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Mr. Anderson,
An honest question, if I may?
Does the extensive welding for full penetration stay bolts add weakness to the surrounding sheet? Likewise, does it weaken the stay itself in that first portion inside the sheet?
Or is any weakening just too minor to be a concern? A local boiler inspector mentioned he was seeing potential problems in all-welded boilers with the longitudinal seams -- apparent wasting caused by the changes to the metal from welding on either side of the seam.
Another possible plus of threaded stays is that the process, in many jurisdictions, is considered a mechanical repair not requiring special certifications.
Thank you.

The only exception I have ever heard of welds damaging or weakening the metal is welding on cast iron. Since cast iron is not flexible heating up the metal (especially small pieces) will result in expansion and when it cools it will often crack. I was always told you need to heat the entire piece up to 1200 degrees, do the weld, then put it back in the oven for gradual cooling. I have seen some guys get away with welding very large pieces of cast as the metal soaks up so much heat, it is often just warm after welding. The cracking issue might be why locomotive, passenger car, and freight car trucks were all made (and still are) using cast steel instead of cast iron. Cast steel can take a incredible amount of abuse.

As far as welding stay bolts. The weld is obviously many times superior to the metal you are working on measured by tensile strength. I would imagine just about everything else would fail on that boiler before a weld broke on a stay bolt. Not saying it wouldn't happen as there are some terrible welders out there and the weld will only be as good as the guy welding it. But considering how thick the average locomotive boiler is, the boiler metal will absorb and dissipate the heat rather quickly. There has been millions of pressure vessels made that use boiler tubes (nuclear, marine boilers, industrial boilers, power plant boilers, etc) and they have a excellent safety record.

Every weld will create a heat affected zone (HAZ) in the surrounding base material. How large this zone is depends mostly on the thickness of the material being welded and the amount of heat which was put into it. The material in the heat affected zone isn't necessarily weaker, but does have internal stresses from the heating and cooling cycle(s) inherent in welding. This is why many WPS' (Weld Procedure Specification- the instructions for welders on how to properly execute a welding procedure) have requirements for pre-and-post heating within a specified range, as well as minimum/maximum "inter-pass" temperatures to prevent the weld piece from getting too hot or cold- which can reduce the amount of stress within the HAZ and the size of the HAZ. The WPS may also have a post weld heat treatment, which as we know brings the whole piece up to specified temperature, and gradually and evenly cools it back down, relieving these internal stresses.

Having said all that, a properly made weld will be stronger than the original base material. So no, there's no merit to the idea that welding makes the stay or surrounding material inherently weaker.

I have been apart of three firebox constructions/replacements (involved in some of these more than others) which utilized full-pen welded bolts throughout. We TIG welded the roots in, and filled/capped with ER-7018 SMAW (Stick welded). It is time consuming to do, and I'd bet that a significant portion of the stays had at least part of the root not get adequate penetration. When there's thousands of bolts, there's no good way to visually inspect the root of every single one in a space like a water leg, and maintain that quality control. Code doesn't require the X-ray of these welds either, iirc.

As Kelly said, they're difficult to cut out to replace, and unlike a threaded stay, you'll need a competent (read certified) welder to do it. However, the weld is going to be less likely to leak at the sheet over time than the contemporary threaded stay, so short of them breaking- they'll be more maitenance free. This particularly matters on industrial boilers that don't see inspections or maintenance quite like locomotives do.

Generally speaking, for your locomotive's average firebox staybolt I'll personally take a threaded bolt over full-pen welded one, and I'll take that over a fillet welded bolt (Don't get me started on those, I just don't like them. Largely because they're commonly installed with just one pass of weld to seal and support the full load on the staybolt, and don't have any meaningful weld penetration, which can make them more susceptible to crevice corrosion on the water side) That doesn't make one inherently "better" than the others though- all three types are approved for use in code construction. Depending on your application, one may be better suited for instalation than another- your milage may vary.

"...my former employer, we did testing on the strength of the various types of attaching staybolts to sheets, and found that full penetration welded in staybolts were:
1. The weakest method. When subjected to a straight-line tensile pull, the full penetration welded bolts consistently failed first."

Do you have pictures for the pople like me at a certain level of (non) understanding?

How did you apply this straight line force? Is there a particular legacy apparatus or machine for that purpose or did you have to get creative in applying this force? How do you measure the force? How does the composition affect the breakpoint (if that's the right term)?

Thanks everyone; some of you even answered my question.

I am intimately familiar with "Destruct" testing as it was done at my former employer constantly in the Inspection Lab; the "Fractures" caused incredible noise; the pressures are recorded and catalogued.

Welding is an ever evolving science. I have witnessed modern power plant boiler repair and they adhere to the TIG root pass and 7018 fill and this is sufficient for the 3klb.range. I still can't believe a threaded connection, as good as they are, to be superior.

New "Package" boilers that arrive on a skid all have welded staying.

PJF

Did the weld bead itself break? did it pull from the base metal?

Look at what is staybolted in a typical Stephenson boiler: water legs and chamber. The pressurization is entirely between the inner and outer wrappers, and for most of the staybolts the resultant force outward is from the areas of plate at each end, normal to the staybolt axis Obviously this is comparable to the straight-line pull-test force in question, and you can generate it in a pressure test either with steam pressure or a hydro test. I suspect in Kelly's testing, this was a normal tensile test apparatus with the plate secured in a common manner for all the options.

I don't see why anyone compares staybolted firebox construction with water-tube boilers when it comes to welding. One of the principal issues with full-pen welding is the fillet formed on the inside of the full-pen welds, which was difficult to access with appropriate imaging until comparatively recently, and it can be difficult to use x-ray or other NDT to visualize the weld in-plane as it is made, and after it has been made.

Now, part of Tross' concerns, and very important when assessing why the inside fillets matter, is that staybolting is usually welded with the boiler as a whole relatively cold. When it heats up, the staybolts bend in a variety of ways -- this being the reason for the use of flexible staybolts, but as these only have ball joimts and thimbles on the outside, there is still some bending moment at the point where the screwed or welded staybolt goes into the inner wrapper. One of the approaches has been the use of necked or waisted staybolts to keep the bending momenta away from the joint in the plate and any stress raisers due to cutting or preparation and then insertion.

It would be interesting to see a boiler welded-staybolted at operating temperature, perhaps prestressed in some way, with assured reaming and careful NDT during and after each staybolt is installed. John Rimmasch emphatically believed that could be done. I'm now not sure he knew what was involved in doing it, and if I'm not mistaken the NABC ESC never approved fillet-welded staybolts in locomotive practice. That was not a decision made in ignorance.

I have been wrong before, but I think to criticize Kelly on the subject of locomotive staybolts as if he missed something elementarily obvious is not something I would do.

Mr. Ellsworth,

You mentioned pre-stressing a totally welded staybolt boiler?

An All-Welded boiler is/supposed to be put into a "Normalizing" Furnace to relieve the stresses from the welding before tubes are installed.

You questioned my stating the similarities between water-tube vs. a fire-tube boiler in regards to the welding; I'm not a welding tech but in my experience, they are very similar and I may be wrong on this; water is inside of one but outside on the other.

I am on this site totally for the educational value and exchange of ideas as applied to railroading; the 90% of the "other stuff", I find entertaining.

Paul J. Ferry

Normalizing, though desirable (and in my opinion, necessary) does not help the thermal-expansion problems involved with welded staybolts. The only real whole-boiler normalizing furnace was built at Alco in the late 1940s, essentially providing a controlled-atmosphere furnace around the sort of facility used to fabricate and work boilers vertically to relieve stress. It was mistakenly deemed useless and demolished when Alco stopped marketing steam power... just in time for the 'revised' Alco, repurposed as a nuclear company, to want and need it for normalizing large fabricated vessels...

The differential-expansion issue, as I indicated with the reference to Flannery staybolts, is between a cold firebox and a hot and pressurized one -- this comprises much of the discussion in the Tross paper. A secondary purpose of a whole-boiler CA normalizing furnace is that it can be used to resolve stress at working temperature and internal pressure, allowing the boiler to be constructed with staybolt geometry that is correct at intended working temperature and pressure. And problems with strain will then be present in the cold boiler, where failures are far less critical. Much of the import of cold-boiler strains can be solved with proper heating and pressurization, for example with a Direct Steam system, instead of the method of heating a cold boiler via actual firing, which either takes a protracted time or involves avoidable stresses.

Welding of high-pressure watertube boilers is very different from the 'normal' practice used in locomotive fireboxes -- although it would be found in watertube fireboxes like Emersons, or in the kind of modified LaMont forced-circulation waterwall firebox that people at SACA were so enthusiastic about a decade ago. These are usually tube-to-tube or tube-to-vessel circumferential welds, and don't involve a solid rod in a piece of heavy plate. The watertube boilers also usually use more exotic alloys and much more expensive attention to detail. On the other hand, when I was carefully looking at using watertube boilers for high-speed locomotives in the mid-Seventies, I had my boilermakers lined up at $35,000 per year plus benefits... they were all ex-Navy, where real-world welding of watertube boilers, blind in confined spaces with full penetration integrity, is a commonplace.

.

There is a series of photos of destruct testing on the UP in '49 that I couldn't transfer.

These show 3 staybolt heads after driving with 3 different types of snaps before welding.

From a previous article on this site from 2013

viewtopic.php?f=1&t=35461

As can be seen, the results of the "Destruction Test" in the tensile machine is still in the neighborhood of (30-38)Klbs which is respectable; I have a question from what I can see in the photos, is, the "Weld Bead" appears to have held, the bolt itself tore away. In the photo, it doesn't appear that the holes were countersunk for the full pen.

I didn't find any photos of Destruct Testing of the bolt ends with out welding, nor the pressures exerted for a comparison.

Thanks for Looking

Paul Ferry

If I'm seeing the same photos you're referring to, they're threaded staybolts which are seal welded to help prevent them from weeping/leaking. A small seal weld does not require any beveling or "countersinking" (as you call it)- as the weld is not meant to serve as any structural component for the staybolt to support the load- that job is still being done by the threads.

Mr. Boilermaker,

My question remains: what is the Tensile Machine value for the Countersunk/Beveled full penetration weld vs the threads vs the threads and "Seal Weld".

Thanks,

PJF

Kelly, on the threaded stays that you tested, were these Whitworth threads or UN? I have been told that staybolts are Whitworth threads. Curious that this might either settle or throw gas on the fire of which is stronger.