A friend of mine is devdeloping a working museum dedicated to the steel mills. As part of this museum, he is putting together a small, working rolling mill. This mill is to be driven by an Ames Unaflow steam engine. The engine has about a 17" bore x 20" stroke.  The cylinder bore was badly pitted by condensate that laid in the cylinder after the engine was shut down for some years. Having some working knowledge of steam engines and repairs to them, my friend asked me to recommend a "fix".

Since this engine will be run on 150 psi  saturated steam & run occasionally at light loads,  I told him to follow conventional steam engine repair practice. This would consist of:
  -boring the cylinder oversize to get a clean bore,

  -turning the piston undersized by about 0.060"

  -baking the piston (first drilling/tapping a relief hole in the piston since it is cast hollow)

  -abrasive blasting

  -set the piston up on an arbor (or the piston rod) in a lathe or on roller stands so it can be revolved to position it for brazing

  -apply and maintain a preheat on the piston (a preheat of about 200 degrees F is what my own instincts are telling me)

  -brazing a bronze overlay on the piston

  -machining the bronze overlay to the correct diameter for the rebored cylinder & cleaning up the ring grooves

The piston is made of cast gray iron. It is a hollow casting (Unaflow engines typically had a long piston body to cover the exhaust ports).

Solid steam engine pistons on stationary engines & railroad locomotives typically were built up by oxyfuel brazing. This worked quite well in engines running on saturated steam. it also provided a good wearing surface for the piston to ride against the cylinder wall at about 6:00. 

Since this piston is a large hollow casting, I felt that oxyfuel brazing would put too much heat into it. My thought is to use a MIG brazing process with the piston either in a lathe or on some kind of roller stands. In that way, the piston could be revolved so the brazing was done at 12:00 in the flat position.

I checked one source of MIG brazing wire. For buildup work on cast iron, their procedure was to deposit an intial layer of what amounted to a nickel repair alloy ( "Ni Rod"). This is not a viable thing on a steam engine piston as the ring grooves have to be remachined, aside from the amount the piston would have to be turned undersized. I am lookng for a MIG brazing procedure that will enable a layer of bronze to be deposited right on the cast iron piston.

Spray welding to deposit bronze or iron on worn/turned down steam engine pistons has been tried by steam engine erectors in the past. The erector who taught me had told me years ago that spray welding never worked on steam engine pistons, and to avoid it at.

Thus far, my own instincts are to turn the piston undersized by about 0.060" and leave a fairly coarse surface finish. The cylinder will be bored over by about 0.200". The piston has seen years of service with steam containing boiler compounds as well as steam cylinder oil. For this reason, I am leaning towards a 0.060" cut on the piston, followed by baking to get rid of any residual oil. I also suggested abrasive blasting to get rid of any surface film of oxide as might devlop on raw cast iron being baked. This is how I've prepared a few smaller iron castings in the past for repair brazing.

I would appreciate any recommendations as to brazing alloys and welding parameters.

                                                              Joe Michaels