PSI - Issue 36

Odarka Prokhorenko et al. / Procedia Structural Integrity 36 (2022) 254–261 6 Odarka Prokhorenko , Serhii Hainutdinov, Volodymyr Prokhorenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000 In Fig. 4 (a), the initial proportion of ferrite in the solid metal is equal to 1, when the temperature rises to 1300 °C, it is sharply decreases to a negligible value of 7.57813e-4. 259

( а )

(c)

(b)

(d)

Fig. 4. Kinetics of temperature and phase processes during welding by «Fr_72» scheme : (a), (b) in node 69997; (c), (d) in node 63153.

Together with that, the proportion of austenite in the metal increases sharply from 0 to 0.999242 and then remains at this level up to time moment t=63.9987 s. By this moment, the temperature in the node decreases from 1300 °C to ~798 °C and the reverse processes of phase structu re transformation begin. Ferrite intensively increases in proportion to 0.997755, while austenite decreases to 1.50206e-3. All these processes in the metal occur at the point of the metal transition into the molten stage, but it still remains in a solid state, which justifies the use of FEM. From time moment t=97.5 s, bainite appears in the node (Fig. 4 (b)), the proportion of which reaches its maximum value of 1.33819e-3 at t=112.5 s and remains unchanged until the end of the cooling process at t=1200 s. At t=125 s a martensite phase appears in the node due to existing austenite, which is gradually decreases to value 1.98415e-6 at the time t=1200 s. In the cooled state, such phases exist in the node in the following proportions: ferrite - 0.997755, austenite - 1.98415e- 6, bainite - 1.33819e-3, martensite - 9.04386e-4, which are in total amount equal to - 0.99999 or ~1. The analysis of kinetic processes in the node 69997 ends at this moment. For further analysis, consider the kinetics of temperatures and phase processes at node 63153 shown in Fig. 4 (c) and (d). The initial proportion of the ferrite phase in the node is equal to 1. With the increase in the metal temperature, ferrite transforms into austenite, similarly to how it was discussed above for node 69997. The minimum proportion of ferrite occurs at temperature 1300 °C and is equal to 7.51838e -4, meanwhile the proportion of austenite in the metal increases to the value of 0.999248. This is almost the same proportion of austenite as in node 69997. Further, starting from time moment t=122 s, the temperature in the node begins to decrease from 1300° C and at t=184.344 s, the proportion of ferrite in the node is equal to 0.986188, and austenite to 0.0138117. At the next step (t=218.516 s), the proportion of ferrite is equal to 0.999381, and it does not change until the end of the welding and cooling process (t=1200 s). The proportion of austenite in the node is 6.19401e-4. At t=269.773 s, the proportion of austenite becomes even lower ~1.31617e-4. The difference between the proportion of austenite between these two step is 4.87784e-4, which is exactly equal to the proportion of martensite in the node at t=269.773 s. Thus, by tracing and analyzing the kinetic processes of changing proportions of various structural phases in the metal at node 63153, the kinetics of formation and changes of the martensite phase in the node was determined. Until the end of the cooling process of the node under consideration, the martensite phase is formed due to a corresponding decrease in the austenite phase. There is no bainite in the metal. 3.2. A back-step welding of a butt joint with six 100 mm long sections (technological scheme – «Fr_82»). The distribution of phase structure components in the plate in longitudinal sections parallel to the weld, with coordinates |y|=0, 5, 10, 15 and 20 mm, is shown in Fig. 5. It is obvious that the distribution character is similar, and the quantitative values are comparable with those for "Fr_72" welding scheme. Characteristic cone-shaped surfaces of the maximum values of the considered phase structure parameter (ferrite, bainite, martensite, and austenite) are also formed in this case. They are localized at five places above the back-step weld in the vicinity of the adjacent sections joining with each other, and their maximum values do not coincide with the coordinates of nodes where the welded sections being joined. The maximum values of the phase proportions in the longitudinal sections are also