Copper-nickel welds well. It does not need preheat, it does not sensitize, and it does not harden in the heat affected zone. What it does do is punish contamination and punish heat, and shops that treat it like carbon steel get porosity and cracking.
Filler metal
The standard practice is to weld both 90/10 and 70/30 with a 70/30 type consumable, which means a filler around the ERCuNi or ENiCu composition depending on the process.
Using a higher nickel filler on 90/10 base metal is deliberate, not a substitution. The extra nickel covers dilution from the base metal and keeps the weld metal sound and corrosion resistant. Matching filler to a 90/10 base gives a weaker, more porosity prone deposit.
Consumables for copper-nickel also carry deliberate additions of titanium or manganese to scavenge oxygen and nitrogen. That is what keeps the weld metal free of porosity, so keep filler dry, clean and correctly identified. Do not improvise with a copper filler.
Contamination is the main enemy
Copper alloys are far more sensitive to pickup than steel, and two contaminants matter most.
Iron
Iron picked up from steel brushes, steel grinding wheels, steel layout tables or shared fixtures causes inclusions and local corrosion sites in the finished weld. Keep dedicated non-ferrous or stainless tooling for copper alloy work, and do not stage copper-nickel parts on the same table as carbon steel fabrication.
Note the distinction from alloy chemistry. The iron deliberately alloyed into C70600 is essential and protective. Iron smeared onto the surface from a grinding wheel is contamination. Same element, opposite effect.
Sulfur
Sulfur causes hot cracking in copper alloys. It arrives from cutting oils, greases, marker pens, crayons, tape residue and unwashed hands. Degrease the prep properly and mark with approved low-sulfur markers only.
Heat control
Copper-nickel conducts heat better than steel but far less than pure copper, so it does not need the heavy preheat that pure copper does. In practice:
- No preheat beyond warming to drive off moisture and condensation.
- Keep interpass temperature low. Let it cool between passes. Heat build-up widens the heat affected zone and increases distortion without any metallurgical benefit.
- Do not weave excessively. Stringer beads with controlled heat input give a sounder deposit.
- No post weld heat treatment is normally required. Copper-nickel does not need stress relief the way alloy steel does.
Protecting the bore
This is specific to seawater service and it is often overlooked. The inside surface of the pipe is the surface that has to grow the protective cuprous oxide film. A weld root that is heavily oxidised, sugared or full of slag gives that film a poor start exactly where the water is fastest.
Use an argon purge inside the joint for the root pass. On a weld neck flange the root is in the bore and you cannot dress it afterwards, which makes the purge more important than it is on a slip-on where the internal weld is accessible.
Slip-on versus weld neck
Most copper-nickel seawater piping uses slip-on flanges, welded inside and out. They are easier to align, they tolerate small fit-up errors, and seawater distribution rarely has the fatigue duty that would justify a butt weld.
Weld neck belongs where the line sees pressure cycling, vibration, thermal movement, or where a code requires a full penetration butt weld and radiography. The tapered hub also gives a smoother flow transition, which matters more in copper-nickel than in steel because turbulence is what strips the protective film.
Brazing
Silver brazing is common on naval copper-nickel work and appears in the naval fitting specifications alongside welding. It puts much less heat into the part and suits small bore and socket type joints. It needs clean, closely controlled joint clearance and the correct flux, and the joint has to be flushed afterwards, because residual flux is corrosive.
Checklist before you strike an arc
- Prep degreased, oxide removed, bright metal for at least an inch back
- Dedicated non-ferrous tooling only, no steel brushes or wheels
- Correct filler, dry, identified, and a higher nickel type than the base metal
- Argon purge set on the inside of the joint
- Interpass temperature control agreed, and no preheat beyond moisture removal
- Approved marking materials only, nothing sulfur bearing
- Procedure qualified for the actual alloy and thickness
For grade selection before fabrication, see copper-nickel flanges and choosing 90/10 or 70/30.