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, coal, and limestone to high temperatures while forcing air through the mixture. The coal 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 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.
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.
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. Production in the
United States alone peaked at 80 million tons in 1945.
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 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, but
the ore in Korea was refined using electric
furnaces drawing on the considerable hydroelectric
potential at sites such as Fusenko.
Coking coal
came from the Kaiping
and Luanchow
fields of North China.
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. Production
peaked at 5.6 million tons in 1944.
Iron prodution 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.
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. 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.
References
Ellis (1995)U.S. Geological Survey (accessed 29 December 2006)
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