Designed for use in changing diameters in a piping system. A reducing flange consists of a flange with one specified diameter having a bore of a different and smaller, diameter.
Specifications:

Reducing Flanges
Size: 1/2” to 60”, DN 12mm to DN 1500mm view...
Pressure class: Class 150 to Class 2,500, PN 2.5 to PN 250
Facing: RF / RTJ
Material:view...
Cabon steel: A105, A350 LF2
Stainless steel: A182 F304/F304L/F316/F316L/F321
Alloy steel: F11, F12
Packing: plywood case and plywood pallet
Except for the bore and hub dimensions, the flange will have dimensions of the larger pipe size. Reducing flanges are normally provided as one of three types:
Welding Neck flange
Slip-On flange
Threaded flange
Uses of Reducing Flanges:
Reducing flanges are used in pipe-to-pipe connections.
Reducing flanges are a little easier to bolt up than non-reducing flanges.
They provide the most perfect solution when you need to connect different size flanges.
These flanges are available in all types and pressures.
Reducing flanges are fixed by welding, gluing or clamping flanges of equal dimensions provided with different connecting pieces.
They offer the simplest and least expensive method of mating two different sized flanges.