Feb 05 2015

A Better Steel

Material science seems to me to be an underappreciated discipline. Perhaps because its benefits are not seen directly by the consumer, but only indirectly. Material scientists don’t make a better gadget, but they make a better gadget possible. Sometimes a breakthrough can even be a complete game-changer for certain technologies.

Humans have been using an alloy of carbon and iron for over three thousand years. Iron is a very common element, making up about 5% of the Earth’s crust. Steel is iron with 0.2-1.5% carbon alloy. Carbon makes steel hard but brittle, and so carefully controlling the amount of carbon to optimize hardness but keep it malleable enough not to be brittle is what makes steel.

Steel is still on the cutting edge (pun intended) of material science. Researchers are still discovering ways to make steel lighter, stronger, and better suited to specific purposes. A recent paper, for example, presented a new technique for making blended steel that results in light, strong, and ductile steel – perfect for making more fuel efficient cars, for example.

A brief sidenote on terminology: “hardness” is the resistance to deformation by a force. There are different kinds of hardness, such as scratch hardness and compression hardness. “Strength” is the measure of a substances elastic range. “Toughness” is a measure of how much total energy a material can absorb before it breaks.

Different combinations of these properties are suited for difference purposes. In a cable you want a high tensile strength. For your stainless flatware you want hardness to resist scratching.

Because I’m a nerd, I wanted to know what the current state of the technology is in terms of material science for certain applications. Specifically, if you were going to make a sword that was the best possible sword modern technology could make, what would it be made out of? I was a bit surprised to find that the answer is still steel. But this led me to a deeper understanding of just how advanced modern steel technology is.

Variables in making steel include its total carbon content, types and amounts of other alloys, impurities, and the precise way in which it is heated and cooled. There is no one perfect steel – there are different kinds of steels for different applications.

For example, chromium alloy makes steel corrosion resistant resulting in stainless steel. You would not want a stainless steel sword, however, because stainless steel is more brittle. Here is a demonstration of why you would not want to be in a battle with a stainless steel sword.

Tool steels use alloys to make them heat resistant and more durable. Tungsten and molybdenum have higher melting points than iron, while cobalt and vanadium make the steel more durable.

Other alloys include manganese, silicon, nickel, titanium, copper, and aluminum. By varying these alloys you can make steel lighter, more springy, or more malleable.

The new techniques described above uses alloys of nickel, aluminum, and magnesium. It also, however, involves heating and cooling to affect the microstructure of the steel. It is the microstructure that affects the properties of the steel. When the iron atoms form a crystalline structure, that makes the steel soft and malleable because the atoms can easily slide past each other. An amorphous arrangement is much harder because there is no plane of slippage, but a completely amorphous steel would be brittle.

The techniques currently being developed attempt to use alloys, heating, and cooling to create just the right combinations of microstructures to have essentially the best of both worlds.

It seems incredible that after centuries of perfecting steel manufacturing, material scientists are still making advances in steel technology. This makes me wonder how much potential there is left in steel tech. Specifically, will nanofabrication techniques one day result in a “nanosteel” that has the optimal properties this element can have?

How light and strong can we possibly make a nanosteel alloy? Will our spacecraft in 500 years be made of a steel alloy? In which applications will other elements entirely, such as carbon allotropes like graphene, replace steel?

The one thing material scientists can’t do is invent new stable elements. They are limited to finding new ways to arrange existing elements. Still there is a great deal left to be explored in terms of new arrangements of atoms of those elements. It seems likely that iron and carbon will remain central to our technology infrastructure for a long time, perhaps forever.

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