PSI - Issue 68

Ahmed W. Abdelghany et al. / Procedia Structural Integrity 68 (2025) 520–526 Abdelghany et al. / Structural Integrity Procedia 00 (2025) 000–000

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range of 27 and 29 µm, respectively, well below the AWGS of the initial sample A (49 µm). Moreover, the grain size distributions for each of the TMCP samples subjected two-step uniaxial hot compression schedules (i.e., I – VI) are shown in Fig. 3 (c). The cumulative fraction data indicates that 80% of the grain counts are below 45 µm for the initial condition (sample A), which was reduced to 27 and 29 µm for schedules III and IV, respectively. Vickers hardness measurements (HV10) were performed for all the samples across the various TMCP schedules, with the evolving hardness graphically represented in Fig. 4 (a). It is observed that the hardness tends to inversely correlate with the temperature of the second-hit compression step. Accordingly, as the temperature of the second deformation increases, the overall hardness of the material decreases. Notably, after the second compression, there was a significant increase in hardness values regardless of the second-hit compression temperature (T 2 ), compared to those recorded on recrystallized microstructures after the first deformation hit at 1100 °C (180 HV). This indicates the pronounced effect of the TMCP schedule on material hardening. As regards schedule IV, which employed a controlled cooling rate of 1 °C/s, the sample displayed almost negligible drop in hardness level compared to that subjected to air cooling following the second compression hit at the same temperature, i.e., T 2 =900 °C (schedule III), possibly due to sluggish restoration. In schedule II, the isothermal holding time t 2 was extended to 60 s after the second-hit compression step. Compared to schedule I with t 2 = 30 s at the same second-hit compression temperature (i.e., T 2 =850 °C), the hardness remained practically unchanged (slightly increased) despite longer holding, though the effect is within the limits of scatter. An illustration of Kernel average misorientation (KAM) map shown in Fig. 4 (b) represents the local misorientation results from higher dislocation densities in the microstructure. The KAM map for sample I showed high degree of misorientation in the pancaked grains (non-crystallized regions) signifying high dislocation densities.

Fig. 4. (a) Average Vickers hardness (HV10) measured in the centre of the 201LN alloy for all TMCP schedules, and (b) Kernel average misorientation (KAM) map for Sample I, subjected to a second-hit compression at T 2 =850 °C.

Variation in the cooling rate did not have any noticeable effect on either the hardness or the AWGS following second-hit compression at T 2 =900 °C. However, small traces of statically recrystallized grains were observed under slower cooling condition after the final deformation pass. At the lowest second pass temperature (T 2 ) of 850 °C, the AWGS were recorded as 42 µm and 44 µm, with corresponding hardness values of 272 and 280 HV, for schedules I and II, respectively. This indicates that an extended holding after the second compression (t 2 ) had no significant effect at this lower deformation temperature in realizing the substructure strengthening. Realization of desired pancaking in the microstructure with a concomitant hardness enhancement is presently under evaluation for the structure-property correlations and austenite stability of the 201LN alloy, particularly in cryogenic conditions, to further optimize the TMCP conditions. 4. Conclusion Based on the results obtained through a comprehensive microstructure analysis and hardness measurements on the 201LN alloy, TMCP processed via a unique two-step uniaxial hot compression testing in a Gleeble simulator, the following conclusions can be drawn: