Malleable iron fittings are lighter fittings in 150# and 300# pressure class. They are made for light industrial and plumbing use up to 300 psi. Some malleable fittings such as floor flange, lateral, street tee and bullhead tees are not commonly available in forged pipe fittings.
Malleable iron offers more ductility that is often required in light industrial use. Malleable iron pipe fitting is not good for welding (if you ever need to weld something to it).
Malleable iron fittings, also called black iron fittings, are available up to 6 inch nominal pipe size, though they are more common to 4 inches. Malleable fittings include elbows, tees, couplings and floor flange etc. Forged Pipe Fittings are available as socket weld pipe fitting or threaded pipe fitting. Threaded pipe fittings and socket weld pipe fittings are available up to 4 inch nominal pipe size.
Forged Iron Threaded or Socket Weld Fittings are more expensive in sizes bigger than 2 inch compared to their malleable iron or butt weld counter parts.
Malleable Iron Fitting and Forged Pipe Fitting differ in material metallurgy, pressure class and application. It is important to select appropriately between them for the intended application.