PSI - Issue 60

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

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morphology of the 20,000 hrs thermally aged weld specimen reveals no significant difference in comparison to the as-welded weld specimen. The reason behind in the negligible microstructural changes subsequent to thermal aging at 400 °C is that the phase transformations i.e. spinodal decomposition and G-phase precipitation take place at these temperatures are visualized at nano-metre scale as stated in the literatures [Chandra et al. (2012)], which is highly difficult to distinguish by optical microscopy.

Fig.5. Microstructure at different magnifications of the weld fusion zone of thermally aged weld specimen at 400 ⁰C for 10,000 hrs: (a) SMAW region at 50 µm scale, (b) GTAW region at 50 µm scale and (c) GTAW region at 20 µm scale.

Fig.6. Microstructure at different magnifications of the weld fusion zone of thermally aged weld specimen at 400 ⁰C for 20,000 hrs: (a) SMAW region at a 50 µm scale, (b) GTAW region at 50 µm scale and (c) GTAW region at 20 µm scale.

3.3. Effect of thermal aging on hardness

3.3.1 Effect of thermal aging on bulk hardness

Fig. 7. Force – indentation depth curves at 200 gf (2000 mN) of the weld fusion zone of SS 304LN for various thermal aging cycles at 400 °C: (a) SMAW (b) GTAW.