Steel toughened by pancakes

New treatment set to improve steel's properties at low cost.

Many US 'Liberty ships' succumbed to brittleness in chilly northerly waters. Credit: Project Liberty Ship

Japanese materials scientists have figured out a way to toughen steel without the need for large amounts of expensive additives, paving the way for better buildings, ships and machinery. The technique involves 'cooking' and deforming the steel to form a mosaic of microscopic, pancake-shaped grains.

Each of these grains is peppered with tinier specks of hard material, explain Yuuji Kimura and colleagues at the National Institute for Materials Science in Tsukuba. The stacked grains suppress brittleness by stopping cracks from spreading.

Steels, like most metals used in structural engineering, are ductile under their normal conditions of use: they bend rather than snap. But if they get too cold, they become brittle and prone to fracturing as tiny cracks surge through them.

This problem was spectacularly evident in the 'Liberty ships' — cargo vessels made cheaply and rapidly in the Second World War for ferrying supplies across the Atlantic to Europe. Cracks occurred in about one-third of these ships, partly because the cold ocean waters turned the steel brittle.

Cold-induced brittleness is also a problem for pipelines and offshore ocean structures that work in cold conditions. One expensive way of improving steel's resistance to cracking is to add large amounts of other metals such as nickel to the alloy mix, making so-called high-alloy steels.

Another approach to toughening involves using heat and pressure to manipulate the microscopic structure of the metal, which is a patchwork of tiny grains. In particular, a layered, laminated microstructure of stacked, flattened grains reduces brittleness because cracks moving through the material may be deflected parallel to the grains.

The laminated layers split apart (delaminate), and in doing so, they absorb and dissipate the energy of the crack before it gets very far. Mother-of-pearl and plywood are toughened by the same principle — but in the case of steel this suppression of brittleness at low temperatures typically comes at the cost of reducing toughness at higher temperatures.

'Tempforming'

Kimura and his team have now found a way to create a laminated microstructure in a low-alloy steel (with low amounts of extra ingredients added to the iron/carbon mix) that retains strength and toughness while reducing the temperature at which the material becomes brittle.

Their process, called 'tempforming', is described in this week's Science 1.

The method uses a low-alloy steel containing small quantities of silicon, chromium and molybdenum. When heated to 500 °C, tiny crystals of iron carbide, a very strong, hard compound, form inside the metal, boosting its strength. Kimura's team then applied a kind of high-pressure 'rolling pin' to the metal, flattening the grains into plate-like shapes that line up to give a fibrous appearance. When the tempformed steel finally does break, it looks rather like snapped bamboo.

This texture improves the material’s toughness at low temperatures, where brittleness threatens to set in, by the 'layering' mechanism that deflects cracks. The steel actually gets tougher as it gets colder, until the toughness drops suddenly at about –60 °C, when it finally becomes brittle.

The resulting metal could be used in steel components that have to withstand very high strains, such as bolts, Kimura suggests. "An ultra-high-strength bolt should not only reduce the number of bolts used in a construction, but could also allow new types of construction, leading to reduced weight of automobiles, buildings and bridges," he explains.

Promising future

“I have not seen a combination of properties this good in anything other than very high-alloy steel,” says John Morris, a metallurgist at the University of California at Berkeley. Patented steel wire, a high-toughness material known for about a century that is made by drawing out steel into wires, also has such a tough, fibrous microstructure. But it can be made only as fine wire strands, not as lumps or sheets.

And crucially, the tempformed steel is no less tough in the ductile region than ordinary steel, because the fibrous microstructure allows delamination to occur without introducing weak boundaries between layers.

Because the amount of other metals added to the alloy is low, says Morris, “the material would, potentially, be much less expensive than the competitive ultra-high-strength alloys with comparable toughness”. But he adds that the fibrous structure makes the new material tougher in some directions than others, like wood. So “it clearly is not suitable for some applications, for example sheets or plates loaded in torsion”, he says. “But it should work for bars or tubes, and perhaps other shapes.”

Tempforming looks promising, but is literally up against tough competition. In March, researchers at the University of Cambridge, UK, and the UK Defence Science and Technology Laboratory reported a process for making a type of steel called Super Bainite, an ultra-hard, non-brittle steel that also does not need expensive alloy additives.

Super Bainite, which Morris calls “fascinating”, has already tested well for use as armour on military vehicles, and the new process requires temperatures of no more than about 150 °C, helping to keep manufacturing costs down.

References

  1. 1

    Kimura, Y., Inoue, T., Yin, F. & Tsuzaki, K. Science 320, 1057-1060 (2008).

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Ball, P. Steel toughened by pancakes. Nature (2008) doi:10.1038/news.2008.851

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