Q1: What are the common non-destructive testing methods for electric resistance welded pipes?
① Ultrasonic testing (UT): can detect internal defects such as weld failure and inclusions, with an accuracy of 0.5mm; ② Eddy current testing (ET): suitable for surface crack detection, with a speed of up to 3m/s; ③ Radiographic testing (RT): judging volume defects such as pores through X-ray imaging; ④ Magnetic particle testing (MT): only applicable to surface defects of ferromagnetic materials; ⑤ Hydrostatic test: verify overall sealing, with a test pressure of 1.5 times the working pressure. API standards require that high-pressure pipelines must be tested with a combination of UT+RT.
Q2: What are the specific requirements of API 5L standards for oil and gas welded pipes?
PSL1 grade pipes only need to meet basic mechanical properties, while PSL2 grade pipes require additional control of chemical composition (such as S≤0.015%), Charpy impact energy (≥40J at -20℃) and stricter dimensional tolerances (such as outer diameter ±0.75%). The weld hardness must be ≤250HV10 to avoid HIC hydrogen-induced cracking. For sour environment (H₂S working condition), HRC≤22 and SSC test are required. The latest version of API 5L (46th edition) adds technical requirements for strain-based pipeline design.
Q3: How to judge weld defects through ultrasonic testing?
Use a focused probe (5MHz) to scan along the axial and transverse direction of the weld. Unfused defects are displayed as continuous linear echoes, and pores are isolated point signals. The defect height can be quantified by TOFD (time difference diffraction method) with an accuracy of 0.1mm. The acceptance criteria usually stipulate: the length of a single defect is ≤6.4mm, and the cumulative defect length is ≤12.5% of the total length of the weld. The automated ultrasonic system (AUT) can generate C-scan images to achieve 100% full inspection.
Q4: What is the role of hydrostatic test in welded pipe inspection?
The strength and sealing of the pipe body are verified by applying a hydrostatic pressure twice the working pressure (such as 20MPa for 10s). The pressure curve needs to be monitored during the test, and the pressure drop is ≤5% to pass. This method can detect macro cracks and penetrating defects, but cannot detect micro defects. For large-diameter pipes (such as Φ1420mm), head sealing and high-pressure water pump systems are required. GB/T 3091 standard stipulates that all galvanized welded pipes must pass a 2.5MPa water pressure test.
Q5: What items are included in the weld mechanical property test?
① Tensile test: weld strength should be ≥ 90% of the parent material; ② Bending test (positive bend/back bend 180° without cracks); ③ Impact test (-20℃ weld zone ≥ 27J); ④ Hardness test (weld/heat-affected zone hardness gradient ≤ 50HV difference); ⑤ Microstructure analysis (check abnormal structures such as martensitic bands). ASTM A370 standard requires that the sampling position be 45° from the center line of the weld, and the number of samples is 1 group per 50 tons of pipe.
