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Free-Cutting Steel Bar


Free cutting steel with a higher component than usual steel of sulfur and phosphorus intended for the fabrication of parts for high-speed automatic and semiautomatic machine tools. Free-cutting steel is produced in the form of rods, and it contains 0.08–0.45 percent carbon, 0.15–0.35 percent silicon, 0.6–1.55 percent manganese, 0.08–0.30 percent sulfur, and 0.05–0.16 percent phosphorus. The high sulfur content leads to the formation of inclusions (for example, manganese sulfide) disposed along the grain. These inclusions facilitate shearing and promote grinding and easy chip formation. For these purposes, free-cutting steel is sometimes alloyed with lead and tellurium. The mechanical properties of free-cutting steel along the grain (depending on the grade of steel and the rod diameter) are as follows: for hot-rolled rods the ultimate strength σγ = 420–750 meganewtons per sq m (MN/m2), or 42–75 kilogram forces per sq mm, and the relative elongation δ = 14–22 percent; for cold-drawn, cold-worked rods σv = 520–840 MN/m2(52–84/mm2), and δ = 6–17 percent. The mechanical properties of the grain are substantially lower than along the grain. The ductility and toughness are lower than those for the ordinary carbon steels because of the sulfur and phosphorus. Weldability is poor. Free-cutting steel parts are usually used without heat treatment or with only tempering to relieve stresses. Free-cutting steels are utilized primarily to make bolts, nuts, certain automobile parts, and instruments.