Brass is an alloy, a mixture of copper and zinc. Unlike pure metals, which melt at a single, precise temperature, brass melts across a range. For most common commercial grades, this melting range typically spans between 900°C and 940°C (1,650°F and 1,720°F). Understanding this thermal behavior is important for any application involving heating the material, such as casting or welding. The specific temperature at which the material fully transitions to a liquid state depends on the exact ratio of copper to zinc and any minor additives.
The Variable Melting Range
The characteristic of melting across a temperature interval is a defining feature of alloys. This range is metallurgically defined by two points: the solidus and the liquidus. The solidus is the temperature at which the material first begins to liquefy. The liquidus is the higher temperature at which the material becomes entirely liquid.
The span between the solidus and liquidus temperatures is known as the freezing range, which can vary widely depending on the brass grade. For instance, C28000 Muntz Metal, an alpha-beta brass with a higher zinc content, has a narrow melting range, becoming fully liquid between 899°C and 904°C (1,650°F and 1,660°F). Conversely, C26000 Cartridge Brass, which contains less zinc, has a slightly higher range, melting between 915°C and 955°C (1,680°F and 1,750°F).
The Impact of Alloy Composition
The ratio of copper to zinc is the most influential factor in determining the specific melting range of any brass alloy. Copper melts at a fixed temperature of 1084°C, while pure zinc melts at 419.5°C. Increasing the percentage of zinc in the alloy generally results in a lower overall solidus and liquidus temperature for the brass.
A chemical challenge arises when brass is heated near its melting point due to the low boiling point of zinc, which is 907°C. Zinc begins to vaporize, or volatilize, before the brass alloy even reaches its liquidus temperature. This vaporization causes toxic zinc oxide fumes to be released and results in a change in the composition of the remaining molten brass.
As the zinc content is driven off, the remaining alloy becomes richer in copper. Since copper has a higher melting point, the loss of zinc effectively raises the melting temperature of the remaining material. This loss of a key component can alter the material’s mechanical properties, requiring careful temperature management during any high-heat process.
Practical Considerations for High-Heat Applications
For industrial applications such as casting, welding, or brazing, precise temperature control is important when working with brass. To achieve a fully liquid state for casting, the material must be heated past its liquidus temperature, but overheating must be avoided to minimize zinc loss. Overheating can lead to significant zinc volatilization, which not only changes the final alloy composition but also produces hazardous fumes.
Because zinc starts to vaporize at temperatures near the solidus, adequate ventilation is required for safety whenever brass is heated. The resulting zinc oxide fumes can cause a temporary illness known as metal fume fever, making respiratory protection necessary in many industrial settings.
The difference between melting temperatures for casting and softening temperatures for heat treatment is also important. Annealing, a process used to soften the material for subsequent forming, occurs at temperatures well below the solidus, typically between 600°C and 700°C. This ensures the brass remains fully solid while the internal structure is homogenized.