Vyksa Steel Works, Vyksa, Russia:

S. V. Zharkov, Chief Specialist, Center for Research Laboratories, Engineering and Technology Center
P. P. Stepanov, Cand. Eng., Director of the Engineering and Technology Center
O. A. Bagmet, Cand. Eng., Head of the Laboratory of the Center for Research Laboratories of the Engineering and Technology Center

Vyksa Steel Works, Vyksa, Russia¹ ; Vyksa branch of the National University of Science and Technology “MISIS”, Vyksa, Russia² ; 
L. I. Efron, Dr. Eng., Prof.², Scientific Supervisor of the Engineering Research Center¹, e-mail: Lefron@omk.ru

The aim of the work was to study the effect of cooling rate on the formation of the structure and impact strength of a welded joint of 0,07C-1,6Mn-0,2Cr-Nb pipe steel. The cooling rate w_8/5 was varied from 0.9 to 12.4 °/s in the temperature range of 800–500 °/s by changing the heat input of welding, the groove width, and the temperature of 22 mm thick plates welded on a laboratory welding stand. The conducted studies confirm the previously obtained results on the weldability of pipe steels, that a decrease in the heat input of welding and, as a result, an increase in the cooling rate of the w8/5 welded joint is accompanied by an increase in impact toughness. For the studied steel, a significant increase in the impact toughness of KCV-40 (more than 2.5 times) occurs in the range of cooling rates w8/5 from 0.9 to 7.3 °/s and is typical not only for tests along the fusion line, but also in the center of the seam weld. It is shown that with an increase in the cooling rate of the welded joint in the structure of the weld metal, the maximum length and width decrease, and, at the same time, the proportion of grain boundary ferrite crystallites. At high cooling rates, the matrix of the weld structure consists entirely of acicular ferrite, which is favorable in terms of increasing the density of high-angle grain boundaries, toughness, and cold resistance. In the heat-affected zone, with an increase in the cooling rate, the maximum and average grain sizes decrease; grain-boundary and Widmanstatt ferrite is eliminated; a completely bainite structure is formed and the proportion of lath bainite increases, which also leads to an increase in the density of high-angle boundaries and contributes to an increase in impact strength. With an increase in the cooling rate, the proportion of the MA-constituents in the structure of the seam and the HAZ are increases. It is assumed that an increase in the cooling rate, simultaneously with a favorable effect on the structure of the weld and the weld pool, reduces the time for the processes occurring in the weld pool, which can be an unfavorable factor in terms of the toughness of the weld.

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