Heat Treating 1008 1

[blackout44]

Hey Folks,

I am a builder of a steel based musical instrument that has a complete dependency on proper heat treatment to achieve the desired tone. The instruments are known as "handpans", descendants of the steel pan from Trinidad and Tobago. A youtube video showing these instruments is here:

http://www.youtube.com/watch?v=Sp5dgy8UDKs

I'll cut to the chase. The instrument is constructed from 18 gauge (1.2mm thick), 1008 grade sheet steel. It is cold worked heavily by hammering the flat sheet into a bowl shape, about the size and shape of a Chinese Wok. At this point, it is Gas Nitrided to add rust resistance and slight surface hardness, although the hardening is not much due to the lack of proper alloying elements in 1008. After the nitriding, notes are pressed and shaped into it, and it is then heated in a kiln at atmospheric pressure..... this is where my issue arose.

Nitriding is great for its rust resistance, however it kills the tone of instrument. It mutes it. A musician would term this as having "no sustain". The notes will not ring for several seconds as they should. However, I have found that heating the already nitrided steel to 730F for an hour, and then cooling at a rate of 30F per hour to room temperature has increased the sustain, or ring, of the instrument. Using faster cooling does not yield the same resulting increase in sustain. The slower cooling would suggest that I'm annealing it in a sense, but it's getting NOWHERE near the critical temperature to begin with. Am I correct in stating that the crystal structure of the metal has not changed? Heck, what is the structure of it? Bainite? Pearlite? Definitely not martensite based upon my reading.

I have had hardness tests done on the metal before and after nitriding, and on instruments that have the desired tone that were not nitrided. However, hardness showed no relation to the amount of sustain of the instrument. Regardless, the hardness is around 65 HRB. It is important to note that these were done on standard rockwell hardness testers, so the results of testing nitriding sheet steel may be quite wrong, or so I'm told.

Another interesting fact is that the instruments have huge amounts of sustain ("ring") if they are not nitrided at all. Of course, the nitriding is mainly done for the rust resistance, and a slight change in tone.

It seems that changes that most metallurgists would right off as insignificant actually have huge effects when they are affecting the acoustical properties of steel.

So, for you experienced metallurgists, what is happening here? Is this a "process" anneal (if not, what is it)? On the microscopic and molecular level, what is happening? Is grain size increasing, decreasing....?

If there are any studies that anyone knows regarding 1008 and heat treating, or even nitriding and heating, please direct me to them. I'd REALLY appreciate it.

Thanks!

 

[CoryPad]

The structure for 1008 steel is almost all ferrite plus a little pearlite. That is supported by the low hardness values.

Nitriding is not that good for corrosion resistance. Can you consider stainless steel? It would be just as formable as 1008, and there would be no need for nitriding.

Your heat treatment after pressing notes will not change any structures, but may do some stress relief.

 

[Goahead]

Would it be possible to consider phosphate conversion coating instead of nitriding for limited corrosion resistance?

http://www.welding-advisers.com/

 

[arunmrao]

I agree with the above points. Can you consider spring steel for your application. It will retain the ring for a longer time without significant dampening.

 

[tbuelna]

blackout44-

As others noted, gas nitriding will not produce a corrosion resistant surface. In fact, it is normal practice to remove (chemically or mechanically) the thin outer surface "white layer" left after nitriding. because it is very brittle and prone to fracture.

I imagine that the nitriding process would have some effect on the modal response of the steel pan. The nitriding process produces residual compression in the outer metal surface, which could produce a different response (or tone) than a stress-relieved pan structure would when it is struck.

If you are performing even moderate metal forming operations after nitriding, then you are likely destroying the nitrided case in the formed areas. A nitrided case is extremely hard and has very limited ductility, so it does not like to be bent.

Since your steel pan relies on lots of manual cold-working (ie. hammering) to fine tune its sound, you might consider using a steel alloy that work-hardens more readily, and skip the nitriding altogether.

Hope that helps.
Terry