.

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If memory serves -- please correct if wrong -- are you to go just one size up when enlarging threaded stays? On one job we did, we went up 2 sizes, simply because the correct tap was readily available in that size and for the length of those crown stays. It worked very well, and the inspector was giddy! There was one small bead of water when we did the hydro, and we pointed that out to the inspector who said go ahead and fire her up and that will take up -- we did and it did.
Always work through your local inspection branch, and keep your inspector as a friend! Use materials with a paper trail, bought from recognized sources. Better to get paperwork on everything.

The fillet-welded 'controversy' was a timeless topic in the discussions over the past decade or so that, in part, culminated in the resuscitation of an ASME locomotive-boiler code. They are predominantly a European development, really a postwar development, and there was a seminal article on them in Glasers Annalen (a German publication) circa 1950 that Wasatch thought important enough they had it translated. Anyone commenting on fillet-welded staybolts needs to have read this carefully, and probably taken some notes and asked some questions about it.

As noted, the point is that (completely unlike seal welding) the fillet weld, though completely outside the 'clearance' hole for the stay, is of adequate dimensions to constitute the high-integrity (as though for full-penetration weld assurance) 125% of stay cross-section that the code calls for. To be sure that both this and the necessary 360-degree integrity of fillet dimension and profile have been achieved, I'd expect some sufficiently-advanced NDT technology, on the level of the radiography used for confirmed weld integrity in some critical fabrication, would have to be used. The problem is that there is an inherent crevice on the waterside on both ends of a fillet-welded stay; moreover, inherent in some of the bending diagrams including the one just provided is the tacit requirement for potential stress raisers due to the hole in the plate to be minimized, e.g. via a combination of reaming and waterside radiusing of the edge. Discussion of these points is often absent from lay discussions of the things.

The fundamental point of threaded staybolts is that the thread be rolled to the right profile, to mitigate cracking at the roots, and the cumulative mating surface of the threads provides the 125%-or-better pressure-bearing surface (which is part of the three-turn requirement, choice of thread pitch, etc.) It has been my understanding that 'best practice' with threaded bolts is that the 'outer' and 'inner' threads are cut separately, but coordinated so they start to 'bite' simultaneously when installed; that the outer thread is at sufficient larger radius (and comparable tooth profile, etc.) that the bolt can be fully inserted and the inner liner properly 'orthogonally' adjusted before either thread actually engages anything, and that the taps for firebox staybolts are made so that they will cut the necessary coordinated threads regardless of whether the sheets were 'precisely' spaced at the time of installation. What Kelly is describing as post-hammering the ends of the bolt is not (if I understand him correctly) to enlarge the cross-section at the threads to give more of a precise interference seal fit, but to relieve any driving galling or stiction so the machined faces of the threads actually contact each other firmly for pressure bearing. Seal welding would be just that: ensuring that nothing gets into the ends of the threads to cause progressive failure -- the not discussed issue being that you can't effectively seal-weld threads on the waterside, which is really the only place you'd be concerned about that, so while you're short-term preventing induced stress allowing deformation of the thread followed by water intrusion from actually weeping into the firebox space under pressure, you're still setting up for solids intrusion, crevice corrosion, etc. when water, especially modern boiler-treated water, gets in along the threads.

My ASSumption was that seal-welding in the firebox also did the same thing as those little hats Union Pacific sometimes used: keeping combustion-gas heating from spot-heating or burning any exposed thread. If the bolt has a telltale it becomes relatively simple to design a cutter that will remove seal weld quickly, effectively, and orthogonally from a staybolt that is to be removed, and by extension to center over a fillet-welded stay to cut it free for servicing.

Note the provision in code that Jason quoted which allows a full-pen staybolt weld to be made entirely from one side. That is explicitly because welding from both sides, while now technically possible with purpose-built robots, isn't really practical in most restoration work -- certainly on #14 under prevailing conditions. So you'd need to bevel substantially, center the stay appropriately, take a full-pen first pass and then confirm 360 degrees of good bead inside the waterspace before finishing the weld and whatever fillet is appropriate to a full-pen weld inside the firespace or at the outer wrapper. I'd be at least tempted to set up a sufficient borescope or camera arrangement through the waterspace to let someone inspect these welds from the waterside shortly after process, when perhaps a little additional heat or rework could fix any waterside shortcomings quickly and definitively.

I do think it's more than mere semantics to use the word 'stay' rather than 'bolt' to describe any fully-welded component. To me a 'bolt' has connection threads, and you do NOT structurally weld something threaded by choice -- not staybolts, not rod, and especially not threaded rod. (This before we get into issues of metallurgy, which is where this situation jumps the shark in utterly improbable ways...)

I don't see any particular drawbacks about using welded bushings to restore threads for staybolting ... if they, themselves, are properly full-pen welded. Someone here can probably advise me why threaded bushings with tappable bores couldn't be used too -- with adequate length of bushing you might even be able to rotate a threaded bushing to full nominal depth and then align it to correct 'register' with threads in the other sheet. That might indeed be better than trying to add weld metal without HAZ or inclusion damage, probably not downhand, in a confined space...

The thing that is bothering me here is that we've had at least one account that says Gary Bensman knowingly installed tubes in the wrong sheet holes, and then John Rimmasch kept them there. That doesn't make sense to me; in fact it doesn't make sense on many levels.

Now, the closest thing I have to firsthand experience with Wasatch in this context is that I went up to look at the butcher job that was done on 3713's firebox when Bruce Mowbray extended the RFC. It still is incomprehensible to me that someone with supposed 'knowledge and expertise' couldn't even drill properly matching holes in a formed inner wrapper (we won't get into syphons) but perhaps Mr. Mowbray could comment on the Wasatch inspection and the details of whatever proposal they sent him afterward. That is a far more demanding application than #14 ever could be.

To me, much of this hinges on the carriage bolts, a whole double row of them with excess length in the gas space, all supposedly hammered to get the little square down so the heads looked more like seal rivets. I want to know who did that, who bought the bolts, and who did the hammering. And I want to hear the careful description of what they thought they were accomplishing thereby.

Henschel fillet-welded staybolt - former boiler of DB 01-1100

Threaded staybolts - large American locomotive

Thanks Kelly and yes I think a similar name was used. Believe in the application I was involved in the "asshole bushings" were used in the newly replaced side sheet area. All holes in the outside wrapper were bushed down to I think 7/8". Probably the bottom 36" of side sheets were replaced that time?

On another project at about the same time frame a boiler with damaged or enlarged stay bolt holes were welded up, ground flush and re-drilled and tapped to 7/8". Two separate jobs, same museum. Not a boiler guy but I thought the "asshole bushing" approach was novel as I had never seen it done before. Not in the same vein as welding the stays, but thought it close enough to mention. I have no pictures that I am aware of. Just to add to the discussion of staybolt replacement. An option if you will. Blessings.

Hi All

Just because one shop has trouble with full penetration welds does not mean that they are bad just that one shop does not understand the technique. Consider the following video https://www.youtube.com/watch?v=dAV2mPbv-EA

And as others have said a bad full penetration weld still has more material than a fillet weld done properly. Note the depth of crevice in the poorly welded stay bolt is still not as deep as the one for a fillet weld. Now this is a topic where the "field" people do not understand the position of those who went to college. In the end there are a lot of issues buried in the staybolt attachment of choice.

Robby Peartree

Lots of good discussion here and I was just recently aware of the full penetration welding. In the video he is most definitely right. The full penetration stay bolt welding is same process used in welding pipe. You start with a 6010 rod to fill in the gap and then typically go to a 7018 rod after. Pipe welding much like boiler welding is very much at the elite level of welding. Most weld test require you to weld two plates together, upward, with maybe a 1/16 inch gap with full penetration. Not that difficult.

When I took a weld test for General Dynamics they wanted a upward weld with a 1/4" gap, no backing plate, no stopping, no undercutting, and top and bottom ends also fused flush on both sides. I looked at them and said "are you guys crazy"? Any pin holes or bubbles was a fail. Then they cut your plate into 4 strips and bent them nearly in half 4 times in opposite directions. Even if the weld didn't crack but the metal did you failed. It seemed to me at the time a really impossible weld. I asked if I could see a demonstration and some advice which they provided.

It required a new learning skill on my part but I quickly learned that molten metal is surprisingly stable if you can keep that arc under it, and it has somewhere to flow. It didn't take long for me to fill that 1/4" gap with at least a 1/8th thick weld on the other side. It doesn't look pretty on the other side but if you grind it you will find that after some practice it is nearly always a full penetration weld.

However the crevice corrosion problem spoken about earlier is very much a problem if you are blind welding pipe, tanks, or other places where you can not see or grind the rear of the weld to check for valleys and pits. Years ago I used to weld holes and cracks in leaking hydraulic cylinders. You would see previous welds over the hole sometimes inches thick like bird crap. No matter how many times you welded that hole it didn't matter. That hole was the weakest point in the cylinder and all that pressure would continue to push into that hole and out the weld. I cut a cross section on top of a big pile of weld with a cutting wheel, using a angle grinder, to see what was going on with this hole. I was amazed at what I saw. A river like channel smooth as silk from that fluid cutting through this weld.

The only solution was to grind off all that weld, drill a hole completely through it, and weld it up 100%. Later a careful hand with a dremel grinder was required to smooth out the inner cylinder back to the way it was before. I understand why the FRA does not allow torch cut holes on that boiler. The pressure and stress should be uniform around a hole. You get that with a nice smooth drill cut hole. With a torch cut hole you get valleys, channels, dips, etc. Uneven stresses. Each V grove is a where a crack would start. When dealing with high pressures you can always bet that pressure will always find that little dip, crevice, grove or other and dig away at it.

I do believe the threaded stay bolts are the best option. I understand the simplicity of the full penetration welding and it will likely cause the least problems. However replacing those stay bolts would require oversize hole drilling like they had previously. Drilling through welds is extremely hard. Even a portable mag drill with a carbide cutter would be difficult. I suppose you could cut the whole thing out using a cutting torch at a angle. However you would still have a lot of work to to smooth out that hole with a grinder.

Someone was asking about oversize holes and how to fix them. Many people fill the entire hole up with weld which is unnecessary since you will be drilling most of it out. Simply running one pass around the inner hole is usually sufficient to build up the wall.

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Remembering to turn the appropriate transition profile between the new principal diameter and the 'roots' -- and to turn a transition at the threads to prevent enhanced SCC there.

Interestingly, this precise issue is discussed in the Glasers Annalen article I referenced earlier -- EDIT: it is apparently from 1953, not 1950 as I thought, and the translation can be found here:

Intro to fillet-welded staybolts: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiCh4qIqr_tAhVBmVkKHa_vAz4QFjARegQILBAC&url=http%3A%2F%2Farizonamechanicalengineering.com%2FLibrary%2520Documents%2FFWS1-Intro.pdf&usg=AOvVaw2dj5tImBU7fA7roBNU6DCg

The translated article: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjz-onCqr_tAhUjpFkKHWNxDI4QFjAAegQIAxAC&url=http%3A%2F%2Farizonamechanicalengineering.com%2FLibrary%2520Documents%2FFWS2-Rear_Boiler_Knowledge.pdf&usg=AOvVaw0lv5NkWtID0FdnHtrGTWzb

(I apologize for the crappy Google URLs but they use an unfamiliar encoding hex in one place and I can't figure out how to reduce them to just the actual URLs...) The relevant part of the article begins on p.4.

Note that reduction of the area of the stay to optimize unavoidable bending behavior pertains in a couple of respects to correct practice of these 'fillet-welded stays' following European practice. I believe the translated term is 'necked' stay, I think from 'aufdornstehbolz' which is a nominally a mandrel-form bolt

My introduction to practical pressure-boiler welding came from a couple of Navy guys when I was doing a startup in Newport in the early '80s. These guys could do a full-pen weld in superheater elements blind in other than assured downhand position -- something I looked upon, and still do, as tantamount to practice of black magic. This being for far higher pressure and temperature than anything in locomotive practice.

There have been some highly interesting developments in multiple-heat keyhole welding in the past several years, which make full-pen-welded stays much more practical as a semiautomated shop procedure. I think we are rapidly approaching a point where this will change boiler fabrication even more dramatically than adoption of 'death ray' NDT has... in some cases. I tend to agree with the previous poster that properly tapped, inserted, and relieved threaded bolts are still better in many contexts.

Thank you gentlemen for the latest batch of helpful replies. This thread has some great information.

I agree that this staybolt thread has been fascinating. Some great photos which are really helpful to visualize what is being discussed. Thanks to all the posters for a great technical discussion.

Staybolts seem to be s very robust technology. On one traction engine project, replacing a number of water leg stays, I found one with the holes badly misaligned from the factory -- perhaps as much as 3/4 of a hole! The holes were reamed and threaded accordingly and the stay bucked in. We over sized it and used the same holes with no leaks. Friends have found similar things. Perhaps the guys back in the factory just let 'er go and never said anything, rather than get chewed out by the foreman.The engine worked hard for many decades and still held water 100 years later, so that stay did its job. There was a good arrangement of stays in that boiler with 4-inch pitch.
On another project, an inspector wanted us to replace a good sampling of the crown stays owing to necking. That was fair, because some were noticeably necked. It was a fair bit of hard and dirty work, but the results were pleasing to us and the inspector.
On removing those necked stays, we found they were still good, strong metal for close to 3/4 the original diameter.
Is there a hard and fast rule for allowable necking of crown or other stay bolts?

I would say 'no', and I would argue that the answer should always be 'no' on general principles -- assuming we are talking about 'accidental' observed necking, not the intentional reduction of diameter to change the bending characteristics etc. of a staybolt.

Staybolts are tension members; like a chain, they fail completely if they fail at any weakest point.

The issue of observed 'necking' is not just the reduction in cross-sectional area, it's the probable development of stress or other cracking in the yielded area. Since it is difficult to impossible to conduct NDT on the necked areas to verify the absence of these or other stress raisers, the sensible thing would be to replace them.

On the other hand, when steam boilers were used in commerce I wouldn't be surprised to find some discussion of allowable limit. As I couldn't separate sensible from crapshoot, I'm not the one to speculate what a "rule" for it might be.

I always figured that if a threaded stay wasn't necked down below root diameter, it was good, since the were installed new prenecked at that point.