Korean Scientists Develop Lightweight Steel Stronger, Cheaper Than Titanium
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Light? Strong? Tough? Cheap? A new material developed in Korea promises all of the above, and could throw open wide doors in many industries, including small arms development.
The weight and efficacy of small arms is highly dependent on the materials used to make them. Lightweight materials like aluminum or polymer are used wherever possible, and only where they are absolutely needed, such as the pressure-bearing barrel and locking surfaces, are heavier materials like steel applied. A good material for small arms construction should be both light and strong, with a high specific strength, while also being ductile and tough. Beyond that, however, materials suitable for small arms production also need to be easy to work in multiple ways (suitable for stamping, machining, welding, forging, etc), and, perhaps above all, cheap. It’s these last two requirements that exclude, with few exceptions, titanium, which is extremely strong and tough, from being a good material for this purpose.
However, this new development could overturn the existing paradigm of aluminum, traditional steel and polymer. The material is an alloy of steel and aluminum, a marriage that previously resulted in a brittle, useless alloy, due to coalesced bands of material that were very hard, but fragile, which acted as “fault lines” in the material, causing them to crack. An article at SAE International explains how the Korean scientists solved this problem:
Probably the most surprising point about the new steel composition is that it gains its mass advantage through the addition of aluminum, a low-density alloying agent that had been tried many times before but had always yielded unsuitably brittle results. Decades ago metallurgists in Russia and elsewhere attempted to add aluminum to steel, and even though the resulting metal was very strong and lightweight, it invariably had little ductility—that is, when subjected to large forces, it would break rather than bend. Manufacturing products from a low-ductility metal is very difficult.
Photomicrography studies subsequently revealed that the experimental aluminum-rich steel alloys contained a very hard but very brittle cubic crystal of iron and aluminum called B2 that made them mostly unusable. B2 is an intermetallic compound—a crystalline material in which different elements replace other more typical elements in certain atomic sites. In the previous high-aluminum steel formulations, the B2 intermetallic compounds tended to arrange themselves into brittle bands at which the material would shear off when stressed.
“My original idea was that if I could somehow induce the formation of these B2 crystals, I might be able to disperse them in the steel,” Kim said. He and his colleagues realized that if nanometer-scale B2 crystallites were uniformly distributed as a secondary phase throughout the steel’s ductile austenite (face-centered cubic crystal) primary alloy phase, they would strengthen the whole by halting microscopic crack propagation much like strong carbon fibers serve to reinforce the more flexible resin matrix in a polymer composite material.
After spending years researching the concept, the trio found that by adding nickel to the mix (which includes carbon and manganese besides iron and aluminum) and then specially annealing, or heat-treating, the solidified metal, B2 precipitates would evenly permeate the metal in nanometer-sized clusters rather than long bands. The small percentage of nickel, which reacts with the aluminum, offered greater control over B2 formation, as nickel made the crystals precipitate out at a much higher temperature.
The benefits of a successful steel-aluminum alloy are very great. Besides the lower density, the SAE International article expounds on its material benefits:
Recently, however, three materials scientists at the Graduate Institute of Ferrous Technology at Pohang University of Science and Technology in South Korea have come up with another potential option—lightweight steel. Professors Hansoo Kim and Nack J. Kim, together with doctoral student Sang-Heon Kim, have developed a low-density steel alloy that exhibits higher specific tensile strength and ductility than titanium alloys—the lightest and strongest metals known, but potentially at one-tenth the cost, according to a paper published in the February 5th issue of the journal Nature. (Seehttp://www.nature.com/nature/journal/v518/n7537/full/nature14144.html)
“Because of its lightness, our steel may find many applications in automotive and aircraft manufacturing,” Hansoo Kim stated in an e-mail communication.
“We developed a new type of flexible, ultra-strong, lightweight steel that is 13% less dense than normal steel and has a strength-to-weight ratio that matches even our best titanium alloys,” Kim said.
What benefits could this bring to the firearms industry? Besides the obvious application as a lighter alternative to other steels in parts like hammers, triggers, gas blocks, flash hiders, and other minor parts, or (if it proves to be stamp-able or forge-able) a new material for receivers, the steel also holds promise as a material for use in military rifle barrels. The current limiting factor in the weight of an infantry rifle barrel is not strength, nor is it stiffness, but heat capacity. Modern select-fire rifles are expected to go through such torturous courses of fire that they are being fitted with heavier and heavier barrels that last longer in fully automatic. The new Korean steel alloy is alloyed with aluminum, meaning it may have a higher specific heat capacity than conventional steel. If so, an infantry rifle barrel made from the Korean steel (perhaps with a 4150 liner) could be made lighter than its counterparts made of 4150, with benefits possibly great enough for a barrel the weight of a standard M4-profile barrel to have a heat capacity equivalent to an M4A1 SOCOM profile barrel, saving a quarter pound of weight. This is pure speculation by someone who is not a materials scientist, of course, but it is illustrative of the variety of applications this material could have in small arms alone.
Perhaps the most promising application is given by the US Army’s new M240L machine gun, which is an example of a weapon where pressure for reduced weight has so completely overridden cost concerns that the receiver has been machined from titanium alloy. Titanium is not only an expensive material, but it is very difficult to machine efficiently, and it requires skilled labor and special techniques to do so. While titanium-machining is an art that is steadily being perfected, it is still very expensive. The new Korean steel could be substituted for titanium in future versions of the M240, or another machine gun design, to give a weapon that has a light, strong receiver while retaining the unstoppable reliability that the Belgian MAG is known for, and most importantly, at a greatly reduced cost versus the titanium-receiver M240L.
All of this depends, however, on how easily the new steel can be worked and machined. Even the greatest wonder-material will receive only limited application if it cannot be easily worked into shape. The SAE article says this on the subject:
Production process tests
In their experiment, the researchers melted about 40 kg (88 lb) of the steel alloy in an induction furnace with a protective argon atmosphere and cast it into a rectangular ingot, Kim reported. Following a homogenization treatment—1150°C (2102°F) for 2 hours—the ingot was hot-rolled into strips 3 mm (0.12 in) in thickness. The hot-rolled strips were cold-rolled into 1-mm (0.04-in) -thick sheets that were next annealed at 870 to 900°C (1598 to 1652°F) for 2 to 60 minutes. The sheets were then immediately water-quenched or rapidly cooled to 25°C (77°F).
“All the steps except for the casting are very similar to the existing processes for industrial sheet steel production,” he noted.
Subsequent joining tests showed that “our steel can be welded by electrical resistance spot welding, laser welding, and argon TIG welding,” Kim said.
He stressed that the team’s B2-dispersion method is really more important than the new alloy: “Steel scientists all over the world can make many variants of our steel for their own applications based on the novel microstructure, which comprises a steel alloy matrix and intermetallic precipitates.”
The Pohang University researchers are now working with the South Korean company Posco, one of the world’s largest steel manufacturers, to scale up their technology.
“We are planning a mill trial production of our steel this year at Posco, not for direct commercialization but for checking possible difficulties that are frequently met during scale-up,” he said. “If everything goes smoothly, you may see our steel on the market in two to three years.”
How long will it take for small arms designers to take advantage of the new steel, if it is successfully brought to market? It is impossible to tell now, but if and when they do, it could cause some exciting changes to the current paradigm.