Page 396 - Handbook of Materials Failure Analysis

394 CHAPTER 15 Welding-associated failures in power boilers

On the other hand, due to the heat input during welding, not only different micro-
structures are obtained within the weld but also microstructural changes take place
in a small area besides the fusion line. It has been shown that there is a great influence
of heat input and heat treatment on the mechanical properties of P91 weld joints,
where the best combination of properties was obtained in low to medium range of
heat input, between 1 and 2.8 kJ mm 1 for both treatments, and a 90% increase in
time to rupture was observed for normalizing/tempering treatment compared to sub-
critical post-weld treatment [23,24].

3 STRESS-CORROSION INDUCED FAILURES

3.1 PITTING CORROSION OF REBOILER 304L TUBES RESULTING
FROM WRONG MATERIAL SELECTION
Some case studies, with special relevance to failure by stress-corrosion induced fail-
ures will be discussed hereafter. Since recently a detailed study on failures associated
with ferritic steels has been presented [25], this work will focus on failure cases on
austenitic and DSS grades (304, 316 and duplex steel) that failed due to combined
environmental and stressing conditions by corrosion- and/or mechanical-induced
mechanisms.
A number of the reported failures on parts made from SSs have been attributed to
either pure corrosion or combined stress-corrosion causes [11,26–28]. However, fail-
ures may be attributed to pure mechanical causes only; the failure of two centrifugal
pump shafts (one made from 316 austenitic and the other from DSS) during use in a
Brazilian petroleum refinery were attributed to fatigue [13], where in both cases, the
failure started with crack nucleation and was promoted by the presence of nonme-
tallic inclusion particles, which appeared in the austenitic SS as a result of the early
stages of steelmaking process (in the 316 SS shaft), and could not be attributed to the
material properties in the case of DSS (where the failure started at an edge, where a
nut locked the rotor in the shaft).
Pitting corrosion and/or SCC were found to be the cause for a number of the
reported failures in SS parts in both welded or none-welded parts, in various struc-
tures made of 304 SSs working in aggressive environmental conditions [29–32].
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads
to the creation of small holes in the metal. The driving power for pitting corrosion is
the lack of oxygen around a small area. This area becomes anodic while the area with
excess of oxygen becomes cathodic, leading to very localized galvanic corrosion.
The corrosion penetrates the mass of the metal, with limited diffusion of ions, further
pronouncing the localized lack of oxygen. The mechanism of pitting corrosion is
probably the same as crevice corrosion [33]. This kind of corrosion is considered
more dangerous than uniform corrosion as it causes little loss of material with small
effect on its surface, while it damages the deep structures of the metal. The pits on the
surface are often covered by corrosion products and the pits penetrate through the