PSI - Issue 68

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S. Ghosh et al. / Procedia Structural Integrity 68 (2025) 1329–1336 S. Ghosh et al. / Structural Integrity Procedia 00 (2025) 000–000

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rolling mill according to a two-stage roll-pass schedule detailed in Table 2. To monitor rolling temperatures accurately, thermocouples were inserted into holes drilled in the edges of the samples to the mid-width at mid-length. Hot rolling in the first stage began in a recrystallization-controlled regime with first four passes intended to reduce the thickness to 26 mm with about 0.2 strain per pass, reaching a fourth-pass temperature of approximately 1030–1050 °C. As the temperature dropped to around 920 °C, the rolling process transitioned into the second stage of no-recrystallization regime comprising of four passes with roughly 0.21 strain per pass, resulting in a final thickness of 11.2 mm and a width of 90 mm. The finish rolling temperature was controlled between 800–820 °C. After the final pass, the DQP steel was quenched to a temperature T Q =150 °C, then transferred to a preheated furnace maintained at 200°C for partitioning. The furnace was then turned off, allowing plates to cool slowly to room temperature over 24 hr.

Table 2. Hot rolling schedules of steel samples prior to DQP treatment. Rolling pass# Thickness (mm) True strain Temperature (°C) - 60 - >1175 I 49 0.2 1140 II 40 0.2 1100 III 32.5 0.2 1060 IV 26 0.2 1030 Cooling to ~920 (°C) - 26 - 920 V 21 0.2 880 VI 17 0.2 860 VII 13.7 0.2 835 VIII 11.2 0.2 815

Transmission electron microscopy (TEM) study was conducted using a JEOL 2200FS EFTEM/STEM operated at 200 kV enabling high magnification resolution to illustrate in-depth structural phenomena that might be occurring during DQP processing. For TEM analyses, thin lamellas were sectioned from the specimens using a FEI Helios DualBeam focused ion beam (FIB) mounted in a field emission scanning electron microscope (FE-SEM). The volume fraction of RA was determined using a Rigaku SmartLab 9 kW XRD unit (40 kV; 135 mA). The measurements were performed using CoK a X-rays with 2θ ranging between 45° and 130° and the rotation performed at 7.2°/min. The obtained diffractograms were analysed by Rietveld refinement using a PDXL2 software. The carbon content of the RA ( C γ ) was also estimated based on the lattice parameter of austenite derived from XRD results. FE-SEM was employed for fractographic studies to examine the characteristic features of FCG surfaces, and corresponding cyclic/monotonic plastic zone size were estimated for both H-Si and M-Si DQP steels. 2.2 Mechanical Testing The HV5 hardness values (average of at least 5 readings) of the DQP specimens were measured using a Duramin A 300 automatic hardness tester. The room temperature tensile tests (3 tests per material) were conducted on specimens machined in the transverse direction at an initial strain rate of 0.008 s⁻¹ in accordance with ASTM E8M standard. The room temperature fatigue crack growth (FCG) tests were carried out on an MTS 100 kN servo-hydraulic machine using compact tension (CT) specimens measuring 7 mm thickness and 10 mm notch length. FCG tests were conducted in the ∆ control mode. In this mode, the stress intensity factor range of the propagating crack is the controlling parameter. Here, a “normalized K gradient” is defined as = & " ! '×& # # " $ ' , which considers the change in stress intensity factor as the crack propagates. ASTM E647-24 mandates that this gradient should not be more negative than -0.08 mm -1 . The tests were conducted in two parts: decreasing ∆ , and increasing ∆ modes for the threshold regime and the Paris law regime, respectively. The propagating crack length was measured by the crack opening displacement (COD) gauge with the span limit of 5 mm. The pre-cracking length was estimated to be 2.5 mm. To calculate the threshold, the crack growth rate below 10 -9 m/cycle was obtained. The test was carried out in the decreasing ΔK method with the normalized K gradient conforming to the ASTM standard, as mentioned above. Once the decreasing