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Return to Failure Analysis Case Histories
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Polythionic Stress Corrosion
Cracking of Stainless Steel Tube
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ENVIRONMENT:
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Oil Refinery |
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EQUIPMENT:
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Heater |
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MATERIAL: |
Type 316H Stainless Steel |
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SERVICE TIME: |
Seven years |
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FAILURE MODE:
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Polythionic Stress Corrosion Cracking |
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Background
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A
hydrotest at 735 psi revealed numerous leaks in four tubes from a heater. All of
the leaks were observed to be on top or fireside of the tubes.
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The heater was newly installed seven years ago. The
design tube metal temperature was 1000°F. In the past, tube metal
temperature was measured in the range of 1200 to 1400°F during the past run in
areas of coke deposits and less than 900°F in areas where there were no coke
deposits. The tubes were reported to be Type 316H stainless steel with minimum
molybdenum content of 2.5%.
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The tubes had been pigged and flushed with a
soda ash treatment to remove coke deposits and inhibit the formation of
polythionic acids. |
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Findings |
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Visual
examination of the tube revealed two areas on the outside diameter (OD) where
leaks had been observed. Both areas had been ground to reveal the underlying
base metal. On the inside diameter (ID) surface a longitudinal strip of coke
deposits approximately 2 ½-inch wide was present along the entire length of the
tube sample. The leaks identified on the OD were within the area of these
deposits.
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The outside and inside diameters were measured with
a digital caliper to determine if the tube was out of round. The OD measurements
determined that the tube was approximately 0.045” out of round. The largest OD
measurement was obtained in the area of the coke deposits. The smallest OD
measurement was obtained 90° to the coke deposit. The ID measurements were all
6.050” ±
0.003”. A portion of the coke was chipped off and the thickness measured to be
from 0.040” to 0.050”.
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An approximately 10” long section was removed
which included the two identified leaks. This section was split
longitudinally to reveal the ID surface. The section represented by Figure 1
indicates the ID of the leak in Area A of Figure 1. This portion was cut out
and used for cross-sectional analysis. The remaining parts of this
longitudinal section were cleaned and polished with abrasive grinding to a
240 grit finish on the ID surface. The polished surfaces were tested using
liquid dye penetrant. One crack was identified, Figure 2. This crack was
located underneath the coke deposits associated with known leak.
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| Figure 3. Cross-section of crack , as polished
(20X Original Magnification) |
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When the tube was cut longitudinally, it was
observed that substantial hoop stresses were present in the tube. When the
longitudinal cut was finished the tube popped open increasing in diameter
from 6.611” to 6.750”. |
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A metallographic cross-section of the crack in
Figure 2 was prepared. Three separate cracks were observed in this area. The
polished cross-section revealed that the cracks had initiated on the ID surface
and appeared to have propagated intergranularly, Figure 3. The cracks were
relatively narrow with no significant branching. No indications of creep voids
were observed.
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The polished cross section was placed into a JEOL
JSM 35 scanning electron microscope equipped with Energy Dispersive x-ray
Spectroscopy (EDS). An EDS analysis of the crack tip did not reveal any
indication of the presence of chloride or sulfur compounds.
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The cross-section was electrolytically etched in
oxalic acid and the microstructure viewed at up to 500X magnification. The
microstructure observed was typical of sensitized austenitic stainless steel
with deep ditches present. The intergranular nature of the cracks was confirmed.
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Chemical analysis by OES (optical emission spectroscopy) confirmed the alloy
to be within the chemical composition for 316H stainless steel. |
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Discussion
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Based on this analysis the
leaks were caused by cracks that initiated on the ID surface. All of the cracks
observed were found underneath the coke deposits. The cracks are intergranular
and narrow with no significant branching. These characteristics are consistent
with stress corrosion cracking due to polythionic acids.
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