Conservation of Cast & Wrought Iron

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
	Machinable

Permissible Working Stresses Tones/sq.in. (N/sq.mm.)

London Building Acts, 1909, the first official specification, calculated on Safety Factors of 5-8 allowing for variable quality, manufacturing defects, etc.

	TENSION	COMPRESSION	SHEAR	BEARING
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.