PSI - Issue 69

Pekka Kantanen et al. / Procedia Structural Integrity 69 (2025) 53–60

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Fig. 5. Nital-etched RD-ND cross-sections of base material (W-EDM) samples, punched holes, and tensile test specimen fracture surfaces of 650 °C and 700 °C IAT material. Locations of the microhardness profile indentations are visible.

Fig. 6. Microhardness measurement results close the fracture surface, 5 mm and 8 mm from the fracture, and under the grip area of tensile specimen samples of (a) 650 °C and (b) 700 °C IAT material. Standard deviations of the measurements are given.

The EBSD microstructures for the IAT 700 °C tensile specimen near the fracture surface, at 5 mm, and at 8 mm distances from the fracture surface are shown in Fig. 7. Fig. 8 and Fig. 9 display EBSD microstructures near quarter thickness of punched material shear-affected zone (SAZ) of IAT 650 °C and 700 °C samples, respectively. The IAT 700 °C base material exhibited a higher fraction of RA (43.3% by XRD and 31.1% by EBSD) compared to the IAT 650 °C base material (7.5% by XRD and 1.1% by EBSD). Strain-induced martensite transformation during punching, attributed to the high degree of localized deformation, explains the increased hardness near the edges. In tensile test specimens, the deformation area is wider which explains the high hardness values observed over 1.6 mm distance from fracture surface (Fig. 4). The actual maximum hardnesses present in both punched and tensile test sample edges are most likely to be higher than the measured values due to the location of the first indentations being 50 µm from the edge. As it can be seen from the EBSD maps, the deformation is more severe closer to the edges. In addition to the degree of localized deformation, there can be differences in local strain rates between the deformation processes in the punching and tensile tests, however, the TRIP effect have been reported to be independent of strain rate at testing temperatures from RT to 150 °C [10].

Fig. 7. EBSD microstructures of IAT 700 °C tensile specimen (a) close to, (b) at 5 mm and (c) at 8 mm from the fracture surface.