**2. Materials and Methods**

Laser welding was conducted on 5.5 mm thick sheets of 'Domex 960' advanced high strength steel from SSAB (Stockholm, Sweden). This steel is thermomechanically processed (TMP) and was received in TMP condition with a yield strength of 960 MPa and elongation (A5) of approximately 8%. The chemical composition of the steel is given in Table 1, but for Thermocalc simulations, a simplified composition of Fe–0.08 wt % C–1.78 wt % Mn–0.5 wt % Mo–0.187 wt % Ti–0.004 wt % N is assumed. All calculations were carried out with the thermodynamic database TCFE8 [24] and mobility database of MOBFE3 [25].


**Table 1.** The chemical composition of the steel used in the experiments (wt %).

For the welding, process the keyhole penetration mode, with ytterbium Fiber Laser (YLR Laser–15000, IPG Photonics, Oxford, MA, USA), 5 kW power, travel speed of 1.1 m/min, and Argon shielding gas with a flow rate of 20 L/min, was used. The welded specimens were subjected to post heat treatment immediately after the laser welding. For that, an induction heater was placed above the specimen. The temperature was monitored during the whole procedure of welding and Q&P, by using welded thermocouples located 1.5 mm from the fusion zone (FZ). For the same quenching temperature (QT) of 355 ◦C (approx. equal to 60% initial martensite), three partitioning temperatures (PT) of 440 ◦C (20 ◦C above Ms), 540 ◦C (half between Ms and Bs), and 640 ◦C (Bs) and three partitioning times (Pt) were tested.

Tensile testing was performed on cross weld specimens; the tensile axis was kept perpendicular to the rolling direction of the steel sheet, while the weld zone is located exactly in the middle of the dog-bone-shaped A50 samples. Charpy V notch samples were prepared according to the standard EN ISO 6892–1:2009 and testing was conducted on transverse specimens with a notch in the center of FZ. Results showed that all the samples that were partitioned at 640 ◦C had very low impact toughness.

In order to investigate the reasons, the samples that showed the best and the worst toughness were selected for comparison, see Table 2.

**Table 2.** Mechanical properties of samples Q&P treated immediately after laser welding. (Energy = Absorbed energy during Charpy V test at 20 ◦C.)


The precipitate characterization was determined from C–replicas taken from weld area (FZ and HAZ) with a FE–SEM Helios Nanolab 600 (FEI, Hillsboro, OR, USA) working with an acceleration voltage of 30 kV and a 1.4 nA beam current. The precipitate sizes were determined by image analysis (I–A) of STEM micrographs using the software A4i© (Aquinto AG, Berlin, Germany). The carbide chemistry was identified in transmission using energy-dispersive X-rays (EDX) (Jeol Ltd., Tokyo, Japan) at 30 kV accelerating voltage. Electron Backscatter Diffraction (EBSD) was carried out on HAZ using an accelerating voltage of 20 kV, a beam current of 11 nA, and a step size of 150 nm and 100 nm for Q&P and base metal (BM) samples, respectively. The raw data were filtered by confidence index (CI) of CI > 0.09 after a CI cleanup and grain dilation (grain tolerance angle 5◦ and minimum grain size 2). The grains were defined as at least two adjacent points with similar orientation within a range of 15◦ misorientation.
