PSI - Issue 36

Odarka Prokhorenko et al. / Procedia Structural Integrity 36 (2022) 254–261 4 Odarka Prokhorenko , Serhii Hainutdinov, Volodymyr Prokhorenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000 distribution along the joint length of the phase structure parameters is relatively regular, as, for example, for the longitudinal section y=20 mm. 257

Fig. 2. Phase composition of the welded joint in longitudinal sections at |y|=0, 5, 10, 15 and 20 mm after welding by «Fr_72» scheme.

Ferrite along the length of the plate in section y=20 mm is distributed almost uniformly at a concentration of ~ 1.0 (100%), austenite ~ 0...9e-7 (0...9e-5) %, bainite ~ 0 (0%), martensite ~ 0...0,0006 (0...0.06) %. Fig. 2 also shows that the phase composition of the weld and HAZ varies quantitatively in strips of ~ 20 mm wide, adjacent to the weld axis on its two sides. In general, there is a dynamic balance of phase proportions at any point in the plate volume at any temperature exist, that is, the sum of the phase proportions at a point is always equal to 1 (100%). To determine the reason for the appearance of two zones with higher proportion values of the phase structure components in the vicinity of the junction of weld sections it is necessary to consider and analyze the kinetics of temperature and structural phases content in the two aforementioned nodes 69780 and 62978, which are correspond to the centers of peaks in Fig. 2. The kinetics charts are shown in Fig. 3: (a) and (c) refer to node 69780, (b) and (d) refer to node 62978. The temperature curves in Fig. 3 for the two nodes under consideration are different and have a peculiar stepped character, which is associated with the sequence and direction of execution of all three sections of the back-step weld according to the "Fr_72" welding scheme. Proportions of the structural phases in the metal of the plate at any node of the mesh model are inextricably linked with the kinetics of temperature at this node. This can be seen in Fig. 3 from the kinetics of temperature and proportion of the structural phases at any current value of time t(s) of the heating and cooling process of the node under consideration. For example, the peak bainite values in Fig. 2 correspond exactly to its same values for the bainite phase curve in Fig. 3 (a) and (b) (horizontal parts of the curves for the bainite phase). The kinetics of temperature, proportions of ferrite, austenite, bainite and martensite in the metal structure in the considered nodes 69780 and 62978 are absolutely and unambiguously related to each other, which is clearly seen in Fig. 3. For example, consider the charts in Fig. 3 (a) and (c), which are referring to node 69780. As the temperature in the node increases to its maximum value, in the first section of the T(t) curve, a sharp decrease of ferrite proportion from the initial value of 1 to the minimum value of 0.264451 (dashed curve) with its transformation in equal proportion of austenite (dotted curve) can be observed in Fig. 3 (a). Transformation process takes place until the maximum temperature T(t) is reached, at this point austenite is 0.735549, while ferrite proportion dropped down to 0.564451. Further, with decreasing of the temperate in the node, the reverse process of increasing the ferrite proportion begins. The dashed curve goes up sharply, while the dotted curve for austenite continues to decline. From the time t=12.4987 s, bainite appears in the node due to the decrease in austenite.