Q1: What NDT method is mandatory for the seam weld of high-pressure pipe (e.g., API 5L PSL2)?
A1: Full-length, 100% non-destructive testing of the weld seam is required. This is typically achieved by Automated Ultrasonic Testing (AUT) or Radiographic Testing (RT).
Q2: How does "Normalizing" heat treatment benefit pipe like API 5L X70?
A2: Normalizing (heating above the transformation temperature and air cooling) refines the grain structure, improves toughness and ductility, and homogenizes mechanical properties, especially important for Thermo-Mechanically Controlled Process (TMCP) plates used for pipe forming.
Q3: What is "Sour Service" pipe, and what special requirements does it have?
A3: Pipe for environments containing wet H2S. Grades like API 5L X65 Sour Service have ultra-strict chemistry limits (very low sulfur, controlled hardness), and may require full-body normalization to minimize the risk of Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC).
Q4: What causes "graphitization" in welded carbon-molybdenum (C-0.5Mo) pipe, and how is it prevented?
A4: Graphitization is the decomposition of carbides into graphite nodules after prolonged exposure above 425°C, severely weakening the steel. It's prevented by using stabilized grades with chromium (e.g., 1.25Cr-0.5Mo, ASTM A213 T11) instead of plain C-0.5Mo for such services.
Q5: Why is post-weld heat treatment (PWHT) often required for alloy steel pipes like P11/P22?
A5: PWHT (stress relief) reduces residual welding stresses and tempers the hard, brittle martensitic microstructure in the HAZ of these air-hardenable Cr-Mo steels, restoring toughness and reducing the risk of stress corrosion cracking in service.
