1. What is the difference between ASTM A335 Grade P11 and P22 welded pipes in terms of alloy content and application temperature range? Answer: The main difference between ASTM A335 Grade P11 and P22 welded pipes lies in their alloy content and application temperature range. P11 is a low Cr-Mo alloy steel with Cr: 1.00-1.50% and Mo: 0.45-0.65%, while P22 has higher Cr and Mo content (Cr: 2.10-2.90%, Mo: 0.87-1.13%). Due to the higher alloy content, P22 has better high-temperature strength, creep resistance, and corrosion resistance than P11. The application temperature range of P11 is up to 550°C, and it is mainly used in medium-temperature, medium-pressure boiler pipelines and petrochemical equipment. P22 can be used at temperatures up to 600°C, and it is suitable for high-temperature, high-pressure boiler systems (such as superheaters, reheaters) and petrochemical pipelines that require higher creep resistance.
2. How does the molybdenum content affect the creep resistance of ASTM A335 Grade P92 welded pipes, and what are their typical application scenarios? Answer: ASTM A335 Grade P92 welded pipes are high Cr-Mo-V-Nb alloy steel (Cr: 8.50-9.50%, Mo: 0.85-1.05%, V: 0.15-0.25%, Nb: 0.04-0.09%), and molybdenum is a key element affecting their creep resistance. Molybdenum can form solid solutions in the steel, refine the grain structure, and prevent the movement of dislocations at high temperatures, thereby improving the pipe's creep strength and creep rupture life. The higher the molybdenum content (within the standard range), the better the creep resistance. Typical application scenarios of P92 welded pipes are high-temperature, high-pressure power generation equipment, such as ultra-supercritical (USC) and advanced ultra-supercritical (AUSC) boiler main steam pipelines, reheater pipelines, and header pipes, where the operating temperature is between 600-650°C and the pressure is above 25 MPa.
3. What are the quality requirements for the weld seam of GB/T 6479-2018 20G welded pipes used in boiler water walls, and how to ensure welding quality? Answer: GB/T 6479-2018 20G welded pipes are carbon steel pipes used in boiler water walls, and their weld seam quality requirements are strict: 1) No cracks, incomplete fusion, incomplete penetration, or slag inclusion; the weld seam surface should be smooth, with no obvious porosity or undercut. 2) The weld seam strength should not be lower than the base metal strength (tensile strength ≥410 MPa, yield strength ≥245 MPa). 3) The weld seam should have good toughness to withstand the thermal cycle of the boiler (alternating heating and cooling). To ensure welding quality: 1) Use welding materials matching the base metal (such as E4303, E4315 electrodes). 2) Preheat the pipe before welding (preheating temperature 80-150°C) to reduce cold crack risk. 3) Control welding parameters (current, voltage, speed) to ensure stable weld formation. 4) Perform post-weld heat treatment (tempering at 600-650°C) to eliminate residual stress. 5) Conduct strict inspection, including visual inspection, UT/RT testing, and mechanical property tests (tensile, impact tests).
4. What are the application limitations of ASTM A335 Grade P5 welded pipes, and in which high-temperature environments are they not suitable? Answer: ASTM A335 Grade P5 welded pipes are Cr-Mo alloy steel (Cr: 4.00-6.00%, Mo: 0.45-0.65%) with good high-temperature strength and corrosion resistance, but they have certain application limitations. Their maximum operating temperature is 550°C, so they are not suitable for high-temperature environments above 550°C, such as ultra-supercritical boiler pipelines (operating temperature ≥600°C), because their creep resistance and high-temperature strength will decrease significantly, leading to pipeline deformation or rupture. In addition, P5 welded pipes have poor resistance to sulfur corrosion, so they should be avoided in environments containing high sulfur content (such as petrochemical plants with high sulfur crude oil processing), as sulfur will react with chromium and molybdenum to form sulfides, reducing the pipe's corrosion resistance and service life.
5. What are the chemical composition and performance characteristics of API 5L X80 welded pipes, and why are they widely used in long-distance oil and gas pipelines? Answer: API 5L X80 welded pipes are high-strength low-alloy (HSLA) steel pipes with the following chemical composition: carbon (C: 0.14% max), manganese (Mn: 1.80% max), chromium (Cr: 0.50% max), molybdenum (Mo: 0.30% max), niobium (Nb: 0.06% max), vanadium (V: 0.06% max), and titanium (Ti: 0.02% max). Their performance characteristics are: high strength (minimum yield strength 551 MPa, tensile strength 620-750 MPa), good toughness (impact energy ≥40 J at -20°C), excellent weldability, and good corrosion resistance. They are widely used in long-distance oil and gas pipelines because: 1) High strength allows for thinner pipe walls under the same pressure, reducing material and transportation costs. 2) Good toughness and weldability ensure the pipeline's integrity and reliability, even in harsh environments (such as cold regions, earthquake zones). 3) Good corrosion resistance (after anti-corrosion coating) extends the pipeline's service life, reducing maintenance costs.
