Steel is made by adding carbon to molten iron. Various quantities of carbon produce different types of steel. Each type of steel is designated by a number. Therefore, 4140 is a designation of only one type of steel, containing a carbon content of .38 percent to .43 percent. It's important, however, to understand the uses for this type of steel and its working characteristics.

When steel first comes out of the melting furnace and is allowed to cool, it's a soft metal. If the steel is heated to almost melting and cooled quickly, usually by dunking in very cold water, it becomes very hard. This process is called quenching and tempering. Steel with a low carbon content will not harden well, so it cannot be used to make tools such as drill bits. Steel with higher carbon contents than 4140 become very hard. These are used to make machine tools such as router bits. The 4140 steel is halfway between soft and tool grade steel. Steel experts at the Interalloy Materials Company note that 4140 has good hardening properties.

Before 4140 is hardened, it's soft and machine cuts easily. A hacksaw blade will cut it. According to Interalloy, it forges as well. Forging is a process in which the steel is heated almost to melting and then pounded on. This process packs the molecules closer together, making for a denser and stronger steel. After hardening, it machines fairly well in operations such as sawing, milling or grinding. Machine shop instructors at Fox Valley Technical College recommend various cutting speeds for mills. If using regular tool steel grade cutting bits, the milling speed is 60 to 100 feet per minute or fpm. If using very hard carbide bits, the milling speed is 275 to 450 fpm. As with all steels, using a cooling fluid on the cutting tools greatly aids in the machining process.

Experts at Speedy Metals Supply Company and Interalloy concur that 4140 does not weld well. Because of its chemical makeup, stress cracks develop in the welds. Therefore, a machine designer or engineer designs a welded area with other types of steel as opposed to 4140. Interalloy recommends if welding is to be done, it should be completed before the steel is hardened.

The 4140 steel is available in round bar, square bar, flat stock and hollow tube. You can order just about any size, shape or quantity you want from a multitude of suppliers. The 4140 steel is widely used to make gears, bearings, machine shafts, rollers and bolts. Overall, this type of steel is cheap and plentiful, so you don't have to worry about the cost if you make a mistake on the first part or two.