Machine screws are a subcategory of screws used to attach pieces of machinery together. They're usually narrow in diameter, have threads from the top to the tip and are inserted into prethreaded holes. Like other types of screws, machine screws come in a wide range of head types and head shapes; they also come in a variety of materials, including stainless steel, carbon steel, zinc-coated steel and nylon.
Stainless steel is known for its extra resistance to corrosion and its ability to withstand high temperatures. Normal carbon steel rusts when it's exposed to moisture, but stainless steel doesn't. To create the rust-resistance, ordinary steel is mixed with other elements, particularly chromium. When the steel contains more than 10 percent chromium, it's categorized as stainless steel. Instead of rusting, when the stainless steel is exposed to moisture, the chromium oxidizes and forms a nearly invisible, protective layer on the metal. The most basic type of stainless steel is a mixture of iron and chromium with a crystal structure of ferrite, which is why it's called ferritic stainless steel. There are other types of stainless steel, too.
Most stainless steel machine screws are made of 18/8 stainless steel, otherwise known as type 304. It has nickel added to it -- as do most types of stainless steel -- in part to create an austenite crystal structure instead of ferrite. Austenitic stainless steel is considered to be both ductile and strong; plus, it's easy to shape. This is why type 304 is the most commonly used kind of stainless steel. Since 18/8 stainless steel is annealed, it isn't magnetic -- and therefore neither are stainless steel machine screws. A lower-carbon version of type 304 stainless steel (called 304L) is used in particularly corrosive environments.