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Failure Analysis Case Histories
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Stress Corrosion Cracking of a Stainless Steel Pipeline
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ENVIRONMENT:
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Steam, 1450 psi |
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EQUIPMENT:
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Pipeline |
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MATERIAL: |
304H Stainless Steel - 4", 6" & 10" Pipe |
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OPERATING TEMPERATURE:
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900 to 1000F |
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SERVICE TIME:
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4 Years Active followed by 6 Years Inactive |
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FAILURE:
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Stress Corrosion Cracking in Sensitized Metal |
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During the four years of active service, several cracks in the
pipeline had been noticed. These cracks extended from the outside
inward, Figure 1. A metallurgical examination during this period
revealed that the failed material was in a highly sensitized
condition. After six years of inactivity, it was desired to
reactivate this steam line. A hydro-test was performed which
revealed several leaks due to cracking along the pipeline. An
analysis to determine the cause of the cracking was made. |
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Metallurgical examination was performed to verify the sensitized
condition of the material. Also on-site replications were made of
the valve bodies to determine if they were in the same sensitized
condition. Photomicrographs below illustrate the ditched structure
indicating a highly sensitized condition. The high carbon content of
304H makes this material susceptible to grain boundary precipitation
of chromium carbides at elevated temperatures (>1000F) as indicated
by the sensitized structure Figures 2 and 3. The sensitized
structure is an indication of the materials susceptibility to
intergranular corrosion and/or cracking.
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Energy Dispersive Spectroscopy (EDS) was used to analyze the surface scale and crack surface. A high chlorine peak was observed in both locations indicating high chloride levels. A chemical analysis of the pipe insulation revealed a high chloride content also (360 ppm). Whether the chlorides were a constituent of the insulation or the result of contamination was not determined.
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In this case, the presence of three detrimental factors - sensitization, high (1450 psi) pressure, presence of chlorides, - resulted in the multiple cracks in the pipe walls.
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