Figure 2a shows the microstructures of base metal for the
low titanium content steel (A steel), which is made up of
the tempered bainite and a small amount of ferrite. The
welding thermal simulation test are carried out by Gleeble-
3500 for base metal; when the welding heat input is
30 kJcm-1, the microstructures of CGHAZ are mainly
made up of the lath bainite and a small amount of granular
bainite, as shown in Fig. 2b. This kind of hybrid microstructure
insures that the CGHAZ of the low titanium
content steel has high hardness and toughness after welding
heat cycle. In addition, the intragranular acicular ferrite is
found in CGHAZ, as shown in Fig. 3a, and it is for another
reason that the toughness of CGHAZ is higher than that of
base metal for low titanium content steel. When the
welding heat input is 50 kJcm-1, the microstructures of
CGHAZ are mainly made up of the lath bainite, but the
granular bainite increases, and the original austenite grain
grows, as shown in Fig. 2c, and the toughness of CGHAZ
begins to decline. When the welding heat input reaches
80 kJcm-1, the microstructure of CGHAZ are mainly
made up of the upper bainite and the original austenite
grain grows up markedly; the lath of bainite and ferrite
coarsens, and the toughness of CGHAZ declines significantly,
as shown in Fig. 2d.