Wrought iron

Wrought iron is commercially pure iron made by melting white cast iron and passing an oxidizing flame over it, leaving the iron in a porous condition, which is then rolled to unite it into one mass. As thus made, it has a fibrous structure, with fibers of slag through the iron in the direction of rolling. It is also made by the Aston process of shooting Bessemer iron into a ladle of molten slag. Modern wrought iron has a fine dispersion of silicate inclusions that interrupt the granular pattern and give it a fibrous nature.

Structurally, wrought iron is a composite material; the base metal and the slag are in physical association, in contrast to the chemical or alloy relationship that generally exists between the constituents of other metals.

The form and distribution of the iron silicate particles may be stringerlike, ribbonlike, or platelets. Practically, the physical effects of the incorporated iron silicate slag must be taken into consideration in bending and forming wrought iron pipe, plate, bars, and shapes, but when properly handled — cold or hot — fabrication is accomplished without difficulty.

Mechanical Properties

The value of wrought iron is in its corrosion resistance and ductility. It is used chiefly for rivets, staybolts, water pipes, tank plates, and forged work. Minimum specifications for ASTM wrought iron call for a tensile strength of 275 MPa, yield strength of 165 MPa, and elongation of 12%, with carbon not over 0.08%, but the physical properties are usually higher. Wrought iron 4D has only 0.02% carbon with 0.12% phosphorus, and the fine fibers are of a controlled composition of silicon, manganese, and phosphorus. This iron has a tensile strength of 330 MPa, elongation 14%, and Brinell hardness 105. Manganese wrought iron has 1% manganese for higher impact strength.


Ordinary wrought iron with slag may contain frequent slag cracks, and the quality grades are now made by controlled additions of silicate, and with controlled working to obtain uniformity. But for tanks and plate work, ingot iron is now usually substituted.

The Norway iron formerly much used for bolts and rivets was a Swedish charcoal iron brought to America in Norwegian ships. This iron, with as low as 0.02% carbon, and extremely low silicon, sulfur, and phosphorus, was valued for its great ductility and toughness and also for its permeability qualities for transformer cores. Commercial wrought iron is now usually ingot iron or fibered low-carbon steel.

The tensile properties of wrought iron are largely those of ferrite plus the strengthening effect of any phosphorus content, which adds approximately 6.8 MPa for each 0.01% above 0.10% of contained phosphorus. Strength, elasticity, and ductility are affected to some degree by small variations in the metalloid content and in even greater degree by the amount of the incorporated slag and the character of its distribution.

Nickel, molybdenum, copper, and phosphorus are added to wrought iron to increase yield and ultimate strengths without materially detracting from toughness as measured by elongation and reduction in area.

Fabrication Forging

Wrought iron is an easy material to forge using any of the common methods. The temperature at which the best results are obtained lies in the range of 1149 to 1316°C. Ordinarily "flat and edge" working is essential for good results. Limited upsetting must be accomplished at "sweating to welding" temperatures.

Bending

Wrought-iron plates, bars, pipe, and structurals may be bent either hot or cold, depending on the severity of the operation, keeping in mind that bending involves the directional ductility of the material. Hot bending ordinarily is accomplished at a dull red heat (704 to 760°C) below the critical "red-short" range of wrought iron (871 to 927°C). The ductility available for hot bending is about twice that available for cold bending. Forming of flanged and dished heads is accomplished hot from special-forming, equal-property plate.

Welding

Wrought iron can be welded easily by any of the commonly used processes, such as forge welding, electric resistance welding, electric metallic arc welding, electric carbon arc welding, and gas or oxyacetylene welding. The iron silicate or slag included in wrought iron melts at a temperature below the fusion point of the iron-base metal, so that the melting of the slag gives the metal surface a greasy appearance. This should not be mistaken for actual fusion of the base metal; heating should be continued until the iron reaches the state of fusion. The siliceous slag content provides a self-fluxing action to the material during the welding operation.

Threading

The machinability or free-cutting characteristics of most ferrous metals are adversely influenced by either excessive hardness or softness. Wrought iron displays almost ideal hardness for good machinability, and the entrained silicate produces chips that crumble and clear the dies.

Standard threading equipment that incorporates minor variations in lip angle, lead, and clearance is usually satisfactory with wrought iron.

Protective Coatings

Wrought iron lends itself readily to such cleaning operations as pickling and sandblasting for the application of protective coatings. Where protective coatings such as paint or hot-dipped metallic coatings are to be applied, the coatings are found to adhere more firmly to wrought iron and a thicker coat will be attained compared with other wrought ferrous metals. This is because the natural surface of wrought iron is microscopically rougher than other metals after cleaning, thus providing a better anchorage for coatings.

Corrosion Resistance

The resistance of wrought iron to corrosion has been demonstrated by long years of service life in many applications. Some have attributed successful performance to the purity of the iron base, the presence of a considerable quantity of phosphorus or copper, freedom from segregation, to the presence of the inert slag fibers disseminated throughout the metal, or to combinations of such attributes.

In actual service, the corrosion resistance of wrought iron has shown superior performance in such applications as radiant heating and snow-melting coils, skating-rink piping, condenser and heat-exchanger equipment, and other industrial and building piping services. Wrought iron has long been specified for steam condensate piping where dissolved oxygen and carbon dioxide present severe corrosion problems. Cooling water cycles of the once-through and open-recirculating variety are solved by the use of wrought-iron pipe.

Applications

Building Construction — Hot and cold potable water, soil, waste, vent, and downspout piping; radiant heating, snow melting, air-conditioning cooling and chilled-water lines; gas, fire protection, and soap lines; condensate and steam returns, ice-rink and swimming-pool piping; underground service lines and electrical conduit.

Industrial — Unfired heat exchangers, brine coils, condenser tubes, caustic soda, concentrated sulfuric acid, ammonia, and miscellaneous process lines; sprinkler systems, boiler feed and blowoff lines, condenser water piping, runner buckets, skimmer bars, smokestacks and standpipes, salt and water well pipe and casing.

Public Works/Infrastructure — Bridge railings, fenders, blast plates, drainage lines and troughs, traffic signal conduit, sludge digestor heating coils, aeration tank piping, sewer outfall lines, large outside diameter intake and discharge lines, trash racks, weir plates, dam gates, pier-protection plates, sludge tanks and lines, dredge pipe.

Railroad and Marine — Tie spacer bars, diesel exhaust- and air-brake piping, ballast and brine protection plates, brine, cargo and washdown lines on ships, hull and deck plating, rudders, fire screens, breechings, tanker heating coils, car retarder and yard piping, spring bands, car charging lines, nipples, pontoons, car and switch deicers.

Others — Gas collection hoods, stay-bolts, flue gas conductors, sulfur mining gut, air and transport lines, coal-handling equipment, chlorine, compressed air lines, distributor arms, cooling tower and spray pond piping.

Wrought iron is available in the form of plates, sheets, bars, structurals, forging blooms and billets, rivets, chain, and a wide range of tubular products, including pipe, tubing and casing, electrical conduit, cold-drawn tubing, and welded fittings.

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