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Stress Corrosion Cracking in a Heat Exchanger Tube

A crack was found in the outer diameter of a coiled heat exchanger tube. The heating tube had water running inside the tube and natural fired gas heating elements on the outside.

Procedure:

  • Photo documentation of all phases of investigation.
  • Hardness and chemical measurements
  • Microstructural analysis of crack
  • X-Ray Fluorescence in crack

Discussion:

Visual and metallographic inspection indicated multiple cracks located parallel to the main fracture direction and perpendicular to the major axis of tube. Chemical analysis indicated a mild carbon steel and hardness measurements were within expected limits. Metallographic examination indicated the network of crack propagation was intergranular. Intergranular cracking can be associated with stress corrosion cracking. X-ray fluorescence indicated chlorine present in the crack.

Conclusion:

For stress corrosion cracking to occur, three characteristics must be present:

  •  Tensile stresses (applied or residual)
  • A corrosive element(s) such as chlorine
  • Susceptible material, such as mild carbon steel
All three characteristics were present in the fractured tube indicating that stress corrosion cracking was the cause of failure.
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Figure 1 - Example of crack found in the heat exchanger tube. The sample was cut perpendicular to the crack and mounted of metallurgial analysis. 


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Figure 2 - Cross-section of the crack showing intergranular crack propagation indicative of stress corrosion cracking.