What’s Free-Machining Steel?

Free-machining steel also referred to as Free-cutting steel or Automatic Steel, is the alloy steel by added elements such as sulfur, lead, calcium, selenium, tellurium, and bismuth to lubricate the cutting tool, reducing the cutting resistance of the steel, thereby reducing the surface roughness of the workpiece and increasing the cutting speed and prolonging the tool service life. Free-cutting steels can be divided into sulfur free-cutting steels, lead free-cutting steels, calcium free-cutting steels and composite free-cutting steels according to different elements. According to their different uses, free-cutting steels are divided into automaton free-cutting steels, structural free-cutting steels and special free-cutting steels (heat resistant steels, stainless steels, tool steels, etc.).

 

Sulfur free-cutting steel

The sulfide in steel mainly exists in the form of (FeMn)S solid solution. Sulfur forms manganese sulfide inclusions with manganese and iron in steel. This kind of inclusions can cut the continuity of the matrix metal, promote the chip to form a small and short crimp that is easy to discharge and reduce the tool wear, reduce the surface roughness and improve the tool life. Generally, the machinability of steel increases with the increase of sulfur content in steel, but the transverse ductility, toughness ratio, fatigue and corrosion resistance of steel also decrease.

When the sulfur content is too high, the hot brittleness makes the hot working of steel difficult. Usually, the sulfur content is 0.08% ~ 0.30%, and some can be increased to 0.4%. The sulfur content in free-cutting tool steel and stainless steel should be between 0.06% ~ 0.10%. Phosphorus (0.04% ~ 0.12%) and sulfur compound into the steel, phosphorus solid dissolved in ferrite will improve the hardness and strength, reduce toughness, so that the chip is easy to break so as to obtain a good machining surface roughness, but too high phosphorus content will significantly reduce the plasticity, improve hardness, making steel more difficult to cut or machine.

Lead free-cutting steel

Lead is uniformly distributed in steel in the form of fine metal particles or attached to the surrounding sulfide. Due to the low melting point of lead, the melting material can be lubricated in cutting, reducing friction and improving machinability, but it does not affect the normal temperature mechanical properties. The lead content in steel is generally 0.10% ~ 0.35%, and too high lead content is easy to cause serious segregation and form large particle inclusions but reduces its favorable influence on machining performance. Adding lead and sulfur to low carbon structural steels can significantly improve the cutting performance of steels.

Ca free cutting steel

Calcium combines with aluminum and silicon to form compound oxides with low melting points (mainly CaO, Al2O3, SiO2). In high-speed cutting, calcium oxides are attached to the surface of the cutting tool to lubricate and reduce friction, thereby increasing tool service life. If calcium sulfur and lead are added at the same time, the compound action will make the cutting effect better.

Se, Te, and Bi free-cutting steels

The contents of Te and Bi are about 0.03% ~ 0.10%, and Se can reach 0.15%. Se exists in steel in the form of selenides such as FESE and MnSe, and its action is similar to that of sulfur. For steels requiring both high machinability and good ductility, selenium is better than sulfur. Te can be added to steel alone, or in the form of composite inclusion with lead or sulfur to reduce the cutting resistance and cutting heat, and significantly improve the machinability of steel. However, Te can reduce the plasticity and toughness of steel. Bi is similar to lead in the form of fine metallic particle inclusions that are uniformly distributed or attached to the sulfide.

Application of free cutting steel

(1) Automobile and motorcycle parts: crankshaft, connecting rod, wheel hub, support steering bar, gasket, gear brake and rack, etc

(2) mechanical equipment: ceramic/plastic/glass machinery, food machinery, construction machinery, textile machinery, hydraulic press, etc.

(3) Electrical appliances and furniture: motor shaft, fan shaft, sewing machine shaft, outdoor furniture, garden tools, anti-theft locks, etc.

(5) Hardware accessories: bolts, screws, nuts, pipe joints, spring seats, hexagonal nails, octagonal nails and other standard parts.