162 / 352
158
Figure 13: Transition region at the fusion line:
WM cast microstructure, left, and coarse-
grained HAZ region, right, with a visible fusion
region between them
Figure 14: Transition region between the
directed fine grains of rolled BM, left, and the
fine-grained bainite of the tempered HAZ, right
The size of the HAZ is limited and the regions of different microstructures too small
for an evaluation of their mechanical properties. In the case of NN70 steel, the HAZ
microstructure consisted of bainite of different grain sizes. Upper bainite of CG (location
1) in the vicinity of the fusion line (with an eventual martensite region, but not clearly
recognized here) is followed by lower bainite (location 2) and FG tempered bainite
similar to the BM (location 3) in the middle, and by lower bainite at the end (location 4,
Fig.15). It should be noticed that the transition between the bainite regions is smooth.
Due to different microstructures in the regions, the mechanical properties, including
impact toughness and crack resistance, can differ significantly, affecting the overall
response of a welded joint specimen.
Regarding structural integrity, the response of the HAZ as a whole is important and
the obtained HAZ microstructure, being of the same type, is favourable, in spite of its
complexity caused by the differences in the size of the grains. Welded joint design has to
accommodate as much as possible to the expected service loading and an X-shaped
welded joint, (Fig. 15), is recommended and accepted here for welding qualification and
WPS. For this reason, the notch root in Charpy V and the crack tip in pre-cracked speci-
mens were located as presented in Fig.15. The obtained results represent an average
response of the loaded HAZ to the involved stress concentration and local constraint.
An evaluation of the mechanical properties is possible by HAZ simulation, which
enables samples of sufficient size (11x11x55 mm) to be obtained. It was successfully
applied to different steels of increased strength for pressurized equipment /21/. The per-
formed analysis with normalized V micro alloyed NIOVAL 47 steel of TSt 460 class
(nominal yield strength 460 MPa) revealed the presence of a very brittle CG region in the
HAZ close to the fusion line, which resulted in a loss of structural integrity of spherical
storage tanks by crack initiation and growth during service /22/. The differences in the
microstructure in the HAZ were not so expressed, and only an average response regarding
local properties, such as crack resistance and impact energy, of the HAZ as a constituent
of welded joints was required.
Applying the approach, proposed by Kfouri /23/, which demonstrated that a fracture
process could be discontinuous, to the complex HAZ microstructure (Fig. 15), it can be
supposed that the final stretch zone, as well as the process zone is not uniform along the
crack tip, being dependent on the local microstructure. It is to expect that these zones are