Gunmetal - the material
Composition and structure of the gunmetal alloy
Gunmetal is a valve and plumbing material standardised in accordance with DIN 50930-6 / DIN EN 1982 that, with its diversified applications is especially suited to sanitary installation engineering, e.g. as a valve material for stop, securing and regulating valves, as fitting material for pipe-system components and as a construction material in water, filtering and recycling engineering.
Drinking water applications
|Gunmetal = CuSn5Zn5Pb2||Elements suitable for drinking water according to DIN 50930-6|
|Tin Sn||No information|
|Zinc Zn||No information|
|Lead Pb||max. 3,0 %|
|Nickel Ni||max. 0,6 %|
|Antimony Sb||max. 0,1 %|
|Impurities||Respective max. 0,02 %|
|Release of elements according to DIN 50930-6*|
* DIN 50930-6 merely provides information about the influence on the composition of drinking water by metal ion migration. It does not specify the corrosion restistance of the material.
Mechanical engineering application
|Gunmetal 5 = CuSn5Zn5Pb5-C||Elements according to DIN EN 1982|
|Copper Cu||83,0 - 87,0 %|
|Tin Sn||4,0 - 6,0 %|
|Zinc Zn||4,0 - 6,0 %|
|Lead Pb||4,0 - 6,0 %|
|Nickel Ni||0,0 - 2,0 %|
|Antimony Sb||0,0 - 0,25 %|
|Gunmetal according to DIN EN 1982|
The alloy elements all go into dissolution with the exception of lead in the alpha phase of the mixed crystal, i.e., with the exception of lead, they can no longer be metallographically detected as individual elements.
The elementary lead is exuded and precipitated on the grain boundary at the end of the solidification and on the cavities arising through the volume contraction. It is available in the form of granular, grey occlusions in the reddish alpha-base – nickel, tin and zinc are completely dissolved in copper.
In this alloy, just as in all other plumbiferous copper alloys, the lead basically has the function of a chip breaker, see photo below. That is one of the very important prerequisites to be able to economically and industrially process this material. They need to be able to be machined quickly and automatically as the price of a material is determined less and less by its acquisition cost and increasingly by its processing and post treatment costs.
The chips illustrated in b) prevent the material from being automatically processed. They entwine themselves in the processing tool, requiring frequent manual intervention by the machine operator. The short chips shown in a) fall down from the processing point without any problems and can be automatically removed by the machine there. The lead has no other function in the alloy.
a) short chip b) long chip
Nickel is added to the alloy to distribute the lead uniformly and finely during large wall thickness differences. That creates the prerequisite that the alloy's mechanical parameters are secured according to DIN EN 1982 without any great casting efforts. We are not currently aware of any solid scientific examinations about the total influence of nickel on the above-mentioned alloy. There is a need for examination here and industry will supply it.
Due to its solidification characteristics and its alloy composition, gunmetal can only be cast in sand and continuous casting. While that is very expensive, it also enables a cast design that is very near the final form of the component, which can be managed with only a small amount of cost-intensive processing. That is a clear contrast to components made of forged or pressed blanks. The argument about more highly pressure-sealed components in forging and pressed parts is equally well answered by the 100 % leak test performed on the cast parts.
The better aerodynamic form of a cast product (see photo 3) speaks for itself in its noise development and functionality when compared with the sharp-edged and mechanically processed interior contours and deflections in brass components.
|c) Cast construction||d) Machining construction|
|Round moulds||Sharp edges|
|Less processing||Increased chip volumes|
|Reduced flow losses||Adverse flowing|
Components made of gunmetal
The quality of copper, together with the hygiene of tin, the easier processing of lead and the low proportion of the reactive zinc has led to gunmetal being used in many areas in life. The close relationship to bronze – gunmetal is also referred to as multi-component bronze – characterises the main attributes of this alloy. The corrosion resistance of gunmetal is especially important. Gunmetal is corrosion resistant against the corrosion forms often found in drinking water, dezincification and stress-crack corrosion. The multitude of applications result from that.
With its outstanding material characteristics, the use of gunmetal ranges from drinking water through aggressive seawater and on up to process water. In the same way, petrochemical, dye and paint products along with gasses and liquid gasses are conducted through components made from this material. At the same time, the material allows operational temperatures of – 176 °C to + 225 °C. That is because the gunmetal itself underlies nearly no measurable embrittlement even at such low temperatures. For that reason, the material is also preferably used as a valve material in gas liquefaction. Even such critical, liquefied gasses such as oxygen, nitrogen etc. are safely conducted, distributed and blocked through valves made of gunmetal at approx. – 176 °C.