Stainless Steel Nipples - Aluminum Housing

[hackie]

Hi,

Am concerned about a mechanical connection I am designing between 316 SS nipples into a 6061-T6 Al housing. Temp range is cyclic, from ambient to 600 deg F. Housing is then cooled back to ambient using PAO heat transfer fluid. This process is likely to occur 500-1000 times during product life cycle. Does anyone see areas of concern? The thermal expansion differential seems small enough.......

Also, does anyone have a recommendation for a high temp thread sealant compatible with PAO's? Using graphite paste, but tends to solidify & flake.

Thanks

 

[metengr]

I see one serious problem with your mechanical connection, based on your stated service conditions. Your reported maximum service temperature of 600 deg F is pushing the limit for 6061. Above 350 deg F, the 0.2% offset yield strength exhibits a noticeable drop.

For example, at 300 deg F, the reported yield strength of 6061-T6 is 31 Ksi. At 400 deg F, the yield strength drops to 15 Ksi. At 500 deg F, the yield strength deceases to 5 Ksi, and at 600 deg F, the yield strength is 2.7 Ksi.

The bottom line is that the 6061-T6 housing at 600 deg F will be subjected to local creep deformation at the attachment locations of the 316 ss nipples. These connections are like stress risers in the housing and will leak after several temperature cycles.

 

[hackie]

Thanks you for your response.

Would the LCD at the NPT connection be exacerbated by a temper of 6061-T0? I ask because during the aluminum housing is actually 2 pieces dip-brazed together to accomodate a coolant channel. During this process, the aluminum is brought to the the T0 state. The brazer then brings the assy back to T6 state. Is there a temper better suited to this type of connection?

Perhaps 7075 or even titanium would be a better alloy solution for the housing?

I do also have a choice in nipple material, and am considering a static radial o-ring seal-type connection instead of NPT. Would I be better off with this type of connection?

Thanks!

 

[CoryPad]

7075 would be no better than 6061. A titanium alloy would be a huge improvement.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EdStainless]

Al will not cut-it at 600F. If you use Ti use an alloy like 6-4. The commercialy pure grades are not very strong and soften quickly at temp. Using 6-4 in the annealed condition should be fine.

If you go to to Ti you will want a fitting with lower CTE. Does this connection need to be diconnected often? If not, I would make it out of Ti also. If it does you might want to consider a ferritic stainless.
If you use an o-ring seal make sure that the o-ring is Aflas. For extened high temp service nothing else will come close.

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