What are the special requirements for cutting 20# steel pipe?
Flame cutting requires controlling the oxygen pressure at 0.5-0.7MPa, and the cut roughness Ra≤50μm. Plasma cutting is suitable for pipes with a wall thickness of <25mm, and the speed is 15%-20% faster than carbon steel. Laser cutting has the highest accuracy (±0.1mm), but the equipment investment is large. All cutting surfaces must be cleaned of slag with an angle grinder and chamfered by 0.5-1mm. The cuts of important load-bearing components must be subjected to magnetic particle inspection, and the inspection depth must be ≥2mm. CNC cutting machines should be used for mass production, equipped with automatic dust removal systems.
What are the key points of process control for cold bending of 20# steel pipes?
The cold bending radius should not be less than 4 times the pipe diameter, and a mandrel must be used when the wall thickness is >6mm. The pipe bending die must be lubricated regularly, and graphite-based lubricants are recommended. The rebound amount is usually 2°-4°, which needs to be compensated by bending. After bending, the ovality should be ≤5%, and the wall thickness reduction rate should be ≤12%. Precision bending requires the bending angle tolerance to be controlled at ±0.5°, and a servo motor-driven CNC pipe bending machine should be used. Stress relief annealing at 600-650°C should be performed after bending.
How to optimize the welding process of 20# steel pipe?
Manual arc welding recommends the use of J427 welding rod (low hydrogen type), and the current is 10% lower than that of Q235 steel. During gas shielded welding, the ratio of Ar+CO2 mixed gas should be 80%/20%. For butt welding, it is recommended to open a 60° V-shaped groove with a root gap of 2-2.5mm. During multi-layer welding, the interlayer temperature should be controlled below 120°C. After welding important structures, stress relief annealing at 580-620°C is required, and the insulation time is calculated as 1min/mm. When the ambient temperature is below -5°C during winter construction, it must be preheated to above 50°C.
How to scientifically determine the support spacing of the 20# steel pipeline system?
The calculation formula for the horizontal pipe support spacing is: L=0.1√(EI/γ), where E is the elastic modulus and I is the moment of inertia. Taking DN200 water pipe as an example, the standard spacing is 4.2m at room temperature, and the hot water pipe is shortened to 3.5m. A guide bracket is set every 3m for vertical pipes, and a fixed bracket must be set within 1.5m at the corner. Vibrating pipes need to be equipped with shock-absorbing rubber pads, and the spacing is reduced by 30%. The bracket should use an arc-shaped bracket with a contact surface angle of 120° to avoid local stress concentration.
What are the key indicators for the acceptance of 20# steel pipeline system?
The pressure test must use clean water, the chloride ion content is <25ppm, and the test pressure is 1.5 times the working pressure. Welds need to be sampled at 20%RT, and level II is qualified. The support and hanger inspection includes: whether the fixation is firm, whether the clearance of the sliding part meets the standard, and whether the anti-corrosion is intact. The system straightness deviation is ≤3mm/m, and the total length is ≤15mm. The water supply pipes also need to be disinfected and flushed, and the turbidity of the water outlet should be <1NTU. The completion data should include material certification, welding records, heat treatment curves, etc.
