PSI - Issue 17

Jan Kec et al. / Procedia Structural Integrity 17 (2019) 230–237 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

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a)

b)

Fig. 7. (a) comparison of morphology of a fractured specimen at (a) 40 °C and (b) -60 °C.

3. Conclusions

The most important results of investigation of mechanical properties and microstructure of gas pipeline X60 steel being in operation since 1974 can be summarized as follows: - Gas pipeline X60 steel did not show any discontinuous yielding during the tensile test; it can be concluded that it has a very low dislocation density arisen from the process of manufacture. The results of the tensile test show that prescribed values of minimum yield point, ultimate tensile strength and ductility were reached. - Microstructure of the steel is of the band type with alternating layers of ferrite and pearlite. In the central part of specimen, bands of granular bainite are observed. Inside the pearlite/granular bainite bands and in their vicinity, elongated MnS inclusions and sharp-edged NbC particles occur. - Higher number of pearlite bands per unit area, higher elongation of MnS inclusions as well as a higher elongation of ferritic grains in T-L orientation caused the decrease of absorbed energy in USE and DBTT. The fracture surface showed action of ductile mechanisms at ambient temperatures, a mixture of ductile and brittle mechanisms at middle temperatures and brittle mechanisms at low temperatures. - In comparison with BM, the decrease of absorbed energy was observed in WCL and FS; it can be attributed to the heterogeneity of microstructure. WCL microstructure consists of acicular ferrite, grain boundary ferrite, polygonal ferrite and Widmanstätten ferrite. Acknowledgements Petrov, R. H., Jonas, J. J., Kestens, L. A., Gray, J. M., 2015. Microstructure and Texture Development in Pipeline Steels. Oil and gas pipelines integrity and safety handbook, 159-185. Hillenbrand, H. G., Graf, M., Kalwa, C., 2001, Development and production of high strength pipeline steels. Proceedings of the conference niobium 2001, Orlando, FL, USA. Koh, S. U., Yang, B. Y., Kim, K. Y., & Kim, J. S., 2004. Effect of line pipe steel microstructure on susceptibility to sulfide stress cracking. Corrosion 60, 244-253. Zadow, L., Gamboa, E., Lavigne, O., 2015. Inclined stress corrosion cracks in gas pipeline steels: Morphology and implications. Materials and Corrosion 66, 1092-1100. Sojka, J., Jonšta, P., Rytířová, L., Sozańska, M., Jerome, M., 2005. Influence of microstructure on sulphide stress cracking of hot rolled tubes. Acta Metallurgica Slovaca 11, 323-330. Feng, R., Gong, B.K., Cui, H.W., Pan, Y.K., Bao, J.H., 2018. The Influence of Abnormal Segregation Band on Mechanical Properties of Hot Rolled Ferrite/Pearlite Steel Plate. Materials Sciences and Applications 9, 81-91. Nagode, A., Resnik, A., Vertnik, R., Bizjak, M., Kosec, B., G ojić , M., Kosec , G., Š arler, B., Zorc, B., 2017. The development of a banded microstructure in S355J2 steel bar. Kovove Materialy - Metallic Materials 55, 51-56. The support of institutional financing, grant MSM2579700001 is gratefully acknowledged. References