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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were subsequently machined from these plates for testing using a Gleeble thermo-mechanical simulator. Fig. 1 (a) provides a schematic representation of the TMCP schedules that were executed in the Gleeble simulator, and Table 2 offers an overview of the temperature profiles and processing steps involved. Initially, each sample was heated at a linear rate of 3 °C/s to 1100 °C, which is well above the no-recrystallization temperature (T nr ). The sample was held at this temperature for 120 s, followed by first uniaxial compression hit at 1 s -1 to achieve a true strain of 0.29. This was immediately followed by a 30 s isothermal hold (t 1 ), intended to ensure the formation of a fully recrystallized microstructure. For the second-hit compression step, the samples were cooled at 3 °C/s down to four predetermined temperatures (T 2 ) close to or well below T nr temperature (i.e.,1000, 950, 900, and 850 °C). Following the second compression-hit of 0.25 true strain at 1 s -1 , the samples were isothermally held for 30 s (t 2 ) prior to continuous cooling in all cases, except schedule II, where a longer hold of t 2 = 60 s was applied. The cooling step involved a phase of accelerated cooling using high pressure air. Notably, in one specific case (schedule IV), this cooling was intentionally planned at a slower rate of 1 °C/s to assess the impact of slow cooling rate on the microstructure and hardness. All the two-step hot compression schemes I – VI imitating the TMCP schedules are listed in Table 2. The table also includes compression schedule A, which undergoes only single-hit compression in the recrystallization regime for comparison purposes. In this reference schedule, the procedure skips the second hit of compression, involving direct cooling to room temperature following 30 s hold at the deformation temperature of 1150 °C. This detailed plan, including variations in holding times and cooling rates, is meticulously designed to analyse the effects of different TMCP conditions on the characteristics of microstructure evolution and related hardness. The temperature control during the TMCP process was precisely managed through a thermocouple welded in the middle of the outer surface of the cylindrical specimen, as per the specified practice. This ensured accurate monitoring and adjustment of the temperature to adhere to the defined schedules.

Table 2. TMCP schedules performed on the 201LN ASS using Gleeble thermo-mechanical simulator. First compression Second compression Schedule# T 1 [°C] T. Strain (ε 1 ) t 1 * [s] T 2 [°C] T. Strain (ε 2 ) t 2 * [s]

Cooling rate

A

1100

0.29

30 30 60 30 30 30 30

-

-

-

AC AC AC AC AC AC

I

850 850 900 900 950

0.25

30 60 30 30 30 30

II

III IV

1 °C/s

V

VI

1000

AC is air cooling - accelerated cooling using high pressure air. *t 1 and t 2 are the isothermal hold time in [s] before next step (i.e. final air cooling)

2.3. Microstructure characterization The compressed specimens were sectioned longitudinally along the compression axis to examine the central region of each specimen, as shown in Fig. 1 (b). The samples were then mounted and ground to a finish of 1200 grit using standard metallographic practice, followed by electro-polishing using a mixture of perchloric acid, ethanol, and distilled water. The electro-polishing was conducted at sub-zero temperatures with an applied voltage of 23-24 V for 3 min., allowing the removal of any strain-induced layers that may result from the grinding process. The samples were then electrolytically etched using a solution consisting of 60% nitric acid and 40% distilled water, with an applied voltage of ~ 1.3 V for a duration ranging between 40 and 70 s. For microstructural investigation, a confocal laser scanning microscope (CLSM Keyence VK-X200) was utilized. MIPAR image analysis software was employed to accurately measure grain size and distribution in the resulting TMCP microstructure. The image processing algorithm was trained specifically for this study to exclude twinning (e.g., Σ3 boundaries) to ensure accurate grain boundary analysis without miscounting twin boundaries. Vickers hardness measurements were performed for all the TMCP specimens using a ZHU2.5 universal hardness testing machine (Germany), using a 10 kg load and standard dwell time of 15 s. An average of 5 to 7 hardness readings are reported.