What are Free Cutting Steels ?

Free cutting steels (also free machining Steels) are extensively applied for machining parts subject to comparatively light loads (bolts, nuts, screws, etc.) and produced on automatic screw machines or multiple spindle automatics. These steels are intended for uses where easy machining is the primary equipment.

The distinguishing features of free cutting steels i.e., high machinability and high quality surface finish after machining., are due to the higher Sulphur and Phosphorous content.

Sulphur exists in the free cutting steel in the form of manganese sulphide which forms inclusions stretched out in the direction of rolling. These inclusions promote the formation of short brittle chips, reduce the friction on the surface being machined, and enable a satisfactory surface finish to be obtained at high cutting speed. Phosphorus dissolves in the ferrite (pure iron) and increases its brittleness. This also makes the chip more brittle and enables a smooth bright surface to be obtained in cutting. The addition of 0.15 to 0.35 per cent phosphorus considerably improves the machinability without reducing the physical and mechanical properties of the steel.

The tool life achieved in machining free cutting steels is from 2 to 2.5 times higher than when carbon steels of the same carbon content are machined. It must be noted., however, that free cutting steels have lower dynamic strength characteristics and are more susceptible to corrosion.

Free cutting steels are frequently supplied in the cold drawn (work - hardened) form. These cold drawn steels have a high tensile strength and hardness but less ductile when compared to ordinary carbon steels.

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