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The world needs hard metals. Steel is used in everything from cars to cruise ships, cables to crescent wrenches. Regardless of alloy type, steel is mostly composed of iron. Iron smelting and limited steel manufacturing has been in use for thousands of years, but it wasn’t until the Bessemer steel process, invented in the mid-1800s, that mass production of high-quality steel was made possible. Thus began the industrial revolution.
As with the soft metals, a small quantity of alloying elements can have a dramatic effect on steel’s properties—the addition of less than 1 percent carbon and manganese, along with a little metallurgical legerdemain, is what makes brittle iron into tough 1018 steel. And 4140 alloy steel, suitable for aircraft use, is made by combining an equally small amount of chromium along with a dusting of molybdenum.
Carbon steels such as these can be hardened to one extent or another, and are easily welded. There’s just one problem: They rust, making plating or painting a requirement for most any application involving carbon steel.
The 300-series stainless steels offered by Protolabs carry at least 20 percent chromium along with a fair amount of nickel, making them more difficult to machine. Still, these popular materials are commonly used for medical instruments, vacuum and pressure vessels, and for food and beverage equipment. 300-series stainless is quite tough, but cannot be hardened like carbon steel. If hardness is a requirement for your application, consider kicking it up a notch with 17-4 PH.
This versatile but very tough material contains nickel, chromium, and copper. Although considered part of the stainless steel family, its machinability in the annealed state approaches superalloy status. When heat treated, it easily achieves hardness of 45 Rc and tensile strength of 150,000 psi or higher, three times that of carbon steel. It’s most commonly used in the medical, aerospace, and nuclear industries, or anywhere a combination of high strength and good corrosion resistance is needed.
Since rust never sleeps, metallurgists developed stainless steel. By increasing the amount of chromium to at least 10.5 percent, corrosion resistance is greatly enhanced. Stainless steel is widely used in the chemical industry, textile processing, and for marine applications. Many stainless steels are temperature resistant as well, and are able to withstand temperatures upwards of 2700˚F, hot enough to turn aluminum, brass and copper into molten puddles. 316 stainless, for example, is excellent for heat exchangers, and sees regular use in steam turbines and exhaust manifolds.
If you’re looking for some truly robust alloys, look no further than cobalt chrome and Inconel. Protolabs doesn’t machine these materials, but its 3D printing service is happy to sinter them for you through a direct metal laser sintering (DMLS) process. Each material has unique, high-performance properties.
Inconel contains 50 percent or more of nickel, giving it excellent strength at a range of temperatures. It’s used for extreme demands such as gas turbine blades, jet engine compressor discs, and even nuclear reactors and jet engine combustion chambers. The high nickel content makes Inconel one of the most difficult materials to a machine, requiring wear resistant coated carbide and a rigid machine tool. Sitting right next to nickel on the periodic table is cobalt, the main ingredient in cobalt chrome alloy. This material is known for superb wear resistance and human biocompatibility, making it ideal for dental implants, hip and knee replacements, and arterial stents.
Finally, there’s titanium. This lightweight element is alloyed with aluminum and vanadium, providing a strong, corrosion-resistant material. Like cobalt chrome, titanium is biocompatible and is used extensively for bone screws, pins, and plates. Its tensile strength is roughly twice that of mild steel but weighs just half as much. This makes titanium appealing to the aerospace industry and high-performance vehicle manufacturers.
Portions of this article were taken from Protolabs’ A Guide to Manufacturing Metal Parts.
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