G. Ferrite Formation
The aluminum-based TRIP steels, as in the case of the
current high-Al TRIP steel under investigation, generally
contain about 1 to 1.3 wt pct of aluminum in order
to suppress the formation of iron carbides and stabilize
the austenite at room temperature. During welding and
subsequent solidification of the molten weld pool, the
equilibrium aluminum content in ferrite can rise as high
as 1.35 wt pct at 1400 C for the steel under consideration
(Figure 13). According to the corresponding
binary Fe-Al phase diagram, if the aluminum content
exceeds 1.15 wt pct, the ferrite is stabilized (Figure 14).
When the liquid weld pool starts to solidify to d ferrite at
the fusion lines and at the columnar grain boundaries,
the aluminum starts to partition to the newly solidified d
ferrite from the liquid weld pool. Once the aluminum in
the d ferrite reaches the critical stabilization limit
(1.15 wt pct), the d ferrite stabilizes at the fusion lines
and at the grain boundaries and does not undergo any
further transformation into intercritical austenite and
subsequent martensite. Thus, soft ferritic zones are
formed close to the fusion lines and columnar grain
boundaries. The hardness variation across the FZs and
HAZs of high-Al welds is shown in Figure 15. It can be
seen that the presence of soft ferrite causes a dip in the
hardness profile close to the fusion line as well as in a
few areas in the FZ.