CA6NM Crack Repair

[lcrahydro]

We have a Francis hydro turbine wheel that has cracks on 11 of the 13 buckets. The crown (top) is cast CA6NM and the blades are EC125 plate. We think the EC125 is the plate equivalent of CA6NM from the OC Carson Foundry. The cracks originate in a high stress area (crown to bucket) weld fillet and running into the EC125 plate. This needs to be a fun penetration high strength repair. Any suggestions on the type of weld material, and where we could get appropiate procedures, etc would be appreciated. The vendor reportedly uses a propriety weld wire, (possible similar to a E309) but we cannot get it from them.

 

[metengr]

FOr CA 6NM you are better off using an ER410 NiMo filler metal that matches the strength of the bucket material.


Remove the cracks by local excavation. Verify defect removal by wet fluorescent mT or Liquid Penentrant testing.

Preheat; 450 deg F and maintain throughout welding. Also, make sure the preheat is held for 15 minutes before welding.

Use the GTAW process.

After welding, slow cool to below 200 deg F.

I would highly recommend a local PWHT of 1100-1150 deg F for 15 minutes at temperature.

Blend grind and perform a final wet fluorescent MT or Liquid Penetrant test for acceptance of the weld repair.

Note; You should qualify a welding procedure and welder for this application.

 

[Guest102023]

First take a chemistry and report back here. (I would love to consult on this one!) Hopefully this puppy is in the shop and not the field.

When matching base metal with 410NiMo, the weld deposit has higher hardness than the HAZ and is at greater risk of H2 cracking. Tight moisture control practices and stringer beads with moderate diameter electrodes, making sure to temper previous passes. When I was in this business I specified limits on certain elements in my FCAW wire. Whatever you do, purchase from a reputable manufacturer who understands the intricacies of this alloy.

309 filler is often substituted where strength is not critical, although blade to band and blade to crown joints are always critical, especially at the trailing edges. Doesn't sound like an option for you here.

Metengr, your recommended preheat is too high and risks excessive interpass temperature. Its the same problem of Ni depressing the lower transformation temperature you are familiar with in P91 >> austenite >> untempered martensite. 300F minimum, do not exceed 500F.

 

[Guest102023]

p.s.,
GTAW will be very slow for this scale of work.
Grind excavated surfaces to bright metal before MT.
Local PWHT of this many repair locations is not really possible; the good news is you can't really exhaust CA-6NM's capacity for PWHT (which is more tempering than stress relief).

 

[metengr]

When matching base metal with 410NiMo, the weld deposit has higher hardness than the HAZ and is at greater risk of H2 cracking.

Respectfully disagree with 309. I have worked with CA6NM and 410NiMo filler materials on many steam turbine and pump components. Use of suitable preheat and low hydrogen consumables will not result in hydrogen cracking. For this type of bucket and strength requirement, 309 filler metal would not be my choice, period.

Metengr, your recommended preheat is too high and risks excessive interpass temperature.

Disagree again.
This is why you cool to below 200 deg F to ensure all martensite transformation is completed.

 

[Guest102023]

Metengr,

I don't think we disagree about 309; I stated that it is frequently used on CA-6NM but I recognized this would probably not be a suitable place for it. Strength would be ~50/90 vs. ~100/130 ksi.

You are correct about cooling down to 200F after welding; what I am saying is do not let the interpass run away to the point where untempered martensite can result. It is easy to do that on thick sections, but the consequence of it is always expensive.

A big problem working with CA-6NM has been that most of the advice given for 410 and other weldable martensitics is not very applicable; this is a unique alloy. (I blame the consumable marketing people for that mostly.) When I fabricated hydro turbines we only began to eliminate consistently massive repairs when we began controlling interpass.

This work should only be attempted using electric resistance heating blankets, thermostatically controlled.

 

[Guest102023]

CCT diagram for CA-6NM attached --