Heat Treating 1008

[blackout44]

Hey Folks,

Firstly I will apologize as this is not a specific thread regarding knife making. However, it seems this community is VERY knowledgeable in the heat treating of steel. I have read quite a bit on these forums, so thanks for the fantastic info. If this should be in a different section please move it.

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 hardness, although the hardening is not much due to the lack of proper alloying elements in 1008. After the nitriding, notes are pressed into it, and it is then heated at atmospheric pressure..... this is where my issues have 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.

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

So, for you experienced metallurgists, what is happening here? Is this a "process" anneal? On the molecular level, what is happening?

Thank you. Again, please move this topic where it is appropriate; sorry for the confusion.

 

[mete]

You're not HTing the steel to harden , only playing with stress relief and sub critical anneal .Any idea what temp the nitriding is done at ?

 

[me2]

Hardness testing on nitrided sheet metal must be done very carefully. The last hardness testing of that nature I did required the pieces to be sectioned, mounted, and microhardness tested. This is pretty interesting though.

 

[Stacy E. Apelt - Bladesmith]

From Stacy - All kinds of things can be happening in a very slow heat and cool cycle at 730F. Structures change into other structures. I would suggest you have metallurgical micrographic studies done to see what the structures are before and after HT.


From the Moderator - That said, this is a knife making sub-forum, and not a metallurgical site for instrument makers. I appreciate your coming here to look fort answers, but I am going to close the thread as it is not relevant to Shop Talk.
You can email or PM any of the folks here you wish to ask off forum.