Sunday, March 09, 2008

Overview of Brass

Brasses are copper zinc alloys. In general, they have good strength and corrosion resistance, although their structure and properties are a function of zinc content. Alloys containing up to approximately 35% zinc are single phase alloys, consisting of a solid solution of zinc and alpha copper. These brasses have good strength and ductility, and are easily cold worked. The strength and ductility of these alloys increases with increasing zinc content. The alpha alloys can be differentiated by a gradual change in color, from golden yellow to red, as the zinc content is increased up to 35%. Gilding 95%, Commercial Bronze, Jewelry Bronze, Red Brass and Cartridge Brass are in this category of brasses. These are known for their ease of fabrication by drawing, high cold worked strength and corrosion resistance. Increasing the zinc content up to 35 % produces a stronger, more elastic brass alloy with a moderate decrease in corrosion resistance. Brasses containing between 32 and 39% zinc have a two phase structure, composed of alpha and beta phases. Yellow brasses are in this intermediate category of brasses. Brasses containing more than 39% zinc, such as Muntz metal, have a predominantly beta structure. The beta phase is harder than the alpha phase. These materials have high strengths and lower ductility at room temperature than the alloys containing less zinc. The two phase brasses are easy to hot work and machine, but cold formability is limited. Brasses are used in applications such as blanking, coining, drawing, piercing, springs, fire extinguishers, jewelry, radiator cores, lamp fixtures, ammunition, flexible hose, and the base for gold plate. Brasses have excellent castability, and a good combination of strength and corrosion resistance. The cast brasses are used in applications such as plumbing fixtures, fittings and low pressure valves, gears, bearings, decorative hardware and architectural trim. The UNS designations for wrought brasses includes C20500 through C28580, and C83300 through C85800 for cast brasses.
Certain brasses can corrode in various environments. Dezincification can be a problem in alloys containing more than 15% zinc in stagnant, acidic aqueous environments. Dezincification begins as the removal of zinc from the surface of the brass, leaving a relatively porous and weak layer of copper and copper oxide. The dezincification can progress through the brass and weaken the entire component. Stress corrosion cracking can also be a problem for brasses containing more than 15% zinc. Stress corrosion cracking of these brasses occurs when the components are subject to a tensile stress in environments containing moist ammonia, amines, and mercury compounds. If either the stress or chemical environment is removed the stress corrosion cracking will not occur. Sometimes a stress relieving treatment is sufficient to prevent stress corrosion cracking from occurring. The microstructure of the single phase brass alloys, with up to 32% zinc, consists of a solid solution of zinc and alpha copper. The as-cast structure of the low zinc brasses consists of alpha dendrites. The first material to solidify is almost pure copper, as the dendrites continue to solidify they become a mixture of copper and zinc. A composition gradient exists across the dendrite, with zero zinc content at the center and highest zinc content at the outer edge. The composition gradient is called coring, and it typically occurs with alloys that freeze over a wide temperature range. Subsequent working and annealing breaks up the dendritic structure. The resulting microstructure consists of twinned, equiaxed grains of alpha brass. The annealed microstructure is made up of equiaxed, twinned grains of alpha copper, similar to the structure of unalloyed copper. The grains appear in different shades due to their different orientations. The twins are parallel lines that extend across individual grains. The twins result from a fault in the staking sequence of the copper atoms, making it difficult to distinguish the individual grains.
Alpha copper is the primary phase in cast alloys containing up to approximately 40% zinc. The beta phase,which is the high zinc phase, is the minor constituent filling in the areas between the alpha dendrites. The microstructure of brasses containing up to approximately 40% zinc consists of alpha dendrites with beta surrounding the dendrites. The wrought materials consist of grains of alpha and beta. Cast alloys with greater than 40% zinc contain primary dendrites of beta phase. If the material is fast-cooled, the structure consists entirely of beta phase. During a slower cool, the alpha precipitates out of solution at the crystal boundaries, forming a structure of beta dendrites surrounded by alpha. This structure is called a Widmanstatten structure, because a geometrical pattern of alpha is formed on the certain crystallographic orientations of the beta lattice. The wrought, two phase material consists of grains of beta and alpha. Hot rolling tends to elongate the grains in the rolling direction.
Brasses frequently contain lead in order to improve machinability. The microstructure of the leaded brasses is similar to that of the unleaded brasses with the addition of almost pure lead particles found in the grain boundaries and inter-dendritic spacings. The lead is observed in the microstructure as discrete, globular particles because it is practically insoluble in solid copper. The number and size of the lead particles increases with increasing lead content.