Advanced armor steel developed

A team of scientists at the U.K. Defense Science and Technology Laboratory (DSTL), in collaboration with Corus, have developed an advanced armor steel called Super Bainite. The material is made through a process called isothermal hardening. With this method, steel is heated to 1,000ºC, air-cooled to 250ºC, then held there for eight hours before finally cooling it to room temperature. The end result is an ultra-hard, crack-free steel. “Other ultra-hard steels need to be quenched and tempered and tend to contain costly elements to avoid cracking during processing,” said Peter Brown, head of the DSTL research team. “This conventional approach is effective, but also more expensive, complicated and energy intensive than the Super Bainite manufacturing route.”

Steel is a solution of carbon in iron (more precisely: mostly of iron — with a carbon content between 0.2 percent and 2.14 percent by weight [C:110–10Fe], depending on grade). It has been the industrial backbone of modern society. Brown said scientists discovered in the early half of the last century that heating and cooling steel in novel ways and adding other elements such as silicon and manganese would change the configuration of iron atoms.

“In the bainite configuration the iron atoms are arranged fairly openly, whereas in the austenite phase they are packed closely together,” Brown adds. “While Super Bainite steel is mainly bainitic it also contains some austenite.”

He explained that Super Bainite’s high carbon content and unusually low final processing temperature makes its mixed microstructure incredibly fine. “As a consequence it’s also extremely hard, which is one reason it’s such a good armor,” he added. Brown said that Super Bainite is still experimental armor steel. “Clearly, if and when it becomes commercially available, it could be used to protect a variety of defense platforms,” he added.

A number of civil applications, from armored limousines to bulletproof furniture for commercial and domestic premises, have also been proposed.

In addition to offering ballistic protection, a good armor needs to be affordable. “Armour that’s prohibitively expensive or very difficult to process is no use to anyone,” said Brown. “Super Bainite’s composition and processing are inexpensive.” He added that much of the process’s cost and complexity has been removed using Cambridge University software. “Recent large-scale production trials with Corus have also demonstrated that the Super Bainite manufacturing route can be conducted in the U.K.,” said Brown. “From a defense perspective, the availability of a secure onshore supply of high-performance steel armor is strategically important.”

During a recent Horizon seminar at Cambridge University, Brown discussed adding “holes” into the steel armor. These holes would actually provide a protective advantage. He said the trick is to think of them as circular “edges” rather than holes. When a bullet hits an edge, it gets deflected and turns from a sharp projectile into a blunt fragment. “The introduction of holes also reduces weight and stops cracks from spreading,” he added. “As a result, perforated Super Bainite steel armor is ballistically very efficient.”

Brown and his research team are also studying manufacturing methods for other kinds of high-performance materials. Kolsterising, in particular, is a new process developed by the company Bodycote to increase surface hardness of stainless steel. “It’s a new process for introducing carbon into the surface of austenitic steels,” he said. “The resulting surface region, although thinner than tissue paper, is twice as hard as Super Bainite.”

Brown added that Kolsterising is still experimental and needs to be scaled up to make thicker material. “Whether it becomes a useful armor material, only time will tell,” he said.