Garage-annealing 17-4PH to H900

[Acousteau]

Hi, I'm new here, thanks for reading my question.

I have some thick-walled cylindrical tubes that were custom machined out of 1.5"D 17-4PH rod. These parts are going to be used in an ultrasonic wave generator which has to have the lowest acoustic damping possible: the SS tubes will be conducting acoustic waves along their length and must not attenuate / absorb the acoustic energy. The less attenuation, the better. It's a critical issue in this case.

I have read that metals carrying longitudinal acoustic vibrations exhibit lesser absorption of the acoustic energy with increasing yield strength. Materials with the highest yield strength have the least acoustic attenuation, the least pillow-like absorption of energy. I hope this rule-of-thumb doesn't have many exceptions.

Age hardening the -PH state alloy into H900 state causes an increase in yield strength, so I'd like to age harden the pieces I have before installing them in the prototype device. I have a simple, cheap ceramics kiln with a homemade temperature regulator that can hold the kiln temperature steady within 3'C.

The information I've googled online gives the consensus that I can slowly (?) heat the 17-4PH pieces to 482'C, hold for 1 hour, then "air quench", which for me means, open the lid of the kiln and let 'em cool down. Then I'll supposedly have pieces of 17-4 H900... if I'm lucky, and don't mess it up somehow. There is supposed to be no deformation from this process (H900 at least) and a very slight shrinkage.

Any opinions? Can I age-harden to H900 in a ceramics kiln, in atmospheric air, as long as the temperature and time are accurate? What does heating "slowly" mean? 17-4PH is supposed to be "simple" to age-harden. Is it *this* simple?

- Most of all, I need to find the alloys of aluminum and SS which have the absolute lowest attenuation of longitudinal ultrasonic waves (low band, between 20 - 80 kHz) possible. So far I am using 7075-T651 aluminum rod (waveguide section, must be a aluminum) and 17-4PH stainless rod (piezoelectric backweight section, must be heavier than aluminum and very strong). I settled on these alloys by opting for the highest yield strength among all the alloys available in low quantity online. Any opinions or suggestions ... or guesses ... would be awesome to hear. There is so much to learn. Thanks guys!

- Graham

 

[kenvlach]

Are you sure that the 17-4 PH is in the solution quenched condition ('Condition A')? If already aged, further aging will soften it. If unknown condition, perhaps do some hardness testing.

MIL-H-6875H HEAT TREATMENT OF STEEL, PROCESS FOR (S/S BY SAE-AMS-H-6875) is freely available:

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=6191

For H900 aging: Slow heating is only required as necessary to prevent damage such as cracking or warping (if no damage, can even put into pre-heated oven). Air cooling after 1 hour is usual.
See Table 1D & footnotes therof.

Carpenter Technologies gives the contraction of 'Condition A' material upon H900 aging as about 400-600 ppm (linear).

 

[EdStainless]

Yes, aging 17-4 is easy. Try a small piece first and make sure that the hardness comes out correct. You can also get some good info from mill data sheets, try AK and Allegheny.

I this is a heavy wall part I would put it into a hot furnace. For max hardness you don't want to heat any longer than 60min.
For cooling just pull it out and place it in the open, but not in a draft.

Since you want this part quiet wrt UT signals I presume that you will finish grind it after heat treatment to remove any surface texture that cause noise.
I believe that internal stress also interferes with UT signals. Uniform heating and cooling will help with this.

There are some Ni based aircraft alloys with very low UT attenuation. It has to do with the crystal structure of the metal.

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http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

One particular problem when using a ceramic kiln that uses an on/off controller for heat treating of any kind is the exposure the part being heat treated to radiant heat form the heating elements unless the elements are covered. Having used kilns for all sorts of things other that their intended use is that some type of muffle or shield is needed to keep the parts from overheating from the radiant heat.

One other caveat is make sure your 17/4 parts are clean prior to heat treating unless you want your fingerprints engraved in the part.