Gunmetal

A number of metallic materials are referred to as gunmetal (also red brass , multi- component or machine bronze ) . This is to alloys of copper base , which are used for many industrial products.

Definition of terms

The term “red brass” has been used as a trade name in German-speaking countries since the beginning of the 20th century and is derived from the copper-red color of the alloys in the CuSnZnPb group. In the older literature, the term “gunmetal” is occasionally considered to be “out of date”, but it is still the common trade name in specialist circles.

“Obsolete” refers to the fact that it was not only since the 12th century that people understood how to create larger sculptures from a copper- tin - zinc - lead alloy using the lost wax process. The four-component alloy typical of gunmetal has been used in pharmacy mortars from the Lower Rhine to Italy since the 15th century . Using modern analytical methods, it was found that sculptures were made from a four-substance system as early as the “classical” Bronze Age , because it was presumably found empirically that these were more resistant to weathering and better castable than those made from pure tin bronze.

Copper-tin-zinc-lead alloys are listed in DIN EN 1982 with the abbreviations CC 490 K to CC 493 K. The more detailed designations, such as CuSn3Zn8Pb5-C (associated DIN number CC490K) indicate that the copper base is 81 to 90% still 1.5–11% tin, 1–9% zinc and, if required, 1–8% lead ; the following C refers to the use for cast products (English cast means “cast”).

Cappenberger Barbarossa head
made of gunmetal ( Kaiser Friedrich I. , 1156, Cappenberg)

The lead addition is different from the color the Rotgusslegierungen similar Tombaken (brass alloys such as Goldtombak, Similor, Talmi), with zinc as the sole, or at least the major alloying element.

The literature on gunmetal is partly contradictory. In English, alloys made from 90% copper with 10% tin are called “gunmetal”. Based on this, gunmetal is equated with gunmetal on various occasions. On the other hand, gunmetal is more precisely named “leaded red brass” (red brass with added lead) or in French “bronze d'étain au zinc” (tin bronze with added zinc), in Italian “ottone rosso” (red brass).

From cannon bronze to machine bronze

Originally, a Cu90Sn10 alloy processed into gun barrels was called "cannon bronze" because of its relative hardness under normal use. In England, this alloy was called "Gunmetal", as was a further development of the 19th century with an additional 1.5% zinc, emphasizing the intended use. Gun barrels made from this were also given a steel core tube to better withstand the gas pressure of the propellant charge. In 1859, gun barrels were manufactured exclusively from steel by Friedrich Krupp for the first time (Krupp steel). Until then, gunmetal pipes were clearly superior to the traditional cast-iron or bronze pipes with a smooth interior. In an effort to further improve the alloys, numerous variations were created. Today, at most, the Rübel and Uchatius bronze are still known, the latter also known as “steel bronze” because it leads to particularly tight casts.

The most important reason for the replacement of the “Gunmetal” by “Kruppstahl” was the better heat resistance of the steel, thus the much lower risk of overheating of the gun barrel with rapid firing sequences. Gunmetal already loses its strength from 200 ° C, normal, heat-resistant cast steel only shows a drop at 600 ° C, heat-resistant, alloyed steel cast only at more than 1000 ° C. Steel gun barrels could still be pulled and drilled; Due to the twist, the accuracy of the projectiles was significantly improved.

After the end of its military use, gunmetal became the basis of easily machinable multi-component alloys for the rapidly developing railway technology. The improved gunmetal had already proven that it is advantageous to add a little zinc to a tin bronze. In order to make it easier to work with, lead has now also been added. These new alloys were called “machine bronzes ”, although, in metallurgy correct, they were low-alloy, copper-rich brass and not bronzes.

The changes in color and properties of the alloys, determined by the amount of zinc added, make the transitions between bronzes and brasses appear fluid. The literature also gives a lot of leeway for a precise definition. For example, machine bronze or gunmetal - replacing the term gunmetal for the first time in German-speaking countries - are alloys which, in addition to the typical bronze components copper and tin, may contain up to 8% zinc and 8% or more lead at the expense of the tin content. The Duden (German Spelling, Vol. 1, Mannheim 1991) calls red brass a cast bronze, which is also not terminologically correct. A clear distinction can only be found with higher zinc additions to the copper base, because here the limit between a multi-component bronze and a brass alloy is determined by the zinc content of the alloy between 42% and 37%; one speaks without exception of armature brass, die-cast brass and wrought brass alloys. The English "naval brass" (German "marine brass") is a four-component alloy made of 62 parts copper, 37 zinc and 0.5 parts each of tin and lead and, given the mass proportions, a brass and not a bronze.

Meyers Konversationslexikon, edition 1897, refers to brass when it comes to gunmetal and names a type of brass based on copper-zinc, "gunmetal" or "red brass". This is in contrast to tombac and other zinc-containing alloys, which, with increasing zinc content, are ultimately assigned to brass, i.e. brass as it is understood today. "Red brass", a correct term in Italian as "ottone rosso" to this day, meant a color differentiation from the brass that was manufactured in the "brass foundry". This was derived in the 18th century from the Dutch term “Geelgieters”. Until the 20th century, only the bronze foundries succeeded in defining the concept of the brass foundry. From the 15th century there are reports of " red casters ", but they are said to have been copper foundries in contrast to tin foundries.

The casting of metals, which was carried out by hand rather than industrially until the 19th century, required a combination of all foundry tasks based on a base metal for economic reasons. “Heavy metal foundries” for bronzes, brasses and gunmetal and “light metal foundries” for aluminum and magnesium alloys emerged, each with the ability to meet a wide range of requirements in terms of knowledge and production technology. Specialists such as die casters were excluded for some time, as were lead, zinc and tin foundries, many more associated with handicrafts than industry.

From machine bronze to gunmetal

After Krupp's invention of cast steel in 1859, gun bronzes, including gunmetal, lost their military importance. However, this loss was quickly offset by the development of the railroad, first in locomotive construction. It turned out that the multicomponent copper-based alloys with the addition of tin, zinc and lead were easy to cast and work with and also had excellent sliding and emergency running properties , which made them the first material to be sought for plain bearings and bushings. With an addition of up to 2.5% nickel, not only was the toughness improved, the associated increase in strength allowed the wall thickness to be reduced and, last but not least, the cavitation resistance to seawater was increased. In rare cases, antimony was also used as an alloy component.

Variations in the composition made it possible to supply the industry with bearings for both normal and high, high-speed loads. Further areas of application were found in machine parts of all kinds subject to movement such as gear wheels and pinions, in pump housings and pressure-tight fittings. In view of this knowledge, which was already gained in the 19th century, the term “machine bronze” was established in Germany. As early as the 20th century, this name was replaced by the trade name “gunmetal”, which is still used today.

Finished parts made of copper-tin and copper-tin-zinc cast alloys
Pump housing made of G-CuSn5ZnPb
Mixing valve housing made of G-CuSn5ZnPb

The standardized copper-tin-zinc alloys

(Table according to the German Copper Institute )

Copper-tin-zinc cast alloys (gunmetal)

Standardization according to DIN EN 1982, previously DIN 1705

CuSn7Zn4Pb7 C CC493K also referred to as RG7 (gunmetal 7), based on the tin content
CuSn7Zn2Pb3 C CC492K
CuSn3Zn8Pb5 C CC490K
CuSn5Zn5Pb5 C CC491K also called RG5 (gunmetal 5).
RG7 and RG5 are the two mostly cast alloys in this group.

Nickel additions of up to 2% are still in accordance with the standard in all cases, since nickel is usually added to the copper content.

When used for gunmetal fittings in drinking water, the lead and nickel content must be reduced according to DIN 50930 Part 6.

With regard to the state of the structure, in the presence of low-melting lead (327 ° C) it is important that it fills the interdendritic cavities resulting from the solidification shrinkage.

Melt management and treatment

The Melt flow in the melting unit, as well as the melt treatment in the interest of optimized casting quality is different for red brass, whether it's casting or formats cast not (mostly as a semifinished product in the form of continuously cast rounds) of what principle applies to all copper alloys that do not Contain alloy components - such as aluminum - that can be oxidized more easily than copper. Copper and its alloys tend to absorb hydrogen. This can already be caused by the ambient air during melting, but it can also be brought along from the mission. Cathode copper ( blister copper ) always contains hydrogen; organic contaminants (oil and grease residues) can adhere to recycled scrap metal. A furnace with excess air, the oxidizing melting method , helps with the combustion of hydrogen and hydrocarbons and leads to a melt with excess oxygen in the form of copper oxide and oxides of the accompanying elements. In this state, which negatively affects castability and the solidification structure, nothing changes due to the fact that at temperatures> 1150 ° C zinc components of the melt become volatile as zinc vapor and become zinc oxide on the bath surface . More controllable than the oxides of the other accompanying elements - tin oxide tends to persist in the melt and cannot be reduced either - the main focus must therefore be on copper oxide (Cu ₂ O) in order to improve the castability of the melt in the long term.

The excess oxygen remaining in copper-rich melts after the oxidation of the hydrogen to volatile H ₂ O (water vapor) is subsequently neutralized by the strongly deoxidizing effect of the added phosphorus, i.e. by binding oxygen atoms to itself. Two phosphorus atoms plus five oxygen atoms coming from the copper oxide present become the compound P ₂ O ₅ ( phosphorus pentoxide ), which is also volatile at melt temperatures . At 358 ° C it sublimes and leaves the melt. As a result of this reduction in the copper oxides, the melt becomes thinner and now allows the tin oxides (ZnO, density 5.6), as well as comparable, non-reducible, oxidic reaction products to rise to the bath surface and enter the slag to be removed.

The problem with any deoxidizing treatment is simply finding a balance between too little oxygen, as this would lead to renewed uptake of hydrogen, and too much effective oxygen. This prevents the absorption of hydrogen, but has a negative effect on the quality of the melt, because oxides generally have a negative effect on the quality of the casting.

For the casting of copper alloys, and this includes the gunmetal multi-component system, which is particularly sensitive in these contexts, the following applies in summary: Between oxidizing and reducing melting, precise control of the furnace operation must be carried out with prior knowledge of the quality of use. The burner is to run with excess air, i.e. oxidizing. Oxygen-releasing additives to the melt are an important aid in the oxidation of hydrogen and hydrogen-containing impurities. Depending on the amount of reactive oxygen released by them from suitable compounds such as nitrates, they can be used to remove or at least noticeably reduce undesirable, easily oxidizable elements such as aluminum. The treatment of the melt is concluded with a fine adjustment, which has the effect that the melt is and remains oxide-free and also has a low residual phosphorus content that prevents renewed oxide formation. This is achieved by adding a copper-phosphorus master alloy (preferably CuP10) to the melt ready for casting. In practice, 0.25% CuP10 is added, for the melt this is 0.025% phosphorus, which after deoxidation still ensures a residual phosphorus content of 0.05% in the cast structure.

Properties and use

The trade name gunmetal alone does not provide any information about the possible uses. The designation machine bronze is more meaningful because it refers to the good sliding properties that are important for machines with their moving parts (gear parts, gears). Added to this are the good casting properties of the alloy group. The alloy CuSn5Zn5Pb5 (called RG 5 for simplicity) is used in the sand casting process (casting in sand molds that can be used once) when heavier parts with a shape linked to the use of cores, such as pump housings or comparable fittings in the field of heating technology such as water supply, are required, But smaller parts can also be made using permanent mold casting (casting in permanent metallic molds). There is only a restriction on the use of lead-containing gunmetal in systems that carry drinking water. The applicable standard DIN 50930 Part 6 requires all parts installed there to have reduced lead and nickel contents. This corresponds to the alloy CuSn7Pb3 with <0.2% nickel instead of the <2.0% otherwise tolerated for red brass. In addition, DVGW approval is required for use as fittings and pipe connections in the drinking water sector.

For standardized parts in mass production 5 provides the alloy RG the advantage in the train - and centrifugal casting to be castable and semi-finished products for mostly mechanized production transitions to provide (bearing bushes of all sizes). The tensile strength is between 180 and 300 N / mm², the higher mechanical values for tensile strength, 0.2% yield strength and elongation at break are provided by the alloy CuSn7Zn4Pb7 (RG 7), which already has emergency running properties (insensitivity to temporary lubrication failure) with a somewhat lower hardness thus fulfills a basic requirement for a plain bearing material. Both alloys are insensitive to salt water, which has proven itself when used in shipbuilding. They are very easy to machine and, depending on the casting process, can be cast from 830 ° C (upper limit 1020/1030 ° C).

The ternary alloy based on copper with added tin and lead, which is still assigned to gunmetal regardless of the missing zinc component, is a bearing material (bearing metal) that is used because of its particularly good emergency running properties.

Economical meaning

The economic importance of gunmetal must not be measured by the much larger numbers for iron castings, but only by its contribution to casting from non-ferrous metals and alloys and here again within the copper sector. For 2006, the GDM states a total of 998,000 t of non-ferrous cast iron in Germany. Almost 99,000 t of this is in the copper sector and within this 67,782 t or 68.7% are red brass, which is divided into sand casting , centrifugal casting and continuous casting .

Literature used

Information printing i25, as well as i025. Publisher DKI (German Copper Institute, Düsseldorf).
Materials, concise dictionary of technical goods and their components, editor Prof. Dr. Paul Krais, published by Johann Ambrosius Barth, Leipzig 1921.
Foundry glossary, 17th edition, 1997, Verlag Schiele & Schön, Berlin, ISBN 3-7949-0606-3 .
Meyers Konversationslexikon, 5th edition, 1897, Bibliographisches Institut Leipzig and Vienna.
Casting of copper alloys (Casting copper-base-alloys) from the American by Dipl.-Ing. Ernst Brunhuber, Schiele & Schön, Berlin 1986, ISBN 3-7949-0444-3 , p. 181 f. and 227 f.
Friedrich Kluge , Etymological Dictionary of the German Language, Walter de Gruyter, Berlin and New York 1975, ISBN 3-11-005709-3 .
Lexicon of metal technology, editor Josef Bersch, A. Hartlebens Verlag, Vienna. Pest. Leipzig, without a year. Full text on wikisource.org .

Web links

Individual evidence

^ Paul Krais, Werkstoffe, 2nd volume GR, JA Barth, Leipzig 1921.
↑ gilded bust of Frederick 1st (Barbarossa) in Cappenberg, s. Bild., Or in Braunschweig the "lion".
↑ s. Heinz Wübbenhorst, "Giessen von Metallen", p. 57 f., Ed. VDG, Gießereiverlag, Düsseldorf, 1984, ISBN 3-87260-060-5 .
↑ "Gunmetal, a bronze formerly used for guns or cannons", according to Webster's New Collegiate Dictonnary, by G&C. Merriam comp., Springfield, Mass., 1980.
^ Ernst Brunhuber, Giesserei-Fachwörterbuch, Verlag Schiele & Schön, Berlin 1977, ISBN 3-7949-0283-1 .
↑ Kluge, etymological dictionary, 20th ed., P. 209.
↑ "Der große Duden", Etymologie, Vol. 7, p. 207.
↑ see also under "further reading"
^ Foundry glossary, 17th edition, 1997, Verlag Schiele & Schön, Berlin, ISBN 3-7949-0606-3 , state diagrams of ternary alloy systems. P. 1455 f.
↑ see also: "The melt treatment of copper alloys", self-published by former Dr. Riedelbauch & Stoffregen GmbH, D 6554 Meisenheim / Glan
↑ Casting copper-base-alloys from the American by Dipl.-Ing. Ernst Brunhuber, Schiele & Schön, Berlin 1986, ISBN 3-7949-0444-3 , p. 181 f. and 227 f.
↑ General Association of German Metal Foundries e. V., Düsseldorf, homepage
↑ Sand casting 24,107 t, continuous casting 37,270 t, centrifugal casting 621 t

[1] Paul Krais, Werkstoffe, 2nd volume GR, JA Barth, Leipzig 1921.

[2] ust of Frederick 1st (Barbarossa) in Cappenberg, s. Bild., Or in Braunschweig the "lion".

[3] s. Heinz Wübbenhorst, "Giessen von Metallen", p. 57 f., Ed. VDG, Gießereiverlag, Düsseldorf, 1984, ISBN 3-87260-060-5 .

[4] "Gunmetal, a bronze formerly used for guns or cannons", according to Webster's New Collegiate Dictonnary, by G&C. Merriam comp., Springfield, Mass., 1980.

[5] Ernst Brunhuber, Giesserei-Fachwörterbuch, Verlag Schiele & Schön, Berlin 1977, ISBN 3-7949-0283-1 .

[6] Kluge, etymological dictionary, 20th ed., P. 209.

[7] "Der große Duden", Etymologie, Vol. 7, p. 207.

[8] see also under "further reading"

[9] Foundry glossary, 17th edition, 1997, Verlag Schiele & Schön, Berlin, ISBN 3-7949-0606-3 , state diagrams of ternary alloy systems. P. 1455 f.

[10] see also: "The melt treatment of copper alloys", self-published by former Dr. Riedelbauch & Stoffregen GmbH, D 6554 Meisenheim / Glan

[11] Casting copper-base-alloys from the American by Dipl.-Ing. Ernst Brunhuber, Schiele & Schön, Berlin 1986, ISBN 3-7949-0444-3 , p. 181 f. and 227 f.

[12] General Association of German Metal Foundries e. V., Düsseldorf, homepage

[13] Sand casting 24,107 t, continuous casting 37,270 t, centrifugal casting 621 t