June 23, 2008

Ferrous metallurgy takes leap forward

Construction Corner

Korky Koroluk

As the concept of sustainability has grown in importance in the construction industry, more and more scientists are working in fields that seem pretty esoteric to most of us.

Nanotechnology is allowing scientists to manipulate materials at the molecular level in order to develop special properties for special applications. But even some older technologies are finding new uses.

Now the ancient field of ferrous metallurgy has taken a leap forward and it could lead to stronger steel for use in everything from cars to buildings, oil rigs to bridges. It could also mean using less steel in those applications.

A team of Japanese materials scientists is using a technique involving cooking and deforming steel to form a mosaic of microscopic, pancake-shaped grains.

Each grain is peppered with tinier specks of hard material, and the end result is stacks of grains that suppress brittleness in steel by stopping cracks from spreading.

Steel, like most metals used in structural engineering, is ductile under normal conditions — that is, they bend rather than snap. But if they get too cold, they are prone to fracturing as tiny cracks surge through them.

This cold-induced brittleness is also a problem for pipelines and offshore ocean structures in cold conditions.

So far, improving resistance to cracking has been done by adding significant amounts of other metals such as nickel to the alloy mix, making what are called high-alloy steels. They’re expensive.

But the Japanese team, led by Yuuji Kimura, of the National Institute for Materials Science, has devised a process they call tempforming.

It involves low-alloy steel containing small amounts of silicon, chromium and molybdenum. When heated to 500 degrees, tiny crystals of iron carbide form inside the metal, increasing its strength.

What follows is heating, rolling, heating, rolling again until a crystalline structure develops, with the grains (which by this time have taken on the shape of a grain of rice) lined up along the bar, rather than across it.

This, the researchers found, reduces brittleness by absorbing the cracks’ energy. And that, in turn, means the steel doesn’t become brittle until its temperature drops to minus 100° C .

Kimura believes the tempformed steel could be used in steel components that have to withstand high strains, such as bolts.

“The 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. . . buildings and bridges,” he said.

An early fan of the steel is Bill Morris, Jr., a materials scientist at the University of California, Berkeley.

“(It) 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,” he said

Kimura foresees the use of his steel to reduce both the number of components needed in a construction job as well as their weight — by replacing solid members with hollow tubes, for example. That would mean less steel used in total on the project.

The work, said Morris, shows that there are still very interesting things to be done —and learned — in ferrous metallurgy.

Kimura has described his work in a paper published in the journal, Science, vol. 320, p 1057. Structural engineers who want to read it can download a copy from www.sciencemag.org/cgi/content/figsonly/320/5879/1057. It’s a free download for members of the American Association for the Advancement of Science. Non-members may purchase a copy.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@journalofcommerce.com.

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