PSI - Issue 69

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

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EDM) was additionally utilized to produce edges for comparison without the punching-induced deformation and damage, to represent baseline hardness of the materials. Prior to hole punching and W-EDM, rolling surfaces of the test pieces were sandblasted to remove excess furnace and rolling scale. Punching was conducted with a 12.5% cutting clearance and a punching speed of 10 mm/min using an Erichsen universal sheet metal testing machine at Lapland University of Applied Science in Kemi. Flat tensile test specimens were machined to a gauge length of 50 mm, a parallel length of 75 mm, and a width of 12.5 mm. The original material thickness was retained as the specimen thickness. Tests were performed according to the EN ISO 6892-1 standard, with the specimen axis oriented longitudinally to the rolling direction. Tensile tests were done with 3 specimens for both IA temperatures. Strain hardening exponents (n 4–6 ) were determined from the tensile test data in the plastic strain range between 4% and 6%. To investigate hardness differences between the sheared edge and base material, Vickers microhardness profiles were measured with three measurement lines inwards from the hole edge on polished rolling direction – normal direction (RD-ND) cross-sectional samples. The measurement location was at quarter thickness from the top surface, and 0.981N testing force was applied. Each measurement profile had 22 indentations with 70 µm spacing between the indents. Microhardness measurements were also performed on tensile test specimens to compare possible differences in mechanical stability of RA between the punching and tensile tests. Similar microhardness profile measurements to the hole edge samples were done for tensile specimen RD-ND cross-section samples near the fracture surface. Additional 3 x 3 indentation matrix measurements were performed at 5 mm and 8 mm distance from the fracture surface and at the tensile specimen grip area. 2.3. Microstructural characterization Microstructures of the base materials were characterized using a Zeiss Sigma field emission scanning electron microscope (FESEM). The samples were prepared from the RD-ND cross-section and polished with the standard metallographic sample preparation. Samples were etched with 2% Nital before investigation at material quarter thickness. Detailed microstructural investigations were performed using a JEOL JSM-7900F FESEM equipped with an Oxford electron backscatter scanning diffraction (EBSD) detector. Each sample for EBSD and X-ray diffraction (XRD) analysis were polished to a mirror finish using a silica suspension (0.04 µm) in the final step. Base material EBSD samples (RD-ND cross-section) were investigated approximately at the quarter thickness of the materials with the acceleration voltage of 20 kV and a step size of 0.02 µm. In addition to the base materials, EBSD investigations were performed near quarter thickness of RD-ND cross-section samples of the punched edges of both IAT materials and for an IAT 700 °C tensile test specimen near the fracture surface. The data was post-processed using Oxford AZtecCrystal and Channel 5 software. RA fractions were quantified utilizing XRD analysis using a 9 kW Rigaku SmartLab XRD unit operating with a Co-Kα source. XRD data was processed with PDXL2 software. A de-texturing method was adopted in the XRD analysis, and a Rietveld refinement technique was utilized for the quantification of the phase fractions. Volume fractions of RA (XRD) were measured at the material quarter thickness of rolling direction – transverse direction (RD TD) cross-sections. 3. Results and discussion 3.1. Base material characterization FESEM images of the base material microstructures at quarter thickness are presented in Fig. 2a and Fig. 2d. The IAT 650 °C material mostly consisted of fully tempered lath-shaped martensite with RA and some carbides. A small amount of cementite was detected through XRD measurements. Similarly, the IAT 700 °C material exhibited a mixed the microstructure of fully tempered lath-shaped martensite/ferrite (carbon depleted martensite) and RA (higher fraction than IAT 650 °C) with some carbide precipitation [6]. XRD analysis revealed a significant difference in RA fraction between the two IAT materials, with 7.5% for the 650 °C specimen and 43.3% for the 700 °C counterpart. As