Iron and Steel

Photograph of scrap steel being processed

Library of Congress. Via Wikimedia Commons

Iron is the second most abundant metal in the Earth’s crust, and its alloys (steels) were the sinews of the industrial powers of 1941. Steels can be manufactured with a wide range of properties using abundant iron ore, coal, and limestone and less abundant alloying metals such as manganese, nickel, chromium, tungsten, and molybdenum.

Iron is obtained from its ore by heating a mixture of iron ore, coke, and limestone to high temperatures in a blast furnace while forcing air through the mixture. The coke is oxidized to carbon monoxide, which combines with the oxygen in iron ore (which is mostly a mixture of iron oxides) to produce carbon dioxide and iron metal. The limestone combines with impurities in the iron, particularly silicon and phosphorus, to form a slag that separates from the liquid iron and can be easily removed.

The coke used in iron smelting is produced from coal that is heated in airless ovens to drive off volatile impurities. The volatiles are themselves flammable and may be burned to provide part of the energy to heat the coke ovens. If the coal is of the appropriate quality, the resulting coke is composed of hard lumps of carbon mixed with residual minerals ("ash") that hold up well against crushing when packed into the blast furnace.This is important to ensure that heated air can flow freely through the mixture during smelting.

The iron produced in this process, known as “pig iron”, still has substantial impurities, primarily silicon and carbon. These make the iron hard and resistant to corrosion, but also brittle and unworkable. Further refining removes some of the silicon and carbon to make various grades of carbon steel. Steels with little carbon, known as mild steels, are tough, ductile, and easily worked, but lack hardness and durability. High-carbon steels are very hard, but must be carefully treated to give them acceptable workability and toughness. Medium steels strike a balance between hardness and workability and make up most structural steel. Iron with the highest carbon content is so unworkable that it must be cast rather than machined; cast iron is extremely hard and durable, but so brittle that it is used only where resistance to wearing or blows is paramount.

Most of the world's steel in 1941 was produced in open hearth furnaces, which converted a mixture of scrap iron and pig iron into a batch of fresh steel in a process taking approximately 8 hours. In the United States, ample scrap steel was available as girders from demolished buildings, junked automobiles and railroad cars, obsolete machinery, and scrapped ships, but Japan had only recently become an industrial power and lacked a base of scrap steel for new steel production by the open hearth process. As a result, Japan imported considerable quantities of scrap steel from the United States in the 1930s, and 66,118 tons of scrap steel from the Netherlands East Indies in 1940. Scrap steel became a target of some of the early embargoes against Japan. However, by then Japan had accumulated a stockpile of 5.7 million tons of scrap.

The most advanced alloy steel was manufactured in electric furnaces, which required a great deal of electrical power but provided hotter temperatures and more precise control. Electric furnaces accounted for just 2% of American steel production in 1940, but Republic Steel and Bethlehem Steel both began construction of two giant electric steel furnaces each as war clouds gathered. Japan had electric steel furnaces at Hungnam that used power from extensive hydroelectric development of northern Korea.

It is also possible to improve the qualities of pig iron by working the iron until its impurities separate into grains or flakes embedded in a matrix of nearly pure iron. This wrought iron is very ductile, but also quite soft.

Steel production in the United States peaked at 80 million tons in 1945. However, even the vast production capacity of the United States was considered inadequate early in the war, making steel plate a limiting resource, and additional steel mills for rolling steel plate were completed by 1944. In the meantime, strip mills that had been producing thin strip steel for the automobile industry were converted to production of steel plate, and shipyards changed their methods to make use of unsheared plate, since the strip mills were not equipped to shear plate. Consideration was given to modifying Liberty Ship construction methods to use steel plates no more than 72 inches (1.83m) wide, the maximum width considered practical for strip mills. However, redesigning the Liberty Ships proved costly, and when many of the strip  mills devised ways to roll acceptable plate of greater than 72" width, the redesign was dropped. The use of strip mills permitted production of steel plate to increase by 50 per cent by April 1942.

An unfortunate consequence of the combination of rushed wartime production and the increased use of welding in shipbuilding was an increase in the incidence of catastrophic failure by brittle fracture. Low-quality steel, particularly when chilled, becomes brittle and prone to cracking. In a riveted ship, a crack cannot easily propagate further once it hits the edge of a plate. In a welded ship, a crack can propagate across welds to great distances. The most spectacular such failure was that of SS Schenectady, a brand-new T2 tanker that abruptly broke in half while docked at Swan Island on 16 January 1943. The entire strength deck and sides of the hull were split and only a narrow strip of plating on the very bottom of the hull remained intact as the ship jackknifed upwards out of the water. The Schenectady incident received wide press coverage, but some 24 other ships suffered catastrophic fractures, and eight of these were lost at sea as a result.

Investigators concluded that the fractures were the result of poor welds that locked in stress. Design flaws that produced stress concentrations, such as square hatch corners, were also blamed, as was poor quality steel plate. Designs were revised, crack arrestors were added to new construction, and considerable effort was made to improve welding technique.  Shipyards enforced new welding sequences that purportedly reduced the potential for locking in stress. Later investigators dismissed the role of locked-in stress, concluding that quality of steel was at least as serious a culprit as poor welding, and that a higher standard of quality for toughness of steel plate was necessary.

Most of the world’s great industrial powers of 1941 were nations that were blessed with iron ore in close proximity to coal fields. Britain, the first great industrial power, had both coal and iron ore within a few miles of each other in its Midlands region. This led to the growth of the industrial cities of Manchester and Birmingham. Germany likewise had coal and iron ore in close proximity in its Ruhr region, along with navigable waterways for transporting the raw materials to the refineries. The United States had vast coal fields in the Appalachians and huge iron ore deposits in Minnesota, with the Great Lakes and a network of canals and rivers forming a highway between the two. 

Japan had no such advantage as it began its industrialization at the end of the 19th century. There are significant coal fields in Hokkaido and Kyushu, but these are not coking quality, and Japan has no large deposits of high quality iron ore. Japan therefore sought iron ore and coking coal from overseas, as it still does today.

In 1941, the Japanese Empire included Korea and Manchuria, which had large deposits of iron ore. Manchuria also had large deposits of coal, such as that at Fushun, as did Korea, and additional coking coal came from the Kaiping and Luanchow fields of North China. Special steel alloys were manufactured from pig iron in Korea using electric furnaces drawing on the considerable hydroelectric potential at sites such as Fusenko. Japan would seize additional iron ore fields in the Philippines and the Netherlands East Indies, but there was never enough shipping to adequately exploit these. Japanese iron production peaked at 5.6 million tons in 1944.

Iron production in China was badly disrupted by the war, peaking at just 10,000 tons per year in 1943. This was a tenth of the 1931 figure.

Alloying Metals

Most steels contain at least traces of alloying metals. Most U.S. steel manufacturers of 1941 used 20 to 40 pounds of manganese per ton of steel. The manganese removes residual sulfur and hardens the steel (especially in combination with silicon) and is nearly indispensable for producing high-quality steel from marginal ore. Steel that is not completely desulfured is brittle, particularly at low temperatures, and the use of low-quality steel led to structural failure of a number of Maritime Commission standard ships.

Nickel increases toughness and chromium hardens the steel; the combination of the two yields very hard, tough steel for armor plate. Stainless steels, with high resistance to corrosion, are also produced using nickel and chromium. Molybdenum can be used to increase the hardness of low-carbon steels, which is particularly useful for producing weldable steel and armor, though this technique was not fully developed until after the Pacific War. Vanadium increases both hardness and the resistance of steel to fatigue from repeated loading and unloading, but it is very expensive. Tungsten is used in high-temperature steels such as those used in machine tools and for turbocharger blades.

In a pinch, unconventional alloying metals can replace the more conventional metals. For example, the Japanese sometimes used copper in place of nickel in armor plate, because Japan had very little access to nickel, while copper is one of the few metals that was mined in any quantity in the Japanese home islands.

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