1. Q: What does "pipe schedule" (SCH) refer to?
A: It is an industry standard designation for the wall thickness of a pipe. For a given nominal pipe size (NPS), a higher schedule number (e.g., SCH 80 vs. SCH 40) indicates a thicker wall and thus a higher pressure rating. Common schedules include SCH 5S, 10S, 40S, and 80S for stainless steel -7.
2. Q: Can you explain the significance of UNS numbers like N08367 in pipe specifications?
A: UNS (Unified Numbering System) numbers provide a unique and unambiguous identifier for metals and alloys. UNS N08367 is a specific designation for a high-alloy austenitic stainless steel (often known as AL-6XN®), with high molybdenum and nitrogen content, used for its exceptional resistance to seawater corrosion. Standards like ASTM B804 specifically cover welded pipe in this grade -4.
3. Q: What is the "heat-affected zone" (HAZ) and why is it important?
A: The HAZ is the area of the base metal adjacent to the weld that was not melted but whose microstructure and mechanical properties were altered by the welding heat. It is often the weakest part of a welded joint, potentially suffering from grain growth, hardening, or loss of toughness, making it susceptible to cracking -2-10.
4. Q: What causes "lack of fusion" defects in ERW pipes?
A: Lack of fusion (or cold weld) occurs when the two edges of the strip do not properly bond during the welding process. Causes include insufficient welding temperature, inadequate squeeze force from the rolls, or contamination on the strip edges. It is a critical defect as it creates a discontinuity in the pipe wall -6.
5. Q: What is the purpose of post-weld heat treatment (PWHT)?
A: PWHT is performed to relieve residual stresses introduced by welding, to temper hard microstructures (like martensite) in the HAZ, and to improve dimensional stability. It is often required for thicker walls, for certain high-strength or creep-resistant grades, or when welding dissimilar materials.
