range being analyzed does not affect the steels microhardness
or microstructure. According to data reported in (Ref 13), it is
not possible to observe the scatter of microhardness on sheet
thickness, which indicates homogeneity of the microstructure
and the properties of a laser-welded joint. The average hardness
of the base metal amounts to approximately 280 HV0.1. On the
curves it is possible to observe an increase in hardness from
approximately 280 to 510 HV0.1. This rise is related to the
intercritical heat-affected zone (ICHAZ), where a ferrite
fraction decreases gradually and, correspondingly, an increase
in the fraction of lath bainitic-martensitic constituents takes
place (Fig. 3).
The greatest hardness, i.e., 500-510 HV0.1, is present in the
heat-affected zone. The HAZ has the highest hardness because
it is usually the most affected by the highest residual stresses,
especially if there is a high cooling rate (Ref 24). It is the
combining effect of material inhomogeneity and thermal
inhomogeneity that results in increased hardness. The peak in
the hardness is present for fine-grained HAZ, where the fine
martensitic-bainitic laths contribute significantly to its increase.
In the coarse-grained HAZ there is a small decrease in
hardness. More apparent is a further hardness decrease
occurring in the fusion zone. In this region, microhardness
decreases and stabilizes at a level between 450 and 490 HV0.1,
independent of the heat input value that is applied. This
behavior can be ascribed to the total dissolution of Nb/Ti
complex carbonitrides and a corresponding lack of the effect of
precipitation strengthening. The hardness is nearly constant
throughout the whole width of the weld. This is due to the
homogeneous microstructure of the columnar grains in the
fusion zone.