Failure Analysis finding:
High temperature studded piping having two sections of
316H austenitic stainless steel were found to be leaking in one
location each during hydrotesting. A sample, including one of
the leaking areas, was cut and investigated for a failure reason
to check the leaking issue. Following preliminary examination
and Dye Penetrant Inspection (PT), the sample was further
investigated for metallurgical investigation and SEM/EDS
examination.
A general view of the sample having crack is shown in figure 1.
As visible below, the failed area was marked in yellow, and by
means of a white band taken for failure analysis investigation.
Technical Background
The leaking pipe section is part of a convection module for heat
recovery. The pipe is made from grade AISI 316H to which some
carbon steel studs were joined by high-frequency resistance
welding. Before commissioning, the system was subjected to hydrotesting, and two leaks were found in the tube under
investigation.
Preliminary Visual Examination and Dye Penetrant
Inspection
Before any further sampling operation, the internal surface
of the pipe was cleaned and then subjected to dye penetrant
inspection (PT). PT confirmed the presence of only one
indication open on the pipe surface as marked by a black box in
figure 2. As better visible in figure 3, the leaking area is affected
by a linear indication consistent with a longitudinal crack.
Furthermore, the pipe appears affected by localized bulging
along the circumferential areas marked by yellow arrows as
illustrated in in figure 2.
Optical Emission Spectroscopy (OES) was carried out on a sample cut from the pipe section. The results were then compared with
the nominal chemical composition for grade AISI 316H. As presented in table 1, the analysis results are consistent with the nominal
composition for the grade under investigation.
Visual and Stereo-Microscopy
Examination
A general view of both sides of the
sample, selected for further investigation,
is shown in figure 4. In detail, the crack
location is marked in figure 4a and 4b
for the external and internal surface,
respectively. As evident in figure 4b, the pipe surface exhibits heat tints.
A general view of the crack path, as
better visible by stereomicroscopy, is
shown in figure 5 and 6 close-up view
of the crack. As shown in the close-ups
below, the internal surface exhibits an
intergranular pattern.
Micro sectioning was carried out as
illustrated in figure 7. As visible below,
the right portion of the crack was
forced open to allow stereo and SEM
fractography (fig. 8). The remaining part
of the crack was used for metallographic
examination along the transverse
direction. Two views of the crack
propagation path, with respect to the
stud positioning and welded joint was investigated for crack propagation
pattern to study.
Metallographic examination
The representative micro-section of the
cracked area, taken transverse to the leak
as illustrated in figure 7, was subjected
to metallographic preparation and
examination prior and post etching.
A general view of the selected area is
visible in figure 8 where the regions subjected to observation are illustrated. As evident below,
the main crack path runs transversally through the pipe wall
thickness with one end located at the lack of fusion between the
right stud and the pipe external surface (figures 9 and 10b).
Additional micrographs of the cracked area are verified prior
and post etching respectively. Based on the metallographic
evidence, the crack path appears to be predominantly
intergranular in nature. Presence of a copperish-coloured
matter was also observed along the crack internal surfaces.
The sample exhibits a microstructure considered consistent
with the condition of the austenitic grade under examination.
SEM examination and EDS Analysis
Following cleaning in ultrasonic bath, Scanning Electron
Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)
were performed in the most relevant and representative
regions of fracture surface in order to investigate the mode of
fracture.
Based on SEM fractography, the crack propagation path,
through the pipe material, was confirmed to be intergranular
nature as evident in figures 11-12 at increasing magnification,
this confirming a brittle mode of failure.
Evidence of additional features, considered not genuine of
the base metal intergranular pattern, was observed and better
investigated by EDS elemental analysis as illustrated in figures
12. Elemental analysis confirmed the additional features to be
mainly composed of copper. Copper was also detected during the examination and analysis of the crack internal surface (in
cross- section) as shown by elemental mapping in figures 13-14.
An additional microsection was cut through a region far from
the leaking one as shown in figure 15. Following metallographic
preparation, a crack was observed in area B as illustrated below.
Area B and A, in the HAZ, were selected for EDS analysis.
In detail, two regions of the crack in area B were analysed as shown in figures 16-19. Based on EDS results, the first region
analysed showed presence of oxygen and carbon inside the
crack whereas only carbon was fond in the second region.
The HAZ area denominated as A, and subjected to EDS
analysis, is shown figures 20-21. As presented by elemental
mapping, no evidence of contamination but the silicon coming,
likely, from the preparation consumables, was detected.
No evidence of copper was found in these additional three
areas selected for further investigation.
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