Cast vs Forged Steel

Quick Answer

Cast steel is made by pouring molten metal into a mold. Forged steel is shaped by applying compressive force to heated solid steel. Forgings have a denser, refined grain structure that delivers higher tensile strength, better fatigue resistance, and superior impact toughness. Castings allow complex shapes and are more cost-effective for large valve bodies. Most piping systems use both: cast valve bodies above 2” and forged flanges, fittings, and small-bore valves.

Process Comparison

The fundamental difference lies in how the steel takes its shape. Casting solidifies liquid metal in a mold cavity. Forging mechanically deforms solid metal under high pressure.

Factor	Cast Steel	Forged Steel
Process	Molten metal poured into mold	Heated solid billet shaped under pressure
Grain structure	Coarse, random (dendritic)	Fine, directional (flow lines follow part geometry)
Porosity	Possible shrinkage cavities, gas porosity	Virtually zero internal porosity
Tensile strength	Standard for grade	10-20% higher than equivalent cast grade
Fatigue resistance	Lower	Significantly higher
Impact toughness	Lower (more variability)	Higher and more consistent
Shape complexity	Unlimited (complex internal passages)	Limited to simpler shapes
Size range	Small to very large (60”+ valve bodies)	Practical limits above ~30-36” diameter
Surface finish	Rough as-cast; requires machining	Better; less machining needed
Cost (small parts)	Higher (mold cost amortization)	Lower for standard shapes
Cost (large parts)	Lower	Higher (heavy equipment, more energy)
Typical NDT	RT, MT, PT	UT, MT
Lead time	Longer (pattern + casting + machining)	Shorter for standard items

Why Forgings Are Stronger

The forging process compresses the steel’s grain structure, eliminating voids and aligning grain flow with the part geometry. This produces:

• Higher tensile and yield strength for the same chemical composition
• Superior fatigue resistance: critical for components subject to pressure cycling
• Better impact toughness: fewer initiation sites for brittle fracture
• Greater consistency: less heat-to-heat variability than castings

A forged ASTM A105 flange and a cast ASTM A216 WCB valve body have similar chemistry but differ in mechanical performance due to manufacturing method alone.

Why Castings Exist

Despite mechanical disadvantages, castings are necessary:

• Complex internal geometries: Valve bodies with tortuous flow paths, multiple ports, and integrated bonnets are impractical to forge
• Large sizes: Gate valves in 24”, 30”, or 48” are routinely cast. Forging billets that size is prohibitively expensive
• Cost efficiency: For complex shapes, casting eliminates extensive machining from solid blocks

Piping Industry Conventions

Standard piping practice assigns:

Component	Typical Manufacturing	Common Specs
Valve bodies (>2”)	Cast	A216 WCB, A351 CF8M, A217 WC6/WC9
Valve bodies (<2”)	Forged	A105, A182 F316
Flanges (all sizes)	Forged	A105, A182, A350
BW fittings	Forged + formed	ASTM A234, A403
Forged fittings	Forged	A105, A182 per B16.11

Both cast and forged components require mill test certificates, appropriate NDT, and hydrostatic testing per the applicable standard.

Read the full guide to valve materials