1. Q: Explain the significance of the Carbon Equivalent (CE) value for weldable grades like X70.
A: CE (e.g., Pcm, IIW formula) is a calculated value from the steel's chemical composition that predicts the hardness and risk of cold cracking in the heat-affected zone (HAZ) after welding. For high-grade line pipes, a low CE is crucial to ensure weldability without extensive pre-heat, allowing for efficient field welding, especially in automated welding projects.
2. Q: What is a "Clad Pipe" or "Mechanically Lined Pipe" (MLP), and when is it used?
A: It's a pipe with a carbon steel (e.g., API 5L X65) outer layer for structural strength and a thin inner liner (e.g., 316L stainless steel, Alloy 625) for corrosion resistance. It's used for transporting highly corrosive fluids (e.g., wet CO₂, acidic multiphase flow) where solid corrosion-resistant alloy pipe would be prohibitively expensive.
3. Q: When is ASTM A335 P11 pipe specified?
A: A335 P11 (1.25Cr-0.5Mo-Si) is used for high-temperature power plant piping (e.g., steam lines, reheat lines) and refinery piping where temperatures range from 450°C to 550°C (842°F to 1022°F). It offers better creep strength than carbon steel but is less costly than higher alloy grades.
4. Q: What are the key advantages of ASTM A335 P22 over P11?
A: P22 (2.25Cr-1Mo) has higher chromium and molybdenum content, providing superior oxidation resistance, higher creep strength, and better microstructural stability at temperatures up to about 580°C (1076°F). It's a workhorse material for high-temperature, high-pressure headers and steam lines.
5. Q: Why is ASTM A335 P91 a revolutionary grade for fossil power plants?
A: P91 (9Cr-1Mo-V) is a creep strength enhanced ferritic (CSEF) steel. Its addition of vanadium, niobium, and nitrogen, along with a specific heat treatment, gives it creep strength comparable to austenitic stainless steels (like 304H) at temperatures around 600°C, but with much better thermal conductivity and lower thermal expansion, reducing thermal fatigue.
