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Failure Analysis Case Histories
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Fatigue Cracking of an Inlet Nozzle on a Crude Unit
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ENVIRONMENT:
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Oil Refinery |
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EQUIPMENT:
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Vapor Inlet Nozzle on a Vacuum Flasher Column in a
Crude Unit |
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MATERIAL: |
Type 316 Stainless Steel |
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SERVICE TIME: |
Less than 1 year |
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FAILURE MODE:
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Fatigue Cracking Corrosion |
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Background |
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The nozzle in question
was from a crude unit, and was the west vapor inlet nozzle to the vacuum
flasher column. The weld that failed was a flange-to-pipe stub weld.
The piping connection to this nozzle was from a vacuum column feed heater.
The leak was discovered on startup of the crude unit.
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The material for this
nozzle was specified as A-240 Type 316 stainless steel, 3/8” wall
thickness. This alloy was confirmed at the site with a portable alloy
analyzer.
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The line was installed as a replacement in
twenty-one years ago. A crack developed in the mating flange
connection twelve years ago. That crack was ground out and re-welded
and reinforcing gussets added.
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Findings |
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Clearly visible in Figure
1 is one crack running alongside the weld that completely
penetrates the wall and extends slightly more than half the length of the
sample provided. The
crack runs parallel to, but is not touching, the circumferential weld on
the nozzle OD surface. |
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Figure 1. Close-up view of portion of Figure 1 showing end of
major crack. (6X original magnification) |
Figure 2. A view of a portion of the upper fracture
surface. (6X original magnification) |
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The part was sectioned.
The section was made in such a way as to allow
close examination of the mating surfaces of the through crack fracture
surface, as shown in Figure 2. The fracture was a classic example
of fatigue cracking of wrought stainless steel. The initiation point for
this crack was not seen in the sample provided, which meant that the crack
started elsewhere on the nozzle and propagated to this point.
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The macroscopic features, coupled with the fact that the
crack was longer on the nozzle OD than on the ID, indicated that the crack
first occurred on the nozzle OD and propagated both around the nozzle and
through the nozzle wall simultaneously.
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Closer examination of the
OD surface revealed several additional cracks, some in the weld itself and
others adjacent to the weld on both sides. Two cracks following the
ripples on the weld bead, as well as cracking alongside the weld on the
side away from the large fatigue crack, were documented in Figure 3. More
cracks along weld ripples, most visible where a grinding wheel contacted
the surface, were observed elsewhere on the sample. |
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Figure 3. Close-up view of part of the OD surface, showing
cracks in two weld ripples (marked by arrows) and also cracking
alongside the weld. (6X original magnification) |
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| The cross-section showed at least three additional cracks – one at the edge of the
weld and two within the weld. The crack at the edge of the weld is shown
at higher magnification in Figure 4. There is much oxidation along the
sides of the crack, including at the very crack tip. |
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Figure 4. Magnified view of a crack at the left edge of
the weld.
(60X original magnification)
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| Another section revealed
an even tighter secondary crack in the part. This crack, as viewed in the
scanning electron microscope, was observed in Figure 5. As seen in the bottom of the inverted view in Figure
5, there was also a shallow depression or “pit” in the metal surface at
this crack location. |
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Figure 5. An SEM image of a smaller secondary crack. Note also the depression or “pit” in the
metal surface, shown at the bottom in this view. (60X original
magnification)
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| An EDS analysis of the
base metal and the oxidation product within the crack were made. The
dense product within the surface of the shallow “pit” depression was
similarly analyzed. A black coating was observed on the OD surfaces of
the sample, and this, too, was analyzed. The results were summarized in
Table I. |
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TABLE I
– EDS ELEMENTAL ANALYSES
(Expressed
as approximate weight percents)
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Base
Metal |
Oxide
in
Crack |
Product
In
“pit” |
Surface
Coating |
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Iron |
62 |
21 |
71 |
7 |
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Chromium |
17 |
5 |
13 |
2 |
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Nickel |
11 |
3 |
6 |
-- |
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Molybdenum |
3 |
-- |
-- |
-- |
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Silicon |
3 |
1 |
1 |
44 |
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Sulfur |
-- |
2 |
4 |
-- |
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Zinc |
-- |
1 |
1 |
- |
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Aluminum |
2 |
- |
- |
16 |
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Lead |
-- |
-- |
-- |
22 |
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To further analyze the
zinc distribution in the oxides within the crack and “pit”, an EDS map was
performed. The magnification was increased to include only
the bulbous oxide at the crack tip, as shown in Figure 6. A dot
map for zinc in this area is shown in Figure 7. It was seen that zinc
was dispersed non-uniformly throughout the oxide products within and
adjacent to the crack. (The individual, widely scattered dots throughout
the base metal areas in these two maps were believed to be background from
the analysis and not indicative of zinc in the base metal.)
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Figure 6. A higher magnification view at the top end of the
crack shown in Figure 6. (Original magnification 220x) |
Figure 7. EDS dot map of zinc distribution in Figure 7. |
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A sample of the base
metal was mounted, polished, and prepared with two different etchants that
selectively reveal certain brittle phases in the grain boundaries.
Etching with Vilella’s reagent would reveal both metal carbides and
“sigma” phase in the structure. As shown in Figure 8 there was
significant precipitation in this sample when treated with Vilella’s
reagent. Murikami’s reagent would show metal carbides, but not sigma
phase. As shown in Figure 9, there was clearly no carbide precipitation
visible when etched with Murikami’s reagent. It was clear,
therefore, that the grain boundary precipitates in Figure 8 were sigma phase.
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Figure 8. The metal structure after etching with Vilella’s
reagent. (250X original magnification) |
Figure 9. The same area shown in Figure 10 after re-polishing
and etching with Murikami’s reagent.
(250X original magnification)
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Discussion
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The primary cracking
failure in this sample was metal fatigue. No obvious initiation point for
the fatigue was visible in this sample. Apparently the crack began
elsewhere on the nozzle and propagated to the portion of the nozzle
represented by this sample when the process was shut down.
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The question, then, was
whether there were contributing factors in the metal that might have
initiated and/or promoted fatigue failure. At least two such contributing
factors were found.
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The numerous secondary
cracks in and around the weld appear to be promoted by zinc contamination
– a phenomenon referred to as “Type I Embrittlement”. When unstressed
Type 316 stainless steel is exposed to zinc contamination at temperatures
above approximately 1050° F,
penetration of the metal occurs accompanied by formation of a zinc/nickel
intermetallic compound. With stress present, this penetration occurs in
the form of cracks that form perpendicular to the localized stresses.
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The sample had a black
coating on its outside surfaces. This material appeared to be a baked
hydrocarbon product, presumably from process leaks near this location.
This coating contained significant levels of metals, including lead,
calcium, aluminum and zinc. It was probably this coating that provided
the zinc for reaction with the base metal.
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Metallurgical analysis
and selective etching revealed sigma phase precipitation along grain
boundaries, but no significant carbide precipitation. Sigma phase is a
brittle metallic phase that, if present, will help to promote fatigue
failure along grain boundaries. Type 316 stainless steel will normally
exhibit carbide precipitation in the temperature range of 800 to 1500°
F, but those carbides will be re-dissolved into the metal structure at
temperatures above approximately 1600°
F. Sigma phase forms in this alloy at temperatures in the range of 1100
to about 1700° F. The presence
of sigma but absence of carbides indicates that this part was heated in
the range of approximately 1600 to 1700°
F for an extended period.
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