Issue 46

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. M ECHANICAL 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