Copper and Copper Alloy Castings

This article describes application of copper and copper alloy casings, specifically: castings required for high strength and resistance to fatigue, castings required for pressure tightness, castings required for resistance to corrosion, castings required for service at elevated temperatures.

Castings required for high strength and resistance to fatigue

For those applications in which high strength or resistance to fatigue are important the alloys normally used are the high tensile brasses, aluminum bronzes or copper-manganese-aluminum alloys.

It is very important that the designer should be aware of the precise factors, which are significant in any particular case. For example, tensile strength is rarely significant in that the designer is not interested in the stress under which a component will break but rather in the stress under which it will start to deform; therefore, under conditions of static stress, proof stress is the more useful indication of the suitability of the material.

While proof stress in itself indicates deformation under load, the property most readily determined as a means of testing material is the permanent set stress, the values for which, in the case of the high tensile brasses and aluminum bronzes, can be taken as being approximately 14MPa (1 ton per square inch) higher than the corresponding proof stress values. Under conditions of cyclic loading or vibration, the significant factor is fatigue, or, if corrosion is also involved, corrosion fatigue.

It is important that full consideration should be given to other properties which are required in any particular application, such as general corrosion resistance or wear resistance. It can be generally accepted that the aluminum bronzes and copper-manganese-aluminum alloys (CMA) alloys are superior to high tensile brasses in both these respects. Under marine conditions the high tensile brasses are suitable, provided a suitable composition is used. Generally, the high tensile brasses should not be used in applications involving rubbing. More details are given concerning corrosion and wear resistance in the appropriate sections.

High tensile brasses, aluminum bronzes and the CMA alloys can be supplied in the form of sand or die-castings. Sand castings can be undertaken of the order of 60 tons or more in cast weight.

Castings required for pressure tightness

Hydraulic or gas pressure is a particularly searching test of the quality of a casting, revealing defects which might have quite insignificant effects on the strength of the casting. Any discontinuities through the metal forming the wall of the casting, however small, are potential sources of leakage.

Given a reasonable design, it is possible to make pressure tight castings from any of the copper base alloys. The aluminium bronzes, high tensile brasses and CMA alloys require careful foundry techniques, but it is possible to make excellent pressure tight castings from these alloys. Because of the greatly increased mechanical properties it is possible to make weight reductions in the castings which should more than compensate for the extra costs involved in producing them.

The best alloys of all for the production of pressure tight castings are those containing substantial amounts of lead and the majority of pressure tight castings are made either from leaded gunmetals or plumbers’ brass. These leaded alloys are also very much more easily machined than other copper base alloys; an important consideration with such castings as valves and pump bodies.

In designing castings for these applications sudden changes in thickness in adjacent sections should be avoided as far as possible. Where this cannot be done the angles should be rounded or filleted. The greatest number of failures in pressure tightness occurs round areas where there are sudden changes of wall thickness. Machining allowances should be kept to a minimum to avoid taking away too much of the close-grain metal near the skin.

A test of pressure tightness frequently applied to small valve bodies and similar castings is that in which air is applied to the casting submerged in water. Air at 100 lb. per sq. in. (0,07 bar) is generally used. This test is applied to castings such as valve bodies with weights between 4oz (0,1 kg) and 24lb (10 kg) approximately. For larger castings it is more usual to test under hydraulic pressure.

Castings required for resistance to corrosion

Copper and copper base alloys are noteworthy for their resistance to corrosion and this is often the main reason for their use. For certain applications, some of the alloys have better corrosion resistance than others and these notes are intended to give general guidance on the selection of an alloy.

It must be emphasized most strongly that it is impossible to do more than give general guidance as local conditions can materially alter the behavior of an alloy so that full details of the service conditions must always be taken into account. The user is strongly recommended to consult his supplier unless he has previous experience of the behavior of copper alloys in the particular circumstances concerned.

Atmospheric Corrosion. All cast copper alloys have good resistance to atmospheric corrosion, although most undergo superficial tarnishing generally resulting in the development of the well-known greenish patina. Corrosion rates of copper base alloys are higher in sulphur bearing atmospheres, and are not, therefore, so suitable where the concentration of sulphur dioxide in the atmosphere reaches a high level as in chimneys and railway tunnels, with the exception of alloy G3 which has now been included in the standard because of its suitability for this application.

Natural Waters. Corrosion rates in natural waters are generally negligible and the cast brasses are traditionally used for plumbing and similar fittings. Some mine waters may be appreciably acid in character and these are more aggressive, especially where they contain iron salts, particularly ferric chloride. The phosphor bronzes, aluminum bronze alloys are the most suitable alloys for such applications.

Seawater. The phosphor bronzes and gunmetals have notably good resistance to corrosion by seawater and are used for such purposes as pipe fittings, cocks and pump bodies. The high zinc brasses tend to undergo slow de-zincification, but this is very much reduced by the addition of tin and for most applications where temperatures are normal they are satisfactory. High tensile brasses of suitable composition are widely used for marine propellers. (De-zincification is selective attack on the zinc-rich constituents in a brass and can be deeply penetrative. It is confined mainly to high zinc brasses and to a large extent is inhibited by the inclusion of tin in an alloy of suitable composition.) Aluminium bronze suffers de-aluminification under some circumstances in sea-water selective form of attack similar to de-zincification.

Waters. Phosphor bronzes and gunmetals are used for handling boiler feed waters. Aluminium bronze AB2 and the CMA alloys are also satisfactory for this purpose. The brasses tend to undergo de-zincification and are not so suitable; de-aluminification of aluminum bronze AB1 may sometimes occur.

Acids. Copper alloys are not completely resistant to attack by acids, but rates of attack in dilute acids where conditions are non-oxidizing are very low, ranging from about 0.002-0.08 inch per year according to the concentration and degree of aeration. The best resistance to attack is afforded by aluminum bronze AB2. The phosphor bronzes are also very suitable for handling dilute acids. Leaded bronzes are sometimes recommended for dilute sulphuric acid. Brasses are not generally so satisfactory. Corrosion rates are higher with hydrochloric acid than with sulphuric acid, but phosphor bronze, aluminium bronze and the CMA alloys are frequently used. Strong aeration of the solution or the presence of oxidizing salts can considerably increase the rate of attack. Oxidizing acids such as nitric acid or strong sulphuric acid cannot be handled with copper alloys.

Alkalis. The resistance of the copper alloys to alkaline solutions is not so high as to acid solutions and, although they can be used for handling dilute caustic alkalis, ferrous materials are generally more satisfactory. All the alloys with the exception of the CMA alloys suffer considerable attack in solutions of ammonia or ammonium salts and they are unsatisfactory for these applications.

Food Products. Copper alloys are widely used for handling food products, though in many cases they are given a heavy coating of tin. This is not so much to protect the alloys against attack, but rather to avoid risk of traces of copper affecting the food. Very small amounts of copper can cause discoloration or an alteration in the flavor of certain foods.

Stress Corrosion. There is a danger of stress corrosion with highly stressed components cast in beta brass HTB3. Failure takes the form of cracks spreading rapidly with little or no general corrosion. Two conditions are necessary: first, the presence of high stresses and, secondly, the presence of a corrosive medium such as seawater or an industrial or marine environment.

Castings required for service at elevated temperatures

When considering service at elevated temperatures, important factors are load carrying capacity, structural stability and resistance to oxidation.

Resistance to Oxidation. Some of the copper base alloys contain additions of aluminum and these have exceptional resistance to oxidation. The aluminum bronzes and certain of the high tensile brasses remain practically unaffected by oxidation almost up to the melting point. This property is used in certain applications such as moulds for glassware, which are frequently made from aluminum bronze, an application involving very high operating temperatures but where the stresses are quite low. The casting alloys containing no aluminum are less resistant to oxidation but suffer no more than superficial tarnishing at temperatures up to 320°C.

Load Carrying Capacity. Despite the relatively good room temperature mechanical properties of some of the alloys, none of the cast copper base alloys is suitable for sustaining high loads at high temperatures. Their high temperature applications are mainly in cases where resistance to corrosion and oxidation are important and steel is unsuitable.

In connection with load carrying capacity at elevated temperatures, it must be emphasized that the mechanical properties of an alloy at room temperature are not a reliable guide to its performance at elevated temperatures nor is it safe to base design stresses on the results of short time tensile tests carried out at the operating temperature.

Safe working stresses can only be determined from the results of creep tests of several thousand hours duration in which the deformation of the specimen under load is recorded as time proceeds. Under sustained stress at high temperatures metals undergo slow permanent deformation (plastic strain) and the most useful information to the designer is the load which will cause not more than a certain amount of plastic strain in a given time.

Although they have good room temperature properties, all the brasses begin to fall in strength at temperatures above 150°C and they are not suitable for load carrying applications at higher temperatures, but there are many applications where the loads involved are very low and the resistance of the brasses to oxidation and corrosion makes them a good choice.

Superheated Steam. Many years of service experience have proved the suitability of gunmetal components for handling superheated steam at temperatures up to 290°C (550°F). Aluminum bronzes have also been used for similar applications, but under service conditions where the steam contains chemically active impurities selective attack on these alloys has been experienced. The aluminum bronzes are not recommended for handling steam at high temperatures if the steam is contaminated with small amounts of sulphur dioxide or chlorides.

August, 2004