What makes a steel "stainless" is high chromium and nickel content with iron and carbon. There are 2 really bad things about chromium in the boiler world. Chromium REALLY LIKES both chlorine and carbon atoms.

At moderate temperatures, 400F+, chromium loves to mate with chlorine dissolved in water to form chromium chloride (a salt) which permeates through the grain boundaries to failure. Lifetime at 400F temps with normal stress is in the 1000's of hours time frame.

If you weld "stainless", the chromium LOVES to mate with carbon forming chromium carbides in the heat affected zone of the weld, which with normal boiler stress causes failure in the 1000's of hours time frame.

Woe be unto he who welds a "stainless" boiler then uses chlorinated tap water. It's like building an Interstate hiway through the heat affected zone of the weld for the chloride salts to form in.

Any welding of "stainless" that is expected to last has to be post weld heat treated to dissolve the grain boundary carbon back into the alloy matrix.

The words in the Code that deal with shutting down this whole topic forever are AMSE Section I PG-5.5 "...the use of such austenitic alloys for boiler pressure parts that are water wetted in normal service is prohibited."

M Austin

Matt always has the coolest pictures.

dave

Gentlemen,

This sounds like an interesting modification to say the least, and quite innovative, but consider this:

Most (if not all) backhead throttles utilize a machined "bearing" of sorts to accept and support the throttle rod where it intersects the backhead. The bore of this piece is usually not much larger than the throttle rod itself, effectively producing a mechanical restriction so that boiler pressure cannot "catastrophically" enter the cab.

The packing rings that are used within the stuffing box surrounding the throttle rod are only responsible for sealing out the minute amount of steam that would be escaping around the diameter of the throttle rod. If this packing were to fail, the inherent construction of the assembly would prevent a dangerous volume of steam from passing through and injuring individuals within the cab.

Also, it is worth considering that the likelihood of a complete failure of packing occurring, to the point where packing would not even diffuse the bypassing steam, seems relatively unlikely.

Obviously, this should be taken into consideration with the requirements of the group's restoration, along with what condition the aforementioned parts are in, or are being capable of being restored to. If the entirety of the throttle assembly, including the throttle rod and the backhead "bearing" has to be replaced, it would be worth machining this to a careful tolerance to minimize the amount of potential steam space, while still providing enough "slop" in the bore for expansion and movement.