Page 25 - Handbook of Materials Failure Analysis

3 Case Studies 17

per 3 months) for filter replacements were recorded due to the failure of the stainless
steel grid. It should be mentioned that similar failure incidents of steel strainer used
in the filtration of liquid naphtha streams in the petrochemical industry were pre-
sented in Ref. [31].

3.2.3 Failure analysis
Elemental microanalysis of the stainless steel performed by EDS, concerning mainly
the principal alloying elements (Cr, Ni), showed that the construction material
matches approximately the AISI 304 stainless steel grade (see Table 1.4). Macro-
scopic observation of the failed grid after its removal from the operation site showed
clearly pronounced fragility and facile metal removal due to severe degradation pro-
cesses involved (Figure 1.12b).
The cracks were in a direction normal to the perpendicular axis of the grid which
coincides with the direction of mechanical loading due to the component’s own
weight and the fluid’s pressure. In addition, severe fouling and contamination cov-
ering almost all the flat surface as well as the interior of the holes was observed. Pre-
liminary observations indicated the nonmetallic nature of the contamination scale,
which likely have formed as the result of the interaction between the flowing medium
and the grid material.

3.2.3.1 Stress corrosion cracking
Figure 1.13 shows optical micrographs from representative cross-section areas of the
holes of the failed grid. Extensive microcracking was evident exhibiting multiple
branching, a typical fingerprint of SCC. Cracks emanated from the circumference
of the holes and propagated in the interior forming also secondary cracks. Crack
propagation most likely follows a transgranular direction as it is commonly met
in austenitic stainless steels.
As a first stage, a crack was initiated at pitted areas close to the surface area where
the metal had been exposed to the liquid naphtha environment (at 130 °C approxi-
mately), see also Figure 1.14. Pits constitute significant stress concentration areas
and thus most sensitive to the nucleation of fatigue or stress corrosion cracks. In
the second stage, the crack propagated through the material forming branches, con-
sistently to SCC mechanism. In the final stage, crack propagation led to ultimate cat-
astrophic fracture of the grid.

Table 1.4 Chemical composition of the grid steel obtained by EDS analysis
(wt%) [24]
Material/Standard C Cr Ni Mn Si Fe

Specimen – 18.2 8.2 – – Balance
AISI 304 grade (UNS S 0.08 18.0-20.0 8.0-10.5 2.00 0.75 Balance
30400) a
a
¨
Stahlschlussel (Key to Steel), Verlag Stahlschlussel Wegst, GmbH, D-71672 Marbach, 2004.
¨