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ASTM A743 grade CF-8M is a common stainless steel alloy for making medical implants, pumps and valves. It is composed of iron, chromium, nickel and molybdenum, and is austenitic in structure. It is highly corrosion-resistant, is heat-treatable and can be deep drawn, bent and welded. The composition of ASTM A743 grade CF-8M is as follows: Iron (Fe) - Balance; Chromium (Cr) - 18%-20%; Nickel (Ni) - 8%-10.5%; and Molybdenum (Mo) - 2%-2.5%.
The microstructure of the ASTM A743 grade CF-8M alloy is a mixture of ferrite and austenite. Ferrite has a body-centered cubic (BCC) lattice structure, while austenite has a face-centered cubic (FCC) lattice structure. The ratio of ferrite to austenite in this alloy varies, depending on the amount of heat that is applied during the heat-treating process. The higher the temperature and the longer it is held, the greater the amount of austenite will be present. Lower temperatures and shorter heating times will produce more ferrite. This mix of ferrite and austenite is commonly referred to as a duplex microstructure.
The macrostructure of the ASTM A743 grade CF-8M alloy consists of ferritic grains, austenitic grains, and a small amount of precipitates. The ferritic and austenitic grains are typically equal in size and shape, and don’t show any directional preference. The precipitates, on the other hand, are generally small in size, and are usually located at the boundaries of the austenite and ferrite grains.
The primary corrosion resistance of the ASTM A743 grade CF-8M alloy is provided by its chromium content, as chromium forms an adherent, corrosion-resistant oxide layer on the surface. The high levels of silicon and molybdenum also contribute to its corrosion resistance by providing resistance to chloride-induced pitting and crevice corrosion. The nickel content helps improve the alloy’s resistance to stress-corrosion cracking, and helps maintain its ductility in the cold-worked condition.
All in all, the microstructure and composition of ASTM A743 grade CF-8M alloy makes it ideal for use in a variety of medical applications, including implants, pumps and valves. Its high corrosion resistance and mechanical strength, combined with its ductility and weldability, make it an ideal choice for many medical device applications.