--> *Non Engine*Cast Iron Crack Repair - How To?

Not an engine...
Was asked by a piano shop if it is possible to repair stress cracks on the harp of a 100 year old piano. Removing it from the piano is the easy
part.
The cracks are on the "struts" that run from the tuning area up to the curvy part of the harp. A repair has obviously been done once before (probably not a good thing) and this too has cracked. At this point I don't know if that was a braze or a weld. This is a known weak point on all of these harps from this period. The stress crack is about 1"-2" long and essentially cuts the strut along its width, 10" or so from where the tuners are located.
The question I have is can this sort of crack be TIG'd or MIG'd with or without a pre-heat and if so, what alloy would likely work. Let's assume for the discussion that there were no prior repairs... then factor in the variables.
On my mind are thermal expansion coefficients as well as hardening of the casting in the weld area (making it easier to crack?).
We have access to a local foundry that would make us a section of this strut *if* it made sense to cut a section out and splice in a newer better designed (better profile, less likely to stress crack) section. We can precisely cut it along a long path, for example.... (assuming some sort of weld/braze would *work*)
Heating the entire harp is not likely to be practical. ( possible but unlikely to find a large enough or hot enough oven?? This is a Baby Grand size)
The cracks, btw are near where a large screw fastens the harp to the soundboard - and look like the force of the screw holding the harp down (wood expansion, etc...) has pulled the harp *closer* to the soundboard and caused the crack to propagate from the bottom up... forces *up* at the ends, and down at the screw... crrraaaaccckkk...
Anyone with experience and expertise ("the answer") please also email me at "bear at bearlabs dot com" - TIA.
_-_-bear
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I've had good luch repairing other cast items with tig and stainless wire with preheat. Mostly high nickle cast manifolds, but also some old tractor castings etc. Works better than the old stick-welder and Ni-Rod.
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wrote:

I did the exhaust manifold with mig and plain steel wire, it held up for 2 years of playing and plowing Pat
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There's an _awful_ lot of stress on a piano's harp, though. Many tons, with fatal failure modes if it lets loose.
These struts are under compression, if I got your description right, yes? Can they be replaced outright? Does it need to be cast iron, or could steel be machined to the correct shapes?
Dave Hinz
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Dave Hinz wrote:

Yes. Think of it as a "D" shape, where the struts run more or less on top of the "D" and ---> this way across the face of the "D", as do the strings.
Compression, and here obviously some bending... although now that you mention it the cracks show in the opposite direction as you would expect if it was due to the tension created by the strings... thus my guess that it is/was due to the expansion of the underlying wood sound board that in essence pushed the ends of the "D" this way <--->
So, whatever is used for the repair will need to be equal or stronger than the original casting, AND not mess up the surrounding cast metal in the process.
Replacing these struts entirely is not an option due to the way that the harp is made, as far as I can see... plus, even if you could replace it, you can't put in stuff that is going to move at a different rate than the rest of the harp or you're back where you were before, merely transferring the nasty stresses around until a weak point is found again.
It's some what different than repairing the crack in a block or a head because of this, and the different aim in the repair - the automotive repair usually only cares about a pressure leak proof result, the item is usually not under stress the same way. Or so it seems to me.
_-_-bear
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Try that on a high nickel thinwall automotive manifold, and it will often let go in less than 2 months. Yes, you CAN get lucky -----
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Why weld at all? Use locks to bridge the cracks and stop them from moving or spreading.
http://www.locknstitch.com/AboutLocks.htm for some pointers. I have made some and used them for repairs and they work great. Even repaired a cracked engine block using a lock under a head gasket. Held until the guy wrecked the car...
--
Steve Williams

"BEAR" < snipped-for-privacy@netzero.net> wrote in message
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(snip)
OK A repair may be possible. A lot depends on the quailty of the casting. In your favor are the facts that the piece is very clean, and was likely cast with a great deal of care.
I have done many cast iron repairs of all sizes.
The main enemy is heat or more precisely uneven heat.
I would recommend TIG welding with pure nickle rod or TIG brazing with silicon bronze rod.
Either way the procedure is the same. Preheat as large an area as you can. It doesn't need to be red hot. 500 degF is plenty. You don't need a furnace, just a propane weedburner
http://www.flameengineering.com/Back-pack_Kits.html
and a tempilsik
http://www.tempil.com /
Use the tempilstik to mark various areas. Heat until you get the color change in the mark.
Once the whole piece is warm, bury it under heavy wool blankets or powdered lime. Leave the areas to be welded exposed. By trapping in the heat you shouldn't have to worry about the rest of the piece for a while.
Clean the areas around the cracks with a die grinder so you have clean metal. Now tack weld or braze a few points on each crack. Go in with a carbide burr in a die grinder and V-out the cracks between the tacks. LEAVE THE TACKS IN PLACE.
You want to V-grind out a nice deep groove from both sides if possible.
Now whether you use nickle or bronze the same rules apply. Low welding heat. Lots of small weld passes rather than one big one. As little diffusion of the filler metal into the base metal as possible. Scatter the welds around, once again so you don't get too much heat in one place.
When the welds are all done, reheat it with the weedburner and cover it again. This time just let it cool slowly on it's own.
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On Fri, 06 May 2005 23:06:03 GMT, Ernie Leimkuhler

Greetings Ernie, I have tig weld repaired cast iron with bronze several times and have had great success with the process. The only times I had trouble were when the weld area needed to be re-machined. Twice there were very hard inclusions in the bronze that could only be machined with carbide cutters. Were the hard spots caused by too much heat and too much mixing of the cast iron with the bronze? All your above advice is great and made me remember a couple things you've said before about cast iron repair. As evidence of the strength of using bronze is a bearing bore I repaired on a 1945 7HP Wisconsin engine. The flywheel struck a rock while the engine was running and the block cracked big time right through the bearing bore and into the block surrounding the bore. A Timken tapered roller bearing goes in this bore. Since the repair about 8 or 9 years ago the repair has not failed. And the engine is always run flat out, never babied. Eric R Snow
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Likely, but cast iron is a weird character, and there are so many variations and types of castings that it is difficult to state one set of rules. The only cast iron repair method that encourages dissolution of the filler into the parent metal is gas welding with cast iron filler rod.

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I wish I could find more info on this particular technique. The only thing I've read about it is a story, many years ago, about the guy that Harrah brought in from Italy or somewhere to weld the cracked block on his Bugatti Royale. According to Harrah, the guy was supposed to be the most renowned practitioner of this particular art.
It worked, BTW. The crack was in a cylinder wall. However, after it was fixed and the engine was re-bored, etc., Harrah ran it for less than a minute and then shut it down again, possibly forever. So we won't get any benefit of experience from that particular job.
-- Ed Huntress
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I think this is a challenging repair mostly due to the stresses that are involved after the repair is made, and any change to the casting, including the addition of the weld metal and the resulting HAZ in the cast iron is going to have an affest on the sound quality coming from the piano.
Having said that if you decide to attempt the repair I would us Inco Ni-Rod 55 for the repair with a MIG setup. There will be some presheat needed, peeening between passes, and ideally some slow cooling after all the lasses are made, perhaps covering the weld with a thermal blanket. I don't have my brochure available on Inco Ni-Rod 55 to tell you maximum interpass temperature, and the recommended preheat, etc. But, if you contact International Nickel Co. (INCO) then you could request your own brochure.
I would recommend the MIG repair with that rod over a SMAW process with stick electrodes.
You will find that the nickel wire or rods (approx 55% nickel) are the best materials to repair cast irons.
Unless you are replacing a significant section that is missing I would not fool around with a redesign because you will change the nature of the sounding plate and the piano will not sound the same. It's never going to sound the exact same as when it was new from the crack and repair anyway.
BTW I am not a welder but a metallurgist. I wish I knew how to do the welding itself but my expertise is the metallurgy involved, microstructure, etc. Someday I hope to purchase my own welder and practice at home to know the manual skills part better.
Mark

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I don't believe that welding a crack in the metal harp of a piano is going to change the sound. The metal harp (or plate) only holds the strings allowing them to be stretched to a specific tantness. The sound comes from the string and sounding board. Lane
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yes it is, make sure they know what they are doing
simple answer huh <g>

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On Sun, 15 Jan 2012 18:37:29 -0500, snipped-for-privacy@nospam.com wrote:

I sure would not want the job - but here's what to consider. You need to end stop the cracks - which means finding the ends and drilling them. Hoping not to hit any carbide inclusions in the casting (they are hell on small drill bits) Then you need to clean and grind out the cracks, from both sides - and then evenly preheat the whole casting to prevent warpage, and then , when it is so hot you can hardly stand to be close to it, you need to tig it with a high nickel rod - I like stainless steel, myself, and pean the weld while it is still hot. Best to have 2 welders going at once - one on each side of the crack - and when it's all done you need to carefully cool it, evenly, back down to below 200 degrees F - all the time praying it doesn't warp, making the whole process a big, expensive, waste of time and effort. Would have to be quite the piano to make it worth the effort, in my estimation.

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Search on cold welding in SEJW - I've described the process at least twice, it works, but if the casting is poorly designed in the first place it will probably fail again anyway. I was taught the process in part because it is a way to salvage cracked engine blocks without a huge oven. Crack prep about the same, no preheat, weld a short section with nickel rod, then peen it until cold, weld another short section and peen, stop if the thing starts to get hot. The peening is rather important. I'm not going t retype the whole thing, it's out there.
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