PSI - Issue 26

P. Ferro et al. / Procedia Structural Integrity 26 (2020) 11–19

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Ferro et al. / Structural Integrity Procedia 00 (2019) 000 – 000

grains as parallel plates forming the so- called α colony. The α colonies continue to grow until they meet another α colony. With increasing cooling rate, the size of the α colonies as well as the thickness of the individual α plates becomes smaller. At higher cooling rates α plates start to nucleate also at the α colony boundaries and start to grow perpendicular to the nucleation site forming basketweave structure (Fig. 8). The SR heat treatments didn’t affect significantly the original microstructure of the joints , while great amount of coarse acicular α occurred in the FZ of SQA heat treated samples. Furthermore, m artensitic α’ decomposes to equilibrium α and β even if the decomposition is not completed at the aging temperature up to 600°C. 3.2 Microhardness Fig. 9 collects the microhardness profiles as a function of the butt-welded joint condition. All of them show a quite flat shape with a little increment of the microhardness by approaching the FZ. The lowest values are measured in the as-welded joints while the biggest ones are observed in the SQA1 post heat treated samples. This is attributed to the higher amount of  ’ phase in the last ones, induced by the solution and quenching and only partially decomposed during the 1h aging heat treatment at 600 °C. By increasing the aging dwell time, the increased decomposition of  ’ reduces the hardness of the joint as shown in Fig. 9.

 colonie s

Fig. 8. HAZ of as-welded butt joint

Fig. 9. Microhardness profiles