Hardness Profile: Fig. 1 shows the macrostructure of
dissimilar RSW of DP600 and low carbon steel indicating
that there are three distinct microstructural zones:
Weld nugget (WN) or fusion zone (FZ) which is
melted during the welding process and is resolidified
showing a cast structure. Macrostructure of the weld
nugget consists of columnar grains.
Heat affected zone (HAZ) which is not melted but
undergoes microstructural changes.
Base metal (BM)
Fig. 2 shows a typical hardness profile of the spot
welds. Hardness of FZ is higher than that of the BM. This
can be attributed to the formation of martensite in FZ.
Martensite formation in FZ of dual phase steel RSWs is
well documented in literatures [9, 10, 20].
A reduction in the hardness (softening) with
respect to the BM was observed in the HAZ of DP780
and DP980 spot welds. HAZ softening during welding
of DP steels has been reported in the literature [11-13].