Gunmetals

Gunmetals

Alloys of copper with tin, zinc and lead have been used for at least 2,000 years due to their ease of casting, good strength and corrosion resistance. Early uses were for brooches, mirror cases, church doors, fonts and statues. The use for cannon in Medieval times led to the term ‘gun metal’ being adopted; this use is now obsolete with the manufacture of steel ordnance.

Today, in the UK, the term ‘gunmetal’ is applied to a family of copper-based casting alloys containing between 2-11% tin and 1-10% zinc. Modified forms may contain, in addition, such elements as lead (up to 7%) and nickel (up to 6%) when the alloys are classified as ‘leaded gunmetal’ and ‘nickel gunmetal’.

Gunmetals are noted for the manufacture of intricate castings required to be pressure tight such as valves, pipe fittings and pumps. They are also frequently used for bearings where loads and speeds are moderate.

Statue of Sir John Betjeman at St Pancras Station, London

Gunmetals are the first choice for modern statues, an example being the 2.1 metre high sculpture of Sir John Betjeman by Martin Jennings in St Pancras International Station, London.

FAQs

I am designing a cast architectural feature as part of a town centre display board, which the public can access. I have been offered LG3, leaded gunmetal CuSn7Pb4Zn2 for the project.. Please advise.

This alloy has been widely used for some notable contemporary statues since it patinates easily and is capable of being deformed to remove any distortion produced on casting. However, since no current standard exists for this alloy there might be a problem since the public has access to the structure, so it is suggested that leaded gunmetals CC491K (LG2) or CC492K (LG4) are used.

I am trying to identify the brass Hpb59-1. It is proposed to use this brass in the cast condition for valves used in fire protection systems in buildings.

This is a Chinese Specification composition 57-60 copper, 0.8-1.9% lead, balance zinc. It is a leaded Muntz Metal. A more suitable alloy for this application would be the gunmetal LG2 (CC491K) 85%copper, 5% tin, 5% zinc, 5% lead. It has better corrosion resistance and is easily cast.

I am using a leaded gunmetal casting which is machined and then crimped onto a copper pipe. I have had a number of failures due to leakage. Can you explain this?

Leaded Gunmetal is a cast alloy which is not designed to be cold worked. Crimping involves extreme deformation which will not usually be tolerated by the gunmetal without cracking. However, castings do not have a uniform structure and in the areas free from lead and possibly a smaller grain size some ductility is present. This will account for some of the castings surviving the crimping without cracking.

I have been asked to produce 22 valves in gunmetal from a drawing which originates from 1958. The company has been taken over and the patterns and casting drawings lost. Due to the small quantity that will have to be made from bar, what should I use?

Suggest that you use continuously cast CC491K (LG2) bar or a wrought free-machining phosphor bronze.

What material is recommended for underground pipe fittings?

Gunmetal or dezincification resistant brass.