PSI - Issue 60

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

716

8

The typical force-indentation depth curves measured at 200 gf (2000 mN) in the weld fusion zone of as-welded and thermal aged conditions are shown in Figures 7 (a and b). After thermal aging, the load required to achieve the similar indentation depths has increased slightly and the total indentation depth has decreased for both SMAW and GTAW regions. This is an indicative of increase in hardness along the weld fusion zone due to the effect of thermal aging, the effect is marginal. However, there is practically very less difference in the force-indentation depth curves for samples aged at 10,000 and 20,000 hrs [Chandra et al. (2010)]. Vickers micro hardness was calculated based on the obtained indentation curves and the results were presented in table 4. In agreement with the indentation curves as shown in figure7, the Vickers hardness of both GTAW and SMAW regions has increased for the thermal aged specimens. However thermal aged weld specimens at 400 ℃ for 10,000 hrs and 20,000 hrs, the Vickers hardness value remained the same at the GTAW region and decreased in the case of specimens at SMAW region. Table 4. Bulk hardness of welds in different aging conditions. SI. No. Sample condition Vickers hardness (HV) GTAW SMAW 1 As-welded 242 ± 5 226 ± 3 2 400 ⁰C / 10,000 h 259 ± 10 263 ± 10 3 400 ⁰C / 20,000 h 256 ± 13 250 ± 11 Further the indents were examined under an optical microscope corresponding to the bulk hardness at 20 µm scale bar for the weld specimens as shown in figures 8 (a, b and c). Vickers indentations corresponding to the bulk hardness on the dark and light-etched phases were  -ferrite and austenite respectively. The significance of  -ferrite is very low due to the size of the indentations corresponding to the macro-scale hardness measurement where the indentation is carried out on austenite and  -ferrite phases.

Fig. 8. Optical micrographs showing indentations corresponding to the bulk hardness at 20 µm for the weld specimens: (a) As-welded, (b) Thermal aged at 10,000 hrs and (c) Thermal aged at 20,000 hrs. 3.3.2 Effect of thermal aging on micro hardness

Fig. 9. Force – indentation depth curves at 1 gf (10 mN) of the weld fusion zone of SS 304LN for various thermal aging cycles at 400 °C.