Half-remembered advice

[AtomicClock]

Several years ago, I was having some work done to a damaged slide on a Bach 42. I don't recall exactly what the problem or fix was. But the tech said he could probably fix it (and was ultimately successful), but can't always. Apparently, the same repair done over and over changes the metal, rendering future fixes impossible or temporary. He said that they typically can perform the repair three times before just replacing the part instead. I think it was outer slide tubes, or maybe the slide crook.

Does this sound familiar to anyone? What could he have been talking about?

[JohnL]

Yes, it's a thing. It's the same principal as when you bend wire back and forth to break it. Too many dent/undent cycles and the material will crack.

[tbonesullivan]

Definitely true. Just look up "work hardening". Different metals have different properties, and being deformed changes those. There are some guitar makers that use nickel silver frets that have been bent back and forth once, as it makes them harder. This is also why you will see some people use a torch to "anneal" really nasty brass creases, as in a way it can help reset the crystal structure in the alloy.

[izMadman]

It seems like doing the same repair repeatedly can change the metal enough that eventually, you have to swap out the part. I’m not 100% sure which part it was, but it makes sense that after a few fixes, it’s better to replace it entirely.

[Burgerbob]

That can go for any part, really. A bell that has been run over and repaired to the right shape isn't going to play the same, much less after 3 or more times.

[Crazy4Tbone86]

Well folks……..I won’t say that metal has nine lives like a cat, but the “dented three times and it is done” phrase is quite false.

First of all, metal tends to “work harden” much faster when dent work is done with a metal hammer. Even that has different degrees of severity. An alloy will work harden faster if it is struck at a direct 90 degree angle during dent work. To harden the metal less, a tech should attempt to strike the metal with sideways “glancing “ blows. This gives the alloy a chance to stay softer and allows more dent removal cycles before it work hardens.

Another way to reduce the metal hardening process is to use non-metal hammers. For some types of dent work, I like to use delrin and canvas hammers. Canvas hammers do a wonderful job of fixing dents with no marring (and almost no hardening) of the metal.

Of course I am oversimplifying this topic, but there is the whole area of annealing. Annealing is the process of heating an alloy to the point of being cherry red and allowing it to cool slowly. Annealing realigns the molecular structure of the metal and can make the alloy very soft in many cases. At that point, it might be best to temper the metal a little bit to get it back to its original state. I like to burnish the metal a little bit to temper it back to a desirable hardness.

To address the original topic. No…..a piece of metal is NOT necessarily done after it has been dented three times. I have seen student trombones come into my shop with mangled tuning slide crooks (dented and fixed at least 5 or 6 times). I pop the crook off, anneal it and remove the remaining dents (which can actually temper it back to the original hardness) and the crook is ready to begin a new life cycle. Believe it or not, an annealed (and lightly tempered) component can sometimes make the instrument play better than the original.

The one part of a trombone that is tough to anneal is the hand slide. The outer tubes can sometimes be too thin to anneal. However, I have been able to anneal quite a few outer slide tubes with success. You just need to burnish them quite a bit because outer tubes are usually much more tempered than other parts of a trombone. The tubes need to be rigid in order to stay straight.

Another part of a trombone that can be annealed successfully is the bell. I have annealed 7 or 8 Elkhart 8H/88H bells with great success over the years. Again, you need to “work” the metal after the annealing because the red brass becomes dead soft in the annealing process. I keep rolling them on a tool called the “fast Eddie” and working the throat and stem on bell mandrels and those bells come back to life pretty well!

[AtomicClock]

The one part of a trombone that is tough to anneal is the hand slide.

Mine is a "lightweight" nickel silver slide, if that makes a difference.

[BGuttman]

Brian was describing a technique called "torch annealing" which can be difficult to do on precision drawn tubing like a slide. You can anneal a slide in an enclosed oven, but those are expensive to make and run. No repair shop is going to have such a device on hand -- takes up too much room and isn't needed that frequently.

Atomic -- nickel silver is an alloy much like brass and can be annealed much the same way. The nickel in the alloy makes it harder and dent removal is more difficult.

[Crazy4Tbone86]

Nickel Silver can be annealed as well, but it responds (acoustically) differently to tempering. In general, the softer the metal, the “warmer” the sound it will produce. If the metal is too soft after annealing, the can be very dull or dead. Thus, the need for tempering the metal to achieve the vibrant sound needed for instrument components. If you temper too much, the sound can get too bright or even edgy/brittle.

Nickel Silver is unique in that the more you temper it, the warmer the sound. That is why the Elkhart Conn 8D horns were so popular. Conn got very good at tempering those nickel silver bells the correct amount so that they were vibrant and warm sounding. I believe Mick Rath got pretty good at tempering his nickel silver bells as well. Every one of those I played at shows was rather impressive.

I have not worked much with annealing/tempering nickel silver very much. For a nickel silver slide, I imagine that everything would be similar to a brass slide, but it would require more repetitions of tempering with a roller tool and/or burnishing tool.

[timothy42b]

If I am remembering that long ago class correctly, brass alloys only work harden, unlike steels.

As you heat steel, the crystal structure can change to a harder version, and if you cool it fast enough it stays hard. And if you heat it to the right temperature, it can relax back to the softer state. (one of multiple states. Steel precipitation hardens)

Brass doesn't do that. But as you work it and stretch the crystals, moving the dislocations etc., it work hardens. Annealing it is heating it just enough it can relax back to a softer state, but it might not be the identical one.

If there are residual stresses, they are going to relax and release also, and you might not want that. Ever put the wrong piece of plastic kitchen wear in a microwave? You get a mangled mess, as the plastic gets soft enough to let the manufacturing retained stresses move the material. Can that happen to brass? Very likely, especially if thin.

[Chronos91]

Looking at a phase diagram between copper and zinc, if the zinc weight percentage is 30% or less (as it is for yellow, gold, and red brass), you just have a single phase system, so you can't use heat treatment to get higher hardness. Like Tomthy42b said, with steels you can manipulate the phases that are present with heat treatment cycles.

[BGuttman]

Looking at a phase diagram between copper and zinc, if the zinc weight percentage is 30% or less (as it is for yellow, gold, and red brass), you just have a single phase system, so you can't use heat treatment to get higher hardness. Like Tomthy42b said, with steels you can manipulate the phases that are present with heat treatment cycles.

Working the metal increases the hardness. Annealing (heat treatment with very slow cooldown) allows the dislocations in the metal to disappear allowing the metal to get softer.

Back when I was working with brass parts (not for musical instruments) we had three degrees of hardness: soft, half-hard, and hard. I'll bet there were durometer values to describe them, but I can't remember what they were.

[JohnL]

I'll bet there were durometer values to describe them, but I can't remember what they were.

More likely defined by yield strength and/or hardness (Rockwell B in thicker gauges, Vickers in thinner gauges).