Page 23 - Handbook of Materials Failure Analysis

3 Case Studies 15

• In both failure cases, rupture always occurred within the low-alloy ferritic steel
area, probably corresponding to 13Mo3 steel grade; while the presence of
various inclusions, such as MnS stringers and round Al 2 O 3 , CaO particles, did
not seem to have been contributed to the premature failure.

The above evidence suggests potential review and redesign of the continuous oper-
ation of pipeline taking into consideration the heat transfer requirements and calo-
rific value of lignite raw materials for the combustion chamber of the unit. The
13Mo3 low carbon steel grade should be likely revised and substituted by more
heat-resistant steel grades as superheater construction materials. Regarding the bent
tube failures where cracks/separations were observed at the inner bent area, the influ-
ence of geometry change on creep life for pressurized pipe bent areas has been exten-
sively reported [22], mainly using the numerical approach by FEA.

3.2 CORROSION FAILURE OF A STAINLESS STEEL FILTER GRID
OPERATED IN A PETROCHEMICAL PLANT
3.2.1 Summary
The present case study concerns the failure analysis of an austenitic stainless steel grid,
operating in a shell-and-tube heat exchanger of a petrochemical industry [24]. Macro-
scopic examination of the grid indicated extensive brittleness and severe cracking per-
pendicularly to applied stress, while both grid surfaces as well as the interior of the
filtrationholeswerecoveredsignificantlybyscaledeposits.Metallurgicalinvestigation
of selected grid areas and surface deposits examination indicated severe cracking exhi-
biting multiple branching, which suggested strongly the action of SCC mechanism.
Besides the extensive cracking areas, pits surrounded by twinning and slip bands were
observed, while elemental microanalysis carried out in the areas around and in the inte-
riorofthe pitsindicated mainlythe presenceofchromium-rich oxides. Thecatastrophic
failure of the stainless grid occurred as a synergistic effect of acting degradation mech-
anisms, such as SCC, high-temperature oxidation, while the potential formation of det-
rimental σ-phase cannot be discounted.

3.2.2 Background information
The products coming from crude oil refinery reach constant temperature conditions
for their further postprocessing, commonly achieved using appropriate heat exchange
units. The present shell-and-tube heat exchanger, used for the cooling of a naphtha
stream with the simultaneous heating of a crude oil, is illustrated in Figure 1.11.
Naphtha flows in the tubes (inlet temperature: 137 °C, outlet temperature: 90 °C) of
the heat exchanger and crude oil in the shell (inlet temperature: 15 °C, outlet temp-
erature: 74 °C). The operating pressure of the shell was up to 20 bar, while the corre-
sponding pressure of the tubes was up to 7 bar. The present investigation aims to
outline the degradation phenomena contributed to the failure of a perforated austenitic
stainless steel screen (grid) of a T-type pump (Figure 1.12a).