Lead in Steels

Lead in Steels  Lead (Pb) (atomic number 82 and atomic weight 207.21) has density of 11.34 gm/cc. Melting point of Pb is 327.5 deg C and boiling point is 1749 deg C. The phase diagram of the Fe-Pb binary system is at Fig 1. Pb has a face centered cubic (f.c.c.) crystal structure.  Fig 1 Fe-Pb binary system Pb is only slightly soluble in liquid or solid steel. When added , Pb does not remain in the metal but it remains separate from and mechanically dispersed in the steel  as submicroscopic metallic inclusions when it solidifies. It does not change the mechanical properties of the steel appreciably. The sole function of Pb is to improve the machinability of steel. This accounts for the wide use of Pb for high strength steel for which machinability is required. Also, Pb only slightly influences mechanical properties of steel. However, with the growing consciousness of environmental issues in recent years, there is a tendency toward eliminating Pb because it is seen to be one of the pollutants of the environment. Addition agents and addition practice Elemental Pb is available as cored wire (Pb micro shots), pellets and fine shots suitable for addition in the continuous casting tundish, ladle metallurgical furnace, or degassing furnace. Alloys containing bismuth (bi) are also available for addition. Addition of Pb in liquid steel is carefully controlled to ensure its even dispersion in the solidifying steel. Segregation of Pb can be an issue. Pb segregation in steel  can lead to problems during cold working and machining of steel. Due to this Pb bearing steels are normally checked for segregation. The steel sample is usually heated at 700 deg C and is examined for uniform distribution of Pb. This test is known as sweat test. Pb does not...

Free Cutting Steels

Free Cutting Steels Free cutting steels also known as free machining steels are those steels which form small chips when machined. This increases the machinability of the material by breaking the chips into small pieces, thus avoiding their entanglement in the machinery. This enables automatic run of the equipment without human interaction. Free cutting steels with lead also allow for higher machining rates. As a thumb rule, free cutting steel normally costs 15 % to 20 % more than the standard steel. However this is made up by increased machining speeds, larger cuts, and longer tool life. The cutting (machining) operation is shown in Fig 1. Fig 1 Cutting operation In turning, milling and drilling operations commonly known as machining operations, deformation/welding of the tool/work piece interface occurs rather than chip formation. During the machining operations surface finish is impaired, cutting temperature increased and tool life reduced significantly. A large ‘built-up edge’ is formed on the tool tip at very low sulfur contents. This requires frequent dressing or changing of tools, reduced productivity and higher costs. The term machinability is characterized by the following three parameters. Speed of machining Surface finish of the machined components Tool life of the cutting tools employed for machining operation. The term machinability relates to the ease and cost of achieving a production schedule for machined parts. It deals with consistent production of machined components which are able to satisfy product property specifications and in service performance requirements, at minimum through cost. Machinability can be measured in terms of surface finish, chip form, tool life, power consumption, and production rate. Machinability is not a unique material property like tensile strength, since it depends on the criterion selected, the type of cutting tool, cutting operation, cutting conditions and the machine tool...