Stainless Steel: By adding a minimum of 10.5 percent chromium, reducing the carbon content to a maximum of 1.2 percent, and tossing in some alloying elements like nickel and molybdenum, metallurgists convert ordinary rust-prone steel into stainless steel, the corrosion-proof switch hitter of the manufacturing world. With dozens of grades and classes to choose from, though, it might be difficult to determine which is best for a given application. Austenitic stainless steels 304 and 316L, for example, have a crystalline structure that make them non-magnetic, non-hardenable, ductile, and quite tough. Martensitic stainless steels (grade 420 is one), on the other hand, are both magnetic and hardenable, making them a good choice for surgical instruments and various wear components. There are also ferritic stainless steels (most are in the 400-series), duplex steels (think oil and gas), and the precipitation hardening stainless steels 15-5 PH and 17-4 PH, both favored for their excellent mechanical properties. Machinability ranges from quite good (416 SS) to moderately poor (347 SS).