Tin Brasses

Overview

Tin brass is used for its increased corrosion resistance and somewhat higher strength than straight brass. This family of alloys is made with zinc contents ranging form 2 to 40% zinc, and 0.2 to 3.0% tin. Tin reduces susceptibility of the high zinc brass to dezincification. Dezincification is the selective leaching of zinc from the brass leaving a porous copper structure. Arsenic, antimony and phosphorus also reduce the susceptibility of brasses to dezincification. The tin brasses are economical and have slightly better properties than the straight brasses. They have good hot forgeability and reasonably good cold formability. The tin brasses are used in applications such as high strength fasteners, electrical connectors, springs, corrosion resistant mechanical products, marine hardware, pump shafts, and corrosion resistant screw machine parts. This category of brasses includes admiralty brasses, naval brasses and free machining tin brasses. The cast tin brasses are called cast red brasses. Alloys that contain less than 8% zinc are a red copper like color, and hence the name red brass. Semi red brasses contain up to 15% zinc and are lighter in color than the red brasses. They have reduced corrosion resistance, but retain their good strength. These materials have moderate strength, high atmospheric and aqueous corrosion resistance, and excellent electrical conductivity. Cast red brasses are also made containing lead which increases their machinability and pressure tightness. The wrought tin brasses are designated by UNS C40400 through C48600. The cast red brasses are labeled UNS C83300 through C83810 and the cast semi red brasses are UNS C84200 through C84800.

The microstructure of the tin brasses is dependent on the zinc and tin content of the alloy. Tin brasses with low zinc and low tin contents are single phase alloys. The structure consists of alpha copper solid solution containing zinc and tin. The cast structures contain cored dendrites, the zinc and tin content of the dendrites increasing from the center to the edge of the dendrites. The wrought microstructure contains twinned grains of alpha solid solution. Alloys with higher zinc contents have multi phase structures, made up of alpha and beta. The wrought microstructure contains twinned grains of alpha copper solid solution and beta grains. The cast red brasses have less than 12% zinc and less than 7% tin. While the semi red cast brasses have up to 17% zinc and less than 6% tin. These alloys have a wide freezing range and segregation of the alloying elements occurs during solidification. The formation of zinc rich and tin rich beta occurs during solidification. The tin rich phase transforms to alpha plus delta, which fills the areas between the alpha dendrite arms. The wrought structure consists of grains of alpha solid solution and grains of zinc and tin rich alpha and delta phases.

NOTE: The file size of the Larger and Largest View of the Micrographs are substantially larger than the thumbnail shown. The Larger View images range in size from 11K to 120K depending on the image. The Largest View images range in size from 125K to almost 500K.