1. Industry
Send to a Friend via Email

Your suggestion is on its way!

An email with a link to:

http://metals.about.com/od/properties/a/Metal-Profile-Brass.htm

was emailed to:

Thanks for sharing About.com with others!

You can opt-out at any time. Please refer to our privacy policy for contact information.

Discuss in my forum

Metal Profile: Brass

What is brass?

By

Metal Profile: Brass
Wikipedia Commons/Public Domain

Brass is a binary alloy composed of copper and zinc that has been produced for millennia and is valued for its workability, hardness, corrosion resistance and attractive appearance.

Properties:

  • Alloy Type: Binary
  • Content: Copper & Zinc
  • Density: 8.3-8.7 g/cm3
  • Melting Point: 1652-1724 °F (900-940 °C)
  • Moh's Hardness: 3-4

Characteristics:
The exact properties of different brasses depend on the composition of the brass alloy, particularly the copper-zinc ratio.

In general, however, all brasses are valued for their machinability, or the ease with which the metal can be formed into desired shapes and forms while retaining high strength.

While there are differences between brasses with high and low zinc contents, all brasses are considered malleable and ductile (low zinc brasses more so).  Due to its low melting point, brass can also be cast relatively easily. However, for casting applications, a high zinc content is usually preferred.

Brasses with a lower zinc content can be easily cold worked, welded and brazed. A high copper content also allows the metal to form a protective oxide layer (patina) on its surface that guards against further corrosion, a valuable property in applications that expose the metal to moisture and weathering.

The metal has both good heat and electrical conductivity (it's electrical conductivity can from 23% to 44% that of pure copper), and it is wear and spark resistant.

Like copper, its bacteriostatic properties have resulted in its use in bathroom fixtures and healthcare facilities.

Brass is considered a low friction and non-magnetic alloy, while its acoustic properties have resulted in its use in many 'brass band' musical instruments.

Artists and architects value the metal's aesthetic properties, as it can be produced in a range of colors, from deep red to golden yellow.

History:
Copper-zinc alloys were produced as early the 5th century BC in China and were widely used in central Asia by the 2nd and 3rd century BC.

These decorative metal pieces, however, can be best referred to as 'natural alloys', as there is no evidence that their producers consciously alloyed copper and zinc. Instead, it is likely that the alloys were smelted from zinc-rich copper ores, producing crude brass-like metals.

Greek and Roman documents suggest that the intentional production of alloys similar to modern brass, using copper and a zinc oxide rich ore known as calamine, occurred around the 1st century BC.

Calamine brass was produced using a cementation process, whereby copper was melted in a crucible with ground smithsonite (or calamine) ore. At high temperatures, zinc present in such ore turns to vapor and permeates the copper, thereby producing a relatively pure brass with 17-30% zinc content. This method of brass production was used for nearly 2000 years until the early 19th century.

Not long after the Romans had discovered how to produce brass, the alloy was being used for coinage in areas of modern day Turkey. This soon spread throughout the Roman Empire.

Read more about the History of Brass.

Types:
'Brass' is a generic term that refers to a wide range of copper-zinc alloys. In fact, there are over 60 different types of brass specified by EN (European Norm) Standards. These alloys can have a wide range of different compositions depending upon the properties required for a particular application.

Brasses can also be classified in a variety of ways, including by their mechanical properties, crystal structure, zinc content and color.

The most essential distinction, however, is made by their crystal structures. This is because the combination of copper and zinc is characterized by peritectic solidification, an academic way of saying that the two elements have dissimilar atomic structures, making them combine in unique ways depending upon content ratios and temperatures.

Three different types of crystal structure can form as a result of these factors:

1. Alpha Brasses: Alpha brasses contain less than 37% zinc melted into copper and are named for their formation of a homogenous (alpha) crystal structure. Such brasses are softer than their counterparts and, therefore, more easily cold worked, welded and brazed.

2. Alpha-Beta Brasses: Alpha-beta brasses contain between 37-45% zinc and are made-up of both the alpha grain structure, as well as a beta grain structure that is more similar to that of pure zinc. More common than alpha brass, alpha-beta brass is both harder and stronger and, consequently, is usually hot worked by extrusion or stamping and die-casting.

3. Beta Brasses: Although much more rarely used than alpha or alpha-beta brasses, beta brasses make-up a third group of the alloy that contain greater than 45% zinc content. Such brasses form a beta structure crystal and are harder and stronger than both alpha and alpha-beta brasses. As such, they can only be hot worked or cast.

In contrast to crystal structure categorization, identifying brass alloys by their properties allows us to consider the affect of alloying metals on brass. Common categories include:

  • Free machining brass (3% lead)
  • High tensile brasses (aluminum, manganese and iron inclusions)
  • Naval brasses (~1% tin)
  • Dezincification resistant brasses (arsenic inclusion)
  • Brasses for cold working (70/30 brass)
  • Casting brasses (60/40 brass)

The terms 'yellow brass' and 'red brass' - often heard in the US - are also used to identify certain types of brasses. Red brass refers to a high copper (85%) alloy that contains tin (Cu-Zn-Sn), which is also known as gunmetal (C23000), while yellow brass is used to refer to a brass alloy with a higher zinc content (33% zinc), thereby making the brass appear a golden yellow color.

For a list of standard brass designations, see here.

Production:
Brass is most often produced from copper scrap and zinc ingots. Scrap copper is selected based on its impurities, as certain additional elements are desired in order to produce the exact grade of brass required.

Because zinc begins to boil and vaporize at 1665°F (907°C), below copper's melting point 1981° F (1083°C), the copper must first be melted. Once melted, zinc is added at a ratio appropriate for the grade of brass being produced. While some allowance is still made for zinc loss to vaporization.

At this point, any other additional metals, such as lead, aluminum, silicon or arsenic, are added to the mixture to create the desired alloy.

Once the molten alloy is ready, it is poured into molds where it solidifies into large slabs or billets

Billets - most often of alpha-beta brass - can directly be processed into wires, pipes and tubes via hot extrusion, which involves pushing the heated metal through a die, or hot forging.

If not extruded or forged, the billets are then reheated and fed through steel rollers (a process known as hot rolling). The result is slabs with a thickness of less than half an inch (<13mm).

After cooling, the brass is then fed through a milling machine, or scalper, that cuts a thin layer from the metal in order to remove surface casting defects and oxide.

Under a gas atmosphere to prevent oxidization, the alloy is heated and rolled again, a process known as annealing, before it is rolled again at cooler temperatures (cold rolling) to sheets of about 0.1" (2.5mm) thick.

The cold rolling process deforms the internal grain structure of the brass, resulting in a much stronger and harder metal. This step can be repeated until the desired thickness or hardness is achieved.

Finally, the sheets are sawed and sheared to produce the width and length required.

All sheets, cast, forged and extruded brass materials are given a chemical bath, usually used hydrochloric and sulfuric acid to remove black copper oxide scale and tarnish.

Applications:
Brass's valuable properties and relative ease of production has made it one of the mostly widely used alloys. Compiling a complete list of all of brass's applications would be a colossal task, but to get an idea of industries and the types of products in which brass is found we can categorize and summarize some end-uses based on the grade of brass used:

Free cutting brass (e.g. C38500 or 60/40 brass):

  • Nuts, bolts, threaded parts
  • Terminals
  • Jets
  • Taps
  • Injectors

For the full list of brass applications, click here.

Sources:
Copper Development Association. Brass
URL: www.copperinfo.co.uk
The Copper Development Association Inc. www.copper.org
Madehow.com. Brass.
URL: www.madehow.com

Follow Terence on Google+

©2014 About.com. All rights reserved.