Analysis of cracking cause of the tee in diluted steam tube of cracking furnace
In view of the cracking occurred in 304H heat-resistant stainless steel tee for dilute steam tube in an ethylene cracking device after one year of operation,the material composition,mechanical properties,metallographic structure,residual stress and fracture characteristics were tested and analyzed.The results show that the cracks are originated in the inner wall and the outer wall of the tee respectively,most of the cracks are accompanied by intercrystalline corrosion characteristics,and the crack propagation characteristics are mainly intercrystalline.Acid corrosion media such as O,S and Cl exist in the corrosion products near the fracture site.The maximum axial stress on the main surface of the tee reaches 147 MPa,and the highest hoop stress at 20 mm away from the crack reaches 114 MPa,which means the surface stress in the operating tee reached 1/2 times the yield strength of the material.It is considered that the generation of cracks is attributed to the intergranular stress corrosion cracking under the combined action of stress and corrosion medium after the material sensitization.Replacing the tee material or reducing the residual stress in the tee through the appropriate solid solution treatment is an important way to avoid the tee cracking.
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