*3.3. Martensite Transformation and Retained Austenite*

Figure 7 shows the crystallography of CGHAZ and FGHAZ by EBSD. It is clear that CGHAZ had the larger grain (Figure 7a) compared with FGHAZ (Figure 7c). The EBSD superimposed figure in Figure 7b–d of band contrast and grain boundaries indicated that the number of boundaries with the misorientation >15◦ in FGHAZ was greater than in CGHAZ, which means that the effective grain size in FGHAZ was smaller than that in CGHAZ. The comparison is presented in Figure 7e. This agreed well with the in situ observation of martensite transformation.

**Figure 7.** Crystallographic analysis in sample Z direction of simulated (**a**) CGHAZ, (**c**) FGHAZ, EBSD superimposed figure of band contrast and grain boundaries (misorientation < 15◦ in red line, misorientation > 15◦ in green line) of simulated (**b**) CGHAZ, (**d**) FGHAZ with t8/5 of 25 s. (**e**) Curves of misorientation angle-relative frequency of simulated HAZs.

Apart from martensite transformation, some austenite was retained in FGHAZ because of the fast continuous cooling transformation (shown in Figure 4), as well as the shorter transformation temperature range. The volume fraction of retained austenite in FGHAZ was measured to be 9~11% by XRD, while only 0.5~1.2% retained austenite in CGHAZ (Figure 8a,b).

**Figure 8.** Comparison of volume fraction of retained austenite by XRD in simulated (**a**) CGHAZ and (**b**) FGHAZ.

The retained austenite morphology in FGHAZ by TEM is presented in Figure 9. The retained austenite was found along the martensite lath, as shown in Figure 9a. In the bright field micrograph (Figure 9b), the retained austenite was found to be film-like, and thickness was around ~150 nm. The retained austenite is demonstrated by the field micrograph in Figure 9c and the diffraction pattern in Figure 9d. By using EDS line-scan, the concentration of Mn in retained austenite was measured and there was ~1.8 enrichment factor of Mn found in retained austenite, which is shown in Figure 10.

**Figure 9.** TEM showing (**a**) RA morphology, (**b**) Bright field micrograph and (**c**) Dark field micrograph (**d**) diffraction pattern showing austenite in simulated FGHAZ at t8/5 of 10 s.

**Figure 10.** TEM micrograph of retained austenite and EDS line-scan along the red line in retained austenite of simulated FGHAZ at t8/5 of 10 s.

The enrichment of Mn of 9% in retained austenite lowers the martensite transformation temperature. It is possible for retained austenite to transform into martensite in an impact toughness test temperature of −60 ◦C due to the lower stability of retained austenite.

### *3.4. Hardness*

The SHCCT diagram in Figure 4 shows that the hardness of FGHAZ was around 363~393 HV (0.2) and around 354~384 HV (0.2) of CGHAZ. Both at the peak temperature of 1320 ◦C and 850 ◦C, the continuous cooling transformed product was martensite. Figure 4 also reveals that at the same t8/5 range of 5~60 s, each average hardness value of FGHAZ was larger than that of CGHAZ by around 10 HV. The harder martensite in FGHAZ was attributed to the higher transformation rate, as well as fine grain.
