Iron vs Steel: Pure Iron, Cast Iron, Wrought Iron and Steel Compared

Iron is a chemical element, symbol Fe, and steel is an alloy of iron with less than about 2.1 percent carbon. In ordinary use the word iron also covers two historic commercial materials, cast iron and wrought iron, and the three are told apart by how much carbon they contain and in what form. Cast iron has more than about 2.1 percent carbon, most grades 3 to 4 percent, with 1 to 3 percent silicon; it melts at a lower temperature than steel and is shaped by pouring into moulds, but it cannot be rolled or forged. Wrought iron is almost pure iron, with under 0.1 percent carbon and up to about 2 percent of slag drawn out into fibres; it was worked by hammer and rolls, and its commercial production ended in the 1970s. Steel holds enough carbon, often with other alloying elements, to be strong and to respond to heat treatment, and little enough to stay ductile, so it can be rolled, forged and welded. In service the difference shows at failure: a grey iron casting is strong in compression but snaps in tension with almost no stretch, while a steel bar bends a long way before it breaks.

What iron is

Pure iron is a soft, ductile, silver-grey metal with a density of 7,874 kilograms per cubic metre and a melting point of 1,538 degrees Celsius. It is too weak for most structural work. Nearly all iron leaves the blast furnace as pig iron, which carries about 4 percent carbon together with silicon, manganese, sulphur and phosphorus taken up from the ore and the coke. Pig iron is brittle and is not used as it comes from the furnace. It is remelted with scrap to make cast iron, or refined to steel by blowing oxygen through it to burn the carbon down. Steel is also made by melting scrap in an electric arc furnace, a route that bypasses pig iron altogether.

Where iron ends and steel begins

The dividing line comes from the iron-carbon phase diagram. At high temperature iron takes a face-centred cubic structure called austenite, which dissolves carbon up to a maximum of about 2.1 percent at 1,147 degrees Celsius. An alloy below that limit can be heated until all of its carbon is in solid solution, and then rolled, forged and heat treated as a single solid: that alloy is steel. Above the limit, part of the metal stays liquid down to the eutectic temperature of 1,147 degrees Celsius, reached at 4.3 percent carbon. That is nearly 400 degrees below the melting point of pure iron, which is why cast iron is cheap to melt and runs easily into a mould. On cooling, the carbon that cannot dissolve comes out as graphite or as iron carbide, and a metal full of those particles cannot be worked hot. Published values for the limit run from 2.08 to 2.14 percent, and most references round it to 2.1. The melting point article covers how carbon lowers the melting range.

Cast iron: grey, ductile, white and malleable

Cast iron is named after the process that shapes it. Its properties depend on the form the surplus carbon takes, and that is set by the silicon content, the cooling rate in the mould and any treatment of the liquid metal.

Grey cast iron holds its carbon as flakes of graphite, which give a fractured surface its grey colour. The flakes act as internal notches, so grey iron is weak in tension and stretches less than 1 percent before it breaks, but it is several times stronger in compression than in tension. The same flakes break up the chips in machining and absorb vibration. These properties make grey iron the usual material for machine tool beds, cylinder blocks, brake drums and discs, and pump and valve bodies. The standard EN 1561 classifies it by minimum tensile strength, from EN-GJL-100 to EN-GJL-350 in megapascals, and ASTM A48 does the same in thousands of pounds per square inch, from class 20 to class 60.

Ductile iron, also called spheroidal graphite or nodular iron, is cast iron treated with magnesium just before casting, so that the graphite forms round nodules instead of flakes. Without the sharp edges of the flakes the metal bends before it breaks. The EN 1563 designations run from EN-GJS-350-22, with 350 megapascals tensile strength and 22 percent elongation, to EN-GJS-900-2. Ductile iron was developed in the 1940s and is used for water and sewer pipe, crankshafts, gears and vehicle suspension parts.

White cast iron holds its carbon as iron carbide, called cementite, because low silicon or fast cooling prevents graphite from forming. The fracture is white, and the metal is hard, resistant to abrasion and brittle. It is too hard to cut with ordinary tools and is finished by grinding. Alloyed white irons are used for mill liners, crusher parts and slurry pumps. Malleable iron starts as a white iron casting and is annealed for a long time. The anneal turns the carbide into compact clusters of graphite and gives a ductile casting, used for pipe fittings and other small parts.

Wrought iron

Wrought iron is iron that was never fully melted. In the bloomery, and later in the puddling furnace patented by Henry Cort in 1784, the iron was brought to a pasty mass below its melting point, then hammered and rolled to squeeze out most of the slag. What remained was nearly pure iron with fibres of siliceous slag drawn out along the bar. The fibres give wrought iron a grain like wood, which shows when the metal is etched, rusted or bent to failure. The metal is tough and ductile, and it forge welds readily because the slag acts as a flux, although it is harder to weld by electric arc. It contains too little carbon to harden by quenching. Wrought iron was the engineering iron of the nineteenth century, used for chain, rivets, bolts, rails, pipes and structural members. The Eiffel Tower, completed in 1889, contains about 7,300 tonnes of puddled iron.

Steel

Steel is defined by the 2.1 percent limit, but nearly all steel contains far less carbon. Structural mild steel holds up to about 0.25 percent, shaft and axle steels such as EN8 about 0.4 percent, and bearing steels about 1 percent. Manganese and silicon are present in every steel. Chromium, nickel, molybdenum and vanadium are added to make alloy steels, and at 10.5 percent chromium or more the alloy becomes stainless steel, described in the stainless steel manufacturing article. At rolling temperature a carbon steel is a single solid phase, austenite, so it can be cast into billets and then hot rolled or forged to any section. Above about 0.3 percent carbon it can also be hardened by quenching and tempered to a chosen strength. Mild steel has a yield strength of about 250 megapascals and stretches more than 20 percent before breaking. Hardened and tempered alloy steel can exceed 1,000 megapascals. The carbon, alloy and stainless steel comparison sets out the three steel families.

Cast iron, wrought iron and steel compared

Why steel replaced wrought iron

Until the middle of the nineteenth century steel was made in small batches and cost too much for structures, and wrought iron was produced by puddlers who stirred each charge by hand. Henry Bessemer patented his converter in 1856. It blew air through molten pig iron and turned a whole charge into steel in minutes. The open hearth furnace followed in 1865, when Pierre-Emile Martin applied the regenerative furnace of Carl Wilhelm Siemens to steelmaking. Mild steel from these processes did the work of wrought iron at a lower cost, and demand for wrought iron fell away. The last wrought iron maker, the Atlas Forge in Bolton, England, closed in 1973. In the second half of the twentieth century the basic oxygen converter, first operated at Linz in 1952, and the electric arc furnace replaced the open hearth, and most open hearth furnaces had shut by the early 1990s.

Where each is used today

Cast iron remains the economical choice for complex shapes that carry load in compression, damp vibration or resist wear, and ductile iron is used where a casting must also bend without breaking. Wrought iron is no longer made from new iron. Small quantities are reworked from recycled wrought iron for the repair of historic structures, and most gates and railings sold today as wrought iron are mild steel. Steel is used for nearly every product that is rolled, forged or drawn, from reinforcing bar and structural sections to precision shafts and fasteners.

Steel at Laxcon Steels

Laxcon Steels makes steel, not cast iron or wrought iron. It melts stainless, alloy and carbon grades in an electric arc furnace, refines them in an argon oxygen decarburisation converter and casts them as continuous cast billets and blooms. The billets are rolled in the company's own mills into products such as hot rolled round bar from 16 to 125 mm. The carbon limit of each grade is listed in the grade reference.

Frequently asked questions

Is steel stronger than iron?

In tension, yes, for the common forms. Mild steel has a tensile strength of about 400 megapascals and stretches more than 20 percent before it breaks, while grey cast iron is graded from 100 to 350 megapascals and stretches less than 1 percent. Grey iron is several times stronger in compression than in tension, and the stronger grades of ductile iron overlap many steels.

What is the difference between cast iron and wrought iron?

The difference lies in carbon and slag. Cast iron has more than about 2.1 percent carbon, is melted and poured into moulds, and is brittle in tension. Wrought iron has under 0.1 percent carbon with fibres of slag, was never fully melted, and was shaped by hammering and rolling, which makes it tough and ductile.

Is wrought iron still made?

Not from new iron. Commercial production ended in the 1970s, and wrought iron for the repair of historic structures now comes from reworked recycled material. Most gates, railings and furniture sold as wrought iron are made of mild steel.

Does cast iron rust?

Yes. Cast iron, wrought iron and carbon steel contain too little chromium to form a protective oxide film, and all of them rust in damp air unless painted, coated or kept dry. Stainless steel, with at least 10.5 percent chromium, is the iron alloy that resists rusting.

Sources

• ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys: classification of cast irons, wrought iron.
• ASM Handbook, Volume 3, Alloy Phase Diagrams: the iron-carbon system.
• EN 1561, Founding: grey cast irons; EN 1563, Founding: spheroidal graphite cast irons.
• ASTM A48, Standard Specification for Gray Iron Castings; ASTM A536, Standard Specification for Ductile Iron Castings.
• IS 2062, Hot rolled medium and high tensile structural steel; BS 970-1, Wrought steels for mechanical and allied engineering purposes.
• Callister, W. D., Materials Science and Engineering: An Introduction, Wiley: the iron-carbon system and cast irons.