Copper based alloy

Details Copper based alloy Copper based alloy

1991-09-05

1992-11-17

Dean, R.

Alloys or metallic compositions

Copper base

Nickel containing

148414, 148435, C22C 906

Patent

active

051641577

DESCRIPTION:

BRIEF SUMMARY
This invention relates to copper based alloys, the copper being present in an amount of about 70% to 80% by weight.
Copper-nickel-manganese alloys have been known for many years, and such alloys have found many uses not least in marine environments. In the particular application of alloys for fasteners and shafts, in a marine environment, high strength combined with good ductility is required preferably with minimum properties as indicated below:


______________________________________ Cross sectional thickness of fastener- up to 75 mm After suitable hot working, followed by heat treatment; Minimum 0.2% proof stress 700 N/mm.sup.2 Minimum tensile strength 870 N/mm.sup.2 Minimum elongation, 12% Cross sectional thickness of fastener- over 75 mm After suitable hot working, followed by heat treatment; Minimum 0.2% proof stress 650 N/mm.sup.2 Minimum tensile strength 840 N/mm.sup.2 Minimum elongation, 15% ______________________________________
This level of strength and ductility can be achieved by high strength duplex stainless steels and other alloys by cold working, and also by certain low alloy carbon steels, and by certain nickel-based alloys, but not by the general run of copper based alloys. (An exception is beryllium-copper alloy but this is not generally acceptable because of the toxicity of beryllium and high cost.)
Moreover, high strength and ductility are not the only necessary requirements of an alloy which is intended to be used to fabricate fasteners for use in marine environments. In such environments, cathodic protection systems are employed in which an electric current is generated between a sacrificial anode such as zinc and the remainder of the structure. Under these conditions the sacrificial anode corrodes in preference to the other material and hydrogen is generated in atomic form by electrolysis of the seawater.
Galvanic coupling between dissimilar metals can also lead to corrosion currents, the generation of hydrogen due to electrolysis of seawater, and absorption of hydrogen and resultant embrittlement of the more noble cathodic metal.
It has been found that premature failures of fastenings, in particular bolts, have occured due to embrittlement resulting from the passage of this hydrogen into the high strength steels and nickel-base alloys from which the bolts are manufactured.
Hydrogen embrittlement adversely affects most bolting materials, including high carbon steels, nickel base alloys, titanium alloys, and duplex steels.
Therefore there exists a need for an alloy which in a marine, offshore environment is essentially immune to hydrogen embrittlement and which is capable of being processed and heat treated to give levels of strenght and ductility which equate with those indicated above. These levels of strength and ductility must also be retained after prolonged exposure to hydrogen for say 1500 hours in seawater.
The alloy should also be resistant to corrosion in seawater and should also preferably be resistant to galling, a phenomenon in which surfaces tend to adhere together when in sliding contact as for example during the tightening of a nut on a bolt. This last requirement is met if the alloy has a relatively low coefficient of friction even when under high load.
The present invention is based upon the belief that a useful copper based alloy will result if when the alloy is melted, cast and heat treated, a hardening precipitate is formed which is of the type Ni.sub.3 Al, but which in all probability will be (Ni,Mn).sub.3 (Al,Nb) so that some of the nickel and aluminium atoms in the crystal lattice of the precipitate are substituted by manganese and niobium atoms respectively. A further benefit arises if some of the strengthening of the alloy is achieved by precipitation of chromium in that a higher ductility can be achieved at a given strength level.
The alloy is intended, in particular, for the production of fasteners, and it will be recalled that the alloy should respond to appropriate hot working and subsequent heat treatment to acqu

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