J Fail. Anal. and Preven. (2008) 8:320-326
DOl 10.1007/s11668-008-9143-8
CASE
H IS T O R Y -P E E R -R E V IE W E D
F a ilu r e
A n a ly s is
o f T w o S ta in le s s
S te e l B a se d
C o m p o n e n ts
U sed
in a n O il R e fin e r y
C a s s io B a r b o s a '
Joneo
J o s e L u iz F e r n a n d e s '
L o p e s d o N a s c im e n to '
I b r a h im
d e C e r q u e ir a
Abud
Submitted: 26 February 2007/ in revised form: 28 April 2008/ Published
© ASM International 2008
online: 29 May 2008
The petroleum industry has changed significantly over the past decades. For example, in Brazil, oil
extraction under very deep sea water is growing very
quickly. As a consequence, materials and components used
for such applications must have properties required to
withstand adverse conditions and ensure satisfactory
performance and reliability in service. Nonetheless, components that normally fulfill these standard requirements
can fail under severe service conditions such as high
pressure and temperatures and high concentrations of H 2 S
and CO 2 , Among the factors that can cause the premature
failure in metallic components are the use of inadequate
materials, the presence of defects that appeared during the
production, and errors of project, assembly, or maintenance. Failure analysis allows the identification of causes
and thus contributes to improvements in the operation and
performance of similar equipment. I n this work, light
optical microscopy and scanning electron microscopy
(SEM) were used to analyze the microstructure and fracture surface of two centrifugal pump shafts that failed
during use in a Brazilian petroleum refinery. T h e results
showed that one shaft, made of duplex stainless steel, failed
by fatigue fracture, and the other, made of 316 austenitic
stainless steel, experienced a similar fracture, which was
promoted by the presence of nonmetallic inclusion
particles.
K eyw ords
A b str a c t
C. Barbosa
([8]) . J. L. do Nascimento·
1. de Cerqueira
Abud
Instituto Nacional de Tecnologia (INT), Avenida Venezuela,
room 626, 20081-312 Rio de Janeiro, Brazil
e-mail: cassiob@int.gov.br
J. L. do Nascimento·
J. L. Fernandes
Centro Federal de Educa<;ao Tecnologica
(CEFET-RJ), Rio de Janeiro, Brazil
do Rio de Janeiro
82,
Failure analysis·
Stainless steels· Fracture·
Fatigue
The petrochemical industry is one of the most dynamic
segments of the world economy, and its growth in the last
years has led to the development of new materials to meet
new requirements that are getting more demanding. Stainless
steels have been developed for many applications that
require a high resistance to corrosive environments. Duplex
stainless steels (DSSs), which were first developed in 1927
and improved in the following decades, have been mentioned
as a possible option to replace more traditional stainless
steels. A NACEIISO standard describes the requirements for
such applications [I]. DSSs have a two-phase microstructure
(austenite and ferrite) and have some advantages, mainly
higher strength, higher resistance to intergranular corrosion,
and lower cost when compared with austenitic stainless
steels. On the other hand, DSSs have some disadvantages,
such as difficult thermomechanical processing and low
resistance to pitting corrosion [2]. I n order to meet the typical
requirements for toughness, strength, and corrosion resistance, phases such as (J phase, for instance, must be
minimized [3-5]. Another deleterious phase, formed during
thermomechanical processing of DSSs, is Cr2N. The Cr2N
phase is also the main precipitate found in the heat-affected
zone (HAZ) of welded joints. The conditions that favor
the formation of austenite, such as high nitrogen content and
low cooling rate, minimize the formation of Cr2N precipitates [6] and increase the volume fraction of austenite. The (J
phase is formed as a consequence of the decomposition
of ferrite and also increases the volume fraction of austenite. This (J phase reaction is favored by aging at higher
temperatures (650-900 0c) and longer periods (30 min to
8 h), and the kinetics of the reaction to form (J phase are
slower than for Cr2N precipitation [3,5].
Austenitic stainless steels have many outstanding properties, apart from a high resistance to corrosion in many
environments: they retain ductility and toughness under a
range of exposure conditions, are less sensitive to embrittlement than ferritic stainless steels, and have better
forming characteristics. Nevertheless, these good properties and characteristics are dependent on chemistry, in the
sense that minor elements such as sulfur and phosphorus
must be kept at levels below limits specified in standards
[7-9].
Fatigue failure is consequence of cyclic loading. When a
crack nucleates and propagates in the material, rupture will
occur at a stress level much lower than that necessary to
cause fracture under static loading. The main factors
that influence fatigue failure are the range of variation
of applied stresses, the number of cycles, stress risers,
corrosion, temperature, residual stresses, and combined
stresses. According to Fernandes and Castro [10], fatigue is
a local, progressive, and accumulative mechanical failure,
as a result of the nucleation and progressive propagation of
a crack caused by cyclic loading. According to Suresh [II]
the phenomenology of fatigue cracks can be analyzed as a
problem of local plastic deformation and can be explained
by the appearance of shear bands. When there is dislocation movement, certain grains form persistent slip bands,
which appear mainly on the surface of the piece.
Brittle fracture can be associated with several different
causes, including the presence of harmful nonmetallic
inclusions. The effect of such inclusions depends on the
amount, shape, size, and distribution. Inclusions can act as
stress raisers and thus serve as preferential sites for the
nucleation of cracks [12].
Failure analysis uses several kinds of techniques to
investigate the causes of failure in equipment or structures.
Generally, the causes are related to the use of inadequate
materials, the presence of defects, errors in design,
improper assembly, or inadequate service during use.
Knowledge about the causes and the correction of the
anomalies allow improving the performance of similar
equipment and prevent the reappearance of the same kind
of failure [13]. Frequently, failure analysis attempts to
correlate the topography of a fracture surface to the possible causes of fracture by using a scanning electron
microscope (SEM) [14].
In this work a fracture analysis of two centrifugal pump
shafts used in the petrochemical industry is conducted
using techniques such as SEM fractography, microstructural observation of duplex stainless steel in an optical
microscope, and hardness tests. These techniques complement each other and thus, in an efficient way, allow the
identification of the cause of the component failure.
A duplex stainless steel shaft and an austemtlc stainless
steel shaft that failed in service were investigated. There is
not much information about the history of use of both
components, but it is known that, in the first case, there was
a problem in the frame of the pump that made necessary a
repair work. After this work, the pump started to work, but
after less than 3 months the shaft was broken. In the second
case, the shaft was installed in the pump and after around
2 months it was broken on the edge where a nut locked
the rotor in the shaft. The chemical composition of duplex
stainless steel, obtained through x-ray fluorescence analysis, is presented in Table 1, and the chemical composition
of the austenitic stainless steel is shown in Table 2.
Table 3 shows the limits of chemical composition for
316L austenitic stainless steel prescribed by standard
ASTM A 276-92 [7] (maximum or range). The nitrogen
content is above and the nickel content is below the limits
determined by ASTM A 276-92 for 316L grade austenitic
stainless steel.
The first centrifugal pump shaft (duplex stainless steel)
was sectioned to obtain samples for microstructural analysis, SEM fractographic analysis, and hardness tests.
Samples for microstructural analysis were subjected to
standard metallographic
preparation:
grinding (100600 meshes), diamond paste polishing (6-1 llm), and
etching with a 30 mL nitric acid, 10 mL chloridic acid, and
60 mL distilled water etchant. Samples for fractographic
analysis were observed and photographed in equipment
operating at 20 kV. Rockwell C hardness test comprised
five measurements at different points of the sample, and the
average value was calculated and considered to be representative of the sample hardness.
The second centrifugal pump shaft (austenitic stainless
steel) was subjected to the same preparation for optical
microscopy and SEM observation (though the etchant in
this case was a 20 g picric acid and 100 mL chloridic acid
Table
steel
3 Limits of chemical
composition
for 316 austenitic
stainless
reagent), while the hardness tests were performed
Rockwell B scale, because of the lower hardness.
on
Figure I(a) shows the as-received fractured centrifugal
pump shaft, while Fig. I (b), obtained in a stereomicroscope, presents the macroscopic overview of the fracture
surface. Figure 2 shows the microstructure of the duplex
stainless steel, with austenite delineated in a ferritic matrix,
which is in accordance to the NACEIISO standard [1]. No (J
phase was found through optical microscopy. In Fig. 2 (a)
(transverse section image), the average size of the austenitic
grains (austenitic islands distributed in a continuous
ferritic matrix) is about 50 Jlm, which can be considered an
acceptable value for a duplex stainless steel. The slightly
elongated shape of the austenitic islands is also normal for
a duplex stainless steel. Figure 3, obtained in a scanning
electron microscope (SEM), presents microscopic aspects
of the fracture surface, in which striations are clearly
visible. These striations indicate failure by fatigue [IS].
Fig. 1 (a) Fractured centrifugal
pump axis. (b) Fracture surface:
macroscopic overview. Fracture
origin indicated
Figure 4 shows a detailed aspect of the same region of this
fracture surface. Table 4 shows results of Rockwell C
hardness test.
The average hardness value (HRC 22.4) is lower then
the upper limit (HRC 25) prescribed by the NACEIISO
standard for the application of this component, and thus it
can be considered acceptable [1].
According to Reick [2], duplex stainless steels, with
chemical composition very similar to the one analyzed in
this study, have tensile strength around 640 to 750 MFa,
yield strength between 400 and 450 MFa, and total elongation around 25%, and these values are compatible with
the application defined for this material in the present case.
If one considers the results found in this study, with
different techniques, it is evident that factors related to the
intrinsic characteristics of the material (chemical composition and microstructure) cannot be linked to the causes
of failure, which probably can be attributed to external
factors.
Figure 5 shows an unetched aspect of the longitudinal
section of the austenitic stainless steel pump axis. Many
nonmetallic inclusion particles (sulfide, oxide, and silicate
particles) can be seen. The microstructure of the same
material can be observed in Fig. 6 (transverse section) and
Fig. 7 (longitudinal section): austenitic twinned grains and
nonmetallic inclusion particles are clearly visible in both
sections. In Fig. 6 (transverse section image) the average
austenitic grain size is about 50 Jlm, 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 60 Jlm.
The general macroscopic overview of the fracture
surface on this pump axis is shown in Fig. 8. The arrow
Fig. 2 Duplex stainless steel
microstructure.
(a) Transverse
section. (b) Longitudinal section
I
22
22
2
3
23
4
23
22
5
A verage (Rockwell
indicates the site where crack begins, and from this region
radial marks diverge.
Scanning electron micrograph images of the fracture
surface are presented in Fig. 9 and 10, which reveal characteristic aspects of fatigue, even if not as clear as in the
C)
22.4
first case. I n the same figures, arrows depict some small
holes that contain inclusion particles; these holes are shown
in detail in Fig. 10. These particles were analyzed by an
EDS (x-ray energy dispersion spectrum) microprobe
whose result is presented in Fig. II. I n this spectrum, a
clear sulfur (S) peak can be seen, compared with the
spectrum obtained from the matrix (Fig. 12), where such
kind of peak does not exist. This analysis shows that, in
spite of the sulfur content (0.026%) being slightly below
the upper limit prescribed by the standard (0.030%
according to ASTM A 276-92), sulfide inclusions were
observed in the material. Sulfur is well known for
Fig. 6 Transverse section:
particles (black dots)
twinned
austenitic
Fig. 7 Longitudinal section: austenitic
(black and elongated)
Fig. 8 General macroscopic overview
pump axis. Fracture origin indicated
grains
and inclusion
grains and inclusion
of the fracture
Fig. 9 Fracture
particles
surface
(cleavage).
Arrows:
holes
with
inclusion
particles
surface on the
degrading the mechanical properties of stainless steels. In
some applications, when ductility and toughness are
extremely important, it is necessary to lower the sulfur
content below 0.020%, since sulfide 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 do not seem to be
harmful to the material used in this application.
It is clear that the high sulfide inclusions content contributes to fatigue fracture which led to the failure of the 316
austenitic stainless steel pump axis analyzed in this work. It
is well known in the literature [16-18] that nonmetallic
inclusions appear in the early stages of steelmaking process,
mainly as a consequence of the presence of impurity in the
raw material, which is retained in the liquid steel, or as a
result of contamination from several sources. It is very
difficult, perhaps impossible, to remove these inclusion
particles by heat treatment or any other postfabrication
J Fail. Anal. and Preven. (2008) 8:320-326
325
Full.cale
Fig. 11 EDS spectrum:
particles
coun15: JOOI
F (inclusao)
10.221 keY
13 CounlS
Cursor:
Fe
3000
2500
2000
Cr
1500
1000
OFe
Cr
Fe
500
S
Zn
0
0
3
4
5
keV
klm ·1·H
Fig. 12 EDS spectrum:
matrix
F u ll s c a le
10
8
F (matrll)
c o u n ts: 3 0 0 0
Cursor:
10.221 keY
20 CounlS
Fe
3000
o
6
o
klm.l.H
procedure, then, besides a costly steelmaking procedure,
the only possibility of prevention lies on a careful inspection
routine, comprising metallographic observation and determination of inclusion contents in stainless steel samples.
The average hardness value (HRB 95.48: Table 5) is
compatible with a 316 AISI-SAE austenitic stainless steel
(around HRB 95) [16].
According to the literature [16], 316 austenitic stainless
steel has ultimate tensile strength (UTS) around 515 MPa,
yield strength (YS) around 205 MPa, and total elongation
l
94.6
2
96.4
3
95.2
4
95.8
95.4
5
Average
(HRB)
95.48
around 40%, which can be considered adequate properties
for this kind of application.
Stainless steels are widely used in the fabrication of
components operating in the petrochemical industry for
some reasons, mainly due to their excellent resistance to
corrosion coupled with good mechanical properties.
Nevertheless, some factors can lead to the failure of
components, even if fabricated with an adequate material.
In the present study, two cases of failure of components
made of stainless steel were analyzed.
In the first case, the failure of a centrifugal pump shaft
fabricated in duplex stainless steel based on observation
using optical microscopy, scanning electron microscopy
and Rockwell C hardness tests, led to the following results:
2.
3.
4.
5.
alloys) and other alloys," NACEIANSIIISO
(corrosion-resistant
(2003)
Reick, W., Pohl, M., Padilha, A.F.: Desenvolvimento
em a<;:os
inoxidaveis
feerftico-austenfticos
com microestrutura
duplex
in the stainless
ferritic-austenitic
steels with
(Development
duplex microstructure).
Met. Mater. 48(409), 551-563
(Sept
1992) (in Portuguese)
Lee, KM., Cho, H.S., Choi, D.C.: Effect of isothermal treatment
of SAF 2205 duplex stainless steel on migration of fJly interface
boundary and growth of austenite. J. Alloys Compd., 285, 156161 (1999)
Chen, T.H., Yang, J.R.: Effects of solution treatment and continuous cooling on a-phase precipitation
in a 2205 duplex
stainless steel, Mater. Sci. Eng. A, 311, 2841 (2001)
Chen T.H., Weng KL., Yang J.R.: The effect of high-temperature
exposure on the microstructural
stability, toughness property in a
2205 duplex stainless steel. Mater. Sci. Eng. A, 338, 259-270
(2002)
6. Liou, H.-Y., Hsieh, R.-!., Tsai, W.-T.: Microstructure
and stress
corrosion cracking in simulated heat-affected
zones of duplex
stainless steels. Corros. Sci., 4 4 , 2841-2856 (2002)
for Stainless and Heat-Resisting
Steel
7. "Standard Specification
Bars and Shapes," ASTM A 276-92, Annual Book of ASTM
Microstructure, composed by austenite in a ferritic
matrix, which is in accordance to the standard, and no (J
phase was observed in optical microscope.
The hardness of the duplex stainless steel sample
(HRC 22.4) also meets the specified values (lower than
HRC 25).
There is no evidence of corrosion neither any other kind
of degradation in the material.
SEM images reveal evident presence of fatigue striations on the fracture surface.
Standards
8. Colombier,
L., Hochmann,
J.: Aciers
Inoxydables
Aciers
Refractaires. Dunod, Paris, 620 pages (1965)
9. Peckner, D., Bernstein,
!.M.: Handbook
of Stainless Steels.
McGraw-Hili Book Company, New York, NY (1977)
10. Fernandes, J.L., Castro, J.T.P.: Fatigue Crack Propagation
in
API-5L-X60,
Technology
and Equipments
Conference-VI
COTEQ, Aug, 10 pages (2002)
11. Suresh, S.: Fatigue of Materials. Cambridge
University,
605
The previous results suggest that the fatigue failure was
probably caused by external factors not related to the
intrinsic characteristics of the material.
In the second case, the failure of a centrifugal pump axis
made of austenitic stainless steel, the presence of sulfide
inclusions contributed to fatigue fracture on this component, in spite of adequate microstructure and hardness. The
most viable way of prevention is minimizing the inclusion
contents in stainless steel samples.
Casos Selecionados
(1933-2003)
(Failure Analysis and Metallography,
Selected
Cases
(1933-2003)),
IPT (Technology
Research Institute), Sao Paulo, Brazil, 1st ed., 416 pages (2004)
(in Portuguese)
Wouters, R., Froyen, L.: Scanning electron microscope fractography in failure analysis of steels, Mater. Charact., 36, 357-364
(1996)
Properties and Selection: Stainless Steels, Tool Materials and
Metals, Vol. 3, 9th ed., Metals Handbook,
Special-Purpose
American Society for Metals, Metals Park, OH (1980)
Cabalfn, L.M., Mateo, M.P., Laserna, J.J.: Large area mapping of
non-metallic inclusions in stainless steel by an automated system
based on laser ablation, Spectrochim. Acta Part B, 59, 567-575
(2004)
Perkins, KM., Bache, M.R.: The influence of inclusions on the
fatigue performance of a low pressure turbine blade steel, Int. J.
Fatigue, 27, 610-616 (2005)
Maropoulos, S., Ridley, N.: Inclusions and fracture characteristics
of HSLA steel forgings, Mater. Sci. Eng. A, 384, 64-69 (2004)
•
•
•
•
pages (1991)
12. Failure Analysis and Prevention, Vol. II, ASM Handbook, ASM
International, Materials Park, OH, 1164 pages
13. Azevedo, C.R.F., Cescon, T.: Analise de Falha e Metalografia,
14.
15.
16.
17.
1. "Petroleum
and natural gas industries-Materials
for use in
H 2S-containing
environments in oil and gas production,"
NACE
MR 0175/1S0 15156-3,
"Part 3: Cracking-resistant
CRAs
18.