PSI - Issue 36

Odarka Prokhorenko et al. / Procedia Structural Integrity 36 (2022) 254–261 Odarka Prokhorenko , Serhii Hainutdinov, Volodymyr Prokhorenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000 5

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(b)

( а )

(c)

(d)

Fig. 3. Kinetics of temperature and phase processes in nodes 69780 and 62978 of the plate mesh model during welding by «Fr_72» scheme.

At t=24.9987 s, the proportion of bainite reaches its maximum value of 0.292714, ferrite reaches a value of 0.69505, and from the time t=19.9987 s, remaining austenite transforms into martensite, however these are very small proportions, as can be seen from the martensite curve in Fig. 3 (c). Further, with the accomplishment of the welding of the second section, the heating source approaches the node under consideration, and at the time t=75.9908 s, temperature in the node rises to 1300 °C and remains at this level until t=92.5 s. At time t=80 s, welding of the last third section of the back-step weld begins. A sharp increase in the temperature at the node to 1300 °C led to the same sharp decrease to almost 0 value (~ 0.07%) of ferrite (dashed curve in Fig. 3, (a)), and the same sharp increase of austenite (ferrite transformed into austenite). Portion of austenite by the time t=75.9908 s was used for the martensite formation. Nothing happened to bainite from t=24.9987 s and until the considered time moment t=75.9908 s. Except for the negligibly small transformations of austenite into martensite, the process of phase transformations in the node practically does not change until the time t=112.5 s, at which the temperature in the node decreases to 843.427 °C. Starting from this moment, austenite is transformed into ferrite again to a proportion of 0.692707, which is slightly less than it was before (0.69505) in the time interval t=(17.4987… 71.9987) s. A small portion of ferrite transformed into the martensite through the austenite phase. In time interval t= (161.563… 184.344) s, the proportion of bainite increases due to existing austenite, from 0.292714 to 0.295048 value, and which does not change until t=1200 s. At this moment, in the node, in addition to the already indicated bainite, the proportion of ferrite is equal to 0.692707, austenite to 2.91385e-5, martensite to 0.0122161, temperature is T= 51.2723 °C. The total value of the proportions of all structural phases is 1.0000002385 = ~ 1. This completes the analysis of the kinetics of thermostructural processes at node 69780. In similarly way, the kinetics of such processes to be analyzed in any other nodes of interest of the welded joint mesh model. In Fig. 3 (b), (d) the kinetic charts for node 62978 are shown. The joint consideration and analysis of the kinetics of temperature and phase processes for nodes 69997 (x = -200 mm, y = 0 mm) and 63153 (x = 200 mm, y = 0 mm) in the middle of each of two extreme sections of the back-step weld ( Fig. 4) in connection with the subsequent analysis of similar kinetics for the process flow chart of back-step welding by "Fr_82" scheme, for which these nodes are the joints of sections 1 and 2, as well as 5 and 6, which of them is 100 mm long, is of a considerable interest. For further analysis, node 69997 will be considered. For the convenience of analyzing the kinetic processes in this node and their relationship with temperature, the temperature charts in the node, similar to those shown above, are shown in Fig. 4 (a) and (b). A back-step welding of the joint with three sections according to "Fr_72" scheme begins on the X-axis from the coordinate x=-100 mm, which is 200 mm from the front edge of the plate. In this case, the heating source moves from x=-100 mm in direction of the front edge of the plate to the coordinate x=-200 mm, on which considered node 69997 is located, and then to the front edge of the plate (coordinate x=-300 mm). The welding arc reaches the node 69997 in ~20 s (distance 100 mm). Temperature in the node rises sharply and reaches the nominal melting temperature of 1300 °C at ~20 s. At this temperature, a crystal lattice is still preserved in the metal, the so-called "long-range order" and the metal has not yet passed into a molten state. In order for the metal to melt, a portion of heat must still be introduced into it in the form of latent heat of melting, which will destroy the crystal lattice and then a stage of melt with freely moving atoms of iron and other impurities in it will take pla ce.At temperatures up to 911 ° C, ferrite exists in its α allotrope (Fe α ) with a body centered cube crystal lattice. Further, at temperatures (911...1392) °C it changes to γ allotrope (Fe γ ) with a face centered cube crystal lattice.