PSI - Issue 60

Cyril Reuben Raj et al. / Procedia Structural Integrity 60 (2024) 709–722 Cyril Reuben Raj / StructuralIntegrity Procedia 00 (2024) 000 – 000

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GTAW section retained higher delta ferrite (around 8-10%) which is desirable to avoid hot cracking in the weld zone [Lippold and Kotecki (2005)]. Upon accelerated thermal aging, the ferrite content along and across the SMAW and GTAW sections reported minimal deviation. The ferrite precipitation into secondary phases was not observed in the ferrite measurement due to very low ferrite content and analysis performed at macroscopic levels.

3.2. Effect of thermal aging on microstructure analysis

Fig. 4. Microstructure at different magnifications of the as-welded specimen at weld fusion zone (conventional groove): (a) SMAW region at 50 µm scale, (b) SMAW region at 20 µm scale, (c) GTAW region at 50 µm scale and (d) GTAW region at 20 µm scale.

Figure 4 shows the microstructure at different magnifications of the weld fusion zone of the as-welded pipe as observed under the optical microscope. Figures 4(a) and (b) consist of microstructural morphology of SMAW region at a lower and a higher magnification respectively. Similarly, the microstructural morphology of the GTAW region is shown in Figures 4(c) and (d) at lower and higher magnification scale bar. The microstructure reveals phases of austenite and δ - ferrite. The majority of δ -ferrite is characterized by vermicular morphology and a very few areas accounts for lath morphology. This major difference between the morphologies of SMAW and GTAW regions is due to the high content of inclusions within the SMAW region as evident in the Figure 4(b). High inclusion density in SMAW region is caused due to the use of flux and consequently the formation of manganese and silicon rich oxides during de-oxidation process as reported in literature [Mills (1997)]. These oxides are generally trapped in the molten pool during solidification, thereby accounting for the high inclusion density. Figure 5 shows the microstructure at different magnifications of the weld fusion zone of thermally aged welds at 400 °C for 10,000 hrs as observed under the optical microscope. It is clearly evident that no significant change in the microstructural morphology of the 10,000 hrs thermally aged welds specimen either in the SMAW region or in the GTAW region in comparison to the as-welded specimen as shown in figure 5 as examined under the optical microscope. Similarly figure 6 shows the microstructure at different magnifications of the weld fusion zone of thermally aged welds at 400 ℃ upto 20,000 hrs as observed from the optical microscope. Here the microstructural