PSI - Issue 68

Sakari Pallaspuro et al. / Procedia Structural Integrity 68 (2025) 802–808 Pallaspuro S. et al./ Structural Integrity Procedia 00 (2025) 000–000

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detail by (Somani et al., 2018); the yield strength of the DQ&P is ~1130 MPa, and ~1360 MPa for DQ. Hardness of the here studied DQ&P is 455 HV 1 , and 494 HV 1 for DQ (Pallaspuro et al., 2022). The welding practice for the 10 mm plates used in this study is the same as in (Pallaspuro et al., 2022). Pro-beam K26-3 welding chamber was used for the electron-beam welding of the grinded, cleaned, and demagnetized plates. Welding was carried out at a speed of 10 mm/s with 120 kV voltage and 37 mA beam current, focusing on the top surface of the specimens at a focal distance of 626 mm. Figure 8 pattern beam oscillation (600 Hz, 1.3 mm amplitude) was applied with an aim to produce a straight fusion line. The welding direction was transverse to the rolling direction, and the resulting cooling time t 8/5 was about 2.2 seconds based on the measurements with thermocouples positioned near the fusion line (FL). Hardness of the weld seam is ~460 HV 1 , very close to the DQ&P base material (Pallaspuro et al., 2022), but fusion line hardness drops to min. 338 HV 1 . Post-weld heat treatment (PWHT) somewhat like the partitioning step was done to some of the plates. This was done by taking the welded plates out of the flooded chamber, temperature reading 90 °C at this point, and inserting the warm plates to a furnace for one hour holding time at 275 °C, and finally cooled in air down to room temperature. This PWHT treatment lowered hardness values of all the cases with about 20 HV 1 (Pallaspuro et al., 2022). The weldments were characterised with optical and electron microscopy (SEM), the ultrasonic contact impedance (UCI) testing to map the HV 1 hardness distribution in the weldments, and the retained austenite content of the weld seam was determined with a Rigaku SmartLab XRD through analysis using Rietveld refinement with PDXL2 software for austenite (fcc) and martensite (bcc/bct) structures. Average carbon contents in retained austenite were estimated according to (Dyson & Holmes, 1970). The mechanical testing consisted of fracture toughness testing according to the standard ASTM E1921 to assess the toughness properties at the onset of cleavage cracking, determining elastic-plastic K Jc and the fracture toughness reference temperature T 0 (a temperature, where median fracture toughness is 100 MPa√m). Single-edge notched bend (SENB) specimens were machined to a 9.0 mm thickness and 18.0 mm height, and the fatigue pre-cracking was done to a 9 mm final length (a/W = 0.5). The tests were done both at room temperature, and with a weather cabinet to do low-temperature testing at -40 °C and -60 °C. In addition to the base materials, the welds were tested with the notches positioned directly in the middle of the weld seam (WS) and at the FL / coarse-grained heat affected zone (HAZ).

Fig. 1. Microstructures of the studied notch positions: a) Weld seam (optical microscopy), b) lath-like microstructure of the weld seam, c) fusion line and the coarse-grained heat-affected zone (HAZ), and d) martensitic matrix of the DQ&P base material (BM). The hardness variations in the weldments are presented in e) – h).