I work at a pumped storage generating plant currently going through a "Life extension & modernization". Each of the units here can be run as a generator (during peak electric demand hours), or as a pump (during off peak hours when there is surplus pwoer in the grid). In the pump mode, the units are run as pumps to pump the same water used in generating back to the upper reservoir. Each unit has a "runner", which is really a hybrid between a centrifugal pump impeller & a Francis type of hydro turbine runner. In service, these runners are subjet to cavitation erosion. Each runner weighs about 90 tons. The original runners were installed new in 1970-72. These were a cast medium-carbon steel alloy. Areas subject to cavitation erosion were repaired during maintainence outages. This repair took the form of plasma gouging to sound metal, grinding, and then pad welding. First layers of the pad welding were done using 309L, with successive layers welded with a specially formulated stainless electrode from the turbine builders. This was something like a 302 stainless with cobalt added. All welding was done with the runner in place inside the turbine casing, which meant mostly overhead as well as some vertical welding. SMAW was the only process that produced sound welds. After pad welding beyond the original contours of the runner uckets ('blades") grinding and blending and polishing were done. On average, 250-300 lbs of electrode was burned during each 2 year maintainence outage.

The units are now being upgraded with new runners, giving higher efficiency and higher output. These runners are made of a Martensitic stainless steel. The runner crowns and bands are castings, while the "buckets" (or "blades") were formed from thick martensitic stainless plate and machined and manually ground to the required contours. These buckets were then welded to the crowns and bands in the vendor's own plant, so all welding was done with the runners on positioners, as well as with proper preheat and postheat.

The inevitable fact about hydro turbine runners is that no matter how good a design is, every runner will be subject to cavitation. It's a matter of how much cavitation, ranging from a light surface " frosting"  to a  deep erosion looking like badly weathered concrete.
The new runners, after the first 2 years in service, are showing the light frosting indicative of cavitation erosion. We polished these areas with flapper wheels, and put the unit back in service. I know that in two more years, there will be cavitation damage needing to be addressed in those same areas.

The matter of doing welded repairs in place to these Martensitic runners is complicated by the fact the runners are located inside turbine casings which are unwatered for the outage. The overall environment is cold and damp. We put in a temporary staging which creates a working compartment, then we turn on large forced air heating blowers for 48-72 hours prior to doing anythng with the runners.

I would appreciate any suggestions for doing pad weld repairs to Martensitic Stainless, heavy sections over 2" thick, in place, in what amounts to Horizontal, Vertical and Overhead positions. The turbine vendor has only qualified a GTAW procedure for the flat position, using 309 filler metal. While the 309 filler metal will make a good weld, it defeats the whole purpose for having a Martensitic Stainless Steel Runner. 309, as an austenitic material, is simply too soft and does not resist cavitation erosion all that well. A welding procedure is needed to run the pad welded repairs with somethng similar to an austenitic stainless steel. As I noted, a real preheating and postheating by controlled methods are a practically impossible with the runners in place in the turbines. By way of general information: The runner is the bottom-most part of the rotating element of each unit. COupled with the shaft, thrust runner disc and generator rotor, each rotating element weighs about 850 tons. To get a runner out requires dismantling the generator and turbine head cover- over a month's work for  sizeable and experienced crews working double shifts.

I would apprecaite hearing from anyone who has had experience with this type of pad welding on Martensitic Stainless Steels.

                                           Joe Michaels