Why do welded pipe welds need heat treatment? What are the common methods?
Reason: Residual stress and brittle structure will be generated during welding, reducing the toughness and pressure resistance of the weld.
Common methods:
Annealing: Heating to 600~700°C and then slowly cooling to eliminate stress and refine grains.
Normalizing: Heating to above the critical temperature and air cooling to improve weld strength and uniformity.
Local heat treatment: Only heat the weld area to save energy (applicable to large-diameter pipes).
Application scenarios: Oil pipelines (API 5L standard), high-pressure boiler tubes, etc.
How to improve the pressure resistance of welds through mechanical processing?
Internal and external burr removal:
Internal burrs: Remove with a tool or high-pressure water jet to avoid fluid resistance and stress concentration.
External burrs: Grinding or scraping with a grinding wheel to improve surface flatness.
Weld rolling: Apply pressure to the weld through a hydraulic roller to improve density and fatigue life.
Whole pipe expansion: make the pipe diameter uniform and reduce the wall thickness deviation at the weld.
Effect: can increase the pressure bearing capacity of the weld by 10%~20%.
Which anti-corrosion technologies can significantly extend the life of welded pipes?
Galvanizing: hot-dip galvanizing (HDG) or electrogalvanizing, suitable for building scaffolding and water pipes (rust-proof life of 20~30 years).
3PE anti-corrosion: three-layer structure (epoxy powder + adhesive + polyethylene), used for oil/gas pipelines (corrosion resistance life of more than 30 years).
Epoxy coating: inner wall coating to prevent corrosion of the conveying medium (such as sewage, chemicals).
Cathodic protection: with sacrificial anode or impressed current, suitable for buried pipelines.
Industry case: West-East Gas Transmission Project adopts 3PE+cathodic protection dual protection.
How to detect weld defects to ensure pressure safety?
Ultrasonic testing (UT): high-frequency sound waves detect internal cracks and pores (high sensitivity, suitable for thick-walled pipes).
Radiographic testing (RT): X-rays or gamma rays can see through the weld and visually display defects (such as incomplete penetration and slag inclusions).
Eddy current testing (ET): Used for surface or near-surface defect detection (high efficiency, suitable for thin-walled pipes).
Hydrostatic test: Pressurize to 1.5 times the working pressure and maintain pressure to test the weld tightness.
Standard requirements: API 5L stipulates that high-pressure welded pipes require 100% UT or RT testing.
How do new welding technologies improve weld quality?
Laser welding:
Energy is concentrated, the heat-affected zone is small, and the weld is narrow and deep (suitable for precision thin-walled pipes, such as automobile exhaust pipes).
Tandem MIG
Two welding wires work together to improve deposition efficiency and reduce pores (suitable for large-diameter thick-walled pipes).
Friction stir welding (FSW):
No fusion welding process, avoids thermal cracks, and is suitable for non-ferrous metal welded pipes such as aluminum alloys.
Advantages: New technologies can reduce the need for subsequent processing and directly improve weld strength and corrosion resistance.
