step is 0.5 mm in the HAZ and 1 mm in the base metal and the weld metal. The hardness profile measured across the average diameter changes to a letter M as shown in Fig. 6. In addition, Tab. 5 shows the average values of the Vickers hardness for each region of the welded joint. The large values of hardness of HAZ zone related to the existence of acicular ferrite in its microstructure (presents the greatest value of the mechanical properties by virtue of the fine-grained and interlocked microstructures) [42-44], on the other hand, the low values are in the BM respect to welded joint can be related by the reverse cold deformation caused by the rolling (Bauschinger effect). The hardness results representing the weld seam showed a uniformity of values between base metal and seam weld sign a good property. Figure 6: Hardness profile of the welded joint. Region Average value BM 207 HAZ 230 WM 227 Table 5: Results of HV10 hardness along the weld joint FATIGUE TESTS atigue cracking tests were performed on CT50 specimen’s thickness of 7 mm, in accordance with ASTME-E647 whose dimensions given by the Fig. 7. These tests were carried out in laboratory of materials and reactive systems (LMSR), Mechanical Department of University of Sidi Belabbes. These tests were conducted in ambient air for the same load ration value R at a nominal frequency of 20Hz in the three zones. Welded structures present a gradient of microstructure and mechanical behaviours from the weld metal to base metal. The study of fatigue crack growth in welded joints of pipeline steels, particularly in the heat affected zone, is difficult research field. In order to more clearly compare the evolution of the cracking rate in the three zones, we chose to represent them by their respective linear regression lines obtained from the experimental points (seven point method) on the linear parts of the curves. F A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 311 Figure 7: Diagram and dimensions of CT50 specimens used for cracking tests Figure 8: Orientation of the CT specimens with respect to the welded joints According to the recommendations of ASTM E399 and AFNOR A03 404, the values of the stress intensity factor K in the case of CT geometry are calculated using the following relationship: 3 2 2     1 a P aw K f w a B w w               (2) where 𝑃 the amplitude of loading [N], w is the width of specimen [m], B is thickness of specimen [m] and a, is the crack length. The compliance function a f w       is on the form [26, 45]: 2 3 4 0.886 4.46  13.31 14.72   5.6 a a a a a f w w w w w                                  (3) A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 312 Figure 9: Evolution of the crack length according to the number of cycles. In order to explore the field of medium and high cracking rates, we imposed a sinusoidal load of constant amplitude 7P KN  for the duration of the test, with a load ratio 0.1R  . The curves of evolution of the length of the crack “a” as a function of number of cycles of BM, WM and HAZ regions of welded joint are presented in Fig 9. The difference in advanced of the crack between these curves in higher as the number of cycles increases. These results influenced on the evolution of the nominal cracking rate versus plotted on a bi- logarithmic curve shown in Fig. 10. Values of da dN are between 10-6 and 10-2 mm/cycle and the values of K values change from: - 13.8 to 36.9 MPa m in the heat affected zone. - 13.2 to 36.6 MPa m in the weld metal. - 11.7 to 35.4 MPa m in the base metal. The curves have an almost rectilinear look on a big part of the explorer domain, which can be presented a law of Paris of the form [46]:     m da C K dN   (4) The results of the fatigue tests obtained in the three zones recorded respectively in Tab. 6. Table 6: Paris law in the different zones studied. Regions Paris law BM  4.87  111.1   da E K dN   WM  4.55  115.2  da E K dN   HAZ  3.82  118.1   da E K dN   A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 313 Figure10: Fatigue crack growth rates for as a function of ΔK for the three zones studied The results of Fig. 10 have shown that the welded metal and heat affected zone have an important higher crack propagation resistance at the small ΔK values (The mechanical properties of weld material are better (Tabs. 3 and 4). The Paris equation coefficients obtained for the different X70 welded joint regions mentioned in Tab. 6 indicate that the growth rate of the crack influenced by the microstructure of the welded joint zones. The high resistance to propagation of heat affected zone can be related to the existence of acicular ferrite in microstructure (Acicular ferrite microstructure in heat-affected zone give high mechanical behaviors in terms of strength and toughness) [47-49]. At the high values of ΔK, the affinity between cracking rate plots can be related for the decrease of crack closure. For low values of ΔK this phenomenon as more marked by the crack tip plasticity following its advance (thus forming a wake plasticized). These results are in good agreement with the results of Beltrao et al.[50] but, they noticed that the fatigue crack propagation is get more independent from the high load ratio values(R=0.5) EVOLUTION OF ENERGY PARAMETERS arious experimental techniques have been developed to measure the surface creation energy U. Most of these techniques are based on stable mechanical hysteresis loops, which measure work in an area near the crack. Ikeda et al. [51] had measured the quantity for steel of low carbon content and for high resistance aluminium alloy from hysteresis loops in the plastic zone using strain micro gages. Using the differential method of Kikukawa et al. [52], Ranganathan measured the hysteric work U [53], which represents the dissipated energy in the plastic zone by unit created surface on the aluminium alloy (2024 T351), testing CT specimens modified in order to be able to measure the crack tip opening displacement (CTOD) in the loading axial direction. The progression of the crack tip opening displacement δ, and δ’ with respect to the load P were registered at a frequency of 0.05 Hz (δ evaluated by a clip gage). The differential displacement δ’ is calculated by the expression: δ δ α P   (5) : The specimen compliance at a singular crack length. The measurements were realized during one cycle for constant amplitude. Characteristic δ and δ’ with respect to the load P layouts for constant amplitude loading are shown in Fig. 11. (Crack opening load Pop was measured at the beginning of the horizontal segment on δ’ with respect to P diagram [54]. Fig. 12 shows the evolution of the hysteretic energy Q dissipated during a cycle as a function of ΔK, for a load ratio R = 0.1, in the three zones studied. This energy is determined by a numerical integration of the cycles (P- δ‘), its expression is obtained by calculating the area of this loop obtained by acquisition and processing by a program written under LABVIEW. V A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 314 Figure 11: Diagrams of δ and δ 'with respect to charge P. Figure12: Evolution of the hysterical energy of the crack as a function of ΔK The advantage of this program is to be able to estimate this hysteretic energy for the low values of ΔK. We notice that Q increases when ΔK increases for the three zones studied. Fig. 13 represents the evolution of the specific energy expended U per cycle in function of ΔK in the three zones studied. This energy is given by the following relation [31], [55]: A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 315             ‘        2 2 Area of the curve PQ U da da B B dN dN                (6) where B the specimen thickness and da dN is the fatigue crack growth. U decreases with K at small K values to achieve a constant value at high K levels. These results indicated that the specific energy for the different zones of the weld joint studied is not a constant and depends the microstructure and on the decrease of crack closure phenomenon. These results are consistent with the work of [53], [57-58] where they consider that the hysterical work is essentially dispelled in the plasticized zone and that in the case where the closing phenomena are important, it is conceivable that a part of the energy U is dissipated in the zone situated in the wake plasticized along the crack front. Figure 13: Evolution of specific energy as a function of ΔK CONCLUSION atigue behavior of welded structures is a more complicated than that of base material due to the presence of heterogeneous metallurgical zones produced during the welding process. Fatigue crack propagation in base metal BM, weld metal WM and heat affected zone HAZ of welded joints of an API X70 pipeline steel was investigated. The most generally model used for the fatigue crack growth based is LEFM and the crack propagation is the dominant stage of the fatigue life. The fatigue crack growth behavior of welded joints API X70 pipeline steel affected by variation of microstructure, weld materials and the loading parameter. Fatigue crack propagation in heat affected zone of X70 steel is delayed compared to the propagation to the other zones. The hysteretic energy Q dissipated during one cycle increases when ΔK increases for the three zones studied and specific energy U reaches a constant value at high ΔK levels. 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