Free-cutting steel

Family overview

Free-cutting steel is steel to which elements such as sulfur (S) or lead (Pb) are added on purpose to make machining easier. During cutting these elements help break the chip short and reduce friction between the tool and the chip, which benefits tool life, surface finish and chip handling. A long, continuous chip can wrap around the tool and stop machining, so these steels are mainly used where the same shape is cut repeatedly, as on automatic lathes.

This family includes the SUM series of JIS G 4804 (SUM22, SUM23, SUM24L, SUM43), the SAE designations 1215, 12L14 and 1144, and 11SMn30 and 11SMnPb30 of EN 10087. Because the standard systems differ, grades of a similar kind cannot be assumed to share the same composition range or specified items, so correspondences between grades are for reference only. The designation also shows whether lead is added. The L in SUM24L and 12L14 and the Pb in 11SMnPb30 mark leaded grades, and grades without such a mark gain machinability mainly from sulfur, in some cases with phosphorus raised as well.

Neither leaded nor lead-free grades can be called better in every case, because their advantages and drawbacks differ. Lead additions are known to help chip handling and tool life, but welding and hot working need care. Leaded grades also cannot be used in some parts covered by regulations that restrict hazardous substances including lead, or by customer environmental standards. Lead-free grades are an option for such parts, but chip form, tool life and cutting conditions can change, so trial machining is often used to confirm them.

Machinability is gained at the expense of some other properties. Manganese sulfide (MnS) inclusions stretch along the rolling or drawing direction and tend to lower ductility, toughness and fatigue properties in the direction perpendicular to it, and sulfur and phosphorus reduce weldability. These steels are therefore used for small precision parts machined in large quantities, and for parts that are welded or carry heavy impact or cyclic loads they are compared with ordinary carbon and alloy steels.

Role of the main elements

• Sulfur (S): combines with manganese to form manganese sulfide (MnS) inclusions. MnS gives the chip a point at which to break and reduces both friction and the built-up edge, the workpiece material that sticks to the cutting edge. Machinability generally improves as sulfur increases, but ductility, impact toughness, weldability and cold formability tend to decrease.
• Manganese (Mn): combines with sulfur first to form MnS. When manganese is short, the sulfur combines with iron as iron sulfide (FeS), which collects at grain boundaries and causes hot shortness, cracking during hot working. A steel with added sulfur therefore also needs enough manganese to match it. Manganese also raises the strength and hardenability of the matrix.
• Lead (Pb): barely dissolves in iron and is dispersed as fine particles, often attached to MnS. It softens or melts with the cutting heat and acts like a lubricant between the tool and the chip, reducing cutting resistance and friction. At high temperature the lead melts and can cause cracking in hot working or welding.
• Phosphorus (P): hardens ferrite and makes it more brittle, which helps the chip break and leaves a smoother machined surface. It is used in grades with phosphorus raised along with sulfur, such as the SAE 12xx series (1215, 12L14); more phosphorus lowers ductility, toughness and weldability.
• Carbon (C): sets the base strength and the response to heat treatment. The low-carbon grades (SUM22, SUM23, SUM24L, 1215, 12L14, 11SMn30, 11SMnPb30) put machinability ahead of strength and do not harden deeply on quenching. The medium-carbon grades (SUM43, 1144) raise carbon and manganese to gain strength and hardenability, at the cost of lower machinability than the low-carbon grades.
• Other elements: free-cutting steels are also known in which bismuth (Bi) replaces lead, or tellurium (Te) and selenium (Se) are used together with bismuth. Which elements are used, and how much, differs by grade and is confirmed against the applicable standard.

Heat treatment and delivery condition

• Delivery condition: typically classified as hot-rolled, cold-drawn, or surface-finished by peeling or grinding. The cold-drawn condition gains strength from work hardening and tends to have smaller dimensional variation and a smoother surface than the hot-rolled condition. Even for the same grade, cutting conditions and tool life can differ with the delivery condition.
• Low-carbon grades: they do not harden deeply on quenching and are often used as rolled or as drawn. Where only the surface needs to be hard, surface hardening such as carburizing, carbonitriding or nitrocarburizing is sometimes applied, and heat treatment conditions are confirmed against the standard of each grade.
• Medium-carbon grades (SUM43, 1144): strength and hardness can be raised by quenching and tempering or by induction surface hardening, and the steel is sometimes used cold-drawn and stress-relieved without heat treatment. Sulfides act as stress concentrators, so toughness and fatigue properties can be lower than in ordinary steel of the same carbon content.
• Leaded grades (SUM24L, 12L14, 11SMnPb30): lead melts at high temperature, so cracking can occur in hot working or welding and the conditions are set separately.

Grades

Composition range

Confirm the composition range against the applicable standard.

Typical applications

• Low-carbon free-cutting steels (SUM22, SUM23, 1215, 11SMn30 and others): fastening and connecting parts where machinability matters more than strength, such as small screws, bolts and nuts, pins, bushings and spacers machined in large quantities on automatic lathes.
• Leaded grades (SUM24L, 12L14, 11SMnPb30): examples include small precision parts for which surface finish and tool life matter, and fittings and connectors for hydraulic and pneumatic systems.
• Medium-carbon free-cutting steels (SUM43, 1144): parts that need some strength and are used without heat treatment or with surface hardening only, such as shafts, pins, spindles, rollers and gear blanks.
• Small precision parts such as shafts, gears and bushings in office equipment, home appliances and electronic equipment, where dimensional accuracy and surface condition are required.

Products covering these grades

• Cold-drawn bar · bar-to-barSUM22 · SUM24L

Grades not listed on a product page (SUM23, SUM43, 1215, 12L14, 1144, 11SMn30, 11SMnPb30): send us the standard, size and quantity, and we will confirm availability. Quote & technical inquiry