*3.2. Residual Austenite*

In order to evaluate the RA content on the above considered specimens, XRD patterns have been acquired. Results reported in Figure 13 for the reference material and for Variant III show no evidence of variation between the different conditions, since the RA fraction is low enough to stand below the intrinsic sensibility threshold of the technique (equal to about 1 wt.%). Therefore, X-ray diffraction technique showed to be not suitable for RA determination in HAZ for the considered set of samples.

**Figure 13.** XRD spectra for reference material (**a**) and variant III (**b**) as a function of second temperature peak.

RA content has also been evaluated by the analysis of phase maps as obtained by EBSD (Figures 14–17).

**Figure 14.** EBSD phase maps. Red zones: RA phase (reference material).

**Figure 15.** EBSD phase maps. Red zones: RA phase (variant I).

**Figure 16.** EBSD phase maps. Red zones: RA phase (variant II).

**Figure 17.** EBSD phase maps. Red zones: RA phase (variant III).

Quantitative optical microscopy metallography after LePerà selective etching has been applied to the same specimens and the micrographs referring to Variant I and Variant II are shown as an example in Figures 18 and 19.

**Figure 18.** Microstructures of the considered specimens after LePerà etching. Red zones: RA phase (Variant I).

**Figure 19.** Microstructures of the considered specimens after LePerà etching. Red zones: RA phase (Variant II).

RA fractions as a function of second peak temperature for the considered steels are reported in Figure 20, as obtained by the analysis of EBSD phase maps (Figure 20a) and by optical metallography after selective etching (Figure 20b), giving scope for a comparison. RA % values as obtained by light microscopy analysis after selective etching appears to be higher in magnitude than those by crystallographic information given by EBSD. Moreover, the reference material shows more residual austenite than the rest of the variants in the selective etching technique. Such a result, not predicted, is already mentioned in literature (e.g., [18]) and it is related to the fact that the selective etching could partially affect some zones neighboring the austenite areas, together with some limitations in the measurements accuracy. Therefore, although the EBSD technique applies for smaller investigation areas, it is still more accurate for evaluating RA % values compared to selective etching. As a consequence, the values obtained by analyzing the EBSD phase maps were considered more reliable. However, the capability of observing an extended area by optical microscopy enables the estimation of localization and distribution of residual austenite. In fact, as shown in Figures 18 and 19, the constituent RA is arranged along bands, precisely in correspondence with the segregated areas of the original microstructure (Figure 1), where the stabilizing elements of austenite (such as C of Mn) are more concentrated [28]. It is also worth to mention that the RA values determined by means of EBSD technique are well below the X-ray diffraction threshold (equal to 1 wt.%), thus confirming the unsuitability of such a technique in the analysis of RA content in inter-critical zones of welded joints of considered steels. Furthermore, taking each steel variant as a separate reference, RA values shown in Figure 20 prove that the temperature of the second peak does not seem to influence the content of RA regardless of the technique used (either EBSD or LePerà selective etching). In this regard it is useful to consider these two methods as complementary, EBSD to determine the numerical value and LePerà selective etching method to give a localization overview of RA in HAZ of welded joint. Moreover, the experimental results of RA % obtained by EBSD technique (Figure 20a) show that V addition promotes the formation of residual austenite in agreement with [33]. However, there is no evidence that such low RA contents which were determined can be considered responsible for the hardness behavior reported in Figure 10. In this regard, in order to understand the hardness peak at 735 ◦C shown in Figure 10, the analysis of the precipitation state conducted by TEM is reported in the Section 3.3.

**Figure 20.** Quantified RA % with EBSD phase maps (**a**) and selective etching (**b**) as a function of the second peak temperature for the different considered steels.
