## 2 Materials and Methods  
HSLA Strenx(r) 700 MCE steel plates, with a thickness of 3 mm, were used in order to perform butt-welded joints. This steel is composed of a fine-grain ferritic-bainitic microstructure. The chemical composition can be consulted in Table 1 and this material showed a yield strength of 808 MPa, a tensile strength of 838 MPa and 15% of elongation at failure [29]. Five series were welded using the parameters presented in Table 2. The series S1-S4 were welded with a single pass in one side of the plates (top side) while the S5 series was welded by applying two passes: one in each plates' side (top and bottom side). The welding equipment used was a disk laser Trumpf TruDisk 2000 with laser maximum output of 2000 W, beam wavelength of 1020 \(\upmu\)m, beam parameter product of 2 mm-mrad, fiber diameter of 200 \(\upmu\)m and focus diameter of 350 \(\upmu\)m. The equipment was used in continuous mode and did not use filler material. It is important to mention that in the S5 series the welding speed on the top side was different for each sample (1.75, 1.80, 1.90, 1.95 and 2.00 m/min) while the weld pass on the bottom side was constant for all samples (2.50 m/min). The heat inputs used in welding were chosen to achieve different hardnesses in the welded series, but at the same time were maintained at a low heat input to obtain small HAZs, low softening and similar tensile mechanical properties to the BM as shown in [29]. The fatigue crack growth tests were carried out in a Dartec hydraulic machine using the ASTM E647-13a standard [30] as a guideline. The FCGR rate determination for the BM and welded series was achieved with constant load range \(\Delta\)P tests (\(\Delta\)K increasing) and M(T) specimens' geometry was used. The samples' geometry used in the FCGR test can be consulted in Figure 1. As is possible to see in Figure 1, pre-cracks were machined by electrical discharge machining (EDM) and were localized on the underfill (see Figure 2). The specimens were loaded perpendicularly to the weld direction at stress ratio R = 0, at room temperature and with 15 Hz of frequency. The FCGR tests were performed in three conditions: not welded (BM), excess weld removed and both sides removed. The crack length was measured by optical observation through a travelling microscope (45\(\times\)) with digital micrometers and the crack growth rates were calculated according to the 5-point incremental polynomial method, following the ASTM E647-13a standard [30]. Once the specimen had a minimum crack size of 1 mm, at intervals with crack size increments (\(\Delta\)a) of 0.25 mm, the number of cycles was registered. During the test, it was verified that the measurements of the lengths of the two cracks did not differ more than 1.25 mm. The stress intensity factor range (SIFR) was calculated according the expression that corresponds to the M (T) specimen given in [30]. The results were displayed as da/dN vs. \(\Delta\)K curves, only for regime II of the Paris law, for each condition. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**C** & **Mn** & **Si** & **P** & **S** & **Cr** & **V** & **Nb** & **Ni** & **Cu** & **Al** & **Mo** & **Ti** & **Co** & **Fe** \\ \hline
0.07 & 1.69 & 0.01 & 0.012 & 0.006 & 0.03 & 0.02 & 0.046 & 0.04 & 0.011 & 0.044 & 0.016 & 0.117 & 0.016 & balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of base material (wt %). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Series** & **Laser Power (kW)** & **Welding Speed (m/min)** & **Heat Input (J/mm)** \\ \hline S1 & 2.00 & 1.60 & 75.0 \\ S2 & 1.75 & 1.60 & 65.6 \\ S3 & 2.00 & 2.00 & 60.0 \\ S4 & 1.75 & 2.00 & 52.5 \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline
**S5** & 1.75 \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline
**1.25** & 2.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Welding parameters used in the experimental work.