The 3PE anti-corrosion coating consists of an epoxy powder layer, an adhesive layer, and a polyethylene layer. The epoxy powder layer provides adhesion and chemical resistance. The thickness of the epoxy powder layer varies depending on the pipe diameter and grade requirements: no less than 120 μm for DN≤500 mm and no less than 150 μm for DN>500 mm; the total thickness can reach 2.7–2.9 mm in reinforced anti-corrosion applications.

1. Dimensional Tolerance: This refers to the allowable dimensional deviation range during the production of shaped steel pipes. The size of the dimensional tolerance directly affects the quality and performance of the shaped steel pipe. Generally, the smaller the dimensional tolerance, the higher the quality of the shaped steel pipe.
2. Appearance Quality: This refers to the absence of obvious defects on the surface of the shaped steel pipe, such as cracks, inclusions, and rust. Good appearance quality not only improves the aesthetics of the product but also reduces problems during subsequent processing and use.
3. Mechanical Properties: This refers to the good strength, toughness, and hardness of the shaped steel pipe. The levels of these properties directly determine the safety and reliability of the shaped steel pipe during use.
4. Chemical Composition: This refers to the content of each element in the shaped steel pipe meeting specific standard requirements. A reasonable chemical composition not only ensures that the performance indicators of the shaped steel pipe meet the requirements but also improves its corrosion resistance and wear resistance.

Regarding the classification of special-shaped pipes:
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The core distinction between Q345B and Q345D seamless steel pipes lies in Q345D's superior low-temperature impact toughness at -20°C, achieved through stricter composition control and heat treatment processes. This makes it suitable for critical engineering projects in cold environments, whereas Q345B is primarily used in ambient temperature scenarios. Their differing performance characteristics result in distinct pricing and application boundaries.