There was a recent discussion on FB about a home made park train with a stainless steel boiler.

One of the comments made was that it would be impossible to get that certified, since it was stainless steel.

I'm not sure if I misunderstood and the comment was really saying "Good luck getting a home made boiler certified", which obviously makes sense, but it seemed to imply that there was simply no way to certify it if it's made from Stainless Steel.

Is there such a thing as a stainless steel locomotive boiler? Can one be made? I don't recall ever hearing about one, but I haven't done any real research. Is the expense the issue? Is there a hardness issue with the steel? Other reasons it wouldn't work?

To clarify, I mean the actual boiler, not just stainless steel jacketing.

Stainless steel is VERY bad at conducting heat compared to the alternatives. Put an aluminum and a SS stockpot side by side on the same stove, and the SS one will take twice as long to boil. This is why noted brands use copper cladding on the bottom of their SS cookware.

I would suggest the only reason someone wold take this approach is because they somehow got a free SS cylinder and worked from there.......

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There are 'stainless' alloys used in water-tube boiler fabrication, but that's a different application under very different fabrication and operation conditions.

In addition to the issue of lower heat transmission, many common alloys suffer work-hardening under thermal or stress cycling. And the HAZ when welding, say, 304 with 308 rod is a terrible crapshoot.

If you’ve ever seen the book “So You Want to Build a Live Steam Locomotive”, there’s a chapter on a 1” scale B&O 4-6-2 with a stainless steel boiler. Based on the 1974 publication date of the book, I’d guess this boiler was built around 1970. It’d be interesting to see how it worked out long term. Apparently it steamed well, but it should be noted that it had copper flues (as do many live steamers). The builder surmised that the reduced heat transfer in the firebox would be offset by increased heat transfer in the tubes, and apparently that worked out.

Correct.

The main reason, as I understand it, is cracking. Section I of the ASME boiler code (high pressure power boilers) prohibits stainless steel.

However.......there is always a however....

Section VIII of the ASME boiler code is for unfired pressure vessels. Strangely, there are "fired unfired pressure vessels", which means that the object is a pressure vessel, and is heated via electricity or flame/fuel and develops pressure, but is not considered a boiler. Common examples are cooking pots that have double walls (double boiler) where water is heated and steam is created within to cook soup or stew. These are made of stainless steel, are fired, and can contain steam up to 50psi, and are "U" stamped as a Section VIII vessel.

Go figure....

I believe the material property of interest is the thermal diffusivity. This is a combination of the thermal conductivity of the material (ie "resistance" against the flow of heat) and the thermal capacity (ie "how much" heat a material can contain per degree of temperature increase).

The thermal diffusivity is effectively a measure of the "speed of heat" and represents how quickly a thermal gradient can move though a material.

Among commonly used materials copper has a very high thermal diffusivity and aluminum is slightly lower on the list. Thus copper is widely used for electrical "heat sinks" with aluminum a second more economical choice.

Diamond has one of the highest thermal diffusivities and has been used for "cooling" semiconductor devices with very high current densities by quickly conducting the heat away.

For a boiler shell stainless steel (with a menu choice selected to resist cracking) combined with copper tubes (for most rapid heat transport) might make a very good choice for a boiler. After all you want the shell to "contain" the heat and the tubes to "transfer" the heat into the working fluid.

The "heritage" ASME boiler codes probably never fully evaluated stainless steel boilers because of the historic high cost of stainless steel.

I believe some of the latest RR boilers (NYCRR Niagara's) used "Nickel Steel" (an early "poor man's version" of stainless steel) where is was thought that the addition of Nickel would reduce corrosion, but over the length of that experiment the Nickel Steel was found to be prone to cracking.

Also, there are hundreds of "flavors" of Stainless Steel, I suspect there are some modern menu selections that are just as ductile (crack resistant) as historic non-stainless steels that have been fully evaluated for use in fired pressure vessels. If RR steam engines were to ever make a "comeback" and be widely used I'm pretty sure someone would find (and or mix up) a stainless steel that would work just fine.

From a model perspective - the following is the first episode of a builder making a model PRR A3 0-4-0 switcher.

https://www.youtube.com/watch?v=yiAWsopdr0A

There are currently more than 20 episodes of the build.

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The fundamental reason for not using stainless steel has to do with the heat affected zones in and about welds. Carbide precipitation at the grain boundaries lead to cracking, preferential corrosion, and an all around complex metallurgical problem. With aggressive water treatment the issue of corrosion of steel boilers can be adequately addressed.

The reason for the 'nickel steels' that caused such agony in the late 1940s was lighter weight for higher operating pressure. No one sane would say that those steels demonstrated 'reduced corrosion'. I was told that some areas of PRR Q2 boilers corroded more than 1" in thickness during only a year or two of dead storage -- admittedly, near the Pittsburgh area.

That’s pretty crazy. I have an excerpt here somewhere from a ~1948 mechanical engineer’s handbook that mentions the problems with cracking around the rivet holes on nickel steel locomotive boilers. It notes that generally a fully welded boiler of conventional steel was about the same overall weight as a riveted nickel steel boiler, I’d presume due to the elimination of extra material required for all the lapped joints and rivet heads.

Hugh, look at your reference and see whether correct reaming of the rivet holes might have led to less in-service cracking for some of these steels.

The weight, strength, facility of cleanout, and pressure/temperature cycling tolerance advantages of a properly fully-welded convection section are reasonably well-established. Alco thought enough of the approval of welded construction in 1943 to have installed a vertical annealing furnace capable of taking an entire large boiler -- then scrapped it after the late '40s when there was no longer a perceived need for better big steam. (Just a few years before it would have become invaluable for Alco's rebranded mission as a nuclear component supplier... but there ya go!)

Both the British and the Australians, however, learned the hard way that you don't take specs for riveted fabrication and just change to welded fitment.

I’ll dig it out and check. I remember reading that holes were typically punched, not drilled, in boiler sheets. While this had been completely successful with conventional materials, it resulted in micro-cracking around the rivet holes in nickel-steel which eventually lead to full-blown cracks.

Not a steam guy, but it seems to me that you’d want a boiler to be able to flex given the service conditions on a railroad. Carbon steel gives you that. Nickel steel has a higher tensile strength and work hardens and thus would probably stress crack.

When you’re punching holes in thicker material, you get some significant deformation of the material around the holes. Would I be correct to assume that the deformation in the harder nickel steel would lead to cracking around those holes when under stress?

Or should I shut up and sit down and stick to playing with diesels?