Overheating of 18Cr-18Mn stainless steel 1

[CWicker]

We have recently had an incident in a turbine generator where an earth fault caused excessive damage to the stator and rotor. Melted copper was deposited on the outer surface of the 18Cr-18Mn coil retaining ring. Copper melts at 1085degC. I would expect that as the copper cooled, excessive chromium carbide deposition would occur at the grain boundaries. However, depth of overheating will be difficult to determine, unless the CRR is machined in steps of 100 microns and etched/replicated.
What factors should I consider in determining suitability for refurbishment and return to service of component, bearing in mind that the lead time on a replacement is 6 months.

 

[metengr]

Your question concerns the integrity of nonmagnetic retaining rings on a large generator. We have evaluated corrosion pitting attack on the surface of our 18-5 retaining rings that were still in service using local micro polishing techniques, etching, and evaluating under a portable field microscope or surface replication for viewing back in the lab. Once we determined the corrosion pit was removed and no stress corrosion crack was at the base of the pit, blend grinding was performed to reduce stress concentration.

I see no reason why this approach cannot be used in your situation. Keep in mind that molten copper will solidify very quickly on a retaining ring of this mass. I doubt that the heat will adversely affect the ring material microstructure. These rings consist of an austenitic structure that is roll formed to achieve high strength.

Therefore, before deciding on purchasing replacement ring forgings I would locally remove the solidified copper, micro polish and etch the affected surface to evaluate the microstructure using the replication technique or using a portable optical microscope. You can use this method in another area of the ring to compare microstructures.

 

[metengr]

Also, forgot to mention that in addition to evaluating the retaining ring microstructure, you should perform portable microhardness testing and fluorescent liquid penetrant testing to compare between affected and unaffected areas of the retaining rings. This information coupled with the microstructural evaluation should enable you to decide on suitability for service.