Terminology

• Puddling: process that decarburizes pig iron into wrought iron
• Bloom: spongy mass of iron and slag worked by hammering
• Forge welding: joining by hammering at red heat
• Slag inclusions: fibrous silicate stringers, roughly 250,000 per square inch

History and demand

Invention

• 1784: Henry Cort patented puddling, widely used after 1800

First uses in construction

• 13th century: wrought iron gates at Westminster Abbey
• 1860s: peak demand; 1865 Bessemer process began replacing iron for rails
• 1876: UK output over 4 million tons of puddled iron
• By 1890: steel had largely replaced wrought iron for structural use

Demand trend

• Last US plant closed in 1969; last plant worldwide, Atlas Forge in Bolton, UK, closed in 1973
• Supply today is mostly reclaimed material such as old bridges and anchor chains; limited production for heritage restoration from recycled scrap

Composition and production

Raw materials

• About 99.4 percent iron by mass; carbon under 0.05 percent (cast iron 2.1 to 4.5 percent, mild steel about 0.25 percent)
• Slag up to 2 percent by weight; impurities (sulfur, phosphorus, silicon, manganese) under 0.25 percent total

Manufacturing process

• Puddling furnace removes carbon; the pasty iron is balled and rolled or hammered to squeeze out slag

Properties

Physical

• Specific gravity: 7.6 to 7.9
• Melting point: 2,800°F (1,540°C); softens at red heat

Mechanical

• Tensile strength: 34 to 54 ksi (234 to 372 MPa)
• Yield point: 23 to 32 ksi (159 to 221 MPa)
• Modulus (tension): 28,000 ksi (193,100 MPa)

Fire behavior

• Softens at red heat and can be forge welded

Durability

• Fibrous slag gives corrosion resistance; zinc galvanizing on wrought iron is 25 to 40 percent thicker than on steel

Uses in buildings

Building elements

• Guards and Railings: historic and restoration ironwork
• Site Fences and Gates: forged gates and fences

Pros and cons

Pros

• Workable by forging and forge welding
• Slag fibers resist corrosion

Cons

• Out of production; supply is reclaimed or limited
• About twice the cost of low-carbon steel (1960s)
• Modern look-alike products are mild steel

Design considerations

Compatibility

• Hot-dip galvanizing is possible on mild steel imitations, not on wrought iron

Manufacturing and cost

Main manufacturers and suppliers

Trade association: Artist-Blacksmith's Association of North America.

Sources

• Wikipedia: Wrought iron, https://en.wikipedia.org/wiki/Wrought_iron
• Wikimedia Commons: Wrought iron railing.jpg, photo by BenFrant (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Wrought_iron_railing.jpg