1.What are the root causes of deformation?
Heating stage: The high temperature of 450℃ causes the yield strength of steel to drop by 60%, and the elbow undergoes plastic deformation under its own weight.
Zinc immersion: The hydrodynamic impact of liquid zinc causes instability in thin-walled areas (such as side walls)
Cooling stage: The difference in thermal expansion coefficient between the zinc layer and the steel substrate causes shrinkage stress deformation
Structural weaknesses: Small radius elbows with R=1.5D and long arm elbows (center distance>3D) are more prone to distortion
2.What are the process optimization solutions?
Prefabricated structure reinforcement
Wall thickness compensation: Increase the wall thickness of the high-risk area for deformation during the design stage (such as +15% thickness on the outside of the elbow) Reinforcement welding: Weld the cross-shaped temporary support frame on the non-sealed surface of the elbow (removed after galvanizing)
Galvanizing tooling optimization
Precision temperature-time control
Post-processing shaping technology
Hot shaping: Before the zinc layer is fully hardened (>300℃), use a hydraulic push rod + arc mold to correct the ovality
Control standard: Ovality ≤1.5% DN
Cold pressing correction: Use a three-roller rounding machine to process the deformed parts at room temperature.
3.How can materials and design be improved?
Substrate upgrade
Prefer Q345R instead of Q235 (yield strength is 30% higher, and the ability to resist high temperature softening is stronger) Add micro-alloy elements to refine the grains and reduce thermal deformation
Structural optimization
Avoid using long arm elbows with a center distance of ＞2.5D (use two 45° elbows instead)
The deformation rate of elbows with R≥2D is 40% lower than that of R=1.5D
4.What are the optimization measures for galvanizing tooling?
Special hanger: Use a three-point suspension fixture to make the elbow axis and the zinc liquid surface at a 75°~80° entry angle (reduce fluid impact).
Rotary zinc dipping: Rotate at a constant speed of 2-4rpm during dipping to ensure uniform coverage of the zinc layer (avoid unilateral overheating and deformation).
V-shaped cooling rack: Place it on a 120° V-shaped graphite bracket immediately after leaving the zinc pot to limit radial shrinkage deformation.
5.What are the testing standards?
Ovality: Laser 3D scanning, tolerance: ≤1.0mm for DN≤80mm; ≤2.5mm for DN＞200mm.
Flatness: Flange surface is tested with flat plate + feeler gauge, deviation ≤0.3mm/m.
Repair specification: Local heating correction temperature ≤380℃ (zinc layer melting point 419℃), zinc-rich paint (containing zinc ≥85%) needs to be sprayed after correction.