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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Keywords: Advanced high-strength steel, Electron-beam welding, Fracture toughness, Martensite, Retained austenite.

1. Introduction Advanced high-strength steels (AHSS) with yield strength ≳ 1100 MPa gain their good mechanical properties from tailored combinations of martensite and/or bainite (bcc/bct) and metastable retained austenite (RA, fcc). These steels are desired materials for mobile/automotive applications due to their very high strength and improved deformability over traditional hardened fully martensitic steels. One such type of novel AHSSs is low-alloy (direct-)quenched and partitioned (DQ&P) steels that consist of a fine lath-martensitic matrix enriched with film-like inter-lath retained austenite (Ghosh et al., 2022; Somani et al., 2018). These DQ&P steels can have very good low-temperature impact toughness (Kantanen et al., 2019; Pallaspuro et al., 2022) by having a relatively high fraction of only small-scale film like RA. With these metastable base material (BM) microstructures, the challenge comes with retaining the originally good properties also after welding, and how to achieve low-mismatch welds as AHSSs are prone to softening in the heat affected zone (HAZ) (M. K. John et al., 2022). Minimising this softening is possible via low heat-input / high energy density beam welding processes, e.g., laser beam welding and electron beam welding (EB). Especially EB can produce deep yet narrow weld seams. Quick manipulation of the electron beam position and pattern can facilitate shaping of the fusion line to achieve tailored results. In addition, joining with EB facilitates chemically heterogeneous joints, and welding in vacuum eliminates atmospheric influences. Alloying AHSSs with a moderate amount of nickel (Ni) has proven to be beneficial for retaining austenite in the EB weld seam and achieving good low-temperature toughness properties (Khodir et al., 2014; Qiu et al., 2013). Khodir et al. (2014) achieved the best impact toughness at -40 °C with 1.5 vol.% of film-like RA in a 3 wt.% Ni containing steel. Qiu et al. (2013) were able to maintain upper-shelf fracture toughness of EB-welded Cr-Ni-Mo steels even down to 200 K, explaining the results with the positive effect of strain-induced martensite formation. Pallaspuro et al. (2022) investigated impact toughness properties of an electron-beam welded DQ&P steel. They reported good low temperature impact toughness with T 28J of -66 °C, 91 J/cm 2 at room temperature, and the specimens notched at the fusion line for sampling the coarse-grain heat-affected zone had even lower T 28J . Compared to the base material conditions, EB weld seam had approximately +40 °C higher T 28J . Whether post-weld heat treatment (PWHT) is beneficial for these steels remains still an open question. Stress relief tempering can improve the mechanical properties after quenching and partitioning (Zurnadzhy et al., 2019), and post weld quenching & partitioning treatment improved performance of S960 steel (Forouzan et al., 2017). However, the change in properties is fully dependent on the initial and PWHT treated microstructures. Pallaspuro et al. (2022) reported increase in T 28J with PWHT executed at the same temperature as the partitioning treatment in the given DQ&P steel. In this study, we investigate low-temperature fracture toughness properties of 0.2C martensitic-austenitic steel, its electron-beam welded seam and heat-affected zone, testing also the post-weld heat treated condition. The results show that very good toughness levels can be achieved with low heat-input and alloying that facilitates retaining austenite also in the fast cooled weld seam. 2. Materials and Methods The study material is a laboratory-made 0.22 C – 1.50 Mn – 0.53 Si – 0.83 Al – 1.10 Cr – 0.75 Ni (wt.%) low-alloy steel that has considerable carbon-equivalent levels (CE IIW = 0.74, CET = 0.44) that can indicate poor weldability and need for pre-heating and/or post-weld beat treatment (PWHT). Blocks cut from the vacuum-cast were subjected to hot-rolling and direct-quenching and partitioning treatment (DQ&P), where the partitioning is executed directly after the interrupted cooling in water to a quench-stop temperature of slightly above 275 °C followed by transferring the plates to a large-mass furnace pre-heated to 275 °C for very slow cooling. This yields an effectively low-temperature tempered lath-martensitic microstructure with about 7 vol.% of very finely divided inter-lath retained austenite (RA). Essentially martensitic direct-quenched (DQ) state that was cooled directly to room temperature was produced for comparison. The processing of the materials, microstructures, and mechanical properties are previously presented in