Page 28 - Handbook of Materials Failure Analysis

20 CHAPTER 1 Progressive failures of components

slip band areas cannot be ruled out, as strain-induced martensite formation is a com-
mon phenomenon in austenitic stainless steels. This can effectively aggravate crack-
ing conditions and facilitate crack propagation, due to the inherent brittleness of the
martensite phase.
Thermal degradation phenomena also involve the formation of σ-phase. This is
normally related, in austenitic stainless steels, with the tendency of Fe to react with
Cr forming an intermetallic compound, during exposure at temperatures higher than
600 °C, resulting in severe loss of ductility and fracture toughness of the steel [26].
The coexistence of σ-phase and Cr-carbides, which result in embrittlement effects on
the base metal, has been predicted thermodynamically, during aging of an AISI 310
stainless steel from 600 to 900 °C temperature range; the relevant stoichiometry of
σ-phase and Cr-carbide were found dependent on the aging temperature and holding
time [27]. A transformation mechanism γ!M 23 C 6 !σ was also referred to outline
the intergranular precipitation of σ-phase in austenitic steel [32].
Presumably, the maximum normal working temperature (around 140 °C) does not
suggest favorable conditions for the formation of neither σ-phase nor Cr-carbides.

3.2.3.2 Selective/preferential oxidation
Damage grid surface revealed localized scale deposits on the pitted areas with var-
ious morphologies associated to nonmetallic reaction products (Figure 1.16). EDS
microanalysis identified areas rich in chromium oxides, which also exhibited a brittle
behavior (Figures 1.16a and 1.17a), and areas containing complex reaction products
originated from the flowing medium (Figures 1.16b and 1.17b). The presence of Cl
constitutes an additional supportive evidence of the occurrence of SCC mechanism
as a failure contributor.
Selective oxidation leading to the formation of chromium oxides was observed
mainly around the middle area of each separate grain, attributed to the nucleation of
chromium oxide favored by Cr migration from the interior of the grain toward the

Acc.V Spot Magn Det WD 100 µm Acc.V Spot Magn Det WD 20 µm
20.0 kV 3.0 260x BSE 4.9 Elkeme 20.0 kV 3.0 1041x BSE 4.9 Elkeme
(a) (b)
FIGURE 1.16
(a) SEM micrograph showing the morphology of typical corrosion and scale product deposits
in the interior of the pits; (b) detail of (a).