STEEL is valued for its reliability when making everything from bolts to boats 鈥� but not when it gets cold. Most forms of steel abruptly become brittle at temperatures below about -25 掳C unless they are mixed with other metals. Now, though, a novel type of steel has been developed that resists fractures at much lower temperatures, while retaining its strength and toughness 鈥� without the need for expensive additives.
Steel鈥檚 fragility at low temperatures first became a major concern during the second world war. After German U-boats torpedoed numerous British ships, a 2700-strong fleet of cheap-and-cheerful 鈥淟iberty ships鈥� was introduced to replace the lost vessels, providing a lifeline to the beleaguered British. But the steel hulls of hundreds of the ships cracked in the frigid north Atlantic, and 12 broke in half and sank.
Brittleness remains a problem when building steel structures in cold conditions, such as oil rigs in the Arctic. So metallurgists have strived to lower the temperature at which steel 鈥� an alloy of iron and carbon 鈥� switches from ductile to brittle, usually by mixing it with expensive metals such as nickel and cobalt.
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Yuuji Kimura and colleagues at the National Institute for Materials Science in Tsukuba, Japan, tried a more physical approach. Rather than adding other metals, Kimura鈥檚 team developed a complex mechanical process involving repeated heating and very severe mechanical deformation. Because the process relies on temperature and deformation, the researchers call it tempforming.
First, a 100-kilogram ingot of steel is heated to 1200 掳C and flattened to form a 4-centimetre-thick steel plate. This plate is then rolled into a bar with a cross section of about 9 square centimetres, and quenched with water. The bar is then 鈥渢empered鈥� at 500 掳C for an hour, before being rolled 鈥� or more accurately, crushed 鈥� into a still smaller bar whose cross section is just 2 square centimetres.
The resulting steel has a crystalline structure, made up of rice-shaped grains, that induces any cracks to propagate along the bar, rather than across it. This reduces brittleness by absorbing the cracks鈥� energy. Kimura鈥檚 team found that tempformed steel only became brittle at -100 掳C (Science, vol 320, p 1057).
鈥淭his low-alloy steel appears to achieve a combination of strength and toughness that is comparable to that of modern steels that are very rich in alloy content and, therefore, very expensive,鈥� says Bill Morris, a materials scientist at the University of California, Berkeley. 鈥淚t shows that there are still very interesting things to be done 鈥� and learned 鈥� in the ancient field of ferrous metallurgy.鈥�
Kimura鈥檚 team intends to use its tempformed steel to make ultra-high strength parts, such as bolts. They hope to reduce both the number of components needed in a construction job and their weight 鈥� by replacing solid supports with hollow tubes, for example. This could reduce the amount of steel needed to make everything from automobiles to buildings and bridges.
