Page 402 - Handbook of Materials Failure Analysis

400 CHAPTER 15 Welding-associated failures in power boilers

FIGURE 15.6
The microstructure at the failed area. The failure starts at the fusion zone (UMZ) between
the weld metal and the HAZ [37].

For both cases, welding heat input was varied within the range from 1 to 5 kJ/mm. The
effectofweldingheatinputonthemicrostructureandmicrosegregationintheUMZ,and
the corrosion properties was investigated for both cases. The increase in heat input to
5 kJ/mm led to slight decrease in the Mo microsegregation in the UMZ and appreciable
amount of NbC precipitation in the weld metal accompanied by slight increase in the
weight loss due to corrosion, for the similar weld joints [40]; whereas, for dissimilar
welding using Incoloy 28 the increase in heat input to 5 kJ/mm led to slight decrease
in the Mo microsegregation in the UMZ accompanied by improvement in the corrosion
resistance of the weld joint [41]. Thus, it was recommended to keep the heat input for
similar welds of SASS at 1 kJ/mm and at 5 kJ/mm for dissimilar welding.
A study has linked failure types, in the presence of mechanical loads, in weld austen-
itic SS joints to the mechanical and microstructural properties resulting from different
welding conditions; Andres et. al [14] showed that some welding conditions promote
the appearance of ferrite, formation of nonmetallic inclusions, and grain size and mor-
phology variations of both austenite and ferrite phases. The behavior of the different
structures under stress conditions varied and different causes for crack nucleation and
propagation occurred to the GMAW and SMAW processes. Three different fracture
modes were reported in this study at the welding joints; the FCAW process was influ-
enced mainly by the first failure mode, while the other two had a mixture of the three
different failure modes. The first failure mode was a geometrical stress concentrator at
the weld root, due to a variation of the cross-section area of the weld that acted as the
nucleation site of the crack, and further propagated through the weld or the HAZ. The
second failure mode was generated at the HAZ, where the crack nucleated due to a var-
iation in the grain size produced by the welding process, and further propagated through
the edge or the center of the weld. The third failure mode appeared due to the presence of
exogenous inclusions or slag generated by the welding process, which acted as stress
raisers in the weld and produced the initiation and further propagation of the crack.