**2. Materials and Methods**

S355 steel grade (EN10025-2) plates for structural application were manufactured by Vacuum Induction Melting (VIM) plant in the form of three 80 kg ingots (diameter 120 mm) in four variants, including its base reference. The nominal chemical composition of the considered steels are reported in Table 1.


**Table 1.** Nominal chemical composition of the considered steels (wt.%) (Fe to balance).

The ingots have been hot rolled down to 16 mm thickness in 10 passes. The steels chemical compositions to be investigated were designed in order to have a Carbon Equivalent Content (Ceq) value lower than 0.42%, according to International Institute of Welding (IIW) Equation (1), as a function of weight percentage (%) [40]:

$$\text{Ceq} = \% \,\text{C} + \frac{\% \,\text{Mn}}{6} + \frac{\% \,\text{Cr} + \% \,\text{Mo} + \% \,\text{V}}{5} + \frac{\% \,\text{Cu} + \% \,\text{Ni}}{15} \tag{1}$$

The hot rolled microstructures are reported in Figure 1 after 2% Nital etching. Starting from the hot rolled material, cylindrical specimens (10 mm in length, 4 mm in diameter) were machined to be heat treated in controlled conditions by using a dilatometer. The IC GC HAZ thermal cycles, in accordance with Figure 2, were designed to simulate a double pass submerged arc welding process with heat input of 2.5 kJ/mm in 16 mm thick plate [41]. The initial temperature of the first pass was assumed at the room temperature (25 ◦C) while, for the second pass, the value was set to 150 ◦C. Because of the technological limitations of the dilatometer, the samples were heated up to 1100 ◦C with a heating rate of 100 ◦C/s whilst the holding time was set at 3 s. The cooling profile was set in order to guarantee the cooling time between 800 ◦C and 500 ◦C (t8/5) of about 25 s [41,42]. The second peak of weld conditions, with a peak temperature in the inter-critical zone, was selected by considering the values of critical temperature Ac1 and Ac3. In fact, these temperatures, obtained by dilatometric test, were reported in Table 2 and they are dependent on steel variants. Ac1 and Ac3 can be estimated through empirical equations taking into account the alloy elements [43]. However, in this study Ac1 and Ac3 were assumed equal to 715 ◦C and 815 ◦C respectively so that the peak temperature of the second pass was in the inter-critical zone for all steel variants. Therefore, the heat treatments were designed with the aim to reproduce different microstructures corresponding to different positions of the HAZ in a welded joint. In particular, the inter-critical zone of the second welding pass has been simulated with five different peak temperatures: 720, 735, 750, 775 and 790 ◦C (see Figure 2).

**Figure 1.** Hot rolled material (2% Nital etching) ((**a**) Reference material, (**b**) Variant I, (**c**) Variant II, (**d**) Variant III).

In order to investigate the presence of residual austenite (RA) and to define the most suitable methodology to assess the RA presence in the considered steels after the heat treatment, three different methods have been applied: X-ray Diffraction (XRD), Electron Backscattered Diffraction (EBSD), and LePerà selective etching. XRD analysis was carried out by using a Smartlab Rigaku diffractometer equipped with Cu kα source radiation and a D/teX Ultra 250 SL detector, operated at 40 kV and 30 mA in continuous mode in the angular range 30–110 2θ degree. The automated sample alignment routine has been used. EBSD measurements were performed with the aim to detect the presence and position of RA islands, by means of a field emission gun scanning electron microscope (FEG-SEM) (Ultra-Plus Carl-Zeiss-Oberkochen, Jena, Germany) equipped with an EBSD detector (C Nano Oxford Instruments, Stockholm, Sweden), using a 0.1 μm scanning step size. RA was revealed by building up phase maps, taking into account both face-centered cube (fcc)

and body-centered cube (bcc) phases: automatic image analysis of such maps allowed to determine RA volume fraction.

**Figure 2.** Experimental thermal profiles as acquired by thermocouples as obtained by dilatometry.


**Table 2.** Critical temperature Ac1 and Ac3 evaluated by means of dilatometric test.

LePerà solution (1 g H2S2O5 + 100 mL H2O+4gC6H3N3O7 + 100 mL C2H5OH) for about 60–90 s was conducted for selective etching. The microstructure was then analyzed by optical microscopy (OM) (Eclipse LV150 NL, Nikon, Tokyo, Japan) whilst the image analysis was performed using dedicated software (AlexaSoft, X-Plus, serial number: 6308919690486393, Florence, Italy), in order to determine the RA fraction. The procedures were performed on low magnification image and on three different fields: the RA % reported refers to the average of three values. Vickers hardness tests were made by means of a HV50 (Remet, Bologna, Italy) instrument by using a load of 10 kg. Three hardness tests were performed on each sample. Precipitation state was analyzed by transmission electron microscope (TEM) on extraction replica specimens. The observations were performed with a JEOL 200CX transmission electron microscopy (JEOL Ltd., Tokyo, Japan). The analysis was carried out over a significant area, evaluating the chemical composition (by means of EDX analysis) and the average size of the precipitates, within a limit of 50 precipitates for each sample analysed.
