Gunmetals and Leaded Bronzes


The term gunmetal applied today originates from the mid-19th century, when zinc was first added to binary bronze ordnance parts to improve their casting characteristics. British Admiralty gunmetal, with its nominal composition 88% copper, 10% tin, and, 2% zinc, was thus developed.
Although these alloys are now no longer used for ordnance parts the term "gunmetal" has remained; they are extensively used today in many different engineering fields.

The term gunmetal applied today originates from the mid-19th century, when zinc was first added to binary bronze ordnance parts to improve their casting characteristics. British Admiralty gunmetal, with its nominal composition 88% copper, 10% tin, and, 2% zinc, was thus developed. Similar specifications also became standard, the French ordnance alloy being Cu-90%, Sn-6% Zn-4%, and the U.S. Ordnance alloy Cu-88%, Sn-8% Zn-4%, this latter specification being now covered by G2-C.

Although these alloys are now no longer used for ordnance parts the term "gunmetal" has remained; they are extensively used today in many different engineering fields. Gunmetal has good casting characteristics, particularly as a sand casting, and so is often employed in the production of pump casings and for similar components where comparatively high strength, coupled with pressure tightness and corrosion resistance are important requirements.

G1-C is often specified for various types of valve guides, bearings and bushes, particularly in the gas and oil engine field and where bearing/shaft alignments can be ensured and lubrication is good. Gunmetal has a low coefficient of friction. Very good corrosion resisting properties makes its use commonplace in marine engine ring.

With nickel content at 1%, gunmetals are often employed for valves and gears or for those applications where hardness and toughness are important attributes. Overall, the wear resistance of gunmetal is not as good as phosphor bronze.

G3 alloy, particularly in the sand-cast condition "as cast", shows improved tensile strength, elongation, compressive strength, hardness and impact resistance compared with Admiralty Gunmetal G1. However, its production is slightly more difficult and calls for special care of techniques of manufacture. These mechanical properties can be improved still further by heat treatment but at the expense of some impact resistance and ductility.

The G3 alloy finds particular use in atmospheric corrosive conditions, especially where the concentration of sulfur dioxide reaches a high level. Wear resistance properties are better in the "as cast" condition than the heat treated condition and for slow moving bearings, it is quite satisfactory for many applications. In the heat treated condition, it is used for bearing cages, pump and valve parts, impellers, liners, electrical switch gear components, trolley hangers, etc. This alloy is, however, not satisfactory for use at elevated temperatures because it is age hardenable, and becomes embrittled.

Leaded Gunmetals

The alloys in this group (LGI-C, LG2-C, LG3-C and LG4-C) have lower tin contents than the gunmetals and contain lead.

LG1-C Sn=2.0-4.0%; Pb=3.0-6.0%; Zn=7.0-10.0%; Ni=1.0%; Cu=Rem.
LG2-C Sn=4.0-6.0%; Pb=4.0-6.0%; Zn=4.0-6.0%; Ni=2.0%; Cu=Rem.
LG3-C Sn=6.0-8.0%; Pb=1.0-3.0%; Zn=3.0-5.0%; Ni=2.0%; Cu=Rem.
LG4-C Sn=6.5-7.5%; Pb=2.5-3.0%; Zn=1.5-3.0%; Ni=2.0%; Cu=Rem.

Their most common application is for all types of general and constructional castings, particularly in the form of pump casings, valve bodies, miscellaneous housing and water pump fitments where strength is not an important factor. They possess excellent pressure tightness qualities, which, many authorities claim, improve with increase of zinc content. The corrosion resistance properties of leaded gunmetals are generally good.

Chill cast leaded gunmetals are widely used for miscellaneous small bushes of low stressing but they are, of course, considerably less wear resistant than gunmetal and phosphor bronzes.

Of the group, probably leaded gunmetal LG2-C is most commonly employed where pressure tightness is essential. Similarly, leaded gunmetal of the LG4 type is extensively used for low stressed bushes.

Principal Uses

Gl-C (DIN1705-Rg.10; ASTM.B.143-52.1A, etc). This is the most commonly used lead free gunmetal and is considered a high grade alloy used extensively for corrosion resistance in marine conditions and for pressure tightness.

The alloy has many uses such as for bearings and bushes, pumps and pump fittings, valves, valve bodies and valve guides. It is also used to some extent for gears, particularly when the nickel content, which gives the toughness and hardness required, is near the top limit of 1%. It must, however, be remembered that its overall wear resistance properties are inferior to the phosphor bronzes.

G2-C. Because of its slightly lower tin content this alloy is sometimes favored on economic grounds. Whilst its applications are similar to those of G1-C, it is not nearly so commonly used.

G3-C. This alloy is used for corrosive conditions and is finding increasing use in the electrification of the railways for insulator hanger supports. It is also used for cast components, such as actuating nuts, valve and pump and similar components at normal temperatures. It responds to heat treatment on the improved permanent set stress and ultimate strength with some loss of elongation.

LG1-C. This alloy is by far the least used of the leaded gunmetals, although its uses must be considered as similar in some respects to LG2-C.

LG2-C. The principal uses of this alloy are for valve bodies, pump bodies, elbows, pipes, taps and cocks and other hydraulic fittings where pressure tight properties are important. Its corrosion resistance properties rate fairly high and it also finds application in low-stressed bearings and moving parts.

LG3-C. A general purpose alloy for pressure components used for pump and valve parts, elbows, pipes, etc.

LG4-C. A very old established alloy extensively used on the railways and elsewhere for bearing and engine components, tractor parts, etc., and also for pressure components, pumps, valve parts, elbows, pipes, etc.

Leaded Bronzes

Leaded bronzes are alloys containing tin, lead and copper as the main constituents, sometimes with the addition of zinc and nickel. The structure consists of a matrix of copper with tin in solid solution, known as the alpha phase, which is comparatively soft, with a hard tin rich delta phase interspersed throughout the matrix.

Lead is insoluble in the copper base alloys and is distributed in fine globules throughout the alloy. Lead additions make the alloys more plastic and in bearing practice this is useful with softer shafts; if any slight misalignment has to be accommodated where variable or alternating loading is involved or where only limited lubrication is possible.

The general strength, toughness and resistance to shock of the bronzes is considerably lowered by the addition of lead; nevertheless the alloys find considerable use in certain limited fields. Because of the high working temperature possible with high-leaded bronze LB5, compared with white metals, it is used a great deal for steel backed bearings in the motor and aircraft industries.

All the leaded bronzes can be sand cast, chill cast or continuously cast and LB1, LB2, LB3 and LB4 can also be centrifugally cast.

These alloys are used for bearings and bushes for mining machinery, to resist corrosive waters and for poor lubrication conditions. As unlined bearings for certain rolling mills, and for liners in the paper making industry where resistance to corrosion is necessary, railway bearings, and steam packing metals, pump parts for mildly corrosive conditions, bearings for use in the oil industry and the high leaded bronze as steel backed bearing for the high working temperature which it supports.

Sn=8.0-10.0%; Pb=13.0-17.0%; Zn=max.1.0%; Ni= max.2.0%; Cu=Rem.
Sn=9.0-11.0%; Pb=8.5-11.0%; Zn= max.0.75%; Ni=max.2.0%; Cu=Rem.
Sn=9.0-11.0%; Pb=4.0-6.0%; Zn=max.1.0%; Ni=max.2.0%; Cu=Rem.
Sn=4.0-6.0%; Pb=8.0-11.0%; Zn=max.2.0%; Ni=max.2.0%; Cu=Rem.
Sn=4.0-6.0%; Pb=18.0-23.00%; Zn=max.1.0%; Ni=max.2.0%; Cu=Rem.

LB1. It is sometimes used for bearings in mining machinery in corrosive water conditions and for unlined bearings under poor lubrication conditions. The alloy is soft enough to allow for some misalignment of the bearings.

LB2. The alloy is used for lower loads and medium speeds than straight tin bronzes and has good resistance to wear and to corrosive mine waters. Very extensively used in chill cast, centrifugally cast and continuously cast form for bushes for general purposes. It is also used for mill bearings, railway bearings and in the oil industry.

LB3. A general purpose alloy of reasonable mechanical properties; has good resistance to wear and to corrosive mine waters; for any applications of lower loads and medium speeds. It is principally used in the chill cast, centrifugally cast and continuously cast form for bushes.

LB4. A general purpose bearing bronze, which is reasonably tolerant of faulty lubrication and minor misalignment conditions. It has similar applications to LB2 but has a lower tin content and wider composition tolerances.

LB5. This alloy is of high lead content and is used for steel backed motor and aero engine bearings for use at higher temperatures than the traditional white metal linings. It is also used for its self-lubricating properties under conditions where lubrication is difficult.

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