Brass

Background

Brass is a metal composed primarily of copper and zinc. Copper is the main component, and brass is usually classified as a copper alloy. The color of brass varies from a dark reddish brown to a light silvery yellow depending on the amount of zinc present; the more zinc, the lighter the color. Brass is stronger and harder than copper, but not as strong or hard as steel. It is easy to form into various shapes, a good conductor of heat, and generally resistant to corrosion from salt water. Because of these properties, brass is used to make pipes and tubes, weather-stripping and other architectural trim pieces, screws, radiators, musical instruments, and cartridge casings for firearms.

History

Ancient metalworkers in the area now known as Syria or eastern Turkey knew how to melt copper with tin to make a metal called bronze as early as 3000 B.C. Sometimes they also made brass without knowing it, because tin and zinc ore deposits are sometimes found together, and the two materials have similar colors and properties.

By about 20 B.C.-A.D. 20, metalworkers around the Mediterranean Sea were able to distinguish zinc ores from those containing tin and began blending zinc with copper to make brass coins and other items. Most of the zinc was derived by heating a mineral known as calamine, which contains various zinc compounds. Starting in about 300 A.D., the brass metalworking industry flourished in what is now Germany and The Netherlands.

Although these early metalworkers could recognize the difference between zinc ore and tin ore, they still didn't understand that zinc was a metal. It wasn't until 1746 that a German scientist named Andreas Sigismund Marggraf (1709-1782) identified zinc and determined its properties. The process for combining metallic copper and zinc to make brass was patented in England in 1781.

The first metal cartridge casings for firearms were introduced in 1852. Although several different metals were tried, brass was the most successful because of it's ability to expand and seal the breech under pressure when the cartridge was first fired, then contract immediately to allow the empty cartridge casing to be extracted from the firearm. This property led to the development of rapid-fire automatic weapons.

Raw Materials

The main component of brass is copper. The amount of copper varies between 55% and 95% by weight depending on the type of brass and its intended use. Brasses containing a high percentage of copper are made from electrically refined copper that is at least 99.3% pure to minimize the amount of other materials. Brasses containing a lower percentage of copper can also be made from electrically refined copper, but are more commonly made from less-expensive recycled copper alloy scrap. When recycled scrap is used, the percentages of copper and other materials in the scrap must be known so that the manufacturer can adjust the amounts of materials to be added in order to achieve the desired brass composition.

The second component of brass is zinc. The amount of zinc varies between 5% and 40% by weight depending on the type of brass.

Brasses with a higher percentages of zinc are stronger and harder, but they are also more difficult to form and have less corrosion resistance. The zinc used to make brass is a commercial grade sometimes known as spelter.

Some brasses also contain small percentages of other materials to improve certain characteristics. Up to 3.8% by weight of lead may be added to improve machinability. The addition of tin improves corrosion resistance. Iron makes the brass harder and makes the internal grain structure smaller so that the metal can be shaped by repeated impacts in a process called forging. Arsenic and antimony are sometimes added to brasses that contain more than 20% zinc in order to inhibit corrosion. Other materials that may be used in very small amounts are manganese, silicon, and phosphorus.

Design

The traditional names for various types of brass usually reflected either the color of the material or the intended use. For example, red brass contained 15% zinc and had a reddish color, while yellow brass contained 35% zinc and had a yellowish color. Cartridge brass contained 30% zinc and was used to make cartridges for firearms. Naval brasses had up to 39.7% zinc and were used in various applications on ships.

Unfortunately, scattered among the traditional brass names were a number of misnomers. Brass with 10% zinc was called commercial bronze, even though it did not contain any tin and was not a bronze. Brass with 40% zinc and 3.8% lead was called architectural bronze, even though it was actually a leaded brass.

As a result of these sometimes confusing names, brasses in the United States are now designated by the Unified Numbering System for metals and alloys. This system uses a letter—in this case the letter "C" for copper, because brass is a copper alloy—followed by five digits. Brasses whose chemical composition makes them suitable for being formed into the final product by mechanical methods, such as rolling or forging, are called wrought brasses, and the first digit of their designation is I through 7. Brasses whose chemical composition makes them suitable for being formed into the final product by pouring molten metal into a mold are called cast brasses, and the first digit of their designation is 8 or 9.

The Manufacturing Process

The manufacturing process used to produce brass involves combining the appropriate raw materials into a molten metal, which is allowed to solidify. The shape and properties of the solidified metal are then altered through a series of carefully controlled operations to produce the desired brass stock.

Brass stock is available in a variety of forms including plate, sheet, strip, foil, rod, bar, wire, and billet depending on the final application. For example, brass screws are cut from lengths of rod. The zigzag fins used in some vehicle radiators are bent from strip. Pipes and tubes are formed by extruding, or squeezing rectangular billets of hot brass through a shaped opening, called a die, to form long, hollow cylinders.

The differences between plate, sheet, strip, and foil are the overall size and thickness of the materials. Plate is a large, flat, rectangular piece of brass with a thickness greater than about 0.2 in. (5 mm)—like a piece of plywood used in building construction. Sheet usually has the same overall size as plate, but is thinner. Strip is made from sheet that has been cut into long, narrow pieces. Foil is like strip, only much thinner. Some brass foil can be as thin as 0.0005 in (0.013 mm).

The actual manufacturing process depends on the desired shape and properties of the brass stock, as well as the particular machinery and practices used in different brass plants. Here is a typical manufacturing process used to produce brass sheet and strip.

Melting

• 1 The appropriate amount of suitable copper alloy scrap is weighed and transferred into an electric furnace where it is melted at about 1,920°F (1,050°C). After adjusting for the amount of zinc in the scrap alloy, an appropriate amount of zinc is added after the copper melts. A small amount of additional zinc, about 50% of the total zinc required, may be added to compensate for any zinc that vaporizes during the melting operation. If any other materials are required for the particular brass formulation, they are also added if they were not present in the copper scrap.
• 2 The molten metal is poured into molds about 8 in x 18 in x 10 ft (20 cm x 46 cm x 3 m) and allowed to solidify into slabs called cakes. In some operations, the melting and pouring are done semi-continuously to produce very long slabs.
• 3 When the cakes are cool enough to be moved, they are dumped out of the molds and moved to the rolling area where they are stored.

Hot rolling

• 4 The cakes are placed in a furnace and are reheated until they reach the desired temperature. The temperature depends on the final shape and properties of the brass stock.
• 5 The heated cakes are then fed through a series of opposing steel rollers which reduce the thickness of the brass step-by-step to about 0.5 in (13 mm) or less. At the same time, the width of the brass increases. This process is sometimes called breakdown rolling.
• 6 The brass, which is now much cooler, passes through a milling machine called a scalper. This machine cuts a thin layer off the outer faces of the brass to remove any oxides which may have formed on the surfaces as a result of the hot metal's exposure to the air.

Annealing and cold rolling

• 7 As the brass is hot rolled it gets harder and more difficult to work. It also loses its ductility, or ability to be stretched further. Before the brass can be rolled further, it must first be heated to relieve some of its hardness and make it more ductile. This process is called annealing. The annealing temperatures and times vary according to the brass composition and desired properties. Larger pieces of hot-rolled brass may be placed in a sealed furnace and annealed together in a batch. Smaller pieces may be placed on a metal belt conveyor and fed continuously through a furnace with airtight seals at each end. In either method, the atmosphere inside the furnace is filled with a neutral gas like nitrogen to prevent the brass from reacting with oxygen and forming undesirable oxides on its surface.
• 8 The annealed pieces of brass are then fed through another series of rollers to further reduce their thickness to about 0.1 in (2.5 mm). This process is called cold rolling because the temperature of the brass is much lower than the temperature during hot rolling. Cold rolling deforms the internal structure of the brass, or grain, and increases its strength and hardness. The more the thickness is reduced, the stronger and harder the material becomes. The cold-rolling mills are designed to minimize deflection across the width of the rollers in order to produce brass sheets with near-uniform thickness.
• 9 Steps 7 and 8 may be repeated many times to achieve the desired thickness, strength, and degree of hardness. In some plants, the pieces of brass are connected together into one long, continuous sheet and are fed through a series of annealing furnaces and rolling mills arranged in a vertical serpentine pattern.
• 10 At this point, the wide sheets may be slit into narrower sections to produce brass strip. The strip may then be given an acid bath and rinse to clean it.

Finish rolling

• 11 The sheets may be given a final cold rolling to tighten the tolerances on the thickness or to produce a very smooth surface finish. They are then cut to size, stacked or coiled depending on their thickness and intended use, and sent to the ware-house for distribution.
• 12 The strip may also be given a final finish rolling before it is cut to length, coiled, and sent to the warehouse.

Quality Control

During production, brass is subject to constant evaluation and control of the materials and processes used to form specific brass stock. The chemical compositions of the raw materials are checked and adjusted before melting. The heating and cooling times and temperatures are specified and monitored. The thickness of the sheet and strip are measured at each step. Finally, samples of the finished product are tested for hardness, strength, dimensions, and other factors to ensure they meet the required specifications.

The Future

Brass has a combination of strength, corrosion resistance, and formability that will continue to make it a useful material for many applications in the foreseeable future. Brass also has an advantage over other materials in that most products made from brass are recycled or reused, rather than being discarded in a landfill, which will help ensure a continued supply for many years.