What Is Free-Machining Steel?
Free-machining steel is a steel alloy that includes additives such as sulfur, lead, phosphorus, and manganese. These additions are made to enhance machinability and facilitate cutting during machining operations using machining centers and lathes.
By improving machinability, free-cutting steel is widely used for continuous unmanned machining and high-speed machining using NC machine tools. It is an indispensable material for increasing the efficiency of parts production.
Free-machining steel is denoted by SUM, and numbers such as SUM21 and SUM22 indicate the carbon content, with the number depending on whether it is a low-carbon steel or a medium-carbon steel.
Uses of Free-Machining Steel
Free-machining steel is often used as a material for bolts and nuts produced in large quantities because of its good cutting performance and continuous high-speed machining.
In the automotive field, free-machining steel is used for engine crankshafts, connecting rods, and hydraulic parts, and free-machining steel is also widely used for the shafts of printers.
Free-machining steel is also used for parts of home appliances such as digital cameras, DVD players, and televisions, as well as air conditioners, gas appliances, and other devices that are indispensable in our daily lives.
Principle of Free-Machining Steel
Free-machining steel is classified into low-carbon free-machining steel and medium-carbon free-machining steel according to their carbon content and has different characteristics.
Low-carbon free-machining steel is described as “SUM21-SUM22” and represents free-machining steel with a carbon content of 0.13% or less. Low-carbon free-machining steel is characterized by its emphasis on machinability rather than strength and is used for parts that do not require much strength.
Medium-carbon free-machining steel, described as “SUM31-SUM43”, is stronger than low-carbon free-machining steel and has hardness equivalent to widely used steel materials such as “S35C,” “SS400”. It is quenched for tempering, and since the added sulfur accelerates deterioration, more manganese is added to prevent deterioration.
A small amount of sulfur added to free-machining steel (0.16-0.23%) is added because too much sulfur is harmful to the human body, and higher sulfur content results in properties such as slightly inferior toughness and ductility in the rolling direction.