Conservation of Cast & Wrought Iron

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2  Properties of Cast and Wrought Iron

Cast & wrought iron have widely differing properties which complement each other. In the table below, steel has been included for comparison.
Wrought iron Cast iron Mild steel
Almost pure iron, < 1% C

Silicate slag content up to 4%

Around 4%C 0.15-0.4 %C 
Fibrous wood-like structure Crystalline structure Crystalline structure
Ductile, malleable (forgeable)  Brittle, poor resistance to  mechanical/thermal shock Ductile, malleable
Good in tension Good in compression Good in t & c
Readily fire-welded Difficult to weld Readily welded
More corrosion-resistant than steel More corrosion-resistant than wrought iron Corrodible
Relatively soft Can chill hard in the mould Tough
  Good vibration-damping Fatigue-resistant

Permissible Working Stresses Tones/ (N/

London Building Acts, 1909, the first official specification, calculated on Safety Factors of 5-8 allowing for variable quality, manufacturing defects, etc.
Cast iron 1.5 (23) 8 (124) 1.5 (23) 10 (154)
Wrought iron 5 (77) 5 (77) 4 (62)  7 (108)
Mild steel 7.5 (116) 7.5 (116) 5.5 (85)  11 (170)

Distinguishing Cast & Wrought Iron

Shape, Design - Iron castings are often heavier than wrought iron or steel sections, and may form recognisable components such as columns, beams, monolithic panels etc. Cast iron is often used in compression, wrought iron in tension.

Mould line may be visible, and there may be a mismatch between pattern halves, indicating C.I.

Construction - Riveted structures are usually wrought iron, possibly steel, never cast iron. Fire/forge welded joints indicate wrought iron.

Surface - If hammered or incised, probably wrought iron. Blow holes, porosity and inclusions indicate cast iron.

Cut a sliver with a cold chisel - Wrought iron gives a curled shaving, cast iron chips.

Partially cut then bend - Cast iron breaks with a crystalline fracture, wrought iron bends, showing a fibrous woodlike structure.

Place against a power grinder, observe sparks - Cast iron gives ragged reddish sparks, wrought iron narrow white sparks.

Laboratory tests - to establish mechanical properties (yield/tensile strength, % elongation at fracture) and metallurgical tests may be required to distinguish wrought iron from steel.

©2007 University of the West of England, Bristol
except where acknowledged
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