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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Micro- hardness study on individual phases of austenite and δ -ferrite along the weld fusion zone of welds was performed to characterize the embrittlement nature with the effect of thermal aging. Here the load-indentation is considered on the ferrite phase at 1 gf (10 mN) for the as-welded and thermal aged samples are shown in figure9. In addition, the micro-hardness examinations were conducted on the ferrite phase of the GTAW region, whereas tests were not performed in the SMAW region due to very low size of the ferrite phase i.e. 5 – 8 µm. In contrast to the bulk hardness measurement, there is drastic decrease in the total indentation depth for the thermally aged specimens in comparison to the as-welded specimen. From the above curves, the Vickers hardness is measured for individual phases of ferrite and austenite as detailed in table 5. Table 5. Micro hardness of welds in different aging conditions. SI. No. Sample condition Vickers hardness (HV) Ferrite Austenite 1 As-welded 415 ± 22 392 ± 15 2 400 ⁰C / 10,000 h 679 ± 81 360 ± 32 3 400 ⁰C / 20,000 h 752 ± 96 377 ± 30 In agreement with the indentation curves, a large increase in the ferrite hardness is observed due to the effect of thermal aging at 10,000 hrs. Further, at thermal aging at 20,000 hrs, the incremental hardness is also significant. In addition, the effect of thermal aging on the hardness of austenite phase is very low, slight deviation is caused due to experimental variation. Using optical microscope, indents were examined on ferrite and austenite phase of the weld specimens at 10 µm scale bar as shown in figure 10. Blue and red arrow denotes the location of indents at austenite and δ -ferrite phase, which also resembles as light and dark etched phases. The size of the indent on ferrite phase reduces for the thermal aged specimens in comparison to as-welded specimens at micro-level examinations, whereas on austenite phase the size of the indent remains the same [Chandra et al. (2010)]. The variation in indent size is caused due to thermal aging embrittlement and hence the hardness value increases in the ferrite phase and remains the same in the austenite phase as mentioned in table 5.

Fig. 10. Optical micrographs showing indentations corresponding to the micro hardness at 10 µm for the weld specimens: (a) As-welded, (b) Thermal aged at 10,000 hrs and (c) Thermal aged at 20,000 hrs. Embrittlement manifests increase in hardness and strength of the weld zone is due to the phase transformation occurring in the ferrite phase during thermal aging. However, no phase transformation occurs in the austenite phase, which directs no significant change in hardness value. The phase transformation in the δ -ferrite separates into Fe rich α and Cr -rich α′ by spinodal decomposition mechanism together with the formation of G -phase precipitation which can be noticed only at the micro-scale. The higher ferrite hardness of the thermal aged weld specimens is due to the hardening mechanism based on α – α′ misfit inducing an elastic stress . Even though there is a large increase in ferrite hardness at micro-scale, the effect on bulk hardness is very low. The incremental hardness after thermal aging in GTAW region is accounted to be ~60% in comparison to as-welded specimens, whereas incremental ferrite phase hardness after thermal aging is accounted to be ~80% in comparison to as-welded specimens. The larger increase in the ferrite hardness is not reflecting in the bulk hardness of the weld specimens could be because of smaller content of ferrite (4 – 10%) in the weld fusion zone.