Specify forged flanges for all pressure classes above Class 300, all low-temperature service below -29°C, all sour-service (NACE) applications, and any cyclic-service condition. Cast flanges are acceptable for Class 150 and Class 300 utility service where cost is the primary driver. Forging refines the grain structure, eliminating porosity and improving mechanical properties.

Visual: forged flanges have a smooth, fine-grained surface texture; cast flanges have a coarse, slightly rough surface with visible parting lines. Manufacturing: the flange is forged when the raw material is a forged bar or billet machined to shape; cast when molten metal is poured into a sand or investment mold. Per ASME B16.5, forged flanges are mandatory for all pressure classes above Class 250.

Forged flanges are typically 40-80% more expensive than cast flanges of equivalent size and class. For Class 150 NPS 2": forged ~$8-12/piece vs cast ~$4-6/piece. For Class 600 NPS 12": forged ~$120-180/piece vs cast ~$60-100/piece. The cost difference increases with size and pressure class.