4. Current Status
of Basic Technology Development
Since the 1970s, much research has been devoted to
microstructural control of weld metal. Microstructural
control in a Ti-B system by an acicular ferrite microstructure
was considered optimal and has been widely
applied in order to achieve high toughness. However,
the formation mechanism of that microstructure has still
not been clarified, and it is currently applied by empirical
phenomenology. Although various studies have been
carried out up to now, new knowledge continues to be
obtained by utilizing state-of-the-art observation techniques,
for example, techniques for identifying the
inclusions that acicular ferrite forms by direct observation
of the acicular ferrite formation process by the high
temperature laser scanning confocal microscope, and
detailed analysis of their structures, and detailed analysis
of the crystal orientation relationship of inclusions
and ferrite and ferrite and austenite15).
This kind of basic study and other important studies
related to welded joints are reviewed in the following.
As particular issues, techniques for improving fracture
and fatigue properties of stress concentration at the weld
toe are important, and a variety of studies have examined
this issue. Up to the present, improvement of HAZ
properties by composition design of the steel material
has been used as a technique for improving the fracture
properties of welds. However, NBFWTM (Non-Brittle
Facture Welding) method16) was developed as a technique
for improving performance from the welding process.
In the NBFWTM method, the toughness of the HAZ
around the weld toe is improved by applying ingenuity
to the pass sequence and the heat input in the final layer
of multi-layer welds.
Where the fatigue characteristics of welds are concerned,
due to tensile residual stress and stress concentration
in the weld toe, it was not possible to improve
the fatigue strength of joints even if the strength of the
base metal was increased, and this was a large barrier to
the use of high strength materials. What was studied in
response to this problem was a method for controlling
the smooth bead shape so as to mitigate stress concentration
in the weld toe by a new welding technology, and
peening treatment to smoothen the bead shape after
welding and give compressive residual stress to parts
where tensile residual stress was present.
A hybrid welding technology of plasma welding and