What are the common failure modes of alloy steel pipes?
Including: ① stress corrosion cracking (SCC, such as chloride ion environment); ② hydrogen embrittlement (hydrogen atom penetration in acidic medium); ③ fatigue fracture (caused by alternating load); ④ erosion (including particle fluid wear); ⑤ high temperature oxidation (such as furnace tube coking). Failure analysis needs to be combined with macroscopic morphology, metallography and fracture scanning electron microscope (SEM) observation.
How to prevent high temperature creep failure of alloy steel pipes?
Measures include: ① Select high chromium molybdenum steel (such as P92); ② Control the working temperature below the creep limit of the material; ③ Regularly check the wall thickness (ultrasonic thickness gauge); ④ Avoid overpressure operation; ⑤ Perform metallographic monitoring and replace grain boundary cavities in time. For example, the main steam pipe of a power plant needs to be fully inspected every 5 years.
How to clean the scale on the inner wall of alloy steel pipes?
Mechanical methods (such as PIG pipe cleaners), chemical cleaning (pickling to remove Fe₃O₄ scale) or high-pressure water jets (above 3000bar). The concentration of pickling needs to be controlled (such as 10% citric acid) to avoid corrosion of the matrix, and neutralization and rinsing are required after completion. Severe scaling may cause local overheating, and regular preventive maintenance is required.
How to determine whether the alloy steel pipe should be scrapped and replaced?
The basis includes: ① The remaining wall thickness is lower than the design minimum value (such as corrosion thinning exceeding 20%); ② The crack depth exceeds the safety standard; ③ Material performance degradation (such as hardness reduction of 30%); ④ Frequent leakage or maintenance records exceed the standard; ⑤ Economic evaluation (maintenance cost exceeds 60% of new pipes).
How to conduct root cause analysis (RCA) after alloy steel pipe failure?
The steps include: ① Collect working condition data (temperature, pressure, medium); ② Macro inspection (photographing and recording); ③ Sampling for chemical composition and mechanical property testing; ④ Micro analysis (SEM/EDS to determine the origin of cracks); ⑤ Simulation experiment (such as accelerated corrosion test); ⑥ Compare historical cases and finally propose improvement plans (such as changing materials or optimizing processes).
