Q1: What are the common failure modes of alloy structural pipes?
Stress corrosion cracking (SCC) accounts for more than 40% of chemical pipeline failures, especially in chloride ion environments. Fatigue fractures often occur in vibration load areas, such as compressor connecting pipes. Hydrogen embrittlement is common in high-strength steel welding areas, resulting in delayed cracks. High-temperature creep will gradually reduce the wall thickness of power station pipelines. Erosion corrosion causes local perforation in sand-transporting oil and gas pipelines.
Q2: How to determine the cause of failure through metallographic analysis?
Scanning electron microscopy (SEM) can observe the fracture morphology: cleavage section indicates brittle fracture, and dimple shape is plastic fracture. Energy spectrum analysis (EDS) detects the composition of corrosion products, such as sulfides pointing to H2S corrosion. Intergranular cracks suggest improper heat treatment or sensitization problems. Inclusion rating (ASTM E45) traces smelting defects. Case: The rupture of a certain oil pipeline was caused by excessive MnS inclusions.
Q3: What are the specific measures to prevent hydrogen embrittlement?
Select low-hydrogen welding rods (such as E7018) and dry them strictly (350℃×1h). After pickling, dehydrogenation treatment must be carried out at 190℃×4h. Design to avoid high-constraint joints and reduce residual stress. Hardness is controlled below HRC22 (NACE MR0175 regulations). Cathodic overprotection is prohibited during operation (potential <-1.1V vs CSE).
Q4: How to evaluate the remaining life of high-temperature pipelines?
The current wall thickness is determined by ultrasonic thickness measurement, and the corrosion allowance is calculated by comparing the designed minimum wall thickness. Hardness testing and carbide analysis are used to evaluate the degree of material degradation (such as ASTM E112). Creep damage is graded according to ISO 6303 using the composite metallographic method. The remaining time is calculated based on the Larson-Miller parameter. The main steam pipe of a power plant was evaluated and extended to operate for 20,000 hours.
Q5: How can online monitoring technology prevent sudden failures?
Acoustic emission technology (AE) captures crack propagation signals in real time. Fiber optic sensors monitor strain and temperature distribution. Intelligent pipeline cleaning devices (PIGs) regularly detect corrosion on the inner wall of pipelines. Big data platforms analyze pressure fluctuations to predict fatigue risks. British BP uses AI algorithms to provide 72 hours of early warning of leaks.
