The eccentric reducer, also known as the size head, eccentric head, or size transition, is a type of chemical pipe fitting used to connect pipes of different diameters. Depending on their shape, eccentric reducers can be classified into concentric and eccentric types. The materials for size heads include stainless steel (304, 304l, 316, 316l), alloy steel, and carbon steel. The materials for size heads include No. 20 steel, Q235, Q345, 16Mn, and others.

Materials are classified as follows:
1. Carbon steel: ASTM/ASME A234 WPB, WPC

2. Alloy steel: ASTM/ASME A234 WP 1-WP 12-WP 11-WP 22-WP 5-WP 91-WP911, 15Mo3, 15CrMoV, 35CrMoV

3. Stainless steel: ASTM/ASME A403 WP 304-304L-304H-304LN-304N

4. ASTM/ASME A403 WP 316-316L-316H-316LN-316N-316Ti

5. ASTM/ASME A403 WP 321-321H, ASTM/ASME A403 WP 347-347H

6. Low-temperature steel: ASTM/ASME A402 WPL3-WPL 6

7. Cast steel, alloy steel, stainless steel, copper, aluminum alloy, plastic, argon asphalt, PVC, PPR, RFPP (reinforced polypropylene), and others.

Eccentric reducers can also be classified according to their manufacturing methods such as extrusion, pressing, forging, and casting. Additionally, they can be classified according to manufacturing standards such as Chinese standard, British standard, American standard, German standard, Japanese standard, Russian standard, and others.