Cavitation repair for cast steel hydraulic turbine runner blades

[KMP04]

I'm looking for some feedback in regards to standard cavitation repair practices for turbine runner blades in the hydro electric industry. There are various opinions regarding pre-heat of runners prior to a gouging operation and welding Stainless overlays onto cavitated areas.

Typically our turbine blades are low to medium carbon steel and we overlay with 309L stainless Steel electrodes.

Preheating has not been practiced in the past however recent procedures have been issued requiring 250 deg. F preheat. The reasoning behind this change is to avoid crack initiation. Historically some cracks have been found but many of these are atributed to runner fatigue and not a result of past cavitation repairs.

when performing a repair in winter and spring the turbine temperature can be as low as 35 deg. F. This large steel mass takes quite some time to heat up. Is there a preheat benefit for cavitation surface repair up to 1/4" deep?

Thanks for your comments!

KMP04

 

[metengr]

KMP04;

when performing a repair in winter and spring the turbine temperature can be as low as 35 deg. F. This large steel mass takes quite some time to heat up. Is there a preheat benefit for cavitation surface repair up to 1/4" deep?

Yes. The main benefit of preheat in your specific application is to remove moisture from the repair area especially on "cold" metal.

 

[21121956]

Hello everybody:

Hi metengr, I am a little bit confused because reading the thread725-154993, a post of gtaw said:

2) Where does the water or moisture come from when preheating a thick plate before welding? Answer: The moisture is a by product of combustion. The moisture condenses on the cold surface as water. You'll be surprised by the number of welders that will tell you its moisture coming out of the plate.

Any comments on this subject?

 

[metengr]

21121956;
Yes. Local heat generated from arc welding directly on steel plate that is less than 32 deg F in temperature can run the risk of forming condensation on the surface of the steel plate adjacent to the weld region. The condensation is caused by moisture in air that is warmed from the heat of welding and directly contacts colder areas of the steel plate that surround the weld region (similar to moisture that forms on the surface of a glass of ice water that is left out on a warm summer day). Torch preheating will result in some water vapor being generated, but it is the warmed air heated by the torch in contact with a colder surface that results in condensation. Moisture is not driven out of the steel.

The presence of surface condensation can provide a source for hydrogen. The water evaporates as the plate begins to heat from welding and can dissociate into hydrogen atoms under intense heat generated by the welding arc. The hydrogen atoms diffuse into weld region, which is not desirable.

I might as well mention that additional benefits of preheating cold steel is
-to reduce susceptibility to brittle fracture, if the ductile to brittle transition temperature of the steel is at or above 32 deg F,
-and to reduce harmful thermal gradients.