70-30 Copper Nickel Welding

[sepehr1007]

Hello,

I have been working on a welding procedure for welding 70-30 copper nickel alloy (UNS C71500)using GTAW that will reduce the number of porosity related weld repairs on small diameter piping.

The specifications we are working to are very exacting - internal pores larger than 0.5 mm are unacceptable (radiography).


So far, the shielding gas and back purge gas has been 100% Argon. The material near the weld joints and the weld rods are scrubbed with non-oxide abrasive pads and wiped with acetone; the welders are skilled and follow the techniques recommended for welding the alloy; the shielding gas supplier is reputable (Praxair); hoses, fittings and gas connections are new and checked (and rechecked) constantly. Yet the repairs are frequent, and seem to be at random; although some welders are better than others, even the best welders seem to have trouble getting clean radiographs consistently.

In an attempt reduce the problem, we've experimented with 95% argon + 5% hydrogen mixed shielding gas. The additional hydrogen seems to be very effective at reducing porosity, which we suspect is caused by oxygen contamination. The procedure qualification tensile tests and bend tests (ASME BPV Code Sec. IX) have not revealed any obvious problems.

I'm aware that hydrogen in the weld pool can cause major problems ("hydrogen embrittlement") when welding oxygen containing copper alloys, which I understand is associated with water vapour formation in the solidifying copper. I'm not entirely familiar with welding-related hydrogen embrittlement in copper alloys. Is it a delayed phenomenon as with hydrogen cracking in steels? From my reading, the mechanisms seem to be different in the 2 alloy systems.

My question is: Will the addition of hydrogen to the shielding gas in a de-oxidized copper alloy such as 90-10 or 70-30 copper-nickel cause any problems? It seems to be quite effective at cutting down porosity. I would like to make certain there is no delayed effect on the joint that does not show up during standard procedure qualification testing.

Some more details:

Process - Manual GTAW
Base Metal - UNS C71500 OD 12 mm through 75 mm, WT 3 mm through 6 mm
Filler Metal - ERCuNi (1.6 mm, 2.4 mm)
Shielding Gas - 95% Ar + 5% H₂
Back Purge Gas - 100% Ar
Welding Position - 5G and 6G under limited access conditions
Preheat - None
Max. Interpass - 150 °C
Welding Current - DCEN, 65 - 150 A
Electrode - EWTh-2 (2.4 mm)

 

[metengr]

Hydrogen in the shielding gas is typically not recommended, except for increasing heat input. For copper-nickel alloy GTAW, the shielding gas should be Ar or Ar and He. The porosity is from contamination with air due to shielding or surface contamination with the filler wire. If you are using hydrogen to correct a porosity issue, there is no delayed effect.

 

[EdStainless]

So you have measured oxygen and dew point at the weld location, and they are both very low?
And you back purge using an oxygen meter to tell you when it is good?
And you are not using too high of a flow rate for your shield gas (causing turbulence and drawing more air into the weld)?


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[sepehr1007]

Thank you for your replies!

EdStainless, the oxygen in the purge is measured every few passes and is kept below 0.2%. The ambient temperature and humidity is controlled.

We are using a 25 mm gas nozzle and lens, with a flow rate of ~30 cfph (sorry for mixing units).

 

[metengr]

Did you check on wire contamination? All of the literature I have on 70/30 welding recommends either straight Ar or Ar-He for GTAW.

 

[sepehr1007]

Metengr, yes, immediately prior to use the rods are scrubbed with abrasive pads to remove oxidation, and wiped with lint free cloths soaked in acetone. We've considered residue from the acetone as a possible source of contamination; the intent is to try absolute ethanol, xylene and ethanol. HSE policies precludes the use of methanol. The difficult in carrying out a root cause analysis in this case is that the incidence of defects is frustratingly random; I cannot seem to find any trends or correlations.

I did consider using Ar-He, as that is indeed what I found the literature recommendeds; however, my client's customer has (reportedly) successfully used the Ar/H₂ mix. The choice was partly driven by the client's desire to try the hydrogen mix first.

 

[metengr]

sepehr1007
Sounds like you have fully investigated potential problems. I did come across one application related to the use of 5% hydrogen with Ar to reduce porosity in pure nickel. So, this may be something you are seeing with the Cu-Ni alloy.

 

[sepehr1007]

Thank you everyone, your input was much appreciated.