1. Question: Why is 10# steel pipe, despite its relatively low tensile strength, still widely used? What are its core advantages?
Answer: Although 10# steel pipe has a relatively low tensile strength (335-475MPa), it is still widely used. Its core advantages lie in its excellent plasticity, toughness, and processing performance: Firstly, it has good plasticity, capable of withstanding large cold bending and cold stamping deformation without easily breaking, making it suitable for manufacturing parts requiring forming, such as elbows, flanges, and thin-walled containers. Secondly, it has excellent weldability, with minimal cracking after welding, and the weld joint strength can approach that of the base material, making it suitable for structures requiring welded assembly. Thirdly, it has good toughness, not easily fracturing under low temperatures or slight impacts, and its price is relatively low, making it suitable for low-load applications where strength requirements are not high, but processing performance and toughness requirements are high.
III. Processing and Welding
2. Question: What processing techniques are suitable for Q255 steel pipe? Which processing techniques should be avoided? Why? Answer: Suitable processing techniques for Q255 steel pipes include hot rolling, cold rolling, shearing, cutting, drilling, and simple bending (small angle). Processing techniques to be avoided include high-frequency cold stamping, large-angle cold bending, and precision forging. This is because Q255 steel pipes have a high carbon content and impurities, limiting their plasticity and toughness. High-frequency cold stamping and large-angle cold bending can easily cause cracks and fractures. Precision forging requires steel with good plasticity and uniform chemical composition, while Q255 steel pipes have poor chemical composition uniformity, leading to uneven microstructure and unstable performance after forging.
3. Question: What are the differences in welding performance between 10# and 20# steel pipes? What precautions should be taken during welding? Answer: Both are high-quality carbon structural steels with good weldability, but the weldability of #10 steel pipe is superior to that of #20 steel pipe. This is because #10 steel has a lower carbon content and fewer impurities, making it less prone to defects such as porosity and cracks during welding. The strength and toughness of the welded joint are closer to the base material. Welding precautions: For #10 steel pipe, conventional welding methods such as arc welding and gas welding can be used, without preheating (slight preheating is acceptable for thicknesses greater than 15mm), and no heat treatment is required after welding. For #20 steel pipe, if the thickness is greater than 12mm, preheating is required (preheating temperature 150-200℃). Stress-relief annealing can be performed after welding to prevent stress cracking in the welded joint. Low-carbon steel welding rods (such as J422) that match the base material should be used for welding.
4. Question: What problems are prone to occur when welding Q275 steel pipe? How can these welding defects be avoided? Answer: Common problems during welding of Q275 steel pipes include: welding cracks (hot cracks, cold cracks), porosity, slag inclusions, and insufficient weld joint strength. Methods to avoid these defects include: 1. Before welding, clean the welding area of the steel pipe to remove rust, oil, and scale, ensuring a clean welding surface; 2. Use low-carbon steel or low-alloy steel welding rods (such as J422, J423), avoiding the use of high-carbon steel welding rods; 3. For steel pipes thicker than 10mm, preheat before welding (preheating temperature 150-250℃) to reduce welding stress; 4. Control the welding current and welding speed, avoiding excessively high or low welding temperatures to reduce the brittle structure of the heat-affected zone; 5. Perform stress-relief annealing after welding to eliminate welding stress and improve the toughness of the weld joint.
5. Question: Can 20# steel pipes be heat-treated to improve their strength? What are the commonly used heat treatment processes? What are the effects? Answer: 20# steel pipes can have their strength improved through heat treatment because they are high-quality carbon structural steel with a uniform chemical composition, making them suitable for various heat treatment processes. Commonly used heat treatment processes and their effects: 1. Normalizing: Heating the steel pipe to 870-920℃, holding at that temperature, and then air-cooling. This treatment improves the strength and hardness of the steel pipe, increasing the tensile strength to 450-550MPa while maintaining good plasticity and toughness. It is suitable for applications requiring a certain increase in strength while still maintaining plasticity. 2. Quenching and tempering: Quenching temperature 850-880℃, water or oil cooling, tempering temperature 200-300℃. This treatment significantly improves the strength and hardness of the steel pipe (tensile strength can reach over 600MPa), but plasticity and toughness will decrease somewhat. It is suitable for mechanical parts with high strength requirements, such as shafts and gears.
