Information about Copper Nickels and their Properties

ALLOYS

Types of Cu-Ni Alloy

The Cu-Ni alloys are single-phase throughout the full range of compositions and many standard alloys exist within this range, usually with small additions of other elements for special purposes. The first of these special uses was for marine condenser tubes and there are now many others. Other Cu-Ni alloys are used for coinage and wire mesh for the paper industry but the two most popular for marine applications contain 10 or 30% nickel with iron and manganese additions (C70600 and C71500). Table 1 lists typical international and national standards to which the materials may be ordered in wrought and cast forms.

The effect of alloying additions to Cu-Nis are:-

Manganese is invariably present in the commercial alloys as a de-oxidant and de-sulfurizer; it improves working characteristics and additionally contributes to corrosion resistance in seawater.

Iron is added (up to about 2%) to the alloys required for marine applications. It confers resistance to impingement attack by flowing seawater and increases the strength. The initial development of the optimum compositions of the copper-nickel-iron alloys took place in the 1930's. This work was done to meet naval requirements for improved corrosion-resistant materials for tubes, condensers and other applications involving contact with seawater. Throughout the term "copper-nickel (Cu-Ni)" refers in fact to copper-nickel-iron alloys.

Chromium can be used to replace some of the iron content and at one per cent or more provides higher strength. It is used in a 30% nickel casting alloy (IN-768) {See footnote 1.} A low-chromium 16% nickel wrought alloy (C72200) {See footnote 2.} has been developed in the USA.

Footnote 1: INCO designation.

Footnote 2: UNS designation. A description of copper alloys listed under the UNS numbering system can be found in CDA's Standards & Properties section .

Niobium can be used as a hardening element in cast versions of both the 10% and 30% nickel alloys (in place of chromium). It also improves weldability of the cast alloys.

Silicon improves the casting characteristics of the Cu-Ni alloys and is used in conjunction with either chromium or niobium.

Tin confers an improved resistance to atmospheric tarnishing and at 2% is used with 9% nickel to produce the alloy C72500. This alloy has useful spring properties and is used in the electronics industry. It is not often recommended for marine applications.

Impurities

Impurity elements such as lead, sulfur, carbon, bismuth, antimony and phosphorus, in the amounts to be found in commercial material, have little or no effect on corrosion performance, but because of their influence on hot ductility may impair weldability and hot workability and are, therefore, carefully controlled.

The two main marine grades of wrought Cu-Ni have 10% (C70600) or 30% (C71500) of nickel and popularly known as 90-10 and 70-30 Cu-Ni respectively. The iron levels they contain were carefully chosen to provide the best combination of resistance to flowing seawater and localised corrosion resistance. The 70-30 nickel alloy can withstand higher seawater velocities and is stronger but, for most applications, the 90-10 alloy provides good service at a lower cost. Both alloys are available in the main product forms.

A third alloy (C71640) is being used particularly in the heat rejection section of multistage flash desalination units where higher resistance to impingement corrosion is required. It is a modified 30%Ni alloy containing 2% Mn and 2% Fe and referred to here for simplicity as 66-30-2-2 Cu-Ni. It is only commercially available as condenser tubing.

Variations in the common national and international specifications for the 90-10, 70-30 and 66-30-2-2 alloys are shown in Table 2. From this, the extent to which various standard materials overlap may be compared. In some standards the impurities are more closely controlled than others are. In cases of doubt, the supplier's advice should be obtained.

Table 3 shows the common production limits on the sizes of these materials. This is a guide to what is commonly made to order. It may also be possible to make material outside these sizes by arrangement with the supplier.

Provided foundry practice is good, satisfactory complex castings can be made in these types of alloys. The 90-10 composition has a lower melting and pouring temperature than the 70-30 alloy. Normally for small castings, additions of some extra alloying elements are made for improved properties.

The introduction of electric furnace melting in foundries has led to a greater interest in 70-30 alloys, in particular an alloy containing 1.5-2.5% chromium which has exceptional resistance to impingement corrosion, making it ideal for heavy duty marine pump and piping applications. An alternative contains 0.5-1.5% niobium and other closely specified elements in a 70-30 alloy also specified for naval use. Since these alloys have tight limits on impurities, only certified ingots are used. Electric melting practice is essential for these alloys; attaining the correct melting temperature in reasonable time and giving a cleaner furnace atmosphere that avoids contamination and gas pick up. They are therefore only available from specialty castings suppliers.

For security and other reasons the Cu-Ni alloys used for a large percentage of the world's coinage requirements do not necessarily conform to any of the common specifications quoted. Generally, they do not include the iron, manganese or other significant additions.

High strength and other Cu-Ni alloys:

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STANDARDS

Table 1. This summary covers American (ASTM), European (EN - replaces all previous national standards) and International Standards (ISO).
StandardsApplicable Standard Numbers
CompositionPlateSheetStripTubeRodWireForgingsCastings
ASTM 122
171
122 122 111
359
395
466
467
469§
552
543
608
122
151
369
505‡
EN 1652
1653
1652
1653
12451
12449
12163 12420 1982
ISO 429 1634 1634 1634 1635,
16362
1637 1639 1638 1640
‡ 70-30 alloy only
§ 90-10 alloy only
Table 2. Standard Compositions of Wrought Cu-Ni Alloys (UNS Copper Alloy Numbers) Maximum or range.
UNS Number
CuPbFeZnNi (incl Co)MnOther
C70600 Rem 0.05* 1.0-1.8 1.0* 9.0-11.0 1.0 *
C71500 Rem 0.05* 0.4-1.0 1.0* 29.0-33.0 1.0 *
C71640 Rem 0.01 1.7-2.3 - 29.0-32.0 1.5-2.5 0.03S 0.06C
*Special limits apply when the product is to be welded and is so specified by the purchaser: 0.5% Zn, 0.02%P, 0.02%Pb, 0.02%S and 0.05%C
(European Copper Alloy Numbers)
Designation
SymbolNumberCuCCo*FeMnNiPPbSnZnOth-er#
CuNi10-Fe1Mn CW3-52H Rem 0.05 0.1 1.0-2.0 0.5-1.0 9.0-10.0 0.02 0.02 0.03 0.5 0.2
CuNi30-Fe2Mn2 CW3-53H Rem 0.05 0.1 1.5-2.5 1.5-2.5 29.0-32.0 0.02 0.02 0.05 0.5 0.2
CuNi30-Mn1Fe CW3-54H Rem 0.05 0.1 0.4-1.0 0.5-1.5 30.0-32.0 0.02 0.02 0.05 0.5 0.2
*Co max 0.1% is counted as Ni
#S 0.05%max

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PRODUCT FORMS

Table 3. The sizes below represent typical manufacturing capabilities. They are not necessarily available from stock, nor in every alloy. Larger sizes may be available to special order.
FormSizes
MetricUS Customary
Plate up to 3000 mm wide, 10 to 150 mm thick up to 10ft wide, to 6" thick
Clad steel plate to order only to order only
Sheet & Strip up to 2000 mm wide, 0.2 to 10 mm thick up to 6ft wide, 0.008" to 0.25" thick
Seamless Tubes:
  • Pipeline
6 to 420 mm OD, 0.8 to 5.0 mm wall thickness 0.3" to 16" mm OD, 0.03 to 0.2" wall thickness
  • Condenser
8 to 35 mm OD, 0.75 to 2.0 mm wall thickness 0.3 to 1.5" OD, 0.03 to 0.1" wall thickness
  • Coiled
6 to 22 mm OD, 0.5 to 3 mm wall thickness 0.25 to 1" OD, 0.02 to 0.12" wall thickness
Tubes - longitudinally welded 270 to 1600 mm OD, 2.0 to 10 mm wall thickness 10 to 63" OD, 0.1 to 0.5" wall thickness
Fabrications by arrangement by arrangement
Wire all common wire and wire mesh sizes all common wire and wire mesh sizes
Rod & Section all common sections up to 180 mm diameter all common sections up to 7" diameter
Welding Consumables all common sizes all common sizes
66-30-2-2 Cu-Ni is normally available as seamless tubing only.
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PHYSICAL PROPERTIES

Table 4. Typical Physical Properties.
Units90-1070-3066-30-2-2
Density kg/dm3 8.90 8.95 8.86
Melting Range °C 1100-1145 1170-1240
Specific Heat J/kg °K 377 377 377
Thermal Conductivity W/m°K 50 29 25
Coeff. of Linear Expansion
10-300°C
10-6/°K 17 16 15.5
Electrical Resistivity microhm. cm 19 34 50
Modulus of Elasticity GPa 135 152 156
Modulus of Rigidity GPa 50 56

In some applications, such as minesweepers, low magnetic permeability is required. Although the 70-30 alloy is essentially non-magnetic, the 90-10 alloy has a higher iron content and can have a permeability between 1.01 and in excess of 1.2 depending on final heat treatment conditions. A fast cool from the final solution heat-treatment temperature is required to achieve low permeability.

Additional information on permeability is given in:

M. Jasner, M. Hecht and W. Beckmann. KME Publication. Osnabruck. The Behaviour of CuNi 90/10 vs 6Mo Superaustenitic and Superduplex Steels in Marine Environments (2000) View Abstract & Order

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MECHANICAL PROPERTIES

(Data is for guidance only; it is advisable to check that values are current for design and specification)

Table 5. Minimum mechanical properties for 90-10 Cu-Ni (CW 352H) based on Euronorm Standards
FormConditionThickness t*Tensile Strength0.2%Proof StrengthElongationHardness
mmN/mm2N/mm2%HV

Tubes

EN 12449: 1999

R290 20max 290 90 30 -
H075 20 max - - - 75-110
R310 6 max 310 220 12 -
H105 6 max - - - 105
R480 4 max 480 400 8
H150 4 max - - - 150

Sheet/Plate

EN 1652: 1997

R300 0.3-15 300 (100) 20 for t<and=2.5

30 for t>2.5mm

-
H070 0.3-15 - - - 70-120
R320 0.3-15 320 (200) 15 for >2.5mm t
H100 0.3-15 - - - 100

Plate for pressure vessels, heat exchangers

EN 1653: (1997)

R270 2.5-125 270 100 30 (85)
R280 over10 280 110 20 (85)
R300 2.5-10 300 120 25 (90)
R320 2.5-60 320 200 15 (110)
R350 10-40 350 250 14 (120)
Figures in parentheses are not requirements but are given for information only.
1N/mm2 is equivalent to 1MPa
*For tubes t =wall thickness
Table 6. Minimum mechanical properties for 90-10 Cu-Ni(C70600) based on ASTM Standards
FormTemperTensile StrengthYield StrengthElongation in 50mmHardness
StandardFormerMPa (N/mm2)MPa (N/mm2)%Rockwell B
Seamless Condenser Tubes B111M-98 O61 Annealed 275 105 - -
H55 Light drawn 310 240 - -
Plate Sheet Strip and Rolled Bar B122/122M-95 M20 As hot rolled 275-425 - - -
H01 Quarter hard 350-460 - - 51-78
H02 Half hard 400-495 - - 66-81
H04 Hard 490-570 - - 76-86
H06 Extra Hard 505-585 - - 80-88
H08 Spring 540-605 - - 83-91
Plate and sheet for pressure vessels, condensers and heat exchangers B171M-96 M20 and 025 (80mm and under) 275 105 30
(>60 to 140mm) 275 105 30
1N/mm2 is equivalent to 1MPa
Table 7. Minimum mechanical properties for 70-30 Cu-Ni(CW 354H) based on Euronorm Standards
FormConditionThickness t*Tensile Strength0.2% Proof StrengthElongationHardness
mmN/mm2N/mm2%HV

Tube

EN 12449: 1999

R370 10 max 370 120 35 -
H085 10 max - - - 85-120
R480 5 max 480 300 12 -
H135 5 max - - - 135

Sheet/plate

EN 1652: 1997

R350 0.3-15 350-420 (120) 35 for t>2.5mm -
H080 0.3-15 - - - 80-120
R410 0.3-15 410 (300) 14 for t>2.5mm -
H110 0.3-15 - - - 110

Heavy PlateFor pressure vessels

EN 1653:1997

R320 2.5-125 320 120 30 (100)
R410 10-40 410 300 14 (140)
Figures in parentheses are not requirements but are given for information only.
1N/mm2 is equivalent to 1MPa
*For tubes t= wall thickness
Table 8. Minimum mechanical properties for 70-30 Cu-Ni(C71500) based on ASTM Standards
Form>TemperTensile StrengthYield StrengthElongation in 50mmHardness
StandardFormerMPa (N/mm2)MPa (N/mm2)%Rockwell B
Seamless Condenser Tubes B111M-98 O61 Annealed 360 125 - -
HR50 ( up to 1.21mm wall) Drawn, stress relieved 495 345 12 -
HR50 ( over 1.21mm wall) Drawn, stress relieved 495 345 15
Plate Sheet Strip and Rolled Bar B122/122M-95 M20 As hot rolled 310-450 - - -
H01 Quarter hard 400-495 - - 67-81
H02 Half hard 455-550 - - 76-85
H04 Hard 515-605 - - 83-89
H06 Extra Hard 550-635 - - 85-91
H08 Spring 580-650 - - 87-91
Plate and sheet for pressure vessels, condensers and heat exchangers B171M-96 M20 and 025 (80mm and under) 345 140 30
(>60 to 140mm) 310 125 30
1N/mm2 is equivalent to 1MPa

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CRYOGENIC PROPERTIES

Copper and copper alloys were the first metals used in the fabrication of low temperature equipment for the liquefaction and storage of cryogenic fluids. Copper and many copper alloys retain ductility at low temperatures. This plus their good thermal conductivity makes an unusual combination of properties for heat exchangers and other components in cryogenic plants and in low temperature processing and storage equipment.

90-10 and 70-30 Cu-Ni alloys become stronger while still retaining good ductility as the temperature goes down as shown in Table 9. They also retain excellent impact properties to 20°K. Stress stain curves are given in Figures 1 and 2.

Table 9. Average Properties of Copper Nickels at Low Temperatures.
AlloyTest temperature, °KElastic propertiesPlastic Properties
UniaxialTriaxial
No.Name And treat-mentYoungs Modulus 106psi (5%)Tensile Strength psiYield Strength psiElongation % in 4DReduction in Area %Notch tensile Strength (KT=5.0) psiImpact Charpy Energy Absorbed, ft-lb
706 90-10
Cu-Ni,
(ann-ealed)
295 17.7 49,600 21,400 37 79 65,000 114
195 54,700 24,700 42 77 73,100 113
76 19.5 72,000 24,800 50 77 87,200 115
20 82,500 30,200 50 73 96,800 116
4 20.5 80,600 24,900 53 73 100,000
715 70-30
Cu-Ni,
(ann-ealed)
295 22.0 57,800 18,700 47 68 79,400 115
195 68,000 22,200 48 70 90,500 114
76 23.0 89,800 31,600 52 70 112,900 114
20 103,100 38,100 51 66 127,600 114
4 23.2 104,600 40,100 48 65 130,500
Taken from CDA Application Data Sheet 144/8R . View Paper
Data spread in most instances was 1%.
Figure 1 . Copper Alloy No. 706 (Annealed) Figure 1 . Copper Alloy No. 706 (Annealed)
Figure 2 . Copper Alloy No. 715 (Annealed) Figure 2 . Copper Alloy No. 715 (Annealed)

CORROSION RESISTANCE

Cu-Ni alloys have been chosen for their excellent resistance to sea water for several decades. The Interactive Presentation includes a visual overview of their corrosion behaviour and the operating conditions required to achieve optimum service. Additionally, the references and links below review overall corrosion properties although more specific papers can be found, many of which are downloadable, in the Reference Section.

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ANTIMICROBIAL CHARACTERISTICS OF COPPER FOR TOUCH SURFACES

Well before micro-organisms were discovered, the Egyptians, Greeks, Romans and Aztecs used copper-based preparations to treat sore throats and skin rashes, as well as for day-to-day hygiene. Copper was also used to ward off infection in battlefield wounds.

In the 19th century, with the discovery of the cause-and-effect relationship between germs and the development of disease, scientific evidence started to be gathered. In the last few decades, work has been done on the antimicrobial properties of copper and its alloys – including copper nickels – against a range of micro-organisms threatening public health in food processing, healthcare and air conditioning applications.

There is now a solid body of laboratory and clinical evidence to demonstrate rapid, broad spectrum antimicrobial efficacy of copper against the most important pathogens challenging public health such as VRE, MRSA and C. difficile. It has been demonstrated that bacteria, viruses and fungi cannot survive on copper or copper alloy surfaces.  In 2008, the US Environmental Protection Agency (EPA), following extensive laboratory evaluations, allowed registration of 282 alloys (now increased to over 400) to make public health claims.

Teams around the world have also led clinical trials to assess copper’s role in reducing bioburden in the clinical environment and any associated improvement in patient outcomes.  Published results show a median reduction of more than 90% in contamination on the copper alloy vs control surfaces.

Copper nickel alloys are listed by the EPA and the Copper Alliance stewardship program, AntimicrobialCopper. Further information can be found at a dedicated website: www.antimicrobialcopper.org.

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BIOFOULING RESISTANCE

Cu-Ni alloys have an inherent high resistance to biofouling which has been used to good effect for boat hulls and other applications, including sheathing structures, in marine environments. A visual overview of their response to biofouling can be found in the Interactive Presentation and further information is provided in the following references and links.

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WELDING AND FABRICATION

(Main reference: Copper-nickel Welding and Fabrication Published by CDA In as A7020-99/13. Nickel Institute as 12014. Second Edition and CDA UK as Publication 139 Second Edition. View PDF Paper

General Handling

The precautions required for handling copper-nickels will be familiar to any fabricator who routinely handles materials like stainless steels and aluminium alloys, but may be new to those used to dealing with only carbon steels.

Cleanliness is most important as contamination can cause cracking and porosity during heat treatment or welding and may affect the corrosion resistance of the alloy. Ideally fabrication should take place in an area devoted solely to Cu-Ni alloys. Where this is impracticable, the standard of care of the material should be well above that necessary for carbon steels.

  • Sheets should remain in their packing until needed and should be separated by protective material or other means to avoid abrasion.
  • Plates and sheets are best stored vertically in racks which have the steel frames covered.
  • Walking over sheets should be avoided.
  • Plastic film may be interposed between the sheet and rolls when roll forming.
  • Grease and paint should be kept away from the surface particularly near edges of weld preparations, while all trace of marking crayons must be removed before making a joint.
  • Stainless steel brushes should be used and tools such as grinding discs should not be used on Cu-Ni alloys if they have been used on other materials.
  • Openings of pipes and fittings must be protected on completion of fabrication to prevent ingress of dirt etc. before installation.
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Cutting and Machining

Most normal cutting processes are acceptable for Cu-Ni such as shearing, abrasive disc cutting and plasma arc. High-speed abrasive wheels work well for bevelling edges and trimming material. Laser and abrasive water jet cutting are also possible.

Oxy-acetylene cutting is not appropriate for these materials. Band saws or shears may be used for cutting, but allowance must be made for the fact the alloys are relatively soft and ductile.

Although Cu-Nis are not as readily machined as free cutting materials such as brass, they are not difficult to machine and can be ranked along with aluminium bronze and phosphor bronze alloys. They are much easier to machine than say stainless steels and other alloys which work-harden rapidly.

More details and recommended speeds and oils are detailed in:

Machining Brass, Copper and its Alloys, CDA Publication TN 44. View PDF Paper

Recommended Machining parameters for Copper and Copper Alloys, DKI Monograph [i018e]. View PDF Paper

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Forming

Cu-Nis can be hot and cold formed although cold working is preferred. If cold forming is used a full inter-stage anneal may become necessary when the amount of cold work exceeds about 40-50%. A 20% cold reduction approximately halves the as -annealed elongation and doubles the proof strength.

Tubes can be bent by a range of methods, including rotary draw bending, 3-roll bending, compression bending and ram bending (press bending). When bending copper-nickel, a mandrel and wiper die are also applied for support (mandrel bending). Care must be taken to get smooth bends and avoid wrinkling, because liquid turbulence in service can lead to impingement attack. Bends with a tube bend radius of twice the tube diameter can be produced. Smaller radii require prefabricated bends.

More details on bending are given in the Tube and Pipe Bending Section.

Hot working Cu-Nis can lead to hot cracking and therefore should be avoided or only attempted with advice from a supplier. The temperature ranges are:

90-10 850-950°C
70-30 925-1025°C
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Heat Treatments

The work-piece should be clean and free from any contamination before and during heating.
Cu-Nis can embrittle if heated in the presence of contaminants such as sulfur, lead, phosphorus and other low melting point metals. Sources of contamination include paints, marking crayons, lubricating grease and fluids, and fuels. Fuels used must be low in sulfur; normally, fuel oils containing less than 0.5% by weight sulfur are satisfactory.

Oxidising atmospheres will cause surface scaling. Furnace atmospheres should be neutral to slightly reducing and must not fluctuate between oxidising and reducing conditions. Flame impingement must be avoided.

For a full anneal, soaking times of 3-5 minutes per mm thickness can be used. Recommended temperatures are:

90-10 750-825°C
70-30 650-850°C

Stress relieving is seldom used but if required the recommended temperatures are:

90-10 250-500°C
70-30 300-400°C
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Descaling

The surface oxide films on both alloys can be very tenacious. Oxides and discolouration adjacent to welds can be removed with very fine abrasive belts or discs. If pickling is required, a hot 5-10% sulfuric acid solution containing 0.35g/l potassium dichromate is satisfactory. Before pickling, oxides can be broken up by a grit blast. The pickled components should be rinsed thoroughly in hot, fresh water and finally dried in hot air.

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Welding

Copper-nickels can be readily welded by all conventional processes and, since they have a simple metallurgical structure, do not require preheat or post-weld heat treatment. However, it is essential that requirements for preparation, particularly cleanliness, are carefully followed and that welders undergo a period of familiarisation with the particular characteristics of these alloys if they are not to encounter problems. Automatic welding, including orbital welding of pipe, may also be appropriate.

In some applications, insurance and inspection bodies may require qualification of both welders and welding procedures to appropriate standards. A welding procedure specification (WPS) should be prepared in all cases.

Since the predominant application of copper-nickels is in the form of relatively thin-walled pipe, the gas-shielded tungsten-arc welding process (known as TIG or GTAW) is frequently used, both for joining pipe sections and for attaching fittings and flanges.

The most widely available welding process is the manual metal arc process(known as MMA or SMAW) using flux coated stick electrodes. This is quite suitable for welding Cu-Ni alloys and has the advantage of using relatively inexpensive equipment.

For thicker materials, above 6 mm, the TIG (GTAW) process can be used for the root run before completion of the weld with by the MMA (SMAW) process. The gas-shielded metal arc process (known as MIG or GMAW), using a continuous wire feed, is faster and can be closely controlled with modern sophisticated equipment.

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Mechanical Properties of Welds

A copper-30% nickel filler material is recommended for the welding of the 90-10 and 70-30 Cu-Ni alloys. Because of the higher nickel content, the weld metal is stronger than the 90-10 Cu-Ni base metal. In making a transverse tensile weld test in a 90-10 Cu-Ni weld qualification plate test, all the elongation can be concentrated in the heat affected zone if the specimen moves. This can be prevented by using a longitudinal bend test specimen instead. Typical properties of the all weld metal are shown in Table 10:

Table 10. Typical All Weld Metal Mechanical Properties
(not to be used for design purposes, weld properties must always be used in compliance with design standards)
Welding process0.2% Proof Strength N/mm2Tensile Strength N/mm2%Elongation 5dHardness Hv
TIG (bare wire) 200 385 40 105
MMA ( flux coated electrode) 270 420 34 120
d is the diameter of the testpiece gauge length
1 N/mm2 equals 145 psi
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Preparation for Welding

If stored correctly, material to be welded should be in a generally clean condition. Dirt of any kind must be removed along with residual oil and grease. Particular attention should be given to sources of the elements that can cause cracking or microfissuring in the weld, which can originate from crayon or paint identification markings, temperature indication markers, and other contaminants. (Fittings of other alloys, such as gunmetal - copper-tin-zinc alloy - are also a source of detrimental elements and should not be welded to copper-nickel alloys.)

The joint area should be thoroughly cleaned before welding starts. Particular attention should be paid to the weld preparation and an adjacent area at least 10mm wide, preferably wider, either side of the preparation, which can be degreased with an uncontaminated organic solvent applied with a fine abrasive pad or a clean cloth. The area should be dried with clean cloths. Their appearance after use is an indicator of cleanliness: they should be free from any residue.

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Weld Preparations

It is possible to weld copper-nickel up to 3mm thick with a square butt preparation. However, autogenous welding should not be attempted, since this will result in porous welds due to the absence of effective deoxidisers in the alloys. Above this thickness, a bevelled preparation must be used; the included angle of the V should be larger than for carbon steel - typically, 70° or more - because the molten weld metal is not as fluid as with carbon steels, and manipulation of the electrode or torch is necessary to ensure fusion with the side walls.

Although it is possible to weld in all customary welding positions, it is desirable to weld down-hand, which allows higher deposition rates and may demand less skill. It will often be impracticable to turn large or complex structures into this most favourable position for welding, but it is worth the effort of manipulating subassemblies for down-hand welding, where possible.

There is no need to pre-heat the base metal before tacking or welding unless this is necessary to ensure that the base metal is dry. To avoid microfissuring, the interpass temperature is maintained below 150°C.

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Tack Welding

Because of their high coefficient of thermal expansion relative to carbon steel, Cu-Ni's have a greater potential for distortion when welded. Welding fixtures can help but their use is limited to sub-assemblies. Tack welds should therefore be made to maintain a uniform gap and alignment between the parts being welded. They must be positioned at about half the spacing usual for carbon steel and are preferably quite short. The TIG (GTAW)process is often used for tacking, although, where the equipment has the facility available, the MIG (GMAW)spot-welding process is a convenient and well controlled technique for the purpose. Tacks should be wire brushed or ground to clean metal where they are to be incorporated into the weld metal of the joint.

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Welding Consumables

While consumables are available that deposit weld metal similar in composition to the 90-10 copper-nickel alloy, welds made with them may not have adequate corrosion resistance for all applications. Consumables for the 70-30 alloy, on the other hand, offer superior deposition characteristics and the corrosion resistance of 70-30 weld metal is at least comparable to each of the base metal alloys. These consumables are therefore recommended for both types of alloy.

For welding copper-nickel to steel, nickel-copper consumables containing about 65% Ni are used as the weld metal can absorb more iron dilution from the steel without cracking than copper-nickel weld metals.

Many weld consumable manufacturers offer Cu-Ni and nickel-copper electrodes and filler wires to recognised specifications, Table 11. These contain additions of titanium and manganese to react with nitrogen and oxygen from the atmosphere which would otherwise create porosity. If weld metal porosity persists despite the use of the correct filler material, the most likely causes are inadequate shielding of the weld pool and improper weld joint cleaning. Other possible causes include an excessively long arc, moisture on the weld preparation or the use of coated electrodes which are not fully dry.

Table 11. Welding Consumables
Welding ProcessFormTypeAWS SpecBS Spec
MMA
(SMAW)
Flux-coated electrode Cu-30%Ni A5.6 ECuNi In draft
65%Ni-Cu A5.11 ENiCu-7 BS EN ISO 14172
E Ni 4060

TIG (GTAW)

MIG (GMAW)

Wire in straight lengths or on spools Cu-30%Ni A5.7 ERCuNi BS EN ISO 24373
S Cu 7158
65%Ni-Cu A5.14 ERNiCu-7 BS EN ISO 18274
S Ni 4060
AWS - American Welding Society
BS – British Standards Institution
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For the Manual Metal Arc (MMA or SMAW) Process:

  • Flux-coated electrodes are designed to operate with direct current, electrode positive.
  • No special electrode baking or drying treatment is required unless they have been exposed to the atmosphere for some time. In this case, they should be dried in an oven, e.g. for 1-2 hours at 250C.
  • An electrode size slightly smaller than that of a carbon steel electrode under comparable conditions is preferred taking into account the need for manipulation.
  • Any weaving should not be more than three times the electrode diameter.
  • A long arc should be avoided, since this results in weld porosity through reaction with the surrounding atmosphere.
  • Start positions can be unsound and reversing the electrode direction to remelt initially deposited weld metal or the crater at the end of a run can help to avoid problems.
  • Slag must be removed between runs by chipping and brushing to leave a clean surface for the next run.
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For the Gas-shielded Tungsten Arc (TIG or GTAW) process:

Compared to MMA (SMAW), separate control of heat input via the arc and filler material addition gives TIG (GTAW) a degree of flexibility which is an advantage when welding shaped joints. In general, the process is suitable for joining materials up to 6 mm thick and is the method commonly used for welding thin walled pipes. It is also preferred for tacking and for inserting root runs in thicker joints that are completed by the MMA process with flux-coated electrodes. Automatic equipment is available for orbital welding of pipe and other applications.

To offset a greater risk of weld metal porosity than with the other processes, the weld pool must be protected as far as possible from contact with the atmosphere by maintenance of a short arc and adoption of a stringer bead technique; weaving of the torch is undesirable. It is essential that the weld pool is fully deoxidised by addition of filler metal throughout a run. Autogenous welds are very likely to be porous. If the filler metal is accidentally withdrawn at any point, that part of the weld should be ground out and repaired.

Argon is recommended as the shielding gas. Gas flow should be maintained at the end of a run until the weld pool has solidified; crater filling devices are beneficial. The interior of pipes should be purged with argon prior to and during welding. Where joints are made with backing bars, these should be made of copper or copper-nickel alloy.

Direct current should be used.

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For the Gas-shielded Metal Arc (MIG or GMAW) process:

Due to the higher capital cost of equipment and the necessity of buying spools of filler wire, MIG (GMAW) is more appropriate for extensive welding operations.

MIG (GMAW) can be operated over a range of currents to provide various transfer modes:

Dip (or short circuiting) transfer
- low heat-input and used for thinner sections
Spray transfer
- relatively high heat-input and only suitable for thicker materials, say above 6 mm thickness and down hand welding
Pulsed-arc transfer
- a technique in which metal transfer is closely controlled providing a combination of low overall heat input and adequate fusion to the base metal. It is suitable for a range of thicknesses.

Because of the range of transfer conditions which are possible with the gas-shielded metal arc process, welding parameters can vary widely. In all cases, these should be set for the equipment and position and thickness of the material by careful welding procedure trials directed towards stable transfer conditions and welds of good appearance.

  • Argon or a mixture of argon and helium is preferred as shielding gas.
  • The spooled filler wire must be kept dry and not exposed to contamination.
  • Attention should be paid to the effectiveness of the wire feeding system when welds have to be made some distance from the welding equipment since filler wire is relatively soft.
  • Low friction liners are essential for the feed hose.
Post-weld treatment
No heat treatment is necessary after welding. All traces of slag should be removed from joints made by the manual metal-arc process and the weld area may be cleaned e.g. with a rotating flap wheel or stainless steel brush, to leave a bright finish.
Inspection
Welds should be inspected visually for defects such as cracks, undercut, lack of fusion and penetration, and weld contour. Liquid dye penetrant inspection is a simple method for ensuring that there is no cracking at the surface. For critical applications, more advanced inspection techniques are adopted, such as radiography, but these are not required for general fabrications.
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Clad Plate-Preparation and Welding

An economical way of constructing thicker plate section can be to use steel plate which has been roll-clad with 90-10 or 70-30 Cu-Ni alloy. Examples are tube sheets and water boxes. Also 8mm (2mm copper- nickel and 6mm steel) thick plate has been successfully used to build four fire boats in Italy. This type of material can offer significant advantages in some situations but is not so readily available as the solid Cu-Ni alloy itself.

Clad plate should be handled with special care appropriate to the Cu-Ni alloy and not treated as normal structural steel.

Unlike solid Cu-Ni plate, it is possible to use oxyacetylene equipment for cutting clad plate if the ratio of steel to clad thickness is 4 to 1 or greater (20% clad or less). The clad side of the plate should face downwards so that cutting is initiated from the steel side to allow the slag stream from the backing steel to act as a cutting agent for the cladding. This precaution is not necessary for plasma-arc cutting but some trials may be required to find the most suitable settings for either cutting procedure. It is essential that the cut face is ground or machined to clean metal in forming the weld preparation.

When designing weld procedures for clad plate, it is necessary to treat the cladding and backing material as separate components and to avoid the respective weld metals being mixed. Otherwise, cracking is likely to occur from copper in carbon steel weld metal or iron in Cu-Ni weld metal. The region adjacent to the interface between the backing material and the cladding is welded with the 65% nickel-copper filler material which can cope with iron pick-up from the carbon steel side. When the clad thickness is about 10mm or less, the 65% nickel-copper filler metal is often used for the complete weld.

When it is possible to weld from either side, the steel side is welded first and the assembly is then inverted. The cladding is prepared for welding, cutting into the steel weld and allowing for at least two runs; the first being of the 65% nickel -copper alloy referred to earlier followed by the 70-30 Cu-Ni.

When access is only possible from the steel side, the joint is prepared to leave the copper- nickel cladding protruding, so that it can be welded the same as for the solid alloy. The weld joint in the steel backing is then made with the 65% nickel-copper followed by the steel filler runs.

If the preparation is made from the cladding side, the joint is partially filled with steel weld metal and then completed with the combination of the 65% nickel-copper filler and then 70-30 Cu-Ni filler.

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Linings

Lining a vessel with Cu-Ni sheet can be a convenient and economic alternative to the use of the solid alloy or clad plate. An early example was the construction of a water box in which the lining was fabricated as a separate component from 1.2mm thick 90-10 Cu-Ni sheet, made to fit closely into a carbon steel shell. It was then attached to the shell by a pattern of MIG (GMAW) spot welds, using an automatically timed sequence. It was necessary in this case to ensure that the lining fitted closely in the shell and was in intimate contact with it when the welds were made. Seal welds made around the flanged opening completed the lining process. Automatic spot welding allowed welds to be made with a 70-30 Cu-Ni alloy filler wire with reproducibly low iron dilution.

In recent years, lining techniques have been extensively developed for lining vessels and ducting with corrosion-resistant alloys, particularly in the power generation industry. Usually, spot welds are used to minimise bulging due to differences of thermal expansion between the backing material and the lining or from the pressure variations and the lining is attached as sheets or strips by a carefully designed welding procedure. It is important that the backing material surface is thoroughly cleaned, e.g. by grinding and blast cleaning with abrasives to produce an uncontaminated surface. The final surface should be closely inspected and any areas of localised thinning must be repaired before commencement of lining.

Two welding procedures are commonly adopted for lining:

  • In the first, each sheet is fillet welded to the backing material and then a third, covering bead is deposited to complete the joint.
  • In the second procedure, each strip is tack welded to the backing material, overlapping the adjacent sheet by a few centimetres. A seal weld is then made directly between the strips.

With both procedures, it is advisable to use the 65% nickel-copper filler material, although the 70-30 Cu-Ni filler can be used for the seal weld in the second procedure.

The number and pattern of spot welds is determined by the area of sheet or strip between the welds. The reproducibility of the technique also makes it ideal for the repetitive sequence of tack welds. Fillet and seal welds are best made by the MIG (GMAW) process since it operates at relatively high speeds and can be closely controlled by modern power sources. Details and regions of complex shape may be welded by the TIG (GTAW) process which, although slow, is flexible and facilitates manipulation of the torch by the welder.

Throughout fabrication of a lining, care must be taken to avoid surface damage to the Cu-Ni sheet and, on completion, any weld spatter and discoloration must be removed. Welds should be examined visually for defects and the absence of porosity or cracks breaking the surface of welds can be confirmed by a penetrant inspection technique.

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Brazing

Copper-nickel alloys are readily brazed by all processes although torch brazing is more common. Since the process relies on wetting of the surfaces to be joined by the brazing alloy, absolute cleanliness is essential. Fluxes alone are not capable of removing all contamination, particularly those containing lead or sulfur, and oils, paint etc. which should be removed carefully with solvents and degreasing agents. Oxides and dirt can be eliminated with emery paper or a chemical cleaning process.

If parts have been cold formed, they may contain significant internal stresses, which promote intergranular penetration by molten filler material during brazing, resulting in cracking at the joint. Removal of stresses by full annealing is not necessary; heating to 600-650°C for a few minutes is sufficient for adequate stress relief and this can be done simply with an oxy-fuel torch, taking care that the part is heated uniformly.

While phosphorus-bearing brazing alloys are often recommended for joining copper alloys, they are not suitable for copper-nickels because the nickel reacts with phosphorus to form a brittle nickel phosphide phase. Silver-based brazing alloys (‘silver solders’) should be used. They offer a useful combination of melting range, flow characteristics and mechanical properties. They also perform well in brazed joints where copper-nickels are exposed to seawater. Alloys containing cadmium are no longer recommended because of health hazards in application, but there is a range of silver-copper-zinc alloys which are suitable and safe.

For brazing pipe and fittings, pre-placed brazing alloy rings are preferred over manual feeding, providing better control of quality and minimising the use of flux, residues of which must always be removed after the joint has been made, usually by washing with hot water. The larger the pipe size, the more difficult it is to achieve uniform heating around the diameter to reach the brazing temperatures. Some organisations limit brazing to pipe diameters up to and including about 50mm.

Furnace brazing is possible and advantageous where significant numbers of assemblies are to be joined. Exothermic, endothermic or dissociated ammonia atmospheres are suitable, together with inert gas because of the high vapour pressure of some brazing alloy constituents, vacuum brazing is less suitable.

USEFUL REFERENCES/LINKS:

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Painting

Although painting of Cu-Ni is seldom required as the alloys already have inherent corrosion and biofouling resisting properties, there are occasions when painting is desirable e.g. for aesthetic reasons or to reduce the exposed metal area in a bimetallic couple and reduce the risk of galvanic corrosion.

Cu-Ni can be painted. A thorough roughening by grit or sand blasting is all important before paint is applied. Above the water line on boat hulls, appropriate epoxy followed polyurethane coatings can be applied. Leading paint suppliers will normally prefer to recommend appropriate paint specifications based on their proprietary products for specific applications.

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ENVIRONMENTAL

  • 90-10 Cu-Ni is a single phase alloy with a melting point above 1100C. In the case of a hydrocarbon fire:-
    • Pipes do not need additional fire insulation for protection
    • copper nickel does not generate toxic fumes
    • it will not show degradation in an uncontrolled manner
  • Cu-Ni has inherent high resistance to biofouling:
    • it does not need chlorination
  • Cu-Ni can be recycled.
    • it can be 100% remelted and reused.

Leaching

Copper is naturally present in aquatic environments and is an essential element in the life of aquatic organisms. Cu-Ni surface films are antifouling. Copper, which is present in the Cu-Ni surface film, seems to be uninviting to a large number of marine micro-organisms because they prefer to settle elsewhere. This keeps heat exchangers free of fouling so that the calculated effective heat transfer is maintained. The amount of released copper is small, often difficult to detect by normal methods of analysis, and quickly diluted in aquatic environments.

To resist the corrosive attack from cooling waters, Cu-Ni alloys need to develop the protective film. In general the film consists of copper oxides and other elements whose chemical composition depends primarily on water quality. Film formation in oxygen containing waters takes place at exponential growth rates, the biggest portion being formed in a few hours and only a small remainder still being formed after weeks or months. Consequently, a short-term increase in copper concentration at the cooling water outlet will be noticed during initial start up of the heat exchanger but this decreases with time.

The copper released both in the course of film formation and during service is available in two different forms:-

  • Biologically active (e.g. ionic copper)
  • Biologically inactive (e.g. elemental metal)

It is important to make this distinction rather than refer to the total copper content because ionic copper is the environmentally active form. Since analytical testing and apparatus may contain more copper than the water samples being tested, proper testing practices should be employed particularly in regard to sample storage and method of analysis.

Copper release or leaching data is discussed for Multistage Flash Desalination Units in a paper by J.W. Oldfield and B. Todd, Environmental Aspects of Corrosion in MSF and RO Desalination Plants. Desalination 108 (1996) 27-36 Published by Elsevier Science BV:

"Deaerated brine Tubing: Copper in brine blow down in modern plants with good process control, can be expected to be less than 0.1ppm

  • Copper in distillate: For acid and additive dosed plants, the copper content is expected to be less than 0.1ppm; in most cases being below 0.035ppm."

OTHER REFERENCES:

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Touch Contact

Nickel has the ability to sensitize susceptible individuals and cause an allergic reaction. This is explained in a thorough overview of Allergic Nickel Dermatitis (ACD) on the Nickel Institute web site.

Notably the European Union's Nickel Directive 94/27/EC (amended by Directive 2004/96/EC) prohibits the use of nickel in products intended for close and prolonged skin contact which will result in solubilization of nickel at a rate exceeding 0.5 micrograms per square centimetre per week.

Copper-nickel alloys contain nickel in solid solution which when in prolonged contact with sweat can produce nickel ions and therefore the potential for ACD in susceptible individuals. Essentially, most of the studies related to copper-nickel have involved coinage rather than engineering alloys as these have the most public exposure. For example in the UK, the 5p, 10p and 50p coins are currently 75Cu and 25Ni; the 20p coin 84Cu and 16Ni. If tested in artificial sweat, all will release nickel at a rate greater than the 0.5ug/cm2/week specified in the Directive. However because of the transient nature of the contact, the handling of coins rarely causes dermatitis. Also, used coins have been shown to have lower levels of nickel release than coins when they first enter circulation, probably because of surface dirt, oils and fats from the skin, and products of corrosion (e.g. nickel oxide).

Similarly for non-coinage articles, contact of copper-nickel with skin per se is not the direct cause of ACD but it is the soluble nickel products resulting from the corrosion of the metal by sweat. This requires a lapse of a period of time for the concentration of the soluble nickel corrosion products to reach a critical dose. Thus transient contact with skin is rarely damaging. For longer exposures, the rate of reaction may differ markedly between persons as a result of differences in the corrosivity of their sweat. Menné (1994) concluded that elicitation is unlikely below 15 μg/cm2 non-occluded exposure. Prolonged exposure will require protection or avoidance of skin contact.

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