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
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
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
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.