Welding resulfurized steel

[tmoritz]

A contractor managed to gouge our generator rotor in the process of removing the rotor poles. We've taken a metal sample and found the base metal is similar to AISI 1137 but with low manganese and high copper. My primary concern is the high sulfur content.

The contractor needs to repair the surface and the proposal is to fill the gouges with weld material then grind and machine the surface true. I have read several places that there is considerable risk of hot cracking due to the high sulfur content. Is it possible to weld this base metal?

I've attached the OES metal analysis results.


Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[stanweld]

You are correct regarding the potential for hot cracking.
Welding will also induce residual stresses which may create unfavorable distortion of the shaft. Low heat input weld processes are recommended to mitigate the propensity for hot cracking.

Have you considered electroless nickel or hard chrome plating to fill the gouges?

 

[tmoritz]

The gouges were the result of the contractor trying to drill out a stubborn drive and stand key. The drill bit drifted out of the key set and into the rotor rim. The gouges vary from very shallow to over a half inch deep in the worst case. I don't think plating processes will be sufficient to repair this damage.

Can you expand on your suggestion for low heat input weld processes?

Thanks,

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[metengr]

The contractor needs to repair the surface and the proposal is to fill the gouges with weld material then grind and machine the surface true. I have read several places that there is considerable risk of hot cracking due to the high sulfur content. Is it possible to weld this base metal?

No, I would not recommend weld repair of this base material, period. What about braze metal repair?

 

[stanweld]

Pulsed GMAW with .030" diameter E70S-6 with heat input < 15 KJ/inch might work. There are no guarantees.

You may also wish to consider metengr's proposal or the use of one of the epoxy/metal cold weld materials.

 

[MiketheEngineer]

Dumb question - could it just be smoothed out and not welded??

 

[metengr]

tmoritz;
Let me discuss the welding option another way - if, and I mean if, you want to perform local weld repairs to a generator rotor shaft first off you need to qualify a welding procedure. Simply welding to add material in this situation is not enough and should there be a problem than what?

Second, if by chance you can try to weld with no hot short cracking along the base metal HAZ of the weld coupon, there is no guarantee that what you did for coupon welding to qualify the procedure would work on a generator rotor shaft because of the inherent poor weldability of this material (from high copper and sulfur).

Third, what nondestructive testing (NDT) requirements are you going to specify after welding to ensure no defects? Catch my drift for all of this? Is this equipment insured?


Last, you have possible distortion concerns – the shaft and pole slot geometry are final machined. If you do any welding to rotating equipment you need to understand all of the risks and steps required and weigh against other options.

 

[unclesyd]

Would it be possible to do as posted by MiketheEngineer and cut a new keyway using this machine or something similar?

 

[tmoritz]

It appears, due to the base metal composition, we will not be able to weld repair. A new dovetail slot is not an option because the rotor poles are equally spaced around the rotor. Moving one would require moving all and the geometry does not allow this. We would have to replace the rim.

We are now looking at stress levels. If they are low enough then we will fill and mill. The filler will be brazing material or metal/ceramic filled polymer like Chesterton ARC 858. If the stress levels are too high then we may have to fabricate a new rim. Nobody will like that option.

Thanks for all your input.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[unclesyd]

It's a bummer if you can't recut the keyway. You might want to file the information at the site posted below for future repairs.

http://www.cpmt.com/video/km3000-operational-video