Lead

Bluish-gray, flat-lustrous,  relatively soft heavy metal with a low melting point.

SymbolPb
Atomic number82
Atomic weight207.21
Specific weight at 20 °C111.3 g/cm3
Melting point327 °C
Boiling point1740 °C
Melting heat1090 kJ/kg
Specific thermal capacity at 20 °C0.13 kJ/(kg·K)
Thermal conductivity35 kJ/(m·K)
Therm. coefficient for linear expansion Coefficient for linear expansion29·10-6/K


Lead is used for production of accumulator plates and soft solders or as material for bearings and for type metals and cable sheaths.

Lead vapor and lead-containing dust is toxic.

Lead in steel
Lead is insoluble in molten iron and usually does not bond with the accompanying and alloy elements present in steel, either. In the solid materials, lead is present in the form of metal particles of sub-microscopic size and finest distribution. The most types of iron ore only contain very low quantities of lead. Usually, the contents in steel are less than 0.01 %.

However, lead is frequently used for steel production to achieve improvement of the material’s machinability at high cutting speeds. The element is added in metal form in quantities of around 0.25% during casting of the steel into ingots. The addition of lead improves the workability of unalloyed or low-alloyed steels even if the machinability of such types of steel is already very good due to relatively high sulfur and phosphorous contents. The improved machinability properties are attributed to the fact that the finely distributed lead particles have a chip-breaking effect. This prevents excessive generation of heat, which would otherwise develop due to friction between the cutting tool and the long, spiral-shaped chips.
Steels treated with lead can be processed at cutting speeds that are around 30% higher than for lead-free steels. These types of free-cutting steels can be soldered and welded without problems and are perfectly suitable for heat treatment and surface finishing

By the addition of leadhardenability is slightly decreased, while it has hardly any influence on mechanical properties, for example. The latter is in contrast with sulfur- and phosphorous-containing free-cutting steels, with which improved machinability entails embrittlement.

Lead in cast iron
Solubility of lead in cast iron is very low; however, some types of pig iron may contain up to 0.003 % Pb. Due to lead-containing raw materials in the furnace charge, the lead content may be increased to 0.02 %.

Lead contents of more than 0.003 % in nodular graphite cast iron cause formation of Widmanstätten graphite and drastically reduce resistance against formation of cracks. Lead contents of this extent stabilize the pearlite phase (see also Pearlite stabilization).

If only magnesium is added to cast iron to promote the formation of nodular graphite (s. Nodular graphite cast iron), lead contents of more than 0.003 % have a rather negative effect. However, this effect is reversed if low Cer quantities are added together with the magnesium.

 

 

 

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