What are the common defects of high-frequency welded pipes? How to detect?
The main defects of high-frequency welded pipes include incomplete penetration (insufficient current or pressure), inclusions (impurities in steel strips), pores (too fast cooling) and cracks (stress concentration). Detection methods include ultrasonic flaw detection (UT, detecting internal defects), eddy current detection (surface cracks), hydrostatic test (verification of pressure bearing capacity) and visual inspection (weld appearance). API 5L standard requires non-destructive testing of each welded pipe.
How to control the weld strength of high-frequency welded pipes?
The weld strength depends on the matching of welding current, extrusion pressure and welding speed. Too low current will lead to incomplete fusion, too high current will easily burn through; insufficient extrusion pressure will form a virtual weld. The factory needs to determine the optimal parameters through process tests and regularly sample for tensile tests and bending tests (such as GB/T 3091 standard). In addition, the carbon equivalent (CE) of the raw steel strip needs to be ≤0.4% to ensure weldability.
How to ensure the dimensional accuracy of high-frequency welded pipes?
The dimensional accuracy is controlled by the sizing machine and the straightening machine. The sizing machine calibrates the welded pipe to the target outer diameter (tolerance is usually ±0.2mm) through multiple rollers. The straightening machine eliminates bending, and the straightness error must be ≤1.5mm/m. The online laser diameter gauge can monitor the pipe diameter in real time and adjust the roller pressure by feedback. High-precision welded pipes (such as pipes for automobile drive shafts) also require inner diameter honing.
Why do high-frequency welded pipes need to be galvanized or coated?
Galvanizing (such as hot-dip galvanizing) can prevent steel pipes from rusting and extend the outdoor service life (such as scaffolding and guardrails). The thickness of the galvanized layer must be ≥40μm (GB/T 13912 standard). Coating treatment (such as epoxy resin) is suitable for corrosive environments (such as chemical pipelines), and corrosion resistance must be verified by salt spray testing. Pickling is required to remove oxide scale before galvanizing, and sandblasting is required to increase adhesion before coating.
What are the key indicators of international standards for high-frequency welded pipes (such as API 5L)?
API 5L requires the chemical composition (such as S, P content), mechanical properties (yield strength, tensile strength), hydraulic test pressure (usually 1.5 times the design pressure) and non-destructive testing pass rate (100% UT or eddy current) of welded pipes. PSL2 grade pipelines must also meet the Charpy impact test (impact energy ≥ 40J at -20℃). Manufacturers must hold API monogram certification.
