Machinability of Metals

What is Machinability?

Machinability is how easily a metal material can be cut or machined. After forging, virtually all parts are machined to remove excess material and provide the final shape and dimensions.

How Machinability is Determined

The American Iron and Steel Institute adapted ratings for metal materials that determine their machinability. The weighted average of normal cutting speed, surface finish, and tool life are calculated for each material to determine machinability.  Materials that have a less than 100% rating are more difficult to machine and ratings higher than 100% is easier.

Machinability of Stainless Steels

Stainless steel is known as a steel alloy, containing a minimum of 10.5% chromium with a strong resistance to corrosion. Although typically harder to machine than carbon or alloy steels, stainless steels are ideal for high-stress environments, including gas turbines.

Machinability of Superalloys

Superalloys are known for their heat resistance and mechanical strength but are typically much harder to machine than other forging materials. They typically have a high resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation and their primary application for such alloys is in the hot-section of aircraft and industrial turbine engines.

Machinability of Carbon Steel

Carbon material grades typically start with a “1” with the last two digits of the grade representing the average carbon content. 11 series carbons are free machining steels while the 12 series will have a higher and better machinability rating.

Sulfur

Sulfur (S) is normally regarded as an impurity in steel and is required to reduce to the limits of practicality. However, steels which are machined need a certain minimum S content for proper chip formation. Where machining constitutes a major fraction of the end products cost, many types of steel including carbon, alloy, and less often stainless are intentionally re-sulphurised just for this reason.

Hardness

Hardness provides a reliable guide to machinabil­ity with harder materials usually more difficult to machine than softer ones.  Some customers prefer parts to be provided in the annealed or normalized condition for rough machining prior to final heat treatment and finish machining.  Parts supplied in the annealed or normalized state are typically softer than after final heat treatment.