1. What are the application limitations of ASTM A312 Grade 321 welded pipes, and in which corrosive environments should they be avoided? Answer: ASTM A312 Grade 321 welded pipes are austenitic stainless steel containing titanium (Ti: 5×C-0.70%), which is added to prevent intergranular corrosion by forming titanium carbides instead of chromium carbides. However, they have the following application limitations: 1) Poor resistance to pitting corrosion and crevice corrosion in high-chloride environments (such as marine water, saltwater, or chemical media with high Cl⁻ content), because they do not contain molybdenum (unlike Grade 316). 2) Not suitable for high-temperature environments above 870℃, as the titanium carbides will decompose, reducing the pipe's strength and corrosion resistance. 3) Higher cost than Grade 304 and 304L, so they are not cost-effective for general corrosion-resistant applications. Therefore, Grade 321 welded pipes should be avoided in marine environments, chemical plants with high chloride content, and high-temperature applications above 870℃.
2. How to detect intergranular corrosion in ASTM A312 Grade 304L welded pipes, and what measures can be taken to repair defective pipes? Answer: Common methods to detect intergranular corrosion in ASTM A312 Grade 304L welded pipes include: 1) Strauss test: immerse the pipe sample in a boiling nitric acid solution for a certain period, then measure the weight loss; if the weight loss exceeds the standard, it indicates intergranular corrosion. 2) Huey test: immerse the sample in boiling 65% nitric acid solution, repeat the test for several cycles, and check for corrosion. 3) Electrochemical test: use electrochemical methods to detect the corrosion potential and current, judging the presence of intergranular corrosion. For pipes with intergranular corrosion defects, repair measures include: 1) Grinding the defective area with a grinder until the corrosion is completely removed, then re-welding the area using matching welding materials and appropriate welding parameters. 2) Performing solution annealing on the repaired area to restore the corrosion resistance. 3) If the corrosion is severe (exceeding the allowable range), replace the defective pipe section with a new one that meets the standard.
3. What are the chemical composition and mechanical properties of ASTM A335 Grade P91 welded pipes, and what are their main applications? Answer: ASTM A335 Grade P91 welded pipes are ferritic-martensitic alloy steel with the following chemical composition: carbon (C: 0.08-0.12%), chromium (Cr: 8.0-9.5%), molybdenum (Mo: 0.85-1.05%), vanadium (V: 0.18-0.25%), niobium (Nb: 0.06-0.10%), and iron (Fe: balance). Their mechanical properties are excellent: minimum yield strength of 415 MPa, minimum tensile strength of 585 MPa, and good toughness at high temperatures. Due to their high-temperature strength, creep resistance, and corrosion resistance, P91 welded pipes are mainly used in high-temperature, high-pressure boiler systems, such as superheaters, reheaters, and main steam pipelines in thermal power plants, as well as in petrochemical plants where the operating temperature is between 550-650℃.
4. Why is heat treatment essential for ASTM A335 Grade P22 welded pipes, and what is the standard heat treatment process? Answer: Heat treatment is essential for ASTM A335 Grade P22 welded pipes because P22 is a Cr-Mo alloy steel (Cr: 2.10-2.90%, Mo: 0.87-1.13%), and the welding process will cause changes in the microstructure (such as the formation of martensite and bainite), leading to high residual stress, brittleness, and reduced toughness. Heat treatment can eliminate residual stress, adjust the microstructure, and improve the pipe's mechanical properties and corrosion resistance. The standard heat treatment process for P22 welded pipes includes: 1) Normalization: heat the pipe to 890-910℃, hold for a certain time (according to the wall thickness), then air cool to room temperature. This refines the grain structure and improves strength. 2) Tempering: heat the pipe to 620-680℃, hold for a sufficient time, then air cool or furnace cool. This eliminates residual stress, reduces brittleness, and improves toughness.
5. What are the main welding challenges of GB/T 9948-2013 15CrMoG welded pipes, and how to overcome them? Answer: GB/T 9948-2013 15CrMoG welded pipes are Cr-Mo alloy steel (Cr: 1.00-1.50%, Mo: 0.40-0.60%), and their main welding challenges are: 1) High hardenability: the weld seam and heat-affected zone (HAZ) are prone to form hard martensite, leading to cold cracks. 2) Welding residual stress: the large temperature gradient during welding causes high residual stress, which increases the risk of cracking. 3) Poor weldability at room temperature: the pipe is prone to cracking during welding if preheating is not performed. To overcome these challenges: 1) Preheat the pipe before welding: the preheating temperature is usually 150-250℃, which reduces the temperature gradient and prevents martensite formation. 2) Use low-hydrogen welding electrodes (such as E5015-G) or welding wires to reduce hydrogen content and avoid hydrogen-induced cracks. 3) Control welding parameters: use small welding current, slow welding speed, and multi-layer multi-pass welding to reduce heat input and avoid overheating. 4) Perform post-weld heat treatment (tempering at 600-650℃) to eliminate residual stress and improve toughness.
