Introduction:
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.

Figure 1 - Example of crack found in the heat exchanger tube. The sample was cut perpendicular to the crack and mounted of metallurgial analysis.  

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