The addition of silver to tough pitch copper raises its recrystallisation (softening) temperature considerably with very little effect on the electrical conductivity. Silver also improves the mechanical properties especially the creep resistance. The resistance to recrystallisation and creep increases with increasing silver content. These materials are therefore preferred for electrical machinery when resistance to softening is required, as in commutators, or when the material is expected to sustain stresses for long times at elevated working temperatures, as in large alternators and motors.
The term ‘tough pitch’ originates from the time when molten copper, after refining, was cast into ingot moulds. During refining the copper was oxidised to remove impurities then reduced by hydrogen to give the correct oxygen level. To monitor this process, a small sample was taken and the solidification surface observed. If the surface sunk there was too much oxygen; if it was raised there was too much hydrogen. If it was level (correct pitch), the oxygen was correct, and the properties good; in other words ‘tough’, hence tough pitch.
A level of 0.02% to 0.04% oxygen is maintained in ETP copper to oxidise the remaining impurities to oxides which would otherwise dissolve in the copper forming solid solutions which would reduce conductivity. Oxides have little effect on conductivity. Silver-bearing tough pitch coppers are the most widely used grade for components subjected to elevated temperatures such as in electrical machinery or overhead conductors.
Today all copper is continuously or semi-continuously cast - there are no static ingots. The copper is fed all the time and a rolled product is produced at the end of the process.
Silver is present naturally in many copper ores so copper-silver alloys will have been in use for thousands of years exploiting their attractive colour, high ductility and ability to be hardened by cold working. The significance of silver as an alloying element to enhance the mechanical and electrical properties of copper was not exploited until the late 1930s.
Silver-bearing coppers cannot be hardened by heat treatment. They do not become harder with time; they do not have a ‘shelf life’. Silver-bearing coppers may be stress relieved between 250 and 350oC though this does not soften the copper. If this is required then a full anneal at 350 to 650oC is needed. However, it is rarely called for in the annealed condition since its outstanding property is the retention of work hardness at temperatures up to 250oC and short times at 350oC.
A summary of the properties from the annealed (soft) to the hard condition is shown below:
- Tensile strength: 220-385 N/mm2
- Proof strength: 60-325 N/mm2
- % Elongation: 55-4
- Hardness (HV): 45-155
- Electrical conductivity: 98% IACS
- Thermal conductivity: 377-388 W/moC
| Process || Rating |
| Cold formability |
| Excellent |
| Hot formability |
| Good |
| Soldering || Excellent |
| Brazing || Good |
| Oxyacetylene welding |
| Not recommended |
| Gas shield arc welding |
| Fair |
| Resistance welding |
| Not recommended |
This copper will suffer from steam (hydrogen) embrittlement when heated in a hydrogen (reducing) atmosphere.
Silver-bearing tough pitch copper, like other pure coppers, cannot be regarded as a free-machining material yet it is not difficult to machine, especially in the work-hardened condition, and has a machinability rating of 20% (free-machining brass is 100%).
Resistance to Corrosion
All coppers corrode at negligible rates in unpolluted air and water due to the formation of a protective oxide surface layer. Copper artefacts have been found in near pristine condition after being in the earth and under the sea for thousands of years. However, copper is susceptible to more rapid attack in the presence of ammonia, sulphur, hydrogen sulphide and mercury. All coppers are virtually immune to stress corrosion cracking.
Resistance to Softening
Softening is time and temperature dependent and it is difficult to estimate precisely the time at which it starts and finishes. It is usual, therefore, to consider the time to ‘half-softening’, i.e. the time taken for the hardness to fall by 50% of the original increase in hardness caused by cold reduction. In the case of silver-bearing tough pitch copper, this softening occurs at temperatures above 150oC. It has been established experimentally that such coppers would operate successfully at a temperature of 105oC for periods of 20-25 years, and that it could withstand short-circuit conditions as high as 250oC for a few seconds without any adverse effect.
Creep is slow deformation of a material with time at temperature and must be allowed for in the design of any component operating above room temperature. The resistance of silver-bearing tough pitch copper to creep at moderately elevated temperatures is considerably better than non-silver-bearing coppers, especially when the material is in the cold worked condition. Tests have shown that at normal working stresses, non-silver-bearing coppers will begin to creep at temperatures as low as 130oC, whilst even at 230oC no creep was observed in silver-bearing tough pitch copper.
As the silver content increases (up to a maximum of 0.12%) the resistance to creep increases, so the more demanding applications such as rotating electrical machinery require the higher silver content.
- Chemical process equipment
- Printed circuit foil
- Radio parts
- Fuse clips
- Commutator bars and segments
- Generator coils
- Alternator windings
- Trolley wires
- Electrodes for spot welding
Silver-bearing tough pitch copper is available as bar, plate, sheet, strip, tube and wire.
Below are the specifications for Europe and the US. Note that for USA, some compositions are not identical. For equivalent standards from other countries visit the Copper Key
• UK: C101 (British Standard BS designation). UK Standards are superseded by European Standards.
• Europe: CW011A (CuAg 0.04), CW012A (CuAg 0.07), CW013A (CuAg 0.10) (European Standard EN designation).
• USA: C11400, C11600 (American Society for Testing and Materials ASTM designation).
Further information on tough pitch copper silver, and other conductivity materials, is available at the Copper Alloys Knowledge Base:
CuAg 0.04 CuAg 0.10
Application Example 1: Overhead Contact Wire
The overhead contact wire is CW 013A. It is stronger and has better creep resistance than Cu-ETP.
Application Example 2: Busbars
For busbars operating at high temperatures silver bearing tough pitch copper is ideal since it can operate at temperatures up to 250o
C and short times at 350o