ation of the tendency to cold cracking in the HAZ during welding. The permissible level of hardness is 315 HV, which reflects a certain amount of bainitic-martensitic mixture in the HAZ structure, is established by norms of Det Norske Veritas (DNV-OS-F101) and is applicable for welding pipes with a wall thickness of 20 mm or more. (This criterion applies to the evaluation of field joints of pipelines welded with high cooling rates, when partial quenching of HAZ site is possible in the case of increased stability of the austenite). As shown in Fig. 10, neither steel exceeds the 315-HV limit up to cooling rate of 70°C/s. It should be noted that the increase in Nb content up to ~0.10% at medium level of Mn and small amounts of Cr did not affect the propensity to quenching of HAZ metal and thus the compositions studied are not at risk of cold cracking during welding, even with very low heat input. Conclusions 1. Weldability assessment was performed based on careful investigations of two Nb-containing industrial steel grades of X70 and X80, respectively, with 0.056 and 0.094% Nb. 2. The resistance of the two steels to brittle fracture in the HAZ was evaluated on samples of the steels after high-temperature heating and cooling to simulate the weld thermal cycle of welded joints at different heat inputs. 3. Use of different criteria of resistance to brittle fracture including the temperature of 50% shelf impact toughness and temperature of minimum specified impact toughness (here 70 J/cm2), have shown that the HAZ of both investigated steels ensure performance of pipelines down to 30°C in SAW of thick-walled pipes using high heat input. 4. CCT diagrams developed and measurements of microhardness of microstructures, formed by the transformation of austenite at different cooling rates from 1300°C, have shown that the investigated steels with Nb content up to ~0.1% do not have a tendency to cold cracking in the HAZ, even at very low heat input. References 1. Gray, M. 2011. Evolution of microalloyed linepipe steels with particular emphasis on the “near stoichiometry” low carbon 0.1 percent niobium “HTP” concept. Proc. the 6th International Conf. on High Strength Low Alloy Steel (HSLA Steels 2011), Beijing, China, pp. 652–657. 2. Frantov, I., Permyakov, I., and Bortsov, A. 2011. Improvement of weldability and criterion of reliability of high strength pipes steels. Metallurgist, No. 12, pp. 74–81. 3. Li, Y., Crowther, D. N., Green, M. J. W., et al. 2001. The effect of V and Nb on the properties and microstructure of the intercritically reheated coarse grained HAZ in low-carbon microalloyed steels. ISIJ Int., 41, 1, pp. 46–55. 4. Stepanov, P. P., Zikeev, V. N., Efron, L. M., Frantov, I. I., Morozov, Y. D. 2010. Metallurg, No. 11, pp. 62–67. 5. Hattingh, R., and Pienaar, G. 1998. Weld HAZ embrittlement of Nb-containing C-Mn steels. International Journal of Pressure Vessels and Piping — Internt. J. Pressure Vessels Piping, 75(9): 661–677. 6. Yang, J., Hyang, C., and Chou, C. 1999. Microstructure of heat-affected zone in Nbcontaining steels. Material Transactions, JIM, 40(3): 199–208. 7. El Kashif, E., and Koseki, T. 2007. Effect of Nb on HAZ microstructure and toughness of HSLA steels. Materials Science Forum (Vols. 539–543), THERMEC-2006, pp. 4838–4843. 8. Sakai, S., Sakai, T., and Takeshi, K. 1977. Hot deformation of austenite in a plain carbon steel. Trans. of ISIJ, 17, pp. 718–725. 9. Guagnelli, M., Di Schino, A., Cesile, M. C., and Pontremoli, M. 2011. Effect of Nb microalloying on the heat-affected zone microstructure of X80 large diameter pipeline after in-field girth welding. Proc. of CBMM workshop "Weldability of Nb Containing Steels.” 10. Fujibayashi, S., and Endo, T. 2002. Creep behavior at the intercritical HAZ of a 1.25 Cr-0.5 Mo Steel. ISIJ Int.,Vol. 42, No. 11, pp. 1309–1317. 11. Kawano, H., Shibata, M., Okano, S., Kobayashi, Y., and Okazaki, Y. 2004. TMCP steel plate with excellent HAZ toughness for high-rise buildings. R&D Kobe Steel Engineering Reports, Tokyo, Vol. 54, pp. 110–113. 12. Bate, A. D., and Kirkwood, P. R. 1988. Development in the weldability and toughness of steels with offshore structure. ASM Intern. Symposium, Microalloying 88, Chicago, Sept. 1988, pp. 175–188. 13. Graf, M., and Niederhoff, K. 2000. Properties of HAZ in two-pass submerged arc welded large-diameter pipe. Europipe publication. 14. Miao, C., Shang, C., Wang, X., Zhang, L., and Subramanian, M. 2010. Microstructure and toughness of HAZ in X80 pipeline steel with high Nb content. Acta metallurgica Sinica. 46(5): 541–546. 15. Mitchell, P. S., Hart, P. H. M., and Morrison, W. B. 1995. The effect of microalloying on HAZ toughness. Proc. of Microalloying 95. ISS, Warrendale, Pa., pp. 149–162. 16. Rykalin, N. N. 1957. Berchung der Warmervorgange beim Scweisen, VEB Verlag Technik, Berlin, Germany. WELDING JOURNAL 29-s WELDING RESEARCH Fig. 9 — The cooling rate, depending on the heat input at multipass welding of butt joints (figures show the temperature before the next weld pass, °C), independent of pipe wall thickness. Fig. 10 — Determination of the critical cooling rate, preventing cold cracking in the HAZ, based on the maximum permissible hardness of 315 HV.
Welding Journal | January 2014
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