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Failure of Stainless-steel-based Components Used in an Oil Refinery – Analysis and Implications
Figure 4: Intergranular Cracks Near the Edge – Transverse Section Table 2: Hardness of the Sample
Points Hardness (HRC)
1 45.4
2 45.5
3 44.4
4 44.8
5 44.5
Average Rockwell C 44.92
Average Brinell 420
composition, hardness and microstructure) cannot be linked to the
90µm
causes of failure, which can probably be attributed to external factors
such as inadequate assembly of this component in the main structure.
Figure 5: Transition from Brittle (right) to Ductile (left) Fracture
In the second case, an austenitic-stainless-steel-based pump shaft was
Region of the Sample
analysed using the same techniques. Microstructural observation using a
light optical microscope revealed the presence of many non-metallic
inclusion particles (sulphide, oxide and silicate particles). Many of the
grains were twinned and the average austenitic grain size was about
50µm, which can be considered a normal value since in most austenitic
stainless steels used for this kind of application the average grain size
varies between 30 and 60µm. Fractographic SEM analysis revealed
characteristic aspects of fatigue, although they were not as evident as
those seen in the previous case. Inclusion particles were observed and
analysed by an X-ray energy dispersion spectrum (EDS) microprobe,
whose results showed a clear sulphur peak that was absent in the
surrounding matrix. Although the sulphur content (0.026%) was slightly
below the upper limit prescribed by the standard (0.030% according to
Figure 6: Brittle Fracture Near the Edge – Intergranular Cracks (arrows)
Can Be Seen
ASTM A 276-92), sulphide inclusions were observed in the material.
Sulphur is well known as an element that degrades the mechanical
properties of stainless steels. In applications where ductility and
toughness are extremely important, it is necessary to reduce the sulphur
content to below 0.020%, since sulphide inclusions aligned on the
rolling direction are sources of mechanical anisotropy and reduce
resistance to corrosion.
8,9
The high nitrogen content and the low nickel
content did not seem to be harmful to the material used in this
application. It is clear that the high level of sulphide inclusions
contributed to fatigue fracture, which led to the failure of this austenitic-
stainless-steel-based component.
It is well-known in the literature
12–14
that non-metallic inclusions appear
in the early stages of the steel-making process, mainly as a consequence
Previous Cases of the presence of impurities in the raw material that are retained in the
Two other cases of failure of stainless-steel-based components used in liquid steel, or as result of contamination from various sources. It is very
refineries, which have been described in detail elsewhere,
18
are briefly difficult – perhaps impossible – to remove these inclusion particles by
presented below. heat treatment or any other post-fabrication procedure. Therefore,
other than incorporating prevention into the steel-making process, the
In the first case, a fractured centrifugal pump shaft made of duplex only possibility of prevention lies in a careful inspection routine
stainless steel was analysed by SEM fractography, microstructural comprising metallographic observation and determination of inclusion
observation in an optical microscope and hardness tests. Its contents in stainless-steel samples. The average hardness value
microstructure, with islands of austenite delineated in a ferritic matrix, (95.48HRB) is compatible with a 316 AISI-SAE austenitic stainless steel
was in accordance with the NACE/ISO standard.
5
The average size of the (around 95HRB).
2
austenitic grains is about 50µm, which can be considered an acceptable
value for a duplex stainless steel. SEM revealed fatigue striations. HRC Implications of Failures/Accidents in Refineries
tests showed an average value of 22.4HRC, which is lower than the A complete analysis of component failure in refineries must focus not only
upper limit (25HRC) prescribed by the same standard for this component; on technical issues, such as those described above, but also on more
thus it can be considered acceptable. After considering the results general aspects, since these undesirable events usually have serious
reported here, obtained using various techniques, it is evident that consequences in various contexts. These consequences (implications) can
factors related to the intrinsic characteristics of the material (chemical be classified into three main groups:
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HYDROCARBON WORLD VOLUME 4 ISSUE 1
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