*3.5. Hardness*

The hardness distribution of various welds is depicted in Figure 8. Lower hardness is observed in the weld metal region. The hardness increases when moving from the weld metal to the CGHAZ and FGHAZ. After that, the hardness decreases again to the lowest point when it comes to the over-tempered region. Finally, the hardness would increase continuously until it reaches the hardness value of the base metal (495 Hv). The hardness variation trend is very similar to that found in the armour steel weld produced with conventional fusion welding, as shown by Reddy et al. [23]. In addition, the overall hardness decreases with increasing heat input. Once the heat input reaches 10.22 kJ/cm and beyond, the overall hardness decrease becomes evident.

**Figure 7.** Microstructure in weld metal and heat-affected zone (HAZ); (**a**) Test 1 (HI = 7.78 kJ/cm); (**b**) Test 7 (HI = 11.16 kJ/cm) Note: M and B represent martensite and bainite respectively.

**Figure 8.** Hardness distribution across weld with varying heat input.

The variation trend of hardness across the weld is in good agreement with the microstructure change. As martensite and smaller grain size favours higher hardness, the increase in hardness from weld metal to FGHAZ is unexpected because of the increased fraction of martensite and decreased grain size from weld metal to FGHAZ, as shown in Figure 7. In the over-tempered region, the martensite was heavily tempered, which led to the reduction in dislocation density, precipitation of carbides and reduction in solid solution strengthening in the matrix. Thus, the hardness in the over-tempered region decreased dramatically compared with the HHA base metal (495Hv).

When the heat input was increased, the fraction of martensite decreased in both the weld metal and HAZ, as shown in Figure 7, because of the slower cooling rate induced from higher heat input. Furthermore, the softening behaviour in the over-tempered region became severer because of prolonged holding time in this region with increasing heat input, leading to more significant reduction in dislocation density, precipitation of carbides and reduction in solid solution strengthening. That is why the overall hardness of the welded joints decreased with increasing heat input.
