SINTERING

the welding together of small particles of metal by applying heat below the melting point. The process may be used in steel manufacturingto form complex shapes, to produce alloys, or to work in metals with very high melting points. In a steel-sintering plant a bed of powdered iron ore, mixed with coke or anthracite, is ignited by a gas burner and then moved along a traveling grate. Air is drawn down through the grate to produce downdraft combustion. As the bed moves forward, a very high heat (1,3251,500 C [2,4002,700 F]) is generated that converts the tiny particles into lumps about 2.5 cm (1 inch) in diameter suitable for burning in the blast furnace to convert them to steel. Sintering is also used in the preliminary molding of ceramic or glass powders into forms that can then be permanently fixed by firing. The driving force in sintering is decreasing surface energy; as the sintering proceeds, adjacent particles partially coalesce owing to viscous flow (as in glass) or to diffusion processes (as in crystalline materials), and consequently the total surface area decreases. The result is improved mechanical and physical properties of the material. See also powder metallurgy.