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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Yield Strength (YS) of the weld (GTAW) reduced by 17% and 9% for 10,000 hrs and 20,000 hrs respectively with respect to as welded condition. However, there is no significant difference in Ultimate Tensile Strength (UTS) after thermal ageing for 10,000 hrs and 20,000 hrs with respect to as welded condition. Percentage elongation increased to 23.36% for 10,000 hrs after thermal ageing. There is marginal decrease in yield strength and significant increase in ductility due to thermal aging at 400 ℃ for 10,000 hrs with respect to as welded in case of GTAW. However, there is not significant change after thermal aging at 400 ℃ for 20,000 hrs with respect to as welded condition. This anomalous behavior of decreased yield strength and increased ductility after thermal ageing for weld of GTAW may be due to the relaxation or redistribution of locked in residual stresses in the weld due to thermal ageing. Since, specimens have been prepared from different location across thickness; weld specimen may be having tensile or compressive residual stresses. Locked in residual stresses (tensile or compressive) in the weld specimen could have lead to differences in thermal ageing effect for 10,000 and 20,000 hrs with respect to as-welded condition. However in contrast to the GTAW, the results of weld (SMAW) show a significant reduction in the ductility i.e. 40.38% and 38.65% for 10,000 and 20,000 respectively after thermal aging in comparison to as-welded condition. There is no significant change in strength properties. Although ferrite hardness has increased significantly in section 3.3.2 and there is no change in hardness of austenite phase after thermal ageing. The same observation is missing in tensile properties because the content of the ferrite phase is very low compared to austenite phase. Although ferrite hardness has increased significantly in section 3.3.2 and no change in austenite phase after thermal ageing but the same is not reflected in tensile properties because ferrite phase content is very less compared to austenite phase in macro scale. Further a least change in the tensile properties due to thermal aging indicates yielding, plasticity and fracture can be accounted in the austenitic stainless steel under the slow strain rates and low constraint conditions in the tensile specimen as disputed to the significant change in the hardness property calculated under very high strain rates during Vickers hardness measurement tests [Vitek et al. (1991)]. 3.4.2 Effect on thermal aging at Heat affected zone Figure 12 shows the stress vs strain curves obtained during the tensile examinations for heat affected specimens considered along the heat affected zone for different thermal aging conditions is characterized at GTAW and SMAW zone. The aging conditions were denoted for heat affected specimens are as follows: SSW-R for as-welded, SSW-T41 for thermal aging at 400 ℃ upto 10,000 hrs and SSW-T42 for thermal aging at 400 ℃ upto 20,000 hrs along the heat affected zone. The tensile properties obtained from the stress vs strain curve for each aging conditions were listed in table 7. From the results at GTAW zone, it is observed that there was slight change in the tensile properties of heat affected specimen after thermal aging with respect to as-welded. In comparison to the tensile properties of as-welded, YS was reduced by 8% for 10,000 hrs and 1% for 20,000 hrs. However, UTS was reduced by 5.43% for 10,000 hrs and increased to 1.25% for 20,000 hrs and % elongation or ductility was increased to 2.74% for 10,000 hrs and reduced by 3.44% for 20,000 hrs. Due to the effect of thermal aging, a reduction in YS for thermally aged specimens in comparison to the as-welded specimens proves slight increase toughness in the heat affected zone but not significant with respect to thermal aged weld specimens. Whereas, the change in the values of UTS and ductility is less than 5% and hence considers not significant effect to thermal aging. From the tensile results at heat affected specimens considered at SMAW zone, it is observed that there was a slight reduction in both yield strength and ultimate tensile strength for all the cases of heat affected SS 304LN specimens. In comparison to the tensile properties of as-welded heat affected specimens, YS were reduced by 2.25% for 10,000 hrs thermal aged specimens and UTS of welds were reduced by 4.63% for 10,000 hrs and 1% for 20,000 hrs as these small deviation are within the range of less than 5% and hence not significant. Similarly ductility was also reduced by 2.88% and 5.4% for 10,000 and 20,000 thermal aging hours implies very less change and considered not significant. From the above results, the effect of thermal aging is not significant in the heat affected specimens upon comparison to the weld specimens due to less ferrite content in the heat affected portion. Table 7. Tensile tests along the heat affected zone of GTAW and SMAW for heat affected specimens in different aging conditions zone. SI. No Specimen ID Specimen location Yield Strength (MPa) Ultimate Tensile Strength (MPa) % Elongation 1 SSHAZ-R GTAW 414 626 85.97 2 SSHAZ-R SMAW 444 647 81.45 3 SSHAZ-T41 GTAW 381 592 88.4