Magnetite formation on Stainless fracture surface

[Gem1]

Can anyone tell me what times and/or temperatures would be required to grow approximately 50 microns of magnetite on a 12%Cr, martensitic stainless steel?

I have a failed turbine blade has seen 65,000 hours service at less than 100 degC (steam) and I have found magnetite on the fracture surface of a crack at a brazed erosion shield. I'm wondering if the magnetite means it must be a quench crack or if it could have occured when an erosion shield was brazed onto it at nominal temps of up to 750 degrees.

Thanks

 

[metengr]

Gem1;
Normally, quench cracks will not be a cause of failure in turbine blades. The reason being all blades receive a wet fluorescent MT before final fabrication and at that time cracks or other defects would have been detected. Second, with 65,000 operating hours, a pre-existing crack would result in rapid failure of the blade under these service conditions.

I would look very closely at the brazed region because I would bet the brazing temperature exceeded the lower critical temperature for the 403/410 stainless steel blade resulting in fatigue crack propagation along or in the hardened heat affected zone. I have seen this before. Brazing that is performed using a manual process can result in local overheating of the blade; seen it too many times.

 

[EdStainless]

A micro-hardness traverse in the region of the failure should tell you if the temp limits were violated.

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Plymouth Tube

 

[Gem1]

Thanks Guys. This was my first reaction too, I didn't think a crack could escape detection prior to installation. I have investigated local hardnesses and microstructures, which are telling me that the brazing temperatures were violated and that there are hardness variations around the shield.

It is just that I am confused by the relatively high level of scaling on the fracture surface and the observation of some corrosion penetration into the material from the FS indicating it is quite old. Could the brazing have caused the crack and then still had enough time/temp to cause a 50 micon hematite layer on the FS? Operating temps just don't seem high enough to form it?

 

[metengr]

Gem1;
Yes, the heat from brazing along with fluxing used for silver brazing can result in this type of surface appearance and will certainly result in discoloration from heat. There will be little in-service oxidation because the back end of the LP steam turbine operates under vacuum service.

I think you found your smoking gun.

 

[TVP]

100 C is definitely too low to form hematite, no matter how long the exposure was. It had to be either from the brazing operation or prior to that (quench crack, etc.).

 

[Gem1]

Sorry, I meant magnetite. There is a very adherent layer of the stuff on the fracture surface. The coating is on the whole fracture surface.

I guess I'll go with the brazing crack option, despite a 'fatiguey' look to the crack - the shape of FS and some beach mark-like indications. (The surface was too scaled for useful SEM and too etched after scale removal for concrete conclusions).

I agree that service temps wouldn't scale up the FS like that. Maybe they had a couple of go's at the initial brazing.