The optical micrographs of the weld nugget, HAZ and
transition region from HAZ to base metal in the weldments
are shown in Fig. 8.
As seen in Fig. 8, the weld nugget has a columnar structure.
An increasing in energy input caused coarsening of
the microstructure of weld nugget and also of HAZ. When the images are generally evaluated, the microstructures of
these areas are considerably different from those of original
base metal. Unlike the base metal, it is noted that grain
growth occurs due to the heat transfer. Therefore, the area
of HAZ grain growth region was found wider on IF steel
side of weldment that have higher thermal coefficient compared
with stainless steel. Interstitial free steel shows little
grain refinement through transformation from austenite
to ferrite and they also showed rapid grain growth at high
temperatures. In the austenitic stainless steel, the austenitic
matrix shows relatively low alloying element diffusion
rates, a high thermal expansion coefficient and high electric
resistance. Therefore, when compared with ferritic stainless
steels, HAZ grain growth and solid state transformation
tend to occur slowly, but welding shrinkage strains can
be very high [12].